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Q. Last week I canned a few quarts of crushed and whole tomatoes. I thought I had done everything correctly, including allowing proper headspace in my jars. However, after processing some of the jars leaked some of their juices while cooling. At first I was concerned, but it seemed that the jars had sealed properly so I cleaned them and put them away. I checked on the jars yesterday and found that the two quarts of crushed tomatoes had leakage signs. I tossed them out
A: Ah, siphoning—the canner’s bane. The problem you’ve described is fairly typical, especially in pressure canning (see this earlier Ask the Goats on a related problem). Whether you’re water-bath canning or pressure canning, the cause is the same: a sudden change in temperature or pressure can cause trapped air in the jars to suddenly expand, forcing liquids out. In a pressure canner, you can reduce the chance of the problem by leaving the jars to cool in the canner, but obviously, this strategy won’t work in the water-bath. What you can do is turn off the heat and take off the lid when the processing time is done, then wait five minutes. This cools things down a bit and reduces the chance that you’ll get siphoning without overcooking the product too much. I had never noticed this before your question, but it’s actually the strategy recommended in many canning books, including the Ball Blue Book and Put ‘Em Up.
The main danger from siphoning is that it can interfere with your seal. If this is the case, refrigerate or reprocess them. Depending on their contents, you might even be able to add more liquid before trying again. If you do get a seal, the jars are safe, but you’ll want to eat them first, as they’ll be more prone to oxidation. Again, this isn’t a safety issue, but a quality issue.
As for preventing the problem in the first place: siphoning is much more likely to happen in liquid-y products (peaches in light syrup, pickled peppers, tomatoes in water, etc.) than in thick, gloppy canned goods (jams, chutneys, relishes). Be sure to remove air bubbles, and watch that headspace. Then cross your fingers, spin three times, and hum.