Why Is Liquid Lost During Canning?

There are four things that can cause liquid to be lost from the jar during processing. 

1. Food packed into a jar raw is more likely to lose liquid as the raw food contains air that is driven from the jar during the heating/canning process. The recommendation is to heat most fruits before packing into jars. Recipes identify this as a hot pack.
2. Rapid fluctuation of the temperature in the canner can also cause loss of liquid. When a boiling water canner comes to a rapid boil, adjust the temperature of the stove to maintain a steady boil, but not a ferocious boil. When a pressure canner reaches the designated pressure, reduce the temperature of the stove to maintain that pressure without making sudden changes in the pressure.
3. Removing jars from the canner too quickly after processing causes stress that sends liquid out of the jar. After processing is complete, remove the canner from an electric burner, remove the lid from a boiling water bath canner and allow the jars to sit in the hot water 5 minutes before removing to a counter. Remove an atmospheric steam canner from heat and allow jars to sit in the canner for 5 minutes with the lid still in place before removing jars to a counter. Pressure canner directions include a 10-minute wait after the pressure returns to zero and the pressure regulator or weight is removed. This allows the temperature in the jars to equalize to the surrounding temperature and reduces siphoning.
4. Overfilling jars not only encourages liquid loss but also increases the risk of seal failure. During heat processing, the contents of the jar expand.  Too little space between the lid and food/liquid forces liquid out of the jar when it is heated.  Use correct headspace when filling jars.

If your jars siphon, check the jars for a solid seal. Clean jars thoroughly, and be careful to remove all food that may be trapped in the threads at the top of the jar. If food is left on the jar, it will mold. Check these jars occasionally in storage to make sure they stay sealed. Mark the jars to use first. Sometimes, a jar can come unsealed months after it was canned. 

If liquid is lost during processing, it does not cause the food to spoil, but the food above the liquid may darken and dry. According to the National Center for Home Food Preservation, if the loss is excessive, for example, half of the liquid, refrigerate the jar(s) and use within two to three days.

Resources:

National Center for Home Food Preservation. (n.d.) "Frequently Asked Canning Questions—Should Liquid Lost During Processing be Replaced?" University of Georgia.

National Center for Home Food Preservation. (n.d.)  "General Canning Information—Causes and Possible Solution for Problems with Canned Foods." University of Georgia.