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Local government food safety guidelines for storing foods (p26) which have already been cooked stipulate food internal temperature must be brought from above 140F (serving temperature) to 70F in less than 2 hours and from 70F to 40F in less than four hours.

Why then do the rules for defrosting (p23) (the reverse) say food should be at room temperature "for very short periods during preparation" and foods can only be defrosted in one of three ways: in refrigerator, under running water, or in the microwave provided there is no interruption in the cooking process (ie. partially in the microwave then cooked or all in microwave). If its just time and temperature, why is the cook trusted to watch the clock for cooling and not defrosting?

Previous posts have asked for guidelines and best practices; however, my question accepts the practical advise and hopes to discover the political (creating effective guidelines) and biological (does warming from 40F generate more/more harmful bacteria than cooling from >140F?) reasons. I have my own theories but what are yours?

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Cooked foods are starting with a very low bacterial count, because the cooking process killed everything. Defrosting foods are starting from an unknown count, because they picked up some during the cooling process before they were frozen. Bacteria multiplies exponentially, so the starting point matters greatly.

I'm glad you asked for theories because I don't have anything to back this up.

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  • I was focused on the fact cooking does not kill all the bacteria. Your's is an interesting position. How quickly we need empirical evidence! Yet the cooling process seems to be the worse for me. When a product cools through the danger zone to refrigeration temperature, the bacteria are not killed. Instead they enter a dormant state. Whereas a food thawed through the danger zone to cooking is entering a state where bacteria is killed. In the latter the concern is for when bac count is too great to be made safe. While the former hopes to slow the bac count only.
    – xtian
    Oct 10, 2011 at 16:39
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The very document you link to clearly explains the reason:

It is important to use methods that will allow the entire mass to thaw evenly. Any method that allows one part, for example, the outside surface, to defrost before the inner portion is not acceptable, because the portion that thaws out first will be in the danger zone before the other portion is thawed.

Bacteria grow and produce toxins while in the danger zone. If you thaw on the counter then the exterior will be crawling with them long before the interior is thawed.

The other methods are acceptable because:

  1. Keeping the food refrigerated never permits any part of the food to enter the danger zone.
  2. Very cold water keeps the exterior cold, which is close enough to refrigeration as long as the food will be cooked immediately.
  3. Microwaves partially penetrate the interior and cook the entire portion very quickly; the food will be in the danger zone but for too short a time period to matter.

It's also very important to note that the cooling rules are not about simply "watching the clock". If it were that simple, they wouldn't bother with rules, because room temperature is in a pretty consistent range no matter where you are. Cooling hot, potentially-dangerous food on a countertop is every bit as inappropriate as defrosting it there. Food service professionals will hold the food above 140° F until it is ready to serve or store, and if storing, they'll cool it very quickly by dividing into smaller portions and/or using an ice water bath or even specialized equipment (e.g. flash freezing).

There's really no practical difference here. The extra 1 hour permitted to cool from 140° F to 70° F is really only because you've just cooked it, so (a) it's been pasteurized and (b) temperatures very close to 140° F are not very hospitable for bacteria should the food become contaminated again. When defrosting from frozen, it's assumed that the food (cooked or raw) is already contaminated either from factory handling or from prior cooling in the danger zone, so you need to strictly follow the 2-hour rule.

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  • I want to accept this answer; however, these rules are about time and temperature. Again this is not a practical question (see below) about why or how. That said if you want to elaborate on the last paragraph about bacteria counts after pasteurization versus bacteria counts in raw food I will accept that as the best answer. (I'm not sure what the "extra hour" is about. Both cooling from 140-70 and defrosting to room temperature using running water are allotted 2h.)
    – xtian
    Oct 27, 2011 at 20:01
  • @xtian: Your question asked why those are the rules, not about bacteria counts, and this post answers the "why". Discussion of bacterial counts would probably be better suited to a forum of microbiologists, if there is such a thing. At a conceptual level it's clearly obvious that pasteurized food will have (much) fewer bacteria than raw food, so I really don't think that bears elaboration.
    – Aaronut
    Oct 27, 2011 at 23:09
  • That's a blatant misrepresentation of my question. And, its not "clearly obvious" as you suggest given: 1) misunderstanding here, 2) fact, pasteurization does not kill all bacteria only reduces the count to acceptable levels, 3) fact, waste product toxins created by bacteria are not destroyed by cooking. The question is very much about food and bacteria. If you will allow me to suggest, your confusion about the question may originate not in the question but the domain. These rules are easy to remember for all food service workers as a matter of public policy, not science or logic.
    – xtian
    Oct 27, 2011 at 23:27
  • @xtian: You're repeating points I've already made myself several times on this site. None of those in fact have anything to do with your question. Pasteurization doesn't have to kill all bacteria in order to be effective and protein toxins are a non-issue as long as the bacteria are kept in check. You certainly don't have to accept my answer but please note the section of our FAQ where it says "...practical, answerable questions based on actual problems that you face." We're here to answer questions about food and cooking, not speculate on political motives or microbiology.
    – Aaronut
    Oct 27, 2011 at 23:44
  • I'm not suggesting Public Health guidelines are "political". You have again missed the point of my inquiry. Ultimately, the quesion is asking users who know the food-biology facts to enlighten me on the reasoning for the rules (Are there more bacteria in raw foods then toxins in cooked foods? Presumably since I eat rare beef and sushi and vegetables its not at all guarnateed). This is a factual inquiry. You can't bully me to accepting your answer. I don't know the answer. I'm fine with that.
    – xtian
    Oct 28, 2011 at 17:00
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When defrosting there could be a big difference in temperature between the outside and middle of the item. The middle could still be rock hard when the outside is warmed into the danger zone on a summer's day. Thus the recommendation is to defrost within a refrigerator as the max temperature any part gets to will be controlled. Defrosting in running water or microwave are also acceptable as they are fast and so little has time to grow and go bad.

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  • Strangely, it seems you are suggesting the interior would have less bacteria than the interior, when in either case (defrosting or cooling) the time and temperature are the important controls (ie. cooling: 140-70F @ <2h, 70F-40F @ <4h). Thus, your point goes toward the concern of exceeding time & temperature limits and still not be defrosted. The question is not practical, but theoretical... (>_<)
    – xtian
    Oct 10, 2011 at 13:30
  • I think it's a combination of this, and the higher initial bacterial count. This especially applies to frozen raw meat. The surface of the meat, where the bacteria count will usually be higher, reaches the danger zone fairly quickly and stays there for the majority of the defrosting time.
    – Bob
    Oct 10, 2011 at 13:48
  • There is still a guideline. Different bac counts on surface and interior are irrelevant. Concern for the rapid growth is not voiced the same for cooling and thawing in guidelines. I'm sorry to dog the topic. I still hear only one point; initial bacteria counts may be greater.
    – xtian
    Oct 10, 2011 at 16:20
  • @xtian: I think another point might that the amount of time to wait for the centre to completely thaw could be long enough for the outer regions (which have been thawed for a while) to collect new bacteria (even if the object was bacteria-free when first taken out of the freezer) which then start to multiply.
    – FrustratedWithFormsDesigner
    Oct 10, 2011 at 21:12
  • @FWFD, Then what you're suggesting is bacteria growth in defrosting foods is higher in the period of 6 hours at room temerature than for cooling foods over same period because defroting foods have a temperature difference that cooling foods do not?
    – xtian
    Oct 11, 2011 at 15:38
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We do seem to be missing an important point here... cooling is not allowed to happen at "room temperature" at product temperatures below 140 F; only until reaching 140 F from some higher temp. In fact, no food that supports bacterial growth is allowed to be kept at "room temperature", or any temperature inside the danger zone (40 - 140 F) for any period of time except in preparation for chilling, cooking, or service, and that only very carefully under sanitary conditions. Check the USDA guidelines (I just did, courtesy of NDSU: http://www.ag.ndsu.edu/pubs/yf/foods/fn572-1.htm#Safe )

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