56 DAIRY BACTERIOLOGY 



While clean metal ware may be sterilised by simply prolonging 

 the steaming sufficiently, wood work presents a more difficult 

 problem, and should the dairy be troubled with an infection of 

 harmful organisms, the usual cleaning should be supplemented 

 with a washing with 1 to 2 per cent, formaldehyde made by 

 diluting commercial formalin with twenty to forty times its bulk 

 of water ; it is advisable to protect the hands from the action of 

 this chemical, which is best distributed by means of a garden 

 syringe. In dealing with churns or combined churns and workers, 

 the disinfecting action may be intensified by keeping them well 

 closed for several hours after washing, as the formaldehyde vapour 

 has a disinfecting action. A 2 per cent, solution of ammonium 

 bifluoride is also a very effective disinfectant. The vessels must 

 be rinsed and dried before using again. The growth of moulds is 

 best prevented by avoiding excessive dampness ; even in cellars 

 used for storing cheese, the humidity need not exceed 90 per cent. 

 If the walls of the cellar have become mouldy (Cladosporium 

 herbarum often appears only as very small black spots) they 

 must immediately be limed. Two per cent, of copper sulphate 

 may be added to the lime with advantage. In investigating 

 the cause of any defect in cheese, it should not be forgotten 

 that the cutting and stirring appliances and the cheese cloths 

 may be infected, and that the wooden thermometer case may 

 harbour bacteria. 



In dealing with the subject of cleaning, it may be well to give a 

 more explicit definition of what is understood by infection. By 

 an infection in the broadest sense of the term, we understand any 

 admixture which may minimise the value of the product in ques- 

 tion. If the foreign substance is sterile, the resulting condition is 

 a purely chemical matter. If the foreign matter is not sterile, 

 we shall be dealing with what is generally known as an infection 

 (opposite disinfection), and this is a purely biological matter. 

 The latter trouble will generally be more serious in its consequences 

 than the former, as the effects of the biological action increase on 

 keeping. Milk especially is subject to rapid alteration, not only 

 because it is an ideal nutrient medium for many microorganisms, 

 but also because, unlike most other dairy products, it is a liquid. 

 A solid medium can only be spoilt gradually by an infection from 

 without spreading inwards, and then only if the medium is not too 

 dry to allow of the growth of microorganisms. 



THE PROCUREMENT OF MILK 



The most troublesome sources of milk infection are the udder 

 and teats of the cow, and the vessels with which the milk comes 



