CANNING Canning involves, first, the application to foods of temperatures high enough to destroy essentially all microorganisms present, both vegetative cells and spores, and second, the sealing of the heated product in sterilized airtight containers to prevent recontamination. The degree of heat and the length of heating vary with the type of food and the kinds of microorganisms likely to occur. Fruits and tomatoes that are sufficiently acid are successfully canned at the temperature of boiling water.