Zeta-Potenetial of Casein Micelles as a Factor in Age Gelation of Ultra-High Temperature Processed Concentrated Skim Milk

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Olson, Douglas W.
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Age Gelation; High Temperature; Skim Milk; Processed; Casein Micelles; Food Science; Nutrition
thesis / dissertation description
The effect of ultra-high temperature processing by direct steam injection and room temperature storage of pH-adjusted and unadjusted 3X (volume reduction) skim milk retentate and the effect of storage at various temperatures of 3X skim milk retentate without pH adjustment on their ζ-potential, viscosity, and pH were determined. Pasteurized skim milk was concentrated to 3X by ultrafiltration. In the pH study portions of skim milk retentate were adjusted to pH 6.38 ± .02 with HCl and 6.85 ± .01 and 7.32 ± .02 with NaOH between ultrafiltration and ultra-high temperature processing. In the storage temperature study, storage temperatures used for pH-unadjusted retentate samples were 11°C, 23°C (room temperature), and 37°C.Although pH 6.38 samples had the lowest viscosity in the pH study before ultra-high temperature processing, these samples precipitated during ultra-high temperature processing. For non-acidified samples, increasing pH of retentate resulted in higher viscosities and quicker age gelation times. Destabilization occurred more rapidly at 37°C than at 23°C or 11°C.The pH drop tended to be greater for samples stored at a higher temperature or adjusted to a higher pH. During 28 weeks of 37°C storage the pH decreased from 6.54 to 6.06. During 32 weeks of 23°C storage pH of samples initially adjusted to pH 7.32 dropped to 7.06.ζ-Potentials of casein micelles were calculated from electrophoretic mobility obtained by measuring Doppler frequency shifts of scattered laser light in samples that had been diluted 300 fold with their own ultra.filtrate. Absolute values of ζ-potential of samples stored at 37°C decreased from -23.4 millivolts immediately after ultra-high temperature processing to -18.5 millivolts after 28 weeks of storage. For samples stored at 11°C and 23°C in the storage temperature study and control samples in the pH study, absolute values of ζ-potential decreased approximately 1.5 to 2.0 millivolts during 28 or 32 weeks of storage.