Optimization of crude protein production feed and process for semi-solid-state fermentation of brewing sorghum straw

This study utilized alkali-assisted, high-temperature-pretreated brewing sorghum straw (BSS) for semi-solid-state fermentation to produce high-protein feed. Solid-state fermentation with A. niger for enzyme production was used to promote stalk biodegradation, and single-cell strains P. kudriavzevii 26 and B. subtilis D3-1 were chosen for liquid-state fermentation and propagation to enhance crude protein content. Baijiu distiller's grains were added to balance the carbon-to-nitrogen ratio of BSS. Using single-factor and orthogonal experimental designs, the optimal solid-state fermentation conditions were established: a 4:1 ratio of BSS-to-distiller's grains, inorganic salt nutrient solution at pH 4, and an 18% inoculation of A. niger for enzyme production, followed by 3 days of fermentation. After increasing the temperature for enzymatic hydrolysis and saccharification, 2% (v/v) urea was added to supplement the nitrogen source. P. kudriavzevii 26 and B. subtilis D3-1 were added sequentially at a 2:1 ratio, with a 12% inoculation amount and a 24-h interval between inoculations. The fermentation was carried out at 34 °C for 5 days. Following process optimization, the fermented feed achieved a crude protein content from 10.41% to 18.06%, a 1.6-fold increase compared to pre-optimization. HighlightsDevelopment of a process for the production of feed protein by semi-solid-state fermentation of BSS and distiller's grainFermentation for the production of feed proteins using A. niger, P. kudriavzevii 26 and B. subtilis D3-1 which three strains are used for the sequential fermentation to produce feed proteinsThe addition of distiller's grains to BSS balanced the C/N ratio and eliminated reliance on synthetic nitrogen through a dual-substrate strategy.