The present disclosure provides an aqueous composition comprising a purified thermostable polypeptide with biological activity, and a purified thermolabile water-soluble substance which in a functional conformation at a permissive temperature absorbs light and/or exhibits fluorescence, wherein the maximal permissive temperature of the polypeptide is above the maximal permissive temperature of the substance. The present disclosure further provides methods employing such a composition, and kits containing ingredients to form such a composition.