Potential of the solid-state fermentation of soy fibre residues by native microbial populations for bench-scale alkaline protease production

The production of alkaline proteases by solid-state fermentation (SSF) was evaluated. The effect of three agro-industrial residues was examined: coffee husk, hair waste from the tanning industry and soy fibre residues. Soy fibre presented the highest yield for protease production at the laboratory scale (37 °C, 100 g samples). Consequently, experiments with soy fibre (F) and soy fibre with 10% compost as an inoculum (FC) were performed for 14 days in 4.5 L bench-scale aerobic near-adiabatic reactors. The highest activity occurred under thermophilic conditions and a high respiration activity of 47,331 ± 1391 U/g dry matter for F and 18,750 ± 1596 U/g dry matter for FC, which are much higher values than those reported in other studies. Alkaline proteases showed maximum stability at pH 11 and temperatures of 43.8 °C (F) and 30 °C (FC), which was determined by a full factorial experimental design. Storage assays demonstrated that 90% of enzyme activity was preserved for three months by lyophilising or freezing the samples at −80 °C.

► Diverse organic wastes present high potential as substrates for protease production.
► Our findings consolidate solid-state fermentation as a waste valorisation process.
► Respiration indices are useful to determine the maximum enzyme activity.
► Soy fibre exhibits the highest potential as a source of protease.
► Alkaline pH and mesophilic temperature were optimal for protease activity.