Every year the agri‑food chain—from fields and orchards to slaughterhouses and dairies—generates hundreds of millions of tonnes of residues and by‑products. In the past these “side streams” were treated as waste, relegated to low‑value animal fodder, burned in situ or simply landfilled. Today, however, volatile input prices, tightening climate legislation and the European Union’s commitment to a circular bioeconomy are transforming the perception of agricultural residues. Valorisation—the systematic conversion of discarded biomass into valuable food, feed, chemicals, materials or energy—is emerging as both an environmental imperative and a business opportunity.

Scientific studies show that side‑stream valorisation can cut greenhouse‑gas (GHG) emissions, enhance soil fertility, diversify rural incomes, and replace fossil‑derived commodities with bio‑based alternatives (Scarlat et al. 2019; Lehmann et al. 2011). This article synthesises the state of the art, surveys leading technologies, analyses policy and market drivers, and highlights how pan‑European collaborations such as the Horizon‑Europe CROSSPATHS project are helping to mainstream these solutions.

1  What are agricultural side streams?

Agricultural side streams encompass every biomass fraction that remains after the primary commodity has been harvested or processed. On‑farm examples include straw, husks, stalks, pruning residues, manure and on‑field losses; processing residues range from fruit pomace, cereal brans and oilseed cakes to whey, molasses or slaughterhouse offal (Mirabella et al. 2014). A scientometric review of 520 peer‑reviewed papers found that food‑industry side streams frequently retain up to 50 % of the proteins, fibres and micronutrients contained in the original feedstock.

Importantly, the biochemical composition of a side stream dictates the most suitable valorisation route. Lignocellulosic residues such as wheat straw, rich in cellulose, hemicellulose and lignin, lend themselves to bioenergy, biochar and fibre‑composite applications, whereas high‑moisture streams like fruit peel or whey—rich in fermentable sugars, pectins and proteins—are prime substrates for microbial fermentation, enzyme recovery and precision fermentation (Sadh et al. 2018).

2  Why valorise? The environmental‑economic rationale

Climate mitigation and soil health. Open‑burning of crop residues emits large amounts of methane, nitrous oxide and particulate matter. By contrast, converting these residues into biogas, biochar or compost drastically lowers emissions and returns stable carbon and nutrients to soils, improving structure and water retention (Lehmann et al. 2011; Zhao et al. 2024).

Resource efficiency and circularity. The EU Circular Economy Action Plan mandates a cascading use of biomass—first food and feed, then materials and chemicals, and finally energy (European Commission 2020). Side‑stream valorisation fulfils this hierarchy and helps reach the Farm‑to‑Fork Strategy’s target of halving per‑capita food waste by 2030 (European Commission 2020).

Rural income diversification. Life‑cycle analyses indicate that integrating side‑stream biorefineries can capture €40–120 of added value per tonne of dry residue, depending on the product portfolio (Scarlat et al. 2019). Biogas plants can supply heat and power for farm operations, while insect rearing or precision‑fermentation facilities create high‑margin protein ingredients from low‑value biomass (GFI 2024).

Feed and fertiliser security. The EU currently imports about 60 % of its protein feed. Insect meal, microbial protein or press‑cake concentrates derived from local residues reduce that dependency, and digestate or biochar can partly substitute synthetic fertilisers whose prices have surged since 2022 (Van Huis 2013).

3  Technological pathways for valorisation

3.1  Biochemical routes

Anaerobic digestion (AD). Mixed consortia of archaea and bacteria break down complex organics in manure, crop residues or processing effluents, producing biogas (55–65 % CH₄) and nutrient‑rich digestate. Comprehensive meta‑analyses report specific methane yields of 200–400 m³ t⁻¹ volatile solids for maize stover or co‑digested food waste, with energy payback times below one year in pilot plants. Recent innovations include thermophilic two‑stage reactors and membrane upgrading that inject biomethane directly into national gas grids.

Fermentation and enzymatic hydrolysis. High‑sugar or protein streams such as whey, potato pulp or fruit pomace can be hydrolysed and fermented into lactic acid, polyhydroxyalkanoate (PHA) bioplastics or single‑cell proteins. Engineered microbial strains now achieve lactic‑acid productivities above 4 g L⁻¹ h⁻¹ on citrus‑peel hydrolysates, while side‑stream yeast grown on spent sulfite liquor is already commercial as aquaculture feed (Sadh et al. 2018).

3.2  Thermochemical routes

Pyrolysis to biochar. Heating lignocellulosic residues at 350–600 °C in limited oxygen produces a carbon‑rich solid (biochar), bio‑oil and syngas. Biochar sequesters stabilised carbon for centuries and can improve cation‑exchange capacity, water‑holding and nutrient‑use efficiency. A 2024 rapid review of 87 field trials found an average 10 % crop‑yield increase and net GHG savings of 2.8 t CO₂e ha⁻¹ yr⁻¹ (Zhao et al. 2024). Emerging reactors produce “designer” chars doped with micronutrients or configured for pollutant adsorption.

Gasification and fast pyrolysis. At 800–1 000 °C, residues are converted to syngas (H₂, CO, CO₂, CH₄) or bio‑oil, which can be catalytically upgraded into drop‑in fuels, methanol or platform chemicals. Although commercial deployment is still limited by feedstock heterogeneity and capital intensity, modular 5–20 MW units co‑located with farms are being piloted in the Nordic countries (Scarlat et al. 2019).

3.3  Mechanical and physicochemical extraction

Dry or wet fractionation isolates fibres, proteins or phytochemicals without harsh solvents. Twin‑screw extrusion of cold‑pressed rapeseed cake, for example, yields a 60 % protein concentrate suitable for meat analogues, while the fibrous fraction is integrated into particle boards. Supercritical CO₂ and deep‑eutectic solvents provide greener extraction of carotenoids, polyphenols and essential oils from herb or fruit by‑products, capturing high‑value nutraceutical markets.

3.4  Insect bioconversion

The black soldier fly (Hermetia illucens) is the most extensively studied insect for waste conversion. Meta‑analysis of 45 trials reports substrate reduction rates > 60 % and conversion of 25 % of intake into larval biomass containing 40–45 % protein and 20–30 % lipids. New studies demonstrate efficient conversion of spent mushroom substrate and wet distiller grains, broadening the feedstock palette. Frass (larval excrement plus residual substrate) is increasingly marketed as an organic fertiliser rich in nitrogen and chitin.

3.5  Integrated biorefineries

Cascading biorefineries sequentially extract high‑value compounds, convert remaining biomass to energy, and recycle nutrients. A citrus‑peel case study in Spain first recovers d‑limonene, then extracts pectin, ferments the hydrolysed sugars into ethanol and digests the solids to biogas—achieving material valorisation of over 90 % of the original mass. Such integrated schemes epitomise the circular‑bioeconomy principle embedded in EU policy.

4  Policy, regulatory and market drivers

EU Green Deal and Circular Economy Action Plan. Revised waste‑framework directives require separate collection of bio‑waste across Member States, creating a reliable feedstock pool and providing legal impetus for valorisation (European Commission 2020).

Farm‑to‑Fork Strategy. Besides the 50 % food‑waste‑reduction target, the strategy earmarks Horizon‑Europe funds for novel up‑cycling technologies, explicitly prioritising protein autonomy and soil carbon sequestration (European Commission 2020).

Renewable Energy Directive (RED III). Agricultural residues qualify as advanced biofuel feedstocks with specific sub‑targets for transport energy, boosting demand for lignocellulosic ethanol and biomethane pathways.

Carbon farming and the forthcoming EU Carbon Removal Certification Framework. Biochar, enhanced weathering and soil‑applied digestate are slated for inclusion in certified removal methodologies, creating potential revenue streams through voluntary and compliance carbon markets (European Commission 2023).

Consumer and brand pull. Large food manufacturers increasingly adopt clean‑label, up‑cycled ingredients to meet ESG goals. Market analyses project a five‑fold increase in demand for side‑stream‑derived fibres and proteins by 2030 (GFI 2024).

5  Challenges and future outlook

Heterogeneity and logistics. Side streams are often bulky, wet and geographically dispersed, raising collection costs and complicating consistent feedstock supply. Digital traceability platforms and regional hub‑and‑spoke models are emerging to mitigate this bottleneck (Optimising Agri‑food SC 2025).

Regulatory clarity. EU feed and food laws still restrict certain waste‑derived ingredients (e.g., animal by‑products in insect feed). Harmonised safety standards and risk‑assessment protocols are urgently needed to unlock scale (European Commission 2023).

Scale‑up financing. Capital‑intensive biorefineries face risk premiums; blended finance, advance offtake contracts and carbon‑credit revenue can improve bankability (Scarlat et al. 2019).

Knowledge gaps. Holistic environmental‑economic‑social life‑cycle assessments are required to avoid burden‑shifting, and qualitative studies on consumer acceptance must accompany technological deployment. Notwithstanding these hurdles, modelling suggests that by 2030 up to 30 % of EU agricultural residues could be valorised, avoiding 130 Mt CO₂e annually and adding €15 billion in gross value (Scarlat et al. 2019).

6  The role of collaborative initiatives such as CROSSPATHS

Transforming side streams into high‑value products demands multidisciplinary expertise—from agronomy and process engineering to toxicology, business modelling and socio‑economic analysis. The Horizon‑Europe CROSSPATHS project exemplifies how such collaboration can be orchestrated. By linking cutting‑edge agri‑food research infrastructures in Estonia, Poland and Portugal, CROSSPATHS has established opportunities to develop cutting edge research and services addressing side stream valorisation.

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Conclusion

Agricultural side streams are not waste but latent value awaiting capture. Biochemical, thermochemical, mechanical and biological technologies—many already commercial—can convert residues into profitable, climate‑positive products. Supportive EU policy, growing consumer demand for sustainable ingredients and collaborative initiatives like CROSSPATHS are aligning environmental, economic and social incentives. While challenges in logistics, regulation and finance remain, the evidence is clear: valorising side streams is a strategic pathway to climate resilience, circular resource use and rural prosperity—literally turning waste into worth.

References

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European Commission (2020). Circular Economy Action Plan and Farm‑to‑Fork Strategy. Brussels. https://food.ec.europa.eu/system/files/2020-05/f2f_action-plan_2020_strategy-info_en.pdf

European Commission (2023). Proposal for a Regulation on the Certification of Carbon Removals. Brussels. https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=COM%3A2022%3A672%3AFIN

GFI (The Good Food Institute) (2024). Sidestreams Analysis: Unlocking Low‑Cost Inputs for Alternative Proteins. https://gfi.org/wp-content/uploads/2024/02/Sidestreams-analysis.pdf

Lehmann J., Rillig M.C., Thies J., et al. (2011). Biochar effects on soil carbon sequestration and fertility: a review. Soil Biology & Biochemistry, 43, 1812‑1836. https://doi.org/10.1016/j.soilbio.2011.04.022

Mirabella N., Castellani V., Sala S. (2014). Current options for the valorisation of food manufacturing waste: a review. Journal of Cleaner Production, 65, 28‑41. https://doi.org/10.1016/j.jclepro.2013.09.062

Remijnse M., Rohmer S.U.K., Marandi A., van Woensel T. (2025). Optimising agri‑food supply chains: Managing food waste through harvest and side‑stream valorisation. Journal of Cleaner Production, 503, 145349. https://doi.org/10.1016/j.jclepro.2025.145349

Sadh P.K., Duhan S., Duhan J.S. (2018). Agro‑industrial wastes and their utilization using solid state fermentation: a review. Bioresources and Bioprocessing, 5, 1‑15. https://doi.org/10.1186/s40643-017-0187-z

Scarlat N., Dallemand J.F., Monforti‑Ferrario F., Nita V. (2015). The role of biomass and bioenergy in a future bioeconomy: Policies and facts. Environmental Development, 15, 3‑34. https://doi.org/10.1016/j.envdev.2015.03.006

Van Huis A. (2013). Potential of insects as food and feed in assuring food security. Annual Review of Entomology, 58, 563‑583. https://doi.org/10.1146/annurev-ento-120811-153704

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