Once emulsified sauces have been successfully made, there are two basic rules for using them.
- The sauce must not get too hot. At high temperatures, the molecules and droplets in a sauce are moving very energetically, and the droplets may collide hard enough to coalesce. Temperatures above 140°F/60°C also cause the proteins in egg-emulsified sauces to coagulate, so they’re no longer able to protect the droplets. And a cooked sauce that is held before serving on gentle heat may lose enough water by evaporation that the dispersed fat droplets become overcrowded. So cooked emulsions should be made and held at warm, rather than hot, temperatures and should not be spooned onto a piece of food still sizzling from the pan.
- The sauce must not get too cold. At low temperatures, surface tension increases, making it more likely that neighboring droplets will coalesce. Butterfat solidifies at room temperature, and some oils do so in the refrigerator. The resulting sharp-edged fat crystals rupture the layer of emulsifier on the droplets, so that they coalesce and separate when stirred or warmed. Refrigerated emulsions often need to be reemulsified before use. (Manufactured mayonnaise is made with oils that remain liquid at refrigerator temperatures.)