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Some people call this siphoning, but it isn't. Siphoning PULLS liquid. Canning PUSHES liquid. So it isn't siphoning. It is just liquid loss.
I have found several factors that affect it, and that overall, you can avoid it about 90% of the time. The other 10%, either things just did not work out due to some unavoidable mistake, or there is just something about that food that is really finicky and you'll always have liquid loss, or the APPEARANCE of liquid loss.
Not all drops in liquid ARE liquid loss.
Some foods will absorb liquid during the canning process, making it appear that liquid was lost, when it was not.
Some foods will swell a bit and rise up above the water line, and the water level will drop, again, making it APPEAR that there was loss when there was not.
REAL liquid loss is pretty much always an air or temperature change issue. And REAL liquid loss means the water in the canner will have some of the liquid from your canning in it, and will usually smell of the food. A significant odor of the food you are canning, in the kitchen, coming out of the canner during processing is a good indicator that some liquid has been lost.
I've tested and observed waterbath canning, pressure canning, and steam canning. I've had liquid loss with all of them, and when I canned like my mother said to can, every jar had some liquid loss, usually quite a lot - about 1" was average for quarts.
Since I've been canning as an adult, I've minimized that, and rarely have issues with it unless something goes wrong.
These are the factors I've isolated that have a strong affect on liquid levels in the jars during or after processing.
1. Fill level. If you overfill, they will lose liquid.
2. Heat up time. This affects it, but not as much as the cool down time. I heat it up a little slower than I could - I don't put the stove on Hi, but on about 7.5.
3. Cold or hot pack heat up time. If I cold pack, or I'm canning jars with cold stuff in them, I heat it on 7, even slower. I've tested this, and it makes a difference in fluid loss.
4. Cool down time. Do it slowly, if you rush it, there is more fluid loss. For pressure canning, or Steam canning, I turn off the burner and leave the canner on it (slower cool down that way), then remove the jiggler (for pressure canning - no jiggler with steam canning) when the pressure is down all the way. I then remove the lid and set it at an angle on top so the steam comes out, for about 5 minutes (no sudden air temp change on the jars), and then take the lid off and let it sit for another 5-10 minutes, and then remove the jars from the canner. For waterbath canning, never remove the jars when the water is still boiling. Cool it down some before you take them out. This ONE factor affects the liquid levels more than any other factor!
5. Overall canning time. The longer it is canned, the more likely it is to lose fluid due to boiling in the jar.
6. Residual air in jar or dryish foods. Some foods trap air between the pieces, so releasing that is important before you can up the food. Air expands more than water, so it pushes water out when it does. Some foods also are more porous and have more air in them to begin with. Peaches do that, so do apples, especially if they are older. Potatoes will absorb water during canning if you raw pack older potatoes that are not in prime condition (soak them in cold water for 2 hours before you pack them to avoid this).
You may never eliminate fluid loss entirely, but you can reduce it significantly with a little care.
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