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Rotary evaporation is used in traditional laboratories to distil solvents by a process of evaporation and condensation, i.e. taking a product into a liquid state, then gas and then back to liquid again. 

Rotary Evaporation

In the kitchen the process of evaporation and condensation can be used to prepare a delicious plate of food with a separate pure flavoured liquid.  For example, if we want to distil and extract aromas say from the cooking of a piece of meat with truffles and recover them in the form of a clear flavoured liquid we can place the meat and truffle seasoning inside a cooking flask which is then immersed in a water bath held at temperature of 60° C.  Much like in the sous vide technique the flask is held under vacuum which lowers its boiling point and cooking temperature, but unlike sous vide the flask is rotated to increase the surface space of the product thus ensuring absolute heat stability and even evaporation.  What happens then is that the closed system of the rotary evaporator allows the capture of the meat and truffle vapour in the cooking flask to be recovered in a separate flask called the recovery flask. Once the vapour condenses back into liquid in the recovery flask it condenses back into its liquid state as a flavoured liquid of pure meat and truffle aroma and taste. The result is a perfectly cooked piece of meat and a recovered truffle flavour liquid which can be used to season a dish.

Chefs can therefore use a rotary evaporator for four main purposes:

1) To extract aromas from a specific product and recover them in the form of clear flavoured liquid.

2) To cook at low temperatures and under vacuum this making sure that the original product flavour and texture characteristics are kept in tact.

3) To concentrate non-volatile components in a mixture through soft migration of water under vacuum and continuous rotation, e.g. concentrating the purest and freshest flavours from a blood orange by removing the water, as well as extracting volatile aroma and flavour molecules from mixtures without heating the mixture too high, e.g. blends of alcohol, herbs and fruit.

4)  To avoid dispersion of taste and flavour that would otherwise occur with conventional cooking.

According to Dave Arnold, Director of Culinary Technology at The French Culinary Institute, "The key to understanding any distillation is to remember that it is a separation.  Sugars, acids, colours, and most bitter compounds are separated from aromas, alcohols, water, and small flavour molecules, etc..  What is phenomenal about rotary evaporation distillation, as opposed to standard distillation, is that it can separate food compounds from one another without altering them." Reference: http://www.cookingissues.com/primers/rotovap/