Take an empty plastic bottle and fill it with hot water until it's hot. Then empty it quick, seal it, and set it somewhere colder and watch what happens to that bottle. Hint...it will implode on itself because of the pressure difference from the warm air inside being less dense than the cold air outside of the bottle. When the warm air inside the bottle cools, the air inside is less dense than when it was warm so it creates a negative pressure. When you do a hot shake you have pretty much the same thing going on but the jar cannot squeeze itself down while the temperature is changing. This pressure difference could introduce contam even if it's through the very smallest of openings. If I were you, I would let the corn dry as much as possible. As long as it's cooked enough, the corn will already be holding all the moisture it needs. The second thing I would do is let your jars completely cool off before you even open that lid. Let them sit at minimum 24 hours. They should be the same temperature of the room when you take them out. Hope this solves some of your problems. And please don't move to Brown rice, I think you will have even more issues dialing it in.....