L. A. ROGERS 397 



sterilization. The temperature at which the milk may be sterilized without coagula- 

 tion is affected in some degree by the growth of bacteria in the milk before concen- 

 tration. A very small increase in the H-ion concentration usually causes a distinct 

 lowering of the coagulation point of the concentrated milk, although in some cases 

 it may have the reverse effect.' 



Evaporated milk, like other canned foods, is subject to various types of spoilage 

 due to spore-forming bacteria which survive the sterilization process. Two common 

 troubles are acid fermentations in which there is no gas production and consequent- 

 ly no indication of spoilage before the can is opened. "Flat sour" is caused by an or- 

 ganism to which Hammer- has given the name B. coagidans. In a somewhat similar 

 fermentation a marked bitter flavor is produced, sometimes without visible change in 

 the milk. Bacteria of the butyric acid group and the spore-forming putrefactive type 

 are responsible for gaseous fermentations causing swelling and sometimes bursting 

 of the cans. 



MILK POWDER 



Two distinct types of manufacturing processes have come into general use in 

 making milk powder. In one process the milk is spread in a thin film on the surface 

 of steam-heated revolving drums held either at atmospheric pressure or in a vacuum. 

 The dried film is removed from the drums by scrapers and ground to a powder. In 

 the more generally used spray process the milk is atomized into a current of heated 

 air from which the dry particles of milk are removed by special devices. 



In the drum process, particularly drums at atmospheric pressure, the tempera- 

 ture of the film of milk is sufficient to destroy vegetative bacteria, and consequent- 

 ly the bacterial content of the powder is very low. Supplee and Ashbaugh^ have 

 shown that the bacteria are usually limited to spore-formers and that the count of 

 powder made on atmospheric drums is not over a few hundreds per gram of powder. 

 Pathogenic bacteria, including the tuberculosis organism, are destroyed by this proc- 

 ess.'' 



Since the moisture content of the milk powder is below 5 per cent there is no in- 

 crease of bacteria on standing, but it has been pointed out that there may be contam- 

 ination in the process of collection and packing and that the bacterial count may be 

 used as a check on the manufacturing methods. ^ 



In the spray process the temperature of the air is high (190° F.) but the evapora- 

 tion from the surface of the minute particles of milk is so rapid that it is probable 

 the bacteria are not exposed to temperatures above their thermal death-point 

 before they are in a dry state. However, in the operation of this process the milk is 

 almost invariably exposed to pasteurizing temperature or a higher temperature be- 

 fore drying either in precondensing which is commonly practiced or to check bacterial 

 change when it is necessary to hold the milk before drying. There is a gradual falling 

 off in the numbers of bacteria in milk powder in storage. The changes in flavor and 



' Benton, A. G., and Albery, H. G.: /. Biol. Chem., 68, 251. 1926. 



' Hammer, B. W.: Iowa Agric. Exper. Sta. Research Bull. 52. 1919. 



3 Supplee, G. C, and Ashbaugh, V. J.: J. Dairy Sc, 5, 216. 1922. 



■t Hunwicke, R. F., and Jephcott, IL: ibid., 8, 206. 1925. 



5 Jephcott, H., Hunwicke, R. F., and RatclilTe, N.: Proc. World's Dairy Cong., 2, 1265. 1923. 



