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Why don't you need to stir water in your cooker to get homogeneous temperature in your bath?


Why don't you need to stir water in your cooker to get homogeneous temperature in your bath?

When using SousVideMagic, commercial rice cookers are always recommended for their better heat transfer characteristics.

To understand why, we need to a have a brief lesson in thermodynamics.

Cooking food is all about transfer of heat and there are three modes of heat transfer: Conduction, Convection and Radiation.

Conduction involves the transfer of heat by the interactions of atoms or
molecules of a material through which the heat is being transferred. In our case, it is the thermo transfer conducted from heated water to food pouches.

Convection involves the transfer of heat by the mixing and motion of macroscopic portions of a fluid.  In our case, the only natural convection we get from a rice cooker is its natural rising of heat from the bottom heater.

Radiation, or radiant heat transfer, involves the transfer of heat by electromagnetic radiation that arises due to the temperature of a body. In our case, the heated lid and side wall of the vessel  radiate heat throughout the vessel.

Take the 14L Water Oven cooker for example.

Conduction-  Water Oven cooker does a much better job than scientific water baths or baths heated by immersion  heater circulators. It is a very well insulated vessel and with surround heat technology, there is little heat loss, thus making rate of heat transfer (heat flux) very efficient.

Convection -  With the aid of noisy artificial stirrers/agitators, scientific water baths and immersion heater circulators win on this mode of heat transfer. Rice cookers do get some natural convection due to bottom heating, that explains why we recommend rice cookers over slow cookers (side heating).

Radiation - Since rice cooker's heat is also being delivered from the side surrounding the vessel, there are helpful amount of heat radiation to make heat transfer better. The other baths get little marks for this form of heat transfer.

What matters is the end result of the cooking process and rice cookers give you better overall heat transfer characteristics. In due time, laws of thermodunamics and the very nature of sous-vide's long term cooking are on our side, heat will reach homogeneousness throughout the whole vessel  including the food. This will happen whether you have the water being agitated or not.

For short time (less than 30 minutes) sous vide cooking, baths with agitator can reach homogeneousness temperature faster and will keep it that way as long as it is stirring. But for long term cooking at core-temperature, your food will always get cooked properly with even doneness throughout the whole volume of the food item with or without artificial agitating.

That's why we always recommend cooking at core temperature when using the SousVideMagic approach. Even heat may not distributed homogeneously initially, since cooking at core temperature one can never over-cook the food, so different parts of the food item will all reach the same desired core temperature eventually. Cooking at core temperature is very foolproof for even doneness.

Stirring the water will undoubtedly make bath temperature reaches homogeneousness sooner. It is very useful for short term sous-vide cooking like cooking seafood dishes for less than 30 minutes. But if you are cooking beef ribs for 36 to 72 hours hours you are wasting lot of energy and making lot of noise by mechanically stirring the bath when you don't really need it. Most SVM users, if water circulation is needed, a simple aquarium air pump diffuser system is used.


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