If the enzymes in fruits and vegetables aren't inactivated, they continue to work after harvest, causing flavor and texture changes. Canning is the process in which foods are placed in jars or cans and heated to a temperature that destroys microorganisms and inactivates enzymes. This heating and later cooling forms a vacuum seal. The vacuum seal prevents other microorganisms from recontaminating the food within the jar or can. Pickling is another form of canning.
Pickled products have an increased acidity that makes it difficult for most bacteria to grow.
Pickled products are also heated in jars at boiling temperatures to destroy any other microorganisms present and form a vacuum in the jar. Jams and jellies are high in sugar. The sugar binds with liquid making it difficult for microorganisms to grow. To prevent surface contamination after the product is made and thus prevent possible yeast or mold growth, these are either canned, frozen, or refrigerated. Freezing reduces the temperature of the food so that microorganisms cannot grow.