Effect of seasonality and processing on physicochemical characteristics of goat and sheep milk : a thesis presented in partial fulfilment of the requirements for the degree of Master of Food Technology at Massey University, Palmerston North, New Zealand

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The interest toward goat and sheep milk consumption is growing with the scientific evidence for the unique health benefits of milk from non-bovine species. However, it is not fully understood whether goat and sheep milk will have the same processability when subjected to standard dairy processing developed for cow milk. Besides, it is of interest to the dairy industry how seasonality interacts with the processing treatments in affecting the properties of goat and sheep milk. The aim of this research was to understand the interspecies difference in physicochemical characteristics of milk, and the processing-induced changes as influenced by seasonality. Fresh goat and sheep milk samples were collected from local producers and characterised for their compositional and physicochemical properties over three different seasons. The fresh whole milk (goat and sheep) was subjected to various processing conditions: 75°C/15s with or without homogenisation and 95°C/5 min with homogenisation. The seasonal and processing-induced changes in goat and sheep milk were analysed using conventional methods. The gelation properties of the milk (acid and rennet) were also investigated. Goat milk was similar to cow milk in major components, but the protein composition was different. It contained a significantly lower amount of αs1-casein. Seasonality showed minimal effect on the composition and physicochemical properties of goat milk due to non-seasonal kidding management. Sheep milk was characterised with a higher content of macronutrients and minerals, and the casein micelle was more mineralised. It had higher buffering capacity and higher viscosity due to the rich amount of buffering components and total solids. The composition and physicochemical properties were broadly same across early and mid-season but changed during late milking season. Heat treatment (95°C/5 min) increased the size of casein micelles in both types of milk, and the effect was more pronounced for sheep milk (48% versus 24% increase in goat milk). The heat-induced increase in micelles size was much bigger in goat and sheep milk than in cow milk, suggesting different mechanisms of casein micelle modification. After heated at 95°C for 5 min, the denaturation level of whey protein and their association was lower in goat milk than in sheep milk. The goat α-lactalbumin was found more heat stable than the sheep counterpart. Goat milk formed weak gels when inoculated with rennet or acidified. Homogenisation decreased final storage modulus (G') and the final loss tangent of goat milk rennet gel. Heating (95°C/5 min) increased the final G' of acid gels made from goat milk, but the impact was far less pronounced than that in cow milk. Sheep milk formed much stronger rennet and acid gels compared to goat milk. Heat treatment (95°C/5 min) improved the acid gelation properties of sheep milk significantly (shorter gelation time, higher gelation pH and increased G' value). However, the extent of improvement was less pronounced in the late season despite the higher protein content.
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