Why is Liquid Lost During Canning?

A frequently asked food preservation question is what causes liquid to be lost from the jar during processing?
Why is Liquid Lost During Canning? - Articles
Why is Liquid Lost During Canning?

Stacy Reed, Pennsylvania State University

There are three 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.
  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. In a water bath, remove the cover and let jars sit 5 minutes in the canner before removing; in a pressure canner, once the canner returns to 0 psi and the weight is removed, let jars sit in the canner for 10 minutes before removing.