Three questions about pressure canning low-acid foods

August 13, 2011 9:31 PM

Three questions about pressure canning low-acid foods, on water covering the jars, headspace, and recipe mismatches.

1) when using a pressure canner, it's specified that you only need 2-3 inches of water. The Ball Complete Book of Home Preserving specifies to heat the jars halfway filled and then to empty the jars into the pot before filling them. The extra water from inside the jars caused them to be completely covered with water during processing. That's okay because the water will still reach 240F before boiling off (10PSI), right? It did cause the water to take a very long time to boil, an additional 30-45 minutes.

2) When recipes specify head space, is that minimum air space or maximum air space? Is it safe to process jars that are not as fully filled?

3) The meat sauce recipe in that book specifies starting with 30 pounds of tomatoes, to ultimately yield 5 quart jars of sauce. My 30 pound box of tomatoes yielded about 12 quarts of puree. There is no long reduction step that would cut down on the volume, and they even suggest that you might add water to make a thinner sauce. Has anyone else made this recipe?

posted by Caviar to food & drink (12 answers total)

No experience using a pressure canner, but to answer question 2, you can not have more head space than recommended. A partially filled jar should just be put in the fridge to consume as soon as you can get to it.
posted by Gilbert at 10:40 PM on August 13, 2011

I'm no expert, but I've been canning a lot lately.

1) A bit more than 3 inches is okay, but I don't think you want to completely cover the jars with water. I'd just pour the excess into the sink if you can. That's my method.

2) That is the minimum air space. It needs that at least that much to form a seal. You can get a way with a little more space, but if there is too much it won't seal properly because it would need extra time under pressure.

3) I haven't tried that recipe, but some tomatoes are meatier than others. Sounds like you can make extra!
posted by meta87 at 10:49 PM on August 13, 2011

#2: Headspace simply means the air pocket between the top of the food and the top of the jar. Always use the headspace measurement that your recipe calls for! This air pocket is what contracts when heated, and allows a seal to form with the rubber gasket under the lid.
posted by MonkeyToes at 5:35 AM on August 14, 2011

#3, FWIW: My husband and I like a thicker sauce, so we always cook it down. Make a note of the pound-to-yield numbers, too. I've found that my year-to-year numbers tend to be closer to each other than to what Ball suggests. If you really don't need that much meat sauce, then you could always water-bath can some of it as straight soup base (one quart of canned soup plus a bullion cube or two, plus about 1/3 c. of alphabet pasta makes a soup that my kids love, year in and year out. We have also added milk/cream to the base to make creamy tomato soup with grilled cheese. The beauty of making a soup base out of extras is that it very quickly deals with everything you have left over after your pressure canner is going, and it's already 7 p.m.).
posted by MonkeyToes at 6:03 AM on August 14, 2011

Ok, so - if the seal was good, there are no issues with the headspace?
posted by Caviar at 6:36 AM on August 14, 2011

Caviar, I can't answer that safely and with certainty, but you could always ask the folks at The National Center for Home Food Preservation, or at your local/area cooperative extension service. I will say that going forward, you should grab a ruler and measure the top of your jars so that you have a clear mental picture of, say, what 1/2" below the jar top is.

Pssst, I have an awesome recipe/tutorial for pizza sauce in my profile.
posted by MonkeyToes at 7:03 AM on August 14, 2011 [2 favorites]

#1 - I would keep the water depth at the manufacturers recommended level.

The purpose for partially filling and heating the jars is to reduce thermal shock. This can be achieved by removing jars from a just run dishwasher or by keeping the jars in a sink of hot water. If processing time is over 10 min. the jars do not have to be sterilized - just clean and warm for hot filled foods.

#2 - 1/4 inch headspace is exactly that. A pinch more is ok, less and product can bubble up and get under the lid and cause sealing problems. Too much headspace and air can be left in the jar, causing discoloration and loss of quality. Each recipe is developed to have the correct amount of time to remove a specific amount of air from the jar without overcooking the food. Partial jars you can just put in the fridge and use soon or freeze.

There are a few inexpensive canning tool sets that come with a wand which has notches. The notches measure headspace, the other side of the wand is used to remove air bubbles from pickles and such. This is a really handy thing to have.

#3 - Many recipes for sauce are written assuming you will be blanching, skinning and seeding the tomatoes. Some varieties benefit from squeezing the tomato meat gently to remove excess water. Roma tomatoes are often stipulated for sauce - they tend to be very meaty- resulting in a thicker sauce. Breaking down some or all of the tomatoes with a potato masher will speed up the cooking time (preserving flavor and nutrients) and thicken the sauce.

If sauce has too much water it will eventually separate in the jar. It will still be edible, just not of "blue ribbon" quality.
posted by cat_link at 9:50 AM on August 14, 2011 [1 favorite]

Headspace isn't as hard a rule as most books will tell you.

Too little and you risk
* if the contents swell (think starchy stuff) they'll push the lid open
* if the contents boil during processing they can force goo over the lip, leading to eventual seal failure

Too much and you risk
* enough oxygen inside to react with some foods and discolor them; mostly cosmetic, I think, though I suppose it could change flavor in the extreme
* the jar not sealing hard because you can't get enough of a pressure drop across the lid. Though all the old canners I've talked to say this, I've never had it happen. And when my wife and I first started canning we were really sloppy about headspace, partially filling the last jar, etc. YMMV

Our rule is to do our best and, if we have a "questionable by the book rules" jar (half-filled, sugar boiled over through the lid seam, etc.) we stick a bit of masking tape on it and either refrigerate it for immediate consumption or put it on the shelf where, if it ferments and explodes, it won't make a mess (and we don't give those away to people for Christmas, etc.). So far we're batting 1.000 on not spoiling any food.

Go forth, can, and enjoy the fruits of the year even in the dead of winter! OM NOM.
posted by introp at 9:52 AM on August 14, 2011 [1 favorite]

You do not want to completely cover the jars when pressure canning. The steam in pressure canning is at a higher temperature than boiling water can achieve; the high temperature is what allows you to safely can low-acid foods. Ideally you use a small amount (see your pressure canner's instructions for the amount) of water to provide sufficient steam.

Whenever I've wound up with a half-filled jar during canning, either pressure or water bath, I've treated the jar (whether it sealed or not) as unsealed, stored it in the fridge, and used it promptly. I don't like to mess with food safety. As introp mentions above, I treat the headspace as a target: if the recipe says 1/2" headspace, I get as close to 1/2" as possible in all jars even if that means a partial jar in the final product.
posted by thatdawnperson at 12:06 PM on August 14, 2011

Steam is always a higher temperature than boiling water, regardless of what the pressure is. But under pressure, the water is also a higher temperature than 1 atm boiling water can achieve. The point of the pressure cooker is that it raises the internal pressure of the container so the water reaches a higher temperature before it boils. i.e. in a pressure cooker at 10PSI, the water is at 240F (instead of 212F) when it's boiling. Unless you're arguing that killing botulism spores requires an even more thorough processing than 240F for 70 minutes (to my knowledge, it doesn't).
posted by Caviar at 12:56 PM on August 14, 2011

In general reference to this question - yes, I'll do it differently next time, but I'm wondering if the batch I did this time (with the water covering the jars, and with one jar at ~1" headspace instead of 1/2") is safe.
posted by Caviar at 1:05 PM on August 14, 2011

As to #3, I looked at that recipe this morning and that number felt way off to me. I had just pulled 15# of heirlooms from the garden. I decided NOT to add meat, and just quartered and roasted the tomatoes and pureed them with seasonings, basil, etc. And only 15 pounds pretty much straight tomatoes, with the water already mostly evaporated from roasting, STILL rendered 4.5 quarts. So I think the Ball number is wrong. Maybe they meant "15" not 5?
posted by GardenGal at 7:01 PM on August 14, 2011