THERMAL-DESTRUCTION OF LISTERIA-MONOCYTOGENES IN ICE-CREAM MIX

Thermal destruction of Listeria monocytogenes strain Scott A was studied in refrigerated ice cream mixes to evaluate the relationship of mix composition to heat resistance with differing heat treatments. A central composite response surface design with two independent variables (high fructose com syrup solids content and milkfat content) and one dependent variable (viscosity of the mix) was developed. High fructose com syrup solids (HFCSS) content ranged from 1 to 7%, milkfat (MF) content from 4 to 18%, and total solids content from 28 to 44%. Sucrose content (11%) and milk-solids-not-fat content (10%) were kept constant. D140-degrees-F values were established in the mixes with a simulated batch pasteurization procedure using closed vials. Survivor data indicated sigmoidal responses with initial shoulders and tailing, but shoulder values were not significantly affected by either HFCSS or MF. D and F (F = 7D + shoulder) values were significantly (p = 0.01, R2 = .88; p = 0.01, R2 = .89, respectively) correlated to HCFSS content with increasing heat resistance conferred at higher concentrations. MF content had little or no effect on thermal death time but had a greater effect on viscosity of the mix than did content of HFCSS. Increased thermal resistance of LM was associated with the common ice cream stabilizer used; stabilizer contained guar gum and carrageenan. Pasteurization guidelines for ice cream mix are adequate to ensure inactivation of LM. Since results suggest that major ingredients in ice cream, ice milk, and shake mixes increase thermal resistance of LM, it is important that every precaution be taken to inactivate the organism.