It is necessary, in all cases, to render milk and cream perfectly healthy. To do this the only really safe method known at the present time is that of pasteurisation. Its purpose is to destroy all disease and spore-producing germs in the raw milk. This is accomplished by submitting the milk to such a temperature that the germs will cease to live, and yet at the same time not destroying the good properties of milk and cream. 140o F. have been found, by experiment, to meet the case if sufficient time is allowed for complete germ destruction to take place. In the machinery chiefly used the flow of milk and cream is slowly continuous, and does not occupy the length of time necessary. To counteract this, the milk is generally submitted to a temperature varying between 170° and 180°, the pasteuriser being made to heat up to 194o. The pasteuriser consists of a steam jacket built of heavy steel plates, insulated with a thick layer of felt covered over with polished steel plate. Within this is a paraboloidal shaped pan usually made of stout tinned copper plate. The milk enters into the inner pan at its base, and here is forced round by means of agitators, which gradually raise the milk to, if necessary, some 3 feet above the top. The agitator may be driven by a belt from the engine shaft, or by steam or exhaust steam from the boiler. The top is fitted with a polished copper cover, and this forms a complete enclosure to the milk, which would suffer in its pasteurising process were it exposed to the air. A pasteuriser such as the one illustrated (Fig. 96) would occupy a floor area of approximately 3 feet by 2 feet 6 inches.

Pasteuriser Cooler Regenerative Heater 108

Fig. 96.

Pasteuriser Cooler Regenerative Heater 109

Fig. 97.

Pasteuriser Cooler Regenerative Heater 110

Fig. 98.

The view of a small plant with a steam driven pasteuriser (Fig. 97) gives an idea of a simple, but compact arrangement for the treatment of milk, which is tipped on arrival into the tank, whence it will travel by means of a pipe, entering the pasteuriser at its base, and rising by centrifugal force to above the cooler. To prevent the fermentation of the highly heated milk it has to be cooled down as rapidly as possible to some 38o to 45o. The cylindrical cooler, of 3 feet by 3 feet area, and varying from 2 to 3 feet in height, cools the milk, which falls into the distributing saucer, or mantel at the top, and trickles down the corrugated sides into the saucer beneath; the cooling agency being either cold or iced water or brine, which makes its way along the corrugation on the inside of the cooler. The cooler is connected with the water supply or brine-producing plant. It is made of stout tinned copper in a perfectly cylindrical or slightly conical shape. The inside fluted mantel, connected with the cold water or brine, is quite detachable and easily removed for cleaning purposes. In the case where no pasteuriser is used the milk, being only of low temperature, would be cooled sufficiently by being run over one cooler chilled by cold water, or preferably by a small refrigerating machine. When pasteurisation takes place, the milk, being raised to some 170° to 180° F., will not be sufficiently cooled over a water cooler, but should be passed on to a brine-chilled cooler, where the temperature of the milk is further lowered, and is in a condition to be stored in a tank, from which it is drawn to be distributed to consumers. Cream is taken through the same process, and is then stored to ripen preparatory to its being turned into butter. The milk, remaining after separation from cream, is also chilled, and is then returned to the farmer. A cooler, made by Messrs. Douglas & Sons, is shown in Fig. 98.

Pasteuriser Cooler Regenerative Heater 111

Fig. 99.

Pasteuriser Cooler Regenerative Heater 112

Fig. 100.

Pasteuriser Cooler Regenerative Heater 113

Fig. 101.

An apparatus which tends to economise the use of steam and water is that known as a Regenerative Heater or Temperature Exchanger. It cannot be said that it is in general use, in spite of its many advantages. Its purpose is to exchange temperature between the fresh cold milk and the pasteurised milk. The fresh milk thus heated goes into the pasteuriser to be raised to a complete temperature, whilst the half-cooled milk is further cooled for milk distribution. This heater (Fig. 99), 3 feet by 2 feet, consists of a distributing pan at top, over a thick tinned copper capillary surface of inverted cone shape. Within is a shaft to which is fixed a revolving agitator. The fresh milk is either pumped into the top pan or rises from the pasteuriser; it then flows through the apertures in the mantel over the corrugated surface till it reaches the bottom saucer. Whilst the new milk is travelling downwards over the outside cooler the pasteurised milk enters at the bottom of the apparatus and is circulated by the agitator, so rising upwards, and is eventually discharged through the pipe at top, emptying itself over a cooler.

The plant (Fig. 100), occupying 18 feet by 4 feet, consists of a receiving tank a, from which the milk is pumped to the temperature exchanger d, travelling downwards enters at the base of pasteuriser c, rises into exchanger once more, and rises and flows into cooler f-. The regenerative heater here shown differs somewhat from that explained above. It consists of a series of brass or copper tubes joined together. The apparatus is rectangular, and is fitted with movable ends for cleaning purposes. The cold milk may be made to travel on the outside or on the inside of heater, as may be desired.

A small dairy, erected in Hertfordshire from the designs of Mr. W. G. Horseman, is illustrated in Fig. 101, and shows all the necessary appliances for a "model" establishment, intended to do little more than supply the needs of a large country house. The large number of air inlets will be noticed - for ample ventilation is essential, air outlet being obtained by means of louvres in the gable ends. In larger establishments the retention of equable and exact temperatures at all times of the year is a matter of great importance, and heating and cooling appliances have to be introduced, adding no little to the complexity of the problem of planning a complete dairy, whether it be for the supply of milk only, or of butter, clotted cream, or cheese in addition.

Plate III.

Pasteuriser Cooler Regenerative Heater 114Morris's Electric Machine Bakery, Richmond.

Morris's Electric Machine Bakery, Richmond.