Melibiose is a sugar that appears in the fermenting beer as the result of an enzymatic degradation of raffinose, which is a minor trisaccharide composed of galactose, glucose, and fructose units. Melibiose is the galactose–glucose disaccharide that is left over following the action of beta fructosidase on raffinose. In lagers, however, melibiose eventually disappears broken down by yeast, whereas in ales it remains intact. This is because lager yeasts develop the enzyme alpha galactosidase that catalyzes the hyrolysis of melibiose, whereas ale strains do not produce that enzyme. This makes melibiose a convenient marker sugar for determining whether a beer was fermented by a top- or a bottom-fermenting yeast strain.