26-5 |
Rheological properties, whey separation, and microstructure in set-style yogurt: Effects of heating temperature and incubation temperature |
W. J. LEE and J. A. Lucey. Dept. of Food Science, Univ. of Wisconsin, Madison, 1605 Linden Dr., 10 Babcock Hall, Madison, WI 53706-1519 Whey separation is considered as a major defect in yogurt, and is connected with extensive rearrangements of the gel network. A better understanding of rheological and physical properties of yogurt gels is necessary to control whey separation. Our objective was to understand how process variables, such as heat treatment and incubation temperature, influence the rheological, physical properties and microstructure of yogurt gels. Skim milks were heated at 72, 75, 82.5, 90, or 93°C for 30 min and were incubated with a commercial starter culture at 34.3, 36, 40, 44, or 45.7°C until pH of milk reached at pH 4.6. A central composite experimental design and response surface methodology were used for data analysis. The rheological properties were determined by dynamic low amplitude oscillation with the measurement of storage modulus and loss tangent. The amount of spontaneous whey separation was quantified by volumetric flask test. Confocal scanning laser microscopy was used to observe microstructures of yogurt gels. Second order polynomial models predicted the effects of heating and incubation temperature on the rheological properties, permeability, and whey separation of yogurt gels. The storage moduli of yogurt gels increased with an increase in heating temperature and a decrease in incubation temperature. The maximum loss tangent, permeability, and whey separation of yogurt gels increased with a decrease in heating temperature and an increase in incubation temperature, indicating an increased possibility for rearrangements, which was confirmed by presence of large pores in microstructures of these gels. Whey separation of yogurt gels was negatively correlated with storage modulus (r = -0.66), and was positively correlated with the maximum loss tangent (r = 0.63) and permeability (r = 0.78). This study demonstrates that high milk heat treatment and low incubation temperatures favor less rearrangement of the network and leads to less whey separation of yogurt gels.
Session 26, Dairy Foods: General developments in dairy technology
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