The pH- and freezing point values of milk in the Western and Southern Cape and factors affecting these values

[摘要] English: The milk yield per cow has gradually increased over the past two decades, and resulted in compositional changes of the milk that altered the physical properties of milk.pH a physical property, which is used as a rejection test on the milk receiving platform, is a resultant of the milk constituents and their concentrations. The current South African specification states that the pH of raw milk should fall within the limits, 6.60 and 6.75.Freezing point, a colligative property of milk, is depended on the concentration of solute ions in solution and not on the chemical identity of the solute. The measurement of the freezing point of milk provides the most reliable means of detecting and measuring extraneous water in milk. The current 'accepted South African specification states that the freezing point of raw milk should fall between -0.512 °C and -0.545°C.The specifications for pH and freezing point are at least 13 and 19 years old, respectively, and may be outdated, resulting in the rejection of acceptable milk or vice versa. The purpose of this study was therefore to determine if these specifications are still applicable or whether this topic warrants further research to establish new specifications.Seventeen variables as determined in 21 564 and 28 083 herd milk samples, originating in the Western Cape and the Tsitsikama regions of the Eastern cape of South Africa, were correlated with the pH- and freezing point values of raw cows' milk, respectively.The pH-values obtained showed the following statistically characteristics, namely: mean (6.72), median (6.72), mode (6.70), and range (6.60 to 6.84).The corresponding statistics of the freezing point values were: mean (-0.523°C), median (-0.522°C), mode (-0.521°C), and range (-0.545 to -0.502°C).pH was significantly (p < 0.05) influenced by chloride (r = + 0.35), magnesium (r = + 0.21), calcium (r = - 0.20), average humidity (r = - 0.19), somatic cell count (r = + 0.16), average temperature (r = - 0.15), lactose (r = - 0.10), evaporation (r = - 0.09), fat (r = - 0.07), average rainfall (r = - 0.05), wind speed (r = + 0.04) and total bacterial count (r = -0.02), whereas protein, total solids, solids-non-fat, sunshine, and radiation showed no significant (p < 0.05) influence.Freezing point was significantly (p = 0.05) influenced by wind speed (r = + 0.36), lactose (r = + 0.35), solids-non-fat (r = - 0.35), protein (r = - 0.28), total solids (r = - 0.23), fat (r = - 0.20), somatic cell count (r = - 0.19), radiation (r = + 0.17), chloride (r = + 0.15), total bacterial count (r = - 0.14), average temperature (r = + 0.12), sunshine (r = + 0.06), evaporation (r = - 0.05) and average humidity (r = - 0.05), whereas calcium, magnesium, and rainfall showed no significant (p < 0.05) influence.Statistical analyses indicate that the pH was significantly (p < 0.05) influenced by region, season, breed, and feed and the freezing point by region, breed, feed, and not by season.The findings of this study justify the hypothesis that the pH specification, being �?6.60 and �?6.75, is outdated, because 14.2% of samples fell outside the upper limit. It would therefore appear that the freezing point standard is still valid because a small percentage, 1.88 %, of the samples studied, gave freezing points higher than the current specification, being �?-0.512°C. As samples drawn from bulk tanks might contain water residues, due to unintentional contamination, it may have contributed to the higher freezing points of these samples. The freezing point of an even smaller percentage, 1.28 %, of samples were found to be below the suggested lower limit.

[发布机构] University of the Free State