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 C18H32O16. 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, C6H12O5). 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.
Raffinose occurs normally in cotton seeds, in barley grains, and in manna; also, in small quantities in the beet root, associated with sucrose. It is more soluble in water than is sucrose and hence remains in solution in the molasses from beet-sugar manufacture, which constitutes the commercial source for this sugar. Raffinose crystallizes out of concentrated solutions, with five molecules of water of crystallization, in clusters of glistening prisms. It is strongly dextrorotatory, the anhydrous sugar having a specific rotatory power of +185°, and the crystalline form, C18H32O16, showing a specific rotation of +104.5°. It does not reduce Fehling's solution, nor form an osazone, and in its other properties it closely resembles sucrose.
The hydrolysis of raffinose presents several interesting possibilities. If its structure is represented as follows:
C6H11O5——C6H10O4——C6H11O5
Fructose Glucose Galactose
\_____ _____/ \_____ _____/
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Sucrose Melibiose
it is apparent that it may break down by hydrolysis in three different ways: (1) into sucrose and galactose, (2) into fructose and melibiose, and (3) into fructose, glucose, and galactose. As a matter of fact, it does actually break down in these three different ways, under the influence of different catalysts; invertase or dilute acids break it down into fructose and melibiose, emulsin hydrolyzes it to sucrose and galactose, while strong acids or the enzymes of bottom-yeasts break it down into the three hexoses.
Gentianose, a trisaccharide found in the roots of yellow gentian (Gentiana lutea), is a non-reducing sugar, which when hydrolyzed yields either fructose and gentiobiose, or fructose and two molecules of glucose.