Pyruvate as an Indicator of Quality in Grading Nonfat Dry Milk ¹

Samples of skim milk and nonfat dry milk (NDM) made from it were collected, paired and tested for pyruvate concentration, [P], and Direct Microscopic count (DMC). The skim milk was tested for Standard Plate Count (SPC) and Psychrotrophic Plate Count (PPC). The geometric average DMC of skim milk was more than three times higher than that of the paired NDM samples. However, [P] of NDM was not significantly different from that of the skim milk. Although [P] of skim milk was poorly correlated with SPC and PPC, r = .31 and .26, respectively, it was relatively well correlated with DMC, r = .64. Data were widely dispersed around the regression line when [P] was ≤ 4.0 mg/L. However, [P] increased rapidly when DMCs were > 10⁶/ml. A limit of 10 mg/L of [P] in NDM reconstituted 1:9 was chosen to represent the current U.S. Department of Agriculture Standard for DMC in NDM. This limit failed to classify about 10% of the samples correctly, assuming that each geometric mean DMC was correct. However, the probability that samples meeting the DMC standard would be rejected by the pyruvate test was quite low and the probability was moderate that samples which would be acceptable by the pyruvate test would be rejected by the DMC. For the latter, 28% of the samples having DMCs of ≥ 10⁷/ml contained < 10 mg/L of pyruvate. No sample having ≥ 10 mg/L of pyruvate had a DMC of ≤ 10⁷/ml. Pyruvate concentration in NDM did not change during storage at 5 or 32°C for 90 days.