Effect of ultrasound on the structure and functional properties of transglutaminase-crosslinked whey protein isolate exposed to prior heat treatment

Effects of ultrasound treatment (20 kHz, 41–45 W cm−2, 0, 20, 40 or 60 min) on the physicochemical, functional properties and elements of the secondary structure of transglutaminase (TGase)-crosslinked whey protein isolate (WPI) exposed to prior thermal treatment (75 °C, 15 min) were investigated. The largest molecular size of proteins in the TGase-crosslinked WPI was observed after the ultrasound and thermal pre-treatment (HU-WPI-TGase). HU-WPI-TGase had the maximum intrinsic fluorescence intensity, with highest loss of free amino groups. Ultrasound-treated WPI (U-WPI) showed more (13%) emulsifying activity and more (63%) foaming ability than untreated WPI, but HU-WPI-TGase had higher foam stability and lower emulsifying activity than U-WPI. FTIR analysis indicated that ultrasound, heat treatment and TGase cross-linking had effects on the β-sheet, β-turn and random coil of WPI. The outcomes from this study show a potential application in providing novel functional ingredients for the dairy industry.