A popular pasttime in the fall for gardeners who grow fresh fruits and vegetables is canning their harvest for later. Some important things to remember when canning fruits and vegetables are to use fresh foods, can them soon after picking, follow recommended processing times and procedures, and can no more food than will be used in a year.

Using fresh foods can help ensure the quality of the preserves later. The variety of fruits and vegetables can also affect the quality of the canned product. Use the varieties that you prefer that are well suited to canning. After harvest, the food should be washed and examined carefully. Old, diseased or moldy food should be thrown away. Trim away any small diseased lesions or spots from food that is in otherwise good condition.

Fruits and vegetables should be canned while they are still fresh. Most vegetables should be processed within six to 12 hours after being picked. Apricots, nectarines, peaches, pears and plums should be ripened one or more days between harvest and canning. Spread in a single layer on a clean dry surface to ripen more evenly. If the canning of fresh produce must be delayed, keep it in the refrigerator if possible, or in a cool, dark place.

Recommended processing times should be followed because proper canning practices remove oxygen from food tissues, destroy enzymes, prevent the growth of undesirable bacteria, yeast and molds, and help form a vacuum in jars. Good vacuums form tight seals that keep liquid in and air and microorganisms out.

If proper canning practices are not followed, chances increase for the of the survival of Clostridium botulinum, the bacteria that causes botulism, a deadly form of food poisoning. These bacteria exist either as spores or as vegetative cells.

Botulinum spores, which are similar to seeds, are on most fresh food surfaces. The spores can survive harmlessly in soil and water for many years. However, when ideal conditions exist for growth, they produce vegetative cells that multiply rapidly. The deadly toxin is produced within three to four days.

Ideal conditions for cell production include a moist, low-acid food, a temperature between 40 F and 120 F and less than 2 percent oxygen, which occurs in tightly packaged foods.

Using the recommended processing time ensures destruction of heat resistant microorganisms in home-canned foods. The recommended time is based on the largest number of microorganisms expected to be present on a given food.

The acidity, or pH, of a food determines whether it should be processed in a pressure canner or a water-bath canner. Acid blocks the growth of Clostridium botulinum bacteria or destroys them more rapidly when heated. The acid may be natural, as in most fruits, or it may be added, as in pickled foods. Acid foods are not at risk for botulinum growth. Botulinum spores in low-acid foods, however, must be destroyed with high heat.

Acid foods, with a pH of 4.6 or lower, can be canned in a water-bath canner. They include most fruits and some varieties of tomatoes. Adding lemon juice, citric acid or vinegar can increase the acid content of many other foods. Acid foods should be packed in a water-bath canner using the hot-pack method.

Low-acid foods have a pH higher than 4.6 and must be canned in a pressure canner. They include red meats, seafood, poultry, milk, all fresh vegetables, and some varieties of tomatoes and must be canned in a pressure canner. Mixing low-acid foods with acid foods does not lower the pH below 4.6 unless the recipes include enough lemon juice, citric acids or vinegar to acidify them.

Low-acid foods should be processed at temperatures of 240 F to 250 F. This temperature range can be reached only with pressure canners operated at 10 to 15 pounds per square inch of pressure. At these temperatures the time needed to destroy bacteria in low-acid food ranges from 20 to 100 minutes. The exact time depends on the food being canned, the way it is packed, and the size of the jars.

Always follow time, temperature and pressure recommendations very carefully. Home-canned foods will be free of spoilage if they are properly prepared and processed, the lids are sealed to form a high vacuum and the foods are stored at temperatures between 50 F and 70 F.

To maintain good color and flavor during processing and storage, do the following:

• Use only high-quality, disease- and bruise-free foods in the proper stage of maturity.

• Wash foods thoroughly and trim away small damaged areas.

• Use the hot-pack method, especially for acid foods to be processed in a water-bath canner.

• After preparing foods for canning, protect them from too much exposure to light, heat and air. Can them as soon as possible.

• Fill clean, hot jars with hot foods, leaving the amount of headspace specified in the recipes.

• Remove air bubbles from filled jars and wipe jar rims with a clean damp cloth before adjusting lids.

• Use recommended self-sealing lids and screw bands or rings.

• Tighten screw bands securely but, if you are especially strong, not as tightly as possible. Read manufacturer's instructions.

• After canning, store sealed jars in a relatively cool, dark, dry place, preferably between 50 F and 70 F.

• Can no more food than you will use within a year.

For more information about home canning, contact you county Extension agent.

Prepared by Jana Huggins, Agricultural Journalism Intern