Bioconversion of thin stillage from corn dry-grind ethanol plants into high-value fungal biomass

Conventional dry-grind corn ethanol plants generate considerable amounts of low-value thin stillage. Thin stillage is currently concentrated by flash evaporation, an energy-intensive process, to produce syrup, which is blended with distillers dried grains (DDG) to produce DDGS. The condensate is recycled as process water. Thin stillage is generated in pasteurized condition and is rich in organic compounds and other nutrients, with a chemical oxygen demand (COD) of 90 g/L The low initial pH of 4 and high nutrient levels make it an ideal medium for fungal cultivation. In this research, the fungus Rhizopus oligosporus was cultivated on thin stillage obtained from a local dry-grind ethanol plant. Preliminary batch tests in a 1.75-L reactor showed proliferous fungal growth under non-sterile conditions and COD removal of up to 86%. Suspended solids decreased from an initial 2 to 3% to as low as 0.03%, with partial removal by attachment to the fungal biomass. The effluent was well-clarified with a light yellow tint. Solids separation is very important, and the effluent from the fungal process could potentially be recycled as process water with minimal further treatment. The fungal biomass is high in lysine (corn protein is low) making it a nutritionally beneficial livestock feed; if fungal cell walls are disrupted, it can be co-fed with DDG to monogastrics. The ability to feed more of the byproducts to hogs and chickens would help resolve the anticipated corn grain shortfall created by the booming ethanol industry. The fungal biomass could also be used to extract chitin/chitosan.