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Food waste valorization advocating Circular Bioeconomy - A critical review of potentialities and perspectives of spent coffee grounds biorefinery
Accepted Manuscript Food Waste valorization advocating Circular Bioeconomy -A critical review of potentialities and perspectives of Spent Coffee Grounds Biorefinery
Anastasia Zabaniotou, Paraskevi Kamaterou PII:
S0959-6526(18)33633-3
DOI:
10.1016/j.jclepro.2018.11.230
Reference:
JCLP 14989
To appear in:
Journal of Cleaner Production
Received Date:
13 July 2018
Accepted Date:
23 November 2018
Please cite this article as: Anastasia Zabaniotou, Paraskevi Kamaterou, Food Waste valorization advocating Circular Bioeconomy -A critical review of potentialities and perspectives of Spent Coffee Grounds Biorefinery, Journal of Cleaner Production (2018), doi: 10.1016/j.jclepro.2018.11.230
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Food Waste valorization advocating Circular Bioeconomy -A critical
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review of potentialities and perspectives of Spent Coffee Grounds
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Biorefinery
4 5
Anastasia Zabaniotou*, Paraskevi Kamaterou
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Biomass Group, Department of Chemical Engineering, Aristotle University of
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Thessaloniki, Greece
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Abstract. Waste biorefineries are instrumental for advocating Circular Bioeconomy. Food
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waste valorization is a goal of sustainable development, gaining high interest in resolving
11
environmental and resources challenges. Coffee use generates massive quantities of spent
12
coffee grounds (SCG), a resource rich in fatty acids, amino acids, polyphenols, minerals, and
13
polysaccharides. This review aims to shed light on the potentialities, prospects, and challenges
14
of the transition from a mono-process to a cascade SCG biorefinery, in a circular economy
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thinking. It was found that mono-process approaches of SCG extraction have been investigated
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by many researchers, while SGC biorefining approaches are still at an early stage of research.
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Studies on SCG biorefineries, their environmental and economic assessment are few in the
18
literature, therefore imitations in extrapolating information and comparing the results were
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faced. It was made evident that more studies are needed on the economic assessment of the
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mono-process SCG break down, at higher Technology Reediness Level (TRL) for realistic
21
assessments. Efficient conversion of SCG in a cascade biorefinery depends on the spectrum of
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various end-products and cost-effective processing schemes. Lipids and/or polysaccharides
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extraction followed by the conversion of by-streams to energy and biochar, in a closing loop
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concept, has good potentialities. The review allowed the exploration of knowledge-based
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strategies to unlock the potential of SCG for bio-derived chemicals, carbon materials, fuels and
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fertilizer production and probably impacting waste management regulations. Some guidelines
27
for the sustainable design of SCG biorefineries were provided.
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Keywords: Spent coffee ground, food waste, cascade biorefinery, sustainability, circular
30
economy, bioeconomy.
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Corresponding author. Anastasia Zabaniotou, prof, email: [email protected]
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Address: Dept of Chemical Engineering, Aristotle University of Thessaloniki, U.Box 455, GR24154
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Graphical Abstract
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SCGs Cascade Biorefinery
Products
Syngas st
1 Refining
2nd Refining Biooil
Collection Biochar
Phenols Tannins Polysacharrides
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Highlights
59 60
Food waste valorization advocates Circular Bioeconomy.
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SCG valorization via one-process extraction has been widely investigated.
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SCG biorefinery is still at an early stage of development.
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A sequential SCG valorization is preferable for higher recovery.
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SCG cascade biorefinery needs proofs of economic viability.
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R&D work at higher Technology Reediness Level is needed 3
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Abbreviations ABTS
2,2'-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid)
ACs
Activated carbons
AO7
Acid orange 7
BSG
Brewer's spent grains
CGA
Chlorogenic Acid
CM
Coffee melanoidins
CS
Coffee silverskin
db
Dry base
DSCG
Dried spent coffee grounds
EAE
Enzyme-assisted extraction
FFA
Free fatty acid
FW
Food Waste
HHV
Higher heating value
GAE
Gallic acid equivalents
HTL
Hydrothermal liquefaction
MAE
Microwave assisted extraction
MB
Methylene blue
PEF
Pulsed-electric field extraction
PEI
Potential environmental impact
Ph
Phenol
PHAs
Polyhydroxyalkanoates
PLE
Pressurized liquid extraction
PMHS
Polymethylhydrosiloxane 4
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SCG-GAC
Spent coffee grounds into calcium-alginate beads
67
SCGs
Spent coffee grounds
SDGs
Sustainable Development Goals
SFE
Supercritical fluid extraction
SL
Solid-liquid
TBAs
Tannin-based absorbents
TE
Trolox equivalents
TS
Total solid
UAE
Ultrasound-assisted extraction
UNDP
United Nation Development Program
USDA
United States Department of Agriculture
VOCs
Volatiles Organic Compounds
EMY
effective mass yield
FC
Feature Complexity index
68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107
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1. Introduction
109 110
Beyond climate change, the main challenges the world is facing today, are the substantial
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increase in energy demand, food and material unsustainable consumption and production, and
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anthropogenic wastes generation (Venkata et al., 2016). According to World Resources
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Institution (World Resources Institute, 2018a), the planet is projected to hold 9.6 billion people
114
by 2050, consuming the equivalent of 1.6 planet’s resources, with a consequent high amount of
115
wastes generated. Consequences of the unsustainable consumption and production patterns are
116
resource depletion, climate change, air and water pollution, loss of biodiversity and of fertile
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soil, amongst other environmental, social, and economic challenges (FAO, 2013).
118
The production of the enormous quantities of food waste (FW) is becoming a global
119
concern (Dahiya et al., 2016), because the world need to adequately feed the 9.6 billion people
120
by 2050, in a way that advances economic development and reduces pressure on the
121
environment (World Resources Institute, 2018a). Long-term strategies are needed aligning
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short and medium-term goals. The long-term goal for many countries and especially for Europe,
123
is ensuring a transition from carbon-intensive to low-carbon societies. This transition must be
124
achieved by developing and deploying not only technologies and practices, but also changing
125
behaviors and patterns of production and consumption (World Resources Institute, 2018b), as
126
depicted in the 17 Sustainable Development Goals (SDGs) of United Nation development
127
program (UNDP). New interconnected areas, such as climate change, innovation, sustainable
128
consumption, are included among other in the SDGs (United Nations Development Program,
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2018).
130
A new approach to sustainability has been proposed in Europe. This encompasses the
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Circular Economy model, as a pathway to engage with challenges of sustainable production
132
and consumption. A priority in the EU is to stimulate the transition towards a Circular Economy 6
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that fosters the promotion of sustainable and resource-efficient policies for long-term socio-
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economic and environmental benefits, by adopting strategies of “closing the loop” in industrial
135
production systems (Maina et al., 2017). One of the most relevant goals in the application of
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this approach is to convert low-value side streams/residues/wastes into more valuable products.
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Limitations of this approach related to the social aspects of circularity, result to the call for more
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ethical and socially inclusive approaches to the Circular Economy (Lazell et al., 2018).
139
Food waste (FW) valorization is a goal of sustainable development and it gains high
140
interest since many bio-based products can derived from them, besides energy and fuels (Rama
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Mohan, 2016). Coffee is one of the most popular and appreciated beverages worldwide, and
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plays an important role in the global economy, as it is the second most traded commodity after
143
oil (Murthy and Naidu, 2012). Coffee industries are a key sector in the global economy, due to
144
income reporting and job creation. Based on United States Department of Agriculture (USDA)
145
report (USDA, 2017), global coffee industry reached an estimated production of 9.34 million
146
tons in 2016/17, which generated massive quantities of bio-wastes that are incinerated, dumped
147
in a landfill, or composted. Coffee companies produce annually more than 2 billion tons of by-
148
products, such as coffee spent grounds (SCG) and coffee silverskin (CS), most of which are
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thrown away for landfilling (Jimenez-Zamora et al., 2015).
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The generation of energy and various commodities in an integrated approach addressing
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sustainability, is a challenge and a perspective for Europe (Dahiya et al., 2016). New
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generations of biorefinery can combine innovative bio-waste resources from different origins.
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Integrated and cascade biorefineries are cornerstones of the Circular Bioeconomy. SCG can be
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used as feedstock to produce various bio-based products and bioenergy in a biorefinery concept
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(Mata et al., 2018), by closing loops (Karmee Sanjib Kumar, 2018). Targeting economic
156
viability and sustainability is very important for a Circular Bioeconomy (Vânia et al., 2018).
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This paper aims to review the potentialities and perspectives of SCG biorefinery, as a
158
solution to current bio-waste disposal problems, in a Circular Bioeconomy. The objective is to
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review the mono-process pathways for the valorization of SCG, reported in the international
160
literature, and to ding in the multi-process-multi-product cascade biorefinery concept that is a
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cornerstone in the transition to Circular Bioeconomy. It finally aims to provide general
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guidelines on how to build a sustainable SCG biorefinery, by summarizing various researcher’s
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suggestions, who have worked on the topic of sustainable biorefineries.
164 165
2. Methodology
166 167
In this paper, a systematic literature review was performed following the method
168
proposed by Thürer et al. (2018), for sourcing, screening, and analyzing the published articles.
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The target was to retrieve and select those articles which investigate and define the current state-
170
of-the-art on valorization of spent coffee grounds (SCG), via sustainable pathways in the
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concept of biorefinery. Main effort was devoted to find studies on biorefineries’ economic
172
viability and sustainability. Since the strategy of Circular Economy is recent and mainly
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articulated in Europe, very few studies appeared in the international literature, mainly published
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during 2018.
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2.1. Sourcing, screening, analyzing the articles
177 178
Scopus and open access are the bibliographic database selected for retrieving the articles,
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because it provides extensive literature tools (books, articles). For quality assurance of the
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research, only peer-reviewed articles were selected. The criteria used for the articles’ selection
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were the title, abstract, keywords, and document type (restricted to ‘articles’ and ‘reviews’),
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and the year of publication for a period of 9 years (from 2009 to 2018). The inquiry took place
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in July 2018 using the terms ‘Spent AND Coffee AND Grounds AND Conversion’; ‘Spent
184
AND Coffee AND Grounds AND Extraction’; ‘Spent AND Coffee AND Grounds AND
185
Valorization’; ‘Spent AND Coffee AND Grounds AND Pyrolysis’; ‘Spent AND Coffee AND
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Grounds AND Biorefinery’; ‘Spent AND Coffee AND Grounds AND Sustainable’; ‘Spent
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AND Coffee AND Grounds AND economic’. Other articles, concerning biorefining and
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Circular Economy were also included.
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The literature inquiry brought to light 844 articles (109 for the search term ‘Spent AND
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Coffee AND Grounds AND Conversion’, 252 for the search term ‘Spent AND Coffee AND
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Grounds AND Extraction’, 119 for ‘Spent AND Coffee AND Grounds AND Valorization’, 98
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from ‘Spent AND Coffee AND Grounds AND Pyrolysis’, 33 for ‘Spent AND Coffee AND
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Grounds AND Biorefinery’, 201 for ‘Spent AND Coffee AND Grounds AND Sustainable’,
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while only 3 dealing with economics. Nine review studies were retrieved, which published just
195
recently.
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The original sample contained 470 duplicates articles, which were removed. From the
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374 remaining articles, 39 were subtracted as unrelated and 13 were cut-off due to their year of
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publication, which was out of the time spectrum set in this study. All 320 papers were examined.
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By focusing on extracting, grouping of papers concerning sustainable SCG valorization that
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give information, knowledge, and data, to reply to the research questions set, finally 92 papers
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were used as the most appropriate and interesting for the review. Another 7 internet sources and
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2 books on SGC valorization, and one book on food wastes, sustainability and Circular
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Economy topics were also included. A total of 102 sources were selected for this review (Table
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1).
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The screening of the 320 papers was made by prioritizing papers discussing
206
sustainability, techno economic assessment, biorefinery approach, mono-process approach of
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the SCG and related review papers published within the decade 2017-2018. The research questions for the screening of the 320 papers were:
208 209
Which is SCG’s composition?
210
Which useful components could be extracted from SCG and how?
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What about thermochemical conversion routes for SCG valorization?
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Can SCG produce biochar via pyrolysis?
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Can biorefining be applied for SCG valorization?
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How sustainable is to valorize SCG by biorefinery?
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What is the economic viability of a SCG biorefinery?
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What is the environmental sustainability of the SCG biorefinery?
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Are there any guidelines for a multi process-multi product approach of SCG valorization?
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Which are the prospects arising from this paper?
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Fig. 1 depicts the number of the related papers published per year. Fig. 2 depicts the
220
paper related to the extracted product per year of publication. It is obvious that papers on mono-
221
process approach are the first appeared in the international literature and continue to be
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published in a large number. Papers on biorefinery approach are recent and mainly of the year
223
2018. Papers on sustainability and technoeconomic assessment are few. Concerning review
224
papers, these are showing an increasing trend the last 3 years.
225 226
3. Materials and methods
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The first research question for the screening of the 320 papers is related to SCG’s
228 229
composition. The SCG composition defines the choices for the valorization.
230 231
3.1. SCG composition
232 233
The biochemical composition of SCG as reported in literature, is presented in Table 2.
234
SCG contain 9-16 wt% db lipids, 5-15 wt% db proteins and are considered as an important
235
source of polysaccharides (carbohydrate whose molecules consist of several sugar molecules
236
bonded together). Literature review revealed that almost half of the material (45.3 wt% db) is
237
sugars polymerized into cellulose and hemicellulose structures (Mussato et al., 2011) and that
238
most polysaccharides (around 70 wt% db of total polysaccharides from roasted coffee) remains
239
in SCG (Arya & Rao, 2007). Mussato et al. (2012) have hydrolyzed SCG and efficiently
240
fermented it to ethanol by yeast. SCGs coffee fibers exhibit antioxidant properties suggesting
241
its use as potential dietary supplement (Campos-Vega et al., 2015). Ballesteros et al. (2015)
242
investigated the antimicrobial and antioxidant capacities of polysaccharides extracted from
243
SCG.
244
The ultimate/proximate analysis of reported SCG is presented in Table 3. The carbon
245
content of SCG ranges between 45-53 wt% whereas the values for hydrogen and nitrogen
246
content are 6-7 wt% and 2-4 wt%. The amounts of ash (1-2 wt% db) containing in SCG is low
247
compared to other biomass sources (Zabaniotou et al., 2017). SCG is characterized by high total
248
volatiles content (Tsai et al., 2012).
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3.2. Mono-process approach: What can be extracted from SCG?
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SCG represent a resource rich in valuable components which could be valorized giving
253
a range of commodities. Large amounts of organic compounds are contained in SCG, such as
254
fatty acids, amino acids, polyphenols, minerals, and polysaccharides (Campos-Vega et al.,
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2015). Coffee is considered an important source of polysaccharides (Ballesteros et al., 2017),
256
proteins (Mussato et al., 2011) and lipids (Campos-Vega et al., 2015). SCG is rich in sugars (45
257
ww%, dry weight of the material) which can be polymerized into cellulose and hemicellulose
258
structures. Mannans is the major polysaccharides of SCG which contains 47 wt% db mannose,
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30 wt% db galactose, 19 wt% db % glucose, and 4 wt% db arabinose (Mussatto et al., 2011).
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Remarkable amount of proteins (13-17 wt% db) are contained in SCG Mussato et al. (2011).
261
Other N-containing substances like caffeine, trigonelline, free amines and amino acids are
262
contained in SCG (Delgado et al., 2008).
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Caffeine is a non-protein nitrogenous compound which can be recovered from SCG.
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Panusa et al. (2013) estimated a low content of caffeine (0.96-0.97 mg/g dry sample) in SCG
265
extracts. According to Cruz et al. (2012) the range of caffeine content found higher for the
266
espresso coffee (1.9 -7.9 mg/g (DW)). However, caffeine, tannins and chlorogenic acid are of
267
eco-toxicological concern and can limit their value-adding applications.
268
There is an especial interest in SCG lipids extraction. Total lipids content of espresso
269
coffee residues ranges from 9.3-16.2 wt% db (Cruz et al., 2012) depending on the coffee variety
270
(Jenkins et al., 2014), a promising feedstock to produce biodiesel (Al-Hamamre et al., 2012).
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Hexane is the most popular solvent for oil extraction, but Supercritical Fluid Extraction is a
272
modern environmentally friendly technology which is increasingly being used for SCG oil
273
extraction (Campos-Vega et al., 2015).
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SCG contain phenolic compounds which are known as human health related compounds,
275
with demonstrated antioxidant, anti-bacterial, antiviral, anti-inflammatory and anti-
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carcinogenic activities (de Souza et al., 2004). Polyphenols and chlorogenic acid (CGA) have
277
been extracted from SCG via a conventional solid-liquid method (Campos-Vega et al., 2015). The techniques for extraction of compounds are classified into two categories: the
278 279
conventional and the non-conventional (Azmir et al., 2013). The conventional techniques are:
280
Soxhlet extraction
281
Maceration
282
Hydrodistillation
283
The non-conventional extraction techniques are (Azmir et al., 2013):
284
Ultrasound-assisted extraction (UAE)
285
Pulsed-electric field extraction (PEF)
286
Enzyme-assisted extraction (EAE)
287
Microwave assisted extraction (MAE)
288
Pressurized liquid extraction (PLE)
289
Supercritical fluid extraction (SFE)
290
Table 4 presents the valuable components that can be extracted from SCG, as
291
demonstrated in the literature. By searching the literature, opportunities were found concerning
292
the recovery of oils, polysaccharides, phenolic compounds, tannins that can be used in various
293
commodities, by using mono-extraction processes, as depicted in Fig. 3.
294 295
3.2.1. Oil recovery
296
The oil content of SCG ranges from 10 to 15 wt%, depending on the coffee varieties
297
(Jenkins et al., 2014). Somnuk et al. (2017) studied the effect of four different solvents (hexane,
298
ethanol, hydrous ethanol and methanol) on coffee oil yield, by using a circulation process. The
299
optimal conditions (30.4 min extraction time and 22.5 g/g ratio of DSCG-to-hexane) resulted
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in approximately 11.8 wt% oil yield. Phimsen et al. (2016) utilized a Soxhlet extractor and n-
301
hexane as a solvent in order to extract the oil from dried SCGs. The extracted oil, yielded from
302
10 to 13 wt% db SCG, and then it was hydrotreated in a shaking batch reactor, by using NiMo/γ-
303
Al2O3 and Pd/C as catalysts. It has proved the bio-hydrotreated fuel production and potentiality
304
to be used as a renewable energy.
305 306
3.2.2. Polysaccharides recovery
307
Polysaccharides play multiple role in life process and present an immense potential in
308
healthcare, food, and cosmetic industries, due to their content of bioactivities that have
309
therapeutic effects and relatively low toxicity (Shi L. 2016). Due to their enormous structural
310
heterogeneity, the approaches for isolation and purification of polysaccharides are distinct from
311
that of the other macromolecules such as proteins etc. (Shi L. 2016). Therefore, various methods
312
widely used in isolation and purification of polysaccharides. The extraction of polysaccharides
313
from SCG has been studied, mainly using chemicals as extraction agents. Sodium hydroxide
314
(Ballesteros et al., 2015) and potassium hydroxide have been employed in SCG alkali
315
treatments, while sulfuric acid has been used to recover carbohydrates from SCG dilute acid
316
hydrolysis (Mussatto et al., 2011).
317
Mayanga-Torres et al. (2017) proposed the recovery of sugars compounds from coffee
318
industry residues using subcritical water hydrolysis as the valorization technique. Evaluation of
319
optimal conditions which simultaneously maximize holocellulose hydrolysis and minimize
320
both sugar degradation and dilution is proposed.
321
The extraction of polysaccharides by autohydrolysis of SCG was investigated by
322
Ballesteros et al. (2017). The extracted polysaccharides (29.29 wt%) characterized by high
323
antioxidant activity. The conditions for the extraction were 15ml water/g SCG, for 10 min, at
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160°C. The polysaccharides obtained were thermostable in a large range of temperature and
325
had typical carbohydrate pattern. Their use for industrial applications, mainly in the food area
326
was proposed.
327
The feasibility of microwave superheated water extraction of polysaccharides from SCG
328
was studied by Passos et al. (2013). They found that a maximum of 0.57 g/batch
329
polysaccharides for 1 g SCG: 10 mL water can be recovered. Further extraction of
330
polysaccharides was achieved with a second extraction (re-extraction) of the remaining un-
331
extracted insoluble material, under the same conditions.
332 333
3.2.3. Phenolic compounds recovery
334
Polyphenols are micronutrients. The health benefits of polyphenols and their protective
335
effects in food systems as antioxidant compounds, are well known and have been extensively
336
investigated (Shavandi et al., 2018). Recovery of relevant natural antioxidants for use as
337
nutritional supplements, foods, or cosmetic additives can be achieved by SCG extraction with
338
environmentally friendly procedures (Panusa et al., 2013).
339
The bibliographic search shown that various extraction methods were used. Subcritical
340
water extraction of SCG resulted in significant antioxidative phenolics production (Xu et al.,
341
2015) at temperature range of 160-180°C, time range of 38-55 min and solid-to-liquid ratio of
342
14.1 g/l. 86.2 mg GAE/g of total phenolic compounds were recovered. Shang et al. (2017)
343
optimized the SCG extraction conditions for total phenolics by using pressurized liquid
344
extraction (PLE) method with water and ethanol. Optimal conditions obtained at 195°C
345
extraction temperature. The total phenolics content ranged from 19 to 26 mgGAE/gDW.
346
Al-Dhabi et al. (2017) developed and validated the SCG ultrasound-assisted solid-liquid
347
extraction of phenolic compounds Ultrasonic power, temperature, time, and solid-liquid (SL)
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ratio were studied as parameters. The optimum process conditions were: 244 W of ultrasonic
349
power, 40 °C of temperature, 34 min of time and 1:17 g/ml of SL ratio and the extraction
350
obtained yields reached 33.84 GAE/g of total phenolic content.
351
Coffee cherry pulp is a by-product derived from the process of coffee production. Coffee
352
cherry pulp contains considerable amounts of phenolic compounds and caffeine. An attempt to
353
produce “Cascara”, a refreshing beverage, has been made by Heeger et al. (2017). Six dried
354
coffee pulp samples and Cascara produced out of one of those samples, were investigated in
355
Switzerland. Aqueous extraction of coffee pulps revealed a content of total polyphenols
356
between 4.9 and 9.2 mg gallic acid equivalents (GAE)/g db. The antioxidant capacity was
357
between 51 and 92 lmol Trolox equivalents (TE)/g DM, as measured with ABTS radical.
358
Bourbon variety from Congo and Maragogype variety showed highest caffeine contents, 6.5
359
and 6.8 mg/g DM, respectively. In all samples, chlorogenic acid, protocatechuic acid, gallic
360
acid and rutin, were present. The beverage Cascara contained 226 mg/L of caffeine and 283 mg
361
GAE/l of total polyphenols and an antioxidant capacity of 8.9 mmol TE/l.
362
Jimenez-Zamora et al. (2015) showed the prebiotic, antimicrobial and antioxidant
363
capacity of SCGs and CS, as well as those melanoidins (a coffee component generated during
364
the roasting process), obtained from the former. The prebiotic activity was important in both
365
CSG and CS, although the presence of coffee melanoidins (CM) interfered with such beneficial
366
properties. On the contrary, CM exerted an intense antimicrobial activity that could be used to
367
avoid the growth of pathogenic bacteria in food products. CSG, CS and CM were highly
368
antioxidant. The addition of sugar during coffee roasting, namely torrefaction, increased the
369
antioxidant and antimicrobial activity due to a larger generation of CM, although prebiotic
370
activity was not affected.
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3.2.4. Tannins recovery
373
Tannins extensively assessed as natural alternatives to in-feed antibiotics (Huang et al.,
374
2017). Low et al. (2015) investigated the influence of extraction parameters in SCG tannins’
375
recovery. They found that sodium hydroxide concentration, extraction temperature and liquid
376
to solid ratio considerably effected the SCG tannin extraction yield and its reactivity. Extraction
377
time had only marginal effect in the tannin extraction process. The optimal extraction conditions
378
were found: 5 wt% sodium hydroxide concentration, 100 °C extraction temperature, 30 min
379
extraction time, and 8.2 liquid to solid ratio. These conditions resulted in a high tannin
380
extraction yield (21.02 wt%) and high reactivity.
381
Tannins have traditionally been regarded as “anti-nutritional factor” for poultry. Recent
382
researches have mentioned that when applied in appropriate manner, improved intestinal
383
microbial ecosystem, enhanced gut health and hence increased productive performance (Huang
384
et al., 2017). However, tannins if used as additives in poultry feed to control diseases and to
385
improve animal performance, must ensure a consistent quality (Redondo et al., 2014).
386
Tannins are also low-cost natural biopolymers and excellent candidates to produce bio-
387
sorbents. Low-cost and eco-friendly products, such as adhesive, plastic, polyform can be
388
produced by using the SCG extracted tannins. Tannin-added films can be used as green,
389
nontoxic packaging materials for food and pharmaceutical products (Missio et al., 2018).
390
Tannin-based absorbents (TBAs) have a natural affinity to absorb heavy metals dyes, and
391
pharmaceutical compounds from contaminated waters (Bacelo et al., 2016).
392 393
3.2.5. Caffeine recovery
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Brazinha et al., (2015) optimized the process of producing a natural extract from SCG
395
by using membrane technology, with no organic solvents or adsorbents involved. The extracted
396
product was enriched in caffeine with specific health benefits.
397
Shang et al. (2017) optimized the SCG extraction conditions to caffeine using pressurized
398
liquid extraction (PLE) method with water and ethanol. At the optimal conditions (195°C
399
extraction temperature) caffeine’s yield reached 9 mg/g db.
400 401
3.3.
SCG valorization for bioenergy and carbon materials production
402 403
Energy recovery from biomass is a way to reduce waste, produce fuels, protect the
404
environment and mitigate greenhouse gas (GHG) emissions (Limousy et al, (2017). Energy
405
recovery should be combined with material recovery for enhanced resources efficiency in the
406
concept of a Circular Economy.
407
Energy recovery from SCG was documented in the international literature. Biochemical
408
(transesterification) and thermochemical (pyrolysis, gasification, hydrothermal liquefaction,
409
combustion), enzymatic conversion technologies were used for this, as it is depicted in Fig. 4.
410
Biodiesel, biooil, CHP, heat, biochar, activated carbons, carbon nanotutes, are the main
411
products of the application of a mono-thermochemical process fueled with SCG (Limousy et
412
al., 2017).
413 414
3.3.1. Biodiesel production
415
SCG contain significant amounts of lipids (∼16%w/w), which could potentially be
416
utilized as feedstock in biodiesel production. Many researchers used transferification of the
417
exctracted oil (Loyao et al. (2018). Solvent extraction technologies, with a wide range of
18
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418
solvents for lipid extraction from SCG, were used to determine the effect of solvent selection
419
and process temperature on the extraction efficiency and composition of the obtained oil, by
420
Efthymiopoulos et al. (2018). Al-Hamamre et al. (2012) studied oil extraction from dried spent
421
coffee grounds (DSCGs) for biodiesel production. They obtained 60 g DSCG. Kondamudi et
422
al. (2008) extracted the SCG oil (10-15 %wt) using solvents such as hexane, ether, and
423
dichloromethane under reflux conditions. They transesterified the oil to produce biodiesel and
424
achieved 100% conversion. The produced biodiesel was found to be stable for more than 1
425
month under ambient conditions.
426
The valorization route of lipid recovery followed by transesterification for biodiesel
427
production was also studied by Go and Yeom (2017). Lipid extraction was estimated at 92.7%,
428
using 13.7 mL-hexane/g-WCG, within 30 min extraction time, and 25°C. NaOH was used as
429
an alkaline catalyst. Optimum conditions for transesterification were achieved with the addition
430
of 0.5% catalyst, 1.5 mL methanol/g-lipid, at 45°C, and 9 h of reaction time. Biodiesel
431
production was mainly influenced by reaction time and temperature. Caetano et al. (2013)
432
examined the potential of biodiesel production from SCG. They used various solvents and
433
proposed a two-step process of acid esterification followed by alkaline transesterification for
434
lipids with high free fatty acids, as the best route to biodiesel. However, the properties of the
435
derived biodiesel (iodine number, acid value, and ester content) did not comply with the NP
436
EN 14214:2009 standards (Caetano et al., 2013). For meeting standard requirements, they
437
proceeded with two remediation procedures: a) blending of SCGs lipids with other higher-
438
quality vegetable oils before transesterification, b) mixing the produced biodiesel with higher-
439
quality biodiesel.
440
Döhlerta et al. (2015) studied the catalytic conversion of triglycerides derived from SCGs
441
to produce diesel, by using a cheap reductant agent, the polymethylhydrosiloxane (PMHS)
19
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442
under mild reaction conditions. Significant amounts of PMHS-waste generated as by-products
443
of the process (∼1.7% of the PMHS is required for the oil reduction). They were depolymerized
444
in a subsequent step, resulting in PMHS-waste conversion to methyltrifluorosilane and
445
difluoromethylsilane, which can be useful for new silicones production as building blocks. The
446
acid catalytic solvo-thermal in situ transesterification of SCG was demonstrated by Park et al.,
447
(2018). They suggested themeth for boosting the economic feasibility of biodiesel production
448
from wet SCGs.
449
Kookos (2018), by performing an economic and environmental analysis of biodiesel
450
production from SCG, concluded that the process economics can be attractive only in the case
451
of a centralized large-scale production plant. Biodiesel production from SCG is not
452
economically sound for small scale units.
453 454
3.3.2. Biooil production
455
Fast pyrolysis can convert the SCG into biooil and biochar. The fast pyrolysis of SCG
456
targeting biooil production was studied by Kelkar et al. (2015). The experiments took place in
457
a compact, transportable, screw conveyor reactor. Biooil yields showed a maximum yield of
458
61.8 ww% at 500 °C, while the highest biochar yield was observed at the lowest pyrolysis
459
temperature (429 °C). SCG-biooil contained fatty acids, fatty acid esters, medium-chain
460
paraffins, olefins, and caffeine.
461
Fast pyrolysis of SCG was also studied by Bok et al. (2012). They produced biooil with
462
a maximum yield of 55 ww% at 550 °C pyrolysis temperature, pyrolysed in a fluidized bed
463
reactor. Li et al. (2014) investigated SCG bioenergy production potential using pyrolysis, at
464
two different heating rates (10 and 60 °C/min). Biogas contained mainly CO2, CO, CH4 and the
20
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465
gaseous volatile. An efficiency of 77–85% was achieved in relation to the feedstock moisture
466
content.
467
SCG pyrolysis was performed by Cho et al. (2016) targeting both, waste disposal and
468
biooil recovery. In their study, SCG were pretreated with FeCl3, and carbon dioxide to enhance
469
syngas generation and reduce condensable hydrocarbons, such as tar. Syngas enhancement was
470
achieved via the CO2-induced thermal cracking of VOCs, due to the reaction between CO2 and
471
VOCs. Tar reduction was achieved by using Fe as catalyst in a CO generation from Fe-SCG
472
pyrolysis. This has also resulted in CO2 dramatical increase (up to 8000%), compared to SCG
473
pyrolysis withN2.
474
Hydrothermal liquefaction (HTL) of SCG in hot-compressed water was applied to
475
produce crude bio-oil in a 100 cm3 stainless-steel autoclave reactor, with N2 atmosphere, (Yang
476
et al., 2016). The effects of operating parameters (retention times - 5 min, 10 min, 15 min, 20
477
min and 25 min, reaction temperatures -200 °C, 225 °C, 250 °C, 275 °C and 300 °C,
478
water/feedstock mass ratios -5:1, 10:1, 15:1 and 20:1, process gas initial pressure -2.0 MPa and
479
0.5 MPa), were investigated targeting biooil yield with designed properties. A yield of 47.3ww
480
% of the crude biooil was achieved at 275 °C liquefaction temperature, 10 min retention time,
481
water/feedstock mass ratio of 20:1 and initial pressure of 2.0 MPa. The higher heating value
482
(HHV) of crude biooil was estimated at 31.0 MJ kg−1.
483
Yang et al. (2017) investigated the co-liquefaction in subcritical water of SCG mixed
484
with paper filter, corn stalk and white pine bark, aiming to bio-crude oil production. The
485
optimum reaction temperature was estimated at 250 °C, and the mixing biomass ratio was 1:1.
486
The best feedstock combination was SCG and CS and addition of 5% NaOH, as a catalyst.
487
Biooil quality and high yield suggest SCG as a valuable biooil feedstock.
488
21
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489
3.3.3. Biochar production
490
Slow pyrolysis of SCG can produce biochar. Pyrolytic char from SCG is a much better
491
fertilizer compared to direct SCG use in the fields, as it was proved by various studies. SCG
492
and SCG-char were applied as biochar, by Kim et al., (2014). In the case of SCG application,
493
the soil phytotoxicity increased, because a massive amount of dissolved carbon amount was
494
released from SCG in the soil. In contrast, SCG-char application did not exhibit this
495
phenomenon because any easily released organic matter was removed previously in the
496
pyrolysis process.
497
Tsai et al. (2012) evaluated SCG as a potential feedstock for the production of biochar
498
via pyrolysis. The conditions used were: Tpyrolysis= 400-700°C, heating rate=10 °C/min. It was
499
reported that the produced biochars showed high carbon content (>80 ww%), fixed carbon (>60
500
ww%) and calorific value (>30 .1 MJ/kg). The produced char can be also used also as solid fuel
501
in the industrial sector due to high calorific value.
502
Researchers (Cho et al., 2017) investigated co-pyrolysis of paper mill sludge mixed with
503
SCG, focusing on biochar production. CO2 was used as reaction medium aiming to syngas
504
generation enhancement and biochar’s physico-chemical properties modification. The
505
synergistic effects of CO2 and Fe/Ca caused a decrease in pyrolytic oil. The presence of Fe/Ca
506
in PMS favored CO generation Fe-ions were converted into magnetite (Fe3O4) and porous
507
biochar was created. Cho et al. (2017) concluded that co-pyrolysis of paper mill sludge and
508
SCG, using CO2 as reaction medium could feasibly generate CO and biochar, suitable for
509
environmental applications.
510 511
3.3.4. Activated carbon production
22
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512
Jung et al. (2016) prepared activated carbons from SCG into calcium-alginate beads
513
(SCG-GAC). The activated carbon powder originated from SCG was used for the removal of
514
acid orange 7 (AO7) and methylene blue (MB), from aqueous media. PH played a highly
515
important role in dye adsorption, whereas the influence of ionic effects was essentially neutral.
516
The pore diffusion model describing the adsorption kinetics, revealed that the rate-limiting step
517
during the adsorption process was pore diffusion. The maximum SCG-GAC adsorption
518
capacity for AO7 at pH=3.0 was estimated at 665.9, for MB 986.8 mg/g absorption obtained,
519
at 30 °C and pH= 11.0.
520
Different impregnation ratios of KOH were utilized by Laksaci et al. (2017) for the
521
synthesis of new activated carbons (ACs), from SCG. Many functional groups were identified
522
on the ACs surface. BET measurement revealed a maximal specific surface area of 1778 m2 g-
523
1,
524
methylene blue (MB) molecules was tested.
for an impregnation ratio of 36 mmol of KOH/g. ACs removal efficiency of phenol (Ph) and
525 526
3.3.5. Nanocarbons production
527
Zein et al. (2017) studied the SCG microwave radiation to produce nanocarbons. They
528
found that the optimum condition for maximizing nanocarbons yield (60 ww%) obtained at 200
529
°C, 650W microwave power and 45 min residence time. They concluded that this method could
530
potentially produce spherical shaped nanocarbons, which could be utilized for future scientific
531
innovations.
532 533
3.3.6. Liquid polyols production
534
Soares et al. (2014) investigated the possibility of SCG conversion into liquid polyols,
535
using acid liquefaction at moderate temperature and autogenous pressure. They concluded that
23
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536
the produced polyols have characteristics similar to those of petroleum-based polyols, which
537
are used in the polyurethane foam industry.
538 539
3.3.7. Energy production
540
The possibility of using SCG for energy via combustion, was proposed by Ciesielczuk et
541
al. (2015). They used briquettes made of mixed beech shavings and SCG, for increasing the
542
calorific value. SCG tested as a new bulking agent for biodrying of dewatered sludge (DS). It
543
was proved that SCG is an excellent bulking agent that accelerates DS biodrying and produces
544
a solid fuel with a high calorific value (Hao et al., 2018).
545 546
3.3.8. Cogeneration of heat and power (CHP)
547
Food manufacturers have been piling into the bioenergy sector turning waste and
548
production by-products into energy, producing enough heat and power for their own needs,
549
with surplus energy feeding back to the grid. Combined heat and power (CHP) can be produced
550
by SCG gasification. Cutting-edge, innovative and economical gasification techniques with
551
high efficiencies are a prerequisite for the application of gasification. Feedstock types, the
552
impact of different operating parameters, tar formation and cracking, and modelling approaches
553
for biomass gasification of biomass have widely studied.
554
Steam gasification of SCG was investigated by Pacioni et al. (2016) in the temperature
555
range of 650 to 850 °C, with a steam partial pressure range of 0.05 to 0.3 bar. A magnetic
556
suspension thermobalance was used for the gasification tests which were performed
557
isothermally. Gas chromatograph equipped with TCD/FID detectors was used for gaseous
558
products analysis. Product characterization revealed that the products contained higher carbon
559
and lower volatile matter compared to the original SCG and had high calorific value.
24
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560
Temperature and steam partial pressure influenced syngas production. H2+CO yields were
561
higher for a conversion range of 50–80%. The H2/CO ratio showed an increasing trend with
562
temperature.
563
To improve the feasibility and sustainability of SCG gasification, technological
564
advancement and the minimization of socio-environmental effects are needed (Ingrao et al.,
565
2018a).
566 567
3.4.
Biorefinery approach
568 569
The European Commission has set a long-term goal to develop a competitive, resource
570
efficient and low carbon economy by 2050 (EC, 2011). Bioeconomy is expected to play an
571
important role in the low carbon economy. The European strategy for building a sustainable
572
bio-based economy with emphasis on the sustainable use of natural resources, competitiveness,
573
socioeconomic and environmental issues, is on the spot (Scarlat et al., 2015).
574
Strategies relying on complete biomass disintegration through combustion, gasification,
575
or fermentation only, do not lead to optimal utilization of biomass feedstock. Cascading
576
approaches are required to maximize biomass valorization (Ingrao et al., 2018a, 2018b;
577
Zabaniotou et al., 2017, 2018).
578
In the waste-biorefinery concept, multifunctional processes are integrated in an optimized
579
sequence to utilize waste, with an objective of maximizing the productivity of marketable
580
intermediates and products (chemicals, materials, and bioenergy/biofuels), to enhance of the
581
process economics .
582
By searching carefully the international literature, 6 studies and 2 review papers were
583
found, dedicated to explore the various SCG biorefineries at laboratory scale, which signifies a
25
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584
low Technology Readiness Level (TRL). All 8 papers were published during the last two years
585
(2016-2018). The two review papers published on 2018, summarise various SCG biorefineries.
586
(Kourmentza et al., 2018; Mata et al., 2018). While the authors of the first review support the
587
SCG biorefinery approach, the authors of the second review conclude that most of the studied
588
biorefineries have limited scope and low economic value (Mata et al., 2018).
589
The reported SCG biorefineries (2015-2018) integrate various number of processes and
590
products as shown in Table 5, Fig.5. When compared with the mono process/extraction
591
proposals, it is clear that the biorefinery allows a more complete utilization of SCG, by
592
obtaining high value products, using technologies and process already available at commercial
593
scale. For applications in a Circular Bioeconomy, the biorefinery approach is a corne stone
594
(Karmee Sanjib Kumar, 2018).
595
Mata et al. (2018) in their review paper described several proposals for a SCG bio-
596
refinery, and compared each other. They concluded that for obtaining a wider product portfolio,
597
several separation processes are required and a combination of biological and chemical
598
processes is necessary. They also concluded that the most of them have limited scope and the
599
final products have low economic value.
600 601
4. Discussion on SCG biorefinery approach
602 603
The valorization of SCG by a mono-process pathway has attracted a lot of attention
604
recently from both the academia and industry. However, very few studies dealt with the
605
economic viability assessment of a mono-process approach of SCG valorization.
606
Food wastes create huge environmental, economic, and social problems, being also sources
607
of added-value materials. Coffee industries are a key sector in the global economy due to
26
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608
income reporting and job creation. Coffee companies produce annually more than 2 billion tons
609
of by-products such as coffee spent grounds (SCG) and coffee silverskin (CS). Proper design
610
of a biorefinery system, aiming to a wide range of products generation could serve as a unique
611
sustainable solution to SCG wastes management and valorization in a Circular Economy.
612
Techno-economic analysis and optimization models are crucial to design process scale,
613
understand how major cost categories affect the process and assess their sustainability. It is
614
evident that new business models introducing high-value bioproducts to biorefineries are
615
essential for achieving economic viability of industries within Bioeconomy. Economically
616
feasible production of conventional bioenergy such as biofuels, biopower and bioheat, is a
617
challenge. Biorefineries must compete with the inexpensive fossil fuel energies.
618 619
4.1 Prospects of SCG biorefinery
620 621
Besides, contributing to more sustainable and circular economies, the biorefinery has
622
high commercial value when compared to the ones obtained by currently used waste treatment
623
methods. The major advantage of biorefineries is their suitability for maximizing valorization
624
of structural and energetic potentials lying in biomass (Budzianowski Wojciech, 2017).
625
The prospects of SCG biorefinery, as explored at laboratory level and reported in the
626
international literature, are very encouraging. SCG can feed a biorefenery and via advanced
627
chemical and biotechnological methods, can produce a large number of value-added products
628
(polyhydroxyalkanoates, biosorbent, activated carbon, polyol, polyurethane foam, carotenoid,
629
phenolic antioxidants, green composite) and bioenergy (biodiesel, bio-oil, biogas), due to their
630
rich composition in lipid, carbohydrates, carbonaceous, and nitrogen containing compounds
27
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631
among others. However, exploration at a high Technology Readiness Level (TRL), is still
632
lacking behind.
633 634
4.2 Economic viability and sustainability challenges
635 636
The economic viability is a decisive parameter for the biorefinery deployment. This was
637
made obvious in a study performed by Kookos I.K. (2018), who recommended that research on
638
SCG valorization should be oriented towards the efficient recovery of the bioactive compounds
639
for a more economically attractive conversion. The economic performance of the biodiesel
640
production via a mono-process pathway is only viable at large production capacities, realized
641
at centralized facilities, despite that the environmental assessment of the process showed that
642
biodiesel production has good environmental indicators.
643
Results from a recent study on the techno-economic analysis of food waste biorefineries
644
at European level, showed that the most profitable options are those related to economies of
645
scale. However, the risk of increasing externalities due to logistics is possible (Cristóbal et al.,
646
2018).
647
There is a shortage in studies of a cascade SCG biorefinery. Garcia et al. (2017) reported
648
that hydrogen production via SCG gasification biorefinery is viable, but without referring to
649
the production of high-value bioproducts. Mussatto et al. (2013) have suggested the integrated
650
biorefinery of the Brazilian case of spent grains (BSG) for the production of xylitol, lactic acid,
651
activated carbon and phenolic acids integrated with heat production, as viable pathway, because
652
the economic viability and environmental performance that achieved have shown positive
653
indicators. The obtained economic margin was evaluated at 62.25%, the potential
28
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654
environmental impact was 0.012 PEI/kg products, and the carbon footprint of the processing
655
stage represented 0.96 kg CO2-e/kg of BSG.
656
Apart from the economic viability, the environmental sustainability of the SCG
657
biorefinery is a request (Ingrao et al., 2018a, 2018b). The challenge is to produce value added
658
products by integrating different strategies that lead to an interconnected environmental
659
biorefinery for maintaining the ecological footprint. However, sustainable biorefinery systems
660
are still a challenge, since weak designs lead to not viable solutions, with almost similar
661
environmental burdens with the petrochemical systems. They face socio-economic issues
662
related to land use, labor, food security and others, sdditionaly (Moncada et al., 2016).
663
In the economic analysis, results must be evaluated taking into account the high
664
uncertainty that this kind of study entails, which include the cost estimation and process
665
parameters estimation for low TRL technologies (Cristóbal et al., 2018).
666 667
4.3 Business development and market perspectives
668 669
It was made obvious that there is a need for design procedures of economically feasible
670
sustainable biorefineries that could meet technical and market requirements and improve
671
cascading biomass utilization, (Budzianowski Wojciech, 2017). Methodologies for biorefineriy
672
conceptual design and optimization are needed. Approaches need to consider raw materials,
673
technologies, processing routes, products, and technical, economic, and environmental aspects.
674
Processes must be optimized for the specific feedstock used (due to variations on feedstock
675
composition, cost and logistics of process efficiencies and economics), coupled with energy
676
generated from its residue (Mata et al. (2018).
29
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677
The screening of sustainable SCG biorefinery pathways for the production of bio-based
678
products and energy is a complex challenge. Critical tools for predicting the commercialization
679
feasibility of biorefining pathway include laboratory and pilot-scale experimental results,
680
processes modeling, technoeconomic and market analysis. More R&D is needed at higher
681
Technology Reediness level (TRL). Economic and environmental assessment for the practical
682
implementation of a SCG biorefinery at industrial scale is also needed (Zabaniotou et al., 2017).
683
High-value, low-volume bioproducts coupled to bioenergies, with a potential to improve
684
economic viability of biorefineries and biomass resource utilization, are urgently required
685
(Budzianowski Wojciech M., 2017).
686
It is difficult to assess which biorefinery will have a market perspective because detailed
687
economic analysis should be conducted for each. It is suggested that integrated and holistic
688
approaches for bio-waste utilization, as industrial feedstocks, will boost the transition towards
689
the bioeconomy era, the establishment of which would expand and diversify the market outlets
690
of bio-based products (Maina et al., 2017). SCG biorefineries, as many food waste-based
691
biorefineries should be tailored to the local and regional context, and to be profitable and
692
sustainable in the long term.
693
The scale should be analyzed in every biorefinery, during the preliminary design stages.
694
Different factors define the minimal scale for biorefinery’s feasibility. The number and quantity
695
of high added value products usually is associated with low scales (Kachrimanidou et al. 2013).
696 697
4.4
Policy and regulations
698 699
Policy analysis is a new dimension to the sustainability assessment of food waste
700
reduction and valorization. Regulatory framework and policy actions undertaken by local and
30
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701
global actors are the drivers of change in food-waste reduction and valorization. Today, very
702
different national policies apply to bio-waste management, ranging from small action in some
703
Member States, to ambitious policies, in others. This can lead to increased environmental
704
impacts and can delay the full utilization of advanced bio-waste management methods, while
705
action on national level and community is needed. The lack of a standard universal definition
706
of food waste has impact on the efficient use of by-products for technological and commercial
707
exploitation (Morone et al., 2017).
708
Mathematical mapping methods to assess food consumption impacts and protocols,
709
based on laboratory investigation and demonstration, will formulate pathways for the
710
sustainable valorization of CSG and food waste in general (Morone et al., 2017; Galanakis,
711
2017).
712 713
4.5
Indexes
714 715
A number of indexes related to economy and environmental impacts have estimated by
716
Salazar, 2013. Indexes as a new basic concept need to be applied for understanding the
717
biorefinery efficiency (Moncadam et al., 2016).
718
The effective mass yield (EMY) and the Feature Complexity index (FC) are indexes that
719
have been used in other waste-biorefineries (Zabaniotou et al., 2018). The effective mass yield
720
(EMY) is defined as the percentage of the mass of the desired products relative to the mass of
721
used as feedstock. The Feature Complexity (FC) of the biorefinery has to do with the number
722
of different features: it is increasing by the number of features, by the state of technology of a
723
single feature; it is decreasing with the maturity of the technology (high TRL). This means that
724
a high Technology Readiness Level (TRL) of a feature has lower technical and economic risks
31
ACCEPTED MANUSCRIPT
725
and a lower complexity. This led to the calculation procedure of the Biorefinery Complexity
726
Index that the complexity is directly linked to the number of features and the Technology
727
Readiness Level (TRL) of each single feature involved by the IEA Bioenergy Task 42 ‘Bio
728
refining’ (Jungmeier, 2009).
729
The Feature Complexities (FC) are rating from 1-9 according to the TRL of the process
730
(1-9); TRL1:basic (FC9), TRL2:applied research (FC8), TRL3: critical function or proof of
731
concept established (FC7) , TRL4: lab testing/validation of prototype (FC6), TRL5: prototype
732
system verified (FC5), TRL6: integrated pilot system demonstrate(FC4), TRL7:system
733
incorporated in commercial design (FC3), TRL8: system incorporated in commercial design
734
(FC2), TRL9: system proven and ready for full commercial deployment (FC1) (Jungmeier,
735
2009).
736 737
4.6 Circular Economy
738 739
Food waste prevention is an integral part of the new Circular Economy Package, with
740
benefits such as boost of the global competitiveness, sustainable growth and, generationof new
741
jobs. One of the issues of the Circular Economy model is the collection of SCG and the scale
742
of the endeavor.
743
The circularity of a coffee micro-economy naturally brings up questions related to
744
scalability. The collection of coffee grounds requires storage space, proximity among
745
participating buyers, proximity to additional production facilities in which spent grounds will
746
be used, and numerous other logistical concerns.
747
Due to their high organic matter, SCG sometimes are used as a fertilizer. However, SCG
748
are highly toxic to the plants due to the presence of caffeine, tannins, and polyphenols. In
32
ACCEPTED MANUSCRIPT
749
addition, due to the presence of organic matter in SCG, a huge quantity of oxygen is required
750
for their degradation in landfills. Simultaneously, methane, which is a greenhouse gas and even
751
more harmful than carbon dioxide, is also released in the landfills, contributing to global
752
warming. Therefore, usual disposal methods need to be replaced by more sustainable towards
753
increased resources recovery and higher energy efficiency. Valorization of this waste towards
754
material and energy recovery rather than disposal, is gaining interest.
755
Although, the biorefinery concept is considered as one of the research cornerstones in
756
the last years and as the best option to transform the different waste systems by a multi-process,
757
multi-product pathways, (Moncada et al., 2016), there is a shortage of analysis of the potential
758
benefits on associated business development.
759
Only one paper found in the book of Morone et al., (2017), to assess the logistics,
760
economical and social feasibility to isolate SCG from the catering industry and use them as raw
761
material for a novel process to produce alternative high added value products in a near-perfect
762
circular economy cycle, making use of reverse logistics and generating near-zero waste (Topi
763
and Bilinska, 2017). The study was based on a series of theoretical scenarios corresponding to
764
the different possible logistic and process options that stakeholders could identify. This
765
theoretical approach concluded that the process is technically feasible with available technology
766
within current infrastructure and modest investments and the economic case is very attractive
767
to investors.
768
Some international companies of coffee beverage have started to devote efforts on
769
sustainable valorization of SCG, advocating Circular Economy model, by organizing collection
770
systems and exploring technological pathways for valorization (Bernstein, 2012).
771
Alternative scenarios for using the SCG to produce alternative high added value products
772
should be considered and developed, by using the participatory mapping approach and
33
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773
economic, environmental and social benefits compared with compost production scenarios
774
(Morone et al, 2018).
775 776
4.7
Design guidelines
777 778
Reflections and guidelines for sustainable biorefinery concepts are cited, that are mainly
779
proposed by Moncada et al., 2016. These include the following quotes that could be also
780
suggested for a SCG biorefinery design
781
Integration increases the efficiency.
782
Integrated technologies should have priority over separated technologies.
783
Integration of cogeneration CHP using biorefinery solid residues is suggested.
784
Energy integrations levels are important to reach maximum energy efficiency levels.
785
Include as many as possible products in the biorefinery.
786
Cascade approaches are sustainable pathways in the circular economy.
787
Innovative engineering solutions should be preferred.
788
The CHP gasification technology has a better performance than the technology based on direct combustion.
789 790
technical, and economic impacts is very important.
791 792
The use of modern tools and strategies of analysis and evaluation for environmental,
The implementation of optimization strategies and models could be interesting when coupling with further design.
793 794
Supply chain and logistics are essential part of a green biorefinery.
795
Use of indexes is important.
34
ACCEPTED MANUSCRIPT
796
In addition to the above, conducting sensitivity analyses for comparison of different
797
systems, enables global evaluation and rating of those systems on the same scale of normalized
798
values. LCA can be used as a valid tool to support designers, decision-makers towards
799
promotion of more sustainable options of SCG valorization for energy, fuel, biochar and high
800
added value/ low volume products (Ingrao et al., 2018b), although weighing is based often upon
801
social or political considerations (De Benedetto and Klemes, 2009). Therefore, weighing step
802
could be recognized as mandatory by the subject International Standards (Ingrao et al.,2018b,
803
2018a).
804 805
5.
Conclusions
806 807
This study was conducted to review the field of SCG valorization, its prospects,
808
potentialities and challenges. The review attained the proposed goal, as it brought important
809
issues of the SCG mono- and biorefinery valorization options, as reported in the international
810
literature. Limitations were found in extrapolating information and results from the papers
811
reviewed, since each paper explored different end-products and processes, so evaluations and
812
comparisons were difficult to be made.
813
The review allowed the authors to deepen the knowledge in the SCG biorefinery, that
814
represents the platform to start the development at higher TRL, for further integration and
815
optimization of SCG recycling systems. It allowed the development of knowledge-based
816
strategies to unlock the potential of SCG to produce bio-derived chemicals, fuels and carbon
817
materials, and probably effecting waste management regulations.
818
The review focus was centered upon SCG, which an important food waste of global
819
society, containing substances that make it a valuable bio-resource. Today, food wastes are a
35
ACCEPTED MANUSCRIPT
820
major concern and require for their management advanced techniques with economic benefits
821
and environmental safety. It was made evident, that the development of sustainable and efficient
822
refining of SCG depends on the spectrum of various end-products, market outlets and cost-
823
effective processing schemes. It was documented that polysaccharides, phenolic compounds,
824
tannins, biodiesel, bioethanol can be produced from the SCG, by using mono-extraction
825
processes, while a cascade biorefinery can produce fuels, energy, carbon materials and biochar,
826
in a closing loop approach, in addition.
827
SCG cascade or integrated biorefinery seems to be a more economically viable and
828
resource efficient option, compared to strategies relying on complete biomass disintegration,
829
that do not lead to optimal utilization of biomass feedstock. Cascading approaches are favorable
830
because they maximize biomass recycling with material and energy recovery.
831
contributing to more sustainable and circular economies, SCG cascade biorefinery seems to
832
have a commercial value.
Besides,
833
A shortage of economic and environmental assessments was observed in the international
834
literature, so it was difficult to draw concise economic and environmental conclusions for the
835
SCG biorefinery options, with the exception of a study performed by Brazilian researchers
836
(Mussatto et al., 2013), on integrated biorefinery of the Brazilian case of spent grains (BSG)
837
for the production of xylitol, lactic acid, activated carbon and phenolic acids, integrated with
838
heat production. This study provided good economic and environmental indicators, with an
839
economic margin of 62.25%, potential environmental impact of 0.012 PEI/kg products, and the
840
carbon footprint of the processing stage represented 0.96 kg CO2-e/kg of BSG.
841
Considering findings from this review, it seems that efforts are required to sustainability
842
assessment, policy analysis and national regulatory framework harmonization to the EU, that
843
are the drivers of change in food-waste reduction and valorization. Different national policies
36
ACCEPTED MANUSCRIPT
844
apply to bio-waste management, can delay the utilization of advanced bio-waste valorization
845
methods. There is an urgent need for creation of technical standards and indicators to guide
846
and regulate the assessment of the sustainability of SCGs-based biorefineries. LCA proposed
847
as a valid tool to support designers, and decision-makers towards promoting and developing
848
sustainable solutions.
849
Only one paper found to assess theoretically SCG collection and valorization in a circular
850
economy (Topi and Bilinska, 2017), concluding that collection and valorization of SCG is
851
technically feasible with available technology, within current infrastructure, and modest
852
investments. In this case, the economic case is very attractive to investors.
853
Based upon the analysis of the papers, the authors found that there is an urgent need for
854
R&D, effective regulations, methodological approaches to design and estimation of the SCG
855
collection systems, investment and manufacturing costs, indicators development for
856
assessments prior to the development of new business models within a Circular Bieconomy.
857
Through the review some guidelines were highlighted, useful for the design of biorefineries.
858 859 860 861
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1139
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1144
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1145
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50
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Fig. 1. Published articles concerning SCG, per year of publication. Fig. 2. Published articles on single-extraction process products, per year of publication. Fig. 3. High added value products derived from a single-extraction process. Fig. 4
SCG thermochemical conversion processes and their products.
Fig. 5. SCG biorefineries reported in literature.
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Fig. 5
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TABLES
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Table 1. Screening procedure of peer review articles Number of articles in Screening Step
Sample
1.
First sample
630
2.
Sample after duplicates removal
374
3.
Sample after less relevant articles removal
333
4.
Remaining sample after cut-off point
320
5.
Final sample
6.
Books
7. Internet sources Final Sample
92 3 7 102
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Table 2. SCG biochemical composition (wt%db) Lipids
Carbohydrates
Proteins
(wt%db) n.d.
(wt%db) 14.1
(wt%db) 14.4
Somnuk et al., (2017)
9-16
45-47
13-17
Burniol-Figols et al., (2016)
13±0.04
65.9±6.5
4.9±0.6
Passos and Coimbra., (2013)
13.7±0.1
54.1±2.2
13.8±0.1
Wang et al., (2016)
45.3
13.6
Campos-Vega et al., (2015)
References
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Table 3. Proximate and ultimate analysis of SCG Ultimate Analysis (ww%db) C
H
N
S
O
References
53.0
6.8
2.1
0.1
38.1
Somnuk et al., (2017)
52.5±0.4
7.0±0.03
3.46±0.01 0.10±0.00 34.8±0.1
Tsai et al., (2012)
Proximate Analysis (ww%db) Moisture
Volatile
Ash
Fixed Carbon
-
-
1.6
-
Somnuk et al., (2017)
11.5±0.4
79.5±0.01
0.7±0.2
8.2
Tsai et al., (2012)
Table 4. Single-extraction SGC processes for high added value compounds reported in the literature SCGs single-extraction process Reference
Somnuk et al. (2017)
Process Parameters
Process
Oil extraction
Extraction characteristics (Solvent Extr) Hexane Ethanol anhydr. Ethanol hydrous Methanol
Phimsen (Soxhlet extr.) et al. Hexane (2017) Mussato Sulfuric acid et al. (2011) Polysacharides Ballesteros extraction (Autohydrolysis) et al. Water (2017) (Microwave Passos superheated et al. water extraction) (2013) Water (Subcritical Xu et al. Phenolics water extraction) (2015) extraction Water Shang et Phenolics and (Pressurized
Oil Polysacharides SCG/Solvent Extraction Temperature (w/w% (w/w% d.b.) ratio (g/g) time (min) d.b.) (°C) 22.5 30.4 30 14.7 22.8 33.5 30 13.1 20.3 25.5 30 11.8 23.8
19.6
400g/3l
480
0.5M
45
1g/15ml
30
Product Phenolics (w/w% d.b.) -
Tannins Caffeine (mg/g (mg/g d.b.) d.b.) -
7.5
-
-
-
-
13
-
-
-
-
121
-
45.3
-
-
-
10
160
-
29.29
234.1407 mg GAE/g
-
-
1gSCG/10ml
2
200
-
55
-
-
-
14.1g/l
38-55
160-180
-
-
86.2 mg GAE/g
-
-
195
-
-
19-26
-
3-9
al. (2017)
Caffeine extraction
liquid extraction) Water
mgGAE/gdb
Ethanol Low et al. (2015) Brazinha et al. (2015)
Tannins’ Extraction
Sodium hydroxide 5wt%
Caffeine extraction
Membrane technology
8.2
30
mg/g db
195
-
-
-
-
-
100
-
-
-
21.02
-
-
-
-
-
3-9
Table 5. SCG biorefinery approaches reported in literature. No Reference Processes a. Phenols extraction 1 BurniolFigols et al. (2016) b. Acid hydrolysis
2
3
4
c. Ethanol Fermentation Obruca et al. a. Oil extraction (2015) b. Polyphenols extraction c. Bacterial Fermentation Caetano et a. 1st SCG Extraction al. (2017) b. 2nd SCG Extraction c. Transesterification d. Drying & Pelleting e. Pyrolysis and Torrefaction f. Hydrolysis & Fermentation Vardon et a. Oil extraction al. (2013) b. Transesterification c. Slow pyrolysis *L/S ratio= Liquid to Solid ratio
Parameters Solvent: Ethanol Temperature: 70°C L/S*: 25ml solvent/g TS liquidratioH2SO4 1solid w/w% Temperature: 140°C Time: 45 min L/S*: 10g liquid/g TS Strain: Saccharomyces cerevisiae Solvent: n-hexane Solvent: Ethanol Bacteria: Burkholderia cepacia Solvent: water/ethanol/supercritical CO
Products Chlorogenic acid
Bioethanol Oil Polyphenols PHAs High value extracts: antioxidants, caffeine, tannins, polyphenols, etc. Triglycerides Biodiesel, Glycerin, Hydrogen Pellets Biochar, biooil Ethanol Oil Biodiesel Biochar, biooil
Anastasia Zabaniotou, Paraskevi Kamaterou PII:
S0959-6526(18)33633-3
DOI:
10.1016/j.jclepro.2018.11.230
Reference:
JCLP 14989
To appear in:
Journal of Cleaner Production
Received Date:
13 July 2018
Accepted Date:
23 November 2018
Please cite this article as: Anastasia Zabaniotou, Paraskevi Kamaterou, Food Waste valorization advocating Circular Bioeconomy -A critical review of potentialities and perspectives of Spent Coffee Grounds Biorefinery, Journal of Cleaner Production (2018), doi: 10.1016/j.jclepro.2018.11.230
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Food Waste valorization advocating Circular Bioeconomy -A critical
2
review of potentialities and perspectives of Spent Coffee Grounds
3
Biorefinery
4 5
Anastasia Zabaniotou*, Paraskevi Kamaterou
6
Biomass Group, Department of Chemical Engineering, Aristotle University of
7
Thessaloniki, Greece
8 9
Abstract. Waste biorefineries are instrumental for advocating Circular Bioeconomy. Food
10
waste valorization is a goal of sustainable development, gaining high interest in resolving
11
environmental and resources challenges. Coffee use generates massive quantities of spent
12
coffee grounds (SCG), a resource rich in fatty acids, amino acids, polyphenols, minerals, and
13
polysaccharides. This review aims to shed light on the potentialities, prospects, and challenges
14
of the transition from a mono-process to a cascade SCG biorefinery, in a circular economy
15
thinking. It was found that mono-process approaches of SCG extraction have been investigated
16
by many researchers, while SGC biorefining approaches are still at an early stage of research.
17
Studies on SCG biorefineries, their environmental and economic assessment are few in the
18
literature, therefore imitations in extrapolating information and comparing the results were
19
faced. It was made evident that more studies are needed on the economic assessment of the
20
mono-process SCG break down, at higher Technology Reediness Level (TRL) for realistic
21
assessments. Efficient conversion of SCG in a cascade biorefinery depends on the spectrum of
22
various end-products and cost-effective processing schemes. Lipids and/or polysaccharides
23
extraction followed by the conversion of by-streams to energy and biochar, in a closing loop
1
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concept, has good potentialities. The review allowed the exploration of knowledge-based
25
strategies to unlock the potential of SCG for bio-derived chemicals, carbon materials, fuels and
26
fertilizer production and probably impacting waste management regulations. Some guidelines
27
for the sustainable design of SCG biorefineries were provided.
28 29
Keywords: Spent coffee ground, food waste, cascade biorefinery, sustainability, circular
30
economy, bioeconomy.
31 32
Corresponding author. Anastasia Zabaniotou, prof, email: [email protected]
33
Address: Dept of Chemical Engineering, Aristotle University of Thessaloniki, U.Box 455, GR24154
34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
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50 51
Graphical Abstract
52 53
SCGs Cascade Biorefinery
Products
Syngas st
1 Refining
2nd Refining Biooil
Collection Biochar
Phenols Tannins Polysacharrides
54 55 56 57 58
Highlights
59 60
Food waste valorization advocates Circular Bioeconomy.
61
SCG valorization via one-process extraction has been widely investigated.
62
SCG biorefinery is still at an early stage of development.
63
A sequential SCG valorization is preferable for higher recovery.
64
SCG cascade biorefinery needs proofs of economic viability.
65
R&D work at higher Technology Reediness Level is needed 3
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Abbreviations ABTS
2,2'-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid)
ACs
Activated carbons
AO7
Acid orange 7
BSG
Brewer's spent grains
CGA
Chlorogenic Acid
CM
Coffee melanoidins
CS
Coffee silverskin
db
Dry base
DSCG
Dried spent coffee grounds
EAE
Enzyme-assisted extraction
FFA
Free fatty acid
FW
Food Waste
HHV
Higher heating value
GAE
Gallic acid equivalents
HTL
Hydrothermal liquefaction
MAE
Microwave assisted extraction
MB
Methylene blue
PEF
Pulsed-electric field extraction
PEI
Potential environmental impact
Ph
Phenol
PHAs
Polyhydroxyalkanoates
PLE
Pressurized liquid extraction
PMHS
Polymethylhydrosiloxane 4
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66
SCG-GAC
Spent coffee grounds into calcium-alginate beads
67
SCGs
Spent coffee grounds
SDGs
Sustainable Development Goals
SFE
Supercritical fluid extraction
SL
Solid-liquid
TBAs
Tannin-based absorbents
TE
Trolox equivalents
TS
Total solid
UAE
Ultrasound-assisted extraction
UNDP
United Nation Development Program
USDA
United States Department of Agriculture
VOCs
Volatiles Organic Compounds
EMY
effective mass yield
FC
Feature Complexity index
68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107
5
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1. Introduction
109 110
Beyond climate change, the main challenges the world is facing today, are the substantial
111
increase in energy demand, food and material unsustainable consumption and production, and
112
anthropogenic wastes generation (Venkata et al., 2016). According to World Resources
113
Institution (World Resources Institute, 2018a), the planet is projected to hold 9.6 billion people
114
by 2050, consuming the equivalent of 1.6 planet’s resources, with a consequent high amount of
115
wastes generated. Consequences of the unsustainable consumption and production patterns are
116
resource depletion, climate change, air and water pollution, loss of biodiversity and of fertile
117
soil, amongst other environmental, social, and economic challenges (FAO, 2013).
118
The production of the enormous quantities of food waste (FW) is becoming a global
119
concern (Dahiya et al., 2016), because the world need to adequately feed the 9.6 billion people
120
by 2050, in a way that advances economic development and reduces pressure on the
121
environment (World Resources Institute, 2018a). Long-term strategies are needed aligning
122
short and medium-term goals. The long-term goal for many countries and especially for Europe,
123
is ensuring a transition from carbon-intensive to low-carbon societies. This transition must be
124
achieved by developing and deploying not only technologies and practices, but also changing
125
behaviors and patterns of production and consumption (World Resources Institute, 2018b), as
126
depicted in the 17 Sustainable Development Goals (SDGs) of United Nation development
127
program (UNDP). New interconnected areas, such as climate change, innovation, sustainable
128
consumption, are included among other in the SDGs (United Nations Development Program,
129
2018).
130
A new approach to sustainability has been proposed in Europe. This encompasses the
131
Circular Economy model, as a pathway to engage with challenges of sustainable production
132
and consumption. A priority in the EU is to stimulate the transition towards a Circular Economy 6
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133
that fosters the promotion of sustainable and resource-efficient policies for long-term socio-
134
economic and environmental benefits, by adopting strategies of “closing the loop” in industrial
135
production systems (Maina et al., 2017). One of the most relevant goals in the application of
136
this approach is to convert low-value side streams/residues/wastes into more valuable products.
137
Limitations of this approach related to the social aspects of circularity, result to the call for more
138
ethical and socially inclusive approaches to the Circular Economy (Lazell et al., 2018).
139
Food waste (FW) valorization is a goal of sustainable development and it gains high
140
interest since many bio-based products can derived from them, besides energy and fuels (Rama
141
Mohan, 2016). Coffee is one of the most popular and appreciated beverages worldwide, and
142
plays an important role in the global economy, as it is the second most traded commodity after
143
oil (Murthy and Naidu, 2012). Coffee industries are a key sector in the global economy, due to
144
income reporting and job creation. Based on United States Department of Agriculture (USDA)
145
report (USDA, 2017), global coffee industry reached an estimated production of 9.34 million
146
tons in 2016/17, which generated massive quantities of bio-wastes that are incinerated, dumped
147
in a landfill, or composted. Coffee companies produce annually more than 2 billion tons of by-
148
products, such as coffee spent grounds (SCG) and coffee silverskin (CS), most of which are
149
thrown away for landfilling (Jimenez-Zamora et al., 2015).
150
The generation of energy and various commodities in an integrated approach addressing
151
sustainability, is a challenge and a perspective for Europe (Dahiya et al., 2016). New
152
generations of biorefinery can combine innovative bio-waste resources from different origins.
153
Integrated and cascade biorefineries are cornerstones of the Circular Bioeconomy. SCG can be
154
used as feedstock to produce various bio-based products and bioenergy in a biorefinery concept
155
(Mata et al., 2018), by closing loops (Karmee Sanjib Kumar, 2018). Targeting economic
156
viability and sustainability is very important for a Circular Bioeconomy (Vânia et al., 2018).
7
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157
This paper aims to review the potentialities and perspectives of SCG biorefinery, as a
158
solution to current bio-waste disposal problems, in a Circular Bioeconomy. The objective is to
159
review the mono-process pathways for the valorization of SCG, reported in the international
160
literature, and to ding in the multi-process-multi-product cascade biorefinery concept that is a
161
cornerstone in the transition to Circular Bioeconomy. It finally aims to provide general
162
guidelines on how to build a sustainable SCG biorefinery, by summarizing various researcher’s
163
suggestions, who have worked on the topic of sustainable biorefineries.
164 165
2. Methodology
166 167
In this paper, a systematic literature review was performed following the method
168
proposed by Thürer et al. (2018), for sourcing, screening, and analyzing the published articles.
169
The target was to retrieve and select those articles which investigate and define the current state-
170
of-the-art on valorization of spent coffee grounds (SCG), via sustainable pathways in the
171
concept of biorefinery. Main effort was devoted to find studies on biorefineries’ economic
172
viability and sustainability. Since the strategy of Circular Economy is recent and mainly
173
articulated in Europe, very few studies appeared in the international literature, mainly published
174
during 2018.
175 176
2.1. Sourcing, screening, analyzing the articles
177 178
Scopus and open access are the bibliographic database selected for retrieving the articles,
179
because it provides extensive literature tools (books, articles). For quality assurance of the
180
research, only peer-reviewed articles were selected. The criteria used for the articles’ selection
8
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181
were the title, abstract, keywords, and document type (restricted to ‘articles’ and ‘reviews’),
182
and the year of publication for a period of 9 years (from 2009 to 2018). The inquiry took place
183
in July 2018 using the terms ‘Spent AND Coffee AND Grounds AND Conversion’; ‘Spent
184
AND Coffee AND Grounds AND Extraction’; ‘Spent AND Coffee AND Grounds AND
185
Valorization’; ‘Spent AND Coffee AND Grounds AND Pyrolysis’; ‘Spent AND Coffee AND
186
Grounds AND Biorefinery’; ‘Spent AND Coffee AND Grounds AND Sustainable’; ‘Spent
187
AND Coffee AND Grounds AND economic’. Other articles, concerning biorefining and
188
Circular Economy were also included.
189
The literature inquiry brought to light 844 articles (109 for the search term ‘Spent AND
190
Coffee AND Grounds AND Conversion’, 252 for the search term ‘Spent AND Coffee AND
191
Grounds AND Extraction’, 119 for ‘Spent AND Coffee AND Grounds AND Valorization’, 98
192
from ‘Spent AND Coffee AND Grounds AND Pyrolysis’, 33 for ‘Spent AND Coffee AND
193
Grounds AND Biorefinery’, 201 for ‘Spent AND Coffee AND Grounds AND Sustainable’,
194
while only 3 dealing with economics. Nine review studies were retrieved, which published just
195
recently.
196
The original sample contained 470 duplicates articles, which were removed. From the
197
374 remaining articles, 39 were subtracted as unrelated and 13 were cut-off due to their year of
198
publication, which was out of the time spectrum set in this study. All 320 papers were examined.
199
By focusing on extracting, grouping of papers concerning sustainable SCG valorization that
200
give information, knowledge, and data, to reply to the research questions set, finally 92 papers
201
were used as the most appropriate and interesting for the review. Another 7 internet sources and
202
2 books on SGC valorization, and one book on food wastes, sustainability and Circular
203
Economy topics were also included. A total of 102 sources were selected for this review (Table
204
1).
9
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205
The screening of the 320 papers was made by prioritizing papers discussing
206
sustainability, techno economic assessment, biorefinery approach, mono-process approach of
207
the SCG and related review papers published within the decade 2017-2018. The research questions for the screening of the 320 papers were:
208 209
Which is SCG’s composition?
210
Which useful components could be extracted from SCG and how?
211
What about thermochemical conversion routes for SCG valorization?
212
Can SCG produce biochar via pyrolysis?
213
Can biorefining be applied for SCG valorization?
214
How sustainable is to valorize SCG by biorefinery?
215
What is the economic viability of a SCG biorefinery?
216
What is the environmental sustainability of the SCG biorefinery?
217
Are there any guidelines for a multi process-multi product approach of SCG valorization?
218
Which are the prospects arising from this paper?
219
Fig. 1 depicts the number of the related papers published per year. Fig. 2 depicts the
220
paper related to the extracted product per year of publication. It is obvious that papers on mono-
221
process approach are the first appeared in the international literature and continue to be
222
published in a large number. Papers on biorefinery approach are recent and mainly of the year
223
2018. Papers on sustainability and technoeconomic assessment are few. Concerning review
224
papers, these are showing an increasing trend the last 3 years.
225 226
3. Materials and methods
227
10
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The first research question for the screening of the 320 papers is related to SCG’s
228 229
composition. The SCG composition defines the choices for the valorization.
230 231
3.1. SCG composition
232 233
The biochemical composition of SCG as reported in literature, is presented in Table 2.
234
SCG contain 9-16 wt% db lipids, 5-15 wt% db proteins and are considered as an important
235
source of polysaccharides (carbohydrate whose molecules consist of several sugar molecules
236
bonded together). Literature review revealed that almost half of the material (45.3 wt% db) is
237
sugars polymerized into cellulose and hemicellulose structures (Mussato et al., 2011) and that
238
most polysaccharides (around 70 wt% db of total polysaccharides from roasted coffee) remains
239
in SCG (Arya & Rao, 2007). Mussato et al. (2012) have hydrolyzed SCG and efficiently
240
fermented it to ethanol by yeast. SCGs coffee fibers exhibit antioxidant properties suggesting
241
its use as potential dietary supplement (Campos-Vega et al., 2015). Ballesteros et al. (2015)
242
investigated the antimicrobial and antioxidant capacities of polysaccharides extracted from
243
SCG.
244
The ultimate/proximate analysis of reported SCG is presented in Table 3. The carbon
245
content of SCG ranges between 45-53 wt% whereas the values for hydrogen and nitrogen
246
content are 6-7 wt% and 2-4 wt%. The amounts of ash (1-2 wt% db) containing in SCG is low
247
compared to other biomass sources (Zabaniotou et al., 2017). SCG is characterized by high total
248
volatiles content (Tsai et al., 2012).
249 250
3.2. Mono-process approach: What can be extracted from SCG?
251
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252
SCG represent a resource rich in valuable components which could be valorized giving
253
a range of commodities. Large amounts of organic compounds are contained in SCG, such as
254
fatty acids, amino acids, polyphenols, minerals, and polysaccharides (Campos-Vega et al.,
255
2015). Coffee is considered an important source of polysaccharides (Ballesteros et al., 2017),
256
proteins (Mussato et al., 2011) and lipids (Campos-Vega et al., 2015). SCG is rich in sugars (45
257
ww%, dry weight of the material) which can be polymerized into cellulose and hemicellulose
258
structures. Mannans is the major polysaccharides of SCG which contains 47 wt% db mannose,
259
30 wt% db galactose, 19 wt% db % glucose, and 4 wt% db arabinose (Mussatto et al., 2011).
260
Remarkable amount of proteins (13-17 wt% db) are contained in SCG Mussato et al. (2011).
261
Other N-containing substances like caffeine, trigonelline, free amines and amino acids are
262
contained in SCG (Delgado et al., 2008).
263
Caffeine is a non-protein nitrogenous compound which can be recovered from SCG.
264
Panusa et al. (2013) estimated a low content of caffeine (0.96-0.97 mg/g dry sample) in SCG
265
extracts. According to Cruz et al. (2012) the range of caffeine content found higher for the
266
espresso coffee (1.9 -7.9 mg/g (DW)). However, caffeine, tannins and chlorogenic acid are of
267
eco-toxicological concern and can limit their value-adding applications.
268
There is an especial interest in SCG lipids extraction. Total lipids content of espresso
269
coffee residues ranges from 9.3-16.2 wt% db (Cruz et al., 2012) depending on the coffee variety
270
(Jenkins et al., 2014), a promising feedstock to produce biodiesel (Al-Hamamre et al., 2012).
271
Hexane is the most popular solvent for oil extraction, but Supercritical Fluid Extraction is a
272
modern environmentally friendly technology which is increasingly being used for SCG oil
273
extraction (Campos-Vega et al., 2015).
274
SCG contain phenolic compounds which are known as human health related compounds,
275
with demonstrated antioxidant, anti-bacterial, antiviral, anti-inflammatory and anti-
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carcinogenic activities (de Souza et al., 2004). Polyphenols and chlorogenic acid (CGA) have
277
been extracted from SCG via a conventional solid-liquid method (Campos-Vega et al., 2015). The techniques for extraction of compounds are classified into two categories: the
278 279
conventional and the non-conventional (Azmir et al., 2013). The conventional techniques are:
280
Soxhlet extraction
281
Maceration
282
Hydrodistillation
283
The non-conventional extraction techniques are (Azmir et al., 2013):
284
Ultrasound-assisted extraction (UAE)
285
Pulsed-electric field extraction (PEF)
286
Enzyme-assisted extraction (EAE)
287
Microwave assisted extraction (MAE)
288
Pressurized liquid extraction (PLE)
289
Supercritical fluid extraction (SFE)
290
Table 4 presents the valuable components that can be extracted from SCG, as
291
demonstrated in the literature. By searching the literature, opportunities were found concerning
292
the recovery of oils, polysaccharides, phenolic compounds, tannins that can be used in various
293
commodities, by using mono-extraction processes, as depicted in Fig. 3.
294 295
3.2.1. Oil recovery
296
The oil content of SCG ranges from 10 to 15 wt%, depending on the coffee varieties
297
(Jenkins et al., 2014). Somnuk et al. (2017) studied the effect of four different solvents (hexane,
298
ethanol, hydrous ethanol and methanol) on coffee oil yield, by using a circulation process. The
299
optimal conditions (30.4 min extraction time and 22.5 g/g ratio of DSCG-to-hexane) resulted
13
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300
in approximately 11.8 wt% oil yield. Phimsen et al. (2016) utilized a Soxhlet extractor and n-
301
hexane as a solvent in order to extract the oil from dried SCGs. The extracted oil, yielded from
302
10 to 13 wt% db SCG, and then it was hydrotreated in a shaking batch reactor, by using NiMo/γ-
303
Al2O3 and Pd/C as catalysts. It has proved the bio-hydrotreated fuel production and potentiality
304
to be used as a renewable energy.
305 306
3.2.2. Polysaccharides recovery
307
Polysaccharides play multiple role in life process and present an immense potential in
308
healthcare, food, and cosmetic industries, due to their content of bioactivities that have
309
therapeutic effects and relatively low toxicity (Shi L. 2016). Due to their enormous structural
310
heterogeneity, the approaches for isolation and purification of polysaccharides are distinct from
311
that of the other macromolecules such as proteins etc. (Shi L. 2016). Therefore, various methods
312
widely used in isolation and purification of polysaccharides. The extraction of polysaccharides
313
from SCG has been studied, mainly using chemicals as extraction agents. Sodium hydroxide
314
(Ballesteros et al., 2015) and potassium hydroxide have been employed in SCG alkali
315
treatments, while sulfuric acid has been used to recover carbohydrates from SCG dilute acid
316
hydrolysis (Mussatto et al., 2011).
317
Mayanga-Torres et al. (2017) proposed the recovery of sugars compounds from coffee
318
industry residues using subcritical water hydrolysis as the valorization technique. Evaluation of
319
optimal conditions which simultaneously maximize holocellulose hydrolysis and minimize
320
both sugar degradation and dilution is proposed.
321
The extraction of polysaccharides by autohydrolysis of SCG was investigated by
322
Ballesteros et al. (2017). The extracted polysaccharides (29.29 wt%) characterized by high
323
antioxidant activity. The conditions for the extraction were 15ml water/g SCG, for 10 min, at
14
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160°C. The polysaccharides obtained were thermostable in a large range of temperature and
325
had typical carbohydrate pattern. Their use for industrial applications, mainly in the food area
326
was proposed.
327
The feasibility of microwave superheated water extraction of polysaccharides from SCG
328
was studied by Passos et al. (2013). They found that a maximum of 0.57 g/batch
329
polysaccharides for 1 g SCG: 10 mL water can be recovered. Further extraction of
330
polysaccharides was achieved with a second extraction (re-extraction) of the remaining un-
331
extracted insoluble material, under the same conditions.
332 333
3.2.3. Phenolic compounds recovery
334
Polyphenols are micronutrients. The health benefits of polyphenols and their protective
335
effects in food systems as antioxidant compounds, are well known and have been extensively
336
investigated (Shavandi et al., 2018). Recovery of relevant natural antioxidants for use as
337
nutritional supplements, foods, or cosmetic additives can be achieved by SCG extraction with
338
environmentally friendly procedures (Panusa et al., 2013).
339
The bibliographic search shown that various extraction methods were used. Subcritical
340
water extraction of SCG resulted in significant antioxidative phenolics production (Xu et al.,
341
2015) at temperature range of 160-180°C, time range of 38-55 min and solid-to-liquid ratio of
342
14.1 g/l. 86.2 mg GAE/g of total phenolic compounds were recovered. Shang et al. (2017)
343
optimized the SCG extraction conditions for total phenolics by using pressurized liquid
344
extraction (PLE) method with water and ethanol. Optimal conditions obtained at 195°C
345
extraction temperature. The total phenolics content ranged from 19 to 26 mgGAE/gDW.
346
Al-Dhabi et al. (2017) developed and validated the SCG ultrasound-assisted solid-liquid
347
extraction of phenolic compounds Ultrasonic power, temperature, time, and solid-liquid (SL)
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ratio were studied as parameters. The optimum process conditions were: 244 W of ultrasonic
349
power, 40 °C of temperature, 34 min of time and 1:17 g/ml of SL ratio and the extraction
350
obtained yields reached 33.84 GAE/g of total phenolic content.
351
Coffee cherry pulp is a by-product derived from the process of coffee production. Coffee
352
cherry pulp contains considerable amounts of phenolic compounds and caffeine. An attempt to
353
produce “Cascara”, a refreshing beverage, has been made by Heeger et al. (2017). Six dried
354
coffee pulp samples and Cascara produced out of one of those samples, were investigated in
355
Switzerland. Aqueous extraction of coffee pulps revealed a content of total polyphenols
356
between 4.9 and 9.2 mg gallic acid equivalents (GAE)/g db. The antioxidant capacity was
357
between 51 and 92 lmol Trolox equivalents (TE)/g DM, as measured with ABTS radical.
358
Bourbon variety from Congo and Maragogype variety showed highest caffeine contents, 6.5
359
and 6.8 mg/g DM, respectively. In all samples, chlorogenic acid, protocatechuic acid, gallic
360
acid and rutin, were present. The beverage Cascara contained 226 mg/L of caffeine and 283 mg
361
GAE/l of total polyphenols and an antioxidant capacity of 8.9 mmol TE/l.
362
Jimenez-Zamora et al. (2015) showed the prebiotic, antimicrobial and antioxidant
363
capacity of SCGs and CS, as well as those melanoidins (a coffee component generated during
364
the roasting process), obtained from the former. The prebiotic activity was important in both
365
CSG and CS, although the presence of coffee melanoidins (CM) interfered with such beneficial
366
properties. On the contrary, CM exerted an intense antimicrobial activity that could be used to
367
avoid the growth of pathogenic bacteria in food products. CSG, CS and CM were highly
368
antioxidant. The addition of sugar during coffee roasting, namely torrefaction, increased the
369
antioxidant and antimicrobial activity due to a larger generation of CM, although prebiotic
370
activity was not affected.
371
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3.2.4. Tannins recovery
373
Tannins extensively assessed as natural alternatives to in-feed antibiotics (Huang et al.,
374
2017). Low et al. (2015) investigated the influence of extraction parameters in SCG tannins’
375
recovery. They found that sodium hydroxide concentration, extraction temperature and liquid
376
to solid ratio considerably effected the SCG tannin extraction yield and its reactivity. Extraction
377
time had only marginal effect in the tannin extraction process. The optimal extraction conditions
378
were found: 5 wt% sodium hydroxide concentration, 100 °C extraction temperature, 30 min
379
extraction time, and 8.2 liquid to solid ratio. These conditions resulted in a high tannin
380
extraction yield (21.02 wt%) and high reactivity.
381
Tannins have traditionally been regarded as “anti-nutritional factor” for poultry. Recent
382
researches have mentioned that when applied in appropriate manner, improved intestinal
383
microbial ecosystem, enhanced gut health and hence increased productive performance (Huang
384
et al., 2017). However, tannins if used as additives in poultry feed to control diseases and to
385
improve animal performance, must ensure a consistent quality (Redondo et al., 2014).
386
Tannins are also low-cost natural biopolymers and excellent candidates to produce bio-
387
sorbents. Low-cost and eco-friendly products, such as adhesive, plastic, polyform can be
388
produced by using the SCG extracted tannins. Tannin-added films can be used as green,
389
nontoxic packaging materials for food and pharmaceutical products (Missio et al., 2018).
390
Tannin-based absorbents (TBAs) have a natural affinity to absorb heavy metals dyes, and
391
pharmaceutical compounds from contaminated waters (Bacelo et al., 2016).
392 393
3.2.5. Caffeine recovery
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394
Brazinha et al., (2015) optimized the process of producing a natural extract from SCG
395
by using membrane technology, with no organic solvents or adsorbents involved. The extracted
396
product was enriched in caffeine with specific health benefits.
397
Shang et al. (2017) optimized the SCG extraction conditions to caffeine using pressurized
398
liquid extraction (PLE) method with water and ethanol. At the optimal conditions (195°C
399
extraction temperature) caffeine’s yield reached 9 mg/g db.
400 401
3.3.
SCG valorization for bioenergy and carbon materials production
402 403
Energy recovery from biomass is a way to reduce waste, produce fuels, protect the
404
environment and mitigate greenhouse gas (GHG) emissions (Limousy et al, (2017). Energy
405
recovery should be combined with material recovery for enhanced resources efficiency in the
406
concept of a Circular Economy.
407
Energy recovery from SCG was documented in the international literature. Biochemical
408
(transesterification) and thermochemical (pyrolysis, gasification, hydrothermal liquefaction,
409
combustion), enzymatic conversion technologies were used for this, as it is depicted in Fig. 4.
410
Biodiesel, biooil, CHP, heat, biochar, activated carbons, carbon nanotutes, are the main
411
products of the application of a mono-thermochemical process fueled with SCG (Limousy et
412
al., 2017).
413 414
3.3.1. Biodiesel production
415
SCG contain significant amounts of lipids (∼16%w/w), which could potentially be
416
utilized as feedstock in biodiesel production. Many researchers used transferification of the
417
exctracted oil (Loyao et al. (2018). Solvent extraction technologies, with a wide range of
18
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418
solvents for lipid extraction from SCG, were used to determine the effect of solvent selection
419
and process temperature on the extraction efficiency and composition of the obtained oil, by
420
Efthymiopoulos et al. (2018). Al-Hamamre et al. (2012) studied oil extraction from dried spent
421
coffee grounds (DSCGs) for biodiesel production. They obtained 60 g DSCG. Kondamudi et
422
al. (2008) extracted the SCG oil (10-15 %wt) using solvents such as hexane, ether, and
423
dichloromethane under reflux conditions. They transesterified the oil to produce biodiesel and
424
achieved 100% conversion. The produced biodiesel was found to be stable for more than 1
425
month under ambient conditions.
426
The valorization route of lipid recovery followed by transesterification for biodiesel
427
production was also studied by Go and Yeom (2017). Lipid extraction was estimated at 92.7%,
428
using 13.7 mL-hexane/g-WCG, within 30 min extraction time, and 25°C. NaOH was used as
429
an alkaline catalyst. Optimum conditions for transesterification were achieved with the addition
430
of 0.5% catalyst, 1.5 mL methanol/g-lipid, at 45°C, and 9 h of reaction time. Biodiesel
431
production was mainly influenced by reaction time and temperature. Caetano et al. (2013)
432
examined the potential of biodiesel production from SCG. They used various solvents and
433
proposed a two-step process of acid esterification followed by alkaline transesterification for
434
lipids with high free fatty acids, as the best route to biodiesel. However, the properties of the
435
derived biodiesel (iodine number, acid value, and ester content) did not comply with the NP
436
EN 14214:2009 standards (Caetano et al., 2013). For meeting standard requirements, they
437
proceeded with two remediation procedures: a) blending of SCGs lipids with other higher-
438
quality vegetable oils before transesterification, b) mixing the produced biodiesel with higher-
439
quality biodiesel.
440
Döhlerta et al. (2015) studied the catalytic conversion of triglycerides derived from SCGs
441
to produce diesel, by using a cheap reductant agent, the polymethylhydrosiloxane (PMHS)
19
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442
under mild reaction conditions. Significant amounts of PMHS-waste generated as by-products
443
of the process (∼1.7% of the PMHS is required for the oil reduction). They were depolymerized
444
in a subsequent step, resulting in PMHS-waste conversion to methyltrifluorosilane and
445
difluoromethylsilane, which can be useful for new silicones production as building blocks. The
446
acid catalytic solvo-thermal in situ transesterification of SCG was demonstrated by Park et al.,
447
(2018). They suggested themeth for boosting the economic feasibility of biodiesel production
448
from wet SCGs.
449
Kookos (2018), by performing an economic and environmental analysis of biodiesel
450
production from SCG, concluded that the process economics can be attractive only in the case
451
of a centralized large-scale production plant. Biodiesel production from SCG is not
452
economically sound for small scale units.
453 454
3.3.2. Biooil production
455
Fast pyrolysis can convert the SCG into biooil and biochar. The fast pyrolysis of SCG
456
targeting biooil production was studied by Kelkar et al. (2015). The experiments took place in
457
a compact, transportable, screw conveyor reactor. Biooil yields showed a maximum yield of
458
61.8 ww% at 500 °C, while the highest biochar yield was observed at the lowest pyrolysis
459
temperature (429 °C). SCG-biooil contained fatty acids, fatty acid esters, medium-chain
460
paraffins, olefins, and caffeine.
461
Fast pyrolysis of SCG was also studied by Bok et al. (2012). They produced biooil with
462
a maximum yield of 55 ww% at 550 °C pyrolysis temperature, pyrolysed in a fluidized bed
463
reactor. Li et al. (2014) investigated SCG bioenergy production potential using pyrolysis, at
464
two different heating rates (10 and 60 °C/min). Biogas contained mainly CO2, CO, CH4 and the
20
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465
gaseous volatile. An efficiency of 77–85% was achieved in relation to the feedstock moisture
466
content.
467
SCG pyrolysis was performed by Cho et al. (2016) targeting both, waste disposal and
468
biooil recovery. In their study, SCG were pretreated with FeCl3, and carbon dioxide to enhance
469
syngas generation and reduce condensable hydrocarbons, such as tar. Syngas enhancement was
470
achieved via the CO2-induced thermal cracking of VOCs, due to the reaction between CO2 and
471
VOCs. Tar reduction was achieved by using Fe as catalyst in a CO generation from Fe-SCG
472
pyrolysis. This has also resulted in CO2 dramatical increase (up to 8000%), compared to SCG
473
pyrolysis withN2.
474
Hydrothermal liquefaction (HTL) of SCG in hot-compressed water was applied to
475
produce crude bio-oil in a 100 cm3 stainless-steel autoclave reactor, with N2 atmosphere, (Yang
476
et al., 2016). The effects of operating parameters (retention times - 5 min, 10 min, 15 min, 20
477
min and 25 min, reaction temperatures -200 °C, 225 °C, 250 °C, 275 °C and 300 °C,
478
water/feedstock mass ratios -5:1, 10:1, 15:1 and 20:1, process gas initial pressure -2.0 MPa and
479
0.5 MPa), were investigated targeting biooil yield with designed properties. A yield of 47.3ww
480
% of the crude biooil was achieved at 275 °C liquefaction temperature, 10 min retention time,
481
water/feedstock mass ratio of 20:1 and initial pressure of 2.0 MPa. The higher heating value
482
(HHV) of crude biooil was estimated at 31.0 MJ kg−1.
483
Yang et al. (2017) investigated the co-liquefaction in subcritical water of SCG mixed
484
with paper filter, corn stalk and white pine bark, aiming to bio-crude oil production. The
485
optimum reaction temperature was estimated at 250 °C, and the mixing biomass ratio was 1:1.
486
The best feedstock combination was SCG and CS and addition of 5% NaOH, as a catalyst.
487
Biooil quality and high yield suggest SCG as a valuable biooil feedstock.
488
21
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489
3.3.3. Biochar production
490
Slow pyrolysis of SCG can produce biochar. Pyrolytic char from SCG is a much better
491
fertilizer compared to direct SCG use in the fields, as it was proved by various studies. SCG
492
and SCG-char were applied as biochar, by Kim et al., (2014). In the case of SCG application,
493
the soil phytotoxicity increased, because a massive amount of dissolved carbon amount was
494
released from SCG in the soil. In contrast, SCG-char application did not exhibit this
495
phenomenon because any easily released organic matter was removed previously in the
496
pyrolysis process.
497
Tsai et al. (2012) evaluated SCG as a potential feedstock for the production of biochar
498
via pyrolysis. The conditions used were: Tpyrolysis= 400-700°C, heating rate=10 °C/min. It was
499
reported that the produced biochars showed high carbon content (>80 ww%), fixed carbon (>60
500
ww%) and calorific value (>30 .1 MJ/kg). The produced char can be also used also as solid fuel
501
in the industrial sector due to high calorific value.
502
Researchers (Cho et al., 2017) investigated co-pyrolysis of paper mill sludge mixed with
503
SCG, focusing on biochar production. CO2 was used as reaction medium aiming to syngas
504
generation enhancement and biochar’s physico-chemical properties modification. The
505
synergistic effects of CO2 and Fe/Ca caused a decrease in pyrolytic oil. The presence of Fe/Ca
506
in PMS favored CO generation Fe-ions were converted into magnetite (Fe3O4) and porous
507
biochar was created. Cho et al. (2017) concluded that co-pyrolysis of paper mill sludge and
508
SCG, using CO2 as reaction medium could feasibly generate CO and biochar, suitable for
509
environmental applications.
510 511
3.3.4. Activated carbon production
22
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512
Jung et al. (2016) prepared activated carbons from SCG into calcium-alginate beads
513
(SCG-GAC). The activated carbon powder originated from SCG was used for the removal of
514
acid orange 7 (AO7) and methylene blue (MB), from aqueous media. PH played a highly
515
important role in dye adsorption, whereas the influence of ionic effects was essentially neutral.
516
The pore diffusion model describing the adsorption kinetics, revealed that the rate-limiting step
517
during the adsorption process was pore diffusion. The maximum SCG-GAC adsorption
518
capacity for AO7 at pH=3.0 was estimated at 665.9, for MB 986.8 mg/g absorption obtained,
519
at 30 °C and pH= 11.0.
520
Different impregnation ratios of KOH were utilized by Laksaci et al. (2017) for the
521
synthesis of new activated carbons (ACs), from SCG. Many functional groups were identified
522
on the ACs surface. BET measurement revealed a maximal specific surface area of 1778 m2 g-
523
1,
524
methylene blue (MB) molecules was tested.
for an impregnation ratio of 36 mmol of KOH/g. ACs removal efficiency of phenol (Ph) and
525 526
3.3.5. Nanocarbons production
527
Zein et al. (2017) studied the SCG microwave radiation to produce nanocarbons. They
528
found that the optimum condition for maximizing nanocarbons yield (60 ww%) obtained at 200
529
°C, 650W microwave power and 45 min residence time. They concluded that this method could
530
potentially produce spherical shaped nanocarbons, which could be utilized for future scientific
531
innovations.
532 533
3.3.6. Liquid polyols production
534
Soares et al. (2014) investigated the possibility of SCG conversion into liquid polyols,
535
using acid liquefaction at moderate temperature and autogenous pressure. They concluded that
23
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536
the produced polyols have characteristics similar to those of petroleum-based polyols, which
537
are used in the polyurethane foam industry.
538 539
3.3.7. Energy production
540
The possibility of using SCG for energy via combustion, was proposed by Ciesielczuk et
541
al. (2015). They used briquettes made of mixed beech shavings and SCG, for increasing the
542
calorific value. SCG tested as a new bulking agent for biodrying of dewatered sludge (DS). It
543
was proved that SCG is an excellent bulking agent that accelerates DS biodrying and produces
544
a solid fuel with a high calorific value (Hao et al., 2018).
545 546
3.3.8. Cogeneration of heat and power (CHP)
547
Food manufacturers have been piling into the bioenergy sector turning waste and
548
production by-products into energy, producing enough heat and power for their own needs,
549
with surplus energy feeding back to the grid. Combined heat and power (CHP) can be produced
550
by SCG gasification. Cutting-edge, innovative and economical gasification techniques with
551
high efficiencies are a prerequisite for the application of gasification. Feedstock types, the
552
impact of different operating parameters, tar formation and cracking, and modelling approaches
553
for biomass gasification of biomass have widely studied.
554
Steam gasification of SCG was investigated by Pacioni et al. (2016) in the temperature
555
range of 650 to 850 °C, with a steam partial pressure range of 0.05 to 0.3 bar. A magnetic
556
suspension thermobalance was used for the gasification tests which were performed
557
isothermally. Gas chromatograph equipped with TCD/FID detectors was used for gaseous
558
products analysis. Product characterization revealed that the products contained higher carbon
559
and lower volatile matter compared to the original SCG and had high calorific value.
24
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560
Temperature and steam partial pressure influenced syngas production. H2+CO yields were
561
higher for a conversion range of 50–80%. The H2/CO ratio showed an increasing trend with
562
temperature.
563
To improve the feasibility and sustainability of SCG gasification, technological
564
advancement and the minimization of socio-environmental effects are needed (Ingrao et al.,
565
2018a).
566 567
3.4.
Biorefinery approach
568 569
The European Commission has set a long-term goal to develop a competitive, resource
570
efficient and low carbon economy by 2050 (EC, 2011). Bioeconomy is expected to play an
571
important role in the low carbon economy. The European strategy for building a sustainable
572
bio-based economy with emphasis on the sustainable use of natural resources, competitiveness,
573
socioeconomic and environmental issues, is on the spot (Scarlat et al., 2015).
574
Strategies relying on complete biomass disintegration through combustion, gasification,
575
or fermentation only, do not lead to optimal utilization of biomass feedstock. Cascading
576
approaches are required to maximize biomass valorization (Ingrao et al., 2018a, 2018b;
577
Zabaniotou et al., 2017, 2018).
578
In the waste-biorefinery concept, multifunctional processes are integrated in an optimized
579
sequence to utilize waste, with an objective of maximizing the productivity of marketable
580
intermediates and products (chemicals, materials, and bioenergy/biofuels), to enhance of the
581
process economics .
582
By searching carefully the international literature, 6 studies and 2 review papers were
583
found, dedicated to explore the various SCG biorefineries at laboratory scale, which signifies a
25
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584
low Technology Readiness Level (TRL). All 8 papers were published during the last two years
585
(2016-2018). The two review papers published on 2018, summarise various SCG biorefineries.
586
(Kourmentza et al., 2018; Mata et al., 2018). While the authors of the first review support the
587
SCG biorefinery approach, the authors of the second review conclude that most of the studied
588
biorefineries have limited scope and low economic value (Mata et al., 2018).
589
The reported SCG biorefineries (2015-2018) integrate various number of processes and
590
products as shown in Table 5, Fig.5. When compared with the mono process/extraction
591
proposals, it is clear that the biorefinery allows a more complete utilization of SCG, by
592
obtaining high value products, using technologies and process already available at commercial
593
scale. For applications in a Circular Bioeconomy, the biorefinery approach is a corne stone
594
(Karmee Sanjib Kumar, 2018).
595
Mata et al. (2018) in their review paper described several proposals for a SCG bio-
596
refinery, and compared each other. They concluded that for obtaining a wider product portfolio,
597
several separation processes are required and a combination of biological and chemical
598
processes is necessary. They also concluded that the most of them have limited scope and the
599
final products have low economic value.
600 601
4. Discussion on SCG biorefinery approach
602 603
The valorization of SCG by a mono-process pathway has attracted a lot of attention
604
recently from both the academia and industry. However, very few studies dealt with the
605
economic viability assessment of a mono-process approach of SCG valorization.
606
Food wastes create huge environmental, economic, and social problems, being also sources
607
of added-value materials. Coffee industries are a key sector in the global economy due to
26
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608
income reporting and job creation. Coffee companies produce annually more than 2 billion tons
609
of by-products such as coffee spent grounds (SCG) and coffee silverskin (CS). Proper design
610
of a biorefinery system, aiming to a wide range of products generation could serve as a unique
611
sustainable solution to SCG wastes management and valorization in a Circular Economy.
612
Techno-economic analysis and optimization models are crucial to design process scale,
613
understand how major cost categories affect the process and assess their sustainability. It is
614
evident that new business models introducing high-value bioproducts to biorefineries are
615
essential for achieving economic viability of industries within Bioeconomy. Economically
616
feasible production of conventional bioenergy such as biofuels, biopower and bioheat, is a
617
challenge. Biorefineries must compete with the inexpensive fossil fuel energies.
618 619
4.1 Prospects of SCG biorefinery
620 621
Besides, contributing to more sustainable and circular economies, the biorefinery has
622
high commercial value when compared to the ones obtained by currently used waste treatment
623
methods. The major advantage of biorefineries is their suitability for maximizing valorization
624
of structural and energetic potentials lying in biomass (Budzianowski Wojciech, 2017).
625
The prospects of SCG biorefinery, as explored at laboratory level and reported in the
626
international literature, are very encouraging. SCG can feed a biorefenery and via advanced
627
chemical and biotechnological methods, can produce a large number of value-added products
628
(polyhydroxyalkanoates, biosorbent, activated carbon, polyol, polyurethane foam, carotenoid,
629
phenolic antioxidants, green composite) and bioenergy (biodiesel, bio-oil, biogas), due to their
630
rich composition in lipid, carbohydrates, carbonaceous, and nitrogen containing compounds
27
ACCEPTED MANUSCRIPT
631
among others. However, exploration at a high Technology Readiness Level (TRL), is still
632
lacking behind.
633 634
4.2 Economic viability and sustainability challenges
635 636
The economic viability is a decisive parameter for the biorefinery deployment. This was
637
made obvious in a study performed by Kookos I.K. (2018), who recommended that research on
638
SCG valorization should be oriented towards the efficient recovery of the bioactive compounds
639
for a more economically attractive conversion. The economic performance of the biodiesel
640
production via a mono-process pathway is only viable at large production capacities, realized
641
at centralized facilities, despite that the environmental assessment of the process showed that
642
biodiesel production has good environmental indicators.
643
Results from a recent study on the techno-economic analysis of food waste biorefineries
644
at European level, showed that the most profitable options are those related to economies of
645
scale. However, the risk of increasing externalities due to logistics is possible (Cristóbal et al.,
646
2018).
647
There is a shortage in studies of a cascade SCG biorefinery. Garcia et al. (2017) reported
648
that hydrogen production via SCG gasification biorefinery is viable, but without referring to
649
the production of high-value bioproducts. Mussatto et al. (2013) have suggested the integrated
650
biorefinery of the Brazilian case of spent grains (BSG) for the production of xylitol, lactic acid,
651
activated carbon and phenolic acids integrated with heat production, as viable pathway, because
652
the economic viability and environmental performance that achieved have shown positive
653
indicators. The obtained economic margin was evaluated at 62.25%, the potential
28
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654
environmental impact was 0.012 PEI/kg products, and the carbon footprint of the processing
655
stage represented 0.96 kg CO2-e/kg of BSG.
656
Apart from the economic viability, the environmental sustainability of the SCG
657
biorefinery is a request (Ingrao et al., 2018a, 2018b). The challenge is to produce value added
658
products by integrating different strategies that lead to an interconnected environmental
659
biorefinery for maintaining the ecological footprint. However, sustainable biorefinery systems
660
are still a challenge, since weak designs lead to not viable solutions, with almost similar
661
environmental burdens with the petrochemical systems. They face socio-economic issues
662
related to land use, labor, food security and others, sdditionaly (Moncada et al., 2016).
663
In the economic analysis, results must be evaluated taking into account the high
664
uncertainty that this kind of study entails, which include the cost estimation and process
665
parameters estimation for low TRL technologies (Cristóbal et al., 2018).
666 667
4.3 Business development and market perspectives
668 669
It was made obvious that there is a need for design procedures of economically feasible
670
sustainable biorefineries that could meet technical and market requirements and improve
671
cascading biomass utilization, (Budzianowski Wojciech, 2017). Methodologies for biorefineriy
672
conceptual design and optimization are needed. Approaches need to consider raw materials,
673
technologies, processing routes, products, and technical, economic, and environmental aspects.
674
Processes must be optimized for the specific feedstock used (due to variations on feedstock
675
composition, cost and logistics of process efficiencies and economics), coupled with energy
676
generated from its residue (Mata et al. (2018).
29
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677
The screening of sustainable SCG biorefinery pathways for the production of bio-based
678
products and energy is a complex challenge. Critical tools for predicting the commercialization
679
feasibility of biorefining pathway include laboratory and pilot-scale experimental results,
680
processes modeling, technoeconomic and market analysis. More R&D is needed at higher
681
Technology Reediness level (TRL). Economic and environmental assessment for the practical
682
implementation of a SCG biorefinery at industrial scale is also needed (Zabaniotou et al., 2017).
683
High-value, low-volume bioproducts coupled to bioenergies, with a potential to improve
684
economic viability of biorefineries and biomass resource utilization, are urgently required
685
(Budzianowski Wojciech M., 2017).
686
It is difficult to assess which biorefinery will have a market perspective because detailed
687
economic analysis should be conducted for each. It is suggested that integrated and holistic
688
approaches for bio-waste utilization, as industrial feedstocks, will boost the transition towards
689
the bioeconomy era, the establishment of which would expand and diversify the market outlets
690
of bio-based products (Maina et al., 2017). SCG biorefineries, as many food waste-based
691
biorefineries should be tailored to the local and regional context, and to be profitable and
692
sustainable in the long term.
693
The scale should be analyzed in every biorefinery, during the preliminary design stages.
694
Different factors define the minimal scale for biorefinery’s feasibility. The number and quantity
695
of high added value products usually is associated with low scales (Kachrimanidou et al. 2013).
696 697
4.4
Policy and regulations
698 699
Policy analysis is a new dimension to the sustainability assessment of food waste
700
reduction and valorization. Regulatory framework and policy actions undertaken by local and
30
ACCEPTED MANUSCRIPT
701
global actors are the drivers of change in food-waste reduction and valorization. Today, very
702
different national policies apply to bio-waste management, ranging from small action in some
703
Member States, to ambitious policies, in others. This can lead to increased environmental
704
impacts and can delay the full utilization of advanced bio-waste management methods, while
705
action on national level and community is needed. The lack of a standard universal definition
706
of food waste has impact on the efficient use of by-products for technological and commercial
707
exploitation (Morone et al., 2017).
708
Mathematical mapping methods to assess food consumption impacts and protocols,
709
based on laboratory investigation and demonstration, will formulate pathways for the
710
sustainable valorization of CSG and food waste in general (Morone et al., 2017; Galanakis,
711
2017).
712 713
4.5
Indexes
714 715
A number of indexes related to economy and environmental impacts have estimated by
716
Salazar, 2013. Indexes as a new basic concept need to be applied for understanding the
717
biorefinery efficiency (Moncadam et al., 2016).
718
The effective mass yield (EMY) and the Feature Complexity index (FC) are indexes that
719
have been used in other waste-biorefineries (Zabaniotou et al., 2018). The effective mass yield
720
(EMY) is defined as the percentage of the mass of the desired products relative to the mass of
721
used as feedstock. The Feature Complexity (FC) of the biorefinery has to do with the number
722
of different features: it is increasing by the number of features, by the state of technology of a
723
single feature; it is decreasing with the maturity of the technology (high TRL). This means that
724
a high Technology Readiness Level (TRL) of a feature has lower technical and economic risks
31
ACCEPTED MANUSCRIPT
725
and a lower complexity. This led to the calculation procedure of the Biorefinery Complexity
726
Index that the complexity is directly linked to the number of features and the Technology
727
Readiness Level (TRL) of each single feature involved by the IEA Bioenergy Task 42 ‘Bio
728
refining’ (Jungmeier, 2009).
729
The Feature Complexities (FC) are rating from 1-9 according to the TRL of the process
730
(1-9); TRL1:basic (FC9), TRL2:applied research (FC8), TRL3: critical function or proof of
731
concept established (FC7) , TRL4: lab testing/validation of prototype (FC6), TRL5: prototype
732
system verified (FC5), TRL6: integrated pilot system demonstrate(FC4), TRL7:system
733
incorporated in commercial design (FC3), TRL8: system incorporated in commercial design
734
(FC2), TRL9: system proven and ready for full commercial deployment (FC1) (Jungmeier,
735
2009).
736 737
4.6 Circular Economy
738 739
Food waste prevention is an integral part of the new Circular Economy Package, with
740
benefits such as boost of the global competitiveness, sustainable growth and, generationof new
741
jobs. One of the issues of the Circular Economy model is the collection of SCG and the scale
742
of the endeavor.
743
The circularity of a coffee micro-economy naturally brings up questions related to
744
scalability. The collection of coffee grounds requires storage space, proximity among
745
participating buyers, proximity to additional production facilities in which spent grounds will
746
be used, and numerous other logistical concerns.
747
Due to their high organic matter, SCG sometimes are used as a fertilizer. However, SCG
748
are highly toxic to the plants due to the presence of caffeine, tannins, and polyphenols. In
32
ACCEPTED MANUSCRIPT
749
addition, due to the presence of organic matter in SCG, a huge quantity of oxygen is required
750
for their degradation in landfills. Simultaneously, methane, which is a greenhouse gas and even
751
more harmful than carbon dioxide, is also released in the landfills, contributing to global
752
warming. Therefore, usual disposal methods need to be replaced by more sustainable towards
753
increased resources recovery and higher energy efficiency. Valorization of this waste towards
754
material and energy recovery rather than disposal, is gaining interest.
755
Although, the biorefinery concept is considered as one of the research cornerstones in
756
the last years and as the best option to transform the different waste systems by a multi-process,
757
multi-product pathways, (Moncada et al., 2016), there is a shortage of analysis of the potential
758
benefits on associated business development.
759
Only one paper found in the book of Morone et al., (2017), to assess the logistics,
760
economical and social feasibility to isolate SCG from the catering industry and use them as raw
761
material for a novel process to produce alternative high added value products in a near-perfect
762
circular economy cycle, making use of reverse logistics and generating near-zero waste (Topi
763
and Bilinska, 2017). The study was based on a series of theoretical scenarios corresponding to
764
the different possible logistic and process options that stakeholders could identify. This
765
theoretical approach concluded that the process is technically feasible with available technology
766
within current infrastructure and modest investments and the economic case is very attractive
767
to investors.
768
Some international companies of coffee beverage have started to devote efforts on
769
sustainable valorization of SCG, advocating Circular Economy model, by organizing collection
770
systems and exploring technological pathways for valorization (Bernstein, 2012).
771
Alternative scenarios for using the SCG to produce alternative high added value products
772
should be considered and developed, by using the participatory mapping approach and
33
ACCEPTED MANUSCRIPT
773
economic, environmental and social benefits compared with compost production scenarios
774
(Morone et al, 2018).
775 776
4.7
Design guidelines
777 778
Reflections and guidelines for sustainable biorefinery concepts are cited, that are mainly
779
proposed by Moncada et al., 2016. These include the following quotes that could be also
780
suggested for a SCG biorefinery design
781
Integration increases the efficiency.
782
Integrated technologies should have priority over separated technologies.
783
Integration of cogeneration CHP using biorefinery solid residues is suggested.
784
Energy integrations levels are important to reach maximum energy efficiency levels.
785
Include as many as possible products in the biorefinery.
786
Cascade approaches are sustainable pathways in the circular economy.
787
Innovative engineering solutions should be preferred.
788
The CHP gasification technology has a better performance than the technology based on direct combustion.
789 790
technical, and economic impacts is very important.
791 792
The use of modern tools and strategies of analysis and evaluation for environmental,
The implementation of optimization strategies and models could be interesting when coupling with further design.
793 794
Supply chain and logistics are essential part of a green biorefinery.
795
Use of indexes is important.
34
ACCEPTED MANUSCRIPT
796
In addition to the above, conducting sensitivity analyses for comparison of different
797
systems, enables global evaluation and rating of those systems on the same scale of normalized
798
values. LCA can be used as a valid tool to support designers, decision-makers towards
799
promotion of more sustainable options of SCG valorization for energy, fuel, biochar and high
800
added value/ low volume products (Ingrao et al., 2018b), although weighing is based often upon
801
social or political considerations (De Benedetto and Klemes, 2009). Therefore, weighing step
802
could be recognized as mandatory by the subject International Standards (Ingrao et al.,2018b,
803
2018a).
804 805
5.
Conclusions
806 807
This study was conducted to review the field of SCG valorization, its prospects,
808
potentialities and challenges. The review attained the proposed goal, as it brought important
809
issues of the SCG mono- and biorefinery valorization options, as reported in the international
810
literature. Limitations were found in extrapolating information and results from the papers
811
reviewed, since each paper explored different end-products and processes, so evaluations and
812
comparisons were difficult to be made.
813
The review allowed the authors to deepen the knowledge in the SCG biorefinery, that
814
represents the platform to start the development at higher TRL, for further integration and
815
optimization of SCG recycling systems. It allowed the development of knowledge-based
816
strategies to unlock the potential of SCG to produce bio-derived chemicals, fuels and carbon
817
materials, and probably effecting waste management regulations.
818
The review focus was centered upon SCG, which an important food waste of global
819
society, containing substances that make it a valuable bio-resource. Today, food wastes are a
35
ACCEPTED MANUSCRIPT
820
major concern and require for their management advanced techniques with economic benefits
821
and environmental safety. It was made evident, that the development of sustainable and efficient
822
refining of SCG depends on the spectrum of various end-products, market outlets and cost-
823
effective processing schemes. It was documented that polysaccharides, phenolic compounds,
824
tannins, biodiesel, bioethanol can be produced from the SCG, by using mono-extraction
825
processes, while a cascade biorefinery can produce fuels, energy, carbon materials and biochar,
826
in a closing loop approach, in addition.
827
SCG cascade or integrated biorefinery seems to be a more economically viable and
828
resource efficient option, compared to strategies relying on complete biomass disintegration,
829
that do not lead to optimal utilization of biomass feedstock. Cascading approaches are favorable
830
because they maximize biomass recycling with material and energy recovery.
831
contributing to more sustainable and circular economies, SCG cascade biorefinery seems to
832
have a commercial value.
Besides,
833
A shortage of economic and environmental assessments was observed in the international
834
literature, so it was difficult to draw concise economic and environmental conclusions for the
835
SCG biorefinery options, with the exception of a study performed by Brazilian researchers
836
(Mussatto et al., 2013), on integrated biorefinery of the Brazilian case of spent grains (BSG)
837
for the production of xylitol, lactic acid, activated carbon and phenolic acids, integrated with
838
heat production. This study provided good economic and environmental indicators, with an
839
economic margin of 62.25%, potential environmental impact of 0.012 PEI/kg products, and the
840
carbon footprint of the processing stage represented 0.96 kg CO2-e/kg of BSG.
841
Considering findings from this review, it seems that efforts are required to sustainability
842
assessment, policy analysis and national regulatory framework harmonization to the EU, that
843
are the drivers of change in food-waste reduction and valorization. Different national policies
36
ACCEPTED MANUSCRIPT
844
apply to bio-waste management, can delay the utilization of advanced bio-waste valorization
845
methods. There is an urgent need for creation of technical standards and indicators to guide
846
and regulate the assessment of the sustainability of SCGs-based biorefineries. LCA proposed
847
as a valid tool to support designers, and decision-makers towards promoting and developing
848
sustainable solutions.
849
Only one paper found to assess theoretically SCG collection and valorization in a circular
850
economy (Topi and Bilinska, 2017), concluding that collection and valorization of SCG is
851
technically feasible with available technology, within current infrastructure, and modest
852
investments. In this case, the economic case is very attractive to investors.
853
Based upon the analysis of the papers, the authors found that there is an urgent need for
854
R&D, effective regulations, methodological approaches to design and estimation of the SCG
855
collection systems, investment and manufacturing costs, indicators development for
856
assessments prior to the development of new business models within a Circular Bieconomy.
857
Through the review some guidelines were highlighted, useful for the design of biorefineries.
858 859 860 861
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Fig. 1. Published articles concerning SCG, per year of publication. Fig. 2. Published articles on single-extraction process products, per year of publication. Fig. 3. High added value products derived from a single-extraction process. Fig. 4
SCG thermochemical conversion processes and their products.
Fig. 5. SCG biorefineries reported in literature.
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Fig. 5
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TABLES
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Table 1. Screening procedure of peer review articles Number of articles in Screening Step
Sample
1.
First sample
630
2.
Sample after duplicates removal
374
3.
Sample after less relevant articles removal
333
4.
Remaining sample after cut-off point
320
5.
Final sample
6.
Books
7. Internet sources Final Sample
92 3 7 102
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Table 2. SCG biochemical composition (wt%db) Lipids
Carbohydrates
Proteins
(wt%db) n.d.
(wt%db) 14.1
(wt%db) 14.4
Somnuk et al., (2017)
9-16
45-47
13-17
Burniol-Figols et al., (2016)
13±0.04
65.9±6.5
4.9±0.6
Passos and Coimbra., (2013)
13.7±0.1
54.1±2.2
13.8±0.1
Wang et al., (2016)
45.3
13.6
Campos-Vega et al., (2015)
References
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Table 3. Proximate and ultimate analysis of SCG Ultimate Analysis (ww%db) C
H
N
S
O
References
53.0
6.8
2.1
0.1
38.1
Somnuk et al., (2017)
52.5±0.4
7.0±0.03
3.46±0.01 0.10±0.00 34.8±0.1
Tsai et al., (2012)
Proximate Analysis (ww%db) Moisture
Volatile
Ash
Fixed Carbon
-
-
1.6
-
Somnuk et al., (2017)
11.5±0.4
79.5±0.01
0.7±0.2
8.2
Tsai et al., (2012)
Table 4. Single-extraction SGC processes for high added value compounds reported in the literature SCGs single-extraction process Reference
Somnuk et al. (2017)
Process Parameters
Process
Oil extraction
Extraction characteristics (Solvent Extr) Hexane Ethanol anhydr. Ethanol hydrous Methanol
Phimsen (Soxhlet extr.) et al. Hexane (2017) Mussato Sulfuric acid et al. (2011) Polysacharides Ballesteros extraction (Autohydrolysis) et al. Water (2017) (Microwave Passos superheated et al. water extraction) (2013) Water (Subcritical Xu et al. Phenolics water extraction) (2015) extraction Water Shang et Phenolics and (Pressurized
Oil Polysacharides SCG/Solvent Extraction Temperature (w/w% (w/w% d.b.) ratio (g/g) time (min) d.b.) (°C) 22.5 30.4 30 14.7 22.8 33.5 30 13.1 20.3 25.5 30 11.8 23.8
19.6
400g/3l
480
0.5M
45
1g/15ml
30
Product Phenolics (w/w% d.b.) -
Tannins Caffeine (mg/g (mg/g d.b.) d.b.) -
7.5
-
-
-
-
13
-
-
-
-
121
-
45.3
-
-
-
10
160
-
29.29
234.1407 mg GAE/g
-
-
1gSCG/10ml
2
200
-
55
-
-
-
14.1g/l
38-55
160-180
-
-
86.2 mg GAE/g
-
-
195
-
-
19-26
-
3-9
al. (2017)
Caffeine extraction
liquid extraction) Water
mgGAE/gdb
Ethanol Low et al. (2015) Brazinha et al. (2015)
Tannins’ Extraction
Sodium hydroxide 5wt%
Caffeine extraction
Membrane technology
8.2
30
mg/g db
195
-
-
-
-
-
100
-
-
-
21.02
-
-
-
-
-
3-9
Table 5. SCG biorefinery approaches reported in literature. No Reference Processes a. Phenols extraction 1 BurniolFigols et al. (2016) b. Acid hydrolysis
2
3
4
c. Ethanol Fermentation Obruca et al. a. Oil extraction (2015) b. Polyphenols extraction c. Bacterial Fermentation Caetano et a. 1st SCG Extraction al. (2017) b. 2nd SCG Extraction c. Transesterification d. Drying & Pelleting e. Pyrolysis and Torrefaction f. Hydrolysis & Fermentation Vardon et a. Oil extraction al. (2013) b. Transesterification c. Slow pyrolysis *L/S ratio= Liquid to Solid ratio
Parameters Solvent: Ethanol Temperature: 70°C L/S*: 25ml solvent/g TS liquidratioH2SO4 1solid w/w% Temperature: 140°C Time: 45 min L/S*: 10g liquid/g TS Strain: Saccharomyces cerevisiae Solvent: n-hexane Solvent: Ethanol Bacteria: Burkholderia cepacia Solvent: water/ethanol/supercritical CO
Products Chlorogenic acid
Bioethanol Oil Polyphenols PHAs High value extracts: antioxidants, caffeine, tannins, polyphenols, etc. Triglycerides Biodiesel, Glycerin, Hydrogen Pellets Biochar, biooil Ethanol Oil Biodiesel Biochar, biooil