Excerpt from the Popular Kitchen Series magabook Canning & Preserving with permission from its publisher, BowTie magazines, a division of BowTie Inc. Purchase Canning & Preserving here.

Nothing compares to the sweet, crisp taste of a fresh-picked green bean or the tart burst of a plump blackberry, still warm from the afternoon sun. Unfortunately, it’s tough to retain that freshness because the high percentage of water in fruits and vegetables makes them perishable. According to the National Center for Home Food Preservation, foods spoil or lose their quality due to:

• the growth of undesirable microorganisms, such as bacteria, molds and yeasts that live and multiply on food surfaces and inside bruised, insect-damaged and diseased produce
• food enzymes that naturally break down molecules
• the food’s reaction with oxygen
• moisture loss

Preserving and canning food slows spoilage. When done properly, canning removes oxygen, destroys enzymes, prevents the growth of microorganisms and helps form a high vacuum in jars that keeps liquid and air and microorganisms out. If done improperly, dangerous bacteria, including Clostridium botulinum, thrive.

“Growth of the bacterium Clostridium botulinum in canned food may trigger botulism, a deadly form of food poisoning,” states the United States Department of Agriculture’s Complete Guide to Home Canning. “These bacteria exist either as spores or vegetative cells. These spores, which are comparable to plant seeds, can survive harmlessly in soil and water for many years. When ideal conditions exist for growth, the spores produce vegetative cells that multiply rapidly and may produce a deadly toxin within three to four days of growth in an environment consisting of a moist low-acid food, a temperature between 40 and 120 degrees Fahrenheit, and less than 2 percent oxygen.”

The USDA-approved processing methods destroy Clostridium botulinum and other microorganisms: the boiling-water canning method and the pressure-canning method. The method you use depends on the acidity level of the food that you’re preserving.

“Whether food should be processed in a pressure canner or boiling-water canner to control botulinum bacteria depends on the acidity of the food,” the NCHFP states. “Acidity may be natural, as in most fruits, or added, as in pickled food. Low-acid canned foods are not acidic enough to prevent the growth of botulinum bacteria. Acidic foods contain enough acid to block their growth or destroy them more rapidly when heated. The term ‘pH’ is a measure of acidity; the lower its value, the more acidic the food. The acidity level in foods can be increased by adding lemon juice, citric acid or vinegar.”

The pressure-canning method heats the preserved food to 240 degrees Fahrenheit. Because certain foods, like meats, lack high acidity, the bacteria are free to grow in the vacuum created by canning. Using a pressure canner exposes these low-acid foods to a temperature that is high enough to sterilize the food and destroy the toughest microorganisms, including Clostridium botulinum. Low-acid foods include all vegetables (except tomatoes), meats, seafood and all mixtures such as soups and sauces.

The boiling-water canning method requires submerging packed canning jars of highly acidic food in boiling water, or 212 degrees, long enough for every particle of food to reach a temperature high enough to destroy microorganisms. High-acid foods that are suitable for canning using the boiling-water method includes pickles, relishes, chutneys with vinegar, most fruits, and heavily sweetened spreads like jams, jellies, fruit butters and preserves.

In addition to selecting the canning method appropriate for your food, you’ll need to adjust for altitude. Most recipes list processing time based on altitudes near sea level. To ensure the health of those who enjoy your foods, always follow your recipe’s instructions to ensure that the microorganisms are destroyed. If you’re unsure of your altitude, contact your local Cooperative Extension System or public library. You also can find canning instructions in the USDA’s “Complete Guide to Home Canning,” which you can download free of charge at here.