Principles and practice of agricultural analysis. Volume 3 (of 3), Agricultural products

by Harvey Washington Wiley

Richmond and Boseley propose to detect formalin by means of diphenylamin. A solution of diphenylamin is made with water, with the help of just enough sulfuric acid to secure a proper solvent effect. The liquid to be tested, which is supposed to contain formaldehyd, or the distillate therefrom, is added to this solution and boiled. If formaldehyd be present, a white flocculent precipitate is deposited, which is colored green if the acid used contain nitrates. For other methods of detecting formalin and for a partial literature of the subject the paper mentioned above may be consulted.

One gram of fine-ground mercuric chlorid dissolved in 2,000 grams of milk will preserve it, practically unchanged, for several days. One gram of potassium bichromate dissolved in one liter of milk will also preserve it for some time. Thymol, hydrochloric acid, carbon disulfid, ether and other antiseptics may also be employed. No more of the preserving agent should be used than is required to keep the milk until the analysis is completed.

All methods of preservation are rendered more efficient by the maintenance of a low temperature, whereby the vitality of the bacteria is greatly reduced.

435. Freezing Point of Milk.—By reason of its content of sugar and other dissolved solids, the freezing point of milk is depressed below 0°. A good idea of the purity of whole milk is secured by subjecting it to a kryoscopic test. The apparatus employed for this purpose is that used in general analytical work in the determination of freezing points. Pure full milk freezes at about 0°.55 below zero, and any marked variation from this number shows adulteration or abnormal composition.[414] A simple apparatus, especially adapted to milk, is described by Beckmann.[415] The kryoscopic investigation may also be extended to butter fat dissolved in benzol.

436. Electric Conductivity of Milk.—The electric conductivity of milk may also be used as an index of its composition. The addition of water to milk diminishes its conductivity.[416] This method of investigation has at present but little practical value.

437. Viscosity Of Milk.—The viscosity of milk may be determined by the methods already described. Any variation from the usual degree of fluidity is indicated either by the abstraction of some of the contents of the milk, the addition of some adulterant or the result of fermentation.

438. Acidity and Alkalinity of Milk.—Fresh milk of normal constitution has an amphoteric reaction. It will redden blue and blue red litmus paper. This arises from the presence in the milk of both neutral and acid phosphates of the alkalies. A saturated alkaline phosphate, i. e., one in which all the acid hydrogen of the acid has been replaced by the base has an alkaline reaction while the acid phosphates react acid. When fresh milk is boiled its reaction becomes strongly alkaline and this arises chiefly from the escape of the dissolved carbon dioxid. By the action of micro-organisms on the lactose of milk, the alkaline reaction soon becomes acid, and delicate test paper will show this decomposition long before it becomes perceptible to the taste. It is advisable to test the reactions of the milk as soon as possible after it is drawn from the udder, both before and after boiling.

439. Determination of the Acidity of Milk.—In the determination of the acidity of milk it is important that it first be freed of the carbon dioxid it contains.[417] Van Slyke has found that too high results are obtained by the direct titration of milk for acidity, and when the milk is previously diluted the results are also somewhat too high.[418] Good results are got by diluting the milk with hot water and boiling for a short time to expel the carbon dioxid. Twenty-five cubic centimeters of milk are diluted with water to about a quarter of a liter, as above, two cubic centimeters of a one per cent alcoholic phenolphthalien added and the titration accomplished by decinormal alkali. This variation of the methods of procedure, suggested by Hopkins and Powers, appears to be the best process at present known for the determination of acidity. The reader is referred to the paper cited above for references to other methods which have been proposed.

440. Opacity Of Milk.—The white color and opacity of milk are doubtless due to the presence of the suspended fat particles and to the colloid casein. On the latter it is probably principally dependent since the color of milk is not very sensibly changed after it has passed the extractor, which leaves not to exceed one-tenth of one per cent of fat in it. Some idea of the quality of the milk, however, may be obtained by determining its opacity. This is accomplished by the use of a lactoscope. The one generally employed was devised by Feser and is shown in [Fig. 107].

The instrument consists of a cylindrical glass vessel of a little more than 100 cubic centimeters content, in the lower part of which is set a cone of white glass marked with black lines. Into this part are placed four cubic centimeters of milk. A small quantity of water is added and the contents of the vessel shaken. This operation is repeated until the black lines on the white glass just become visible. The graduations on the left side show the volume of water which is necessary to bring the dark lines into view, while those on the right indicate approximately the percentage of fat present.