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Agri-Food Waste Reduction and Utilization: A Sustainability Perspective
Akinbode A. Adedeji1,*
Published in Journal of the ASABE 65(2): 471-479 (doi: 10.13031/ja.14797). Copyright 2022 American Society of Agricultural and Biological Engineers.
1 Department of Biosystems and Agricultural Engineering, University of Kentucky, Lexington, Kentucky, USA.
Submitted for review on 9 August 2021 as manuscript number PRS 14797; approved for publication as a Review Article and as part of the Circular Food and Agricultural Systems Collection by the Processing Systems Community of ASABE on 8 February 2022.
Highlights
Abstract. As the human population edges closer to nine billion, we must explore how we can sustainably use Earth’s limited resources. Current agricultural production and food processing create significant amounts of wastes that have drastic effects on the environment, on the cost of production, and on human health and well-being. About a third of these wastes are produced domestically, as well as from agricultural production and food processing, and it often constitutes a disposal problem, although it contains many carbon-based materials (proteins, lipids, carbohydrates, micronutrients, bioactive compounds, and dietary fibers) that can be converted into useful value-added products. Converting these wastes into useful products is important because of the impact it has on the environment, including energy consumption, water usage, and the amount of carbon it releases when discarded. The long-term goal is to ensure that all materials from agri-food production and processing are turned into valuable products based on the principle of upcycling and circular bioeconomies. This short review presents succinct information on where food and agricultural wastes and by-products are generated, it summarizes recent advances in waste reduction and value-added utilization, including the need for behavioral changes and improvements in food labeling, and it presents innovations, limitations, and future prospects for circular food systems that focus on total conversion of food and agricultural wastes to value-added products.
Significant quantities of agricultural and food wastes are produced daily through production losses, post-harvest damage, human activities, and processing activities that often create environmental, social, and economic concerns (Torres-León et al., 2018). It is estimated that about a third to half of global food production is wasted, which is roughly 1.3 billion tons of food per year (Freier, 2019; Gustavsson et al., 2011; Jurgilevich et al., 2016). More food is wasted per capita in developed countries than in developing countries, with the latter seeing more losses in the post-harvest and processing stages of the supply chain, while developed countries see more loses at the consumer level (as much as 56% of global food waste) (FAO, 2011; Ishangulyyev et al., 2019). Consumer demand for spotless and immaculate fruits and vegetables means that produce is often discarded that could otherwise serve as food. This disposal creates environmental problems. It also means that farmers have to grow more to achieve the percentage of perfect produce needed, using enormous amounts of energy and water, which increases the carbon footprint of food production. Agricultural production is responsible for over two-thirds of global food waste (Otles et al., 2015; Despoudi et al., 2021), and most of this waste ends up in waterbodies and landfills. In Europe, more than 50% of municipal solid waste is food waste (Ananno et al., 2021).
This widespread waste indicates that interventions to upcycle and reduce waste are needed throughout the agri-food supply chain. Upcycling of food waste has an estimated global value of $700 billion annually (Ellen MacArthur Foundation, 2019). The goal is to convert the take-make-dispose model of a linear economy into the make-use-recycle model of a circular economy (fig. 1). For example, in the brewing industry, spent grains rich in proteins, fiber, cellulosic and hemicellulosic materials that were previously considered waste are now being converted into sources of nutrients in human food applications (Adedeji and Woomer, 2020; Woomer and Adedeji, 2019).
The environmental impact of agri-food waste disposal, the cost of transporting waste from the point of production to the disposal location, and the loss of useful nutrients in waste are current topics of research. Useful carbon-based materials are wasted due to the lack of cost-effective methods that could convert them into value-added products; as a result, agri-food waste accounts for about 7% of global greenhouse gas (GHG) emissions (Reisch et al., 2021; Wieben, 2017). In addition, much of the discarded produce is due to reduced cosmetic value caused by damage during the supply chain. Such losses occur during transport, and much of the damaged produce is wasted, along with the scarce resources, including water and energy, used in production. Several studies have explored how these undervalued materials can be converted to useful products (Castro et al., 2020; Samborska et al., 2019).
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| Figure 2. The three phases of a circular economy in an agri-food system (Jurgilevich et al., 2016). |
Meeting the growing human need for essential nutrients can be achieved either by cultivating more land, which is finite in quantity, or by establishing a circular economy (fig. 2) in which nutrients and waste matter from agricultural production and food processing are converted into new products that serve human needs. Most technologies currently being developed are aimed at attaining near-zero waste from food production and processing. This article profiles the available research on where food and agricultural wastes and by-products are generated, it summarizes some of the recent advances in waste reduction and value-added utilization, and it highlights future prospects for achieving a circular economy in agri-food production and processing.
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| Figure 1. A circular economy for recycling waste versus a linear economy (Boz and Robinson, 2021). |
Value Addition from Fruit and Vegetable Waste
Fruits and vegetables are major sources of energy, micronutrients, and dietary fiber. However, almost 40% of fruits and vegetables are discarded after production because of the consumer demand for cosmetically perfect produce (Freier, 2019). Other wastes include peels, seeds, pulp, trimmings, and bagasse. Groceries stores prefer unblemished fruits and vegetables because even a minor imperfection can make a good product unsellable. An innovative idea for reducing this waste is creating an alternative market for produce with less than perfect aesthetic appeal. This approach, suggested by researchers at the University of Houston and the University of Illinois, is called anthropomorphism, which means humanizing the imperfect but otherwise healthy produce so that it has a psychological appeal to potential consumers (Koo et al., 2019; Patrick et al., 2019). The results have shown that simple messages encouraging consumption of produce with a few blemishes can increase consumer acceptance and reduce waste.
A significant amount of waste is created in juice production (table 1). For example, apple pomace is a major waste product of apple juice production. Pomace is rich in fiber and polyphenols (Masli et al., 2018), and much effort has been made to convert apple pomace into value-added products. Masli et al. (2018) added dried apple pomace to cornstarch for making extruded food products. They reported an improvement in the overall quality of the extrudate, including better expansion for improved texture and higher dietary fiber content. Another approach to reducing waste in juice production is to retain more of the nutrients instead of filtering them out. Samborska et al. (2019) used an ultrafiltration method to turn the cloudy juice stream, which is normally treated as waste, into a low-sugar juice with all the health-promoting properties retained. Castro et al. (2020) converted orange albedo, which is otherwise discarded, into flour by drying and used it as a substitute for wheat flour in cookies that were rich in dietary fiber and phenolic compounds.
Mango is considered a very important fruit because it combines a unique flavor, pleasant fragrance, and appealing color with a rich nutritional profile (Torres-León et al., 2018). The waste from mango processing includes peels (13% to 16%) and seeds (9.5% to 25%), most of which are usually discarded even though they are good sources of macronutrients such as carbohydrates (58% to 80%) that include high fiber, protein (6% to 13%), and lipids (6% to 16%) (Torres-León et al., 2018). Mango seed flour has been used for foods such as cakes, cookies, and breads, either as full flour or combined with other flours (Masud et al., 2020; Patiño-Rodríguez et al., 2020). Residues from mango processing are converted to high-value products such as 5-hydroxymethylfurfural (HMF), a high-value food additive (Muñiz-Valencia et al., 2020), and xylo-oligosaccharides (XOS) extracted from mango seed shells are used for making high-value prebiotics (Monteiro et al., 2021).
Many of the processes (dewatering, bioconversion, thermal treatment, etc.) used for converting produce wastes to value-added products are expensive and energy-intensive, which may be the reason why the value-added products are not yet available in commercial quantities. In a circular economy, these conversion processes can be combined to reduce the costs.
| Table 1. Examples of agricultural and food processing wastes and their value-added products. | |||
| Source | Waste | Value-Added Products | References |
| Fruits and Vegetables | Carrot processing waste | Biofuel, carotenoids | Kaur et al. (2020) |
| Apple pomace | Corn puffs, pharmaceuticals, phlorizin | Masil et al. (2018); Helkar et al. (2016) | |
| Citrus fruits | Essential oils and phenolic compounds | Torres-Leon et al. (2018) | |
| Mango seeds | Flour for confectionery and baked products | Patino-Rodriguez et al. (2020); Masud et al. (2020) | |
| Low-quality mango fruits | Food improvement additive (5-hydroxymethylfurfural, HMF) | Muniz-Velncia et al. (2020) | |
| Mango seed shells | Component for high-value prebiotics (xylo-oligosaccharides, XOS) | Monteiro et al. (2021) | |
| Orange juice by-products | Flour for confectionery products | Castro et al. (2020); Helkar et al. (2016) | |
| Grain processing | Bourbon by-products | High-fiber extruded snacks | Woomer and Adedeji (2019); Adedeji and Woomer (2020) |
| Biogas and fertilizer | Harmon (2015) | ||
| Brewing residue | Animal feed, biomass fermentation, and essential oils | Helkar et al. (2016); Woomer and Adedeji (2019); Amoriello and Ciccoritti (2021) | |
| Grain (corn and rice) bran | Processing aid (oryzanol) using in confectionery items and other foods | Singh et al. (2012); Helkar et al. (2016) | |
| Meat and seafood | Fish processing by-products | Protein hydrolysates, oil for biodiesel | Chudasama et al. (2020); Dash et al. (2020); Melgosa et al (2020) |
| Shrimp processing residue | Protein hydrolysates, carotenoids, and chitins | Nirmal et al. (2020) | |
| Chicken feathers and viscera | Protein hydrolysates, and bioactive compounds with antioxidant and antihypertensive properties | Bezus et al. (2021) | |
| Chicken meat waste | Proteins and antioxidants | Ghosh et al. (2019) | |
| Oilseeds and legumes | Bean (cowpea, soybean, etc.) and pea husks | Protein and fiber for animal feed, antioxidants in food preservation, and proteins (pea husks) | Oomah et al. (2010) |
| Pumpkin seeds and hulls | Phenolic compounds, antioxidants, and lipids | Hien and Minh (2021) | |
| Watermelon seeds | Iodine, oil, and protein | Vinhas et al. (2021); Rezig et al. (2019); Ramadan (2014) | |
| Food oils | Waste cooking oil (WCO) | Biodiesel, soap, carbon dots, and bioplastics | Hu et al. (2014); Wan Mahari et al. (2022) |
| Glycerol from biodiesel production using WCO | Hydrogen, butanol, ethanol, and biopolymers | Rastegari et al. (2019); Rezania et al. (2019) | |
Conversion of Distillery Waste to Value-Added Products
In the U.S., bourbon is the most important whisky and is called “America’s native spirit”. The mash used for bourbon must contain at least 51% corn and must be aged in a newly charred oak barrel. An enormous amount of by-product is produced during the distilling process. Each year, the U.S. bourbon industry produces 637,100 metric tonnes of a by-product called stillage (Coomes and Kornstein, 2019; Harmon, 2015; Schreiner, 2018). Stillage contains 6% to 7% solids and is usually dewatered (by centrifugation and/or sedimentation) to spent grain with about 70% moisture content (wet basis). The spent grain is often shipped to livestock farmers at a cost to the distilleries. The excess is shipped to landfills, where an additional cost is incurred for treatment to reduce the environmental impact. This is an expensive and unsustainable process for the bourbon industry, and significant effort is being made to find value-added uses for stillage and spent grain. A major challenge is the high moisture content of the stillage (93% w.b.) and the inconsistency in the constituents, thereby requiring custom processing to produce products of consistent quality.
Bourbon is produced from different ratios of corn and other ingredients, and the constituents of the stillage and spent grain differ for different products and for different distilleries. Proposed solutions include using the spent grain as a substrate for growing biomaterials or for anaerobic digestion in bioenergy production (Harmon, 2015). Studies have explored the use of bourbon spent grain as an ingredient in human food and other value-added products. Adedeji and Woomer (2020) and Woomer and Adedeji (2019) studied the quality attributes of extruded products made from proso millet and dried bourbon spent grain and found that extruded products with up to 10% spent grain were comparable to the control with no spent grain. The fiber content of the product also exceeded the daily amount of fiber per serving (25 to 30 g) recommended by the U.S. Food and Drug Administration (FDA, 2021). Other researchers have produced food products such as cookies and pancakes from dried spent grain. Other potential uses of spent grain include using it as a substrate in biomaterial synthesis, including extraction of the proteins for nanomaterials and the production of nanolaminates, which can protect food from gases, lipids, and moisture. These materials can also be carriers of colors, flavors, antioxidants, and antimicrobials (Ameta et al., 2020).
Transformation of Dairy, Meat, and Fish Processing Residue
Cattle production keeps increasing every year, primarily for meat and dairy production. In 2020, global dairy cattle production was 906 million tonnes (FAO, 2021). This increasing production requires increasing use of water and energy, as well as several residue streams that constitute wastes and by-products, all of which leads to an increase in the industry’s carbon footprint (Gerhardt et al., 2020). For example, the carbon footprint of a gallon of milk produced in the U.S. is estimated as 7.98 kg of CO2 equivalent (Schönleben et al., 2020; Thoma et al., 2013). The greenhouse gases, including methane (CH4) and nitrous oxide (N2O), from feed production and manure handling are produced on an enormous scale by the global livestock population (Rotz, 2018; Schönleben et al., 2020).
Because of the energy use, including the massive amount of carbon released, and the water use associated with meat, dairy, and fish production, alternative proteins, such as non-dairy milk and plant-based proteins, have become substitutes for animal protein (Bashi et al., 2019; Gerhardt et al., 2020). Several non-dairy milk and meat alternatives from plants, produced with minimal carbon footprints, have entered the market in recent years from companies such as Impossible Foods, Incogmeato, and Beyond Meat. Tyson Foods, one of the largest meat producers in the U.S., is also competing in this market (McKinsey, 2019). Globally, the alternative protein market is projected to have compound annual growth rates of 7.1%, 13.5%, 15.7%, 45%, 11.4%, and 9.5% for peaprotein, cultured meat, insect protein, dairy alternatives, and textured vegetable protein, respectively, between 2020 to 2025, with an estimated $40.52 billion market value (Bashi et al., 2019; MarketsandMarkets.com, 2021). Alternative proteins may not surpass the demand for animal meat and dairy products, but their growth is driven by their sustainability, and they can help reduce GHG emissions.
About 20% of meat processing residues end up in landfills if they are not converted to other useful products (Ghosh et al., 2019). Studies have examined how to convert these residues into useful products. The initial effort was to recover meat processing wastes that were causing surface water contamination, odors, and increased GHG emissions by using acid hydrolysis, anaerobic digestion, pyrolysis, pelletizing by immersion frying, and direct incineration for heat production (Bujak, 2015; Franke-Whittle and Insam, 2013; Hamawand et al., 2017; Ware and Power, 2016). Ghosh et al. (2019) developed a pulsed electric field process for extracting proteins from waste chicken breast muscle to produce antioxidants such as 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,20-and-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS).
Like meat, the global consumption of fish (freshwater and aquaculture) keeps increasing, especially as more people can afford fish. Aquaculture production of shrimp is the largest seafood industry in the world, and about 60% of shrimp processing solids go to waste. These residues (heads, shells, and viscera) contain many bioactive compounds and other macromolecules, including chitin, carotenoids, proteins, enzymes, and minerals that have applications in both human food and animal feed production (Chudasama et al., 2020; Dash et al., 2020; Nirmal et al., 2020; Vázquez et al., 2017). Nirmal et al. (2020) profiled multiple antioxidant, antiproliferative, wound healing, anti-inflammatory, and antidiabetic compounds extracted from shrimp processing waste. Fish processing wastes are used for similar applications as shrimp processing waste, especially as a source of inorganic nitrogen in compounding fish feed (Shanthi et al., 2021; Tugiyono et al., 2020). Some of the conversion processes are expensive and require improvements to become cost-effective. Advances in green technologies such as supercritical CO2 and ultrasound-assisted extraction, among others, have increased yields, improved quality, and reduced the cost of oil extraction from these residues (Adewale et al., 2016; Melgosa et al., 2020; Yasvanthrajan et al., 2021).
Upcycling of Oilseed and Legume Processing By-Products
Oilseeds are major sources of lipids that provide high amounts of calories per unit mass (9 calories per g compared to approximately 4 calories per g for proteins and carbohydrates). They are essential ingredients in several food industries and serve as lubricants in many food processes by reducing friction and stickiness. Legumes are a major source of protein (up to 57%) and carbohydrates (up to 50%) (Margier et al., 2018). Production and processing of oilseeds and legumes lead to the production of several waste streams that present a disposal problem, can negatively impact the environment, and incur costs. However, these wastes can be converted into value-added products. For example, the waste cake left after oil extraction contains a significant amount of proteins and carbohydrates that are often converted into animal feed.
With further research, higher-value products can be made from these wastes. For example, the husks from legumes often contain protein (e.g., about 14.3% for pea husks) (Hussain et al., 2020) and fiber consisting of cellulose, hemicellulose, lignin, and ash (Osorio et al., 2021; Oomah et al., 2010). The seeds from leguminous plants such as pumpkin, Telfairia, and watermelon that are often considered waste are rich sources of phenolic compounds, proteins, and oil (Rezig et al., 2019; Vinhas et al., 2021). The oil extracted from watermelon seeds is reported to be rich in iodine and unsaturated fatty acids, making it a source of healthy lipids, and the cake is rich in protein suitable for animal feed and other higher-value products (Gwana et al., 2014; Rezig et al., 2019; Nunes and Bhat, 2021). Ramadan (2020) profiled the cold press method as a green approach for the extraction of oils from watermelon seeds. Hien and Minh (2021) used a combination of ultrasonication and hexane solvent extraction for the production of pumpkin seed oil, which was high in unsaturated fatty acids. They reported an oil yield as high as 95.6%, which was a significant increase over the yield of 90.0% with chemical extraction.
Legume husks and oilseed hulls have been studied for the array of phenolic compounds they contain, including caffeic, p-coumaric, ferulic, sinapic, protocatechuic, vanillic, and syringic acids and p-hydroxybenzaldehyde, which have therapeutic applications for their antibacterial, hypocholesterolemic, antioxidant, anthelmintic, antimutagenic, and immunomodulatory effects (Krimer-Maleševic, 2020; Nunes and Bhat, 2021).
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| Figure 3. A biorefining approach for waste reduction and reuse in the agri-food supply chain (Paini et al., 2021) (SCo2 = supercritical carbon dioxide; SubCo2 = subcritical carbon dioxide). |
Waste Cooking Oil
A significant amount of waste cooking oil (WCO) is produced every day. If not properly disposed of, it ends up creating significant environmental problems, including sewer blockages, oxygen depletion in water bodies, and even fire hazards. Most WCO is produced by restaurants (Topi, 2020). The most widely studied use for WCO is the production of biodiesel through transesterification due to the low cost and simplicity of the process, as well as the potential use of the by-product glycerol (AbuKhadra et al., 2020). However, transesterification leads to a secondary waste stream of which glycerol constitutes the largest fraction (8% to 10%). Rastegari et al. (2019) reported that the global inventory of glycerol will reach about 41.9 billion liters by 2020. Rezania et al. (2019) profiled several applications of glycerol, including as a substrate for the production of biopolymers, lactic acid, ethanol, hydrogen, and butanol. Hu et al. (2014) developed a method to use WCO for the synthesis of carbon dots, which has applications in electronics (e.g., sensors and photocatalysis devices), pharmaceuticals (e.g., drug delivery), and chemical industries (table 1). Microwave pyrolysis was used to convert WCO to liquid oil with potential application as a fuel oil as well as a feedstock for polyhydroxyalkanoate (PHA) bioplastics (Wan Mahari et al., 2022). The liquid oil was used as a carbon source for the growth of Bacillus species during microbial bioconversion, yielding about one-tenth poly-3-hydroxybutyrate [P(3HB)]. Bioplastics are biodegradable and are not harmful to the environment, unlike synthetic plastics and WCO. The major challenge with the value-added processes for WCO is that they are not cost-effective without further conversion of the secondary residues to other value-added products (Cornejo et al. (2017).
Future Conversions of Food Waste to Value-Added Products
The way forward is to minimize waste in agricultural production and food processing (Bigdeloo et al., 2021). Several innovative approaches are being developed for this purpose, including a holistic approach that treats agri-food production and processing as a type of biorefinery in which all streams out of the system are targeted for value-added products with the aim of reducing residues (fig. 3), just as in petroleum refining (Paini et al., 2021). Other approaches involve converting wastes into a source of energy as in biofuel production, as nanomaterials in food processing and packaging, and as a source of nutrients in human foods and animal feeds (Mahabir et al., 2021; Mahmound et al., 2021). The current technologies for converting or extracting useful materials from these wastes are energy-intensive and create further waste streams. New technologies, such as supercritical CO2 extraction, enzyme-activated extraction, pulsed electric field, high-pressure treatment, and ultrasonication are less energy-intensive and produce minimal wastes (Picot-Allain et al., 2021; Hussain et al., 2020; Chemat et al., 2017; Redondo et al., 2018; Melgosa et al., 2020; Yasvanthrajan et al., 2021). There is also a re-evaluation of how we process certain types of foods, such as fruit juices, to retain more useful nutrients in the primary stream from the process (Samborska et al., 2019). Creating new products from these existing products also minimizes waste.
The quantity of food wasted on the home front ranges from 11% to 43%, depending on the source (Ishangulyyev et al., 2019; Yu and Jaenicke, 2020), and the annual economic loss due to domestic food waste is estimated to be $160 to $240 billion in the U.S. alone (Bandoim, 2020; Buzby et al., 2014). Domestic food waste has been adduced to various causes, including lack of knowledge about how to store food properly. An example of improper storage includes storing partially ripe bananas or starchy baked products at refrigeration temperature (4°C to 8°C), where they they experience physiological damage and rapid retrogradation or staling, respectively. Educating consumers about proper food storage could help reduce domestic food waste.
Another cause of domestic food waste is the lack of clarity in food labeling (Patra et al., 2020). Properly defining when foods turn bad, combined with smart packaging, could reduce food waste. In particular, there is confusion about the shelf-life of foods, and about 20% of food waste is linked to confusion over shelf-life labeling. Foods that are still good, although with reduced quality, are often needlessly discarded. Unifying the terminology of shelf-life labels such as “Use by”, “Best if used by”, and “Best if sold by” (fig. 4) and simple technologies that can be easily interpreted by consumers, such as color indications for the level of freshness, are critical to reduce domestic food waste. Current efforts are aimed at reducing the costs and risks associated with freshness sensors (Freier, 2019; Plasil, 2020).
The ultimate goal is to achieve circular integration of all food processing streams to produce value-added products and eliminate waste. This goal is imperative as humans increase in number while the Earth’s resources remain finite.
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| Figure 4. Inconsistent food labeling (Foodindustry.com, 2018). |
Conclusion
Developing cost-effective processes to create value-added by-products and eliminating domestic food waste through behavioral change could create a sustainable agri-food supply chain. If we change our perceptions about when to discard foods that are still good, we can save tons of food products that can serve as a source of human nutrition. If we anthropomorphize how we see imperfect fruits and vegetables, we can reduce how much fresh produce goes to waste. There is also a need to reduce the cost of the technologies that are being developed for food waste conversion. Innovative approaches are needed to reduce the carbon footprint of food and agricultural production, which could provided increased returns for stakeholders. Awareness is increasing of the environmental damage due to agri-food production and processing wastes, and this awareness is driving research efforts and cultural change. As a result, a circular economy is becoming inevitable for the agri-food supply chain.
Acknowledgements
This work was supported by the Kentucky Agricultural Experiment Station (KAES), and the USDA National Institute of Food and Agriculture (NIFA) (Multistate Project No. 1024529).
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