The supply of milk is conducted, to a large extent, by towns' dairies, which depend for their supplies upon the dairy farm in the country, and it is obvious that a certain period of time must elapse, in the generality of cases, before a town's dairy receives its supply in the ordinary course, and this constitutes the greatest difficulty in modern dairy practice, owing to the liability of the milk to absorb bacteria, which during transit may multiply enormously. The multiplying of bacteria in milk at different temperatures is easily demonstrated, and the result of this has been stated in various forms many times over. As a graphic means, however, of showing the increase that takes place in the numbers of germs present, and the consequent product of acidity, the table below by Conn may be given. The consequent result of the increase in bacteria is the production of lactic acid, which produces the souring so familiar in milk which has been kept in the household at a high temperature. _Numbers of Bacteria per c.cm. in Milk kept at Different Temperatures._ ──────┬───────┬───────┬─────────┬─────────────────────┬──────────┬────────── Number│ In 12│ In 12│ In 50 │ In 50 hrs. or at │ No. hrs. │ No. hrs. at │hrs. at│hrs. at│ hrs. at │ time of curdling │to curdle │to curdle Outset│ 50° F.│ 70° F.│ 50° F. │ at 70° F. │ at 50° F.│ at 70° F. ──────┼───────┼───────┼─────────┼─────────────────────┼──────────┼────────── 46,000│ 39,000│249,500│1,500,000│ 542,000,000 │ 190 │ 56 47,000│ 44,800│360,000│ 127,500│ 792,000,000.36 hrs.│ 289 │ 36 50,000│ 35,000 800,000│ 160,000│2,560,000,000.42 hrs.│ 172 │ 42 ──────┴───────┴───────┴─────────┴─────────────────────┴──────────┴────────── What actually happens is that the lactic acid is produced by the breaking up of the milk sugar, and the appearance of this sourness is an indication that a period has been reached in the age of the milk which may be described as being—unwholesome.[46] It is necessary, therefore, for the town's milk dairy to be equipped in such a way as to deal promptly with the milk supply. We have seen that the milk should first of all be aërated at blood heat, so as to liberate objectionable odours, after which it should be cooled to as low a temperature as possible, by means of well water. When these operations have been performed on the farm, milk should be sent as rapidly as possible to the distributing towns' dairies, and should be transported in refrigerated waggons, cooled preferably with ice, during the journey. On arrival at the town dairy, it will be necessary to pasteurise the milk—that is to say, the milk should be heated to such a temperature as will destroy any pathogenic organisms which may be present, and the pasteurising temperature should therefore be in excess of the thermal death-point of all such organisms.