Measurement of Pyruvate Content in Milk Ultrafiltrates for Evaluation of Bacteriological Quality

A simple manual method for measuring pyruvate in ultrafiltrate of milk samples is described. Milk was placed in a stirred-cell apparatus and ultrafiltrated through a membrane with nominal molecular weight cut-off of 10,000. Pressure was applied by a nitrogen cylinder to a maximum of 3.51 kg/cm2. The ultrafiltrate was reacted with nicotinamide adenine dinucleotide and then lactate dehydrogenase for spectrophotometric determination of pyruvate. The method was compared to a manual method requiring acid precipitation, centrifugation, pH adjustment, and recentrifugation of milk samples to obtain a supernatant liquid. Milks with added pyruvate corresponding to that in refrigerated commercial milks were tested by both methods. The range of recovery for the ultrafiltration method, 93 to 111%, was not significantly different from recovery by the other method. Ten commercially processed milk samples were analyzed for pyruvate at 1, 7, and 10 days. The pyruvate content of these samples as determined by both methods was not significantly different, and the correlation between methods was .96. Bacteriologic quality of samples from 30 commercial processing plants was measured by standard plate count, psychrotrophic bacterial count, and pyruvate content by ultrafiltration. Overall correlations between logarithm standard plate count or logarithm psychotrophic bacteria count and pyruvate content were .73 or .69. Correlation between flavor score and pyruvate content was −.81.