Milk needs to be both fresh and clean. Its purity and freshness may both be destroyed by bacteria. Hence bacteria must be excluded so far as possible, and milk must be kept under conditions that discourage bacterial growth, so that disease and death to infants may not ensue. If milk-bottles are not effectively sterilized before they are re-used, they can breed disease and spread it by contaminating the atmosphere as well as by carrying into milk whatever they contain. To heat milk sufficiently (1800 F.) to destroy the bacteria that may be harmful changes its protein, as noted earlier. Since it has not been conclusively proved that it is assuredly as digestible for children thus, other means of making it safe have been sought.

Certified milk is milk that has had every care of environment, animals, workers, receptacles in its production. Animals, workers, and milk are all scientifically examined. The milk is then bottled in sterile bottles with sterile covers. Even milk so cared for is not germ-free, but it has only a few thousand bacteria where other milk has millions. Only with the rarest exceptions has certified milk been found to contain disease-producing bacteria. It costs nearly twice what is charged for ordinary milk.

Pasteurized milk has been evenly heated for 10-20 win. at 157° F., at which temperature the bacterial life is greatly reduced and milk is changed less than when boiled. This is accomplished by heating milk in bottles in a water-bath at 159° F., so as to avoid high direct heat. Formerly pasteurizing was advocated as a home precaution, then scientifically somewhat discouraged for a while, but is now re-advised as a more general practice for the milk-supply. Such milk is not so palatable, but is safer. Yet bacterial spores (see p. 71) are not destroyed, so its safety is not completely assured.