The denaturation of whey proteins in whole milk during processing by the indirect UHT and vat systems was determined. Whey protein denaturation plateaued at about 88% following a UHT heat treatment of 149°C for 10 s. However, it reached about 88% with a vat heat treatment of 82°C for 5 min and reached 95 to nearly 100% after 10 to 15 min. Vat processing under pasteurizing conditions at 63°C for 30 min resulted in less than 10% denaturation. The yogurts prepared from the vat processed milk had considerably higher structural firmness than those prepared from the UHT milk, as confirmed by the lower value of curd firmness and the higher shear stress of these yogurts. It was also found that the structural or curd firmness of yogurts prepared from UHT milk was highest when the milk was processed for 3.3 s process holding time and was lower when the milk was processed for longer or shorter process holding times. The results suggest that the vat process at 63°C or 82°C brings about a specific change in the whey protein which leads to formation of a firm structure. This change apparently does not occur during the brief UHT treatment.
The flow behavior of whey protein isolates (WPI) was studied in systems processed under different conditions. Experiments were undertaken to study effects of heating conditions (temperature/time), pH, solid content, calcium chloride, and guanidinium hydrochloride addition on the gelation of whey protein isolate solutions. The rheological data demonstrated a power-law frequency dependence of the viscoelastic functions G'() and G"() and a frequency-independent tan ␦ determined from a multifrequency scan of tan ␦ vs gelling time at the gel point. The power-law exponents (n) obtained from these dynamic measurements for determining the sol-gel transition point were between 0.62 and 0.69. Those values suggest a percolation mechanism for the gelation process.
Apparent viscosity of ultra-high-temperature (UHT) treated milk and of yogurt prepared from this milk was studied and compared to that of vat-treated milk and yogurt. UHT-treated milks (149 C, 3.3 sec) had an apparent viscosity of 2.3 to 2.7 cp, while the apparent viscosity of vat-treated milks (63 C and 82 C, 30 min) ranged from 1.9 to 2.0 cp. The apparent viscosity of unheated (raw) milk was 1.7 cp. The apparent viscosity of yogurt prepared from UHT-treated milk became nearly constant at 0.8 cp after 14 min of shearing, while the apparent viscosity of yogurts prepared from vat-treated milks ranged from 1.8 to 3.8 cp under the same conditions. All yogurts exhibited thixotrophic behavior.
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