Bain-Marie (Water Bath / Double Boiler)
Essentially, a bain-marie consists of two cooking vessels, one vessel placed into the second vessel which also holds heated water. Here we’ll look at the use of a bain-marie within the cooking realm, although it is also used in the scientific realm.
Nerd Notification: Etymology
From French bain-marie, via Medieval Latin balneum Mariae (“bath of Mary”). Bain derives from the French baigner “to bath” which itself comes from the Latin balneare (v.) “to bath” (from balneum (n.) “bath”).1
Bain-Marie in Cooking
In the cooking realm, bain-marie generally refers to two methods.
The first is used for melting chocolate, making Hollandaise etc. In this instance the bain-marie consists of a saucepan (pot 1 in the diagram below) with simmering water, over this will sit a heat-proof bowl (pot 2 in the diagram below). This bowl will sit inside the saucepan but above the water. The base of the bowl does not touch the water. In this example the heat source will come from the stove-top element.
The second is used in the oven, for example cakes and baked custards. In this instance the vessel with food (pot 2 in the diagram above) is placed inside a larger vessel (pot 1 in the diagram above) which is filled with boiling water. The water will come up between halfway to two-thirds on the outside of the food vessel. In this example the heat source is the oven. As an example, in the Pear and Cardamom Yoghurt Loaf (pictured below), the loaf tin was placed inside a larger cake tin. The boiling water was poured into the cake tin so that it came two-thirds up the outside of the loaf tin and the whole thing was placed in the oven to bake.
The beauty of a bain-marie, is that it moderates the heat applied to the food. Regardless of how hot the heat source is, the water doesn’t exceed 212*F/100*C thus moderating the cooking temperature. This helps chocolate to melt without burning, custards to set, and cakes to bake with a soft crust.
Why is it Mary’s bath?
I couldn’t find a definitive answer, according Merriam Webster the term refers to Maria Hebraea (Mary the Jewess) inventor of the apparatus used in alchemy. Alternatively, etymonline credits it to French sources, where it perhaps refers to the gentleness of heating.
Further Nerd Notification: Food Science
Water temperature within a bain-marie can vary within a 40*F range depending on the vessel that contains the water and whether it is covered. The water is both heated by the oven, and is also cooled as the water molecules evaporate from the surface. The water temperature is determined by the heating of the water mass through the vessel and the evaporative cooling that takes place on the water’s surface. According to McGee, in a moderate oven a cast-iron bain-marie may reach 195*F/87*C, a glass bain-marie may reach 185*F/83*C, and a stainless steel bain-marie may reach 180*F/80*C. If the water is covered, water evaporation will cease and it then will come to a full boil (212*F/100*C).2
2McGee on Food & Cooking, Harold McGee, Hodder & Stoughton, 2004 (p.96)