Thermal Resistance Characteristics 741 D And Zvalues

Although juice spoilage from heat-resistant molds has been known for decades, the thermal resistance of spore-forming bacteria in low pH fruit juice and beverages was of little concern to fruit juice manufacturers prior to the 1990s. Despite the isolation of thermoacidophilic spore-formers from apple juice in 1982 [1], the importance of this genus to juice stability and consumer acceptance received little attention until reports of spoilage surfaced in the early 1990s from Europe and the U.S. Since that time, considerable research has been published illustrating the abundance of heat-resistant alicyclobacilli in low pH juices and beverages.

Kinetic parameters of A. acidoterrestris have been elucidated by several research teams using various techniques with different controlled conditions and heating menstrua. Early research by Splittstoesser et al. [28,29] produced D- and z-values that supported previous empirical observations of spore survival in thermally treated juices. D-values (the time necessary at a specific temperature to reduce the overall microbial population by 90%)

reported by these researchers in apple and grape juices ranged from almost 60 minutes at 85°C to between 2 and 3 minutes at 95°C (Table 7.2). Since typical commercial thermal process conditions are in the range 85 to 100°C for 10 to 30 seconds, Splittstoesser's results demonstrated conclusively that spores of the alicyclobacilli could survive traditional pasteurization and hot-fill processes to cause spoilage in shelf-stable products.

Similar kinetic results for A. acidoterrestris have been reported in other juices, beverages, model broth systems, and distilled water by various laboratories (Table 7.2). Reported D-values range from 81 minutes at 88°C to about 1 second at 125° C. Although specific D- and z-values from the various studies differ, there are general similarities in magnitude. Average D-values from Table 7.2 are 47 minutes (81 to 85°C), 24 minutes (86 to 90°C), 17 minutes (91 to 95°C), 7 minutes (96 to 100°C), 3.8 minutes (at 110°C), and 0.025 minutes (at 125°C). This is illustrated in Figure 7.3, which shows an overall thermal death time curve for data points in Table 7.2.

A z-value is the temperature increase needed to reduce by 1-log cycle the time necessary to produce a 90% reduction in cell populations. This is a valuable tool when attempting to alter commercial processing conditions to either decrease the time needed to achieve product safety and stability, or decrease the temperature to enhance product quality. In essence, when process time is decreased, the z-value is used to determine the new target processing temperature. Likewise, if a lower temperature is desired to improve product flavor, the z-value provides the increased time needed to achieve the same product safety and stability as with the previous process conditions.

Except for one study that will be discussed below, z-values in Table 7.2 for A. acidoterrestris in juices, beverages, model systems, and water are relatively similar with an average value of 8.3 ± 1.9°C. This means that on average the time needed to inactivate a specific population of spores will decrease by a factor of 10 if the pasteurization temperature is increased by 8.3°C.

7.4.2 Factors Affecting Thermal Resistance

It is important to remember that the inactivation kinetics of wild-type alicyclobacillus strains may differ from those obtained from laboratory strains that have been subjected to long-term cultivation. Many factors affecting inactivation kinetics for other microorganisms have been studied and may provide insights into thermal inactivation of the alicyclobacilli. Factors include, among others, culture and inoculum incubation temperatures, sporulation temperature, nutrient composition and pH of the growth medium, nutrient composition and pH of the heating menstruum (e.g., test juice), presence or absence of divalent cations, storage temperature of inoculum stock, osmolarity of test juice matrix, or presence of antimicrobial compounds. It is also well documented that specific strains within a species can vary considerably in D- and z-values.

While it might be assumed that various factors would affect thermal resistance of the alicyclobacilli in a similar manner, studies on this topic

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