David J. Wilson, Gregory M. Goodel and Tessa Kelly
On-farm pasteurization of waste milk fed to calves has become increasingly common over the last 15 years. This study investigated bacteria counts in milk before pasteurization and for 24 h following pasteurization at varying storage temperatures associated with seasons. Standard plate counts on Aerobic Count Plate PetrifilmTM to enumerate bacterial colony concentration were measured on raw and post-pasteurized milk after pasteurization at 63°C (145°F) for 30 min using commercial pasteurizers on 3 different commercial dairy farms. Each farm was sampled in each of 4 seasons of the year. All 12 batches of milk were divided into 2 aliquots. One aliquot was incubated at controlled temperature to mimic the season (refrigerated, room temperature, or incubated at 37 °C) and the other was incubated at ambient outdoor temperatures. The SPC was determined at pre-pasteurization, 0 (immediately post-pasteurization), 1, 2, 3, 4, 5, 6, 7, 8, 10, 12 and 24 h post-pasteurization. The final general linear model testing for factors associated with LogSPC was highly explanatory (R2=0.71), and significant (P<0.0001). This final model for LogSPC included time since pasteurization, season, farm, the interaction of time and season, and the interaction of season and farm. The model showed that passage of time was associated with increased LogSPC, especially during summer. In a northern temperate climate under the conditions observed during this study, milk - could safely be fed-if defined as SPC<20,000 cfu/ml-for at least 8 h post-pasteurization during fall and spring, and for 24 h during winter, but only for 3 h if milk was stored outside during the summer. These results suggest that for milk kept outdoors during summer, any milk remaining after first feeding following pasteurization should not be fed to calves at subsequent feedings, but instead should be re-pasteurized or discarded.
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