Destruction of Bacteria by Heat. 193 



The Destruction of Bacteria in Milk. 



it is jur purpose to discuss briefly the destruction of bacteria 

 in milk, which aims to free the milk from disease-producing germs 

 and add to the keeping quality of the milk. 



In practice this is accomplished most frequently by heating, 

 in which the following distinctions are made : 



1. Sterilization of the milk; 



2. Simple boiling; 



3. Pasteurization. 



If it is desired to judge the value of these methods of prepa- 

 ration, the question first considered must be, what changes does 

 the milk undergo through heating? Milk is a biological product 

 the properties of which may be considerably influenced by cold 

 and heat. 



It is generally known that after heating milk retains a so- 

 called boiled-milk taste, and that this becomes stronger the longer 

 the milk is subjected to a temperature of from 70 to 100 deg. C. 



The method of heating is important for the appearance of the cooked taste. 

 Open boiling even for a short time produces a marked change in taste when compared 

 with heating in specially closed utensils or in bottles after subsequent cooling. 



The curdling of boiled milk is more difficult than with raw 

 milk; the boiled milk in curdling after a long time forms a loose, 

 coagulum with less uniform consistency. This change is not so 

 pronounced with heating between 70 to 80 deg. as in boiling and 

 in heating to over 100 deg. C. 



Depending on the height of the temperature and on the length 

 of time the heat is applied, globulin (at 75 deg.) and albumin (at 

 80 deg.) are precipitated. Proteids which are precipitated in milk 

 by heating to boiling temperature disappear if the boiling is con- 

 tinued. According to Peiper and Eichloff the intermolecular at- 

 tachments of the proteids become loosened by heating to high 

 temperatures, and leucin, tyrosin, ammonia, sulphureted hydrogen 

 and phosphorated hydrogen are formed. If the heating has been 

 conducted in poor earthenware or glass vessels, especially new 

 ones, potassium silicate will pass into the milk. Fynn noted the 

 absence of sulphureted hydrogen from heated colostral milk. The 

 reaction became apparent only on the fourth day of lactation. The 

 formation of sulphureted hydrogen and phosphoric acid in milk 

 results from the splitting up of casein. 



Hydrogen sulphide can be demonstrated in canned milk even months after heating, 

 whereas in sterile bottled milk, under the influencee of light and in the presence of 

 oxygen, the sulphide of hydrogen is utilized for the formation of water and sulphur. 



In higher heating the milk becomes brownish through caramel- 

 ization of the milk sugar and the lecithin content of the milk 



13 



