[Illustration: FIG. 84.--A battery of horizontal rectangular steam chests in actual use in a canning factory.] [Illustration: FIG. 85.--A battery of cylindrical process kettles in actual use in a canning factory.] A process closely akin to sterilization by heat is _pasteurization_. This means the heating of material at a temperature and for a time which will destroy the actively growing bacteria but not the spores. The methods for doing this vary but are essentially two in principle. 1. The material in small quantities in suitable containers (bottles) is placed in the apparatus; the temperature is raised to 60° to 65° and maintained for twenty to thirty minutes and then the whole is cooled (beer, wine, grape juice, bottled milk) (Figs. 92, 93 and 94). [Illustration: FIG. 86.--A steam chamber used in government disinfection work. Size, 4 feet 4 inches × 5 feet 4 inches × 9 feet.] [Illustration: FIG. 87.--Circular steam chamber used in government disinfection work, 54 inches in diameter.] [Illustration: FIG. 88.--Portable steam chamber used in government disinfection work.] [Illustration: FIG. 89.--Steam chambers on deck of the U. S. quarantine station barge “Defender.”] [Illustration: FIG. 90.--Steam chambers in hold of U. S. quarantine station barge “Protector.” Disinfected space.] [Illustration: FIG. 91.--Municipal disinfecting station, Washington, D. C.] [Illustration: FIG. 92.--A pasteurizer for milk in bottles.] [Illustration: FIG. 93.--A pasteurizer for grape juice, cider, etc., in bottles.] 2. Pasteurizing machines are used and the fluid flows through continuously. In one type the temperature is raised to 60° and by “retarders” is kept at this temperature for twenty to thirty minutes (Figs. 95 to 98). In another type the temperature is raised to as high as 85° for a few seconds only, “flash process” (Fig. 99), and then the material is rapidly cooled. It is certain that all pathogenic microörganisms, except the very few spore formers in that stage, are killed by proper pasteurization. The process is largely employed in the fermentation and dairy industries. [Illustration: FIG. 94.--A pasteurizer for beer in bottles.] [Illustration: FIG. 95.--A continuous milk pasteurizer.] [Illustration: FIG. 96.--A pasteurizer for cream to be used in making ice-cream.] [Illustration: FIG. 97.--A continuous milk pasteurizer with holder; capacity 1500 pounds per hour. _A_, pasteurizer--the milk flows in tubes inside of a jacket of water heated to the proper temperature; _B_, holder; _C_, water cooler; _D_, brine cooler.] [Illustration: FIG. 98.--A continuous pasteurizing plant in operation. Similar to Fig. 97 but larger. Capacity, 12,000 pounds per hour. _A_, pasteurizer; _B_, seven compartment holder; _C_, _D_, coolers.]