Which technique can be used to convey rich flavor in sauces without thickening them?

The technique that effectively conveys rich flavor in sauces without thickening them is liaison. A liaison involves combining egg yolks and cream to achieve a velvety texture and richness without adding significant thickness. This technique allows for the enhancement of flavor and provides a creamy mouthfeel, which can elevate the overall quality of the sauce.

In contrast, reduction focuses on intensifying flavors by evaporating liquid, which can thicken a sauce naturally due to the concentration of ingredients. Emulsification combines two immiscible liquids, like oil and vinegar, which can contribute to texture and flavor but is not specifically aimed at enriching flavor without thickening. Infusion involves steeping ingredients to extract flavors, which can enhance a sauce but may also not be as direct in providing richness without altering viscosity. Thus, liaison stands out as the most suitable technique for achieving this goal.