Effects of high pressure processing (hydrostatic high pressure and ultra-high pressure homogenisation) on whey protein native state and susceptibility to tryptic hydrolysis at atmospheric pressure

Dispersion of whey protein isolate (WPI) prepared at 10% proteins (w/w) and pH 6.5, was pressure-processed using high hydrostatic pressure (HHP) at 300 MPa and 25 degrees C for 15 min, or ultra-high pressure homogenisation (UHPH) at 300 MPa and initial fluid temperature (T-in) of 24 degrees C. UHPH-processing was followed (or not) by rapid cooling of the processed fluid at the immediate HP-valve outlet. Short-time thermal treatment (SITT) at 75 degrees C for 10 s, 43 s or 110 s was studied for comparison. Processing-induced effects were investigated in terms of (i) protein denaturation by differential scanning calorimetry (DSC) and ammonium sulphate (AS) precipitation, (ii) susceptibility to tryptic proteolysis at atmospheric pressure after processing, (iii) protein particle sizes from light scattering measurement during the hydrolysis process and, (iv) residual proteins by SDS-PAGE after 135 min of hydrolysis. UHPH followed by efficient cooling at the HP-valve outlet, limited significantly (p = 0.05) protein denaturation as assessed by DSC and AS precipitation, while increasing notably protein susceptibility to tryptic attack, comparing with the non-processed sample. In opposite, neglecting the cooling step at the HP valve outlet, led to significantly (p = 0.05) lower residual protein native state, then tryptic hydrolysis efficiency. HHP treatment led to intermediate results. The slight protein unfolding that pre-processing could induced, favoured thus the further trypsin attack. Generally speaking, high pressure (avoiding overheating) offers this potential. Process-induced protein aggregation, but also reassembly of protein fragments and peptides released during hydrolysis may affect the proteolysis efficiency. (C) 2015 Elsevier Ltd. All rights reserved.