16 BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE. 



COST OF PASTEURIZING MILK. 



The present cost of pasteurization has been estimated by Bo wen 

 from the cost given in his earlier paper (10) on the assumption that 

 the average price of coal has increased 2.04 times and that milk- 

 plant labor and equipment have increased 50 per cent over the prices 

 of 1913, the year in which his paper was written. He obtained the 

 information from a series of tests in five establishments which were 

 considered to represent the average city milk plant. The pasteuriz- 

 ing equipment in each consisted of a heater, a holding tank, a 

 regenerator, and a cooler. The cost of operation was based on the 

 pasteurizing cycle, starting with the initial temperature of the raw 

 milk and raising it to the pasteurizing temperature, then cooling to 

 the initial temperature of the raw milk. He based the costs on daily 

 interest at 6 per cent per annum on capital invested in pasteurizing 

 equipment, and depreciation and repairs per day at 25 per cent 

 per annum; interest per day at 6 per cent per annum on capital 

 invested in mechanical equipment for pasteurizing, such as engines, 

 boilers, etc., and depreciation and repairs per day at 10 per cent per 

 annum. Other costs figured were labor, coal now estimated at $8.16 

 a ton, cooling water now estimated at $0.75 per 1,000 cubic feet, and 

 refrigeration now estimated at $2 a ton. With these new estimates 

 substituted for the old figures, Bowen calculates that the average 

 cost of pasteurizing 1 gallon of milk is approximately $0.0049, or a 

 little less than one-half cent. 



BACTERIA WHICH SURVIVE PASTEURIZATION. 



It has been stated that about 99 per cent of the bacteria in milk 

 are destroyed by pasteurization; consequently about 1 per cent of 

 the bacteria remain alive, and the kinds left depend entirely on the 

 temperature to which the milk is heated and the number of heat- 

 resistant bacteria in the milk. From studies of the bacteria which 

 survive pasteurization, it is possible to show graphically the hypo- 

 thetical relations of the bacterial groups in raw milk and in milk 

 pasteurized by the holder process at various temperatures under 

 laboratory conditions. 



The bacterial flora of the various kinds of milk is represented in 

 Figure 1 by columns of equal length divided into sections, which, in 

 a general way, show the relative proportion of the bacterial groups. 



From the figure it may be seen that raw milk contains four prin- 

 cipal groups of bacteria the acid, inert, alkali, and peptonizing. 

 The acid group is divided again into two the acid-coagulating, 

 which coagulates milk within 14 days, and the acid group, which 

 merely produces acid and does not coagulate it in less time than that. 

 In raw milk the inert group is the largest. 



