3.3.2. Yeast-like phase
Successful preservation in the yeast phase challenge study was assessed by fungal proliferation as well as by measurement of CO2 production via head-space analysis. As previously mentioned, minimal growthwas observed in the yeast-like phase, despite rapid CO2 production (Fig. 4). Subsequently, natamycin was not essential in preventing outgrowth of M. circinelloides in the yeast-like phase, but because CO2 production was detected in yogurt containers stored at 15 °C and 25 °C, natamycin was necessary to prevent container bloating. Production of excessive CO2 was not, however, detected in yogurt containers incubated under strict temperature control at 4 °C. This suggests that
some temperature abuse is required to induce CO2 production associated with commercial spoilage.As with the hyphal phase challenge study, the concentration of natamycin necessary to prevent CO2 production was also dependent on temperature. At 15 °C, 10 ppm of natamycinwas sufficient to control CO2 production throughout a 30 day incubation period. Natamycin at 5 ppm controlled CO2 production for 14 days at 15 °C, but CO2 levels showed a moderate increase (~10%) by day 30. At 25 °C, 15 ppm natamycin was necessary to control CO2 production by yeast-like phase M. circinelloides.Without any antifungal in place, CO2 production occurred rapidly. CO2 levels reached almost 40% of the total headspace by day 3 of incubation at 25 °C, despite the limited growth observed
in the yeast-like phase. Natamycin yielded a modest fungicidal effect at 25 °C, reducing the counts by about 2 log with 10 ppm natamycin or greater. However, this same ungicidal effectwas not apparent in yogurt incubated at 4 °C and 15 °C, suggesting that rigeration provides some protective effect and increases M. circinelloides survival.