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A. P. P. YANG¹, Y. Kou², Y. Wang, and I. A. Taub. (1) Defense Department, U.S. Army SBCCOM, Kansas Street, Natick, MA 01760-5018, (2) Department of Food Science, University of Massachusetts, Amherst, MA 01003

Justification: Glycerol and sodium lactate are known to be effective humectants, lowering the water activity and either inhibiting bacterial growth or promoting bacterial destruction. However, the influence of these humectants on the structure of the food, on the availability of moisture, and on the main determinant of bacterial growth and death is not clearly understood.

Objective: Determine the relationship between survival of Staphylococcus aureus and water proton NMR relaxation time in intermediate moisture (IM) meats of differing water activities.

Methods: Sliced meat samples containing either no humectant or humectants at different levels were prepared. They were then dried to a water activity (Aw) of 0.92, 0.90, 0.88 and 0.86 using a microwave-assisted freeze dryer. They were inoculated with Staphylococcus aureus and stored at 35 o C to study the semilogarithmic death kinetics. PCT 20/20 NMR analyzer with a pulsed-field gradient (PFG) probe was employed to measure water mobility T1 in the uninoculated IM samples. The operating frequency for protons was 20MHz.

Results: The destruction rate of Staphylococcus aureus in all cases increased with decreased Aw. The sensitivity of the bacteria to Aw was greatest for the lactate-containing samples. The measured T1 values decreased as Aw decreased, reflecting a decrease in water mobility.

Significance: These results demonstrate the very significant influence of the humectants on the mobility of water in IM matrices. The effects on the death kinetics of bacteria subjected to osmotic stress appear to be a combination of water mobility/availability and specific lactic acid influences.