Headspace

Wheather using a boiling water bath canner or a pressure canner, a specific amount of "headspace" must be allowed. Headspace is the space in the jar between the top of the jar and the food or the packing liquid. All USDA approved recipes for home canning give proper headspace measurements. When canning keep a small ruler or hem gauge handy to check headspace as you work.

Too Little Headspace

Food swells and moves arund inside the jar as it is heated during processing. if too little headspace is allowed, swollen, moving food may be forced out of the jar. Besides themess this makes inthe canner, food that ends up on the rim under the lid can precent a seal from forming. Food that lodges around the lid and is not washed away can harbor mold, which in turn can grow its way into the jar and cause a seal failure. Liquid forced out due to too little heaspece causes the liquid level to dip below the level of the food. Food above the liquid can dry out and discolor.

Too Much Headspace

The goal of procesing is to create a sterile environment in the jar and create an air tight seal to protect the product. Too much headspace means an uncecessarily large amount of air in the jar. This air needs to be exhausted during processing to help create the sterile enviroment. When there is too much air, there is not enough processing time to full exhaus the air, thus failing to create the all important sterile environment.

Sources:
Complete Guide to Home Canning USDA
Ball Blue Book

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