Survivor Curve Selection and Customer Relationship Value

The valuation profession employs a variety of statistical techniques, from simple attrition analysis to more complex retirement rate analysis, to assess customer population life characteristics. For many customer populations, the account survival characteristics are assumed to be constant across the various age cohorts of the population such that customer relationship value is indifferent to account age. However, relaxation of the constant attrition rate characteristic can have a direct influence on the magnitude of the value ascribed to the customer relationship value.

Heat Resistance of Salmonella weltevreden in Low-Moisture Environments

The heat resistance of Salmonella weltevreden inoculated into flour and heated in hot air was determined for (a) an initial water activity (aw) range of 0.20 to 0.60 prior to heating, (b) a range of storage relative humidities of 6.0 to 35.5% prior to heating, and (c) temperatures of 57 to 77°C. The death curves obtained were biphasic, demonstrating an initial rapid decline in the numbers of survivors (1.0- to 1.5-log reductions) during the first 5 to 10 min of heating for all the temperature-water activity combinations tested. Following this initial rapid decline in the number of cells, a linear survivor curve was obtained where inactivation occurred at a slower rate. The initial decline in survivors coincided with a rapid decrease in the water activity of all the samples tested. Irrespective of the initial water activity level in the samples prior to heating, the aw decreased to <0.2 during the first 5 to 10 min of heating. The D values obtained for these experimental parameters ranged from a D60–62 of 875 min at an initial aw of 0.4 to a D63–65 of 29 min at an initial aw of 0.5. The results demonstrated that, for any temperature, as the initial water activity of the sample prior to heating decreased, the heat resistance of the cells increased. The z values obtained from these data ranged from 15.2 to 53.9°C. The relative humidity during storage prior to heating did not appear to have a significant effect on the heat resistance of S. weltevreden in flour. These results demonstrate that the amount of available water in foods that are considered to be “dry” (i.e., with a water activity less than 0.60) will significantly influence the effectiveness of the heat processing of foods and, in addition to the temperature, the aw prior to heating is a critical controlling factor during these processes.

Prior Storage Conditions Influence the Destruction of Escherichia coli O157:H7 during Heating of Apple Cider and Juice

In apple beverage manufacture, cider and juice may be stored for a short time prior to pasteurization. Storage time and temperature may affect the subsequent thermotolerance of bacteria in these beverages. This study examined whether prior storage in pH 3.4 apple cider or apple juice affected the thermotolerance of two Escherichia coli O157:H7 strains in the same beverages at 61°C. Both strains exhibited biphasic survivor curves. Strain ATCC 43894 was consistently more thermotolerant than strain ATCC 43889, with 33 to 153% greater D values derived from the linear portion of each survivor curve. Prior storage at 21°C for 2 or 6 h hastened thermal destruction of both strains in apple cider, but not to a statistically significant extent. In apple juice, prior storage at 21 °C for 2 h significantly decreased thermotolerance of strain ATCC 43889, but not of strain ATCC 43894. During 6 h of storage in 21°C apple juice, populations of strains ATCC 43889 and 43894 decreased by 2.1 and 0.5 log10 CFU/ml, respectively, and died rapidly during subsequent heating. Prior storage in apple juice at 4°C for 24 h significantly decreased thermotolerance of both strains, but this effect was not seen after 2 h of storage at 4°C. Experiments with filtered apple cider showed that presence of Alterable pulp enhanced the thermotolerance of both strains. These results show that short-term (≤6 h) room temperature storage of pH 3.4 apple cider and apple juice may enhance the lethality of subsequent pasteurization.

Death rates of bacterial spores: mathematical models

The concave survivor curves produced as a result of spore heterogeneity were analyzed to determine whether they were caused by innate characteristics of the spores or by the acquisition of heat resistance during the heating process. Mathematical models developed for the two hypotheses revealed that the concave survivor curve (on semi-log paper) caused by innate heterogeneity is parabolic and that caused by acquired heat resistance is exponential. The mathematical models were applied to several published survivor curves of different organisms, and heat resistance parameters and the cause of curvilinearity were determined. For the cases studied, the cause of curvilinearity appears to be acquisition of heat resistance rather than innate heterogeneity of spore population.