Sterile food is food free from bacteria of all kinds. Sterilization of food is therefore destroying all bacteria. Dry heat at 350°+ F., steam (moist heat) under pressure, and some chemicals will kill bacteria. The chemicals that will do this would, however, render a food unfit for human use, so heat must be relied upon to sterilize food. The process of rendering food sterile by heat is known as sterilization. The degree of heat necessary may decrease the palatability of many foods, also even the nutritiousness of some. Sterilized milk is less palatable than raw; fats raised to high temperature decompose; sugar changes its form. But such foods as can be sterilized are thus made safe, if not reexposed after being sterilized.

To prevent destruction of a food and yet its deterioration, less intense heat is used. This, however, only checks bacterial growth without destroying the bacteria that may develop later under more favorable conditions for their life. Freezing acts similarly. Bacteria that cause human disease may resist effectively extremes of temperature, either high or low, moist or dry. It is therefore only such as affect the food itself (putrefactive bacteria), not the person directly (as do pathogenic or disease-producing bacteria), which cooking and freezing food destroy or even significantly delay in their activity.

Keeping food clean lessens contamination; cooking it usually decreases the germs it contains; cooling it delays its decomposition. Food requires continuous freedom from contamination. Even when food is to be sterilized, as in canning, it is still important to keep it sound and otherwise free from contamination. Food in which bacteria have grown is not freed of the effects of their growth by later sterilization. Ptomaines, for instance, are chemical substances formed by bacterial growth.