18D-9

C. R. LOSS, Dept. of Food Science, Cornell Univ., 160 Stocking Hall, Cornell University, Ithaca, NY 14853 and J. H. Hotchkiss, Dept. of Food Science, Cornell Univ., 119 Stocking Hall, Ithaca, NY 14853-7201.

Accurate measurement of microbial thermal inactivation rates at times and temperatures relevant to commercial processing has been limited by existing methodologies. Little is known about the effects of high temperature short time treatments (HTST) on inactivation kinetics of bacteria. We have designed a continuous micro-flow submerged micro-coil (CSMC) apparatus that minimizes come-up times (<3.0s), permitting measurement of inactivation kinetics at near HTST conditions. Our objectives were to compare linear and nonlinear (Weibull) models of thermal inactivation kinetics for pure cultures and mixed milk borne bacterial populations at HTST conditions. Using the CSMC, we measured inactivation rates for Listeria monocytogenes, Pseudomonas fluorescens, and native microbial populations in raw milk (gram negatives, thermodurics, psychrotrophs, and standard plate counts) at 72-98°C for 0-40 seconds. Effects of HTST treatments on membrane integrity of L. monocytogenes and P. fluorescens were determined using fluorescence microscopy. Inactivation kinetics of native milk borne bacteria at 72 to 98°C were more accurately described by the Weibull (R²s 0.93 to 0.99) compared to the linear (R²s, 0.30 to 0.77). Inactivation rates (72°C/ 0-10 sec) for pure cultures of L. monocytogenes and P. fluorescens fit the Weibull model better (R² 0.99 for both) compared to the log-linear model (R² 0.8 to 0.93). A 5 sec treatment at 72°C resulted in a 4D reduction of both organisms. Fluorescence microscopy revealed fewer than 50% of L. monocytogenes cells incurred membrane damage whereas a majority of P. fluorescens cells had compromised membranes. These results show that inactivation kinetics at HTST conditions are accurately described by a nonlinear model. This supports results from other model systems suggesting that a nonlinear model, such as the Weibull, should be used in place of the log-linear model to characterize thermal inactivation kinetics. Better understanding of the underlying mechanisms and reaction kinetics will help design process conditions that optimize food safety and quality.

Session 18D, Food Microbiology: General
2:00 PM - 5:30 PM, Sunday PM Room Hall I-2

2005 IFT Annual Meeting, July 15-20 - New Orleans, Louisiana