Consumer confidence in the food industry is severely affected by large-scale spoilage incidents. However, relatively little research exists on spoilage potential of members of the fungal subphylum Mucormycotina (e.g. Mucor), which includes dimorphic spoilage organisms that can switch between a yeast-like and hyphal phase depending on environmental conditions. The presence of Mucor circinelloides in yogurt may not cause spoilage, but growth and subsequent changes in quality (e.g. container bloating) can cause spoilage if not controlled. The purpose of this study was to evaluate the effects on M. circinelloides of pasteurization regimen, natamycin concentrations, and storage temperature in yogurt production, as measured by fungal proliferation and carbon dioxide production.

A strain of M. circinelloides isolated from commercially spoiled yogurt showed greater yogurt-spoilage potential than clinical isolates and other industrial strains. D-values and z-values were determined for the spoilage isolate in milk as an evaluation of the fungus' ability to survive pasteurization. Natamycin was added to yogurt at 0, 5, 10, 15, and 20 ppm (μg/ml) to determine its ability to inhibit M. circinelloides over the course of month-long challenge studies at 4 °C, 15 °C, and 25 °C. Survivors were recovered on acidified PDA and carbon dioxide levels were recorded. The D-values at 54 °C, 56 °C, and 58 °C for hyphae/sporangiospores 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. The D-values at 51 °C, 53 °C, and 55 °C for yeast-like cells were (in min) 14.25 ± 0.12, 6.87 ± 1.19, and 2.44 ± 0.35, respectively, which yielded a z-value of 0.34 °C. These results indicated that M. circinelloides would not survive fluid milk pasteurization if contamination occurred prior to thermal treatment. CO₂ production was only observed when M. circinelloides was incubated under low-oxygen conditions, and occurred only at temperatures above 4 °C. Addition of 10 ppm and greater of natamycin inhibited the growth and CO₂ production of M. circinelloides under moderate temperature abuse when compared to the untreated control.

These data suggest that yogurt spoilage (container bloating) caused by anaerobic growth of M. circinelloides is due to post-pasteurization contamination. Temperature abuse facilitated spoilage as CO₂ production was observed in yogurt incubated at 15 °C and 25 °C, but not at 4 °C. The addition of at least 10 ppm of natamycin prevented M. circinelloides growth in both hyphal and yeast-like phases, as well as CO₂ production in temperatures of up to 15 °C for 30 days.