Determination of freezing point standard for UHT milk

Resumo

The extended shelf life and undemanding storage requirements of UHT (ultra high temperature) milk make it the most consumed fluid milk in Brazil. The thermal treatment to which UHT milk is subjected alters casein micellar structure, leading to protein sedimentation and age gelation. Therefore, the current legislation allows the addition of up to 0.1% stabilizing salts (sodium citrate and/or sodium phosphates) in milk. The adding of stabilizing salts hinders the possibility of inclusion of freezing point and density as quality parameters of UHT milk. Evaluation of freezing point is the main test to detect adulteration of milk with water; however, addition of stabilizers alters its results. This study aimed to measure the alterations in freezing point and density caused by stabilizers in samples of pasteurized milk added with sodium citrate or sodium phosphates, in order to propose standards for freezing point and density for UHT milk. For this purpose, 16 aliquots of milk were prepared in nine repetitions with different concentrations (0%, 0.001%, 0.01%, 0.05%, 0.075%, 0.1%, 0.125%, and 0.15%) of sodium citrate or a commercial mix of sodium mono-, di- and triphosphate. The density and freezing point of all aliquots were tested. As expected, addition of stabilizers lowered the freezing point of milk. Addition of 0.1% citrate and commercial mix promoted an average reduction of –0.021°H (± 0.001) and –0.017°H (± 0.002) in freezing point, respectively. However, no significant change in the density of milk was observed. Given that UHT milk industry usually adds the maximum limit of 0.1% of stabilizers in order to obtain proper effect, the interval from –0.572°H to –0.545°H can be suggested as the standard for freezing point to UHT milk. However, as the addition of stabilizers caused no significant change in the density of UHT milk, the density parameter of raw and pasteurized milk in the range from 1.028 g.mL-1 to 1.034 g.mL-1 could be applied to UHT milk.