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 at Outset | In 12 hrs.at 50° F. | In 12 hrs.at 70° F. | In 50hrs. at 50° F. | In 50 hrs. or at time of curdling at 70° F. | No. hrs. to curdle at 50° F. | No. hrs. to curdle 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.
Pasteurisation owes its origin to Pasteur, and has become an adopted method throughout the dairy industry, and there are many mechanical devices termed "pasteurisers" (see Fig. 8) which are used for the carrying out of this particular operation. The form of one of these is that of a vertical jacketed cylinder with paraboloidal surface, around which steam is made to pass, so as to maintain the temperature at about 176° F. Milk is allowed to flow in at the bottom of the paraboloidal surface, and is caught by mechanical agitating arms, which revolve at a given speed, and by this action milk is distributed centrifugally over the paraboloidal surface, and is forced out by the same action, at the top of the apparatus, after being heated.
