No. G. DijiPARTMENT O^ AGKICU l/riJli lO. i73 



thus treated. The use of preservatives, such as salicylic acid, bor- 

 acic acid, formalin, etc., for the purpose of keeijing milk is not to 

 be recommended. In employing heat to kill bacteria, tv^^o general 

 methods have been used, known as pasteurizing and sterilizing. 



(1.) Pasteurizing Milk. — In pasteuriziiig milk for market, it is 

 heated to 140 degrees F., and held at that temperature for twenty or 

 thirty minutes. Formerly, higher temperatures were used, but it 

 was found that milk, heated above 15G degrees F. acquired a "cooked" 

 taste and that the cream did not readily rise, ^^'hile the lower tem- 

 perature does not destroy all germs, it destroys most of them and 

 greatly improves the keeping power of milk. To secure best results 

 in pasteurizing, it is desirable to use milk as fresh as possible. Milk 

 containing U.2 per cent of lactic acid does not give good results. The 

 fewer bacteria milk contains before pasteurizing, the better will 

 be the results of pasteurization. It is also very essential m pasteur- 

 izing that the heated milk should be cooled down at once to 50 de- 

 grees F., or lower. It should also be stored in germ-free bottles 

 or other vessels and kept cold until it is delivered to the customers. 

 When used by children, pasteurized milk should be consumed within 

 twenty-four hours after treatment. If pasteurization is properly 

 carried out, the milk should remain sweet, even at ordinary tempera- 

 tures, one and one-half to two days lo«ger than ordiuarj^ milk not 

 so treated. 



There are on the market several forms ol machines for pasteurizing 

 milk. The best machine should possess the following qualifications: 

 (a) Compact form, (b) ease of keeping clean, (c) complete control of 

 temperature, (d) ability to heat milk quickly, completely and uni- 

 formly, (e) freedom from liability of additional germs getting into 

 the milk during the operation of pasteurizing. 



The pasteurization of milk has the following advantages: (a) It 

 increases the keeping quality of milk, (b) It destroys disease-germs, 

 especially those of tuberculosis, (c) The taste and digestibility of 

 milk are not changed, (d) The process is practicable on a large 

 or small scale. 



(2.) Sterilizing Milk. — In order to render milk absolutely free from 

 all bacteria and their spores, it is necessary to heat milk above 

 212 degrees F. for an hour on each of three successive days, thus 

 giving all spores a chance to develop into active forms, which are 

 then more readily killed. Simple boiling is effective in destroying 

 practically all living disease-germs. Sterilized milk is objectionable 

 for several reasons: (a) It has the characteristic taste of "cooked" 

 milk, which is unpleasant to most people. If people had only steri- 

 lized milk to drink, very much less would be used. (b) Sterilized 

 milk appears to be less digestible, especially in the case of children 

 and invalids. 



