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Acid Production and Curd Toughness in Milks of Different _s1-Casein Types

Eastern Utilization Research and Development Division and Animal Husbandry Research Division, USDA, Beltsville, Maryland

ABSTRACT

Skimmilks differing in _s1-casein type were inoculated with starter and renneted. Time to reach pH 5.0 and 4.8 was recorded and toughness of the curd was measured at both pH's. Nine Holstein cows of the following phenotypes were used: 1 A, 3 AB, 3 B, and 2 BC. Numbers were limited by the rarity of the A and C types. Milk from cows of the same type was pooled. The study was repeated eight times over a period of four weeks. Analyses of covariance were done with SNF per cent as the independent variable. The A milk was significantly slower than B and BC in reaching pH 4.8 (P<.01). Adjusted means, in minutes, were as follows: BC 289, B 290, AB 304, and A 314. All differences in curd toughness were significant except between AB and B at pH 5.0. The curd was soft when _s1-casein A was involved, while C was associated with toughness. Adjusted means, in grams of force to cut the curd, were: pH 5.0, A 1, AB 19, B 23, BC 119; pH 4.8, A 5, AB 55, B 81, BC 128. The grams of separator slime per 454 g of milk separated also were recorded. Adjusted means were BC .08, B .13, AB .21, A .47. All differences between types were highly significant. Results on a few animals indicate that type of _s1-casein may be of practical importance in some dairy manufacturing operations.

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