Most of the time, mold in applesauce is caused by issues with air. About 25% of an apple’s volume is actually air - this is why it’s so important to both heat the sauce adequately to stop all enzymatic reactions and to also remove air bubbles from the sauce after it has been poured into the jar. Of course, adding lemon juice or ascorbic acid is another way to prevent your applesauce from molding or going bad in storage.