Canning Jars

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What Kinds of Jars Can Be Used For Canning?

What's On This Page

How to Determine If A Jar Of Canned Food Is Sealed?

Is It All Right To Let Jars Cool In The Water In Which They Were Processed?

Is It Safe to Re-Use Canning Jars?

Reasons for Liquid Loss From Jars

Sterilization of Empty Jars

What Causes Jars To Break?

What Is The Best Way To Clean Jars Before Canning?

What Kinds of Jars Can Be Used For Canning?

Why Is Headspace Important In Canning?

Illustration of various glass jars.Use only standard home canning jars. Glass canning jars, also known as mason jars after their nineteenth century American inventor, John L. Mason, are the standard jars to use in home canning.  These reusable jars are produced in a variety of sizes, some with decorative finishes for gifting. They are made to seal properly, to be durable with repeated use and to be used safely in a steam pressure canner. The manufacturer's name is usually blown in the side of the jar.

Recycled jars from the grocery store, also known as packers' jars, are not made for home canning. The jars that contain pickles, mayonnaise or peanut butter are not made for the rigors of home canning. Even though standard home canning lids may seem to fit these jars, the lids may not seal because the glass is not as thick and slightly irregular. The jar neck may be too shallow for a standard home canning band to hold the lid tightly against the jar.

In addition to sealing problems with commercial jars they may also be dangerous. Most of them are made of thin glass and are not heat tempered, as regular home canning jars. They may not withstand the high pressure of canning and break. When you open the canner, the jars may still be under pressure. The quick drop in temperature could cause the recycled jar to explode.

Sterilization of Empty Jars

All jams, jellies, and pickled products processed less than 10 minutes should be filled into sterile empty jars. To sterilize empty jars, put them right side up on the rack in a boiling-water canner. Fill the canner and jars with hot (not boiling) water to 1 inch above the tops of the jars. Boil 10 minutes at altitudes of less than 1,000 ft. At higher elevations, boil 1 additional minute for each additional 1,000 ft. elevation. Remove and drain hot sterilized jars one at a time. Save the hot water for processing filled jars. Fill jars with food, add lids, and tighten screw bands.

Empty jars used for vegetables, meats, and fruits to be processed in a pressure canner need not be presterilized. It is also unnecessary to presterilize jars for fruits, tomatoes, and pickled or fermented foods that will be processed 10 minutes or longer in a boiling-water canner.

How to Determine If A Jar Of Canned Food Is Sealed?

A. Cool jars for 12 to 24 hours, remove the screwbands, and test seals with one of the following options:

  • Press the middle of the lid with a finger or thumb. If the lid springs up when you release your finger, the lid is unsealed.
  • Tap the lid with the bottom of a teaspoon. If it makes a dull sound, the lid is not sealed. If food is in contact with the underside of the lid, it will also cause a dull sound. If the jar is sealed correctly, it will make a ringing, high-pitched sound.
  • Hold the jar at eye level and look across the lid. The lid should be concave (curved down slightly in the center). If center of the lid is flat or bulging, it may not be sealed.

What Causes Jars To Break?

There are several types of breaks that occur. Each break looks different and has specific causes. Thermal shock is characterized by a crack running around the base of the lower part of the jar, sometimes extending up the side. To prevent thermal breakage:

  • Avoid sudden temperature changes, such as putting hot food in a cold jar, putting a cold jar in hot water, or placing a hot jar on a cool or wet surface. Keep jars in hot water until filled.
  • Use a rack in the canner.
  • Avoid using metal knives or spatulas to remove air bubbles or steel wool pads to clean jars.
Internal pressure break is characterized by the origin of the break on the side. It is in the form of a vertical crack that divides and forks into two fissures. To prevent pressure breaks:
  • Provide adequate headspace in jars for food to expand when heated.
  • Keep heat steady during processing.
  • Avoid reducing canner pressure under running water or lifting the pressure control or petcock before pressure drops to zero.
Impact breaks originate at the point of impact and fissures radiate from the point of contact. To prevent impact breaks:
  • Handle jars carefully. Jars that have been dropped, hit, or bumped are susceptible to breakage. Test new jars that may have been mishandled (to see if they break) by immersing them in room-temperature water, bring to a boil, and boil 15 minutes.
  • Avoid the use of metal tools to remove air bubbles.
  • Avoid using old jars. Jars have a life expectancy of about 10 years.

Is It Safe to Re-Use Canning Jars?

You can reuse standard canning jars, but be sure to caredully inspect jars for any cracks or chips along the rim or threads.  Discard any imperfect jars because defects prevent airtight seals.

Wash glass jars in hot, soapy water and rinse well. When you can most food by the boiling-water_bath method and all foods by the pressure-canner method, it is not necessary to sterilize jars before canning. The jars, as well as the food, are sterilized during processing.

You should sterilize jelly containers in boiling water for 10 minutes before using them. Then keep the containers hot -- either in a slow oven or in hot water -- until you use them. This will keep them from breaking when you fill them with hot jelly.

Why Is Headspace Important In Canning?

Headspace is the distance between the surface of food and the underside of the lid. This space allows for the expansion of food solids or bubbling up of liquid during processing. If headspace is not adequate, some food in the container will be forced out, leaving food particles or syrup on the sealing surface and preventing a seal. When too much headspace is allowed, some air may remain in the jar after processing, causing food at the top of the jar to darken. Adequate headspace allows a vacuum to form during the processing of the food.

Reasons for Liquid Loss From Jars

As strange as it may seem, there are many reasons for liquid loss from jars during processing. The following are possible reasons:

  1. The jar is packed too full, that is, you did not leave recommended headspace.
  2. The food is packed so tightly that liquid did not fill the spaces between food pieces.
  3. Starchy foods may absorb some of the liquid.
  4. The liquid you added to cover cold, raw food was not hot enough when you put it in the canner.
  5. You did not remove air bubbles when you packed the food. (You can do this by running a rubber spatula between the food and the jar.)
  6. You did not cover jars of acid foods with one or two inches of water in the boiling water bath canner.
  7. The pressure canner was not sufficiently exhausted.
  8. Pressure fluctuated, or the temperature lowered suddenly during processing, due to uncontrolled heat source.
  9. The temperatures changed suddenly when processing was over. If the pressure canner cools too quickly while the contents of the jar remain at a much higher temperature, the liquid will boil over. The "coming down" period has to be gradual and even.
  10. The petcock was opened before the pressure had returned to zero. When the pressure gets to zero, open the petcock cautiously; if steam escapes, close and wait a few minutes.
  11. The canner stood too long after pressure returned to zero. Open the canner within several minutes after it returns to zero pressure.
  12. You removed the jars too quickly after removing the cover. Let the jars stay in the canner for a few minutes after removing the cover, or until the boiling in the jars goes down.
  13. The gauge's pointer does not rest at zero when not under pressure.

Liquid loss may cause the food to darken, but does not interfere with the keeping qualities. Do not open jars at the end of the processing to replace liquid before you will spoil the food, unless you use the contents immediately.

 

Is It All Right To Let Jars Cool In The Water In Which They Were Processed?

IIt is important to remove jars from a boiling-water canner immediately when the processing time is up. The spores of certain thermophilic, or heat-loving bacteria, can survive boiling-water processing. Because these bacteria thrive at high temperatures, they can multiply and cause spoilage if canning jars are left in the hot water to cool slowly. When processing foods in a steam-pressure canner, the canner is removed from the heat source when the processing time is up. Jars are left in the steam-pressure canner until the pressure returns to zero naturally. This period of time, after removal from heat until the pressure reaches zero, is considered part of the processing time and is necessary for destruction of microorganisms. Do not rush this cooling by placing the canner under water, or by using a fan. Remove the jars immediately when the pressure returns to zero, and cool at room temperature.

What Is The Best Way To Clean Jars Before Canning?

Before every use, wash empty jars in hot water with detergent and rinse well by hand, or wash in a dishwasher. Detergent residue may cause unnatural flavors and colors. These washing methods do not sterilize jars. Scale or hard-water films on jars are easily removed by soaking jars several hours in a solution containing 1 cup of vinegar (5 percent acidity) per gallon of water.