Heat Resistant Psychrotrophic Microorganisms

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Abstract
The title contains two relative terms: heat resistant and psychrotrophic. When micro- organisms are exposed to a destructive agent such as heat, they die logarithmically. From an understanding of the logarithmic nature of microbial death, any microbial species can survive any given heat treatment if the volume of product is large enough. One species might be so sensitive to a given heat treatment, such as pasteurization, that a large volume of product is required after treatment to contain one living cell-perhaps 4 million liters or more. For another species, survival of the same heat treatment might be 100%. There is a spectrum of differences in heat resistance in between the imaginary species 1 have used as examples. It is important to keep the numbers of organisms low in a product that is to be heat treated. The smaller the number of organisms in a raw product, the larger will be the total volume required to contain one living organism after the heat treatment is applied. How much spoilage can a milk company tolerate? One liter in 100? One liter in 1000? I understand that in Europe the goal for shelf life of sterilized milk is spoilage of not more than one bottle in one thousand bottles. The term psychrotroph also is relative. Other terms are used synonymously-ter ms such as: psychrophile, faculative psychrophile, cold tolerant, or psychrotolerant, depending on the author or speaker. Ingraham and Stokes (7) defined psychrophilic bacteria as those that "grow well at 0 C in 2 weeks". Stanier, et al. (16) divide psychrophiles into obligate and facultative psychrophiles and define facultative psychrophiles as "those that can grow at temperatures above 20 C". Witter (19), in a review article published in Journal of Dairy Science, defined psychrophilic bacteria as "bacteria that grow at a relatively rapid rate at refrigeration temperatures" and defined refrig-