Localized surface condensations affect microbial stability of intermediate moisture foods (IMF). Two surface modifications previously reported were tested for their ability to improve microbial surface stability; control of surface preservative concentration and reduced surface pH. The effectiveness of a high surface sorbic acid concentration controlled by a zein coating was confirmed in Staphylococcus aureus S-6 surface challenge experiments under extreme testing conditions. Samples with bulk water activity (aw) of 0.88, stored at 30°C under constant 88% relative humidity (RH) remained stable for over 16 days. Uncoated controls were stable for only 2 days. Samples with bulk aw = 0.85, exposed to cycles of 12 h at85% RH and 12h at 88% RH, remained stable for more than 28 days. Uncoated controls were stable for only 3 days. The reduced surface pH approach to microbial stability required the formulation of an IMF model with low total electrolyte concentration which was coated with a deionized mixture of λ-carrageenan and agarose. The effectiveness of the resulting 2.5 fold increase in the surface availability of the active form of sorbic acid as compared to food bulk was confirmed in a challenge test with S. aureus S-6. In a test with samples at aw = 0.88, stored at RH = 88% and 30°C, we found a stability period of about 20 days and it seems possible to increase it.
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Torres et al. (1985) studied this question.
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