Pyruvate in Producer and Commingled Manufacturing Grade Milk.

Area of Science:

• Food Science
• Microbiology
• Analytical Chemistry

Background:

• Direct microscopic count (DMC) is a standard method for assessing milk quality.
• There is a need for rapid, automated tests to evaluate milk quality efficiently.
• Pyruvate concentration ([P]) has been proposed as a potential indicator of bacterial load in milk.

Purpose of the Study:

• To evaluate an automated pyruvate concentration test as a substitute for direct microscopic count (DMC) in determining manufacturing grade milk quality.
• To assess the correlation between initial pyruvate concentration ([IP]) and various bacterial counts (standard plate count [SPC], psychrotrophic plate count [PPC], DMC) in different milk types.
• To determine the reliability of the pyruvate test in classifying milk quality based on bacterial load thresholds.

Main Methods:

• Collected manufacturing grade milk samples (producer, tank truck, skim milk) from a milk drying plant.
• Performed immediate analyses for SPC, PPC, and DMC.
• Stabilized pyruvate concentration ([P]) by heating samples to 85°C for 10 min for initial pyruvate concentration ([IP]) analysis.
• Stored unheated samples at 4°C and analyzed for [P] and DMC up to 120 hours.

Main Results:

• [IP] showed good correlation with PPC in producer milk (r=0.81) but weaker correlation in skim milk (r=0.26).
• In skim milk, [IP] correlated better with initial DMC (r=0.61) than SPC or PPC.
• The pyruvate test correctly classified 82-91% of producer milk samples based on PPC, SPC, and DMC thresholds (6.5 mg/L pyruvate for 3 × 10^6 bacteria/ml).
• False-negative errors occurred in samples with high bacterial counts, likely due to pyruvate catabolism by bacteria exceeding 10^7/ml.

Conclusions:

• The automated pyruvate test is a viable, rapid substitute for DMC in assessing manufacturing grade milk quality, particularly for producer milk.
• The test's accuracy may be limited in skim milk and when bacterial counts exceed approximately 10^7/ml due to pyruvate consumption.
• Further research may be needed to refine the pyruvate test for diverse milk matrices and very high bacterial loads.