Organisms that cause food spoilage–molds, yeasts and bacteria–are present everywhere in the air, soil and water. Enzymes that may cause undesirable changes in flavor, color and texture are present in raw vegetables. When vegetables are canned, they are heated hot enough and long enough in the jar to destroy organisms that can make people sick in addition to spoilage organisms. This heating, or processing, also stops the action of enzymes that can spoil food quality.
Pressure canning is the only safe method of canning vegetables (except tomatoes-due to their natural acidity). Jars of food are placed in a pressure canner which is heated to an internal temperature of at least 240°F. This temperature can be reached only in a pressure canner. The Clostridium botulinum microorganism is the main reason pressure canning is necessary. Though the bacterial cells are killed at boiling temperatures, the spores they form can withstand boiling.
In any garden soil, these bacteria are naturally found in the spore form. The spores grow well in moist, low-acid foods in the absence of air, such as in canned low acid foods (vegetables and meats). When this happens, the spores change to growing bacterial cells which produce the deadly botulinum toxin. This growth and toxin formation can occur without any noticeable signs of spoilage in the sealed jar. These spores can be destroyed by canning the food at a temperature of at least 240°F. This temperature is above the 212°F boiling point of water, so it can only be reached in a pressure canner at the appropriate pressure. Use the pressure recommended in the manual along with the directions for canning vegetables. Recommended pressures are different for varying altitudes; be sure to check altitude corrections if canning at altitudes over 1000 feet.
Equipment
A pressure canner is necessary for home canning of vegetables. It must have a rack in the bottom, a tight-fitting cover, exhaust vent, or petcock, safety valve and an accurate pressure gauge. The pressure gauge may be a weighted gauge that fits over the vent or a dial gauge on which a needle indicates the pressure inside the canner. Newer canners may also have an extra cover lock as an added safety feature.
Canning jars should be checked closely for cracks or chips. Use jars specifically designed for home canning. Commercial food jars such as mayonnaise or coffee jars break easily, especially in pressure canners, and may not seal with home canning lids. Use only half-pint, pint and quart sizes, and as specified for individual food processing directions. Wash jars in hot soapy water and rinse well. Keep the jars hot until they are filled and placed in the canner. This will help prevent jar breakage.
Two-piece metal canning lids need to be prepared for use. The flat lids can be used only once for sealing new products, but the ring bands can be reused as long as they are in good condition. Pretreat the new flat lids as directed by the manufacturer. Do not reuse lids from commercially canned foods for home canning.
Preparing the Vegetables
Select only fresh, young, tender vegetables for canning. The sooner you can get them from the garden to the jar, the better. If you buy vegetables to can, try to get them from a nearby garden or farm market. Store bought vegetables are often allowed to ripen off the vine for retail transportation.
For ease of packing and even cooking, sort the vegetables for size and maturity. Wash all vegetables thoroughly, whether or not they will be peeled. Dirt contains some of the bacteria hardest to kill. Do not let vegetables soak; they may lose flavor and nutrients. Handle them gently to avoid bruising.
Suggested References for Home Food Preservation
USDA and Cooperative Extension So Easy to Preserve, 5th edition (2006) (University of Georgia Cooperative Extension): http://www.uga.edu/setp/
National Center for Home Food Preservation: http://homefoodpreservation.com
USDA Complete Guide to Home Canning: http://www.uga.edu/nchfp/publications/publications_usda.html
*Manufacturers of Home Food Preservation Supplies
Ball: http://www.freshpreserving.com/home.aspx
Jarden Home Brands (they handle Ball, Kerr, Alltrista): http://www.homecanning.com/
Kraft Foods, Inc. (makers of Sure Jell): http://brands.kraftfoods.com/surejell/
Mrs. Wages: http://www.mrswages.com/
National Presto Industries (www.gopresto.com)
Cottage Food Production Operations Frequently Asked Questions (Texas Department of State Health Services): http://www.dshs.state.tx.us/foodestablishments/cottagefood/
*This list of references is for information purposes only and is not to be viewed as an endorsement for any commercial company.
About the author: Lisa Mapel is an Extension Program Assistant and Better Living for Texans educator with the Brown County AgriLife Extension office. Lisa may be reached at 325.646.0386.