Twenty-five raw milk samples from Friesian crossbred dairy cows were analyzed for milk fat, protein and lactose using an ultrasonic milk analyzer. The samples were then subjected to two different freezing protocols; single freezing and multiple freezing, after which parameters were reanalyzed after freezing and thawing at different freezing times (24, 48, and 72 h). Paired t-test was used to compare the effect of freezing type while the differences in milk constituents with freezing time were analyzed using ANOVA. Linear regression analysis was also performed to study correlations between freezing duration and any change in cattle milk's macronutrient content. The results indicated that milk fat, protein and lactose content decreased significantly with freezing time. However, the decrease was more in multiple frozen samples than single frozen samples. The most decreased macronutrients were lactose (14.1%) in single freezing and fat (25.5%) in multiple freezing. Analysis of the interaction between freezing type and freezing time showed that freezing time significantly affected all the parameters while freezing type (p=0.03) and its interaction with freezing time (p=0.02) affected only the fat content. In conclusion, it should be noted that cattle milk samples frozen at -20°C leads to a significant decrease in fat, protein, and lactose content. The loss of constituents was much more pronounced when samples were frozen, thawed, and refrozen (multiple freezing) than when samples were thawed only once (single freezing).
Key words: Single freezing, multiple freezing, macronutrients, dairy cattle.
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