Maybe it's old age creeping up on me, but I couldn't get the lid off the beetroot jar. Knowing that the pressure inside is lower than the pressure outside, I knocked a nail through the safety button. I was then able to open the jar. It seems that it was not just friction but was also the difference in pressure making it hard to open.
I gather that the food is cooked and then placed in the jar. The lid is screwed on. As the food cools, the air around it cools. The particles are not moving as fast so they don't collide with the lid as much. The pressure decreases. They also take up a smaller volume so the button, which is more flexble than the rest of the lid, is pulled in. By putting a hole in the lid, I allowed more particles in so there were more collisions raising the pressure inside to that outside.
