Purpose: The purpose of this study was to evaluate the effects of pasteurization regime, natamycin concentrations, and storage temperature in yogurt production on M. circinelloides, as measured by mold counts and gas production.
Methods: M. circinelloides was isolated from commercially spoiled yogurt. D-values and z-value were determined for hyphae/asexual spores in milk as an evaluation of the mold’s ability to survive pasteurization. Natamycin was added to yogurt at 0, 5, 10, 15, and 20 ppm to inhibit M. circinelloides over the course of month-long studies at 4°C, 15°C, and 25°C. Survivors were recovered on acidified PDA and container bloating due to gas formation by M. circinelloides was recorded.
Results: The D-values at 54°C, 56°C, and 58°C were (in min) 38.31 ± 0.02, 10.17 ± 0.28, and 1.94 ± 0.53, respectively, which yielded a z-value of 3.09°C. This indicates that hyphae and asexual spores would not survive fluid milk pasteurization if contamination occurred prior to thermal treatment. Gas production was only observed when the mold was incubated under anaerobic conditions, and occurred faster at temperatures above 4°C. Addition of 10 ppm natamycin inhibited the growth and gas production of M. circinelloides when compared to the untreated control.
Significance: These data suggest that yogurt spoilage (container bloating) caused by anaerobic growth of M. circinelloides is due to post-pasteurization contamination. The addition of at least 10 ppm of natamycin was found to be effective at preventing M. cicinelloides growth and subsequent spoilage.