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Canning Basics 2: Ensuring Safe Canned Foods

William Schafer

Growth of the bacterium Clostridium botulinum in canned food may cause botulism, a deadly form of food poisoning. These bacteria exist either as spores or as vegetative cells. The spores, which are dormant and 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 which multiply rapidly and may produce a deadly toxin within three to four days of growth in an environment consisting of:

Botulism spores are on most fresh food surfaces. Because they grow only in the absence of air, they are harmless on fresh foods. Most bacteria, yeasts, and molds are difficult to remove from food surfaces. Washing fresh food reduces their numbers only slightly. Peeling root crops, underground stem crops, and tomatoes reduces their numbers greatly. Blanching also helps, but the vital controls are the method of canning and making sure the recommended research-based process times are used.

The processing times in this publication ensure destruction of the largest expected number of heat-resistant microorganisms in home-canned foods. Properly processed, canned food will be free of spoilage if lids seal and jars are stored below 95°F. Storing jars at 50° to 70°F also enhances retention of quality.

To further reduce the risk of botulism, home canned low-acid and tomato foods should be boiled even if you detect no signs of spoilage. Boil foods for 10 minutes at altitudes below 1,000 feet. Add an additional minute of boiling time for each additional 1,000 feet elevation. In Minnesota, boil food for 11 minutes.

Food Acidity and Processing Methods

Whether food should be processed in a pressure canner or a boiling-water bath to control botulism bacteria depends on the acidity in the food. Acidity may be natural, as in most fruits, or added, as in pickled food. Low-acid canned foods contain too little acidity to prevent the growth of these bacteria. Acid foods contain enough acidity to block their growth, or destroy them more rapidly when heated. The acidity level in foods can be increased by adding lemon juice, citric acid, or vinegar.

The term pH is an index of acidity. The lower its value, the more acid in the food. Low-acid foods have pH values higher than 4.6. They include red meats, seafood, poultry, milk, and all fresh vegetables except for most tomatoes. Most mixtures of low-acid and acid foods also have pH values above 4.6 unless enough lemon juice, citric acid, or vinegar is included to make them acid foods. Acid foods have a pH of 4.6 or lower. They include fruits, pickles, sauerkraut, jams, jellies, marmalades, and fruit butters.

Although tomatoes usually are considered an acid food, some are now known to have pH values slightly above 4.6. Figs also have pH values slightly above 4.6. Therefore, if they are to be canned as acid foods, these products with unknown pH must be acidified to a pH of below 4.6 with lemon juice or citric acid. Properly acidified tomatoes and figs are acid foods and can be safely processed in a boiling-water bath. Processing acid foods at boiling water temperatures will destroy yeast and molds, the most common forms of spoilage microorganisms in these foods. Heat-sensitive bacteria are also killed. Those that are heat resistant, such as C. botulinum spores, are prevented from multiplying because of the high acid conditions of the food.

Acidity of foods helps determine the type of heat processing or home canning required for safe preservation.

Botulism spores are very heat resistant. They may be destroyed at boiling water temperatures, but extremely long times are required. The higher the canner temperature, the more easily and quickly they are destroyed. Therefore, all low-acid foods should be sterilized at temperatures of 240° to 250°F, attainable with pressure canners operated at 10 to 15 PSI. PSI means pounds per square inch of pressure as measured by a gauge. At these temperatures, the time needed to destroy bacteria in low-acid canned food ranges from 20 to 100 minutes. The exact time depends on the kind of food being canned, the way it is packed into jars, and the size of jars. The time needed to safely process low-acid foods in a boiling water canner ranges from 7 to 11 hours. Such long processing times are not researched and are not recommended. Losses in nutrients and quality would be unacceptable. The time needed to process acid foods in boiling water varies from 5 to 85 minutes.

Why There Are More Changes in Home Canning Methods

As with many things in our society, ongoing research continues to increase our knowledge about factors affecting the safety of canned food. Combinations of incorrect ingredients, processing times unadjusted for altitude, faulty equipment, and human error may cause the home-canned food to be unsafe for consumption. The latest changes in home food preservation methods are based on research sponsored by the USDA and conducted at the Extension Service Center for Excellence, Pennsylvania State University. These methods attempt to further minimize the chance of producing an unsafe product. They are more detailed in equipment, ingredients, and procedures. Examples of new method changes are:

  1. Increased processing time and/or pressure with increasing altitude. This was added because water boils at lower temperatures as altitude increases. Lower temperatures are less effective for killing potentially harmful microorganisms. Increasing the process time or canner pressure compensates for lower boiling temperatures.
  2. Increased pounds pressure recommendations for weighted-gauge canners vs. dial-gauge canners. Weighted-gauge canners cannot be set for pressures between 10 and 15 PSI. If desired pressure is above 10, then the 15 PSI setting must be used.
  3. Using a pressure saucepan is not recommended. Pressure saucepans have poor temperature control and a risk of inadequate heat processing. They heat up and cool down too quickly.
  4. Addition of lemon juice or citric acid to tomatoes and tomato products. This ensures that the acid level is great enough to permit water bath processing or allow reduced pressure canning times. Recently a few tomato varieties have been found not to contain enough acid.
  5. Increased processing times for non-water packed whole or halved tomatoes. Some new tomato varieties are more solid or have less liquid. It thus take longer to transfer heat to the coldest point in the jar and kill the microorganisms present.
  6. Increased headspace for many canned vegetables. New methods have increased processing times or pounds pressure and this requires food be heated to higher temperatures. Because of this there is increased expansion of food in the jars and headspace must also be increased.

Unfortunately many home canners think these changes are too complicated or overdone. To help understand and accept these changes, you need to be able to assess and accept risks. There is a potential health risk associated with most things we do. For example, we take a risk of being in an accident when we drive or ride in a car. But we take these risks because we believe the convenience and time we save is worth it. We may choose to reduce driving risks by driving carefully or more slowly and by wearing seat belts. The same is true of the new canning methods in this manual. They involve extra details which make them more difficult and time consuming. However, the risk of food poisoning is further reduced if they are followed. These new USDA methods have been integrated into this publication with methods previously researched at the University of Minnesota and found in previous publications of the University of Minnesota Extension. In the "Tomatoes" section of this publication you are given method options from which to choose, depending on the varieties selected and the level of risk you wish to assume. No method is failure-proof or can assure 100 percent safety under all circumstances of ingredients, processing, and storage. With tomatoes, home canners are allowed to decide on the method which represents an acceptable amount of risk for an acceptable amount of time and energy spent. This risk is further reduced by boiling the food before eating.

Processing Adjustments at High Altitudes

Water boils at lower temperatures as altitude increases.

Using the process time for canning food at sea level may result in spoilage if you live at altitudes of 1,000 feet or more. Water boils at lower temperatures as altitude increases. Lower boiling temperatures are less effective for killing bacteria. Increasing the process time or canner pressure compensates for lower boiling temperatures. The highest inhabited elevation in Minnesota (2,000 ft) is used to determine the recommended processing times found in this publication. However, if you use the charts in the new USDA Canning Guides, select the proper processing time or canner pressure for the altitude where you live. If you do not know the altitude, contact your local county Extension agent or district conservationist with the Soil Conservation Service.

Equipment and Methods Not Recommended

  1. Open-kettle canning and the processing of freshly filled jars in conventional ovens, microwave ovens, and dishwashers are not recommended. These practices have great risk of producing unsafe foods.
  2. Unpressurized steam canners are not recommended because processing times for use with current models have not been adequately researched. Because steam canners may not heat foods in the same manner as boiling water canners, their use with boiling-water process times may result in spoilage.
  3. It is not recommended that pressure processes in excess of 15 PSI be applied when using new pressure canning equipment.
  4. So-called canning powders are useless as preservatives and do not replace the need for proper heat processing.
  5. Jars with wire bails and glass caps make attractive antiques or storage containers for dry food ingredients but are not recommended for use in canning.
  6. One-piece zinc porcelain-lined caps are also no longer recommended. Both glass and zinc caps use flat rubber rings for sealing jars, but too often fail to seal properly.
  7. Pressure saucepans, because of poor temperature control and risk of inadequate heat processing, are not recommended.
  8. Devices for canning food in microwave ovens are not recommended because of incomplete destruction of bacteria due to non-uniform heating.

Reviewed by Suzanne Driessen 2012

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