Diffusing-wave spectroscopy investigation of heated reconstituted skim milks containing calcium chloride

The effect of heat treatment on reconstituted 10 wt% skim milks containing up to 20 mM added CaCl2 at different pH values (pH 6.0–7.2), was investigated both in situ and after cooling of the heat treated milks. Measurements of pH in situ showed that pH decreased at high temperature, that the decrease in pH increases with the increase in the initial pH and that the magnitude of the decrease in pH was greater for milks with added CaCl2. Marked increases in the viscosity at 25 °C of heated milks indicated that milks without added CaCl2 with initial pH ≤6.2 and milks with 10 mM added CaCl2 with initial pH ≤6.4, heated at 90 °C were not heat stable. At a given heating temperature, it was possible to superimpose the measured viscosity of samples with or without added CaCl2 on the same curve when these were plotted as a function of the pH at that temperature instead of the initial milk temperature. To further demonstrate this finding a DWS experimental set-up was built and in situ measurements were performed on the milk samples heated at 75 °C for 10 min. The DWS measurements showed that ηa, the product of the viscosity with particle size, can also be superimposed for all the measured samples, when plotted as a function of the pH at the heating temperatures. Both the viscosity and DWS data demonstrate the importance of the pH at the heating-temperatures in influencing heat-induced changes in milk.