Angiotensin I converting enzyme inhibition and enzymatic resistance in vitro of casein hydrolysate treated by plastein reaction and fractionated with ethanol/water or methanol/water

A casein hydrolysate was modified by the plastein reaction, fractionated with ethanol/water or methanol/water solvents and further hydrolyzed by four proteases to show the potential impact of these treatments on Angiotensin I converting enzyme (ACE) inhibition in vitro. The treated hydrolysate exhibited a higher ACE-inhibitory activity than did the original hydrolysate: the IC50 value decreased from 52.6 μg mL−1 (original) to 14.9 μg mL−1 (treated). Fractionation of the treated hydrolysate with the lowest polarity solvent led to soluble fractions with a higher activity and precipitate fractions with a lower activity. Hydrolysis of the treated or the fractionated hydrolysate by one of four selected proteases for 10 or 30 min reduced the activity; papain, pepsin or trypsin (but not alcalase) digests had residual activities of about 32–50% at 25 μg mL−1, higher than the original hydrolysate (27.8%). It was concluded that the plastein reaction, but not solvent fractionation, produced casein hydrolysates with protease resistance.