Part 11 (2/2)

[Illustration: FIG 25 Pott's pasteurizer]

Another enerally introduced is the Potts'

rotating pasteurizer This apparatus has a central milk cha hot water The whole machine revolves on a horizontal axis, and the crea process

~Continuous-flow pasteurizers~ The dereater capacity than can be secured in the reservoir machines has led to the perfection of several kinds of apparatus where the h the apparatus Most of these were primarily introduced for the treat purposes, but they are frequently ee scale in city h of late years several have been devised in this country

The general principle of construction is much the same in most of them

The milk is spread out in a thin sheet, and is treated by passing it over a surface, heated either with steam directly or preferably with hot water

Where stea on” of the milk proteids to the heated surface

In some of these machines (Thiel, Kuehne, Lawrence, De Laval, and Hochmuth), a ribbed surface is employed over which the milk flohile the opposite surface is heated with hot water or steaal apparatus in which a thin layer ofdrum

In some types of apparatus, as in the Miller machine, an American pasteurizer, the milk is forced in a thin sheet between two heated surfaces, thereby facilitating the heating process In the Farrington h which the milk flows in a continuous stream

One of the enerator type (a Ger surface in a thin strea cold milk so that the heated liquid is partially cooled by the inflowing fresh milk

In machines of this class it requires very much less steam to heat up the milk than in those in which the cold milk is heated wholly by the hot water

A number of machines have been constructed on the principle of a reservoir which is fed by a constantly flowing stream In some kinds of apparatus of this type no atte of the recently introduced milk with that which has been partially heated The pattern for this reservoir type is Fjord's heater, in which the inally designed as a heater for milk before separation, but it has since been materially modified so that it is better adapted to the purposes of pasteurization Reid was the first to introduce this type of machine into America

~Objections to continuous flow pasteurizers~ In all continuous flow pasteurizers certain defects are more or less evident While they fulfill the ie capacity, an absolute essential where large volu handled, it does not of necessity follow that they conforienic and physical requirereatest difficulty is the shortened period of exposure The period which the milk is actually heated is often not more than a minute or so Another serious defect is the inability to heat _all_ of the milk for a uniform period of time At best, the milk is exposed for an extreh the machine much more quickly than do the remainder Those portions in contact with the walls of the apparatus are retarded by friction and are e, while the particles in the center of the streah in the least possible ti simple method enables the factory operator to test the period of exposure in the machine: Start the machine full of water, and after the sae the inflow to full-crea at the sae and also when first evidence of ins to appear at outflow If samples are taken from first appearance of milky condition and thereafter at different intervals for severalthe amount of butter-fat in the sa it takes for the milk to entirely replace the water

Tests made by the writer[143] on the Miller pasteurizer shohen fed at the rate of 1,700 pounds per hour, the minimum period of exposure to be 15 seconds, and the maximum about 60-70 seconds, while about two-thirds of the milk passed the machine in 40-50 seconds This h the machine is undoubtedly the reason why the results of this type of treatment are subject to so much variation Naturally, even a fatal temperature to bacterial life can be reduced to a point where actual destruction of even vegetating cells does not occur

~Bacterial efficiency of reservoir pasteurizers~ The bacterial content of pasteurized anisinally present in the saht to a crea after treatreater than if the raw le farm

An examination of milk and cream pasteurized on a commercial scale in the Russell vat at the Wisconsin Dairy school showed that over 998 per cent of the bacterial life in raw milk or cream was destroyed by the heat employed, i e, 155 F for twenty minutes duration[144] In nearly one-half of the saerm content in the pasteurized sae of twenty-five saer about 25,000 bacteria per cc This h in organise 3,675,000 bacteria per cc De Schweinitz[145]

has reported the gerton which was treated at 158 to 160 F for fifteen ures reported were froar plates Undoubtedly these would have been higher if a longer period of incubation had been e of 82 samples, taken for the period of one year, showed 325 bacteria per cc

[Illustration: FIG 26 Effect of pasteurizing on germ content of milk

Black square represents bacteria of rawafter pasteurization]

~Bacterial efficiency of continuous-flow pasteurizers~ A quantitative determination of the bacteria found in milk and cream when treated in ree of variation in results that is not to be noted in the discontinuous apparatus

[Illustration: FIG 27 Reid's Continuous Pasteurizer]

Harding and Rogers[146] have tested the efficiency of one of the Danish type of continuous pasteurizers These experiments were made at 158, 176 and 185 F They found the efficiency of the machine not wholly satisfactory at the lower teave 15,300 bacteria per cc, with a e from 62,790 to 120 Twenty-five exae from 300 to 20 The results at 185 F

showed practically the same results as noted at 176 F Considerable trouble was experienced with the ”scalding on” of the h acidity was used

Jensen[147] details the results of 139 tests in 1899, en Health Commission In 66 saanisms per cc were found, and in 22 cases from one to five millions Nineteen tests showed less than 10,000 per cc