=Isomaltose= is a synthetic sugar, obtained by Fischer, by condensing two molecules of glucose. Its properties are quite similar to those of maltose, but it yields a slightly different osazone and is not fermentable by yeast. These differences are explained by the assumption that this sugar is a glucose-[beta]-glucoside, while normal maltose is a glucose-[alpha]-glucoside. =Gentiobiose= is a disaccharide which results from the partial hydrolysis of the trisaccharide _gentianose_ (see page 53). It is very similar in its general properties to isomaltose. =Cellobiose= is a disaccharide which results from the hydrolysis of cellulose. It is a reducing sugar, forms an osazone, and resembles maltose. Maltose, isomaltose, gentiobiose, and cellobiose, are all glucose-glucosides, the difference between them being undoubtedly due to linkage being between different alcoholic groups in the glucose molecules. The disaccharide =lactose= is a glucose-galactoside. It is the sugar which is present in the milk of all mammals. It has never been found in plants. =Melibiose=, which is the corresponding vegetable glucose-galactoside, may be obtained by the partial hydrolysis of the trisaccharide _raffinose_ (see below). It is a reducing sugar; forms a characteristic osazone; and exhibits mutarotation. It is not fermented by ordinary top-yeasts, but is first hydrolyzed and then fermented by the enzymes present in bottom-yeasts. TRISACCHARIDES Trisaccharides, as the name indicates, consist of three hexoses (or monosaccharides) linked together by the dropping out of two molecules of water. Their formula is C_{18}H_{32}O_{16}. When completely hydrolyzed, they yield three molecules of monosaccharides; when partially hydrolyzed, one each of a disaccharide and a monosaccharide. One trisaccharide of the reducing sugar type, namely _rhamnose_, exists in plants as a constituent of the glucoside xanthorhamnin. It is composed of one molecule of glucose united to two molecules of rhamnose (methyl pentose, C_{6}H_{12}O_{5}). It is of interest only in connection with the properties of the glucoside in which it is present (see page 84). Three trisaccharides which are non-reducing sugars are found in plants; namely, raffinose, gentianose, and melizitose.