Water Bath Canning Science 101 – Proper Venting

How can I avoid lid buckling or dimpling when I water bath?

This is a sign that pressure has built up inside your jar during the water bath. It is VERY important that the steam is allowed to vent properly during water bath. Whether you are using 2-piece lids or 1-piece lids, the same principles apply.

There are 2 main causes, both of which can result in a compromised seal and/or a lesser vacuum than expected.

1. Lids or rings were applied with too much torque. Only finger-tighten!
2. The jars & contents began to cool before placed in the water bath. This is something to keep in mind when hot filling or hot packing. As soon as you cap that warm jar filled with hot product, it will begin to cool…and seal. If your plastisol begins to form a seal before you water bath, it will be more difficult for the steam to escape, resulting in a compromised seal.

Why is Venting Important?
During the water bath, the venting drives out the steam from the headspace. This accomplishes 2 vital tasks.

1. In forcing out the steam, the oxygen content of that headspace is greatly reduced. This will lower the oxidation rate in the jar – less darkening of product.
2. It increases the potential vacuum of your seal. Generally, a higher level of proper venting will result in a higher vacuum at the end of the cooling period.

Following the proper headspace guideline for each product is also important and can influence proper venting & sealing.

Checking the proper headspace for hot pack green beans at a pressure canning class led by Master Preserver Martha Zepp, held at the Lancaster Farm & Home Center.

In the picture above the preservers are using 3 nifty tools; Magnetic Lid Wand (to retrieve canning lids from the hot water); Canning Funnel (with wide mouth to help avoid spills and dripping; And a Bubble Popper/Head Space Measurer. These tools are part of the 6 Piece Canning Set.

Too little headspace: Food particles or juices may be forced out of the jar. As this happens, you not only end up with a mess, but you will likely get spoilage and/or a compromised seal. Any particle caught in between the liner & the jar can act as a conduit of air & contaminates into your jar.

Too much headspace: Too much oxygen will remain in the jar = more oxidation of your product = darkening or browning in the top layer of your product. This practice also is likely to decrease the potential vacuum of the finished product.

Most good canning cookbooks will include the proper headspace values in recipes. The National Center for Home Food Preservation also includes those values.

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