Acid-Butyrometry

ACID-BUTYROMETRY (N. Gerber) is one of the methods for the quantitative determination of fat in milk; this method consists in the direct measurement of the volume of fat obtained from the coalescence of milk fat globules after the destruction and dissolution of the protein enveloping them by means of sulfuric acid of a specific gravity (hence the name). Milk is known to be an emulsion of fat in an aqueous solution of proteins, carbohydrates, and salts, with the fat contained therein in the form of small droplets. Each of the latter is enveloped by a sort of membrane consisting of milk proteins, which prevents the droplets from coalescing. If these protein membranes are destroyed in some way, the droplets will coalesce, and the fat will float to the surface, where its volume can be determined in one way or another. Therefore, acid-butyrometry consists in mixing in a special graduated vessel (a standard Gerber butyrometer) a defined volume of the milk under study (11 cubic centimeters) with a defined volume of sulfuric acid of specific gravity 1.820–1.825 (10 cubic centimeters) with the addition of amyl alcohol of specific gravity 0.815 (1 cubic centimeter), followed by rapid and vigorous shaking. This causes strong heating, the liquid acquires a yellowish-brown color and becomes clear, the casein flakes dissolve, and a layer of fat appears on the surface of the liquid, the height of which is measured. For better separation of fat droplets from the mixture, the latter is heated for 15 minutes in water at 60–70° and then subjected to centrifugation for 5 minutes at 700–800 revolutions per minute. After this, the butyrometer is removed from the centrifuge and, holding it against the light with the stopper facing downward, the latter is screwed in so that the entire fat layer enters the graduated neck of the butyrometer, then the height of the fat layer is quickly read off by the number of scale divisions. In rich milk, the reading is taken from the lower meniscus; in skimmed milk, from the middle of the upper meniscus. The found number of divisions, multiplied by 1/10, gives the weight percentage of fat. Acid-butyrometry belongs to the simplified methods for the quantitative determination of fat in milk and is distinguished by its speed, accuracy, and convenience. In accuracy, it far surpasses all other simplified methods (Marchand, Feser, etc.) and is inferior only to the gravimetric method. In addition to the study of milk, acid-butyrometry is also used in determining the amount of fat in cream, cheese, and butter, and for obtaining more precise results, butyrometers of special design are used.