Solid agrifood side streams as potential feedstock for microbial biomass and precision fermentation

In this study, grass residues from timothy and red clover, soybean and yellow pea pods, stems and leaves, and brewer's spent grain were investigated as feedstock for Paecilomyces variotii (a biomass fermentation strain) and Trichoderma reesei (a precision fermentation strain). The side streams contained up to 46 % polysaccharides (dry matter) and were pretreated for sugar release using sodium hydroxide (1.6 % (w/v)) treatment at 90 °C for 60 min, or steam explosion at 170–200 °C for 10 min. Solids from the pretreatments were hydrolyzed with Cellic CTec2 enzyme. Combined yields from pretreatment and hydrolysis reached 28–83 % for glucose and 16–71 % for xylose. The glucose and xylose recovery from hydrolysis ranged from 45 %–100 % and 40 %–87 %, respectively. Selected hydrolysates, supplemented with yeast extract and peptone, were used to cultivate the fungi. P. variotii produced significant biomass (12–22 g/L) with all the hydrolysates except those from alkaline-treated brewer's spent grain. The most efficient sugar conversion occurred with hydrolysates from lower temperature steam explosions both with P. variotii and T. reesei. T. reesei growth was inhibited by hydrolysates from higher temperature steam explosions, but substantial growth was measured with yellow pea stems and leaves hydrolysate from 170 °C steam explosion, reaching 14 g/L T. reesei biomass (dry matter). Correlation analysis of the analyzed inhibitory compounds with biomass accumulation revealed that P. variotii showed positive correlations with compounds (e.g., itaconic and methylsuccinic acid, and syringaresinol) that inhibited T. reesei. Further research is needed to validate the findings made with the inhibitory compounds and how to control their accumulation in feedstocks.