Friday, August 22, 2014

Sous Vide Series: Calculating Cooking Time

The Problem

If the great promise of sous-vide is precision, the great irony is that most of us pick our cooking times by guessing. A recipe that gives a fixed temperature and time constitutes a barely-educated guess. A graphic table that accounts for food thickness and starting temperature (like the ones on Douglas Baldwin’s site, or in the Modernist Cuisine Books), constitutes a somewhat more educated guess.

The only way to be certain is with a probe thermometer, which gives real-time feedback. But very few immersion circulators accommodate these, and they’re complicated (you have to use special tape to keep the water from leaking into the bag). And this degree of precision is generally overkill anyway.


Software to the Rescue

I think the most practical level of precision can be achieved with software, specifically with the app SousVide Dash.* This app lets you enter the type of food, the approximate shape of the food, the exact thickness, the starting temperature, and the desired final temperature. It estimates cooking time based on a sophisticated thermodynamic model of the food.

UPDATE: The app has been purchased by PolyScience and is now called the PolyScience Sous-Vide Toolbox. As far as I can tell they just rebranded it and changed the color from yellow to red. As of 6-2016, there's no reason to trade in your old version of the app for the new one.

The following requires some basic familiarity with pasteurization.** It also assumes you've figured out the final temperature you're going for. See our introductory post for links.


Options

SousVide Dash can calculate time for any of three results: Cook to Temperature, Cook and Pasteurize Surface, or Cook and Pasteurize to Core.

There’s another option called Additional Time for Tenderness. This is for long cooks designed to tenderize tough cuts, like what we discussed in the High Steaks for Cheapskates post. We’re going to skip that option here, because this software does not provide any guidance for this type of cooking.

Cook to Temperature: This calculates time to bring the center of the food up to your target temperature.  
It’s the right choice if you’re pasteurizing the surface some other way (searing, or briefly immersing in boiling water), if you’re going to serve the food shortly after cooking, and if the food can be presumed sterile on the inside. 
Whole pieces of meat are presumed sterile everywhere but the surface. Meat that’s been ground, pierced (like with a Jaccard tenderizer), rolled, or pieced together (like with enzymatic meat glue) can be presumed contaminated throughout. These need to be pasteurized to the core. 
Cook and Pasteurize Surface: This calculates time to bring the center to the target temperature, and, if needed, any additional time to pasteurize the surface.  
Often the surface will be pasteurized by the time the center is cooked, but this option will tell you for sure, and will show you a logarithmic graph of dead bacteria vs. time. Cool (at least I think so).  
Use this option under the same circumstances you’d use Cook to Temperature, but if there’s not going to be any additional step that will pasteurize the surface. 
Cook and Pasteurize to Core: This calculates time to bring the center to the target temperature, plus additional time to pasteurize all the way to the core.  
Since pasteurization requires holding the food at temperature (the lower the temp, the longer the time), this option can lead to quite long cooking times. Use this option on foods that can’t be presumed sterile below the surface. This option is the safest bet if you’re serving food to anyone who’s immune-compromised (the pregnant, the elderly, the HIV-positive, etc.).  
Note that the minimum temperature for pasteurization is 54.4°C / 130°F, and at this temperature the times can be longer than optimal for a lot of tender meat. If you must pasteurize a tender steak to the core, it can add over an hour to the cooking time, which may compromise the texture.

Screen-shot showing surface temperature, core temperature, and reduction of  E. Coli, Listeria, and Salmonella


Why are the Cooking Times So F’ing Long?

Typical sous-vide instructions have you set the water bath temperature to the food's target temperature. Simple.

But if you do it this way, SousVide Dash is going to suggest excruciatingly long cooking times—much longer than what you’ll see in the time/temperature tables in places like Douglas Baldwin’s site and the Modernist Cuisine books.

Why the inconsistency? The tables all have a bit of fudge-factor baked into them, in the interests of practicality. The need for this is grounded in thermodynamics: the smaller the temperature gradient between the water bath and the food, the more slowly the heat transfers. So your food will get within a couple of degrees of the target temperature pretty quickly. Those last two degrees will drag on and on.

In other words: SousVide dash is telling you the truth. You just can’t handle the truth!

Kidding. You just have to create your own fudge-factor to speed things along. You have to manipulate the truth, like a pro.


How to Actually Do It

1. Set the water bath temperature 1°C higher than your desired doneness temperature.
2. Set the target temperature 0.5° lower than your desired doneness temperature.

That’s it. Your food will be perfect, and your cooking times reasonable.

Step one is standard practice among cooks who know what they’re doing. Most recipes are calculated with this tweak built-in. They tell you the water bath temperature; they don’t bother telling you that the final core temperature will be a bit lower.

Step two is rarely articulated, but is probably close to what most people do when they work out cooking times by trial and error.

When you set the water bath temperature higher than the target temperature, you’re automatically introducing a temperature gradient to the food. Not the huge kind of gradient you’d see when cooking in a conventional oven, but a barely-perceptible 1°C gradient. Given this, only the very center of the food is going to be cooked to your target temperature. All the rest of it will be, to a slight degree, cooked more. By rigging the parameters to slightly undercook the center, you actually bring a larger portion of the food to your ideal temperature. And! You decrease cooking time significantly.

The following screen shots show two approaches to cooking a thick steak to 54°C. First with no temperature gradient, and then with the method described here. Notice the nearly 100% difference in cooking time. I would actually be worried about the 3 hour steak's texture: that's enough time for tender meat start drying out and getting mushy.


Target temperature set to 54°C. Water bath set to 54°C

Target temperature set to 53.5°C. Water bath set to 55°C.


Longer Cook Times for Tenderizing

This method is useful it you’re not adding significant time for tenderization. Here are the guidelines adjusted for tenderizing:

•If you’re adding more than a couple of hours of extra tenderizing time, set the target temperature exactly to the desired doneness temperature. But keep the water bath temperature 1°C higher.
•If you’re adding more than six hours of tenderizing time, set the target temperature AND the water bath temperature exactly to the desired doneness temperature.

How much time to add for tenderizing? There’s no simple answer to this. Times vary enormously with temperature, the type and cut of meat, and the desired texture. This is a question that’s best answered with recommendations of those who have done it, and then refined by trial and error.



* Dash = "dashboard." I’m not affiliated with the app’s developer

**All sous-vide cooking—and really all cooking of any kind—is most safely done with some familiarity with pasteurization. The realities are quite different from what the USDA and local health departments preach. Douglas Baldwin’s blog includes a good primer on the subject.

2 comments:

  1. Thank you for this detailed post. I used to work for the MathWorks (long time ago), and was thinking as a joke to ask some of the engineers to write an application for me simulating heat transfer in different foods. Of course, it would probably never happen because chefs don't like to pay serious money for software :) Sous-vide dash sounds just like what I've been looking for. Too bad it's not available on Android. Or maybe it is and I didn't look at their site long enough?

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  2. I didn't realize it wasn't cross-platform. That's too bad. I get the impression the developer (Darren Vengroff) is hoping to sell the app to a bigger developer. This could be good news (or bad ... we'll have to see).

    In the mean time it wouldn't hurt to ask him if he's got anything cooking. He wrote back right away to all my questions.

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