Effects of high intensity ultrasound on acid-induced gelation properties of whey protein gel

Pre-heated whey protein isolate (WPI) solution (10% w/v, 85 °C for 30 min) was prepared and subjected to high-intensity ultrasound (HUS, 20 kHz) at different durations (5-40 min) before acidification to determine the effects of HUS on glucono-δ-lactone (GDL)-induced gelation properties of whey protein gel. Results showed that HUS reduced the particle size and increased surface free sulfhydryl groups of pre-heated whey protein solution. Free sulfhydryl (-SH) content of the acid-induced WPI gels (GIWG) and protein solubility in presence of 8 M urea were significantly reduced (P < 0.05) by HUS (20 or 40 min), indicating that HUS facilitated formation of more disulfide bonds during/after the gelation process. HUS significantly increased (P < 0.05) the water holding capacity (WHC), gel strength, gel firmness (G′) as well as the frequency dependence of GIWG. The WHC, gel strength and gel firmness were positively correlated with surface free -SH content of pre-heated WPI and negatively correlated with particle size of pre-heated WPI and free -SH content of GIWG. The results indicated that high intensity ultrasound could be used for modifying whey protein to improve its gelling properties.