Extracellular peptidases from insect- and compost-associated microorganisms: screening and usage for wheat gluten hydrolysis

Abstract

Screening for microorganisms with an enzyme activity for a specific application is challenging. In this study, we showed a successful way to isolate microorganisms from insects and compost samples, which had a desired enzyme activity, namely the hydrolysis of wheat gluten. This was realized by a two-step screening approach. The first step was an agar plate screening assay for extracellular peptidase activity, and the most promising organisms were further tested in a second screening step: the hydrolysis of a wheat gluten suspension. Here, the extracellular peptidases of two microorganisms showed similar hydrolysis performances compared to the commercial enzyme preparation Alcalase®. The best isolate, which was identified as a Bacillus subtilis species, was cultivated in a bioreactor (35 L scale) to produce a larger amount of the extracellular peptidases. The peptidase activities in the cell-free culture medium were partially purified, as it is common for technical enzyme preparations for the food industry. After centrifugation and ultrafiltration, an ammonium sulfate precipitation was performed. This peptidase preparation was biochemically characterized, and the obtained stability was sufficient for application. Wheat gluten hydrolytic approaches (500 mL scale) were performed under controlled conditions (e.g., pH value). The amount of solubilized gluten protein obtained after 24 h hydrolysis was higher using the enzyme preparation from the isolate compared to Alcalase®. The enzyme stability increased under process conditions, compared to the stability in buffer, due to the stabilizing effect of the substrate. The biochemical characteristics and the hydrolysis performance showed the potential of the novel extracellular peptidases for the hydrolysis of the industrially relevant substrate wheat gluten.