Valorization of okara into proteins and fermentable sugars through reduced-pressure alkaline pre-treatment

Abstract

Okara, a soybean by-product from soymilk and tofu production, is generated in large amounts, with approximately 1.1–1.2 kg of wet okara produced per kg of soybean processed. For example, Frias Nutrición (Burgos) produces 7700 tons annually, contributing to approximately 14 million tons worldwide. Due to its high moisture content, storage is costly, requiring drying to prevent spoilage. Discarding it would a significant loss of valuable proteins and fiber. Valorizing okara, especially by recovering proteins and sugars from fiber, without drying pretreatment, would significantly reduce costs and enhance sustainability. The first step to valorize okara was protein hydrolysis using reduced-pressure alkaline pretreatment (RPAP), varying pressure, time, and NaOH concentration. Two optimal conditions were identified at 70 °C using 5 % okara (w/v): (A) 455 mbar, 180 mM NaOH for 1 h (77 ± 3 % yield), and (B) 300 mbar, 100 mM NaOH for 2 h (75 ± 1 % yield). Techno-economic assessment identified process B as more efficient and sustainable at industrial scale. Residual solids from both extractions were subsequently subjected to enzymatic hydrolysis with Cellic® CTec2 (40 FPU/g biomass at 5 % and 10 %), releasing a high content of simple sugars. These hydrolysates were used for lactic acid fermentation with Ligilactobacillus salivarius ATCC 11742 to assess their suitability as substrates. Each hydrolysate was tested alone, supplemented with salts and nitrogen sources typical of MRS medium, and compared to MRS itself. The supplemented hydrolysates showed nearly identical performance to the ideal medium, confirming their suitability for fermentation without inhibitory effects.