What is the sous vide botulism risk if storing meat post-cooking at home?

Question

So I understand that cooking sous vide has a potentially high risk of botulism due to the anaerobic environment (vacuum). As such, you need to be careful with time and temp combinations. If you are storing food for service later, you need to flash chill it and then keep it in a fridge below 4C. This is easy enough at a restaurant with a walk-in, but is somewhat difficult in a home fridge as opening the door creates wide temperature swings.

Douglas Baldwin proposes the following temperature / time guideline for "safe" wrt botulism:

[S]pores of Clostridium botulinum, C. perfringens and B. cereus can all survive the mild heat treatment of pasteurization. Therefore, after rapid chilling, the food must either be frozen or held at

below 36.5°F (2.5°C) for up to 90 days,
below 38°F (3.3°C) for less than 31 days,
below 41°F (5°C) for less than 10 days, or
below 44.5°F (7°C) for less than 5 days

So a couple of questions about those guidelines:

1. How big should I expect the swings in my home fridge to be with normal usage?
2. If I have a second fridge and the door rarely opens, what will the temperature swings be there?
3. Since the botulism concern is due to the vacuum, am I correct in thinking that this concern will disappear if I remove the meat from the vacuum to store it? Obviously, this approach would reintroduce all the normal safety concerns with storing cooked meat.

Answer 1

Flash chilling is very simple. Just make sure you have a fairly large ice bath (ice and water that is at least 50% ice) and put the meat immediately from the heated bath into the ice bath. This will rapidly chill the meat prior to refrigeration. Make sure you leave the meat in the ice bath long enough for it to chill to the center if it's a thick cut or a roast.

1 How big should I expect the swings in my home fridge to be with normal usage?

The mild swings from opening and closing your refrigerator door a couple times a day aren't going to really make big difference. Just make sure you don't leave the door open for extended periods of time.

2 If I have a second fridge and the door rarely opens, what will the temperature swings be there?

There are a lot of things that influence the temperature of a refridgerator besides opening and closing the door. A refrigerator goes through cycles for chilling (and for defrosting) etc where the temperature varies. Some brands of refridgerators (i.e. Samsung) have separate cooling systems for the freezer and fridge portions so freezer defrost cycles do not cause swings in temperature in the fridge section.

Also, an empty refrigerator loses a lot more heat than a full refrigerator when the door is open. Storing plastic bottles of water or cans of soda and beer on empty shelves will actually make the temperature more constant over time since they retain more heat (or "cold") than air. Of course, if you turn a fridge into a beer-fridge, chances are that the door is gonna get opened a lot more.

3 Since the botulism concern is due to the vacuum, am I correct in thinking that this concern will disappear if I remove the meat from the vacuum to store it? Obviously, this approach would reintroduce all the normal safety concerns with storing cooked meat.

Sous-vide cooking should pasteurize the meat if it was cooked long enough and kill most . Keeping the pastuerized food that is sealed at a controlled temperature is going to preserve it for much longer than breaking open the seal and allowing any pathogens in before keeping it at the same temperature.

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The most important thing is to follow the established safety charts for cooking times, temperatures, using correct flash chilling and then following the safety charts for storage temperatures and duration as well.

Answer 2

I'm not sure about your question 3, but as I thought about 1 and 2, I figured the swings would not be too big if you don't open your fridge for extended periods of time: how much air is really exchanged between the inside and the outside if you open your fridge door for 5 seconds - which already gives you plenty of time to get something out?

I figured the internets would have to know something about this so I set about searching for some graphs. Turns out Berkeley has a class that includes an experiment where students measure the temperature in their fridge over a 24 hr period. Most graphs look somewhat like the one on this page, where indeed the swings in temperature from the hysteresis window are larger than occasional increases because of opening the door.

I found two outliers. The first one is from a fridge catering to a large group of students, and the article suggests someone might have put a warm dish in the fridge. The second one is potentially very interesting - the author says she thinks the big upswing is from a wall defrosting cycle in the attached freezer, but that meal time opening of the fridge door didn't do anything to the temperatures.

So it looks like there are only two potential causes for raising the temperature of your fridge by more than a degree or so:

1. putting hot or warm food in your fridge;
2. having your fridge do a wall defrosting cycle for the attached freezer.

Other swings are negligible when compared to the natural operating cycle for your fridge.