Response surface optimization of angiotensin converting enzyme inhibition of milk protein concentrate hydrolysates in vitro after ultrasound pretreatment

Studies were conducted on ultrasound pretreatment of milk protein concentrate (MPC) before enzyme hydrolysis in order to produce hydrolysates with potent angiotensin converting enzyme (ACE) inhibitory activity. The effects of three independent variables of ultrasound pretreatment time (min), hydrolysis time (h) and enzyme-to-substrate ratio (%) on ACE inhibition as determined by central composite rotatable design were investigated in vitro. Optimization was achieved by response surface method. All the independent variables showed significance at 95% confidence level in contributing to ACE inhibition. According to response surface analysis, the optimum conditions for producing hydrolysates with the highest ACE inhibition were ultrasound pretreatment time, hydrolysis time and enzyme-to-substrate ratio of 4.11 min, 232 h and 2.33%, respectively. Under these optimal conditions, the theoretical ACE inhibition was 0.044 mg mL(-1). Model validation yielded satisfactory results indicating relevance of the model to explain ACE inhibition while ultrasound pretreatment showed strongest effect on ACE inhibition of MPC hydrolysates. Industrial relevance: Milk protein concentrate has reportedly produced inferior bioactive compounds when compared to other milk proteins such as whey protein concentrate and sodium caseinate. Through the use of ultrasound pretreatment, milk protein concentrate can be used to produce more potent ACE inhibitory fractions comparable to those from non-aggregated milk protein sources. The use of short ultrasound pretreatment time offers an attractive route for industrial production of bioactive compounds from milk protein concentrate and other complex protein sources. (C) 2013 Published by Elsevier Ltd.