glycosides are naturally occurring molecules made up of two parts joined by a glycosidic linkage; one of the parts is a sugar, frequently glucose, and the other may be a non-sugar, called an aglycone. Common aglycones are phenolic compounds, for example anthocyanins, although terpenoids and a number of other compound types are found glycosylated, in many plant tissues, including fruits. Glycosides present in wines come from the grape and here the sugar is usually glucose, although it too may be connected to a second, non-glucose sugar residue. Formation of a glycoside changes the physical and chemical properties of molecules that are glycosylated. Glycosides are usually more water soluble, and invariably much less volatile than the aglycones from which they were derived. It is still not clear why they form in plants but glycosylation is presumed to aid detoxification and the transport of aglycones, or to make plant tissues rigid. Plants and fruits often have a higher concentration of glycosides than aglycones. The sensory properties of flavour-active aglycones are profoundly diminished by glycosylation and such glycosides, known as flavour precursors, are a reserve of flavour and contributors to varietal flavour expression (as opposed to aspects of flavour that arise from vinification) and bouquet in wines. Following the recognition of glycosides as flavour precursors, a development in the mid 1990s was the determination of this class of compound in grapes and wines through measures of glycosyl-glucose. See also infrared spectroscopy.