Rheology, tribology and sensory properties of ultrapasteurized milk

Yuanzhu Li

Milk may be pasteurized by high-temperature short time (HTST) pasteurization or ultrapasteurization (UP). Literature suggests that UP increases milk astringency, potentially related to changes in mechanical properties. The objective of this study was to determine how storage time, fat content, heat-treatment, and homogenization pressure affect sensory and mechanical properties, and what relationships exist between these properties. Experiments were separated into three phases. First, skim, 1% fat and whole milks were pasteurized by direct or indirect ultrapasteurization, or by HTST pasteurization and homogenized at 20.7 MPa followed by storage at 4°C. Astringency, friction behavior, and viscosity of each milk sample were determined at 25°C at 1, 4, and 8 weeks after processing. Second, skim, 2% fat, and 5% fat milks were pasteurized by indirect UP or by HTST pasteurization, homogenized at 20.7 MPa, and stored at 4°C. Sensory viscosity, astringency, instrumental viscosity, and friction profiles were evaluated at 25°C at 1, 4, and 8 weeks storage. Sodium dodecyl sulfate polyacrylamide gel electrophoresis and confocal laser scanning microscopy were used to determine protein structural changes. Last, 2% fat milk was heated by indirect UP process, homogenized at 2000, 3000, or 4000 psi, and stored at 4°C. Astringency, friction behavior, viscosity, and microstructure of fat of each milk sample were determined at 1, 4, and 8 weeks storage. Increased fat content increased sensory and mechanical viscosity but decreased mixed regime friction profiles and astringency. Increased storage time increased mechanical viscosity, mixed regime friction profiles, and astringency but did not impact sensory viscosity. Compared to HTST pasteurization, UP milk had higher sensory and mechanical viscosity, astringency and mixed regime friction profiles. Addition of saliva did not impact friction behaviors. While 3000-4000 psi homogenization pressure resulted in lower sensory viscosity compared to 2000 psi, it did not significantly affect milk fat droplet mean diameter, astringency, mechanical viscosity, or friction profiles. Correlations were found between sensory viscosity and mechanical viscosity at 50 s-1 shear rate (R2=0.90), and astringency and friction coefficient at 100 mm/s sliding speed (R2=0.60). Thus, sensory and mechanical behaviors in UP milks are linked and mechanical properties can indicate certain sensory attributes.

Rheology, tribology and sensory properties of ultrapasteurized milk

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