Close up of jars of preserves

Methods of preserving food are designed to prevent or control the growth of microorganisms and enzymes that can cause foods to spoil.

Because molds, yeasts and bacteria are found everywhere — in the air and soil, on people and animals and on many surfaces — proper food preservation methods must be used to prevent spoilage. The effects of these microorganisms can range from soft, slimy textures and unpleasant odors to food poisoning that can be deadly.

Molds are recognized by their fuzzy masses that can be nearly any color. They need air and moisture to grow and thrive in the acidic conditions provided by food. Molds are easily destroyed by high temperatures used in processing. Some molds produce invisible filaments that extend into the food and are harmful to eat. Mold can also change the pH of high-acid foods, moving them into the low-acid category and making them unsafe for canning.

Dr. Karen Blakeslee, of Kansas State University, recommends that any food with signs of mold should be discarded. Properly processing all canned foods (including pickles, jams and jellies) controls the growth of molds.

Yeast masses in or on food appear as slime, scum or murkiness. They may cause foods to ferment and can be recognized by gas bubbles, froth or foam. Yeasts are easily destroyed at temperatures between 140 F and 180 F.

Some bacteria can be beneficial — such as in making sauerkraut. Others can be extremely dangerous, as in botulism poisoning. Each type of bacteria differs relative to the temperature and environment in which it thrives. Some need oxygen to grow, while others thrive in lack of oxygen in a sealed jar. Most bacteria grow on low-acid foods, including vegetables and meats. While many bacteria are destroyed by heat, others form spores that can only be killed by temperatures higher than the boiling point of water. Because Clostridium botulinum spores cannot be destroyed or inactivated by boiling water, low-acid vegetables and meats must be processed in a pressure canner, where the temperature can reach at least 240 F. Clostridium botulinum spores that survive processing can germinate, producing the botulism toxin causing serious illness or even death.

Bacteria can multiply rapidly, with millions growing on a gram of food in just a few hours. At this concentration they can spoil food or cause a food-borne illness. Freezing food slows the growth of most bacteria. However, care must be taken to prevent the growth of bacteria in food before it is frozen and after it is thawed.

Enzymes are naturally occurring substances in foods that promote the normal ripening process. If they continue to work after the fruit or vegetable reaches its ideal maturity, they will cause undesirable changes in color, texture, flavor and nutrition. Flavor changes are sometimes described as hay-like, bitter, oxidized or old. Enzymes can be inactivated by heating foods between 170 F to 190 F. Processing foods when canning or blanching vegetables for freezing stops enzyme reactions. The addition of sugar, acidic juices such as lemon or orange juice, and powdered acids such as ascorbic acid or citric acid are frequently used to control enzyme reactions when freezing foods. There are several types of fresh fruit protectors available to reduce enzymatic browning.

Keys to Preventing Spoilage 

The following methods will prevent microorganisms and enzymes from causing spoilage.

• Use top-quality produce free of disease and mold.

• Can foods immediately after harvest.

• Wash your hands often before and during food preparation.

• Use clean equipment and work surfaces.

• Wash produce thoroughly and discard over-ripe produce.

• Use proper canning methods and equipment.

• Always pressure-can low-acid vegetables and meats.

• Acidify tomatoes with lemon juice or citric acid.

• Sterilize jars that will be processed for less than 10 minutes.

• Follow a USDA-tested recipe and process the food for the specified time.

• Adjust canning times and pressure for higher altitudes.

• Work with one canner load at a time so peeled food is not at room temperature for more than two hours.

• When freezing foods, make sure the freezer is at 0 F.

• Never taste a food you suspect of being spoiled. When in doubt, throw it out.

If you have a food preservation question, contact a home economist on Wednesdays from 10 a.m. to 2 p.m. at 717-394-6851 or by writing to Penn State Extension, Lancaster County, 1383 Arcadia Road, Room 140, Lancaster, PA 17601.


The Well Preserved news column is prepared by Penn State Extension.