Storing things in Mason jars is something nearly everyone does. Whether you are canning or just looking to keep something fresh longer, it’s easy to grab a mason jar and screw on a lid. But are canning jars actually airtight?
Canning (Mason) jars will be airtight once the lid is screwed on only if the jar, lid, and ring are not compromised. However, there will still be air inside the jar. The only way to remove air from the jar is by following the safe canning practices recommended by the USDA, which forms a vacuum.
To check if a canning jar is sealed, press the center of the lid to see if it moves. If the center does not move up and down and the lid cannot easily be lifted off after the ring has been removed, the lid has achieved a good vacuum seal.
However, even if not canned with this process, Mason jars can still effectively keep out dust, moisture, insects, or any other contaminants.
Now, let’s cover how to ensure canning jars are airtight when making preserves and then go through some ideas on what can be stored inside Mason jars.
What Makes Canning Jars Airtight?
Canning jars become airtight during or at least as a result of the canning process with the aid of the design and construction of the lid and ring. Canning lids are designed with a flexible rubber gasket that fits snugly over the rim of the jar and is held in place by the ring.
What Part of the Safe Canning Practices Renders Jars Airtight?
- During the canning process, the jars are placed in a pan of boiling water which sterilizes the jars and keeps them hot.
- The jelly or jam is boiling when it is poured into the hot jars.
- The lids are also kept in hot water until they are affixed to the hot jars.
- The filled jars, with lids and rings affixed, are processed in the boiling water bath for the length of time designated in the recipe being used.
- Remove jars from the boiling water bath when the time is up and set them aside to cool completely.
- When the jars are processed, the lid gasket softens and flows slightly to cover the jar, thus sealing the surface but at the same time allowing air to escape from the jar. When all the air has escaped, and a vacuum is created, the center of the lid will not move when pressed.
NOTE: There are actually 2 sets of safe canning practices:
- Low-acid foods such as red meats, seafood, poultry, milk, and all fresh vegetables except for most tomatoes must be processed at 240 – 250 degrees F to kill the bacteria that cause botulism and must be pressure canned. The safe canning practices for pressure canning are basically the same, except a pressure canner is used instead of a boiling water bath to reach the required temperature and the processing times are different.
- High-acid foods like acidic fruits, tomatoes, pickles, and jellied products only have to be processed at 180 – 212 degrees F to stop the growth of the bacterium Clostridium Botulinum in canned food that may cause botulism, so a boiling water bath is normally used for those foods because boiling water will reach the required temperature.
How to Tell If the Seal Has Been Broken?
Press the center of the lid to see if it moves when you touch it. If the center does not move up and down and the lid cannot easily be lifted off after the ring has been removed, the lid has achieved a good vacuum seal. Another sign is if mold is forming on the food.
Common Things to Store in Mason Jars
Things that require a safe canning process to store:
All canned fresh foods including:
- Red meats
- Preserves such as jam and jelly
Things that do not require a safe canning process to store:
1. Frozen foods
Mason jars are also freezer-safe, and jams and any other foods that you are preserving by keeping it in the freezer can be stored in Mason jars.
However, lids can be recycled when storing in the freezer, and new lids are not required because the food is being preserved by freezing and not by storing in an airtight container.
To freeze, follow the process below, steps 1 through 4 and 6 through 11. Then, after pouring the jam into jars and affixing lids, allow the food to cool completely before placing them in the freezer.
I usually allow them cool, then place them in the refrigerator overnight, waiting until the next day to move them to the freezer.
Frozen canned foods, if prepared properly, will last at least 5 years, although the more popular recommendation is that the food should be used within two years.
2. Dry foods
Mason jars are very popular for storing dry foods such as spices, pasta, rice, popcorn, grains, flour, cornmeal, and dry beans. They are safe for storing these foods even though they are only airtight when processed by using safe canning practices during which all the air is removed from the jar.
These foods do not require canning, and the jars prevent the contents from being contaminated by dust, moisture, insects, or any other contaminants that might be found in your kitchen.
The main concern for anyone using Mason jars for storage is to be sure the jars are clean and completely dry before adding any food items. Any moisture left in the jars can cause the food to grow mold and spoil.
As with storing dry foods in Mason jars, storing liquids such as oils, syrups, and cleaning supplies, especially when purchased in bulk, gives you a size container that is more manageable and easier to handle than very large containers.
Mason jars are also great for storing liquids like milk and juices in the refrigerator, which allows you to get rid of larger containers when they are close to being empty.
Other Uses for Mason Jars
Mason Jars can be used to create beautiful and unique decorations to be used throughout the house. They can be:
- Painted, decorated, and used as vases.
- Filled with decorative rocks, leaves, marbles, etc., and grouped to form a centerpiece.
- Filled with a candle to create unique lighting.
- Filled with candies for holiday guests.
- Decorated to use as a pencil, ruler, or other office supply holder.
There are many unique gifts that can be created using Mason jars. Here are a few ideas I have either seen recently or have used myself:
- Decorate and fill with homemade cookies or candies.
- Decorate and fill with trail mix or pretzels.
- Add succulents for a small terrarium.
- Add lamp oil and a wick for a small oil lamp.
- Decorate and fill with flowers.
- Fill with layered dry brownie or cookie ingredients with directions for how to prepare them written on the label.
A Very Brief History of Preserving Food
The method of preserving food by placing it in a sealed container was invented by a French chef named Nicolas Appert. The first canning factory in history was opened by him in 1804. At that time, there was no refrigeration and no reliable method of preserving food.
Later, in 1858, Philadelphia tinsmith John Landis Mason invented the molded glass jars that we use for food storage today, thus the name “Mason” jar.
Mason jars are used for many different things, only one of which is to safely preserve canned foods for an extended period of time.
Though most often bought for canning purposes, they can be recycled indefinitely throughout the house. They can be used as canisters for dry and liquid foods in the kitchen and pantry, to store cleaning supplies in the utility room and bathroom, they can be up-cycled as decorations, and can be used to create unique gift items.
Even after having cleaning supplies stored in a Mason jar, they are non-porous and can safely be washed well in hot, soapy water and used again to store food. Uses for them are unlimited, and they should not be discarded unless they become cracked or chipped.
A new undamaged Mason jar without any production flaws will be one of the best containers you can find for canned foods. By the way, although you can reuse canning jars and rings, just make sure to use new lids for canning to ensure a good seal.
Top 5 reasons to store foods in Mason jars:
- Tempered glass is 5 times stronger than regular glass and is not damaged as easily.
- Mason jars are made from tempered glass that is impermeable and non-porous and doesn’t transmit stains, chemicals, or odors to its contents.
- Jars are clear, and contents can be recognized immediately.
- They will not become discolored or warped.
- Mason jars are recyclable.
What is the safe canning process?
Here is the safe canning process as per the USDA. Note: The steps that contribute to making Mason jars airtight are in bold:
- Gather ingredients, supplies, and equipment which includes half-pint canning jars and rings with new lids.
- Wash and rinse all fruits thoroughly before cooking. For best flavor, use fruit that is 3/4 fully ripe and 1/4 slightly underripe. Remove stems, skins, and pits from fruit and cut into pieces or crush, depending on recipe directions. For berries, remove stems.
- Check jars and discard any that are chipped or cracked. Wash in hot, soapy water and rinse well.
- Sterilize jars by either using the dishwasher sterilize setting, or boil jars in a pan filled with water so that jars sit in at least one inch of water for 10 minutes and reserve jars in boiling water until filled.
- Fit a large pot with a rack, fill 2/3 with water, and bring to a boil. Reduce heat and allow water to simmer until needed.
- In a saucepan, heat 2-3 inches of water to a full rolling boil, add new lids, reduce heat and allow to simmer until needed.
- In a large, heavy, 8- to 10-quart canning pot or Dutch oven, mix fruit with pectin over low heat until sugar dissolves. Then bring to a full rolling boil over high heat, stirring frequently to avoid sticking or burning.
- Add sugar and bring back to a full rolling boil; boil for 1 minute.
- Skim off the foam that forms during the boiling process. The addition of ¼ teaspoon butter before cooking helps cut down on the foam.
- To fill the jars, pour hot fruit mixture into hot sterilized jars, leaving ¼-inch headspace.
- Wipe jar rim with a clean cloth, affix lids and rings, and firmly tighten rings, but do not overtighten.
- Place jars into the rack in the large pot, bring water back to a full rolling boil, and process jam in the boiling water bath for 10 minutes.
- Remove jars from the canner and place them in a prepared area to cool.
- Allow them to cool undisturbed for at least 24 hours. Carefully wipe the outside of the jars with a clean, damp cloth, and store them in a cool, dark, and dry storage area.
Can Mason jars hold pressure?
Mason jars are not designed to hold pressure like aluminum cans are. In fact, too much pressure on the untempered glass could be dangerous.
For more, don’t miss A Guide to Proper Use of Mason Canning Jars | What Are They Safe For?
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