The Waste Crisis and the Circular Economy Opportunity
Amidst the milieu of escalating global urbanization and burgeoning populations, the efficacious management of organic municipal waste has emerged as an imperative concern, necessitating the development of innovative and sustainable technologies. Noteworthy quantities of food waste (FW) produced throughout the food supply chain and its consequential impact on the underutilization of natural resources have evoked widespread global apprehension.
On a global scale, an estimated 1.3 billion metric tons of fruit and vegetable waste increase annually, with a projected escalation to 2.2 billion metric tons by the year 2025. Vast quantities of lignocellulosic residues generated globally, encompassing agricultural byproducts, food cultivation detritus, green grocer’s discards, arboricultural trimmings, the organic and paper fractions of municipal solid waste, further exacerbate the waste crisis.
Traditional methods of waste disposal, including landfilling with its associated challenges of high transportation costs, incineration marked by impracticality due to low calorific value and elevated moisture content, and animal nourishing constrained by the selective appropriateness of waste for diverse animals, not only contribute to environmental degradation but also lead to the inefficient utilization of valuable resources embedded in the discarded materials.
In response to this scenario, the paradigm of waste management is experiencing a transformative evolution towards sustainable methodologies, emphasizing resource recovery as a primary focus. Biological and enzymatic pretreatment methodologies are particularly appealing due to their ability to enhance bioresource recovery from fruit and vegetable waste (FVW) compared to established conventional technologies. This superiority arises from their heightened specificity, diminished necessity for chemical and energy inputs, and absence of process inhibitors in their implementation.
Enzymes: Catalysts for Sustainable Waste Valorization
Enzyme-mediated valorization of organic municipal waste occupies a prominent position in the forefront of this transformative movement, offering a promising pathway for the sustainable and effective conversion of waste into valuable products. Enzymes, serving as inherent catalysts in nature, exhibit a distinctive capacity to accelerate chemical reactions under mild conditions, making them well-suited for the targeted degradation of intricate organic compounds present in organic municipal solid waste.
The bioconversion of organic food and vegetable waste into useful compounds is mostly dependent on enzymes. These biocatalysts help processes like hydrolysis, fermentation, and oxidation break down complex organic molecules into simpler chemicals. Cellulases and hemicellulases, for example, break down cellulose and hemicellulose into fermentable sugars that can subsequently be fermented by microorganisms to produce bioethanol or biogas. Proteins and lipids are broken down into amino acids and fatty acids, respectively, by proteases and lipases. These amino acids can then be converted to biofertilizers or animal feed. Furthermore, pectinases and amylases break down starches and pectin’s, promoting the synthesis of organic acids and other biochemicals.
Utilizing particular enzymes catalyzing specific waste can greatly increase the productivity and efficiency of these bioconversion processes, helping to support resource recovery and sustainable waste management. While the use of pure microbial enzymes for the pretreatment of FVW may be cost prohibitive, employing crude enzymes for pretreatment offers an economical alternative.
Principal agents in the enzyme-mediated depolymerization of FVW encompass pectinases, lignin modifying enzymes, proteases, glycoside hydrolases, lipases, and pectinases, which methodically cleave the particulate constituents of FVW, including lignin, lipids, proteins, holocellulose and pectin, into more elementary subunits. The hydrolyzed form of FVW can undergo subsequent conversion into biofuels, high-value products and platform chemicals through anaerobic digestion and other biological processes.
The necessity to embrace sustainable waste management practices are emphasized by increasing environmental consequences associated with conventional waste disposal methods. Enzyme-mediated processes not only serve to attenuate the production of waste but also align with the principles of a circular economy by facilitating the closure of loops in resource utilization.
Profiling Organic Municipal Waste: Fruits, Vegetables, and Beyond
Organic Municipal waste is mandatorily differentiated as hazardous and non-hazardous waste within a specific geographic area. The preeminent percentage of global waste (approximately 44% of the overall waste generated globally), is food and green waste. Notably, a substantial proportion of this category comes as by-product of fruit and vegetable processing industries, kitchens, marketplaces, mandis etc.
Major fruit and vegetable waste constituents originate from domestic kitchens, marketplaces, and similar sources within municipal areas. Few prominent ones include remains from apple, orange, banana, carrot, tomato, potato etc. and are rich in numerous biomolecules that may be efficiently converted to various value-added products.
Statistical data indicates that apples secured the third position in global fresh fruit popularity in 2019, following bananas and watermelons. Within that particular year, the global production of apples amounted to approximately 87.24 million metric tons. Fruits, such as apples, that deviate from specified standards, including criteria related to dimensions and visual attributes (e.g., colour intensity, discoloration, skin elasticity, and defects like bruises and rots), are frequently subject to disposal by producers, consumers, and retailers. It is estimated that around 3.7 trillion apples are annually directed to landfills due to non-conformity with established quality standards.
Banana and citrus fruits stand as paramount fruit crops on a global scale. Banana production has demonstrated consistent growth, escalating from over 70 million tons in 1999 to approximately 121 million tons in 2021. It has been documented that each ton of harvested bananas yields approximately 4 tons of waste, encompassing 100 kg of discarded fruit, 3 tons of pseudo stems, 160 kg of stems, 480 kg of leaves, and approximately 440 kg of inflorescences and skins. Banana peel (BP), a noteworthy by-product, represents approximately 35% of the total weight of bananas, amounting to an annual production of around 40 million tons.
Citrus fruits are marked by a substantial surge in production over recent decades. The annual citrus fruit output has witnessed a notable escalation, escalating from an average of 62 million tons per annum during the period spanning 1987–1989 to approximately 100 million tons in the year 2010. Citrus fruits are predominantly distributed either in their fresh state or in the form of processed juice. The processing of citrus fruits yields a substantial quantity of peels as a by-product, which, regrettably, lacks inherent value and is commonly disposed of or relegated to waste disposal sites.
Potato (Solanum tuberosum), positioned as the fourth foremost crop in worldwide consumption, assumes considerable dietary significance and finds extensive application in diverse food-processing industries. However, these industrial procedures generate noteworthy volumes of potato peel (PP) as a by-product, traditionally classified as waste and subsequently relegated to disposal.
Carrot (Daucus carota) stands as a pivotal agricultural commodity, exhibiting a global distribution and an annual production exceeding 37 million tons. After industrial processing, about 50% of residues are found that can be used as animal feed, rather than being disposed in landfills.
Tomato (Lycopersicon esculentum), ranking as the second most significant vegetable crop globally, holds a prominent position in the Mediterranean diet. After tomato fruit harvesting, substantial biomass residues, specifically tomato harvest stalks, remain in the field, amounting to approximately 24,000 kg/ha contingent on cultivation conditions.
Unlocking the Value: Bioactive Compounds and High-Value Products from Organic Wastes
The constituent components of fruit and vegetable wastes are rich in numerous biomolecules that may be efficiently converted to various value-added products. Apple pomace, a byproduct of apple processing, serves as an exceptional natural source of beneficial phenolic compounds. These unrefined apple phenolics extract demonstrates the capability to safeguard against DNA damage, enhance barrier function, and impede invasion.
Banana peel primarily consists of biopolymers, including lignin, pectin, cellulose, hemicellulose, fibre, chlorophyll, and various low-molecular-weight compounds. Banana peel serves as a notable reservoir of amino acids, encompassing aspartic acid, threonine, serine, glutamic acid, proline, glycine, alanine, cystine, valine, methionine, isoleucine, leucine, phenylalanine, lysine, and arginine.
Citrus peel waste (CPW) stands out as a promising biomass alternative due to its inherently low lignin content, obviating the need for harsh pretreatments commonly employed in lignocellulosic biomass processing. Recent investigations have explored metabolic engineering strategies aimed at developing yeast strains capable of efficiently utilizing the non-fermentable sugars derived from CPW, enhancing the potential for bioconversion processes.
Carrot peel waste represents a promising substrate for the proposed extraction technique, owing to its abundance in carotenoids and pectin. The inherent composition of carrot peel waste facilitates the formation of a colloidal complex in response to water-induced conditions, enabling the extraction of carotene and pectin.
Tomato residues have also demonstrated promising potential for utilization. Composts derived from tomato crop waste, when blended with peat in various proportions, exhibited enhancements in the processes of rooting, root length, and root weight of cuttings. Researchers have also investigated the use of supplemented tomato waste as a viable alternative culture medium for the synthesis of carotenoids by Rhodotorula glutinis.
Potato peel serves as a valuable reservoir of phenolic acids, including chlorogenic, caffeic, gallic, and protocatechuic acids, which exhibit antioxidant properties exploitable in various applications. Despite processing resulting in a 24–26% reduction in the initial phenolic content, the final potato-based products exhibited higher phenolic levels compared to conventional counterparts.
From Waste to Wealth: Microbial Conversion Strategies
The methodology of using microbial conversion aligns with the tenets of the circular economy and sustainable development. Furthermore, progressions in biotechnological methodologies and waste-to-energy technologies play a pivotal role in enhancing the efficiency and economic viability of these practices.
Platform chemicals represent intermediary substances that can undergo subsequent processing to yield a diverse array of chemicals and materials. Some of the valuable platform chemicals that can be produced from fruit and vegetable wastes include:
Organic Acids: Organic acids widely utilized in industry include citric acid, acetic acid, and lactic acid, although there are numerous others like succinic, ellagic, or ferulic acid that also hold industrial significance. Fruit and vegetable waste has proven to be a significant resource of organic acids, with various experiments employing these remains from the food industry as substrates to acquire distinct organic acids.
Enzymes: Enzymes serve as biological catalysts pivotal for metabolic functions, finding utility across various industries including food, cosmetics, pharmaceuticals, and textiles. Municipal solid organic waste can serve as a potential substrate for growth of varied microbes for production of numerous enzymes usually through solid state or submerged fermentation process.
Biofuels: Biohydrogen, biogas, bioethanol and biobutanol are the major biofuels obtained from fruits and vegetable remains. Carbohydrate-rich raw materials with minimal nitrogen content and compelling marginal pre-treatment are conducive to biohydrogen production. Combinations of fruits and vegetables have been utilized as substrates for biohydrogen production via dry fermentation under both thermophilic and mesophilic conditions.
The incorporation of fruits and vegetable waste into innovative processes plays a significant role in mitigating the environmental impact linked to conventional waste disposal methodologies. Microbial conversion entails employing microorganisms, encompassing bacteria, fungi, or yeast, to mediate the conversion of one substance into another. This approach enjoys broad application across various domains, including industry, environmental science, and scientific research.
The Future of Organic Waste Valorization
Enzyme-mediated procedures have catalysed novel methodologies, substantially reshaping conventional practices. Employing enzymes as environmentally sustainable processing agents or supplements within the food industry has bolstered streamlining efforts, heightened competitive edge, and augmented product innovation. The scientific and industrial research communities are steadfastly dedicating their endeavours to enhance the resilience, efficacy, and diversity of enzymes.
Integration of enzyme-mediated valorisation aligns with the principles of the circular economy, emphasizing resource recovery and waste reduction. Growing economic viability of enzyme-mediated processes becomes apparent as industries recognize the potential for generating marketable products from organic municipal waste. This shift towards a more sustainable waste valorisation model not only addresses environmental concerns but also unfolds new avenues for job creation, innovation, and the establishment of a bio-based economy.
As ongoing research and technological advancements propel the field forward, enzymatic organic municipal waste valorisation holds promise for revolutionizing global waste management practices. This comprehensive and forward-thinking approach underscores the ability to convert municipal waste into an asset, contributing to a more resilient and sustainable future.
