Hydrolyzed whey protein isolate (WPI) is used in the food industry for protein enrichment and modification of functional properties. The purpose of the study was to determine the feasibility of subcritical water hydrolysis (SWH) on WPI and to determine the temperature and reaction time effects on the degree of hydrolysis (DH) and the production of peptides and free amino acids (AAs). Effects of temperature (150 to 320 °C) and time (0 to 20 min) were initially studied with a central composite rotatable design followed by a completely randomized factorial design with temperature (250 and 300 °C) and time (0 to 50 min) as factors. SWH was conducted in an electrically heated, 100-mL batch, high pressure vessel. The DH was determined by a spectrophotometric method after derivatization. The peptide molecular weights (MWs) were analyzed by gel electrophoresis and mass spectrometry, and AAs were quantified by high-performance liquid chromotography. An interaction of temperature and time significantly affected the DH and AA concentration. As the DH increased, the accumulation of lower MW peptides also increased following SWH (and above 10% DH, the majority of peptides were <1000 Da). Hydrolysis at 300 °C for 40 min generated the highest total AA concentration, especially of lysine (8.894 mg/g WPI). Therefore, WPI was successfully hydrolyzed by subcritical water, and with adjustment of treatment parameters there is reasonable control of the end-products.
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