Enhancement of Thermal Stability of Ultra-high Temperature-Sterilized Yak Milk by Different Pretreatment Techniques

The enhancing effects of different pre-treatment techniques (addition of emulsifying salts, electrodialysis, and resin adsorption) on the stability of ultra-high temperature (UHT)-sterilized yak milk with standardized protein content was investigated by measurement of ethanol stability, protein precipitation rate, whey protein denaturation rate and turbidity stability index (TSI) as well as by using Fourier transform infrared spectroscopy (FTIR) and inductively coupled plasma mass spectrometry (ICP-MS). The results showed that all three pre-treatment methods significantly improved the heat stability of UHT-sterilized yak milk. Compared with the standardized control group, whey protein denaturation rate decreased by 92.02% and 90.15%, soluble calcium content by 3.94% and 13.93%, and protein precipitation rate by 11.71% and 20.40% in the samples treated by resin and electrodialysis, with an average TSI of 2.94 and 1.33, respectively. Whey protein denaturation rate decreased by 84.69%, and protein precipitation rate increased 18.39% in the emulsifying salt-treated sample. The resin-treated group had the highest absolute value of the zeta potential (22.83 mV), closely followed by the electrodialysis group (19.57 mV), suggesting that electrodialysis also improved the heat stability of UHT-sterilized yak milk. FTIR analysis revealed that after UHT treatment, both resin and electrodialysis groups showed an increase in the relative content of α-helix and a decrease in the relative content of β-sheet. In conclusion, this study demonstrates that adjusting the salt balance of yak milk can enhance its UHT stability, providing important guidance for the processing of UHT yak milk.