Inactivation of Bacillus stearothermophi/us Spores

The kinetics of thermal inactivation of Bacillus stearothermophilus spores in water extracts of soybeans ("soymilk") was studied using a pilot scale scraped-surface heat exchanger. Survivor curves followed typical first-order inactivation reactions, with D-values ranging from 21.9 sec at 259 F to 5.3 sec at 268 F. The z-value was 15 F, corresponding to an activation energy of 88.6 kcal/mole. Lethal effects in the heating and cooling cylinders accounted for more than 50o/o of the inactivation above a heater outlet (holding) temperature of 265F. Thermal processing is one of the most important methods for preserving food materials. However, concomitant with the inactivation of pathogenic organisms and/or spores may be a reduction in the level of certain nutrients and other quality factors. As processing temperature is increased, however, the rate of microbial inactivation becomes much higher than the rate of destruction of most nutrients or other quality factors. This is the basis for high temperature/short time (HTST) and ultra-high temperature (UHT) processing of foods. Thus, UHT processes, generally in the range of 250-300 F (with the appropriate low holding time) can result in a very high sterilizing effect with relatively little change in chemical, nutritional and organoleptic quality of food. The sterilizing efficiency of UHT processes is assessed primarily in the laboratory using small volumes of spore suspensions sealed in capillary tubes, heating by immersion in a fluid and det~rmining the proportion of surviving spores. Unfortunately, data obtained in this manner do not always match that obtained in actual large-scale plant situations (5. 7). Burton et a! (J) reported that practical sterilization may show a greater rate of change of thermal death at ultra high temperature than would be expected from capillary tube experiments. In addition, because of the very short heating times involved at temperatures exceeding 250 F, the results may have to be extrapolated to estimate sterilization efficiencies in the UHT range. Thus small inaccuracies in the thermal death data obtained at lower temperatures will lead to substantial errors in the UHT region. Therefore it is necessary to evaluate UHT processes with experiments and operating conditions as close to practice as possible rather than by extrapolation from laboratory results. One method particularly suited for obtaining the ultrahigh temperatures and short times required to produce a satisfactory product, especially for proteinaceous liquid foods containing particulates, is the scraped-surface heat exchanger (SSHE). In these units, a concentrically mounted shaft, fitted with blades, rotates within a cylinder. The product to be processed is pumped into one end of the SSHE cylinder and is continuously agitated and scraped from the heat transfer surface by the blades. This minimizes fouling and burn-on problems on the cylinder wall. In addition to the agitation and scraping effects of the blades, the turbulence induced in the bulk of the fluid near the heat transfer surface makes heat transfer more efficient (3). This paper presents results of an investigation of the efficacy of a vertical, liquid-full SSHE for sterilization of a model, low-acid proteinaceous liquid food (water extracts of soybeans or "soymilk"). In low acid foods with pH greater than 4.5, spores of Clostridium botulinum are generally used as the criterion for evaluating the success of a thermal processing treatment (12,20). However, because of the hazards involved with the large-scale experimental use of C. botulinum, B. stearothermophilus spores were used instead. MATERIALS AND METHODS UHT processing system A schematic process flow diagram of the experimental equipment is shown in Fig. 1. Two feed tanks were used. one filled with water and the JOURNAL OF FOOD PROTECTION. VOL 45, FEBRUARY 1982 D ow naded rom hp://m eridianenpress.com /jfp/article-pdf/45/2474/0362-028x-45_2_145. pdf by gest on 28 Feruary 2020