| MadSci Network: Biochemistry |
Freezing is used to stop microbial growth. While it may result in some bacterial death it is not a kill step. During the freezing and thawing processes the product will spend some amount of time at a temperature that allows for microbial growth. This growth can result in an infective dose (for food infection organisms) or toxin development (for food intoxication organisms). The amount of growth is the result of numerous variables, including: - original counts - type of organisms present (differing temp growth optimums and tolerance) - freezing conditions (speed of freezing will dictate amount of growth) - Starting temperature before freezing - thawing conditions - thawing temperature - total time at growth temperatures - holding time post thawing A basic explanation of this rationale appears in the FDA 1999 Food Code Annex 3. The date by which the food must be consumed takes into consideration the differences in growth of Listeria monocytogenes at 5 o C (41 o F) and 7 o C (45 o F). Based on a predictive growth curve modeling program for Listeria monocytogenes, ready-to-eat, potentially hazardous food may be kept at 5 o C (41 o F) a total of 7 days or at 7 o C (45 o F) a total of 4 days. Therefore, the period of time allowed before consumption is shortened for food in refrigerators incapable of maintaining food at 5 o C (41 o F) but capable of maintaining it at 7 o C (45 o F) or below. The impact of temperatue can be seen in the following table (From Dr. Pete Snyder's web site at www.hi-tm.com) ºF ºC 10 Multiplications of Pathogens 30 -1.1 123.8 days 35 1.7 19.3 days 40 4.4 7.5 days 41 5.0 6.5 days 45 7.2 4.0 days 50 10.0 2.4 days 55 12.8 1.7 days 60 15.6 1.2 days 70 21.1 16.9 hours 80 26.7 11.2 hours 90 32.2 7.9 hours 100 37.8 5.9 hours The potential abuse from thawing, re-freezing, and re-thawing certainly creates the potential for the growth of harmful bacteria.
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