=262. Sugars in Evaporated Milks.=—In addition to the lactose normally present in evaporated milks the analyst will, in most cases, find large quantities of sucrose. The latter sugar is added as a preservative and condiment. By reason of the ease with which sucrose is hydrolyzed, evaporated milk containing it may have also some invert sugar among its contents. A method of examination is desirable, therefore, which will secure the determination of lactose, sucrose and invert sugar in mixtures. The probability of the development of galactose and dextrose during the evaporation and conservation of the sample, is not great. The best method of conducting this work is the one developed by Bigelow and McElroy.[217] The principle on which the method is based rests on the fact that in certain conditions, easily supplied, the sucrose and invert sugar present in a sample may be entirely removed by fermentation and the residual lactose secured in an unchanged condition. The lactose is finally estimated by one of the methods already described. The details of the process follow: On opening a package of evaporated milk, its entire contents are transferred to a dish and well mixed. Several portions of about twenty-five grams each are placed in flasks marked at 100 cubic centimeters. To each of the flasks enough water is added to bring all the sugars into solution and normal rotation is made certain by boiling. After cooling, the contents of the flasks are clarified by mercuric iodid in acetic acid solution. The clarifying reagent is prepared by dissolving fifty-three grams of potassium iodid, twenty-two grams of mercuric chlorid, and thirty-two cubic centimeters of strongest acetic acid in water, mixing the solutions and completing the volume to one liter. The clarification is aided by the use of alumina cream (=84=). The flask is filled to the mark, and the contents well shaken and poured on a filter. After rejecting the first portion of the filtrate the residue is polarized in the usual manner. Two or more separate portions of the sample are dissolved in water in flasks of the size mentioned, heated to 55°, half a cake of compressed yeast added to each and the temperature kept at 55° for five hours. The residue in each flask is treated as above described, the mercuric solution being added before cooling to prevent the fermentative action of the yeast, and the polarization noted. By this treatment the sucrose is completely inverted, while the lactose is not affected. The percentage of sucrose is calculated by the formulas given in paragraph =94=, using the factor 142.6. At the temperature noted the yeast exercises no fermentative, but only a diastatic action.