What is the Yellow Liquid on Top of My Tomato Juice?
Making tomato juice using a blender or food processor to blend the tomatoes before heating will speed up the enzymatic breakdown of natural pectin in the tomatoes and cause the juice to separate.
Heating tomatoes quickly helps prevent separation. The yellow liquid contains pectin from the tomatoes plus some red color pigments which when combined with light gives a yellow color. The enzymatic breakdown of pectin will occur when the tomatoes are crushed before they have been thoroughly heated and when the product sets too long before being put into jars. That is why heating quickly will inactivate the enzymes and the pectin remains intact in the tomatoes.
To prevent juice from separating, quickly cut about 1 pound of tomatoes into quarters and put directly into saucepan. Heat immediately to boiling while crushing. Continue to slowly add and crush freshly cut tomato quarters to the boiling mixture. Make sure the mixture boils constantly and vigorously while you add the remaining tomatoes. Simmer 5 minutes after adding all pieces. If juice separation is not a concern, simply slice or quarter tomatoes into a large saucepan. Crush, heat, and simmer for 5 minutes before juicing.
The separation is harmless but is not appealing. Also, it is best to prepare one canner load at a time and process quickly.