EFFECT OF PREHEATING OF MILK ON THE STRUCTURE OF ACIDIFIED MILK GELS

The effect of preheating of milk on the structure of acid-induced skim-milk gels was studied by rheological methods and permeametry. Gels were made from skim-milk solutions prepared from low-heat milk powder (made from skim-milk pasteurized for 15 s at 72-degrees-C) and from high-heat milk powder (made from the same milk that had undergone an additional heating step of 5 min at 85-degrees-C). Gels were formed by quiescent heating of a skim-milk acidified in the cold or by addition of glucono-delta-lactone (GDL). Gels made by addition of GDL to low-heat milk clearly differed from the same gels made from high-heat milk and from the gels prepared by acidification in the cold. The dynamic moduli of gels made by addition of GDL to low-heat milk, were much lower, their dependence on the case in concentration was stronger and the fracture strain was roughly a factor of 2 higher. However the permeability and the overall structure of the large casein aggregates forming the gel was about the same for the different types of gel. These differences were supposed to stem from a difference in the way the large aggregates are linked together, namely by straight or by bent (low-heat GDL gels) strands.