Studies on heat-induced protein interaction and digestion behavior of sheep milk : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Riddet Institute, Massey University, Palmerston North, New Zealand

Abstract

Sheep milk has low heat stability, which results in undesirable changes after heat treatment, such as separation of milk fat, sediment formation, and phase separation. However, the mechanism of low heat stability of sheep milk has not yet been elucidated. Additionally, the protein interactions in sheep milk during heating and the digestion behaviors of differently processed sheep milk are also unknown. Therefore, the aim of this thesis was to explore the protein interactions of sheep milk during heat treatment (67.5–90 °C and 140 °C), and the mechanism of heat coagulation of sheep milk. In addition, the effect of the commercial processing treatment [homogenization (200/50 bar) and thermal (75 °C/15 s and 95 °C/5 min) processing] on the digestion behavior of sheep milk were determined. Sheep skim milk (SSM) was heated under various conditions (including temperatures, heating times and pH values) and the denaturation of whey protein and protein interactions occurring during heating were characterized using high-performance liquid chromatography. Casein micelle diameter increased upon heating, depending on the temperature and time. The association of whey protein with casein micelles and the aggregation of casein micelles occurred simultaneously and contributed to the increase in casein micelle size in SSM. SSM was stable to heat (140 °C) at pH 6.8–6.9 but became unstable at higher or lower pH. The low heat stability of sheep milk was attributed to the low proportion of κ-casein surrounding the casein micelles, high ionic calcium levels and ready dissociation of κ-casein from casein micelles upon heating at pH 7.0. The Human Gastric Simulator was used for in vitro dynamic gastric digestion and pH-stat for simulated small intestinal digestion. Heat treatment of sheep milk resulted in the incorporation of MFGs into the curds through casein‒whey protein or whey protein‒whey protein interactions; this hindered the formation of the closely knitted protein network and led to the formation of fragmented curds. Homogenization of sheep milk resulted in looser and more fragmented curd in comparison with unhomogenized sheep milk; this accelerated the protein hydrolysis and increased the rate of release of protein, fat, and calcium from the curds into the digesta. Processing treatments affected the lipolysis rate but not the lipolysis degree during small intestinal digestion. In conclusion, the findings of this study have advanced our understanding of the heat-induced protein interactions in sheep milk and provided insights into the digestion behavior of differently processed sheep milk within the gastrointestinal tract. This may help to design and develop sheep milk-based products with desired digestive and functional properties.