Integrated processing of plant-derived waste to produce value-added products based on the biorefinery concept

Trends in Food Science & Technology 74 (2018) 119–131

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Review

Integrated processing of plant-derived waste to produce value-added products based on the biorefinery concept

T

Qing Jina, Liangcheng Yangb, Nicholas Poea, Haibo Huanga,∗ a b

Department of Food Science and Technology, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA Department of Health Science, Illinois State University, Normal, IL, 61790, USA

A R T I C L E I N F O

A B S T R A C T

Keywords: Plant-derived waste Biorefinery Value-added products Techno-economic analysis Environmental assessment Social assessment

Background: Plant-derived wastes from agriculture, processing, distribution, and retail are generated in large quantities. The majority of the wastes are underutilized and may cause severe environmental problems if not properly handled. The plant-derived wastes are usually rich in lignocellulose and other valuable compounds including protein, fat, sugar, and phytochemicals. Valorization of these compounds in food waste not only reduces environmental concerns but also improves sustainability and economic competitiveness of agro-food industries. Scope and approach: This review paper first discussed different phases of the biorefinery concepts and their associated applications, and then introduced recent advances in the integrated processing of plant-derived waste for producing various value-added products. Finally, techno-economic, environmental, and social assessments along with relevant policies were introduced and discussed. Key findings and conclusions: During the past ten years, research attentions focused on integrated utilization of plant-derived waste to produce various products have flourished. Compared to production of a single component for food waste valorization, integrated processing of food waste via a combination of different novel technologies to produce multiple products based on a biorefinery concept has significant advantages, including full utilization of feedstocks, minimization of waste generation during processing, synergy effects of different technologies, and diversification of the revenues by covering multiple markets. With the rationale design of biorefinery processes, underutilized plant-based wastes can be valuable resources for the sustainable production of food, chemicals, and biofuels. However, detailed economic, environmental, and social analyses for the biorefinery process are still needed in the future.

1. Introduction Food waste is currently an important issue, both in developing countries and developed countries. Food waste is defined as end products of various food processing industries that have not been recycled or used for other purposes (Garcia-Gonzalez et al., 2015). These end products have economic values less than the cost of recovery; thus, they are discarded as waste. It is estimated that about 1.2 billion tonnes of foods are lost or wasted globally, representing approximately one-third of the edible parts of food produced for human consumption (FAO, 2011). The causes of these wastes include mechanical damage and/or spillage during agricultural production; degradation during storage and transportation; grading, washing, peeling, slicing, extrusion, and canning during food processing; spoilage at market system such as supermarkets and retailers, and at household level (FAO, 2011; Galanakis, 2012).

∗

Food waste creates disposal problems and potentially pollutes the environment, meantime causing abundant loss of valuable nutrients. Food waste has been traditionally recycled as animal feed and fertilizers, or landfilled with other wastes. However, the increasing demand for environmental protection, together with the desire for resource conservation, are encouraging more efficient valorization of food waste for the production of value-added food ingredients, chemicals, and biofuels (Lin et al., 2013). Through developing innovative approaches for valorizing food waste, the food industry can reduce the pressure of waste treatment and improve the sustainability of food production. Although the sources of food waste are highly diverse, they can be classified, based on the original materials, into two main groups (i.e. plant- and animal-derived wastes), and can be subdivided into fruits and vegetables, cereals, oil crops, root and tubers, meat products, fish and seafood, and dairy products (FAO, 2011; Galanakis, 2012). Between the two main groups, plant-derived waste represents the larger

Corresponding author. Human & Agricultural Biosciences Bldg.1, Room 402J, 1230 Washington St. SW, Blacksburg, VA 24061, USA. E-mail addresses: [email protected] (Q. Jin), [email protected] (L. Yang), [email protected] (N. Poe), [email protected] (H. Huang).

https://doi.org/10.1016/j.tifs.2018.02.014 Received 19 August 2017; Received in revised form 24 January 2018; Accepted 12 February 2018 Available online 15 February 2018 0924-2244/ © 2018 Elsevier Ltd. All rights reserved.

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Abbreviation ABE ADP AFEX BSG CNCs DDGS DW EFB FAMEs

GAE GHG GWP LCA MAE MHG ODP PHAs PHB UAE VFAs

Acetone-butanol-ethanol Abiotic depletion potential Ammonia fiber expansion Brewer's spent grain Cellulose nanocrystals Distiller's dried grains with solubles Dry weight Empty fruit bunches Fatty acid methyl esters

Gallic acid equivalent Greenhouse gas Global warming potential Life cycle assessment Microwave-assisted extraction Microwave hydrodiffusion and gravity Ozone layer depletion potential Polyphydroxyalkanoates Poly(3-hydroxybutyrate) Ultrasound-assisted extraction Volatile fatty acids

produce multiple products based on a biorefinery concept has been emerged to solve the complex food waste problem. Biorefinery is defined as the integrated and sustainable processing of biomass into various marketable chemicals, materials, fuels, and power (Cherubini et al., 2007). The biorefinery concept was firstly derived from the petroleum refinery, in which different technologies were used to produce multiple chemicals and fuels from petroleum. In the biorefinery concept, hybrid technologies from various fields, including agriculture, chemistry, engineering, and microbiology, are applied to an integrated process to separate biomass into its building blocks, such as carbohydrates, proteins, and oils. These compounds can

portion (63%) of the whole food supply chain, when compared with animal-derived waste (Pfaltzgraff, Cooper, Budarin, & Clark, 2013). Plant-derived waste presents a major source of carbohydrates, lipids, proteins, minerals, and other phytochemicals. Therefore, there is great potential to recover these compounds or convert them to valuable products through novel processes. As food waste is complex in structure and constituents, valorizing food waste in a profitable way is a highly multidisciplinary problem; a single technology is usually not sufficient enough to fully solve this complex problem (Tuck, Pérez, Horváth, Sheldon, & Poliakoff, 2012). Therefore, integrated processing of food waste via a combination of different novel technologies to recover and

Fig. 1. Corn dry grind biorefinery (phase I), corn wet milling biorefinery (phase II), and plant-derived biomass biorefinery (phase III).

120

10-L high-pressure reactor, atmospheric pressure expansion tank, centrifuge, shaker, bioreactor, municipal waste digester Supercritical CO2 extraction, rotary evaporator, centrifuge, anaerobic digester

Citrus waste

US standard sieve N20, ultrasonic homogenizer, 6-L percolation reactor Blender, Buchner filtration, magnetic stirrer, centrifuge, rotary evaporator, sonication Extractor (14 dm3), gravity separator (5 dm3), cyclonic separator (3 dm3), condenser, heat exchangers, ball mill Incubator, batch reactor, vacuum pump, forced-air oven, 5-L continuously stirred tank reactor Shaker, biogas endeavor system bioreactor, water-jacketed glass bioreactor, suction pump

Corn distiller's dried grains Rice husk and Arundo donax

Cassava peels

Rapeseed straw, cake, glycerol

Olive mill waste

Wheat straw

Grinder, supercritical CO2 extraction, magnetic stirrer, centrifuge, stainless steel reactor, water bath, oven Reactor, continuously stirred tank reactor

Tomato peels and seeds

Grape pomace

Facilities

Crop

Lab-scale/50 g

Lab-scale

Lab- and pilot-scale/7 kg

Lab-scale/10 g of each biomass

Pilot-scale/120–150 kg DM/h capacity Lab-scale/150 g

Lab-scale/10 g

Rapeseed straw, cake, and glycerol, H2O2, NaOH, steam, water, sodium citrate buffer, enzyme, nitrogen, digested manure, electricity Cassava peels, S. cerevisiae, NaOH, enzymes, manure inoculum, electricity

Wheat straw, water, S. cerevisiae, enzyme, acetate buffer, nutrients, N2, digested manure, electricity Corn distiller's dried grains, H2SO4, water, heat, electricity Rice husk and A. donax, HCl, NaOH, water, ethanol, acetic acid, hydrogen peroxide, sulfuric acid, H2SO4, electricity Olive mill waste, ethanol, N2, air, heat, water, electricity

Ethanol, methane

Ethanol, hydrogen, methane

Polyphenols, mono/poly-unsaturatefatty acids, fuels, biochar

Hemicellulose A, hemicellulose B, silica, cellulose nanocrystals, lignin

Arabinose-rich stream, xylose-rich stream

Bioethanol, biohydrogen, biogas

Oils, carotenoids, proteins, sugars, cellulose, lignin

Sweden

Denmark, China

Italy

Italy

The U.S.

Denmark

Tunisia

Italy

Polyphenols, volatile fatty acids, polyhydroxyalkanoates, methane

Grape pomace, CO2, ethanol, water, NaOH, nitrogen, acidogenic microbial consortium, Cupriavidus necator, glucose, nutrients, methanogenic microbial consortium, electricity Tomato peels and seeds, CO2, NaOH, water, N2, electricity

Sweden

-Limonene, pectin, ethanol, biogas

D

Citrus waste, water, sulfuric acid, heat, electricity, steam, S. cerevisiae, nutrients, ethanol

Lab- and pilot-scale/2 kg slurry with 15% solid content Lab- and pilot-scale/ 480 g

Country

Output

Input

Scale/size

Table 1 Facilities, scale, input and output of the biorefinery of plant-derived waste to value-added products.

(Moshi et al., 2015)

(G. Luo et al., 2011)

(Schievano et al., 2015)

(Kaparaju et al., 2009) (Fonseca et al., 2014) (Barana et al., 2016)

(Kehili et al., 2016)

(Martinez et al., 2016)

(Pourbafrani et al., 2010)

Reference

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The advantage of choosing this type of product is that the direct products already have existing market value, thus the likelihood of the product acceptance is high on the market (FitzPatrick, Champagne, Cunningham, & Whitney, 2010). Different from the direct product, an indirect product is a newly developed product with a similar function to an existing product in the market (Clark et al., 2006). However, to make an indirect product successful in the existing market, instead of performing similar functions, the indirect product should also have other advantages such as lower cost or properties which are not found in the existing products (FitzPatrick et al., 2010). Besides producing final products, processes for obtaining intermediate chemicals can also be promising, because these compounds can be coupled into the existing chemical production facilities and converted to higher value-added chemicals (FitzPatrick et al., 2010). For example, butanol, a widelyused intermediate chemical for producing acrylate and acetate, was recently successfully produced from food wastes via anaerobic fermentation (H. Huang, Singh, & Qureshi, 2015). The design of biorefinery processing routes needs to be considered thoroughly. In order to convert feedstock into several value-added products in an integrated process, different technological processes such as mechanical, chemical, thermochemical, and biochemical processes are normally jointly applied. Table 1 summarizes several examples of converting plant-derived wastes to value-added products, the corresponding facilities, scales, inputs and outputs are included to better illustrate the biorefinery processing routes. Mechanical treatment is commonly used in the first step for size reduction. Such technologies include pressing, milling, and pelletization. This process usually does not affect the composition of biomass but only change the particles size and shape, which could improve the mass transfer characteristics, enzymatic hydrolysis, and biodegradability of biomass in following steps (Menon & Rao, 2012). Chemical processes such as acidic or basic hydrolysis, transesterification, hydrogenation, and oxidation are applied to change the chemical structures of biomass (Cherubini, 2010). For example, vegetable oils can be converted to biodiesel by transesterification. Thermochemical processes are often used to produce syngas by gasification (> 700 °C), liquid pyrolytic oil and solid charcoal by pyrolysis (300–600 °C) (Cherubini, 2010). As to biochemical processes, enzymatic conversion, anaerobic digestion, and fermentation are mostly used in biorefinery processing. Structural compounds such as cellulose and hemicellulose in biomass can be enzymatically hydrolyzed to their component sugars, including glucose and xylose. These monosaccharides are then used to produce biofuels such as ethanol, hydrogen, and butanol, as well as organic acids such as succinic acid through fermentation.

be further converted to other value-added products such as platform chemicals, energy, and biofuels (Cherubini, 2010). Compared to obtaining a sole product with conventional processing, biorefinery processing of food waste to produce multiple value-added products has several significant advantages, including (a) full utilization of feedstocks, thus minimizing waste generation during processing; (b) diversification of the revenues by covering multiple markets/niches; (c) synergy effects of different technologies; (d) sharing manpower and equipment; and (e) potential achievements of energy self-efficiency via biogas production or inert fiber material burning. During the past ten years, research attentions focused on integrated utilization of plantderived waste to produce various products have flourished. Thus, the overall objective of this review is to summarize the latest integrated processing routes as they have been applied on various types of plantderived waste or byproducts; in the meantime, economic, environmental, and social assessments along with the relevant policies regarding the biorefinery processing of plant-derived wastes are also introduced and discussed. 2. Biorefinery concept Biorefinery can be divided into three phases (I, II, and III) regarding biomass, targeted products, and processes used (Kamm & Kamm, 2004). A phase I biorefinery has almost no flexibility during the whole process; it normally uses one type of biomass, one process, and one targeted product. For example, in dry grind ethanol process, corn is milled, saccharified, and fermented to ethanol (Fig. 1A). The fixed process converts 1 bushel (56 lb.) of corn to 2.6–2.8 gal of ethanol (Kwiatkowski, McAloon, Taylor, & Johnston, 2006). There is little flexibility in the process. In phase II biorefinery, more products can be produced during the process. An example of this phase is the corn wet milling process (Fig. 1B), in which various products such as starch, lactic acid, ethanol, corn syrup, and corn oil are produced. The spectrum of products from the corn wet milling process can substantially improve the overall economic performance of the biorefinery process. When the ethanol market is weak, a corn wet milling plant can still make profits by producing other products (corn oil, starch, syrup, etc.); however, it is very difficult for a corn dry grind plant to remain at the same profit. A phase III biorefinery has an even higher flexibility than a phase II biorefinery (Fig. 1C). It is not only able to produce multiple value-added products, but also use different types of feedstocks and processing methods. This is very important for the integrated processing of food waste, whose supply is usually highly seasonal. The capability of using different types of feedstocks can ensure a stable supply for the process during the whole year, thereby improving the economic feasibility of food waste valorization. Biomass feedstock, final products, and routes (technologies) to convert biomass to final products are three key points in a biorefinery design. Biomass has a complex chemical composition, which is similar to the petroleum. Plant biomass is made up of carbohydrates, proteins, fats, lignin, vitamins, minerals, aromatic compounds, and dyes (Kamm & Kamm, 2004). Different types of biomass have different physical properties and chemical compositions, which largely determine possible final products and technologies used in the biorefinery process. Thus, it is important to develop a data base of physical property and chemical composition for the development of biorefinery of food wastes. Furthermore, different biomasses can be combined together for biofuels and biomaterials production. For example, fermentation media should contain carbon, nitrogen, vitamins, minerals, and other trace elements, while only one type of biomass may not provide all of them. Therefore, it is advantageous to combine different types of feedstocks that contain complementary and necessary compounds. As to the choice of final products from a biorefinery process, there are direct and indirect product substitution approaches. The direct product from a biorefinery means that the final product exists on the market, and needs to be produced through a newly developed process.

3. Biorefinery of plant-derived waste Large amounts of plant-based waste are generated annually from agricultural activities and subsequent food processing (Table 2). The largest portion of plant-based waste is from the non-grain part of crops, such as wheat straw, rice straw, and corn stover. The high availability and relatively stable composition of these agricultural byproducts make them promising feedstocks in large-scale industrial biorefineries (Kamm & Kamm, 2004). The second largest food waste is from the beverage industry, which generates a large amount of pomace, such as grape pomace and apple pomace. Besides plant fibers, the pomace contains considerable quantities of high-value functional compounds, such as polyphenols, essential oils, and vitamins (Martinez et al., 2016; Yates, Gomez, Martin-Luengo, Ibañez, & Serrano, 2017). Table 3 summarizes the compositions of different plant-derived wastes. It should be noted that the exact composition of a particular waste may vary from different regions or processing methods. For example, the composition of polyphenols in grapes depends on variety, climate, and soil, thus the polyphenol compositions of corresponding waste produced during wine production are different. In addition, different technologies used in winemaking also produce waste with 122

Trends in Food Science & Technology 74 (2018) 119–131

(FAO, 2016a; Moates et al., 2016; Nelson, 2010)

discrepant compositions. For instance, Ganimede fermentation resulted in higher content of anthocyanins compared to traditional fermentation methods in wine, thus leading to different polyphenol compositions in winey waste such as grape pomace (Bai et al., 2013). The major structure of plant-derived waste consists of cellulose, hemicellulose, and lignin (Knauf & Moniruzzaman, 2004). Cellulose can be used to produce biofuels, organic acids, and nano-cellulose materials. Hemicellulose can be degraded to xylose, and then xylite and furfural, which are valuable chemical products. Lignin is used as natural binder and adhesives; moreover, valuable compounds such as phenols can be derived from lignin (Kamm & Kamm, 2004). Besides structural compounds, other reserves rich in plant-derived waste including sugars, proteins, and oils, and phytochemicals also show potential for various applications and are worthy of recovery. Proteins in plant-derived waste with well-balanced essential amino acids can be introduced to food to enhance sensory and functional properties. For example, soy protein has been used in imitation cheese, soy milk, and whipped toppings (Oreopoulou & Tzia, 2007). Phytochemicals, such as polyphenols and carotenoids, have been related to health promoting effects including lowering cholesterol and lipid oxidation (O'Shea, Arendt, & Gallagher, 2012). Besides health promoting effects, the addition of these antioxidant compounds into food matrices can extend their shelf life and delay the formation of off-flavors of food products (Oreopoulou & Tzia, 2007, pp. 209–232). Vegetable oils in plant-derived waste can be used to derive sugar-based surfactants (e.g., alkyl polyglucosides) with low toxicity and good detergent properties compared to traditional surfactants derived from fossil oil (Foley, Beach, & Zimmerman, 2011). Therefore, more research is focusing on the extraction, purification, and production of valuable chemicals, materials, and biofuels from plant-derived wastes.

Animal feed Animal feed Animal feed, compost Compost, heat and electric power generation Animal feed, compost

High volumes from centralized processing facilities, rich in starch, fibers

Kim & Dale, 2004) Kim & Dale, 2004) Waldron, 2014) Moates et al., 2016; Waldron, 2014) 2016a; 2016a; 2016a; 2016a; (FAO, (FAO, (FAO, (FAO,

3.1. Fruits and vegetables

Corn stover Rice straw Rice husk Olive mill residue

Potato byproducts

Corn Rice

Olive

Potato

203.62 (worldwide) 731.34 (worldwide) 110 (worldwide) 30 (Mediterranean basin)

Mainly in the U.S., China Mainly in China, India, Indonesia, Bangladesh Mainly in Spain, Greece, Italy, Morocco Mainly in China, India, Germany, Russia, Ukraine, the U.S.

Fruit and vegetable wastes are generated by agriculture, post-harvest grading, and mass discarded by consumers in industrialized regions such as North America, Europe, and parts of Asia. In developing regions, such as North Africa, Sub-Saharan Africa, South and Southeast Asia, and Latin America, fruit and vegetable wastes are mainly produced at the front end of the food supply chain such as harvesting and storage (FAO, 2011). It is estimated that 10–20% of fruit and vegetable wastes are generated during agriculture and post-harvest stages, and 15–20% of wastes are generated due to processing (FAO, 2011). Currently, most of the fruit and vegetable wastes are used as low-value animal feed and only a small portion of the wastes is used as feedstock for the extraction of phytochemicals, soluble and insoluble dietary fibers (Galanakis, 2012). 3.1.1. Citrus Citrus fruits are among the most abundant crops worldwide, with about 121 million tonnes produced annually (FAO, 2015). The industrial utilization of these citrus fruits for juice production results in large quantities of waste. Citrus wastes include peels, pulps and seeds, which represent about 50% of the fruit weight (Bampidis & Robinson, 2006). Citrus wastes are good sources for sugars, oils, polyphenols, enzymes, vitamins, and minerals. Therefore, there is huge potential of the integrated utilization of citrus wastes to produce multiple highvalue products (Ángel Siles López, Li, & Thompson, 2010). Pourbafrani et al. developed an integrated process using citrus waste to obtain multiple value-added products: D-limonene, ethanol, pectin, and biogas (Fig. 2A) (Pourbafrani, Forgács, Horváth, Niklasson, & Taherzadeh, 2010). In this process, the citrus waste was pretreated with dilute-acid at high pressure to hydrolyze the biomass, followed by explosive pressure reduction resulting in the release of D-limonene (8.9 L/ton) from the hydrolysates. The remained hydrolysates were then centrifuged, where the liquid part was used to obtain pectin (38.8 kg/ton) by solvent precipitation and ethanol (39.6 L/ton) by

4.3 (the U.S., Canada)

(FAO, 2016a; Kim & Dale, 2004) (FAO, 2016a; Onipe, Jideani, & Beswa, 2015)

High volumes worldwide, rich in fibers High volumes worldwide, rich in polyphenols, fibers High volumes worldwide, rich in fibers High volumes worldwide, rich in fibers High volumes worldwide, rich in fibers High volumes, rich in phenols, fibers Wheat straw Wheat bran Wheat

354.35 (worldwide) 90 (worldwide)

Mainly in China, India, Europe, Egypt Mainly in China, India, Russia, Europe Tomato pomace Tomato

4 (Europe)

Apple pomace Apple

3.0–4.2 (worldwide)

Animal feed, compost, polyphenols and oils extraction Animal feed, compost, pectin extraction Animal feed Grape

5.0–9.0 (worldwide)

Animal feed Animal feed

(FAO, 2016a; Moates et al., 2016; Waldron, 2014)

(Đilas et al., 2009; FAO, 2016a; Moates et al., 2016)

(Đilas, Čanadanović-Brunet, & Ćetković, 2009; FAO, 2016a; Moates, Sweet, Bygrave, & Waldron, 2016) (Đilas et al., 2009; FAO, 2016a; Moates et al., 2016)

High volumes, rich in essential oils, pectin, polyphenols, fibers High volumes produced worldwide, rich in polyphenols, oils, fibers High volumes, widespread production, rich in polyphenols, pectin, fibers High volumes, rich source of lycopene Animal feed, pectin extraction

Mainly in Brazil, the U.S., Mediterranean basin Mainly in Europe, the U.S., Australia, China Mainly in Europe, China, the U.S. Citrus peels, pulp, seeds Grape pomace Citrus

15.6 (worldwide)

Reason for selection Current management Geographical location Estimated production volume (million tonnes/year) Waste stream Crop

Table 2 Plant-derived waste streams appropriate for biorefinery process.

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Table 3 Chemical compositions in plant-derived waste. Plant-derived waste

Plant structure (% of dry matter)

Plant reserves (% of dry matter)

References

Cellulose

Hemicellulose

Lignin

Pectin

Protein

Fat

Sugar

Mandarin peel

22.5

6.0

8.6

16.0

7.5

1.6

10.1

Orange peel Citrus pulp Grapefruit peel Grape pomace Apple pomace Pear pomace Carrot pomace Wheat straw Wheat bran Brewer's spent grain Corn stover Soybean hull Beet pulp

37.08 20.9 26.57 9.2–14.5 43.6 34.5 51.6 32.0–49.0 13.0 21.73

11.04 0.4 5.60 4.0–10.3 24.4 18.6 12.3 23.0–39.0 35.5 19.27

7.52 0.31 11.6 11.6–17.2 20.4 59.3 32.2 5.0–19.0 2.84 19.40

23.02

9.06 6.7 12.5 7.0–14.5

4.00

9.57 79.0 8.1

4.50 (Flavonoid) 1.56 (Phenolics)

70.3

6.51 (Phenolics) 0.65 (Phenolics)

(Marín, Soler-Rivas, Benavente-García, Castillo, & Pérez-Alvarez, 2007) (Pfaltzgraff et al., 2013) (Miron, Yosef, & Ben-Ghedalia, 2001) (Marín et al., 2007) (Zheng et al., 2012) (Nawirska & Kwaśniewska, 2005) (Nawirska & Kwaśniewska, 2005) (Nawirska & Kwaśniewska, 2005) (Mielenz, Bardsley, & Wyman, 2009) (Miron et al., 2001) (Meneses, Martins, Teixeira, & Mussatto, 2013)

31.0–41.0 51.2 29.7

20.0–34.0 15.9 12.9

16.0–23.0 1.48 3.35

81.0 76.2

4.19 (Phenolics) 4.87 (Phenolics)

(Mielenz et al., 2009) (Miron et al., 2001) (Miron et al., 2001)

8.5 5.4–5.7 11.7 13.4 3.88

0.9 2.0–6.0 17.3 24.69

0.5

Other valuable compounds

0.2

4.0–9.0 10.1 8.77

consists about 50% skin, 25% seed, and 25% stem (Martinez et al., 2016). Grape pomace contains large amounts of polyphenols, lipid, proteins, fiber, and minerals (Zheng et al., 2012). Da Porto et al. applied ultrasound-assisted extraction method to obtain oils (14%, w/w) and polyphenols (105.2 mg GAE/g flour) from grape pomace (Da Porto, Porretto, & Decorti, 2013). Martinez et al. applied the supercritical CO2 extraction to recover polyphenols (2.7 g/100 g dry biomass) in red grape pomace. The resulting dephenolised residue was then sent to anaerobic digestion, producing a liquid stream rich in volatile fatty acids (20 g/L). This liquid stream was then used as a substrate to produce polyhydroxyalkanoates (accumulated up to 63% of the cells dry weight when fed with 40% acidic effluent) by Cupriavidus necator. Finally, all the solid leftover obtained before underwent an anaerobic digestion to obtain a methane-rich biogas (113 mL/g fed volatile solids) (Fig. 2C) (Martinez et al., 2016). The biorefinery concept was also applied on wine lees, which are the residues obtained at the bottom of tanks after wine fermentation, during storage, or after centrifuge or filtration of wine (Pérez-Serradilla & De Castro, 2011). In the study of Dimou et al. wine lees were centrifuged, and the liquid part was distilled to get ethanol. For the solid part, organosolv extraction was applied to recover antioxidants, and the residual solid was treated with HCl to solubilize tartaric acid. Finally, the solid residue after tartaric acid extraction and the ethanol free liquid part were combined as the feedstock for enzymatic lysis of yeast cells to generate nutrients for further poly(3-hydroxybutyrate) production by aerobic fermentation (Fig. 2D) (Dimou et al., 2015).

fermentation, respectively. The stillage from the distillation of ethanol and the remaining solids were combined to produce biogas (45 m3/ton) via anaerobic digestion. Similarly, Boluda-Aguilar and López-Gómez applied steam-explosion on lemon peels to get essential oils, followed by sequential and simultaneous hydrolysis and fermentation to obtain galacturonic acid and ethanol (Boluda-Aguilar & López-Gómez, 2013). The results showed that more than 60 L ethanol was produced from 1000 kg of fresh lemon peels based on the designed process. Another study conducted by Balu et al. used microwave for obtaining four major fractions (cellulose, pectin, bio-oils, and sugars) from orange peels (Balu et al., 2012). Specifically, orange peels were treated with microwave, followed by filtration to obtain solid cellulose (9%, DW). The filtrate was washed by ethanol to recover pectin (8%, DW). The bio-oils (7%, DW) were extracted from the liquid residue by liquidliquid extraction with ethyl acetate, and all solvents used in this procedure were collected and then evaporated to get sugars (26%, DW). Boukroufa et al. applied microwave, ultrasound, and the recycled “in situ” water of orange peels to obtain essential oils (346 g/100 kg), polyphenols (11.7 g/100 kg), and pectin (4.8 kg/100 kg) from orange peels. This biorefinery process reduced time, energy, and waste water generation (Fig. 2B) (Boukroufa, Boutekedjiret, Petigny, Rakotomanomana, & Chemat, 2015). Similar technology was also applied on pomelo, orange, and lemon peels to get essential oils and pectin (Chen, Hu, Yao, & Liang, 2016; Fidalgo et al., 2016). 3.1.2. Grape Grape is the world's second largest fruit crop with an annual production of more than 60 million tonnes. About 80% of grapes are used to produce wine, which is one of the most important alcoholic beverages in the world, with an increasing demand to 25 billion liters (Oreopoulou & Tzia, 2007). Winery wastes can be divided into four categories: grape stalks, grape pomace (marc), wine lees, and waste water. Many components such as dietary fibers, polyphenols, grape seed oil, and tartrates can be separated from winery waste (Oreopoulou & Tzia, 2007). Grape stalks as the wastes of vineyards are composed of high concentrations of lignin, cellulose, and hemicellulose which, if fractionated, can be an excellent renewable carbon source. Amendola et al. applied autohydrolysis pretreatment on grape stalks followed by a noncatalysed ethanol organosolv extraction of the solid residue (Amendola et al., 2012). Ethanol was used to precipitate hemicellulose in the autohydrolysis liquor, while acid was used to precipitate lignin from both autohydrolysis liquor and organosolv liquid. Grape pomace, the main solid waste generated during winemaking,

3.1.3. Apple The world production of apples was over 70 million tonnes in 2015 (Yates et al., 2017). Approximately 25–30% of the apples are used to produce juice, and the leftover of juice extraction called apple pomace is normally used as animal feed or compost (Dhillon, Kaur, & Brar, 2013; Yates et al., 2017). Since apple pomace contains valuable compounds such as carbohydrates, pectin, and polyphenols, there is an increasing global trend towards the efficient utilization of apple pomace. Recently, Yates et al. used apple pomace to produce various value-added compounds including sugars, polyphenols, pectin, and biomaterials which can be used as biocompatible scaffolds in tissue engineering (Yates et al., 2017). In the integrated process, apple pomace was first extracted by water to get polyphenols and sugars (2%, DW), followed by citric acid treatment, and then precipitated with ethanol to get pectin (10%, DW). The solid residue was further used to produce the biocompatible material for osteoblasts and chondrocytes. 124

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Fig. 2. The flow diagrams of value-added products produced from citrus waste (Pourbafrani et al., 2010), orange peels (Boukroufa et al., 2015), grape pomace (Martinez et al., 2016), and wine lees (Dimou et al., 2015).

hemicellulose to monomer and oligomer sugars.

3.1.4. Tomato Millions tonnes of tomato are processed every year to produce tomato juice, paste, and concentrate (Kehili et al., 2016). During processing, about 4% of the total processed tomatoes are produced as byproducts, including tomato peels and seeds, which are rich in sugars, polyphenols, proteins, oils, and organic acids (Del Valle, Cámara, & Torija, 2006). Kehili et al. developed a biorefinery cascade process for producing value-added products from tomato byproducts (Kehili et al., 2016). The authors used supercritical CO2 technology to extract carotenoids inside the oil fraction of tomato peels and seeds. The residue obtained after supercritical CO2 extraction was used to extract protein by alkali solubilization and acid precipitation. After that, the protein free residue was treated by hot water to hydrolyze cellulose and

3.2. Cereals Cereal grains, the most important sources of calories for the majority of the world's population, have been the primary food source for human since thousands of years ago (ElMekawy, Diels, De Wever, & Pant, 2013). The cereal production in 2012 reached 2087 million tonnes. For each kg of cereal grains harvested, about 1–1.5 kg of cobs, straws, and other residues are produced, leading to large amounts of cereal waste (ElMekawy et al., 2013). Among different cereals, wheat is the major crop in Europe, North America and Oceania, and Industrialized Asia. However, in low-income regions such as densely 125

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liquefaction followed by fermentation was conducted on the solid leftovers to produce bioethanol (0.41 g/g glucose). The dark fermentation of the hydrolysate was conducted to obtain biohydrogen (178 mL/g sugars). Finally, the effluents from both bioethanol and biohydrogen processes were combined together to produce biogas (0.32–0.38 m3/kg volatile solids) (Fig. 3A). In another study, wheat straw was pretreated with 15% NH4OH at 65 °C for 15 h, removing 48% of the original lignin, leaving 98%, 83%, and 78% of the original glucan, xylan, and arabinan, respectively. After that, the solid was treated with hemicellulase to get a liquid fraction rich in xylose and a solid fraction rich in cellulose. Candida mogii and Phaffia rhodozyma were then inoculated into the liquid part to produce xylitol (0.51 g/g xylose consumed) and astaxanthin (1.98 mg/g total sugar consumed), respectively; the solid part was used to conduct simultaneous saccharification and fermentation process to produce ethanol (57 g/L) (Zhang

populated region of South and Southeast Asia, rice is the major crop (FAO, 2011). 3.2.1. Wheat About 21% of the global food supply depends on wheat, and the production is still increasing to meet the growing demand (Ortiz et al., 2008). During harvesting, wastes such as wheat straw are left on the field. Other byproducts such as wheat bran, germ, and parts of the endosperm are generated during wheat milling (Dorado et al., 2009). Wheat straw mainly consists of cellulose, hemicellulose, and lignin. Kaparaju et al. used wheat straw to produce bioethanol, biohydrogen, and methane based on the biorefinery concept (Kaparaju, Serrano, Thomsen, Kongjan, & Angelidaki, 2009). In particular, wheat straw was pretreated hydrothermally to get a liquid fraction (hydrolysate) rich in hemicellulose and a solid fraction rich in cellulose. After that,

Fig. 3. The flow diagrams of value-added products produced from wheat straw (Kaparaju et al., 2009), rice and Arundo donax (Barana et al., 2016), olive mill waste (Schievano et al., 2015), potato peels and mash (Chintagunta et al., 2016).

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then treated with NaOH and further fractioned into hemicellulose B, cellulose nanocrystals, lignin, and silica (Fig. 3B).

& Nghiem, 2014). Celiktas, Kirsch, and Smirnova (2014) designed a cascade process to utilize wheat bran (Celiktas et al., 2014). In this study, 1.98 g/L protein was extracted from wheat bran in a fixed bed reactor, followed by hemicellulose generation using hot water hydrolysis. The solid residue rich in cellulose was then treated by enzyme to obtain soluble sugars, resulting in the final solid rich in lignin.

3.3. Oil crops Among all oil crops cultivated around the world, olives are abundant in West and Central Asia, North America; rapeseeds and sunflower seeds are the major crops consumed in Europe; soybeans are the dominating crops supplied in North America, Oceania, and Industrialized Asia (FAO, 2011). As to the oil crop wastes, about 6–20% of the wastes are generated during agricultural production, followed by post-harvest and processing processes (FAO, 2011). For example, after oil extraction, a large amount of oil pressed cakes remains, which contain substantial quantities of protein, minerals, residual oil, and other nutrients. These compounds can be recovered and reused in the food industry (Tamer & Çopur, 2014).

3.2.2. Corn In 2015, the world production of corn was estimated at 1026 million tonnes, with the U.S. China, and Brazil the thee leading countries in production (FAO, 2016b). The main components in corn are starch, protein, fiber, and oil; therefore, corn is traditionally used as human food or animal feed. During the last ten years, corn has been increasingly used as feedstock to produce ethanol, a renewable biofuel. In the U.S. 38.4% and 37.6% of total corn produced in 2015 were used for ethanol production and as animal feed, respectively (USDA, 2017). Dry grind fermentation is the most popular process used for converting corn to ethanol. During dry grind fermentation, corn is milled, saccharified, and fermented to ethanol using yeast (Fig. 1A). The unfermentable residues, including protein, fiber, and oil are collected and dried as distiller's dried grains with solubles (DDGS). Usually, processing each kg of corn results in 0.37–0.4 L of ethanol and 0.31–0.32 kg of DDGS (H. Huang, Liu, Singh, Danao, & Eckhoff, 2012). DDGS has been long-time used as a source of carbohydrates, protein, and oil for animal feed. However, the increase in DDGS production is expected to drive its value down, thus, there is a great interest to use DDGS as a starting raw material for the production of commodities, platform molecules or specialty chemicals with concomitant economic benefits. One approach is to convert fibers, mainly cellulose and hemicellulose, to sugars for ethanol production. Bals et al. applied ammonia fiber expansion (AFEX) process to pretreat DDGS, followed by enzymatic hydrolysis to produce sugars for ethanol fermentation (Bals, Dale, & Balan, 2006). At the optimal condition, 190 g glucose could be produced from each kg of dry DDGS; meanwhile producing a protein enriched residue as advanced animal feed. In another study, DDGS was processed with dilute acid hydrolysis in a percolation reactor to selectively extract high purity pentoses (Fonseca, Lupitskyy, Timmons, Gupta, & Satyavolu, 2014). After process optimization, a 94% yield of pentoses on the DDGS was obtained, resulting in arabinose-rich and xylose-rich streams using a two-stage hydrolysis. After the pentose extraction, the remaining hexoses could be further processed to produce glucose for ethanol fermentation. This process could be coupled into an integrated biorefinery process of DDGS for production of biofuels, biochemical intermediates and other bioproducts.

3.3.1. Olive According to the report of International Olive Council, the olive oil production reached 2.9 million tonnes globally during 2012–2013 (FAO, 2014). The production of olive oil generates approximately four times more wastes than the commercial oil, which is a heavy burden for the industry as well as the environment. Among the wastes generated during olive oil production, olive mill waste is the one that has drawn attention to many researchers. Schievano et al. has proposed an integrated biorefinery concept for the utilization of olive mill waste to produce polyphenols, mono/polyunsaturated fatty acids, and biofuels (Schievano et al., 2015). The authors used the supercritical CO2 technology with ethanol as the cosolvent to extract polyphenols (10.9 g/kg), poly-unsaturated fatty acids (20 g/kg), and mono-unsaturated fatty acids (601 g/kg). The remaining dry solid leftovers were pyrolyzed and activated to get fuel and biochar (Fig. 3C). Furthermore, the same group applied KOH and CO2-activated biochar to treat contaminated water. As a result, 1 g activated sorbent material could absorb upward of 400 mg metals including Cd, Co, Cu, and Zn. Moreover, 75% of the adsorption ability reached in less than 5 min (Goldfarb et al., 2016). 3.3.2. Rapeseed The rapeseed plant is normally used for the production of vegetable oil for human consumption, animal feed, and biodiesel synthesis (López-Linares et al., 2014). Over 34 million hectares of rapeseed were cultivated globally (FAO, 2013). After rapeseed harvesting, rapeseed straw will be left on the field. In addition, after oil extraction and biodiesel production, other byproducts such as rapeseed cake and glycerol waste are generated. Luo et al. used rapeseed straw along with cake and glycerol obtained after biodiesel production to produce several biofuels (G. Luo et al., 2011). Particularly, rapeseed straw was pretreated with alkaline and subsequent steam. The solid phase was used for ethanol production by enzymatic hydrolysis and fermentation. After distillation of ethanol, the remaining stillage and hydrolysate together with rapeseed cake and glycerol were used for hydrogen and methane production by anaerobic fermentation or digestion, respectively. As a result, the energy recovery efficiency increased from 20% (conventional biodiesel process) to 60% (biorefinery process).

3.2.3. Rice Rice is an important staple crop for more than half of the world's population. The global production of rice is about 671 million tonnes per year (Barana, Salanti, Orlandi, Ali, & Zoia, 2016). Wastes such as rice straw and rice husk are generated during rice harvesting and processing, and both of them contain a large amount of cellulose, hemicellulose, and lignin, which can be good resources for the production of value-added chemicals and biofuels. Patrícia et al. reported a biorefinery framework using rice straw to produce various products (Patrícia et al., 2015). In particular, rice straw was first autohydrolyzed to get a hemicellulose-rich liquid, which can be further purified to oligosaccharides. The solid residue rich in cellulose and lignin was treated with ethanol to get the lignin-rich liquid and cellulose-rich solid. Due to the removal of lignin, the ethanol-treated solids have a 10% higher enzymatic digestibility compared to autohydrolyzed solid. Another integrated biorefinery process was developed to recover lignin, hemicellulose, silica, and cellulose nanocrystals from rice husk and Arundo donax (Barana et al., 2016). Specifically, rice husk and A. donax were leached with HCl, followed by vacuum filtration to separate liquid and solid parts. The liquid portion was concentrated and precipitated with ethanol to get hemicellulose A. The solid portion was

3.4. Roots and tubers As to the roots and tubers, potatoes are the major crops in Europe, North America, Oceania, and Industrialized Asia. The dominating wastes (20% of the total waste) in these areas are produced during agricultural period. This phenomenon mainly depends on the quality standards set by retailers during the grading of post-harvest crop. Cassava is dominated in Sub-Sahara Africa and Latin America, and wastes generated in these areas are mainly due to post-harvest handling and storage stages because fresh roots and tubers are perishable, thus 127

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the second refinery scheme, the solid residues resulting from xylitol, furfural, ethanol and poly(3-hydroxybutyrate) processes were used to produce heat and power using a cogeneration system. The simulation software Aspen Plus was used to simulate the whole process, and MATLAB was used to perform mathematical calculations. The results showed that the first biorefinery scheme had a net profit margin of 53%; while the second only had a net profit margin of 6%, which was mainly due to high capital investment of the cogeneration system. In another study, a techno-economic analysis was conducted for the utilization of BSG to produce xylitol, lactic acid, activated carbon, and phenolic acids for the Brazilian case (Mussatto et al., 2013). Four scenarios based on different levels of heat and mass integration of the biorefinery process were analyzed using Aspen Plus. The results showed that full mass integration and full energy integration was the best scenario with the highest economic margin at 62.3%. The biorefinery of crop residues (e.g. corn stover, wheat straw) and its techno-economic analysis were also investigated. Valdez-Vazquez and Sanchez designed a novel process for the biorefinery of wheat straw and conducted its techno-economic analysis (Valdez-Vazquez & Sanchez, 2017). The biorefinery based on mixed-culture processes included (i) conditioning of feedstock, (ii) hydrogen fermentation, (iii) acetone-butanol-ethanol (ABE) fermentation, (iv) methane digestion, and (v) electricity-steam cogeneration. Butanol was set as the main product in the process. The techno-economic analysis of the process was focused on the impact of residence times (8–120 h) and butanol titers (10–20 g/L) of the ABE fermentation on the production costs of butanol. The results showed that the production costs of butanol from wheat straw via the designed process were between $1.04/L to $1.27/L. Recently, Huang et al. designed a biorefinery process of lignocellulosic biomass for the coproduction of ethanol and 1,5-pentanediol to improve economics of lignocellulosic biofuels. The economic analysis showed that coproduction of 1,5-pentanediol and ethanol from lignocellulosic biomass had economic advantages compared with the ethanol-only lignocellulosic process at the current market price of 1,5pentanediol (K. Huang et al., 2017). It is worthy to mention that although these economic analyses showed promising economic feasibility of the designed biorefinery processes, we have to be cautious with the outcomes because of the inherent uncertainty of techno-economic analysis, especially in the early stage design (Stuart & El-Halwagi, 2012). Early stage technoeconomic analysis relies on the data from lab-scale experiments, publically available information, and factors (such as Lang factor) to estimate total capital and operating costs, all of which can lead to a large uncertainty of the economic analysis.

making these products easily to be damaged during or after harvest activities, especially in the humid and warm regions (FAO, 2011). 3.4.1. Potato Processing of potatoes is conducted mainly for chips, which generates solid wastes including peels or cull potatoes. These solid wastes are usually composed of starch, fiber, polyphenols, and minerals (Tamer & Çopur, 2014). The nutrients rich in potato wastes can be valorized in an integrated process. For example, Chintagunta et al. incubated potato peels and mash with Aspergillus niger and Saccharomyces cerevisiae to obtain ethanol (Chintagunta, Jacob, & Banerjee, 2016). The residue obtained after ethanol production was inoculated with seven different microorganisms to produce biomanure. As a result, biomanure inoculated with Anabaena variabilis could enrich nitrogen, phosphorous, and potassium by nearly 7.7, 21.7, and 15.0 folds than that of the initial concentration (Fig. 3D). 3.4.2. Cassava Cassava is mainly used to produce gari by microorganisms, and a lot of wastes such as peels and pulps are generated during the processing. The cassava wastes contain various valuable compounds such as cellulose (25%), hemicellulose (7%), and protein (5%), which can be readily converted to other value-added products (Aderemi & Nworgu, 2007). Moshi et al. applied the biorefinery concept to fully utilize cassava peels by producing ethanol through simultaneous saccharification and fermentation after alkali pretreatment (Moshi et al., 2015). The residue obtained after ethanol production was used to produce methane by anaerobic digestion. The result showed that the integrated processing to produce ethanol together with methane led to 1.2–1.3 fold fuel energy yield compared to only methane production, and 3–4 fold compared to only ethanol production. 4. Techno-economic, environmental, and social assessment of plant-derived waste biorefinery processes 4.1. Techno-economic assessment Techno-economic assessment is normally warranted before a new process is up-scaled for commercialization. The outcomes of economic assessments can be used as a guideline for developing and modifying capital spending plans, estimating operating and maintenance costs, predicting profitability, and directing future research and development efforts of the process. Techno-economic analysis is especially important to the development of the biorefinery processes of food waste, because many biorefinery technologies are complex than that in traditional processes and require relatively high-capital investments. In recent years techno-economic studies have been performed on the biorefinery of plant-based food processing wastes, such as palm residues (Vaskan, Pachón, & Gnansounou, 2017), olive stones (Hernández, Romero-García, Dávila, Castro, & Cardona, 2014), and brewer's spent grain (BSG) (Mussatto, Moncada, Roberto, & Cardona, 2013). Vaskan et al. in Brazil assessed the economic feasibility of biorefinery of palm empty fruit bunches (EFB), which are the abundant lignocellulosic residues from the palm oil industry (Vaskan et al., 2017). The process consisted of pretreatment of EFB using dilute acid or hot water, separation of soluble pentose to produce animal feed, hydrolysis and fermentation to produce ethanol, and combustion of lignin to produce heat and power. The results showed that the dilute acid pretreatment was more efficient and economical than the hot water pretreatment, but may lead to severe environmental disadvantages due to the large consumption of chemicals. In the study of Hernández et al., techno-economic analysis was conducted for a biorefinery process of olive stones, a byproduct from olive oil production (Hernández et al., 2014) . Techno-economic assessment of two biorefinery schemes of olive stones were presented. The first biorefinery scheme was designed to produce xylitol, furfural, ethanol, and poly(3-hydroxybutyrate); in

4.2. Environmental assessment Environmental sustainability is an important issue when designing a biorefinery process of plant-derived waste. Several criteria such as energy balances, greenhouse gas (GHG) emission savings, air and water pollution, water resources, soil stress, and biodiversity need to be considered thoroughly before implementation of the plant-derived waste biorefinery (Nizami, Mohanakrishna, Mishra, & Pant, 2016). Life cycle assessment (LCA) is a normally used technique to compile and evaluate the environmental impacts of a product or process considering all inputs and outputs throughout its life cycle, which includes biomass production, storage, transportation, utilization, maintenance, disposal or recycling (Nizami et al., 2017). Luo et al. have conducted an environmental assessment of corn stover biorefinery to produce ethanol, succinic acid, acetic acid, and electricity (L. Luo, van der Voet, & Huppes, 2010). In their study, lowvalue ethanol and high-value succinic acid were selected as the main products in a corn stover biorefinery. Acetic acid was a co-product from succinic acid fermentation. Steam and electricity were generated from lignin and other residues. As to the result of eco-efficiency analysis, the author found that the GHG benefits of this biorefinery were remarkable 128

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of biorefinery is one of the advantages over petroleum refinery and also a priority to be considered. For example, according to the policy targeting the European Union, a minimum of 60% GHG saving is required for biofuels production with respect to fossil fuels by 2020 (EU, 2009). Another issue is that the biorefinery plants are normally capital intensive and high-risk plants; therefore, a long-term stable policy for the industry or the investors is important. Social aspect is another important issue to be considered. According to a survey responded by researchers, policy makers, and industrial actors regarding the biorefinery topics, they suggested that policies should be cautious about the potential significant threat to the social acceptance and reputation of biorefinery concepts (Peck, Bennett, Bissett-Amess, Lenhart, & Mozaffarian, 2009). Furthermore, policies should be designed to support a wide range of the development of waste biorefinery, including basic research, collaborative R&D, and facilitation of commercialization of research results.

compared to both the ethanol plant and gasoline refinery mainly because a large amount of CO2 emitted by ethanol fermentation was fixed by acid fermentation. In addition, the eco-efficiency of biorefinery was better than the gasoline refinery in the category of abiotic depletion potential (ADP) and ozone layer depletion potential (ODP). Although the study of such complex integrated biorefinery system is rare, several preliminary analyses suggested that the lignocellulosic waste biorefinery and recycling process can reduce the global warming potential (GWP) by saving GHG emissions, alleviate the cost of landfilling and its associated environmental impact (Nizami et al., 2017; Ouda et al., 2016; Singh et al., 2010). It should be noted that several limitations and challenges of LCA methodologies such as rigid system boundaries, variations in statistical methods, lack of update and accurate data availability, and local conditions and environment are existed, which all affect the accuracy of the assessment (Nizami et al., 2017). Since there is few of plant-derived waste biorefinery system commercially available, uncertainties and parameter sensitivities in LCA studies should be handled carefully. Detailed and various parameters including waste characterization, energy inputs and CO2 emissions of selected technologies, final products selection, local conditions and practices should be considered thoroughly. Parametric LCA might be a useful tool in this situation (Rathore, Nizami, Singh, & Pant, 2016).

4.5. The implementation of biorefinery Most common implementations of biorefinery are focused on using crops to produce first generation biofuels. Driven by tax credits and the Renewable Fuel Standards started in 2005 and extended in 2007, more than 200 operating plants existed in the U.S. use corn as feedstock to produce ethanol (Peplow, 2014). Yet corn ethanol offers only modest savings in GHG emissions compared with petroleum (Van Noorden, 2013). In addition, the corn cultivation could contribute to the increasing food price due to land used for corn-ethanol production would otherwise be used for food. Therefore, according to the Renewable Fuels Association, several biorefineries in Iowa, Kentucky, California, and Kansas states of the U.S. applied plant-based wastes such as corn stover, stalks, stem, and brewery wastes to produce ethanol (RFA, 2017). In addition to biofuels production, some value-added chemical compounds production has also been targeted recently. For example, the largest scale bio-based succinic acid plant of BioAmber was started in Canada in 2015 (Schieb & Philp, 2014). However, as to the integrated biorefinery of crop waste to produce various value-added chemicals and biofuels in a cascade way, although often discussed, have not yet been put into implementation.

4.3. Social assessment The social analysis for the biorefinery is sometimes ignored but important. The research on social perspectives can touch on many potentially interlinked issues which may make the process complex. Some commonly addressed social issues include land ownership rights, local stewardship of common property resources, and labor rights (Nizami et al., 2016). For example, in the case of Kenya, Mumias Sugar Company Ltd. and the state-run Tana Athi River Development Authority planned to utilize 16,000 ha for a sugarcane plantation for the production of biofuels. This project would affect thousands of farmers who currently utilized this land, and the human rights groups complained this project for its potential violations of farmers’ rights and the lack of those farmers inclusion in the early stage of this project (Elbehri, Segerstedt, & Liu, 2013). As to the labor or employment effects, there are some positive effects including new employment opportunities and improving health by reducing indoor air pollution, while some down sides, such as potential negating employment benefits for local communities because the foreign investors may bring their own manpower, should be thoroughly evaluated to keep social sustainability (Elbehri et al., 2013; Nizami et al., 2016).

5. Future perspectives For the plant-derived waste biorefinery design, multiple steps and aspects need to be evaluated carefully before the integrated process being up-scaled from lab- or plot-scale to a commercial level. Although relevant research using food wastes for the biorefinery process design is increasing recently, there are still some issues such as feedstocks selection and supply, transportation, economic, environmental, and social evaluation that need to be considered thoroughly in the future. First, most of the present researches focus on the cereal or oil crop wastes, while others such as fruit and vegetable wastes are lacking of attentions. Fruit and vegetable wastes such as grape pomace and citrus wastes normally contain value-added compounds such as polyphenols and essential oils in addition to cellulose, hemicellulose, and lignin. Those high value compounds can be applied in food, cosmetic, and pharmacy industries, thus are worthy of recovery. In order to better utilize those waste feedstocks, it is important to develop the plant-derived waste compositional database at first place. Those data collected from different types of plant-derived wastes can provide guidelines for the further process design and final products selection. Furthermore, many types of food wastes are generated seasonally and difficult to store, which present challenges of the stable feedstock supply to the biorefinery plant during the whole year. Thus, the capacity of a process to use different types of feedstocks will be important to improve the economic feasibility of food waste valorization. Second, given the large amount of plant-derived wastes generated from agriculture and processing along with the low biomass energy

4.4. Policies The biorefinery of plant-derived waste research and development is still in an early stage of evolution; therefore, the instruction and support of relevant policies could play a significant role in this area. Currently, the most relevant policy instruments for the stimulation of biorefineries are the carbon pricing and mandatory quota (Hellsmark & Söderholm, 2017). Carbon pricing has been used in various regions including Europe and the U.S. For instance, Sweden has carbon tax exemptions for fuels produced from renewable resources, such as food waste. Mandatory quota is an alternative policy instrument that protects a space for renewable chemicals and biofuels which currently cannot directly compete with the fossil-derived products. For instance, the Environment Protection Agency in the U.S. is authorized to set annual quotas dictating the percentage of renewable fuels blended into fossil fuels (EPA, 2018). However, the current policy instruments have not been sufficient enough for stimulating commercialization of large-scale advanced biorefineries, which calls for the development of new innovative policies. Among the criteria of developing innovative policies, sustainability 129

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References

density, transportation of these wastes to a central position could be expensive. One possible solution is to develop small localized biorefineries instead of traditional large production capacity industrial plants. This may also reduce the possibility of fast deterioration of waste with high water content. However, the petrochemicals trend of integrated process at massive facilities makes it hard for these smallscale biorefineries to compete with (Schieb & Philp, 2014). Overall, the food waste logistics and transportation challenges need to be addressed in the future. Third, as to the final products selection, although biofuels may be the mainstay, some bio-based chemicals are also worthy of focusing. In fact, there is a significant government and industry shift from single biofuel production to various bio-based chemicals production. For example, the 2014 U.S. Farm Bill changed the title of USDA 9003 program from the “Biorefinery Assistance Program” to the “Biorefinery, Renewable Chemical, and Biobased Product Manufacturing Program” (Schieb & Philp, 2014). However, compared to the single feedstock and single product models, the integrated biorefinery models are not mature and still need to be developed. Forth, the complexity of biorefinery process of plant-derived waste calls for comprehensive economic, environmental, and social evaluations before it can be up-scaled to a commercial level. However, due to limited or old databases, variable nature of factors in simulation models, and other background methodological choices, the relevant assessment is still in its infancy (Rathore et al., 2016). Therefore, more fundamental and practical research in these aspects will be needed. Moreover, most studies purely focus on the economic or environmental aspects of biorefinery process, while social issues are often ignored and may need to be addressed more in the future. Last but not least, most of the studies and implementation of biorefinery are focused on the U.S. or Europe (Nizami et al., 2017; Ouda et al., 2016). As to some developing countries such as China and India, the relevant research is rare even with high amount of plant-derived wastes such as cereal crop wastes generated annually. Simply adopting the agriculture processes, conversion technologies, and policies from the U.S. or Europe is unwise; therefore, more specific studies targeting particular countries are needed in the future.

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6. Conclusions The integrated processing for the utilization of plant-derived waste satisfies the need to minimize waste generation, reduce energy consumption and increase sustainability of the food industry. Innovative technologies have been investigated to process wastes to various valueadded products. However, when selecting a biorefinery system, the challenges such as finding appropriate products from biomass feedstocks and processing pathways in order to achieve both profitability and reduce environmental impact still remain. Therefore, detailed techno-economic, environmental, and social assessments of plant-derived waste biorefinery are required in the future. The concept of biorefinery is derived from the petroleum industry. It took more than 150 years for the mature development of the petroleum refinery, so we believe the more recent plant-derived waste biorefinery concept will also need time to develop.

Acknowledgements This project is supported by the Virginia Agriculture Experiment State and the Hatch Program of the National Institute of Food and Agriculture (NIFA), the U.S. Department of Agriculture (VA-160079). We want to acknowledge the Department of Food Science and Technology at Virginia Tech for providing the Departmental Assistantship.

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