The different milk constituents vary greatly in their susceptibility to the action of the bacteria that make up the flora commonly found in milk. At ordinary temperatures in raw milk, the lactose is soon attacked and acid formed, but if the acid producing organisms are destroyed by some such procedure as heating, pronounced changes soon take place in the casein. The fat is one of the milk constituents that is more resistant to bacterial action, yet even in it striking changes are observed under certain conditions. Some of the common milk organisms, even some of the acid producing types, are able to change the fat, altho frequently considerable periods of time are required for such action.