Calcium induced skim-milk gelation during heating as affected by pH

Milk gels (acid or rennet) are used by dairy industry to produce dairy products such as yogurt and cheese. Enrichment of milk with calcium salts and heat treatment are known to produce “calcium-milk coagulum” as a new type of milk gels, due to reduction of milk protein charges through calcium binding. The combination of heat treatment and calcium addition to milk results in gel structures, but the effect of calcium addition and pH adjustment during heating of milk is still unclear. The role of added calcium and decreasing pH were investigated by addition of calcium chloride (30 mM) to reconstituted skim milk followed by pH adjustment by hydrochloric acid and sodium hydroxide (4.6<pH<6.6 investigated), followed by heating at 90 °C for 10 min and overnight storage at 22 °C. In parallel, samples with no addition of calcium chloride were produced under the same conditions. The time and temperature to reach the gelation point, as detected by dynamic measurements of storage modulus (G′), were decreasing as pH decreased without addition of calcium, while calcium addition made gelation time and temperature independent of pH except for pH 4.6. Heat treatment combined with calcium addition was found, using confocal laser microscopy, to provide a fine and dense gel structure for skim milk with higher pH, while at pH lower than 5.6, the gel structure was similar to the structure of acid-induced gels. The last observation helps to establish a pH limit for production of calcium gels.