THE GRADING OF MILK 171 



by these dairies is, generally speaking, not so good as might be 

 expected ; this matter deserves attention, for pasteurised milk 

 which is rich in bacteria can only be regarded as a highly objection- 

 able product. It should, therefore, be forbidden to sell pasteurised 

 milk as raw milk. 



In the dairies it is impossible to lay down a definite line of 

 demarcation between first and second grade milk without unduly 

 lowering the standard, for obviously milk cannot be expected to 

 conform to the same standard on a close summer's day as on a 

 frosty day. The author has therefore proposed the adoption of 

 the average reduction time of all the samples tested at one time as 

 the standard for that particular batch of samples. Only milk 

 which is better than the average will be placed in class 1. Milk 

 showing a reduction time equal to or less than the average, but 

 not less than two hours, will be placed in class 2, and other cases 

 can be dealt with as detailed above. As it may be inconvenient to 

 have the samples examined for longer than twelve hours, the 

 reduction time of any samples not finished in this time may be set 

 down as twelve hours in calculating the average ; such cases will 

 only occur on cold winter days. All reduction times may be 

 estimated to the nearest quarter of an hour, the reduction time 

 of milk in class 4 being set down as a quarter of an hour. 



If it is desired to combine the grading according to taste and 

 smell and according to the results of the fermenting test, with the 

 grading according to the reductase test, the following system may 

 be adopted : Samples placed in classes 1, 2 or 3 according to the 

 reductase test are degraded by one class if the taste and smell are 

 decidedly bad, and samples thus placed in classes 2 or 3 are further 

 degraded by one class if the results of the fermentation test show 

 6 2 , 6 3 or s 3 . If the average reduction time should be under five 

 and a half hours, class 1 milk which is decolorised before this 

 time, and which is bad according to the fermenting test, should also 

 be placed in class 2. It will be seen that, excepting in the case 

 just mentioned, the results of the fermenting test are not taken 

 into account in the cases of samples placed in classes 1 and 4, the 

 reasons for this being as follows : Milk which contains relatively 

 few bacteria will generally show a bad result in the fermenting 

 test as it will be particularly poor in true lactic acid bacteria ; 

 moreover, the results observed in the combined reductase and 

 fermenting test in such cases may often be worse than would be 

 shown in the fermenting test alone, as methylene blue exerts a 

 certain toxic effect on bacteria, especially the true lactic acid 

 bacteria, and the fewer the bacteria the more of the poison will 

 each cell have to reduce. On the other hand, the fermenting 

 results are not affected by the small amount of methylene blue 



