Yield and protein quality of thermophilic Bacillus spp. biomass related to thermophilic aerobic digestion of agricultural wastes for animal feed supplementation

Bacillus spp. responsible for thermophilic aerobic digestion (TAD) of agricultural wastes were studied for their growth rate, yield and protein quality (amino acid profile) under conditions that approximate full-scale waste digestion as pointers to the capacity of TAD to achieve protein enrichment of wastes for reuse in animal feeding. Specific growth rates of the thermophiles varied with temperature and aeration rates. For Bacillus coagulans, the highest specific growth rate was 1.98 μh−1; for Bacillus licheniformis 2.56 μh−1 and for Bacillus stearothermophilus 2.63 μh−1. Molar yield of B. stearothermophilus on glucose increased with temperature to a peak of 0.404 g g−1 at 50 °C before declining. Peak concentration of overflow metabolite (acetate) increased from 10 mmol at 45 °C to 34 mmol at 65 °C before declining. Accumulation of biomass in all three isolates decreased with increase in temperature while protein content of biomass increased. Highest biomass protein (79%) was obtained in B. stearothermophilus at 70 °C. Content of most essential amino acids of the biomass improved with temperature. Amino acid profile of the biomass was comparable to or superior to the FAO standard for SCP intended for use in animal feeding. Culture condition (waste digestion condition) may be manipulated to optimize protein yield and quality of waste digested by TAD for recycling in animal feed.