Thursday, April 22, 2010

Sous Vide "Beer Cooler Hack" Calculator

I was introduced to the sous vide cooking method by Kenji Alt in his blog article Cook Your Meat in a Beer Cooler: The World's Best (and Cheapest) Sous-Vide Hack, which is a very interesting read (along with the supporting articles mentioned therein) if you're interested in learning about this exciting and versatile way to cook food.  "Sous vide" is French for "under vacuum", which refers to the fact that you put the food inside of a vacuum-sealed bag (I use a Ziploc bag), and then submerge it in hot water.  To cook sous vide in a beer cooler (ice chest), you simply put water in the ice chest that is slightly hotter than the final desired temperature of the food.  Pop the food into the ice chest, the water cools down, the food warms up, and hopefully you end up with everything reaching equilibrium at the desired final food temperature.

In trying this, the biggest question in my mind was "how much hotter must the water be?"  It obviously depends on several factors, mainly being the amount of meat vs. the amount of water.  The more water there is, the less the water temp will drop as the meat is heated up.  In addition to that, you lose some heat through the walls of the ice chest.

Of course, being the geek that I am, I resorted to Excel to solve my problems.  I put together a spreadsheet, based on some published specific heat capacity data for various foods, that will predict how hot the "bath water" needs to be in order to arrive at the desired result (download HERE.)  I still need to validate it, but it's based on this fairly simple equation for heat transfer between two quantities of two different materials reaching equilibrium in a closed system:

mass(food) * specific heat(food) * deltaT(food) = mass(water) * specific heat(water) * deltaT(water) - some heat lost

DeltaT(food) will be positive, deltaT(water) will be negative, and everything will end up at the same final temperature (the desired done temp). All of these parameters are pretty well-known, so the result should be predictable.  The beauty of having it in a spreadsheet is that you can play with the numbers and realize that you don't need a huge ice chest full of water to do this... it can be done with a fairly small quantity of water.  The last tri-tip I did only used 2 gallons of hot water.  Without this spreadsheet, I would never have guessed that 2 gallons would be enough, but it was!

FYI, I included in the spreadsheet the ability to account for frozen meat by taking into account the latent heat of fusion of the meat as well as its different specific heat capacity as frozen vs. thawed.

Click on the image to download the spreadsheet.

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