Strategies to overcome challenges in upcycling industrial side-streams into microalgae-based foods: Auxenochlorella protothecoides as a case study

Side-streams, encompassing food-safe by-products and edible surpluses from the agri–food industry, represent an abundant source of untapped organic nutrients. Hence, upcycling side-streams into feedstocks for microalgae cultivation presents opportunities to enhance microbial food production and strengthen food security. However, challenges impede widespread implementation. Here, key challenges are identified, and strategies to overcome them are proposed. Strategies are exemplified with Auxenochlorella protothecoides, a microalga with commercial significance as bulk foods and speciality ingredients. Strategies to overcome challenges include, firstly, selecting side-streams rich in assimilable nutrients that fulfil species-specific metabolic demands and optimise growth and/or product yield. Secondly, tailored energy-efficient treatments of side-streams in individualised bioprocesses should be implemented to optimise nutrient recoveries, thereby improving economic and environmental feasibilities. Thirdly, blending different side-streams mitigates side-stream batch variability, nutrient deficiencies, and supply chain disruptions. Fourthly, a comprehensive assessment of side-stream effects on the nutritional, safety, and sensory attributes of the resulting biomass and/or extract is necessary to maintain food product quality. Fifthly, leveraging co-cultures overcomes the restricted substrate utilisation of microalgae monocultures, leading to improved resource recovery and potentially higher product yields. These strategies provide a roadmap for upcycling side-streams as feedstocks for microalgae-based and microbial food production to achieve net-zero waste and meet sustainable development goals.