I always thought the name of this cooking technique was a bit strange because I can’t possibly imagine Mary (or anyone else for that matter) bathing in boiling water. In English it is also called just water bath and it consists in heating up something by putting it in a recipient and that recipient in a bath of boiling water (rather than putting the first recipient directly on the fire). This is generally used for melting chocolate, cooking puddings (such us the Spanish Flan), cheese cake, dulce de leche, etc.

Have you ever wonder why you had to heat up those things in a bath of boiling water? Why not just apply heat to them directly? The reason is that you are exploiting one very useful characteristic of phase transition: the temperature is constant during a phase transition.

To illustrate what I am talking about I did the following experiment. I put a pot with one litre of water on the kitchen fire and measured the temperature every 30 seconds.

Experimental setup

Experimental setup

To do this experiment all you need is a food thermometer (mine cost me 5 pounds) because the usual thermometers that we have at home don’t reach such high temperatures. The temperature increased more or less linearly until the water started boiling (which here in Edinburgh is at 98.6ºC (209.1F). Then it stayed constant regardless the fact that I was still applying heat to it (it stayed constant for more than 10 minutes and then I got bored, in the table I only registered the first 3 minutes of boiling):

Temperature of water from ambient until boiling

Temperature of water from ambient until boiling

The reason why this happens is that while the water is boiling a phase transition is occurring: from liquid water to water vapor. During that time, all the heat applied to the water is used to free the molecules from the pot into water vapor and not to increase the temperature of the liquid.

Now, thermodynamics tells us that, when we put two things with different temperatures the hot one gets colder and the cold one gets hotter and not the other way round. So if we put a recipient inside the water bath it will never go beyond the water temperature (unless of course the temperature in our kitchen is higher than 98.6ºC, in which case we probably wouldn’t be alive to see it).

To prove this, I put a pot with olive oil inside the boiling water:

Heating oil in a water bath

Heating oil in a water bath

In less than 10 minutes the temperature was 95ºC, and so it stayed for the following 10 minutes. Then I put the pot with olive oil and it very quickly reached 200ºC (392F) which is the limit of my thermometer. Since the boiling point of olive oil is 300ºC (572F) it could have even gone higher up. However the smoke point of olive oil is quite low (190ºC/374F), which is where the oil smokes or burns giving food an unpleasant taste, so when you cook you want to avoid reaching such temperatures.

Heating olive oil. Careful: very hot!

Heating olive oil. Careful: very hot!

This illustrates how one control temperature while cooking using the fact that during a phase transition the temperature stays constant. Nowadays cooks have sophisticated techniques to do this but in former times this was a pretty useful thing. If you wanted a different temperature, then you needed to change the substance in the bath (such us oils) since different liquids have different boiling points.

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