Water Bath Canning – Fruits and Tomatoes
The natural acidity of the fruits and the addition of acid (lemon juice, citric acid or vinegar) to tomatoes allows them to be safely processed in a boiling water canner. The heat destroys yeasts and molds that may be present and forces oxygen from the product and jars. This results in the formation of a tight, vacuum seal as the jar cools. Thus, re-contamination of the product by yeasts and molds from the surroundings is prevented.
Pressure Canning – Vegetables and Meats
Because vegetables and meats are low acid foods (pH > 4.6), they must be canned in a pressure canner. The high heat is sufficient to kill yeasts, molds, bacteria and to destroy the spores of bacteria such as Clostridium botulinum. The heat results in removal of oxygen from the product and jar. This results in a tight, vacuum seal as the jar cools. When canned foods are opened, precautions must be taken to ensure their safety.
When foods are pickled, the acidity of the food is increased. This is done by adding acid in the form of vinegar or by allowing fermentation to occur resulting in the natural production of acid in the product. There are two types of vinegar used in pickling. Apple cider vinegar has a fruity flavor and can be used in pickling fruits. Distilled white vinegar is used for light colored products and products where a fruity flavor is undesirable. The natural acid that is produced is lactic acid that preserves the product. The acid level allows the products to be safely canned in a boiling water canner. The heat results in the removal of oxygen from the product and the jar. This results in a tight, vacuum seal as the jar cools.
Jellied Fruit Products
Jellied fruit products rely on acid in the fruit to limit microbial growth. In addition, sugar and/or pectin is added to bind up water in a product making it unavailable for microbes to grow. Yeasts and molds are destroyed by heating in a boiling water canner which also forces oxygen out of the product and jar. The jars form a tight, vacuum seal when cooled.
When foods are frozen the temperature is lowered to a level that halts microbial growth. Water in the product freezes and becomes unavailable for microbes to grow. Freezing, however, does not sterilize the food. When frozen foods are thawed, precautions must be taken to ensure their safety.
Drying removes water from the food and prevents it from being available for microorganisms to use for growth.