Aquaculture by-product: a source of proteolytic enzymes for detergent additives

Intestine proteases of Nile tilapia (Oreochromis niloticus) were partially purified by heat treatment (purification factor of 3.5, enzyme activity remained almost constant) to reach the maximum activity and stability within an alkaline pH range of 7.2–11.0. The optimum temperature and stability over a 120 min period were found to be at 55°C and at 35–45°C, respectively. The proteases’ activity was not affected by a 1 vol. % saponin surfactant, inactivated by 0.01 g mL^?1 sodium dodecylsulphate after 120 min, and it remained stable for 30 min in a 5 vol. % and 10 vol. % hydrogen peroxide solutions. The proteases were slightly activated by Ca²⁺, Mg²⁺, and K⁺ and the substrate most effectively hydrolysed was casein (40.0 U mg^?1). A 2⁴ full factorial design used to evaluated the influence of independent variables showed that the enzyme extract, detergent concentration and the incubation time had a significant influence on the enzymatic activity. The best conditions to be used concerning detergent additive were found with 0.3 mg mL^?1 of protein and 3.0 mg mL^?1 of detergent for 30 min in the presence of Astrus® detergent.