Sous-Vide food safety OR How to keep everyone alive

Hi fellow sous-vide’rs

Just a quick question. I received my Anova Precision Cooker a few weeks back and have tried it a couple of times on salmon. The first time the fish came out very tender but may have been slightly undercooked as my tummy was a little funny the following day. I think the temperature/time combo I tried was around 52C (125F)/40min… The second time I tried the fish I wanted to be safer so I went for the FDA recommended temperature for fish which is 63C (145F) / 40min. However this time the fish although cooked took on a less appetizing paler colour which was pleasant enough to eat but didn’t quite have the flakiness which salmon should have. Anyway, for future I will just fine tune the temperature down slowly to get the desired outcome…

Going through these initial experiments has made me think about the difference between the temperatures which many sous-vide recipes are quoting and that which the FDA or other food satefy bodies stipulate as safe. In most cases the temperature in the sous-vide recipes is lower by 5-10C or more than the recommended FDA temperature for cooking that food. So my question is how does the recipe ensure that the food is safe to eat? Is it the time taken in the lower temperature which kills all the harmful bacteria? or is something else happening which I am missing?

Thanks,
Rick

Yes. The USDA temps are usually the temp for dummies and provide essentially instantaneous bacteria reduction, but at lower temps you’re able to get the same bacteria reduction when held for a longer period of time. See, e.g., http://www.fsis.usda.gov/wps/wcm/connect/9ab2e062-7ac8-49b7-aea1-f070048a113a/RTE_Poultry_Tables.pdf?MOD=AJPERES (or a more readable discussion http://www.seriouseats.com/2010/04/sous-vide-basics-low-temperature-chicken.html).

NB: Just because it’s possible doesn’t mean that that particular recipe follows that guideline.

1 Like

Thanks @jshannon!

That kind of makes sense now… do you know if there are charts which show the ‘safe’ temp/time combinations for most of the common foods (beef, pork, chicken, fish, etc) or is it the case that the chart relates more to the type of bacteria that is being reduced rather than the food type itself?

And now that I know I can cook foods at a lower than the official FDA stated temperature then the time for which the food should be held relates to the moment the sous-vide bag is put into the cooking vessel or from the time that the ‘internal’ temperature of the thickest part of the food has reached the temperature on the chart?

Thanks again
Rick

It would be the time that all of the meat has reached that temp:

The stated temperature is the minimum that must be achieved and maintained in all parts of each
piece of meat for a least the stated time:

The USDA numbers don’t concern themselves with thickness or how you cook it (ie, water vs air), so it’s all about the temp of the meat.

I don’t know what makes a difference in the times. I’d say that all meat is meat and once a bit of meat is at a temp then the results are the same (at least for any given bacteria). But the USDA paper clearly disagrees. You’ll notice that they list tables for chicken with different %s of fat, starting at 25mins (1% fat) and going up to 35 mins (12%) (at 140f). They also list the salmonella numbers for beef. I was thinking that maybe fat made the difference, with fattier meats requiring longer, but similar salmonella reduction in beef is only 12 minutes. So, I don’t know. Maybe the paper goes into details. Or maybe others can explain.

FWIW, I tried searching for a PDF with the same title but for fish and couldn’t find anything.

Thanks @jshannon‌ again!

Ok, so if the temperature stated is for all parts of the meat to reach for the stated time that kind of becomes a slightly harder to establish procedure. I would think then that either you could take the food out of the water/bag to test the temperature with a digital probe to check the thickest part of it or you make a ‘rough-and-ready’ estimate that a food of ‘type x’ with thickness ‘y’ should have a consistent temperature at the level specified on the sous-vide device after ‘z’ minutes… I am going to guess that most reliable recipes are effectively crystallizing this logic in the timings they provide…

I also did a quick google for a nice cheatsheet with the temperature/time curves for various food types but couldn’t see a single place where I could find all the relevant information… that would be handy to have!

Cheers again
Rick

One of the basic fundamental works on Sous Vide mathematics (including temperature curves and pathogens destruction is a paper by Dr. Douglas Baldwin (lately of ChefSteps). You can check his site for info and further reading.

Thanks @vtemkin‌ ! I will take a look at the articles on Dr. Baldwin’s site!

" am going to guess that most reliable recipes are effectively crystallizing this logic in the timings they provide"

Yes, most recipes do imply that (a lot – especially the non-dummy versions like you see on the recipes site here) say “for a 1 inch steak; add another 1/2 for a 2 inch steak”. Also a lot of recipes just assume (how thick can a chicken breast get?).

Really… it’s no different than a standard oven recipe. There are general rules of thumb for how hot your roast will get in the center in an oven considering that roasts are x inches across.

Wow? Baldwin is at Chefsteps? That’s cool. Not sure about the personality fit, but, yay science.

@AnovaRick It should be noted that at 125F the fish will not be pasteurized therefore from a food safety perspective it is essentially the same as eating it raw (though I’m sure at 125F you are killing some pathogens so maybe not quite the same as raw). Thus if it is a piece of fish you would not use for sashimi then you might want to think twice. Also, you should never cook at temperatures under 131F for any extended period of time as bacteria can still grow at this temperature.

Thanks @schilling3003‌ ! Yes, I think that is something that needs to be very clear on recipes on both the Anova and other websites in that anything you are going to cook at these relatively low temperatures really does need to be top quality/super fresh/sashimi grade produce…I did read somewhere that basically the ‘danger zone’ is 40-140F which is what you really need to avoid if you want to eliminate or dramatically reduce the level of bacteria or growth thereof during cooking…

@‌ the FDA designed those guidelines are outdated and not backed by recent science. Additionally, they were designed to be idiot proof so nice even numbers were chosen

For an interesting read check out http://www.foodsafetymagazine.com/magazine-archive1/februarymarch-2004/the-danger-zone-reevaluated/

Essentially, once you hit 122F almost all bacteria cease to propagate. So, already cooked food held at this temperature would remain safe, but this would not kill off existing bacteria. Once you start getting above 125F most bacteria start to die exponentially as temperatures increase, but there are a few rare strains that can actually still grow (but not considered to be a big risk). At 131F only one strain of Bacillus cereus was found to grow at this temp, but only in media not in rice.

And definitely read all of Baldwins’s work.

Not only are the government recommendations overly cautious and “for dummies” and “nice round numbers”, they are, as somebody pointed out above, for “instantaneous” temps.  That is, for air cooking,  you set the oven at a MUCH higher heat than is required for the protein to be considered safe, and you wait for that heat to work its way inward, until the temp is ALMOST the recommended (for dummies) value at the center of mass, and (if you are lucky or proficient) you whip it out and let it rest while that internal heat finishes the migration into the center of the roast or steak or… whatever.  But our whole reason for having turned to sous vide is that while the instantaneous kill-everything-instantly temperature of air-cooking is just being reached in the center of mass, almost all the flesh outside the center had to be overcooked.  Air/oven/bar-bee times and heats are rule-of-thumb conventions that have been found to get the required heat to the middle of the piece of beast with the least dessication (shoe-leathering) of the outer bits. It takes more heat and time than you might hope, because air is not a great medium, and dry air is even less conductive, while also hastening the loss of moisture from the protein. 

In sous vide, we assume a longer time for overall cooking, because we are applying just enough heat to get the bath to the temperature we want the protein to reach.  It necessarily takes much longer for that to happen, and as pointed out, the gentler final temperature is not an instantaneous kill. Instead, it’s a temp that the toughest specimens of pathogens can withstand briefly, but that kills them with a bit of time.  Therefore, we have TEMPERATURES that are set for a given protein for various reasons that have to do with the strains of pathogen that are known to thrive on the particular protein.  The ones that love a good steak are different from the ones that love chicken. So, instead of “for dummies” internal temperature requirements, we have “for dummies” duration requirements. The reason that those times are not vastly longer (given the lower temps involved in sous vide) is that water is a much better thermal transfer medium, and the thin layer of a plastic bag, with no air inside, allows contact with the water bath that’s almost as good as it would be if you poached… but without leaching the flavor as poaching would do.

But just when it started sounding straightforward, we throw in the complication of texture of the protein. We want the collagen to break down, especially in the cuts of beast that start our relatively tough, and that takes a longer exposure to cooking temperature than simply killing off bacteria.   So, a bunch of people have done a ton of informed trial-and-oh-drat-I-ruined-the-fish experimentation to discover the ranges of time/temp exposure that not only kill the bacteria but also bring the meat to a desired texture.  Not too tough, but not mushy or mealy, either. In one sense, it’s inconvenient that we have to wait so long to eat. But on the other hand, it helps that the collagen breakdown is a slow process, with not much change taking place within (say) half an hour. So, we can hold the food until a convenient time (the fashionably-late guests have finally arrived), without overcooking.  

ChefSteps is doing a very commendable job of putting together tables and charts for an ever-increasing assortment of raw materials, recipes, etc.   Kudos.

A great way to learn about time/temperature relationships for sous vide food safety is to use an app like Sous Vide Dash ( http://www.sousvidedash.com/ ). You enter the type of food, and it will show you (with graphs) how to cook it safely. There is also a setting for Anova products because all sous vide machines register temperatures slightly differently.

I cook my fish at a higher temperature because while I like the texture of the lower temperature I don't like lukewarm fish.

What I wonder about is the actual danger from pathogens higher due to the food being held at temperature for a long time.

Would food cooked for 20 minutes be riskier than food cooked for 2 hours or the opposite.

My understanding is that the risk is caused by the chance of bacterial growth during cooking so a long time at a low temperature would be riskier, and a short time at a higher temperature would be less risky. Not real concerned, just curious.


@Helen The area where there is a possible concern is between 122°-131° F. At the lower end live bacteria will cease to propagate at around 125° F almost all bacteria die and above 131°F they all die given sufficient time. The Government published charts on the time required as a function of temperature for food safety. To answer your question you really need to define what you mean by low temperature.

What about botulism? I had read that botulinum bacterial spores can survive past the boiling point, so that is why low acidity foods that are canned(for long storage) must be pressure canned. I guess that foods with a high enough acidity kill of the botulinum, but low acid foods it will continue to live?

I would suggest you read the first chapter of Baldwin’s Practical Guide to Sous Vide Cooking regarding food safety. It sounds like you’re asking about the difference between pasteurization and sterilization. It’s true that pasteurization won’t kill the food pathogen spores, but on their own they won’t make you sick. I think that the line that’s pertinent here would be:

Foods you’ve pasteurized must either be eaten immediately or rapidly chilled and refrigerated to prevent the outgrowth and multiplication of spores.

Clostridium botulinum first and foremost isn’t everywhere. It’s somewhat uncommon in most foods. In the food that’s excised to oxygen, you will not have C. bot. growing because it’s an obligate anaerobe: the air we breathe is poisonous. It needs a vacuum to grow. While you’re cooking food, rarely are the conditions ideal for the germination of botulinum spores. These hearty packages of DNA are designed to protect tyre bacteria’s genetic material until growth conditions are ideal. It is theoretically possible for a few spores to begun germinating, but if there’s a significant amount, your bag will swell, and the odor will be very noticeable. There’s some decent information here: https://www.chefsteps.com/forum/posts/botulism-in-sous-vide-leftovers