Update: The jar sealed, is the food safe? and other questions

Cells and spores of C. botulinum. Image courtesy of the Centers for Disease Control.

Tested recipes recommend glass Mason-style jars and 2-piece self-sealing lids for home canning. In the past few weeks, one of the most frequently asked questions has been about jar sealing. Handling jars and lids correctly will go a long way towards successfully preserving your garden’s bounty for the winter months ahead.

Recommended jars. Regular and wide-mouth Mason-type, threaded, home-canning jars with self-sealing lids are the best choice. Glass canning jars are available in a range of sizes from ½ pint to ½ gallon sizes. Use only the size of jar recommended in a tested recipe. With careful use and handling, Mason jars may be reused many times, requiring only new lids each time. When jars and lids are used properly, jar seals last through storage and jar breakage is rare.

Lid selection. The common self-sealing lid consists of a flat metal lid held in place by a metal screw band during processing. The underside of the lid contains an applied sealing material that, when heated during canning, softens and flows slightly to cover the jar-sealing surface while still allowing air to escape from the jar. The sealing compound then forms an airtight seal as the jar cools. Unused lids, if stored in a cool, dry location, may often be used for up to 5 years from date of manufacture.  To ensure successful jar sealing, buy only the quantity of lids that you will use in a given year. Metal lids are ‘one trip’ they may be used once and are not designed to be reused. The metal screw bands, if handled properly, many be reused many times.

Do I have to sterilize jars before I use them? Not usually. Jars and lids should be cleaned and washed before use, even jars and lids right out of the box. Use mild soapy water for cleaning, followed by a rinse in clean water.  Jars should always be warmed before filling, but they usually don’t have to be sterilized.  Starting with warm jars will help ensure that jars seal on canning.  When the processing time is short, less than 10 minutes, tested recipes recommend filling into pre-sterilized jars.  Jars are sterilized by boiling for 10 minutes.  Many experienced home canners keep jars hot (or sterilize them) in the canner that they will use for processing.

Is it best to boil lids before I use them? NO! Perhaps 10 years ago the companies that manufacture home canning lids changed the sealing compound in the lid. Now information on the packet of lids will instruct the consumer to wash and dry the lids — that’s it!  Boiling newer lids may destroy the sealing compound and lead to seal failure.  Warming the sealing compound before you apply the lids will not harm the lids and may help jars to seal. [I generally warm jars and lids together in a separate pot.] Be sure that any unused lids are dried before storing them for your next canning session.

Why did my jar lids buckle? The most frequent cause of buckling of jar lids is that the screw band was applied too tight.  Screw bands should be applied ‘finger tip tight.’ The screw band holds the metal lid loose enough so that air can escape from the jar during canning and firm enough so that the lid drops in place to form a seal with the jar rim as the jar cools. If the screw band is applied too tight, air trying to escape from the jar during canning may push against the lid forming a buckle. If the band is applied as tight as possible, the bottom may ‘blow out’ of the jar as the air expands and struggles to escape.

Do I invert jars once they are done processing and I have taken them out of the canner?  This is an unusual question from some callers and is not a good idea.  Inverting jars will force the contents against the lid, often forcing the seal to ‘break.’  Inverting jars may also cause liquid or food to foul the sealing surface, resulting in seal failure.

Do I leave the screw band on when I store my jars in the pantry? At the end of a successful canning process and once jars are cool, remove the bands and rinse jars with warm soapy water and dry.  Label jars with the date and name of the recipe. Store in a cool, dry location. Removing the bands helps prevent rust from forming and allows you to notice if the jars become unsealed.

The jar sealed, is the food safe? This is a frequent question during the canning season. Unfortunately the answer is, ‘Not necessarily.’  Just because a canning jar seals does not mean that the food inside is safe. The amount of heat needed to seal a jar is far less than the amount needed to destroy pathogens and spoilage organisms and to ensure shelf stability.

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 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 3 to 4 days. The conditions that allow for toxin production are:

  • a moist, low-acid food (pH greater than 4.6) – meat, vegetables, dairy
  • a temperature between 40° and 120°F, and
  • a low-oxygen environment.

Scientists assume that botulinum spores can be found on the surface of any fresh food. Because they grow only in the absence of air, botulism spores are harmless on fresh foods, but they may be a problem in canned foods.

What is the key to success? Using the right equipment and following an up-to-date recipe will ensure your success. Recipes to help ensure safe, high quality preserved food from the bounty of your garden can be found in the  USDA Complete Guide to Home Canning or from the National Center for Home Food Preservation or in our Wisconsin Safe Food Preservation publications.  Subscribe for updates on safe food preservation and food safety. Stay well and safe preserving! Barb