Physicochemical and functional properties of bovine  milk protein isolate and its associated hydrolysates

Bovine milk protein isolate (MPI) is produced by ultrafiltration (UF) and diafiltration (DF) of skim  milk (followed by spray-drying) and has a protein content of ≥ 90% on a dry matter basis. High  protein milk protein concentrate (MPC) and MPI are well documented to display impaired  rehydration properties. The primary rate limiting factor during hydration is the dispersion of casein  micelles from inter-linked micellar networks formed due to changes in colloidal properties during  manufacturing and storage of MPI.

This thesis describes novel research on the development of MPI enzymatic hydrolysates with  enhanced solubility. It also addresses the comparative functional characteristics of intact and enzymatically hydrolysed MPI particularly in relation to the impact of different enzyme activities and the associated differences in physicochemical characteristics of their hydrolysates on the functional properties.

MPI hydrolysates generated with Flavourzyme™, Neutrase™ and Protamex™ at two hydrolysis  time points per enzyme with degree of hydrolysis (DH) values ranging from 15.6 to 37.1%, were  evaluated in comparison to intact unhydrolysed MPI. Gel permeation chromatography indicated  digestion of individual milk proteins in all hydrolysates. Hydrolysis of MPI with Flavourzyme™,  Neutrase™ and Protamex™ resulted in enhanced nitrogen solubility between pH 4.0 and 7.0  compared to intact MPI. MPI hydrolysates displayed reduced water droplet contact angles (<50°)  compared with intact MPI (~90°), indicating increased wettability. Hydrolysis also resulted in modifications to other functional properties of MPI. Intact MPI was  highly heat stable at 140°C above pH 6.8 (>10 min), while all hydrolysates coagulated within 64s  at 140°C between pH 6.2-7.4. The hydrolysate samples displayed reduced foam capacity and reduced foam half-life (58.1-214s) at 25°C compared to the intact protein (783-850s) despite  displaying reduced surface tension. MPI hydrolysates displayed reduced glass transition  temperatures (48.5-67.3°C) compared with intact MPI (~121°C), indicating reduced powder  storage stability and an increased risk of stickiness, lumping and caking compared with intact MPI.  The hydrolysate samples displayed reduced whiteness and altered yellowness and greenness compared to the MPI control samples. The FlavourzymeTM  hydrolysate (60 min incubation) demonstrated a comparable bitterness (mean bitterness score: 2.14) to the intact protein (1.52-1.57). All other hydrolysates displayed increased bitterness where the NeutraseTM and ProtamexTM hydrolysates had the highest mean bitterness scores (8.80-9.39). Oil-in-water (O/W) emulsions  modelled on a commercially available infant formula prepared using the MPI hydrolysates  demonstrated reduced emulsion stability under accelerated storage conditions compared to the  O/W emulsions containing intact MPI.

Overall, enzymatic hydrolysis enhanced the solubility and rehydration properties of MPI; while  their heat, emulsion and foam stability were reduced and bitterness perception was increased.  Instability and bitterness development was hydrolytic enzyme, DH and molecular mass  distribution dependent. The MPI hydrolysates may find application in specific food and beverage  applications, however, the exploitation of the benefits of hydrolysis (e.g., enhanced solubility) in  complex food matrices may require modification of the formulation (e.g., incorporation of calcium  chelating and viscosity enhancement agents) and storage at controlled ambient temperatures to  achieve improved functional characteristics and emulsion stability. Additionally, the use of FlavourzymeTM  to limit bitterness development may be a key selection factor in the use of MPI  hydrolysate ingredients in specific food and beverage applications.