Posted: July 8, 2012
Molds can be recognized by their fuzzy masses that can be nearly any color. They need air and moisture, but not much moisture to grow. They thrive in the acid conditions provided by food. Molds can easily be destroyed by the high temperatures used in processing canned foods either in the boiling water bath or pressure canner. Some molds produce chemical compounds called mycotoxins which are harmful to eat. Mycotoxins are not visible as are the molds. This is why it is important to correctly process all canned foods including pickles, jams and jellies.
Yeasts usually appear in or on food as slimy masses, scum, or murkiness. Yeasts 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 as in making sauerkraut. Others can be extremely dangerous like the kind that causes botulism poisoning. Each type of bacteria differs as to the temperature and environment in which it thrives. Some need oxygen to grow while others thrive in the lack of oxygen in a sealed jar. Most bacteria grow on low acid foods which includes most vegetables and all meats. While most bacteria are destroyed by heat, others form spores that can only be killed by a temperature higher than the boiling point of water. It is because of the Clostridium botulinum bacteria that low acid vegetables and meats must be processed in a pressure canner where the temperature can reach at least 240°F.
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. Even higher concentrations are needed to be seen by the human eye.
Freezing food stops the growth of most bacteria. Care must be taken however 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 enzymes 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 caused by enzymes are sometimes described as hay-like, bitter, oxidized, or old. Enzymes can be inactivated by heating foods to 170°F to 190°F. Processing foods when canning or blanching vegetables for freezing stops enzyme reactions. Adding ascorbic or citric acid to fruits for freezing slows enzyme activity.
Visit the Penn State Food Preservation web site at http://extension.psu.edu/food-safety/food-preservation or call your local Penn State Cooperative Extension office for more information on home food preservation.