Circular bioeconomy through valorisation of agro-industrial residues by the edible filamentous fungus Neurospora intermedia

Abstract: Prevention of dramatic climate change and ensuring food and nutrition security for subsequent generations necessitates the reduction of greenhouse gas emissions and efficient use of the world’s resources, including efficient waste disposal. The agro-industrial sector generates a large amount of organic waste, that is currently underexploited owing to poor waste management practices. The circular bioeconomy model is expected to play an important role in the transition towards a sustainable future, and a group of microorganisms known as fungi can be part of the solution, owing to their ability to convert organic waste into useful products, including both materials and energy. These processes hold great potential to change waste materials into resources, leading to societal and environmental benefits. The versatility of various fungi, including their ability to grow in both submerged and solid states, enables the valorisation of liquid and solid streams such as ethanol plant residues ‘thin stillage’ and surplus bread. However, fungal strains need to produce marketable products and exhibit good growth characteristics to be considered suitable for industrial applications. An interesting candidate is the edible filamentous fungus Neurospora intermedia. This fast-growing fungus is able to grow on a wide array of substrates, in both liquid and solid states, and produce industrially relevant products, including its own nutrient rich fungal biomass and carotenoid pigments. Submerged fermentation by N. intermedia in semi-synthetic medium showed that the formation of carotenoids can be enhanced by modulating various factors such as light, low pH, high aeration, and the addition of Mg2+ and Mn2+. When cultivated in thin stillage, 6.3 g/L ethanol was produced, along with protein-rich fungal biomass with potential application as feed. However, an additional step was needed to promote pigment production in the fungal biomass. Inspired by the traditional use of N. intermedia for food production by solid-state cultivation, the fungal biomass obtained from cultivation in thin stillage was used as inoculum in a subsequent solid-state fermentation step on surplus bread. The fungal product obtained contained up to 33% proteins and 1.2 mg carotenoids/g total material dry weight. By further combining bread as substrate with brewer’s spent grain, a nutrient-rich fungal-fermented product with an attractive texture was successfully produced. The solid-state cultivation resulted in an improved ratio of essential amino acids, and an increase in dietary fibre, minerals, and vitamins, that added further value to the product. To scale up the process, a novel plug-flow bioreactor was developed and successfully operated semi-continuously, without the addition of an external inoculum. Furthermore, a techno-economic feasibility study of on-site solid-state fermentation in small-scale bakeries revealed that the implementation of this process to sustainably use surplus bread at bakery level is economically feasible. These studies lay the foundation for the development of N. intermedia as a tool to convert waste material into useful products, contributing to a fossil fuel-free future with positive impacts on the economy.