Pyruvate in Producer and Commingled Manufacturing Grade Milk

The automated test for pyruvate concentration, [P], was evaluated as a substitute for the direct microscopic count (DMC) in determining quality of manufacturing grade milk. Samples of manufacturing grade milk from producers and tank trucks as well as skim milk were collected at a large milk drying plant. Each sample was tested immediately for standard plate count (SPC), psychrotrophic plate count (PPC) and DMC. Portions of milk were heated to 85°C for 10 min to stabilize [P] before being returned to the laboratory for analysis of initial pyruvate concentration, [IP]. Unheated samples stored at 4°C were analyzed for [P] and DMC daily for up to 120 h. [IP] was a good indicator of PPC in milk from individual producers (r=0.81). However, [IP] was not highly correlated (r=0.26) with PPC in skim milk samples which were characteristically high in SPC and homogeneous in PPC. With skim milk, [IP] was more highly and significantly correlated with initial DMC (r=0.61) than with initial SPC (r=0.31) or initial PPC (r=0.26). [IP] was a good indicator of [P] in stored fluid samples until counts exceeded about 10⁷/ml. Initial DMC and initial PPC were about equally correlated with [P] determined at 24-h intervals, and the initial DMC was a reasonably good indicator of DMC determined at 24-h intervals. Using 6.5 mg of pyruvate/L to represent bacterial counts of 3 × 10⁶/ml by the three methods tested, the pyruvate test correctly classified 91% of 57 samples of producer milk based on PPC, 88% based on SPC and 82% based on DMC. Most of the error was of the false-negative type in which samples with high counts had less than 6.5 mg of pyruvate/L. This was probably because some bacteria catabolize pyruvate once their numbers exceed 10⁷/ml.