Determination of Specific Gravity and Estimation of Solids-Not-Fat.— Since the specific gravity of milk is raised by all of the other solids and lowered by the fat, it follows that after the influence of each has been deter mined, it should be possible to estimate from the percentage of fat and the specific gravity the percentage of solids-not-fat which the sam ple To determine the specific grav ity it is convenient to use a “Quevennes or a "Soxhletp lactometer, either of which is prac tically a hydrometer of sufficient range to cover the gravity of all ordinary milks and so gradu ated that the thousandths in excess of unity are represented by whole numbers on the scale. Thus a milk with a specific gravity of 1.0315 will give a lactometer reading of 31.5. The temperature should be observed at the same time with the lactometer reading and the latter corrected to 60° F. by adding to the reading 0.1° for each degree F. above the standard tem perature of 60° F. One-fourth of the cor rected lactometer reading plus one-fifth of the percentage of fat gives a fairly close approxi mation to the percentage of solids-not-fat. The lactometer reading may also be useful aside from the estimation of solids-not-fat. Although the addition of cream to milk would lower the gravity, yet in general the lactometer reading is a rough indication of the richness of the milk, because a high percentage of fat is usually accompanied by a high percentage of protein which raises the lactometer reading. Cases in which genuine milk shows a low grav ity as the result of a high percentage of fat are not common and can usually be detected by noticing the viscosity and opacity of the milk as it runs from the bulb of the lactometer. The lactometer reading taken in connection with the appearance is therefore a useful pre liminary test and is used as such by the milk inspectors of many cities.
Detection of Skimmed or Watered Milk. — The most common adulterations of milk are the removal of cream (or the addition of skimmed milk, which amounts to the same thing), and the addition of water. Milk which contains less than 3 per cent of fat has usually been partially skimmed, and milk containing less than 8.5 per cent of solids-not-fat has usually been watered, In most States there are mini mum limits established by law and milk falling below the limit is considered to be adulterated. Thus in New York State milk must contain at least 3 per cent of fat and at least 11.5 per cent of total solids. As stated above, the percentage of fat varies much more than that of solids not-fat. Skimming is therefore more difficult to detect than watering. In fact, it is usually impossible to distinguish by analysis between a genuine sample containing, say, 3.6 per cent of fat, and a sample originally containing 4.5 per cent of fat, one-fifth of which has been re moved. On the other hand, the addition of 20 per cent of water would almost certainly re duce the percentage of solids-not-fat to a figure considerably below the normal minimum. Re cently, too, a more delicate method for the detection of watering has been devised.
Other The addition of chalk, calves' brains, etc., though frequently men tioned in the older works on food adulteration, is now almost unknown. Cane-sugar or starch may sometimes be added to mask the effect of watering. The former may be detected by the rose-red color produced when about 10 cubic centimeters of milk are boiled for five minutes with one cubic centimeter of hydrochloric acid and 0.1 gram of resorcin. To detect the pres
ence of starch, boil about 10 cubic centimeters of milk, cool it thoroughly and then add a few drops of a solution of iodine in potassium iodide. If starch is present a characteristic blue color will be produced.
Artificial Coloring Caramel or yellow coloring matter such as is commonly used in butter may occasionally be added to milk. The detection of these is of little prac tical importance, since they would ordinarily be used only to mask the effects of gross skim ming or watering such as would be readily de tected by the methods already given.
Detection of Preservatives.— FORMALDE HYDE Dilute the milk with an equal bulk of water in a test tube and carefully pour in commercial concentrated sulphuric acid, inclining the tube so that the acid and milk will not mix. If formaldehyde is present a violet ring forms at the junction of the two liquids. If pure acid is substituted for the commercial, a trace of some ferric salt should be added. BORACIC ACID OR BORAX.- Evapo rate a portion of the milk to dryness and burn to ash. Moisten with a few drops of dilute hydrochloric acid and introduce a slip of yellow turmeric paper. This is turned reddish brown by boracic acid and the color may be changed to bluish black by treating with a solution of sodium carbonate. CARBONATES or BICARBO NATES if present in the milk would remain as carbonate after ignition and be shown by an effervescence when the ash was moistened with acid in the test for boracic acid just described.
Bibliography.— Ailcman, C. M., 'Milk: Its Nature and Composition; A Handbook on the Chemistry and Bacteriology of Milk, Butter and Cheese) (New York 1895) •, Allen, A. H., 'Commercial Organic Analysis) (Vol. IV, New York 1898) ; Buchanan, R. E., 'Household Bacteriology' (New York 1913) ; Chapin, H. D., 'Theory and Practice of Infant Feeding' (New York 1902); Conn, H. W., 'Agricultural Bac (%d ed., Philadelphia 1918), 'Bacteria in Milk and its (Philadelphia 1903) ; Farrington and Wall, 'Testing Milk and Its (Madison, Wis., 1912) ; 'Diseases of Infancy' (New York 1902) ; Hunziker, 0. F., 'Condensed Milk and Milk (Lafayette, Ind., 1914) ; Lane-Claypon, 'Milk and its Hy gienic Relations' • Leze, R., 'Les industries du lait) (Paris 1904; Richmond, Chem istry) (1899) ; Rose, ose, M. S., 'Feeding the Family' (New York 1917) ; Rosenau, M. J., 'The Milk Question' (New York 1912); 'Pre ventive Medicine and (New York 1913) ; Rothschild, (Bibliographia (8,375 titles, Paris 1901) ; Russell and Hastings, 'Outlines of Dairy (9th ed., Madison, Wis., 1910) ; Savage, W. G., 'Milk and the Public Health' (London 1912); Sher man, H. C., 'Food (New York 1914) ; 'Chemistry of Food and Nutrition' (2d ed., New York 1918) ; Snyder, H., 'Dairy (New York 1906) • Stocking, W. A, 'Manual of Dairy Products' (Jew York 1916) Van Slyke, L. L., 'Modern Methods of Testing Milk and its Products' (New York 1906) ; Wing, H. H., 'Milk and its Products' (rev. ed., with bibliography, New York 1913). Consult also Reports and Bulletins of the United States Department of Agiculture; the United States Public Health Service; the State departments of agriculture and of health and the agricultural experiment stations. . H. C. SHERMAN, Colombia University, New York,