Canning food correctly means you've got jars of great food that can be stored at room temperature. That means you don't have to worry about taking care of the food if the power goes out. It also adds convenience to a self-sufficient lifestyle: a little effort up front means you can just pop open a jar of soup or sauce later on when you're pressed for time.
But canned incorrectly, not only could you waste a lot of time preparing jars that fail to seal properly, but far, far worse - you could risk fatal botulism.
Yikes! Fortunately, it is very easy to learn to can food safely and successfully, and to avoid these common canning mistakes.
1. The Most Dangerous Mistake: Using a Boiling Water Bath When You Need to Use a Pressure Canner
Acidic foods such as pickles, sweet preserves, fruit and tomatoes (with added acid) may be safely preserved in a boiling water bath, which requires no equipment besides canning jars, lids and a large, deep pot. But non-acidic foods such as soup stocks, unpickled vegetables and meat must be canned in a special piece of gear called a pressure canner (note that this is not the same as a pressure cooker). You can read more about why that is so here, but the important thing to remember is that boiling water bath canning is only safe with acidic foods.
2. Not Adjusting the Canning Time or Pressure for Your Altitude
If you live more than 1000 feet above sea level, you need to adjust either the canning time (for boiling water bath canning) or pressure (for pressure canning). Remember that water doesn't boil at the same temperature at high altitudes as it does at sea level. These adjustments compensate for that, ensuring that your food is safely preserved.
3. Overfilling the Jars
Good canning recipes will instruct you to leave anywhere from an inch down to half an inch of head space. That is the space between the top surface of the food and the rim of the jar. If you fill the jars too high, the canning lids will fail to seal. Unsealed jars are not the end of the world: you can eat the food immediately, transfer it to the refrigerator and use within a week, or reprocess with enough head space and new lids. But you'll waste a lot of time. Better to leave the correct amount of head space to begin with.
4. Reusing Canning Lids
The ring of adhesive on the underside of canning lids wears out with reuse, eventually resulting in unsealed jars. An exception is Tattler reuseable canning lids.
5. Using Cracked or Chipped Canning Jars
Always carefully inspect your canning jars for small chips along the rim, or hairline cracks anywhere on the jar. These could result in unsealed jars or worse, jars that break in the canner.
6. Not Using Enough Water in a Boiling Water Bath
In boiling water bath canning, the lids of the jars must be covered by 1 to 2 inches of water. This ensures that the food is heated evenly from all sides.
7. Not Letting the Jars Cool Undisturbed
Once your jars come out of the boiling water bath or pressure canner, they need to sit undisturbed until the contents have completely cooled. Even though the lids will usually seal well before the end of this cooling down time, if you move the jars you could bring hot food into contact with the adhesive seal of the lids, and that could unseal the jars.
8. Using Inferior Ingredients
A pickle is only going to be as crunchy as the cucumber you started out with; tomato sauce is only going to be as flavorful as the tomatoes that went into it. Use the best ingredients you can get your hands on.