Effects of pulsed electric field processing and ultrasound processing on the physicochemical and functional properties of proteins in milk

摘要

Heat treatment is conventionally used in dairy industry to achieve the microbial safety and desired functional properties of dairy products. However, heat treatment can cause detrimental effects on dairy products such as changes in colour and loss of vitamins. Novel processing technologies including pulsed electric fields (PEF) and ultrasound (US) processing have shown great potential to be the alternatives to heat treatment for dairy processing. In this study, the effects of PEF and US processing on milk protein systems that are altered by pH adjustment, EDTA addition or high-heat treatment are examined. The results show that PEF processing did not affect the structure of the casein micelles or denature the whey proteins in reconstituted skim milk, despite the pH adjustment (pH 6.7 to 8.0) or the EDTA addition (10 to 30 mM). On the other hand, US processing (particularly at 20 kHz) of reconstituted skim milk can disrupt casein micelles and reform micelle-like particles with smaller size and comparable ζ-potential to native micelles. US processing of high-heat treated (90 °C/10 min) reconstituted skim milk can modify the physicochemical properties of casein micelle particles and the heat-induced protein aggregates. Based on the processing effects on the milk proteins, processed milk samples were selected for functionality tests. The results show that milk ultrasonicated at pH 8.0 and adjusted to pH 6.7 had superior renneting properties compared to untreated milks or milk ultrasonicated at pH 6.7. The concentrated milk (& 40 % total solid) made from milk ultrasonicated at pH 8.0 and adjusted to pH 6.7 had higher viscosity, compared to that made from untreated milk at pH 6.7. However, the GDL-induced acid gels made from high-heat treated milks that were ultrasonicated had similar gelation properties, compared to those made from milks that were only high-heat treated. The heat stability of recombined concentrated milks made from high-heat treated milks that were ultrasonicated was unchanged, compared to those made from milks that were only high-heat treated. Therefore, PEF processing can be an alternative for pasteurization of milk with preservation of the important milk proteins, although the processing temperature should be carefully controlled. US processing can be integrated to dairy processing to modify the physicochemical properties of milk proteins and achieve desired functional properties.