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To understand canning basics, you need to know that it is a process that preserves food for long term storage at room temperature by accomplishing three things:
1 - Canning destroys any microorganism that are naturally in and on food. It stops food from spoiling and making you sick. This is done by applying heat to the food, killing bacteria, yeasts, molds and their spores.
2 - Canning destroys enzymes that cause food to start to decompose or loose it's flavor. That's why you should always get the freshest fruit and vegetables possible for canning and freezing.
3 - Canning also (and most importantly) seals the container so that after all those bad critters are killed off, more of them can't get in later.
When food in a canning jar is heated, the air inside (called headspace) expands. Some of that air is forced out around the lid. Since you put the lid on firmly, the rubber compound on the inside of the lid is in full contact with the jar rim and creates a seal.
When it gets hot in the canner, the rubber compound on the lid softens and conforms to the shape of the rim of the jar allowing for a good sealing surface. As the jar cools down, the air inside contracts, causing a partial vacuum inside.
Safety considerations aside, that's the main reason chipped, deformed or damaged jars should NEVER be used, and you should always wipe the rims of the jars clean before applying lids and rings.
It's also a good idea to inspect the rubber compound inside the lids before using them to make sure that none are damaged or deformed. Any of these things could cause a jar not to seal.
Here are some "Canning Basics" general terms and definitions that are helpful to know before you start canning:
Pressure Canning - Canning process that involves placing filled canning jars in to a sealed vessel (pressure canner),to increase pressure inside, which in turn increases temperature. This method is used for low acid foods, because the microorganisms that exist inside low acid foods require higher temperatures to destroy them.
Boiling Water Canning - Also known as hot water bath canning. This process involves placing filled canning jars in a bath of boiling water without additional pressure. This method is used for high acid foods.
High Acid Foods - Foods that have a pH of about 4.5 or less. Most fruit, and a few vegetables fall into this category.
Low Acid Foods - Foods that have a pH of greater than about 4.5. Most vegetables, and all meats, including poultry and seafood, fall into this category.
pH - A scale used by chemists to measure the presence or absence of acid. The range of the scale is 0 to 14. Anything below 7 is considered acidic. Anything above 7 is considered basic or alkaline. A pH reading of 7 is neutral. For what it's worth pH stands for "potential of Hydrogen".
Headspace - The gap of air in a canning jar between the top of the food, and the top of the jar. Headspace is what allows the canning jars to seal. The correct amount of headspace is important. Make sure you follow instructions about correct headspace when canning.
Hot Pack - Packing food in jars for canning that has already been at least partially cooked (and are still hot...).
Raw Pack - Packing raw food in jars for canning. Meats, Vegetables and some fruit can be raw packed. This is also known as cold pack canning.
Blanching - Placing raw fruit or vegetables in boiling water for a brief period of time. This is NOT cooking. Blanching can serve one of two very different purposes:
Blanching loosens the skin on some fruit and vegetables to make it easier to remove the skin. Examples of foods that are blanched to remove the skin are tomatoes and peaches.
Blanching is also done to destroy enzyme activity in foods. This is primarily done when preparing vegetables for freezing. Good examples of food that is blanched to destroy enzymes for freezing would be sweet corn or broccoli.
Learning the canning basics before you get started will help you understand what is happening during the process, and you'll know what the terms used in canning recipes mean.