Protein

Thickeners such as eggs, gelatin, lecithin, condensed and evaporated milk, liver and foie gras, and blood are examples of proteins that thicken when heated. Many proteins are heat sensitive and will break down in boiling mixtures. Because of this, they should be gently heated while stirring. Hot liquids should be gently whisked into blood or mixtures containing it, not the other way around. The sauce should be removed from the heat as soon as it thickens. Giblets, especially the liver, can also be puréed with an equal amount—by volume, but there’s no need to be precise—of butter and be whisked into the warm (not too hot) sauce as thickeners.