Bioconversion of biomass waste into high value chemicals

Journal Pre-proofs Review Bioconversion of biomass waste into high value chemicals Eun Jin Cho, Ly Thi Phi Trinh, Younho Song, Yoon Gyo Lee, Hyeun-Jong Bae PII: DOI: Reference:

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Bioresource Technology

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Please cite this article as: Cho, E.J., Trinh, L.T.P., Song, Y., Lee, Y.G., Bae, H-J., Bioconversion of biomass waste into high value chemicals, Bioresource Technology (2019), doi: https://doi.org/10.1016/j.biortech.2019.122386

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Bioconversion of biomass waste into high value chemicals

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Eun Jin Choa, Ly Thi Phi Trinha,b, Younho Songa, Yoon Gyo Leec, and Hyeun-Jong Baea,c,*

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a

Bio-energy Research center, Chonnam National University, Gwangju 500-757, Republic of Korea

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b

Research Institute for Biotechnology and Environment, Nong Lam University, Hochiminh City,

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Vietnam

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c

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757, Republic of Korea

Department of Bioenergy Science and Technology, Chonnam National University, Gwangju 500-

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* Corresponding author

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Hyeun-Jong Bae. Address: Department of Bioenergy Science and Technology, Chonnam National

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University, Gwangju 61186, Republic of Korea; Tel.: +82 62 530 2097; fax: +82 62 530 0029. E-

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mail address: [email protected]

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Abstract

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Dwindling petroleum resources and increasing environmental concerns have stimulated the

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production of platform chemicals via biochemical processes through the use of renewable carbon

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sources. Various types of biomass wastes, which are biodegradable and vastly underutilized, are

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generated worldwide in huge quantities. They contain diverse chemical constituents, which may

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serve as starting points for the manufacture of a wide range of valuable bio-derived platform

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chemicals, intermediates, or end products via different conversion pathways. The valorization of

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inexpensive, abundantly available, and renewable biomass waste could provide significant benefits

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in response to increasing fossil fuel demands and manufacturing costs, as well as emerging

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environmental concerns. This review explores the potential for the use of available biomass waste

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to produce important chemicals, such as monosaccharides, oligosaccharides, biofuels, bioactive

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molecules, nanocellulose, and lignin, with a focus on commercially viable technologies.

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1. Introduction

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Environmental pollution is one of the largest problems facing humanity today. A fundamental

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pollution-related problem is the disposal of the large quantities of wastes that are continually being

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produced (Muralikrishna and Manickarm, 2017). Approximately 100 billion metric tons of biomass

39

waste are generated annually in the world (TerraGreen, 2019). Biomass waste encompasses a wide

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range of materials that include forestry residues, agricultural wastes, fruit processing waste, and

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waste from the processing of other food. These types of waste can cause serious health or

42

environmental problems if they are not disposed of properly (Alatzas et al., 2019). Therefore,

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developing an ecofriendly and effective strategy for using and managing various types of biomass

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waste is critical. Enzymatic conversion is considered an environment-friendly technology that may

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potentially help to completely replace or reduce the usage of hazardous chemicals in industrial

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processes. Enzymatic conversion offers the potential for higher yields, higher selectivity, and

47

lower energy costs, and generates fewer inhibitory byproducts. Furthermore, enzymes are critically

48

important to the decomposition of biomass into its primary constituents, and can be applied to the

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downstream transformation of biomass components into building blocks or commodity chemicals.

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Biomass waste is currently seen as a low-value material and is largely underutilized.

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However, its role as a resource useful for creating value-added outcomes has become increasingly

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recognized (Koutinas et al., 2014). Biomass waste contains higher fraction of oxygen and lower

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percentage of hydrogen and carbon compared with petroleum resources, and biomass biorefineries

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can likely produce more families of chemicals than petroleum-based manufacturing can (Isikgor

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and Becer, 2015). Efficient development of biomass waste into innovative products can address

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environmental concerns, reduce dependence on petroleum resources, and increase economic

57

efficiency.

58

Today, the sustainable production of chemicals and biopolymers depends entirely on

3

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renewable carbon; therefore, biomass waste has been introduced as a resource dedicated to creating

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value-added products. Valorization of biomass waste is based on the use of chemical constituents

61

such as carbohydrate and non-carbohydrate fractions of biomass to produce commercially viable

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products (Fig. 1). It has also been developed in response to increasing raw material demand,

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production cost, and environmental pollution. This review presents recent developments and future

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trends in the valorization of biomass waste derived from agricultural, forestry, and industrial

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activities to produce important monosaccharides, oligosaccharides, biofuels, bioactive molecules,

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nanocellulose, and lignin. Recently developed technologies associated with the valorization of

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biomass waste are also discussed.

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2. Biomass Waste

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2.1. Environmental impacts of biomass waste

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Large quantities of biomass waste are produced annually worldwide (Perea-Moreno, 2019).

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In the past, they were either burned or naturally converted into organic fertilizers under favorable

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conditions. However, biomass waste has become an increasing concern in recent years because of

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its potential to cause significant environmental problems (Zhang et al., 2012). For example, the

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burning of agricultural waste is a common practice in undeveloped countries despite the

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atmospheric pollution it causes (Sabiiti, 2011). The burning of biomass waste releases pollutants

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into the atmosphere, such as carbon monoxide, nitrous oxide, nitrogen dioxide, and particles. These

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pollutants are accompanied by the formation of ozone and nitric acid, contributing to acid

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deposition, which poses a risk to human and ecological health (Sabiiti, 2011). In addition, fruit and

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vegetable waste are mainly generated during the production and storage stages because of

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overproduction caused by climate change, insufficient skills, natural calamities and a lack of proper

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infrastructure (Wunderlich and Martinez, 2018). A large proportion of these types of wastes is

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dumped into landfills, leading to methane and carbon dioxide emissions, surface water 4

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contamination, ground water contamination, odor, and soil contamination(Singh et al., 2018).

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Methane emitted from landfills is one of the most important contributors to greenhouse gases

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(GHGs) because of its global warming potential. Approximately 60% of the global methane

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emissions come from agriculture, landfills, wastewater, and the production and transport of fossil

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fuels (Singh et al., 2018; Van Dingenen et al., 2018). Furthermore, leachate from landfill contains

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high concentrations of NH3-N, organics, heavy metals, and some hazardous substances. Thus, it has

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a dual influence on plant growth. The use of leachate in irrigation changes soil characteristics, such

91

as salinity and biotoxicity (Youcai, 2018). Thus, biomass waste could cause serious pollution

92

problems and environmental impacts associated with its accumulation, degradation, and treatment.

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2.2. Potential of biomass waste

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The amount of biomass waste generated from agricultural and forestry activities, food

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processing, and other sectors of industry are increasing as a result of the growing population and

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expanding industrialization. The most abundant biomass waste from agriculture emanate from

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sources such as rice straw, wheat straw, corn straw, sugarcane bagasse, and rice husk, generating

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731, 354, 204, 181, and 110 million tons (Mt), respectively, each year (Sarkar et al., 2012). These

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volumes of biomass waste are usually disposed of. Forestry residues produced from harvesting and

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product processing are estimated at approximately 72.5 Mt in the United States and Canada alone

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(Koutinas et al., 2014). The food industry is also responsible for a large amount of biomass waste,

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including 35 Mt of rapeseed meal, 15.6 Mt of citrus waste, 9 Mt of banana waste, 5–9 Mt of grape

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pomace, and 3–4.2 Mt of apple pomace generated worldwide each year (Djilas et al., 2009; Padam

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et al., 2014). Waste produced from the olive oil industry has caused significant environmental

106

concerns, particularly in areas of the Mediterranean where approximately 30 Mt of residues are

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produced per year (Caputo et al, 2003). The coffee agro-industry contributes 7.4 Mt of spent coffee

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grounds and an additional large amount of coffee pulp, cherry husk, and silver skin, which can 5

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damage the environment because of the degradable properties of organic molecules (Kondamudi et

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al., 2008). Therefore, large amount of biomass waste can be utilized to solve disposal problems

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effectively.

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Biomass waste is very diverse in terms of chemical composition and has demonstrated

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significant potential in the development of economical biorefineries. Agricultural and forestry

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residues have high proportions of cellulose and hemicellulose (Table 1), which are useful primarily

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for producing fermentable sugars and biofuels such as ethanol and butanol. Biomass waste from

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food processing, such as processing of carrots and apple/pear pomaces, also has high cellulose and

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hemicellulose content; thus, this type of waste is ideal for sugar conversion without the need for

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complex pretreatment (Nawirska et al., 2005). Spent coffee grounds contain a high fraction of

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hemicellulose (30.1%) with mannan as the major polysaccharide in the residue (19.3 %); this makes

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spent grounds an excellent source for the production of valuable mannose and manno-

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oligosaccharides (Nguyen et al., 2017a). Biomass waste sources that are rich in xylan include corn

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cobs, rice straw, corn stover, sugarcane bagasse, wheat straw, and switchgrass, which have potential

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as a source for xylo-oligosaccharides production. Pectic oligosaccharides, which have emerged as a

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new class of prebiotics, can be obtained from pectin-containing agricultural residues, such as citrus

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waste (30%), apple pulp (20.9%), sugar beet pulp (16.2%), olive pomace (34.4%), potato pulp

126

(15%), soy hull (16.3%), and onion skin (27–34%) (Babbar et al., 2016). Flavonoids, carotenoids,

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phenolic acids, and their derivatives are major bioactive compounds found in fruit and vegetable

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solid

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antimicrobial, and antithrombotic activities with potential for uses in the pharmaceutical, cosmetics,

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and nutraceutical industries (Tournour et al., 2015). The abundant availability of fiber-based waste

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materials and their intrinsic properties have prompted new research on economical nanocellulose

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manufacturing (Garcia et al., 2016). As the second most abundant natural polymer on Earth, lignin

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is isolated mainly from wood wastes and agricultural residues and is used in a broad range of

waste,

exhibiting

antioxidant,

antiallergenic,

anti-arthrogenic,

anti-inflammatory,

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classical chemical applications and innovative future platforms. Effective lignin valorization

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would yield at least 10 times the value of simply burning it to the produce steam or electricity (Wu

136

et al., 2017). Thus, the conversion of biomass waste into energy, chemicals, or polymers for use in

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daily operations will offer not only economic efficiency but also environmental benefits.

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2.3. Key factors in bioconversion of biomass wastes

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Acids such as H2SO4 and HCl are commonly used in the hydrolysis of biomass to manufacture

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sugars. The advantage of acid hydrolysis is a high sugar recovery efficiency, which can be on the

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order of over 90% for both hemicellulose and cellulose sugars. However, the severity of acid

143

hydrolysis is such that toxic degradation products are produced that can interfere with fermentation.

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After the hydrolysis process, large quantities of lime must be used to neutralize the acid in the sugar

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solution. This neutralization forms large quantities of calcium sulfate, which requires disposal and

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creates additional expense. Furthermore, the concentrated acid hydrolysis can lead to equipment

147

corrosion due to high acid consumption. Therefore, the process requires either expensive alloys of

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specialized non-metallic construction, such as ceramic or carbon-brick lining, adding to the

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processing cost and, thus, the cost of the end product.

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There is a growing need for more environmentally acceptable processes in industry. Thus, there

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is a paradigm shift from traditional concepts of chemical-based production and manufacturing to

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bio-based, eco-benign approaches that are equally efficient and economical (Arends et al., 2007).

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Enzyme technology is a promising means of moving toward cleaner industrial production over

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conventional chemical processes. The frequent use of enzymes results in many benefits that cannot

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be obtained with traditional chemical treatment. Enzyme-based technologies are considered more

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desirable for reducing the possible contamination of the end-product with toxic substances. They

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can efficiently utilize raw materials, minimize production cost, and reduce impacts to the

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environment (Bano et al., 2017). In the case of the starch industry, the acid hydrolysis process had

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experienced widespread use in the past. However, the process has now largely been replaced by

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enzymatic processes because it necessitated the use of corrosion resistant materials, resulted in high

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color and salt ash content (after neutralization), required more energy for heating and was relatively

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difficult to control (Betiku et al., 2013). Dwivedi et al. recently reviewed the economics of ethanol

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production from cellulose using different conversion technologies (Dwivedi et al., 2009). The

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economics of several hydrolysis-based conversion technologies show that the cost is highest for the

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process of concentrated acid hydrolysis, neutralization and fermentation ($2.28/gal) and lowest for

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the processes of simultaneous saccharification and fermentation ($1.48/gal).

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Enzymes are critically important for the decomposition of biomass into its primary constituents

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and are applied in the downstream transformation of biomass components into building blocks or

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commodity chemicals (Telekey and Vodnar, 2019). Hydrolytic enzymes, such as cellulase and

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hemicellulase, are known to decompose complex polysaccharides in cell walls to produce soluble

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and simple sugars. Ligninolytic enzymes, including manganese peroxidase, lignin peroxidase,

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versatile peroxidase, and laccase, synergistically and efficiently degrade lignin, enabling the

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complete deconstruction of lignocellulosic biomass (Gupta et al., 2016). Endoxylanase is preferably

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used to depolymerize xylan to xylo-oligosaccharides. Furthermore, endopolygalacturonase

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efficiently degrades onion skin pectin to release a high yield of pectic oligosaccharides (Babbar et

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al., 2016). Lytic polysaccharide monooxygenases (LPMOs) offer potential improvement for

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biomass transformation because they can react with a wide range of polysaccharides including

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cellulose, starch, xyloglucan, cellodextrins, and glucomannan (Hemsworth et al., 2015). In addition

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to cellulase, hemicellulase, and ligninase, other enzymes such as α-amylase, β-glucosidase, tannin

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acyl hydrolase, and ellagitannin acyl hydrolase can also degrade polysaccharides and lignin,

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enhancing the release of phenolic molecules bound to the cell wall matrix. Enzymatic hydrolysis of

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rice bran significantly increases the total phenolic and flavonoid content by 46.24% and 79.13%, 8

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respectively, compared with non-enzyme treatment such as gelatinization and liquefaction (Liu et

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al., 2017). The use of a commercial enzyme cocktail containing polygalacturonase, pectin lyase,

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methylesterase, cellulase, and hemicellulose can significantly improve the extraction of valuable

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polyphenolic compounds, yielding 21.4 g of flavan-3-ols and 227 mg of gallic acid from 1 kg of

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grape seeds (Stambuk et al., 2016). The co-production of bioactive compounds and sugars via one-

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step enzymatic hydrolysis has been presented in recent studies. Treatment of onion peel waste with

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cellulase, pectinase, and xylanase yields 98.5% bio-sugars and simultaneously releases quercetin

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with a 1.6-fold increase (Choi et al., 2015a). However, the action mechanism and synergistic

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interaction of enzymes during the extraction process of bioactive compounds require extensive

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investigation. Such enzymes are produced via solid-state fermentation with microorganisms, and

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the use of renewable and low-cost biomass waste, replacing traditional carbon sources can provide a

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cost-effective and green solution for producing economic and efficient enzymes. Using on-site

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cellulase production demonstrated the potential for hydrolysis and ethanol production from corn

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stover and sorghum stover, with an efficiency of greater than 80% (Idris et al., 2017; Zhao et al.,

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2018).

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3. Bioproducts from the Current Valorization of Biomass Wastes

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3.1. Monosaccharides

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To convert biomass-based materials into biofuels, platform chemicals, or biopolymers, the

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biomass needs to be deconstructed into its constituent sugars. The sugars can be used as carbon

203

sources for fermentation or as raw materials for further transformation into building blocks. The

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United States Department of Energy (US DOE) has identified 12 sugar-derived building blocks,

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including 1,4-diacids (succinic, fumaric and malic), 2,5-furan dicarboxylic acid, 3-hydroxy

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propionic acid, aspartic acid, glucaric acid, glutamic acid, itaconic acid, levulinic acid, 3-

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hydroxybutyrolactone, glycerol, sorbitol, and xylitol/arabinitol, all of which can be converted into

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new chemical classes and potentially replace commonly used petroleum-based materials to produce

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important commodities. Biomass wastes contain a complex mixture of polymers from plant cell

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walls known as cellulose, hemicellulose, and lignin. Generally, cellulose and hemicellulose are

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hydrolyzed to monomeric sugars via acid hydrolysis or enzymatic hydrolysis (Wijaya et al., 2014).

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Enzymatic hydrolysis is the preferred process because it offers the potential for higher conversion

213

yields, higher selectivity, and milder operating conditions; it also generates fewer inhibitory

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byproducts, and does not involve as much corrosion as does acid hydrolysis (Prado et al., 2016). A

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pretreatment step is necessary to reduce the recalcitrance of the biomass, improving the conversion

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yield of hydrolysis. Recent advances in pretreatment technologies have been critically reviewed

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(Rastogi and Shrivastava, 2017). The production of lignocellulosic sugars via pretreatment-assisted

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enzymatic hydrolysis has attracted considerable interest at the laboratory and pilot scale. The

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American Process company has developed a technology using sulfur dioxide and ethanol to

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fractionate and separate biomass components; cellulose is then converted to glucose by enzymatic

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hydrolysis, while a hemicellulose sugar stream is obtained through autohydrolysis. Comet

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Biorefining has announced the construction of a sugar plant slated to begin operation in 2018. This

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sugar plant is capable of producing 60 million pounds of sugar per year, mostly from wheat straw

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and corn stover, by a low-cost process using a new pretreatment method and minimized enzyme

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loading (Comet Biorefining, 2017). Virdia established a cold acid solvent extraction (CASE)

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process that converts wood chips and other non-food, cellulosic biomass into industrial sugars, with

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a yield of 95% – 97%, at a cost competitive with the cost of obtaining sugar from corn (Lane, 2014).

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Technologies that do not use enzymes or acids to produce economically viable sugars have attracted

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investment (Table 2). One example is supercritical hydrolysis, an innovative technology that uses

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supercritical water as a solvent to cleave ether and ester bonds in biomass, producing simpler sugars

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(Wijaya et al., 2014). Renmatix has commercialized a supercritical technology with an expected 10

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annual production capacity of 1 Mt of cellulosic sugar from locally available agricultural residues,

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energy grasses, and woody plants. Subcritical and supercritical water are commonly used to

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fractionate hemicellulose and cellulose into biomass, resulting in separate streams of sugar. Hot-

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compressed water treatment of sugarcane bagasse at 180 oC for 30 min and 1 MPa can extract 85%

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of xylose, based on the initial hemicellulose amount, and the solid fraction is a cellulose-rich stream

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that can be applied further (Sukhbaatar et al., 2014). An emerging mechanical-chemical and dry

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process patented by AMG Energy Group, can produce commercially viable cellulosic sugars from

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agricultural and yard waste, and may offer an efficient alternative to current acid- or enzymatic-

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based processes (AMG Energy Group, 2017).

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In biomass-to-sugar technology, refining sugar products is a crucial step because

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pretreatment and saccharification processes release a significant amount of inhibitors, such as furan

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derivatives (furfural, 5-hydroxymethylfurfural), phenolic compounds (vanillin, phenols, and p-

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hydroxybenzoic acid), and carboxylic acids (acetic, formic, and levulinic acid) (He et al., 2012).

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Membrane technology is considered as a sustainable and flexible process with low energy

246

consumption. It has drawn considerable interest for its unique ability to separate and purify

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intermediate or product streams. Nanofiltration and reverse osmosis can separate C5 and C6 sugars

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from acetic acid, furfural, 5-hydroxymethyl furfural, and vanillin, with high levels of sugar

249

rejection (Nguyen et al., 2015; Wang et al., 2017). Furthermore, membrane processes can

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effectively concentrate the sugar stream, leading to increased downstream product concentrations

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and subsequently reduced energy consumption for the recovery of final products (Qi et al., 2012).

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Forward osmosis has shown immense potential in the enrichment of sugars obtained from rice straw

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(Shibuya et al., 2017). The use of nanofiltration and reverse osmosis to concentrate sugars has

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achieved good results, in which the capability to remove acetic acid, as well as furfural and

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hydroxymethyl furfural, from lignocellulosic hydrolysate has been demonstrated (Gautam et al.,

256

2014). A combination of ultrafiltration and nanofiltration has revealed the feasibility of 11

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concentrating glucose and recycling the cellulase in wheat straw hydrolysate. The recovery of

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hydrolytic enzymes is an important step in improving the economic viability of the production of

259

sugar and its further transformation (Qi at al., 2012). Using membrane technology for purification

260

and concentration in the manufacture of sugars, Toray industries and Mitsui Sugar have established

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a demonstration plant to produce approximately 1400 tons of high quality cellulosic sugar per year

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from surplus bagasse (Nikkei Asian Review, 2017).

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3.2.

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Interest in the production of oligosaccharides from lignocellulosic residues has increased

266

recently because biomass-derived oligomers have been regarded as potential prebiotics with the

267

same desirable properties and functions as common oligosaccharides. Prebiotic oligosaccharides are

268

short-chain carbohydrates that are not degraded by the enzymes of the human digestive tract. These

269

oligosaccharides arrive intact to the lower gut to be selectively fermented by probiotic bacteria,

270

because of its higher stability at low pH and high temperature. They are also non- carcinogenic,

271

stimulate bacterial growth and fermentation, and improve intestinal mineral absorption. In addition,

272

they possess antioxidant, antiallergenic, antimicrobial, immunomodulatory and selective cytotoxic

273

activity, as well as blood and skin health related effects (Fig. 2) (Singh, et al., 2015).

274

Oligosaccharides

Currently commercialized prebiotic oligosaccharides include fructooligosaccharides (FOS),

275

galactooligosaccharides

(GOS),

lactosucrose,

xylooligosaccharides

(XOS),

276

isomaltooligosaccharides (IMO), and soybean oligosaccharides (SOS). Oligosaccharides are

277

produced during the hydrolysis of heterogeneous hemicelluloses, resulting in various

278

oligosaccharides such as XOS, mannooligosaccharide (MOS), and arabinooligosaccharide (AOS).

279

Table 3 lists the production of oligosaccharides from various waste sources via different processes.

280

An oligosaccharide that has garnered immense interest is XOS because of the abundance of xylan

12

281

in biomass waste sources. MOS provides various health-promoting effects in both humans and

282

livestock, but the production of MOS from mannan-rich agro-wastes has just started (Yamabhai et

283

al., 2016). Oligosaccharides can be prepared using direct autohydrolysis, acid hydrolysis, and

284

thermal or chemical pretreatment followed by enzymatic hydrolysis of the hemicellulose-rich

285

fraction (Carvalho et al., 2013). The most common types of autohydrolysis method include

286

steaming or steam explosion, in which lignocellulosic material is heated in an aqueous medium,

287

leading to hydrolytic depolymerization of hemicellulose. Mild acid pretreatment prior to

288

autohydrolysis may solubilize lignin and expose hemicellulose, resulting in enhanced XOS yield

289

from bagasse (92.3%) and switchgrass (84.2%) based on the initial xylan content (Otieno and

290

Ahring, 2012). In a recent study, high oligosaccharide yields were obtained by autohydrolysis of

291

olive stone byproducts at 190 oC for 5 min (14.7 kg/100 kg biomass) (Cuevas et al., 2015). Acid

292

hydrolysis has been used to produce XOSs from wheat straw, tobacco stalks, cotton stalks, and

293

sunflower stalks (Akpinar et al., 2009). However, acid hydrolysis can produce a high level of

294

monosaccharides and generate undesirable byproducts, resulting in a decreased XOS yield and a

295

purification step that is more complex and expensive (Akpinar et al., 2010). The enzymatic

296

conversion of xylan into XOS is the favorable route in nutraceutical and pharmaceutical industries

297

because it does not generate toxic or undesirable products (Carvalho et al., 2013). To produce XOS,

298

the enzyme mixture should have low -xylosidase activity to inhibit xylose formation.

299

Arabinoxylooligosaccharides of 71% purity have been produced through the pretreatment of

300

amylase and protease followed by endo-1,4-β-xylanases hydrolysis of wheat bran (Mathew et al.,

301

2017). Mild alkali pretreatment efficiently exposes xylan in corncobs for endoxylanase action,

302

resulting in 81% XOS yield (Aachary and Prapulla, 2009). Pectic oligosaccharide (POS), a newly

303

important prebiotic, known for its prevention and treatment of various chronic diseases, can be

304

obtained by the depolymerization of pectin-containing biomass waste. POS derived from orange

305

peel waste treated by crude enzymes from Aspergillus japonicus showed comparable prebiotic 13

306

properties to those of FOS and showed antimicrobial activities comparable with those of

307

Staphylococcus aureus, Bacillus subtilis, and Escherichia coli (Li et al., 2016). Controlled

308

enzymatic hydrolysis of onion skins using endopolygalacturonase was found to convert 55.8% of

309

crude pectin into DP2 and DP3 at an enzyme dose of 52 IU/ml for 120 min, and 26.0% of DP4 was

310

released at an enzyme loading of 5.2 IU/ml for 15–30 min (Babbar et al., 2016).

311 312

3.3. Biofuels

313

Biofuels are alternative fuels made from renewable biological sources such as plant and

314

plant-derived resources. Biofuels are commonly advocated as a cost-effective and environmentally

315

benign alternative to petroleum and other fossil fuels, particularly within the context of rising

316

petroleum prices and increased concern over the contributions made by fossil fuels to global

317

warming. Many studies have obtained high ethanol yields from a variety of waste sources, such as

318

agricultural residues, wood wastes, and wastes from the food industry and other manufactures. The

319

effective use of biomass wastes has provided significant economic and social benefits, while also

320

reducing negative environmental effects.

321

Bioethanol, a downstream product of sugar manufacturing, is a common and important

322

liquid fuel. In addition to its role as an alternative to fossil fuels, ethanol serve as an important

323

platform chemical for producing ethylene, ethylene glycol, and their derivative polymers such as

324

polyethylene and polyethylene terephthalate (Koutinas et al., 2014). Global ethanol production is

325

expected to increase from approximately 114 billion liters in 2014 to nearly 134.5 billion liters by

326

2024 (OECD/FAO, 2015). The transition from using edible feedstocks to non-food sources for

327

biofuel is a significant step toward reducing production cost and avoiding competition with global

328

food and feed supplies (Gupta and Verma, 2015). Bioethanol is produced through the microbial

329

fermentation of monosaccharides. Production processes for bioethanol include separate hydrolysis

14

330

and fermentation (SHF), simultaneous saccharification and fermentation (SSF), simultaneous

331

saccharification

332

Saccharomyces cerevisiae and Zymomonas mobilis can efficiently ferment glucose into bioethanol

333

but cannot consume xylose. Natural yeasts such as Pichia stipitis, Scheffersomyces stipitis, Candida

334

shehatae, and C. parapsilosis can convert xylose into bioethanol. The limitation of pentose

335

fermentation can be overcome by using hybrid, genetically engineered yeast or a co-culture of two

336

yeast strains. Table 4 presents recent studies on the use of biomass waste in the production of

337

bioethanol. Co-culture of S. cerevisiae and S. stipitis in a rice husk hydrolysate containing 50 mg L-

338

1

339

culturing S. cerevisiae with hydrolytic enzyme-producing Aspergillus niger was considered an

340

effective and cost-competitive method for simultaneous saccharification and fermentation.

341

Meanwhile, 35.2 mg L-1 of ethanol was produced from potato waste (Izmirlioglu and Demirci,

342

2016). Rice straw and wheat straw are the most abundant examples of lignocellulosic waste, with

343

the potential to produce 205 and 104 billion liters, respectively, of bioethanol annually (Sarkar et al.,

344

2012). The use of mixed biomass, including coffee husks, cassava stems, and coconut coir achieved

345

an ethanol yield of 88.1%, which is comparable with the yield from the individual feedstock

346

sources (Nguyen et al., 2017b). Feedstock source diversification for biofuel production is an

347

important and necessary strategy. Wood residues from forestry harvesting or forestry product

348

processing can also act as suitable feedstock sources for ethanol production. Food industries

349

generate vast amounts of vegetable and fruit waste, which can serve as candidates for the liquid fuel

350

industry because they contain significant amounts of cellulose, hemicellulose, and soluble sugars,

351

which can be readily converted into ethanol via microbial fermentation (Huang et al., 2015).

and

co-fermentation

(SSCF),

and

consolidated

bioprocessing

(CBP).

of pentose and hexose sugars could obtain an ethanol yield of 0.42 g g−1. In a recent study, co-

352

Biobutanol has also received considerable attention because it is more compatible with

353

combustion engines than is bioethanol (Baral et al., 2016). Biobutanol can be produced by

354

microbial fermentation of monomeric sugars via the acetone-butanol-ethanol (ABE) process. The 15

355

most commonly used microorganisms for biobutanol fermentation include Clostridium

356

acetobutylicum, C. beijerinckii, and C. saccharoperbutylacetonicum. However, the use of wild-type

357

strains may produce low butanol concentrations and yields because of butanol feedback inhibition,

358

and heterofermentation, respectively (Tashiro et al., 2013). Thus, researchers have investigated

359

several approaches for improving butanol production, including the genetic modification of

360

microbes (Al-Shorgani et al., 2015) and fermentation engineering technologies (Lipovsky et al.,

361

2016). Additionally, an integrated system of ABE fermentation and a butanol-removal technique

362

have been developed to avoid butanol feedback inhibition, enhancing butanol production (Liu et al.,

363

2014). Another challenge to butanol production is the high cost of the fermentation substrate. Corn

364

stover, a major agricultural residue in the US, is a promising feedstock for biobutanol production

365

because of its high carbohydrate content and widespread availability (Ding et al., 2016). Many

366

investigations have focused on producing butanol from diverse sources of biomass waste, such as

367

barley straw (Qureshi et al., 2010a), sweet sorghum bagasse (Jafari et al., 2016), switch grass

368

(Qureshi et al., 2010b), wood chips (Sasaki et al., 2014), and food waste (Huang et al., 2015).

369 370

3.4. Bioactive compounds

371

The processing of fruits and vegetables generates large quantities of solid wastes, which

372

contains diverse molecules representing potential biological activities. Dry citrus peel waste

373

contains 3.8% D-limonene (Pourbafrani et al., 2010) and flavonoids, such as hesperidin, naringin,

374

nariturin, and eriocitrin (Chen et al., 2017), which can be used in the nutritional, pharmaceutical,

375

and cosmetic industries. Grape pomace is considered an excellent source of polyphenolic

376

compounds and has various potential health benefits such as scavenging activity against free

377

radicals, anti-inflammatory and anti-proliferation properties, and usefulness in cancer therapy. In

378

particular, grape skins contain significant amounts of tannins (16–27%) and other polyphenolic

16

379

compounds (2–6.5%), including catechins, anthocyanins, proanthocyanidins, quercetin, ellagic acid,

380

and resveratrol (Martinez et al., 2016). Grape seeds contain approximately 60% of the polyphenol

381

content in grapes, with high concentrations of flavan-3-ols, catechins, and epicatechins (Da et al.,

382

2014). Apple pomace is a rich source of polyphenols and flavonoids that predominantly resides in

383

the apple peel. Major bioactive compounds isolated and identified in apple pomace include

384

catechins, hydroxycinnamates, phloretin glycosides, quercetin glycosides, and procyanidins (Djilas

385

et al., 2009). Olive pomace, a byproduct of olive processing, contains approximately 98% of the

386

phenolic compounds contained in the entire olive fruit; these compounds include tyrosol,

387

hydroxytyrosol, cinnamic acid derivatives (caffeic acid and verbascoside), flavonoids (apigenin,

388

luteolin,

389

de(carboxymethyl)oleuropein aglycone isomers). Coffee byproducts contain approximately 1.5%

390

total polyphenols, with the highest yield from silver skin (25%), followed by spent coffee grounds

391

(19%) (Campos-Vega et al., 2015). Extracts from coffee byproducts perform excellent antioxidant,

392

anti-inflammatory, and anti-allergenic activities, because of the presence of chlorogenic acids

393

(Zuorro and Lavecchia, 2012). Table 5 lists the potential bioactive compounds extracted from

394

biomass waste sources.

and

rutin),

and

secoiridoids

(oleuropein,

oleuropein

aglycone,

and

395

Generally, bioactive compounds are isolated from biomass by solvent-based extractions.

396

Conventional methods such as solid-liquid extraction and liquid–liquid extraction, require a large

397

amount of organic solvents and long extraction times, and the product quality is usually affected by

398

traces of solvent residues (Santana-Meridas et al., 2012). Supercritical fluid extraction can enhance

399

the extraction yield of polyphenols in grape waste in very short periods of time and at low

400

temperatures; however, it requires expensive and specific equipment to operate at high pressure

401

(Aizpurua-Olaizola et al., 2015). Microwave and ultrasound treatments are used to assist with

402

extraction but still have limitations similar to those of solvent-based processes; these limitations

403

include safety hazards, environmental risks, low product quality, and high-energy input. Enzyme17

404

assisted extraction is an attractive method and has recently gained attention because it provides

405

faster extraction rates and green processing, while achieving higher yields than those of

406

conventional methods. The recovery of lycopene from tomato peel waste was enhanced by 8- to -

407

18-fold through the use of mixed cellulolytic and pectinolytic enzymes as compared with the use of

408

untreated material (Zuorro et al., 2011). Quercetin extraction from onion skin waste increased by

409

1.6-fold after enzymatic hydrolysis with cellulase, pectinase, and xylanase (Choi et al., 2015).

410

Cellulase, pectinase, and hemicellulase are generally used to facilitate the extraction of bioactive

411

molecules because they can hydrolyze the cell wall structure, enabling improved release and

412

recovery of these compounds.

413 414

3.5.

Nanocellulose

415

The use of cellulose-rich sources to develop a novel biomaterial, nanocellulose has garnered

416

a great deal of interest recently. Nanocelluloses have some important properties such as high

417

mechanical strength, high specific surface area, broad surface modification capacity, very low gas

418

permeability, high biodegradability, lack of toxicity, and high absorbability (Lin and Dufresne,

419

2014). Nano-sized cellulosic materials can be divided into two main groups: cellulose nanocrystals

420

(CNCs) and cellulose nanofibers (CNFs), according to morphology, dimension, and size (Garcia et

421

al., 2016). Fig. 3 displays the general process for the production of CNFs and CNCs from cellulosic

422

biomass, which involves sequential basic treatments, including pretreatment and bleaching or

423

cellulose purification. Acid hydrolysis has been used to remove amorphous regions from cellulose

424

biomass, resulting in highly crystalline CNCs (Hiasa et al., 2014). Mechanical treatments, such as

425

milling, microwave treatment, and ultrasound treatment can facilitate acid hydrolysis by decreasing

426

the reaction time and chemical requirements (Silverio et al., 2013). For the production of CNFs,

427

mechanical treatments such as homogenization or sonication have typically been applied following

428

the isolation of cellulose with or without the assistance of acid hydrolysis, steam explosion, or 18

429

enzyme action. Nanocelluloses can be produced from a variety of cellulosic sources, such as

430

mandarin peel waste (Hiasa et al., 2014), rice straw, grape skin (Hsieh, 2013), corncobs (Silverio et

431

al., 2013), and sawdust (Liu et al., 2014). The American Process company designed a commercial-

432

scale production process of nanocellulose from low-cost feedstocks. This process involved the

433

fractionation of biomass with sulfur and aqueous ethanol to generate cellulose-rich solids, and

434

followed by the mechanical treatment of the cellulose-rich stream, to obtain CNFs or CNCs with

435

relatively low energy consumption.

436

Another type of cellulose nanoparticles that should be considered is bacterial nanocellulose

437

(BNC), which is produced mainly by Gluconacetobacter xylinus (Garcia

et al., 2016). This

438

bacterium can consume a variety of substrates as a carbon source, including glucose, fructose,

439

sucrose, xylose, arabinose, mannitol, arabitol, glycerol, and oligosaccharides (Koutinas et al., 2014).

440

High yields of bacterial cellulose require the purification of the carbon source through the addition

441

of various nutrient supplements; however, this process increases the cost of BNC and subsequently

442

limits industrial use (Cerrutti et al., 2016). Molasses is a byproduct of the sugar industry and may be

443

a promising candidate as a low-cost carbon source. Molasses has been investigated as an effective

444

fermentation medium in the production of BNC (Çakar et al., 2014). A combination of molasses

445

and corn steep liquor is an ideal medium because it provides both a carbon source and a nitrogen

446

source for the development of Acetobacter sp. without expensive supplementation, reducing the

447

cost of BNC production (Jung et al., 2010). Grape pomace is one of the most abundant residues of

448

juice extraction and does not require complicated or expensive pretreatments. Grape pomace

449

contains the fermentable sugars D-glucose and D-fructose, which serve as renewable carbon

450

sources. Gluconacetobacter xylinus successfully produced cost-effective BNC in a medium

451

containing both grape pomace extract and corn steep liquor (Cerrutti et al., 2016). Dry olive mill

452

residue, a byproduct of the olive oil industry, has also been exploited as a carbon and nutrient

453

source for the production of BNC (Gomes et al., 2013). Compared with plant cellulose, BNC 19

454

exhibits a greater degree of polymerization and higher crystallinity index. Furthermore, it exhibits

455

higher tensile strength and water-holding capacity, making it more appropriate for certain

456

applications.

457

458

3.6.

Lignin by-products

459

Lignin is a natural amorphous polymer made up of a substituted aromatic structure of

460

monolignols, primarily coniferyl alcohol, sinapyl alcohol, and, in grasses, β-coumaryl alcohol (Gall

461

et al., 2017). Lignin exhibits structural heterogeneity and recalcitrance, which presents both a

462

challenge and an opportunity for use in biorefineries. This section focuses on the advanced

463

processes of lignin isolation from biomass waste.

464

Lignin valorization is a broad topic that has been discussed in detail in several recent articles.

465

Isolated lignin differs in purity, structure of the depolymerized products, and molecular weight of

466

the products, according to the biomass source and process used. The sulfite process used in the

467

lignosulfonate industry generates a large amount of lignin, a profitable commodity commercialized

468

by Borregaard Ligno-Tech, with a maximum production capacity of 1 Mt per year. The soda

469

process is typically applied for non-woody biomass to isolate lignin products with a higher purity

470

and lower molecular weight than the products obtained by the sulfite process. High-quality lignin

471

from a kraft pulp mill was extracted and upgraded by LignoBoost Technology, which launched

472

27,000 tons of kraft lignin in the market in 2013 for potential fuel applications and use as an

473

antioxidant, binder, dispersant, carbon fibers, and activated carbon (Upton and Kasko, 2016).

474

Renmatix commercialized lignin obtained as a co-product from supercritical technology, because it

475

requires low-production cost, is highly reactive, and is clean lignin that can be utilized efficiently in

476

a variety of valuable applications, either as a replacement, such as for wood adhesives, or through

477

transformation, e.g., for ethanol, H2O, ionic-liquids, benzene, toluene, and xylene (BTX). Licella

20

478

Holdings and Canfor Pulp constructed a large biorefinery to convert the woody waste to biocrude

479

oil by supercritical-H2O (Liu et al., 2019).

480

The organosolv process can fractionate and separate each biomass component with

481

relatively high purity and, thus, has recently become attractive for the isolation of lignin at the

482

industrial scale (Upton and Kasko, 2016). Organosolv pretreatment in the presence of an acid or

483

alkali catalyst was employed to enhance the delignification and subsequent alcoholic fermentation

484

process of sorghum bagasse (Teramura et al., 2016) and cotton stalk (Tang et al., 2017a). In a recent

485

study, an 81.7% yield of high-quality, salt-free lignin was recovered from corn stover through a

486

process using aqueous ethanol and n-propylamine as a catalyst, and the properties and

487

functionalities of the lignin were found to offer great potential to upgrade the downstream products

488

to value-added chemicals and materials (Tang et al., 2017b). Mild pretreatment such as

489

autohydrolysis or steaming prior to organosolv pretreatment was effective at fractionating biomass

490

components, yielding 100 g and 140 g of pure lignin from 1 kg of rice straw and poplar chips,

491

respectively (Amiri and Karimi, 2016). Other processes demonstrated a high yield of lignin

492

recovery at the laboratory scale. For example, ionic liquid could help isolate 90.1% of lignin from

493

sugarcane bagasse (Saha et al., 2017), and a new method of applying deep eutectic solvents could

494

extract lignin from woody biomass with high yield (78%) and high purity (95%) (Alvarez-Vasco et al.,

495

2016); however, their high cost presents challenges in commercialization. Microwave reaction was

496

also used for the isolation of lignin from softwood with dilute H2SO4, which offers high lignin yield

497

(82%) and purity (93%) within 10 min at 190 ºC (Cao et al., 2019). However, the scaling up of a

498

biorefinery using microwave reaction requires further investigation as there is limited application

499

experience for industrial-scale reaction vessels.

500

Enzymes involved in lignin degradation can generally be divided into two main groups:

501

lignin-modifying enzymes and lignin-degrading auxiliary enzymes. Lignin-modifying enzymes

502

(lignin peroxidase, manganese-dependent peroxidase, versatile peroxidase, dye-decoloring 21

503

peroxidase, laccase) have been actively used in the paper and pulp, food, dye, and cosmetic

504

industries over the past ten years. Although these enzymes have been applied in these industries,

505

optimal and robust fermentative processes have not been established. Lignin-degrading auxiliary

506

enzymes are unable to degrade lignin on their own; yet, they are necessary to complete the

507

degradation process. Lignin-degrading auxiliary enzymes include glyoxal oxidase, aryl alcohol

508

oxidase, glucose dehydrogenase, and pyranose 2-oxidase (Janusz et al., 2017). These enzymes

509

could be potential candidates for achieving future lignin valorization.

510 511

4. Future directions to enhance enzymatic transfer technology

512

In the routes of waste valorization, enzymes play a crucial role and directly influence the yield

513

of conversion/extraction processes such as enzymatic saccharification, enzyme-assisted extraction

514

of bioactive compounds, and enzyme-catalyzed transformations. However, the cost of enzymes is

515

relatively expensive and currently available enzymes cannot degrade plant cell walls completely

516

without pretreatment. Enzymes are produced in industrial fermentation processes with different

517

microorganisms, in which the use of cheap biomass waste as a fermentation substrate for producing

518

economic and efficient enzymes can be enabled. The recent use of abundant and low-cost biowaste

519

to replace traditional sugar substrate provides a cost-effective and green solution for producing

520

hydrolytic enzymes. Various enzyme companies have also contributed significant technological

521

breakthroughs in cellulosic ethanol through the mass production of enzymes for hydrolysis at

522

competitive prices. In order to reduce the cost of cellulases, which are key enzymes in the

523

production of cellulosic ethanol by enzymatic hydrolysis, Genencor and Novozymes reported

524

reducing the enzyme cost contribution to US$0.30–0.40/gallon (US$ 0.08-0.10/L). However, a

525

further decrease is still necessary to achieve US$ 0.10/gallon (or 0.026/L). The development of

526

novel and efficient enzymes via either metagenomics or metabolic engineering will pave the way

527

for creating innovative and high value products in the bioeconomy. Another strategy to decrease the 22

528

cost of enzyme aims to recover enzymes from the hydrolysis processes and then recycle them for

529

other purposes. More efficient enzyme preparation can also be achieved by various approaches such

530

as selective screening of candidate enzymes, enzyme engineering, and purification of enzyme

531

cocktails, which need to be extensively researched and developed.

532 533

5. Conclusions

534

This review highlights the importance and potential of biomass waste through a biorefinery

535

concept. The available abundance and diverse compositions of biomass waste provide potential for

536

the production of useful chemical platforms and valuable products. These products can be applied

537

in chemical, pharmaceutical, cosmetic, and food industries; thus, they are worthy of recovery. We

538

also demonstrated that enzymatic technology is an important upstream process that acts as a critical

539

driver for the efficient development of biomass waste valorization. However, the technologies

540

associated with biomass waste conversion need to be investigated in greater detail and upgraded to

541

make these products competitive with petroleum-based ones.

542 543

Acknowledgements

544

This research was supported by the National Research Foundation of Korea (NRF) grant funded by

545

the Korea government (MSIP) (NRF-2018R1A2A2A05018238).

23

547

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142.Yamabhai, M., Sak-Ubol, S., Srila, W., Haltrich, D. 2016. Mannan biotechnology: from biofuels to health. Crit. Rev. Biotechnol. 36(1), 32-42. 143.Yang, C.H., Yang, S.F., Liu, W.H., 2007. Production of xylooligosaccharides from xylans by extracellular xylanases from Thermobifida fusca. J. Agr. Food Chem. 55(10), 3955-3959. 144.Youcai, Z., 2018. Pollution control technology for leachate from municipal solid waste; Landfills, incineration plants, and transfer station. Butterworth-Heinemann. pp. 361-376. 145.Zhang, Z., Gonzalez, A.M., Davies, E.G.R., Liu, Y., 2012. Agricultural wastes. Water Environ. Res. 84, 1386-1406.

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146.Zhang, H., Xu, Y., Yu, S., 2017. Co-production of functional xylooligosaccharides and

936

fermentable sugars from corncob with effective acetic acid prehydrolysis. Bioresour. Technol.

937

234, 343-349.

938

147.Zhao, C., Zou, Z., Li, J., Jia, H., Liesche, J., Chen, S., Fang, H., 2018. Efficient bioethanol

939

production from sodium hydroxide pretreated corn stover and rice straw in the context of on-

940

site cellulase production. Renew. Energ. 118, 14-24.

941 942 943

148.Zuorro, A., Fidaleo, M., Lavecchia, R., 2011. Enzyme-assisted extraction of lycopene from tomato processing waste. Enzyme Microb. Tech. 49(6-7), 567-573. 149.Zuorro, A., Lavecchia, R., 2012. Spent coffee grounds as a valuable source of phenolic 39

944

compounds and bioenergy. J. Clean. Prod. 34, 49-56.

945

40

946

Table Captions

947

948

Table 1. Chemical composition of major biomass waste sources.

949

Table 2. Cellulosic sugar production via commercially viable technologies.

950

Table 3. Production of oligosaccharides from waste via different processes.

951

Table 4. Bioethanol production from various biomass waste sources from recent studies

952

Table 5. Bioactive compounds isolated from some typical waste sources from recent studies.

953

41

954

Figure Captions

955 956

Figure 1. Valorization of biomass waste into typical products.

957

Figure 2. The importance of oligosaccharides as source of prebiotics

958

Figure 3. General process for producing BNC, CNC, and CNF from biomass waste.

959

Highlights

960

►This review explores the potential of using a wide range of biomass wastes

961

►Biomass waste provides the potential for the production of useful chemical platforms

962

►Biomass waste can be used for the production of various value-added chemicals.

963

►Enzyme is critically important for the conversion of biomass into valuable products.

964

965

966

Table 1. Chemical compositions of major biomass waste sources. Carbohydrates Type of waste Cellulose (%) Hemicellulose (%) Agricultural residues Rice straw 28–36 19–27 Wheat straw 35–47 20–30 Corn straw 43 21 Bagasse 32–44 27–32 Corn stover 38-40 28 Rice husk 37 18 Corncobs 45 35 Rapeseed straw 49 15 Forestry residues Hardwood stems 40–55 24–40 Softwood stems 45–50 25–35 Wood residues 33–51 20–30 Poplar sawdust 44 19 Wood wastes 45–51 11–18

Lignin (%) 12–24 17-19 8 19–24 7–21 24 15 22

[Sarkar et al., 20 [Saini et al., 201 [Kim and Dale, [Lachos-Perez e [Saini et al., 201 [Phaiboonsilpa e [Sun and Cheng [Pińkowska et al

18–25 25–35 21–32 25 23–25

[Sun and Cheng [Sun and Cheng [Koutinas et al., [Kim et al., 2013 [Cho et al., 2011 42

Waste paper Food and other wastes Pear pomace Apple pomace Carrot pomace Onion skin waste Orange waste Potato peel Cassava pulp Tomato pomace Coffee pulp Coffee husk Spent coffee grounds Seaweed waste

60–70

10–20

5–10

[Sun and Cheng

34 43.6 51.6 36.5

21 24.4 12.3 11.2

20 20.4 32.2 9.4 -

2.3 7 36.7 3.5

17.5 9 4.5

[Rabetafika et al [Nawirska and K [Nawirska and K [Choi et al., 201 [Lin et al., 2013] [Lin et al., 2013] [Lin et al., 2013] [Lin et al., 2013] [Mussatto et al., [Rambabu et al., [Mussatto et al., [Uju et al., 2015

47 69.7 69.9 25–50 63 43 8.6 33.6

967 968

969

Table 2. Cellulosic sugar production via commercially viable technologies. Cellulosic sugar Sugar Feedstocks Capacity producers types AGM Energy Agricultural residues, yard C5 and 250 tons of Group waste and dedicated grasses C6 biomass/day

Technology Mechano-chemical

[AM

American Process

Wood chips and agricultural residues

C5 and C6

3 tons of biomass/day

Pretreatment, enzymatic hydrolysis, [AV and autohydrolysis

Arkenol

Agricultural residues, wood waste, municipal solid waste, purpose grown crops, paper waste, green waste

C5 and C6

500 dry tons of biomass/day

Concentrated acid hydrolysis

Comet Biorefining

Wheat straw, sugarcane bagasse, corn stover and other agricultural wastes

Glucose

60 million pounds Pretreatment and dextrose/year enzymatic hydrolysis (2018)

General Biomass

Renmatix

Agricultural residues, municipal solid waste, forestry residues, or peripheral biomass from fruits, palm, and coffee beans. Sugarcane bagasse, corn stover, citrus waste, lumber mill residue,banana stems, paper mill sludge, rice husks, barley straw, municipal waste

C5 and C6

-

C5 and C6

100,000 tons of sugar /year (2012)

Sweetwater Energy

Agricultural residues, woody waste, municipal solid waste

C5 and C6

Virdia or HCL Clean Tech

Wood chips and agricultural wastes

C5 and C6

[Ark

[Com

Enzymatic hydrolysis

[Gen

Supercritical hydrolysis

[Ren

Dilute acid hydrolysis and enzymatic [Sw hydrolysis 150000 tons Cold acid solvent [Lan of sugar/year extraction 3 tons of biomass/day

43

(2015) 970 971 972

973

Table 3. Production of oligosaccharides from wastes via different processes. Feedstocks

Pretreatment

Hydrolysis

Rye straw Morning light Switchgrass Bagasse Corncob

0.1% H2SO4 0.1% H2SO4 0.1% H2SO4 pH2 acetic acid

Tobacco stalk

24% KOH + 1% NaBH4

Cotton stalk

24% KOH + 1% NaBH4

Sunflower stalk

24% KOH + 1% NaBH4

Wheat straw

24% KOH + 1% NaBH4

Autohydrolysis (208 oC) Autohydrolysis (145 oC) Autohydrolysis (145 oC) Autohydrolysis (145 oC) Autohydrolysis (145 oC) Acid hydrolysis (0. 25M H2SO4) Acid hydrolysis (0. 25M H2SO4) Acid hydrolysis (0. 25M H2SO4) Acid hydrolysis (0. 25M H2SO4) Xylanase from Aspergillus foetidus MTCC 4898 Xylanase from Thermobifida fusca NTU22 Xylanase from Thermobifida fusca NTU22 Xylanase from Thermobifida fusca NTU22 Endoxylanase from Trichoderma sp. Endoxylanase from Aspergillus oryzae MTCC 5154 Endoxylanase from Aspergillus oryzae MTCC 5154 Endoxylanase from Aspergillus oryzae MTCC 5154 Cellulase, pectinase, and xylanase from Aspergillus japonicus

Corncob

1.25 M NaOH

Bagasse

4% NaOH + steam (100 oC, 3h)

Corncob

4% NaOH + steam (100 oC, 3h)

Peanut shell

4% NaOH + steam (100 oC, 3h)

Corncob

2% NaOH

Corncob

0.1% H2SO4

Corncob

2% NaOH

Corncob

0.1% H2SO4 + autoclave (121 oC, 30 min)

Orange peel waste

Water extraction (30 oC, 6h)

Oligosaccharide type

Yield (%)

XOS XOS XOS XOS XOS

69.2 65.0 84.2 92.3 45.9

[G [O [O [O [Z

XOS

13.0

[A

XOS

7.5

[A

XOS

12.6

[A

XOS

10.2

[A

XOS

-

[C

XOS

23.7

[Y

XOS

29.5

[Y

XOS

10.1

[Y

XOS

43.3

[C

XOS

52.0

[A

XOS

81.0

[A

XOS

77.0

[A

POS

38.9

[L

44

Onion skins

-

Endopolygalacturonase

POS

55.8

974 975 976 977 978 979 980 981 982 983 984 985 986

987

Table 4. Bioethanol production from various biomass waste sources in recent studies. Ethanol Fermentation Biowaste source Microorganism concentration process (g L-1) Agricultural residues

Ethanol yield (%)

Rice straw

SSF

S. cerevisiae

12.8

83.2

Rice husk

SSF

S. cerevisiae BIOTECH Hansen 2055

11.6

-

Wheat straw

SHF

E. coli FBR5

18.9

80.2

Wheat straw

SSF

E. coli FBR5

15.1

64.6

Corn stover

SSF

S.cerevisiae DQ1

71.4

80.3

Sugarcane bagasse

SSF

S. cerevisiae

27.1

91.6

Sugarcane bagasse

SSF

S. cerevisiae

18.8

67.1

Sugarcane bagasse

Prehydrolysis-SSF

S. cerevisiae

15.4

50.1



45

[B

Mixed biomass residues

SHF

S. cerevisiae KCTC 7906

9.5

76.8

Mixed biomass residues

SSF

S. cerevisiae KCTC 7906

20.8

83.9

Orange pomace

SHF

S.cerevisiae KCTC 7906

27.1

92.4

Mandarin pomace

SHF

29.5

93.1

Grapefruit pomace

SHF

21.6

90.7

Mixed citrus waste

SHF

20.4

90.2

Potato waste

SSF

35.2

74.4

Apple pomace

SSF

8.6

86.7

Food wastes

Spent coffee grounds Cruciferous vegetable residues

S.cerevisiae KCTC 7906 S.cerevisiae KCTC 7906 S.cerevisiae KCTC 7906 Cocultures of A. niger &.S. cerevisiae Cocultures of T.harzianum, A.sojae & S.cere visiae

SHF

S. cerevisiae

19.0

97.8

SHF

S.cerevisiae KCTC 7906

8.3

85.7

Waste paper

Prehydrolysis-SSF

S. cerevisiae

45.3

90.8

Poplar sawdust

Fed-batch SSF

S. cerevisiae DK 410362

39.9

81.7

Lumber wastes

SHF

P. stipitis KCCM 12009

25.6

90.7

Plywood waste

SHF

P. stipitis KCCM 12009

19.9

90.1

Particle board

SHF

P. stipitis KCCM 12009

19.2

90.5

Pine bark

SHF

S. cerevisiae

15.5

82.2

Other wastes

988 989 990

991

Table 5. Bioactive compounds isolated from some typical waste sources in recent studies 46

Biomass waste Citrus peel waste Orange peel Grape pomace Grape waste Grape seed Apple pomace

Bioactive compounds Essential oils D-limonene Arirutin, hesperidin, nobiletin, and tangeritin Hydroxybenzoic acids and hydroxycinnamic acids, flavan-3-ols, flavanols and anthocyanins Gallic acid, catechin, epicatechin

Extraction methods Microwave- assisted hydro-distillation Alkaline hot water extraction Solid-liquid solvent extraction Supercritical fluid extraction

Catechin, epicatechin, procyanidin Gallic acid, chlorogenic acid, catechin, rutin and phloridzin Lycopene

Enzyme-assisted extraction Microwave-assisted extraction

Chlorogenic acid, caffeic acid, catechin, epicatechin Gallic acid, sytingic acid, mangiferin, ellagic acid -carotene, lutein and violaxanthine Ellagitannins, punicalagin, and punicalin Quercetin

Ultrasound-assisted extraction Solid-liquid solvent extraction

Spent coffee ground Spent coffee ground

Chlorogenic acids 3-O-caffeoylquinic acid, 4-O-caffeoylquinic acid and 5-O-caffeoylquinic acid

Solid-liquid solvent extraction Subcritical water extraction

Olive pomace (solid waste)

Hydroxytyrosol, tyrosol, caffeic protocatechuic, vanillic, p-coumaric and syringic acids, vanillin, oleuropein, apigenin Gallic and syringic acids, kaempferol, naringenin Caffeic acid, naringenin, daidzein α-Tocopherol, cynanidin-3-glucoside, 4hydroxybenzoic acid, and vanillic acid

Solid-liquid extraction with high pressure

Tomato processing waste Carrot pomace Mango peel Pomegranate peels Onion skin waste

Rapeseed meals Soybean meals Rice bran

Enzyme-assisted extraction

Ultrasound-assisted extraction Enzyme-assisted extraction

Microwave-assisted extraction Microwave-assisted extraction Ultrasound-assisted extraction

992 993

47