When should acid be added to a sauce for optimal results?

Adding acid to a sauce after the roux and before the starches gelatinize is optimal because it allows for a better emulsification of flavors and textures. The roux acts as a thickening agent, and introducing acid at this stage helps to maintain the stability and consistency of the sauce as it cooks. If acid is added too early in the cooking process, especially before the roux is fully incorporated, it can hinder the thickening effect of the starches because acid can break down starch molecules, potentially leading to a thinner sauce.

Adding acid later in the cooking process, such as once the sauce has reduced, may not achieve the desired flavor balance derived from the acid. Incorporating it at the right time helps to enhance the overall flavor profile of the sauce while ensuring it remains properly thickened and cohesive.