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 we re made from skim-milk solutions prepared from low-heat milk powder (m ade 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 addit ional heating step of 5 min at 85-degrees-C). Gels were formed by quie scent heating of a skim-milk acidified in the cold or by addition of g lucono-delta-lactone (GDL). Gels made by addition of GDL to low-heat m ilk clearly differed from the same gels made from high-heat milk and f rom 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, th eir dependence on the case in concentration was stronger and the fract ure strain was roughly a factor of 2 higher. However the permeability and the overall structure of the large casein aggregates forming the g el was about the same for the different types of gel. These difference s were supposed to stem from a difference in the way the large aggrega tes are linked together, namely by straight or by bent (low-heat GDL g els) strands.