428 



ENGINEERING ASPECTS OF PASTEURIZATION 



matically opened. Any leakage from the holder past the flush-type seat wiU escape 

 out the leak drain. Any film of leakage not draining out is sterilized by the small 

 amount of steam continuously passing into the closed valve through the steam con- 

 nection. 



On automatic, pocket-type holders the elimination or remodeling of the multiple- 

 port rotary valve has been found necessary on most types. Figure lo shows a holder 

 of this type in which the leakage of unpasteurized milk through the valve was found 



on test to be 2h percent. Figure 

 2 shows this same t^^De of holder 

 satisfactorily equipped with 

 leak-protector inlet and outlet 

 valves. 



So-called "valveless system" 

 pasteurizers now on the market 

 eliminate the necessity for an 

 outlet valve on the bottom of 

 the holder, using either pump 

 vacuum or air pressure to force 

 the milk through an outlet pipe 

 extending through the top down 

 to the bottom of the holder. 



Foam and splash. — Foam 

 consists of air entrained in milk 

 film forming a mass of bubbles 

 causing it to float on top of the 

 milk. Foam present on the milk 

 in a pasteurizer holder is often 

 cooled by the air to a temper- 

 ature below that required for 

 pasteurization. Theobald Smith^ 

 first pointed out that any foam 

 or scum on top of milk in labora- 

 tory test vessels was cooled by 

 the air so that tubercle bacilli, though destroyed in the liquid by heating and hold- 

 ing at a certain temperature, remained in the froth or scum. Whittaker, Archibald, 

 Leete, and Miller report^ that the temperature of foam on milk in a vat during commer- 

 cial pasteurizing runs, as determined by thermocouple tests, showed an average of 

 8.3° F. lower than the milk at the beginning of the holding period on seven runs, 

 where the milk was heated in the vat, and an average of 10.2° F. lower on twelve 

 runs in which the milk was preheated and pumped into the vat. Comparable differ- 

 ences are also reported in the temperatures at the end of the holding period. 



In commercial milk-plant operation the remedy is either to eliminate units caus- 

 ^ Smith, T.: /. Exper. Med., 4, 217. 1899. 



= Whittaker, H. A., Archibald, R. W., Leete, C. S., and Miller, L. F.: Tech. Bull. iS. U.S. Dept. 

 of Agric, Sept., 1927. 



Fig. 8. — Showing leak -protector inlet valve: {a) valve 

 body, {b) valve plug, {c-c) leak-drain grooves, {d-d) stops, 

 (e) stop pin, (f-g) connections to inlet header line and 

 holder, respectively. 



