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C. RODRIGUEZ and D. G. Dalgleish. Dept. of Food Science, Univ. of Guelph, Ontario Agricultural College, Guelph, ON N1G 2W1, Canada Acid gelation of milk is widely employed in the production of fermented dairy products. The viscoelastic characteristics of acidified milk gels depend on the prior heat treatment, which causes casein-whey protein interactions. Research has been done on acid milk gels but still the complete mechanism of gelation and the function of soluble protein complexes in the serum remains unexplained. The objectives were to vary the formation of soluble serum complexes during heat treatment of milk and determine what effect they have on milk gelation. Combinations of heat treatments at 90°C for 10 min. and addition of small amounts of gluteraldehyde, allowed the variation of the amounts of soluble complexes. Milk was acidified using GDL. Time sweep tests were performed during the gelation in a stress-controlled-rheometer. The amounts of complexes were measured by size exclusion chromatography of supernatants. Complexes were also varied by changing the pH of milk before heating from 6.5 to 7.2 . The results show significantly higher values of G’ on heated milk and when glutaraldehyde was added after the heat treatment. When glutaraldehyde was added before the heat treatment the G’ decreased, similar to the untreated sample.The milk heated at pH 6.7 showed the highest G’, followed by milks heated at pH 7.2 and 6.5. The size exclusion chromatography matched these results, showing positive correlation between the amounts of soluble complexes and the G’ of the acid gels, except when milk was heated at pH 7.2. The casein micelle structure showed no significant change during acidification. The soluble protein material that remains in the serum appears to be essential to obtain firmer acid gels. However, results at pH 7.2 suggest an undefined change in the micelles, which makes them less coagulable. These results are helping us to define new methods of controlling the properties of acid gels.
Session 46, Dairy Foods: General
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