What happens when you provide too much headspace when canning?

Question

All directions indicate that headspace is critical. Overfill your jars and liquid or food can expand out the seal and prevent a good seal. Yes I understand this completely.

However, leaving too much headspace? Almost all written directions say this will prevent a good vacuum because the air isn't properly driven out and therefore can't pull back into a vacuum when cooling. This makes zero sense to me and here is why. If you can an empty jar for example, as much air as can be driven out by heat will do. When the air cools, it will decompress into what I would think is the strongest vacuum possible under the circumstances.

How is it that underfilling a jar creates a poor vacuum? Or does it really do that at all?

Answer 1

For a thought experiment: Let's say you could hit the really optimal headspace, for perfect vacuum. In this case, during the cooking phase, the food will expand such that it fills the jar exactly, pushing out all the air. When you cool the jars down, there will be no air sucked back in (the jars are designed in a way to prevent it) and you'll have completely airless vacuum inside the headspace. If you leave too much headspace, some of the air will stay in the jar without getting pushed out, and you'll get a less hard vacuum.

As some anecdotal evidence that this happens, I once pressure-canned some stuff in small old-fashioned Weck jars (they have glass covers with separate rubber seals). I had too little headspace, and liquid from the jar leaked into the canner. It didn't impair the seal in this case, and I ended up with very strongly sealed jars. They're difficult to open, and the gaskets are so deformed that they can't be reused, even for non-canning closure.

The classic safety upside of having stronger vacuum will be that you're better protected from potential spoilage through aerobic bacteria. In principle, following proper canning procedure will leave you with no viable bacteria at all, and in the cases it fails, you'll notice it when the seal fails (or the twist-off lid bulges). But of course, this has only been tested with proper headspace, not with half-full jars, so nominally, you have to follow the recipe to the letter to claim safety.

Also, air leads to discoloration of your food. The less air you have left, the less your food oxidizes, so you keep better quality.

Answer 2

It makes a bit of a difference. Water expands with temperature too, though much less so than gas. As the jars cool to room temperature that leaves an additional vacuum. With a smaller initial headspace, that vacuum is less buffered by air.

It’s not hugely important, though. The point of the vacuum is just to keep the lids sealed and verify that the seal is intact. You don’t need much of a vacuum for that.