Techno-functional and nutritional evaluation of Solein single-cell protein and its application in non-dairy yoghurt alternatives

The growing global population and climate crisis demand expanding non-animal protein options. Single-cell protein biomass, referred to as "Solein", is produced by the hydrogen-oxidising bacterium Xanthobacter sp. SoF1 and is a promising, sustainable source of protein and dietary fibre, especially when created using renewable energy. This study investigates Solein protein powder (SPP) for its composition and techno-functional properties, comparing it to pea protein isolate (PPI). SPP had a lower fat content and higher dietary fibre, while matching the protein content of PPI. SPP met all indispensable amino acid requirements for adults over the age of three, as outlined by the FAO in 2013. A milk alternative resembling semi-skimmed cow's milk was produced from SPP and PPI. These emulsions were fermented with a commercial starter culture containing Streptococcus thermophilus. The fermentation process was monitored by tracking pH, total titratable acidity, and microbial growth. The resulting yoghurt alternative (YA) underwent textural and rheological analysis. Solein protein powder yoghurt alternative (SPP-YA) exhibited faster acidification, greater microbial growth, improved water retention, and a texture similar to dairy yoghurt. Static in vitro digestion revealed moderate protein digestibility of the non-fermented SPP emulsion (63.8-67.5%), based on total amino acids, free amino groups, and total nitrogen, with an in vitro Digestible Indispensable Amino Acid Score (DIAAS) of (51.0 ± 6.1%). Fermentation slightly reduced digestibility (57.8-59.6%) and DIAAS (48.3 ± 1.4%), with isoleucine as the limiting amino acid. This work provides the first insight into the structural and nutritional performance of hydrogen-oxidising bacterial protein in non-dairy YA.