Valorisation of Sugar Beet Processing Waste for Enhanced Production of Fungal Biomass as a Sustainable Alternative Protein Source

This study investigates the use of fungal strains as an eco-friendly and cost-effective approach to enhance the protein content of sugarbeet pulp (SBP) via semi-solid fermentation (SmF). The effects of ultrasonication (US) and hydrothermal (HT) pretreatments, combined with SmF, were evaluated with respect to the nutritional and technological characteristics of the resulting SBP fungal biomass. Fermentation using filamentous fungi, such as Aspergillus nidulans CCF2912, Botrytis cinerea CCF2361, and Rhizopus arrhizus CCF1502, increased the protein content of SBP fungal biomass from 53.7 to 93.82–134.11 mg/g d.w. and improved the essential amino acid (EAA) ratio from 0.81 to 1.96. Nucleic acid (NA) content in the end product was decreased by 20.3–31.5% following additional sterilisation at 90 °C for 30 min, while mono- and polyunsaturated fatty acids increased from 2.36 to 4.93% and from 5.11 to 10.16% of total fatty acids, respectively. Tryptic digestion of SBP fungal biomass proteins allowed in silico prediction of peptides (up to 2 kDa) with potential DPPH, hydroxyl, and ABTS•+ radical scavenging activities. Among the tested fungi, R. arrhizus CCF1502 grown on US-treated SBP substrate showed the highest protein biomass yield and overall nutritional quality. These findings demonstrate that integrating thermal pretreatment with SmF provides a sustainable and efficient strategy to enhance the protein yield and nutritional value of SBP, supporting its potential application as a functional ingredient in food and feed applications.