Phosphates as inhibitors of yeasts isolated from food sources

The inhibition of twenty yeast strains isolated from food sources by six commercial phosphates (polyphosphates A and B; sodium tripolyphosphate - high solubility, TAS; sodium tripolyphosphate, TRI; sodium neutral pyrophosphate, N; sodium acid pyrophosphate, PAS) was investigated. The assays were performed at neutral (all phosphates), acid (2.5, 3.5 and 4.5) (polyphosphates A and 13) and natural (without adjustment). (phosphates TAS, TRI, N and PAS) pH values, inoculating the yeasts in plate count broth with phosphates at concentrations of 0.1, 0.3, 0.5, 1.0 and 1.5% (w/v). The minimal inhibitory concentration (MIC) was determined. The phosphates with chain lengths greater than 15 phosphate units were more inhibitory and had the highest sequestering power. At neutral pH values, these polyphosphates showed MIC values of 0.3% for some yeasts and all the strains were inhibited completely with 1.5% phosphates. At natural pH values (pH obtained after phosphate addition), alkaline phosphates (TAS, TRI and N) were more inhibitory than at a neutral pH. Phosphate TAS (0.5%) increased its inhibitory capacity (percentage of strains inhibited) from 5% (pH= 7.0) to 70% (pH= 8.15). When phosphatcs A and B (chain lengths between 15 and 20 phosphate units) were assayed at acidic pH values, there was a clear inhibitory action that was exclusively due to the addition of these phosphates. These results demonstrate that some phosphates could be used in the food industry to inhibit yeasts associated with food spoilage.