Hi Lisa, Fellow baker here. I have been freezing foods for a long time. You can freeze in a mason jar BUT-
You can't use a jar with "shoulders". The lid has to be the same width as the widest part of the jar. Like the "wide-mouth" jars.
Puree will expand as it freezes and crack the jar top off if it's smaller.
Leave a good amount of air space at the top of the jar, an inch or so, for the puree to expand.
I also think a plastic (non-metal) lid is recommended, but not sure about that.

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