Analytical Food Microbiology. Ahmed E. Yousef. Читать онлайн. Newlib. NEWLIB.NET

Автор: Ahmed E. Yousef
Издательство: John Wiley & Sons Limited
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Жанр произведения: Химия
Год издания: 0
isbn: 9781119428015
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depends on the microorganisms of interest and their natural habitat, as well as presence of competing microbes.

      In food microbiology, several incubation temperatures are typically used. For potentially pathogenic organisms, such as Salmonella enterica, incubation occurs at 32–37°C, a temperature range suitable for mesophiles. A somewhat cooler temperature (e.g., 25°C) is more preferred by spoilage organisms such as yeasts, molds, and psychrotrophic bacteria. “Room temperature” is typically taken to mean 22°C, but this temperature may vary depending on the room used for incubation and even the season and area of the world. Refrigeration at 4°C is typically used to maintain cultures without allowing further growth. A refrigerated incubation can also be used for cold enrichments of psychrotrophic microorganisms such as Listeria monocytogenes.

      Colony Counting

      “Counting” in food microbiology refers to the determination of the size of a microbial population within a specific quantity of food (i.e., population concentration). Enumerating the number of colonies on agar plates may also be referred to as “counting,” therefore careful distinction between these two usages is urged. Throughout this manual, the former will be referred to as “population count” and the later as “colony count.”

      Some enumeration techniques, such as the direct microscopic counting method, allow determination of the number of cells per unit volume or weight of the sample. The plate count technique, however, determines the number of cells or cell clumps capable of forming colonies on agar plates. Since it is impossible to distinguish colonies arising from individual cells and those from cell clumps, the final population count determined by this method is expressed as colony forming units per unit volume or weight, i.e., CFU/ml or CFU/g.

       Counting rules

      In order to obtain counts that can be compared among different laboratories, it is necessary to establish consistent guidelines for counting colonies. In some circumstances, however, different counting or calculation methods may be used in place of, or in conjunction with, the standard counting rules. The following are the rules that will be applied throughout this book for counting bacterial colonies.

      Plates with colonies in the range of 20–200 (the best possible scenario)

      Duplicate plates with colony counts in the 20–200 range are ideally obtained from one dilution only. When this condition is not met, the following rules are applied.

       One plate in the 20–200 range. If the exercise yields only one plate with a colony count in the range of 20–200, calculate the CFU/g in the original sample using the number of colonies on that plate instead of an average.

       Consecutive dilutions producing plates in the 20–200 range. If plates from two consecutive dilutions yield 20–200 colonies, compute the CFU/g resulting for each of the two dilutions. If the two population counts are not appreciably different (e.g., 1.5×104 and 1.2×104 CFU/g), average the numbers and report the average as sample population count. If the numbers are substantially different (e.g., the higher CFU/g is more than twice the lower one), report only the lower computed CFU/g.

      Plates with < 20 colonies

      Plates with no colonies

      It is not necessary in this case to report the count as “est.” because the presence of the less than (<) sign indicates the uncertainty of the count.

      Plates