Adjusting the initial milk pH before freezing affected physico-chemical properties of thawed goat milk

Abstract

This experiment was conducted to study the effects of initial goat milk pH on changes in some physico-chemical properties of milk during freeze-thaw cycles, and to investigate the possible causes by freeze-thawing for 5 cycles to magnify the changes. Raw goat milk was adjusted to 3 alternative pH levels, which were 1) original milk pH, 2) adjusted to original pH-0.2, and 3) adjusted to original pH+0.2. All the milk samples were packed in plastic bags and kept at -18±2°C for 5 days, then thawed at room temperature at 25°C (1 cycle) and this was repeated for 5 cycles. Milk samples from each treatment and each cycle were analyzed for pH, presence of sediment, viscosity, fat droplet size, particle size distribution, heat stability and changes in mineral content of milk serum. The result showed that the goat milk had an original pH of 6.69 (T6.69) that was adjusted to 6.49 (T6.49) and 6.89 (T6.89). During freeze-thawing all the treatment groups exhibited decreasing pH and increasing sediment, milk viscosity and fat droplet size, with only slight differences between treatments. The important differences were in heat stability, as the T6.49 treatment resulted in a heat stability decrease from 80 mins initially to 46 mins (42.50% decrease) at the 5th cycle, whereas T6.89 treatment only had an 8.89% change in heat stability. Average particle diameter (D3,2), volume distribution (D4,3) and uniform distribution (span) increased with freeze-thaw cycles, which clearly showed in T6.49 and T6.69 cases but not distinctly in T6.89. In conclusion, milk pH affected the quality of milk after freeze-thawing. Adjusting pH to 6.89 before freezing in this study reduced changes of pH, sediment formation, coalescence of protein, and heat stability of thawed goat milk; such adjustment is beneficial for further milk processing in the dairy industry.