i MILK 125 



reliable and also easier to judge. The smaller ones have the 

 single advantage of taking up less room, so that a larger 

 number of tests can be made at one time. 



The stand containing the test-tubes can be sunk in a square 

 vessel made of tinplate, which holds water and can be heated 

 with a spirit lamp. In the lid of this water-bath there is 

 placed a thermometer, the bulb of which dips into the water. 



The most important point to be observed in using the 

 fermentation test is to keep the test-tubes as free from bacteria 

 that is, sterile as possible. It is easy to see that if the tubes 

 contain bacteria before the milk is put into them, the changes 

 which take place during the test are due, not to the bacteria of 

 the milk alone, but also to those originally present in the tube. 

 The results in such cases are absolutely worthless. If the care 

 necessary to make the tubes clean and bright is not bestowed 

 upon them, then the test had better riot be attempted. It is 

 preferable, of course, to sterilise the glass tubes, which is easily 

 done in a steam steriliser. Where this is not obtainable the 

 tubes should be cleansed with a warm solution of soda, then 

 washed in hot water, and finally rinsed in cold water which has 

 previously been boiled. The tubes may then be placed in an 

 inverted position and left to drain dry. 



The test-tubes are numbered, and after milk has been poured 

 into them to within 1 cm. of the top, they are closed by the tin 

 or porcelain covers and placed in the bath, the water of which 

 is at a temperature of 40 C., and kept there for twelve hours. 

 During this time the temperature of the water is kept constant 

 between 38 and 40 C. with the help of the spirit lamp. At 

 the end of the time the samples are taken out, and the appear- 

 ance, smell, or other property of the milk serves as an indication 

 of its bacterial contents. If the milk is quite normal and fresh, 

 then it ought to undergo no change other than acquiring a 

 pure sour smell. Should the sample be completely and homo- 

 geneously coagulated, it is not an indication that the milk is not 

 so good, for the lactic acid bacteria apparently still have the 

 upper hand. This condition points, however, to a considerable 

 length of time having elapsed since the milk was drawn. If, 

 on the other hand, the coagulation of the milk is irregularly 

 fiocculent, and there are large quantities of whey, then there 

 has not been a due regard to cleanliness. 



