530 THE CONTROL OF THE MILK SUPPLY 



hagen Dairy Company, the Manchester Pure Milk Supply 

 Company, Bolle of Berlin, and others. The first-named company 

 filter the milk they receive, on their own premises, through four 

 layers of gravel of varying sized pebbles, and three layers of cloth. 

 There is also a small filter with two layers of gravel and two of 

 cloth to be used for smaller amounts of milk. The process of 

 filtration in both cases is upward, first through the lowest layers 

 of coarse gravel, then through the intervening finer layers of 

 gravel, and eventually through the cloth of close texture. The 

 milk rises upwards through the filter under gravitation pressure, 

 and from the top of the filter is run off into coolers. The filter 

 is cleaned by washing the gravel with hydrochloric acid, also 

 with soda and water, and is sterilised at 120'' C. after each time 

 of using. It is then dried in a high-pressure steam oven. Another 

 milk company near Copenhagen filters all milk received through 

 a filter of large sized pebbles by an upward flow, just prior to 

 being bottled. The filtering material is thrown away after each 

 time of using. A third company, the Milk Supply Pasteur, receive 

 all their milk into a reservoir where it is duly weighed and 

 strained, and from this it is pumped to a filter consisting of 

 layers of gravel and felt, the whole apparatus being carefully 

 sterilised daily. The Manchester Pure Milk Supply Company 

 also filtered their milk. On receipt the milk was cooled to 34° Y. in 

 ice tanks, and then filtered through three layers of sand 2 

 inches deep, from which filter the milk was passed to the steriliser, 

 cooled, and bottled. The sand of the filter was regularly cleaned, 

 first with cold water, soda water, and hot water ; then it was 

 steamed, sieved, and dried in an oven. It was said that by means 

 of these gravel and sand filters the number of bacteria contained 

 in the milk was reduced by one-third. Other forms of filtration 

 have been adopted including filtration through felt, compressed 

 absorbent cotton, and such materials. Dr Seibert has reported 

 that milk passed through a half-inch layer of compressed cotton 

 removes seven-eighths of the bacteria, and a second filtration 

 will further reduce the number to one-twentieth. A considerable 

 quantity of extremely fine particles of dust is also waylaid in 

 the filter. But only one quart of milk can thus be filtered in 

 fifteen minutes. Filtering is said to have the disadvantage of 

 holding back some fat, and is at best somewhat cumbersome 

 technique. Hence its adoption has not been very wide. 



Preservatives. — In Great Britain the addition of preservatives 

 to milk is unfortunately widespread. In London from 10 to 40 



