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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.

The leather industry uses a chemical dehairing process that produces alkaline wastewater and hair as solid waste. A cleaner process is proposed in this work to reduce these environmental impacts. It consists of the valorization of hair waste together with sludge from wastewater treatment by a solid-state fermentation process to obtain proteases that can be used in an enzymatic dehairing process. SSF was undertaken in 4.5 L scale reactors. Neither sterilization of the materials nor inoculation of pure microorganisms was necessary for the development of the process, whereas aeration was provided during the assay to ensure the prevalence of aerobic conditions. Alkaline proteases were produced as a consequence of the degradation of hair. The highest activity of the alkaline protease in crude extracts was determined at 14 days of the process (56,270 ± 2632 activity units/g dry sample), after the thermophilic stage. The final organic matter showed a stability degree similar to that of compost in terms of respiration activity. Potential application of the extracted proteases in dehairing cow hides was successfully proved as an alternative to the chemical dehairing process. The results in the efficiency of the dehairing process were similar to those obtained with the chemical process, thus avoiding the need of chemical reagents and strong chemical conditions. The entire process permits the substitution of the chemical process of dehairing by an environmentally friendly enzymatic process, closing the organic matter cycle.