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Properties of the Colostrum of the Dairy Cow. VII. pH, Buffer Capacity and Osmotic Pressure¹

Kansas Agricultural Experiment Station, Manhattan

ABSTRACT

A study was made of changes in pH, in buffering capacity and in osmotic pressure of colostrum and transitional milk produced during the first 2 wk. postpartum. The average pH of first colostrum from 57 cows was 6.28, range 6.00 to 6.61. The pH of the early postpartum mammary secretions from 20 cows increased gradually from an average of 6.32 in first colostrum to 6.50 in milk of the 14th day (composites of 27th and 28th milking). Buffering capacity of early postpartum secretions was greater than that of normal milk and decreased rapidly during the first four milkings. Buffer indices
were calculated at various pH values. The buffer index of first colostrum at maximum buffering (approximately at pH 5) was double that of 14th-day milk and shifted to a slightly lower pH value. Osmotic pressure of colostrum was higher than that of milk collected on the 14th day. The decrease in osmotic pressure was the most marked in the first four postpartum milkings. Milk collected on the 14th day had about two-thirds the osmotic pressure of first colostrum.

¹ Contribution no. 465, Department of Chemistry and no. 205, Department of Dairy Husbandry.