What is formed when a sauce has been thickened through a process of prolonged boiling?

When a sauce is thickened through the process of prolonged boiling, it results in a reduction sauce. This technique involves simmering or boiling a sauce to evaporate some of its water content, which intensifies the flavors and thickens the overall consistency. As the liquid reduces, the remaining components become more concentrated, enriching the taste and viscosity of the sauce.

Reduction sauces are commonly used in culinary practices to enhance the depth of flavor in dishes. This method is widely employed in the preparation of gravies, pan sauces, and coulis, where achieving a robust taste is essential.

The other options do not accurately describe the process of thickening a sauce through boiling. A paste sauce typically refers to a sauce that has been thickened with a starch component. An emulsion sauce relates to the stable mixture of two liquids that typically do not combine, like oil and vinegar, and a thickness sauce is not a recognized term in culinary practices. Thus, the valid and appropriate outcome of prolonged boiling for thickening a sauce aligns with the definition of a reduction sauce.