Influence of Preheating Milk on the Physical Properties and Microstructure of Acid-Induced Milk Gel

The objective of this study was to investigate the influence of preheating skim milk on the physical properties and microstructure of acid-induced milk gel. Skim milk samples preheated at various temperatures (60-90°C, 10min) were acidified by an acidulant, glucono-δ-lactone (GDL). The storage modulus (G') and the loss moduhis (G") were measured using a rheological apparatus. The time courses of changes in G' and G" for the control and preheated samples were examined after acidification. The maximum value of the storage modulus (G'max) and the geiation rate (GR) were found to increase with the increase in temperature of preheating above 70°C, whereas the gelation time decreased. The tan δ(G"/G') at the time of G'max decreased with the increase in preheating temperature. These results indicated that preheating facilitated the subsequent gelation of the skim milk upon acidification. The apparent activation energy of gelation for the control and a sample preheated at 80°C for 10min were 69.5kJ/mol and 70.3kJ/mol, respectively. The effects of preheating on the microstructure of acid-induced milk gels were investigated by scanning electron microscopy (SEM). The SEM micrographs showed that in the case of the control, the aoid-induced gel consisted of a coarse three-dimensional network of casein particles joined together in large clusters, whereas the gels formed from preheated skim milk consisted of a finer network of smaller clusters of casein particles.