- Make sure that you fill to the appropriate amount of head space. For most canned fruits and tomatoes, the head space is usually 1/2 inch, or right where the rings start on the jar. The head space is necessary because the foods will expand under the pressure of the water bath. If there is not enough room for them to expand, they will overflow. Jams and jellies require very little head space, and for this reason rarely overflow.
- Make sure that the jars are covered with an inch of water. If the jars are not covered in the water bath, the pressure will be unequal, and this can cause overflow.
- Make sure that you get out any bubbles. Use a plastic knife or one of the canning spatulas that come with some kits. Run it around the inside of the glass while pushing inward to remove any bubbles that are in between the fruits.
- When the jars have completed their water bath for the suggested time, turn the burner off and remove the lid. Let the jars sit in the water for 5 minutes, and THEN remove them.
- In my experience, I’ve had a lot more problems with overflow when I use a cold pack method of canning. Hot pack canning requires very little extra effort and is worth it for the added security against overflow.
Example—Cold Pack for canning tomatoes—remove skins and core tomatoes, clean and sterilize your jars. To your quart jar add 1/2 teaspoon citric acid (or 2 T. bottled lemon juice) and pack your tomatoes. Pour boiling water over the tomatoes and fill to 1/2 inch headspace. Remove bubbles and adjust liquid as needed. Add 1 teaspoon of canning salt. Place lid and ring on jar. Boil in a water bath canner for 45 minutes.
Hot Pack for canning tomatoes--remove skins and core tomatoes, clean and sterilize your jars. To your quart jar add 1/2 teaspoon citric acid (or 2 T. bottled lemon juice). Place the peeled and cored tomatoes in a pot and cover with water. Bring to a gentle boil and boil for 5 minutes. Place hot tomatoes and liquid in the jar and fill to 1/2 inch headspace. Remove bubbles and adjust liquid as needed. Add 1 teaspoon of canning salt. Place lid and ring on jar. Boil in a water bath canner for 45 minutes.
Go here for information on how to know if food has spoiled in canning jars in case you are worried about that possibility.