20 MILK AND DAIRY PRODUCTS PART 



analysis it is always advisable to add some sort of preservative, 

 for it is never certain if the samples can be analysed immediately 

 on arrival at the laboratory. Many different substances have 

 been used as preservatives, but only a few have shown them- 

 selves to be really suitable for the purpose. In choosing such a 

 preservative material it must not be forgotten that it has not 

 only to protect the milk against changes due to micro-organisms, 

 but it must also cause no alteration in its chemical composition. 



It will suffice to mention here the most important substances 

 used for the preservation of milk samples. In Europe, potassium 

 bichromate, K 2 Cr 2 O 7 and formaldehyde, CH 2 O, are generally 

 employed. Potassium bichromate is a very suitable pre- 

 servative, and was first brought forward by Dr. J. E. Alen, of 

 Gothenburg. Bichromate has the advantage of not in any way 

 altering the chemical composition of the milk, and although it 

 is poisonous, still the yellow colour which it imparts to the 

 milk prevents the sample being inadvertently drunk. About 

 as much bichromate as will lie on the point of a knife (0'5 g. to 

 1 litre of milk) is sufficient to preserve the sample for several 

 weeks, provided the temperature does not rise very high. There 

 is, however, one drawback to the use of bichromate, and that is 

 it causes the cream to rise more quickly than it would normally. 

 This is due to the fact that the dissolved salt raises the specific 

 gravity of the milk, so that a greater difference arises between 

 the specific gravity of the milk fat and that of the milk serum. 

 The cream, therefore, gathers in a thick layer ; but if the milk 

 is warmed to about 40 C., then, as previously mentioned, it is 

 not difficult thoroughly to mix the cream with the milk again. 



For the preservation of small samples of milk a concentrated 

 solution of bichromate may be used, and one drop measured 

 from a dropping bottle into each 10 c.c. of milk/ If the bottles 

 are sent from the laboratory to the place where the samples of 

 milk are to be taken, then it is preferable to put the requisite 

 quantity of bichromate into them before they leave. 



If the preserved milk has to be tested for specific gravity, 

 dry matter, and ash, then potassium chromate cannot of course 

 be used, unless, perhaps, in the case of the dry matter, where 

 the chromate can be specially estimated according to the method 

 given on p. 90, and the amount deducted from the total weight. 

 In such a case as this, however, it is preferable to use form- 



