Effect of milk solids concentration on the pH, soluble calcium and soluble phosphate levels of milk during heating | pH Effet de la concentration du lait en matière sèche sur le pH et les taux de calcium et phosphate solubles au cours du chauffage

When milk is processed to dairy products, the concentration of the milk and the temperature/time combinations to which the milk is exposed can be varied. For example, large differences in milk concentrations and temperature/time combinations are used in the manufacture of milk powder or the various concentrated liquid milk products. Although the milk salts, particularly the concentrations and equilibrium states of calcium and inorganic phosphate, are considered to be important in the stability and functionality of dairy products, there have been few studies on the mineral balance in concentrated milk systems and how this is affected by temperature changes. The objective of this study was to examine the effect of milk concentration (9.6–38.4% w/w total solids), temperature (20–80 °C) and time (0–60 min) on the pH and the concentrations of soluble calcium (Caₛₒₗ) and soluble inorganic phosphate (Pₛₒₗ). At any given temperature, the level of Caₛₒₗ and Pₛₒₗ, as mmol·kg⁻¹, increased and the pH decreased as the milk concentration was increased. However, when measured as a percentage of the total, the level of Caₛₒₗ and Pₛₒₗ decreased as the milk concentration was increased. At any given milk concentration, the level of Caₛₒₗ, Pₛₒₗ and milk pH decreased within the first few minutes of heating, with little further change at longer heating times. For milk samples at each concentration, there was a strong correlation between the final Caₛₒₗ and the final Pₛₒₗ at all temperatures. At any particular milk concentration, there was a strong correlation between the final milk pH and the final Caₛₒₗ or the final Pₛₒₗ levels. These results indicate that substantial decreases in milk pH, Caₛₒₗ and Pₛₒₗ can occur at the temperatures milk will experience in common commercial processes, and the extent of the changes will be dependent on the milk concentration and the processing conditions.