Investigating the impact of αs1-casein genetic variants on sheep milk composition, physico-chemical properties, rennet gelation properties and digestion behavior : a thesis presented in partial fulfilment of the requirements for the degree of Master of Food Technology at Massey University, Palmerston North, New Zealand. EMBARGOED until 18th December 2025.

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2023
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Massey University
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Sheep milk is an important dietary resource in various regions across the world, including the Middle East, Europe, and certain developing countries. The unique composition of sheep milk, with high levels of protein, fat, and essential vitamins, makes it a valuable source of nutrition. Furthermore, the genetic polymorphism of αs1-casein in sheep milk has recently gained attention due to the potential influence on sheep milk properties and dairy production. In this study, the genetic variations in αs1-casein and their effects on the physicochemical properties, composition, rennet gelation properties and digestion behavior of sheep milk were investigated and discussed. The genetic polymorphism of as₁-casein among 39 sheep milk samples were analyzed. Four αs₁-casein phenotypes were identified: AC, BB, BC, and CC and eight sheep milk samples which produced from individual ewes were selected for further investigation. While the fat content remained consistent among the samples, there were significant differences in sheep milk protein and lactose content. Among the selected eight samples, sample 192574 exhibiting the highest protein content and lowest lactose content. In addition, differences were observed in the proportions of major caseins and whey proteins in sheep milk. The mineral content, specifically magnesium, potassium, phosphorus, and sodium, showed variations between samples, and κ-casein, potassium, and phosphorus content seem appeared to be influenced by αs₁-casein phenotypes. The result showed that BB αs₁-casein phenotypes had the highest proportion of κ-casein, while AC αs₁-casein phenotypes had the highest potassium and phosphorus content. The physico-chemical properties including pH, casein micelle size, fat globule size, ethanol stability, heat stability and ionic calcium content were examined. The results showed that, apart from casein micelle size and fat globule size, most physicochemical properties of sheep milk exhibited significant differences among the eight samples. The rennet gelation time, final G', and final tan δ of the sheep milk samples were also examined. Although there were no significant differences in these parameters between the eight selected samples, variations emerged when considering the sampling time, with larger variations in samples from March compared to those from February. Seasonality was found to have a significant impact on the physico-chemical and rennet gelation properties of sheep milk, particularly towards the end of the season. However, no significant differences were observed in the physico-chemical and rennet gelation properties with respect to αs₁-casein phenotypes, it requires for a larger sample size to validate any potential influences in future research. Furthermore, in vitro dynamic gastric digestion was carried out to investigate the digestion behavior of sheep milk. Previous studies primarily focused on bovine milk, and limited information was available on sheep milk digestion behavior, especially concerning the effects of genetic polymorphisms. In this study, eight sheep milk samples from ewes with different αs₁-casein phenotypes were subjected to 20 min in vitro dynamic gastric digestion and three selected samples with different αs₁-casein phenotypes were subjected to 240 min digestion experiment. The curds formed at 240 minutes during gastric digestion displayed a denser and smaller structure, lower dry and wet weights, lower moisture content, and higher G* value compared to the 20-minute digestion curds. The pH profiles of the digesta of the selected three samples did not exhibit significant differences, despite variations in the initial pH values. Furthermore, the SDS-page profiles of the samples revealed that sample 192574 had the most distinct protein hydrolysis level. The distinct digestion behavior of this sample may be attributed to its significantly lower pH and higher κ-casein content compared to the other samples. However, the result showed that αs₁- casein genetic variation had no significant impact on digestion behavior. In conclusion, this research sheds light on the genetic polymorphism of αs₁-casein in sheep milk and its potential effects on composition, physico-chemical properties, rennet gelation properties and digestion behavior. These findings can inform future efforts to improve milk quality and dairy production, as well as enhance the understanding of the complex factors influencing sheep milk properties.
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Embargoed until 18th December 2025
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