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

by Harvey Washington Wiley

431. Micro-Organisms of the Milk.—Milk is a natural culture solution for the growth of micro-organisms, and they multiply therein with almost incredible rapidity. Some of these are useful, as, for instance, those which are active in the ripening of cream, and others are of an injurious nature, producing fermentations which destroy the sugars or proteids of the milk and develop acid, alcohol, mucous or ptomaine products. It is not possible here to even enumerate the kinds of micro-organisms which abound in milk and the reader is referred to the standard works on that subject.[411]

For analytical purposes it is important that the sample be kept as free as possible of all micro-organisms, good or bad, which may be accomplished by some of the methods given below.

432. Sampling Milk.—It is not difficult to secure for examination representative samples of milk, if the proper precautions be taken. On the other hand, the ease and rapidity with which a milk undergoes profound changes render necessary a careful control of the methods of taking samples. The most rapid changes to which a mass of milk is obnoxious are due to the separation of the fat particles and to the action of bacteria. Even after standing for a few minutes, it will be found that the fat globules are not evenly distributed. Before securing the sample for analysis, it is necessary to well stir or mix the milk. A mean sample may also be secured from a can of milk by the sampling tube devised by Scovell, which will be described below.

In securing samples, a full detailed description of the cow or herd furnishing them is desirable, together with all other data which seem to illustrate in any way the general and particular conditions of the dairy. Samples are to be preserved in clean, well stoppered vessels, properly numbered and securely sealed.

Fig. 106.—Scovell’s Milk Sampling Tube.

433. Scovell’s Milk Sampler.—In sampling large quantities of milk in pails or shipping cans, it is exceedingly inconvenient to mix the milk by pouring from one vessel to another or by any easy process of stirring. In order to get representative samples in such conditions, Scovell has put in use a sampler, by means of which a typical portion of the milk may be withdrawn from a can without either pouring or stirring. The construction of the sampler is shown in [Fig. 106], representing it in outline and longitudinal section. The tube a, made of brass, is open at both ends and of any convenient dimensions. Its lower end slides in a large tube b, closed at the bottom and having three elliptical, lateral openings c, which admit the milk as the tube is slowly depressed in the contents of the can. In getting the sample, a is raised as shown in profile. When the bottom of b reaches the bottom of the can a is pushed down as shown in the section. The milk contained in the sampler is then readily withdrawn.

434. Preserving Milk for Analysis.—Pasteurizing or boiling the sample is not advisable by reason of the changes produced in the milk by heat. The milk sample may be preserved by adding to it a little chloroform, one part in 100 being sufficient. Boric and salicylic acids may also be used, but not so advantageously as formaldehyd or mercuric chlorid. Rideal has observed that one part of formaldehyd will preserve 10,000 parts of milk in a fresh state for seven days. The formaldehyd sold in the trade contains about one part of formaldehyd in 320 of the mixture. One-half pint of this commercial article is sufficient for about twenty gallons of milk, corresponding to about one part of pure formaldehyd to 45,000 parts of milk. Rideal much prefers formalin (formaldehyd) to borax or boric acid as a milk preservative. No ill effects due to its toxic action have been observed, even when it is consumed in a one per cent solution.[412]

Samples of milk can be kept in this way from four to six weeks by adding about one drop of the commercial formaldehyd to each ounce of sample. The analyst should remember in such cases that the formaldehyd may not all escape on evaporation, on account of forming some kind of a compound with the constituents of the milk, as is pointed out by Bevan.[413]

Bevan suggests that the formaldehyd may not actually be retained in the sample, but that the increase in the apparent amount of total solids is due to the conversion of the lactose into galactose. This point, however, has not been determined.