Exploration on the Stability of Intermediate Moisture Foods

Microbial stability of intermediate moisture foods (IMF's) has been found to be affected by unsteady state environmental conditions. Temperature fluctuations induce localized surface condensations resulting in potential surface outgrowth before moisture diffuses into the food product. Due to organoleptic, cost and/or safety restrictions it is not always possible to include safety margins by lowering water activity, or increasing preservative content in IMF formulations. The solution explored in our current research has been to consider surface a separate region of the food and, develop processes that enhance its growth inhibiting properties providing a much needed safety margin on the surface. The first approach to enhanced surface resistance to microbial growth was to maintain, for as long as possible, an uneven preservative concentration: high on the surface and low in food bulk. We found that this was possible by the use of zein based coatings that reduced diffusion into food bulk of sorbic acid applied on the surface. The effectiveness of this surface treatment was shown in Staphytococcus auteu6 S-6 surface challenge experiments under extreme testing conditions. Samples with bulk water activity (aw) of 0.88, stored at 30C under constant 88% relative humidity (RH), remained stable for 16 or more days. Uncoated controls were stable for only 2 days. Samples with bulk aw=0.85 exposed at 30C to cycles of 12 hours at 85% RH and 12 at 88% RH, remained stable for 28 or more days. Uncoated controls were stable for only 3 days. Sorbic acid distribution studies showed that the mechanism for the improvement was diffusion control. Apparent diffusion coefficients for sorbic acid were 3-7 x 10~9 cm2/sec in zein coatings and 1 x 10-6 cm2/sec in the IMF model used in this study. The second approach to microbial stability was the establishment of a pH difference between coated surface and food bulk. Preliminary experimental work showed that samples with reduced surface pH remained free of microbial growth significantly longer than untreated controls. However, as soon as the pH difference disappeared rapid growth occurred. The increased resistance to microbial growth at reduced pH is associated with the increased bacteriostatic effectiveness of lipophilic acids such as sorbic acid. Since a surface pH reduction increases availability of the most effective form of the preservative, it results in significant microbial stability improvements. It has been shown that it is possible to establish permanent and controlled pH differences between surface and bulk values. A negatively charged macromolecule can be immobilized in the form of a food surface coating component while other molecules, particularly electrolytes, can move freely. The immobilization of the macromolecule can be described best using a Donnan equilibrium model. This model allowed us to estimate pH differences between surface and food bulk. It also predicted that the key parameters were electrolyte concentration and the number of charged groups of the macromolecule. To prove the validity of the ApH concept we formulated an IMF model with low total electrolyte concentration (about 0.005M) and coated it with a deionized mixture of X-carrageenan and agarose. Measured pH differential was in the 0.3 to 0.5 pH units. Such a reduction resulted in a 2.5 fold increase in the surface availability of the active form of sorbic acid as compared to food bulk. The effectiveness of this surface treatment was shown in S.auteus S-6 challenge tests at aw= 0.88, RH = 88% and 30C. We found a stability period of about 20 days. It seems possible to obtain even longer periods. In our tests the loss of stability was most probably due to growth occurring in the sample bulk and not on the surface.