Fermentation of brewer's spent grains by Pleurotus ostreatus: process optimization by response surface methodology

To achieve high-value utilization of brewer's spent grains and to produce protein-rich food ingredients, brewer's spent grains were upcycled by a liquid fungal fermentation and the fermentation conditions were optimized. Brewer's spent grains (BSG) represent the most abundant by-product of the brewing industry. Although BSG are food-grade, their direct use in food is limited because of sensory changes of the products. In this study, screening experiments revealed that black beer spent grains could be efficiently upcycled by submerged fermentation with the edible fungus Pleurotus ostreatus. The conditions of the fermentation of BSG with P. ostreatus were optimized using response surface methodology, including the parameters substrate concentration, inoculum volume, initial pH value, and temperature. As no separation between BSG and mycelium was possible after the fermentation, ergosterol was used as a biomarker to determine the fungal growth. As optimum conditions, a BSG concentration of 17 g L^- 1 dry matter, an inoculum volume of 4.4% (v/v), an initial pH of 9.3 and a temperature of 30 °C were identified. The fermentation with P. ostreatus increased the true protein content of the biomass compared to the spent grains and reduced the total fat content. The biological value was increased from 88 to 94 (reference standard is whole chicken egg with a biological value of 100). Tryptophan and lysine were limiting in the non-fermented spent grains, while after fermentation, the chemical score of lysine increased from 78 to 92, and tryptophan wasn't limiting anymore. Fermentation by P. ostreatus enhanced the nutritional value of BSG, and the fermentation conditions were optimized by using response surface methodology.