By Dr. Maja Berden Zrimec, researcher and content writer
Agricultural residues that have been so far comprehended as a waste. But in circular bioeconomy, they have a new role as the bioresource for products and energy. This is certainly most welcomed step forward because we produce an enormous amount of waste. The latest Eurostat data show that EU produced 2,337 million tonnes of total waste in 2018. From this, 58.45 million tonnes was vegetal waste stemming from the agriculture and food industry. Worldwide, almost third of the food is never eaten, which is roughly 1.3 billion tons of food waste per year (Bas-Bellver et al. 2020). In the case of fruits and vegetables, 45% of biomass goes to waste; by FAO estimation the residues have even higher share of 60% (Bas-Bellver et al. 2020).
Consequently, the global annual generation of biomass waste is in the order of 140 Gt (Tripathi et al. 2019). In the EU, about 23 Mt of dry biomass per year is available as the residual straw from cereals, and the major global crops (wheat, maize, rice, soybean, barley, rapeseed, sugarcane and sugar beet) produce almost 3.3 Gt of residue (fresh weight per year) in the countries and areas with large biomass potential (Europe, USA, Canada, Brazil, Argentina, China and India) (Tripathi et al. 2019).
Bioresources from agricultural waste
Agricultural waste comes from the cultivation and processing of agricultural products, such as agricultural crops, fruits and vegetables as well as animal products that we’ll not cover in this blog (Gurdil et al. 2021). The waste can be in the form of liquids, slurries, or solids. Its composition depends greatly on the nature and kind of farm activities, seasonal changes, climate and soil quality.
Only a small amount of biomass waste is currently further utilised as a feedstock for the industrial applications and electricity generation. The reason is not that companies simply don’t bother to deal with the recycling of waste. As many industries entering circular economy, also agriculture generally lacks innovative new technologies that enable waste valorisation.
Nevertheless, the utilisation of waste is more than worthwhile. To become truly sustainable, it is imperative that bioresources don’t compete for the agricultural land with the food crops. In this aspect, agricultural waste is an excellent bioresource that can, for example, replace the use of crops (and farm land) for the biofuel production. Waste valorisation also supports the EU’s primary objective that Member States should use biomass for energy purposes without harming the environment, endangering efforts to mitigate climate change, or causing negative social impacts (Ymeri et al. 2020).
Cereal crops are the major contributor to the huge quantities of annual global generation of agricultural residue. Globally, 66% of the residual plant biomass comes from the cereal straw (stem, leaf and sheath material), followed by sugarcane stems and leaves, and other residual biomass including the ‘oil crops’, roots and tubers, nuts, fruits and vegetables (Tripathi et al. 2019).
Valorisation of agricultural waste
Agricultural waste represents an abundant source for biofuel production. Still, most residues are normally discarded or burnt, although many have the potential in energy production (Tripathi et al. 2019). Residues generated from crops, such as sugarcane, rice, ground- and coffee nuts have already been used as a fuel source but are nonetheless significantly under-utilised (Tripathi et al. 2019). A lot of agricultural waste and food also ends up in the biogas plants, where they are used to produce biogas for households and industry. Generating biogas from the agricultural residues enables an important decrease of non-renewable energy consumption and particularly greenhouse gas release from storing manure and agro-waste.
Cellulose, hemicellulose and lignin-rich residues can be used for the production of chemicals, resins and enzymes. They can be also converted into biofuel by biochemical or thermochemical processes. The technique depends on the residues characteristics. Feed-stocks with more than 30% moisture content, C/N ratio of less than 30%, and high cellulose and hemicellulose content are better suited for the biochemical conversion with microorganisms and enzymes, eventually being converted into biogas, bioethanol or biodiesel (Gurdil et al. 2019). Materials with less than 30% moisture, C/N ratio higher than 30%, and a high lignin content are conversed thermochemically and subsequently treated to produce synthesis gas or syngas, bio-oil, biochar, and biocoal (Gurdil et al. 2019). Although thermochemical processing can be used on the broad spectrum of wastes, it is less sustainable biochemical conversion because of the fossil fuels consumption and greenhouse gas emissions (Gurdil et al. 2019).
Fruits and vegetables processing produces wastes, such as seeds, peels, or pulp, which are generated in the different steps of the processing chains. These residues are generally packed with high concentrations of bioactive compounds, often even higher than that of the edible part of fruit. Consumers increasingly prefer products with natural ingredients, that can be produced from the agro-waste. The use of natural bioactive compounds is especially welcome in the commonly consumed processed food where they can improve their nutritional value.
Fruit and vegetable powders are a nit way of consuming food residue’s products. Vegetable waste powdered ingredients have the potential to be used as colouring, savouring, or preservative agents, and to increase the nutritional value of (processed) food, contributing to the development of nutritious and safe diets with a reduced environmental impact. Technologically, some development is needed to produce safe and homogenous products from typically quite heterogenous waste. Processes like cleaning, drying and milling are relevant in achieving the quality powder with the desired functional properties.
High-value applications of agricultural waste
In our blogs we have already written about the valorisable residues from several crops. For example mango peels contain carotenoids, such as provitamin A compound, alpha- and beta-carotene, lutein, and polyphenols, like quercetin, kaempferol, gallic acid, caffeic acid, catechins, magniferin and tannins. Many bioactive compounds in the peel, kernel and pulp are known for their antimicrobial, anti-diabetic, anti-inflammatory, and anti-carcinogenic properties. Waste water from the shea butter production has pesticidal properties and the press cake and husks from processing have potential for fertilisers and fuels.
The cashew shell has a high amount of oil content and the shell derivatives can be used for lubricants, waterproofing, and paints. Cashew nut shell liquid (CNSL — not edible) is an important industrial product obtained while processing the raw nuts. The cashew apple waste is used as a form of energy bagasse’s, fermented cultures used in probiotics or for conditioning of the soil in agriculture. Many phytochemicals can be found also in the pineapple peel. The leaves of some pineapple cultivars can be used for textiles. Bromelain is extracted mostly from stems and is a desirable substance in cosmetics, medicine, and as meat tenderiser.
The production of avocado fruits results in a large quantities of peel and seeds which contain a number of phytochemicals with health benefits, like reducing inflammatory diseases. The residues of avocado contain essential oils with considerable amount of polyphenolic compounds such as proanthocyanidins, catechins and quercetin glycosides. Seeds are a good source of carbohydrates like hemicelluloses, fibers, and starch (30%). The high fibre content of the residues after lipid extraction allows their use in the preparation of flour, which is suitable for the bakery products and pasta.
So the next time you come by the agricultural or food residues, think of them as an opportunity and not as a burden.
Main sources:
- Bas-Bellver C., Barrera C., Betoret N., Segui L. (2020): Turning Agri-Food Cooperative Vegetable Residues into Functional Powdered Ingredients for the Food Industry. Sustainability 12: 1284; doi:10.3390/su12041284
- Eurostat: Generation of waste by waste category, hazardousness and NACE Rev. 2 activity [ENV_WASGEN]
- Gürdil G.A.K., Mengstu M., Kakarash A. (2021): Utilization of Agricultural Wastes for Sustainable Development. Black Sea Journal of Agriculture 4: 146; doi: 10.47115/bsagriculture.953415
- Tripathi N., Hills C.D., Singh R.S., Atkinson C.J. (2019): Biomass waste utilisation in low-carbon products: harnessing a major potential resource. Nature NPJ — Climate and Atmospheric Science 2: 35; https://doi.org/10.1038/s41612-019-0093-5
- Ymeri P., Gyuricza C., Fogarassy C. (2020): Farmers’ Attitudes Towards the Use of Biomass as Renewable Energy — A Case Study from Southeastern Europe. Sustainability 12: 4009; doi:10.3390/su12104009
