Lactose

LACTOSE (milk sugar, lactobiose), C₁₂H₂₂O₁₁, a disaccharide composed of residues of D-galactose and D-glucose, ester-bonded in a monoglycosidic linkage: β(1→5)galactosido-α(1→5)glucose (see Disaccharides). Lactose is found only in the animal organism (in mammals) and under normal conditions is present in milk, being its main carbohydrate. (For the finding and discovery of lactose in urine - see Lactosuria.) L. crystallizes with 1 molecule of water, is easily soluble in water, insoluble in ether and in absolute alcohol; has a slightly sweet taste. The plane of polarization of light rotates to the right, [α]_D = +55.3°. It has reducing properties, reacts with phenylhydrazine (osazone melts, decomposing, at 210-212°), exhibits mutarotation (see). Under acidic or enzymatic hydrolysis [lactase (see), emulsin] it breaks down into D-galactose and D-glucose. In accordance with the content of galactose residue, L., unlike most other reducing disaccharides, upon oxidation with HNO₃ forms a difficultly soluble mucic acid HOOC(CH(OH))₄COOH (melting point 213°). L. under the influence of pure yeast cultures does not ferment (a practically important difference from glucose), but under the influence of many microorganisms it breaks down into fermentable galactose and glucose and passes into lactic acid (see Fermentation), which is the basis for the souring of milk. Under the influence of milk yeast (Torula kephir), which is contained in kefir grains, it ferments with the formation of alcohol, carbonic acid, and lactic acid; this is the basis for the preparation of koumiss, kefir, and yogurt. L. is formed in the mammary gland apparently from glucose, which under the influence of enzymes turns into galactose, which then by synthesis with glucose gives L. Lactose is the main carbohydrate in the food of a breast-fed animal; 1 g of L. gives 3.95 calories upon combustion. Regarding the absorption of L. and the formation of glycogen - see Disaccharides. L. has pharmaceutical application and is obtained from sweet whey, obtained by the action of rennet on milk; the whey is boiled, the precipitated proteins are filtered out, the filtrate is concentrated to the consistency of syrup and left to crystallize. For the quantitative determination of L., the polarimetric method is used, titration with Fehling's solution (0.00676 g of L. reduces 1 cm³ of Fehling's solution).

L. Brode. L. has wide application in bacteriology for the preparation of colored nutrient media. The most common of them are Endo agar, containing L., fuchsin, and sodium sulfite, Conradi-Drigalski agar, consisting of L., litmus tincture, and a solution of crystal violet (the latter to inhibit the growth of saprophytes), and Godlevsky's agar, containing L., ox bile (to enrich the medium when growing typhoid bacilli), malachite green, and sodium sulfite. The basis of these media is the property of L. to break down under certain conditions with the formation of aldehydes and acids, which are detected by means of various indicators by a change in the color of microbial colonies. Colored media are used for the differentiation of microbes, mainly of the intestinal group. L. as a rule is not broken down by any of the pathogenic microbes of this group and is broken down by the bacillus coli, which, when sown on one of the said media, causes a change in the color of the medium. Therefore, the colonies of bacillus coli are colored red (Endo) and green (Godlevsky's agar); on Drigalski's agar, where the indicator is litmus tincture, the colonies of bacillus coli are red. The colonies of typhoid, paratyphoid, and dysentery bacilli are either colorless (Endo and Godlevsky's agar) or colored the color of the medium (Drigalski's agar). Taking into account the ability of L. to change easily, it is recommended when preparing nutrient media to check the reaction of the medium (an acidic reaction has a harmful effect) and to add L. to the ready medium in order not to subject it to repeated heating.