So what is a goat to do? Turn to Twitter, of course. Last year, zoecancan was kind enough to point me to an incredibly helpful article in the peer-reviewed journal Plant Foods for Human Nutrition. Since it’s behind a paywall, I’ve reproduced the abstract below:

Three studies were conducted to evaluate the safety of tomatillos and products containing tomatillos canned by the water-bath processing method. In the first study, plain tomatillos were processed for 25, 37.5, 50 and 62.5 min. In the second study, five tomatillo/onion combinations were prepared while five tomatillo/green chile combinations were prepared in the third study. pH evaluations were conducted to determine safety in all studies using pH 4.2 as the cut-off value. No differences in the pH of plain tomatillos were detected due to processing time. All jars of plain tomatillos had pH values below 4.1. All combinations of tomatillos/onions and tomatillos/green chile containing more than 50% tomatillo had pH values below the 4.2 cut-off value. Results of the three studies indicate (1) acidification of plain tomatillos is probably unnecessary for canning by the water-bath processing method and (2) combinations of acidic tomatillos and low-acid onions or green chile must contain more than 50% tomatillos to have a pH low enough for safe water-bath processing.

So, based on this, I feel pretty comfortable saying that it’s safe to water-bath can this version of salsa verde, or in fact any version of salsa verde where the quantity of tomatillos (by weight) exceeds the combined weight of onions and/or peppers and herbs. The acid safety cut-off is a pH of 4.6, so at 4.1, this is plenty acidic (remember that a lower pH means higher acidity).

For the recipe, along with instructions for water-bath canning (for the scientific), pressure canning (for the conservative), or freezing (for the squeamish), click here. This time, 2 pounds made 4 half-pints. It scales up, so, if you’ve got a bushel full of tomatillos, knock yourself out!

Q: I’ve just read your instructions [for canning chicken stock]. I’ve tried twice now to can a batch of stock in my pressure canner and most of the jars won’t seal. I removed air bubbles, left 1″ in the jar, and followed the other instructions. Any ideas why these jars won’t seal?—Jen

A: After Jen asked me this, we had a long e-mail discussion about the various options. She was indeed doing everything right, but we eventually settled on two things. First, her stock was fairly fatty. While this makes for delicious stock, it can make getting a good seal more difficult if fat gets trapped between the lids and the jar. But secondly, and more importantly, this was a case of opening the pressure cooker lid too soon.

But what, exactly, constitutes “too soon”? Most pressure canning instructions, including mine, tell you to wait about 10 minutes after letting the pressure drop of its own accord before opening the lid. Over time, though, I’ve found that this is too soon. A pressure canner full of chicken stock is very, very hot. Even when the pressure drops inside the canner, the temperature and pressure inside the jars is still quite high. The dramatic change in pressure from removing the lid (even after the indicator has fallen) can be enough to cause liquid to spurt or seep out of the jars. This not only causes you to lose some of your precious chicken stock, but it also interferes with the seal. So now, whenever I pressure can anything with a significant amount of clear liquid (beets, tomatoes, stock, etc.), I always wait at least an hour after the pressure drops before opening the lid. Ideally, I do this at night and just turn off the heat and go to bed. The next morning, I take off the lid, and viola! Everything seals.

Jen e-mailed me about a week later with an update. Success!

What: Beets Two Ways Class
Where: Doris’s house, West Philadelphia
When: Saturday, June 26, 1 PM
How to Register: Send Doris an e-mail

See you then!

You’ve got three basic options: freezing, dehydrating, or canning. More ambitious folks can try fermenting, like I did with my bumper crop of bok choi last year. But let’s stick with the basics here.

Freezing Greens

This is the easiest, and probably the best, solution. It’s very, very easy to freeze greens. You just blanch them, cool them in an ice bath, and freeze. I’ve got step-by-step instructions and photos here. Assuming you’ve got a good freezer, these will keep very well for a year. We used ours all winter in soups, stews, pastas, dips, and even just sauteed with garlic. They shrink dramatically when blanched—plan on about a pound of fresh greens to make about two cups.

For most people, this is the way to go. If, however, you have limited freezer space, you live in an area that experiences frequent power outages, or are planning for a future without electricity, you need other options. Alas, they’re not great. But, you asked, so here goes.

Dehydrating Greens

Are you one of those people who likes to sprinkle seaweed flakes over your rice bowl at the local health food store? Then you might like this. Steam your greens just until they’re wilted. Transfer them to dehydrator trays and dry at a low temperature (say, 110ºF) just until they’re crispy. You’ll want to keep an eye on them—within two hours, mine looked like this:

Now what, you may ask, would you do with dehydrated turnip greens? That’s an excellent question. I rehydrated some with a little bit of room temperature water, and they looked pretty good. The problem was that they had no perceptible taste. In the end, I crumbled them up and stuck them in a spice jar:

I sprinkled an entire leaf’s worth of flakes over a bowl of roasted potatoes and couldn’t tell they were there. I suspect that they’re still fairly nutritious, though, so maybe this isn’t such a bad way to incorporate hearty greens into your diet. I guess.

Canning Greens

The things I do for science.

Yes, it is possible to can hearty greens. It requires a pressure cooker and a processing time of 1 hour, 10 minutes for pints and 1 hour, 30 minutes for quarts. Yes, you read that right. And after you’ve subjected these poor, innocent vegetables to 240ºF to more than an hour, you get this appetizing product:

(And that’s the glamor shot.)

The nutritional value of all of this is questionable. They taste about like you would expect them to. If, however, you have absolutely no other options and feel strongly about having a pantry full of shelf-stable vegetables, this technique will serve you well. But Lordy, do they smell bad. Here’s how you do it.

1) Bring about 3 quarts of water to a boil. You need 2 quarts for the pressure canner, plus some extra to pour over the greens.
2) Steam the greens until wilted. Cut them into manageable pieces.

3) Stuff the greens into canning jars. Add 1/2 t. salt per jar (optional) and cover with boiling water, leaving 1″ headspace. Don’t forget to remove the air bubbles. Adjust two-piece lids.
4) Transfer the jars to a rack on a pressure canner. Pour in 2 quarts of boiling water. Lock the lid into place, but do not place the regulator on the steam vent. Turn on the heat and exhaust steam from the vent for 10 minutes. Then place the regulator on the steam vent and bring the canner to 10 pounds of pressure. Start timing once the regulator starts rocking—70 minutes for pints, 90 minutes for quarts. (If this is confusing, see the step-by-step directions, with pictures, in this post.)
5) Turn off the heat and let the pressure drop of its own accord. When the pressure has dropped, remove the regulator. Now open all your windows. After waiting a few more minutes, open the lid away from your body. The pungent aroma of overcooked turnip greens will fill your house, and you’ll have all the shelf-stable greens you want. Plan on 2 to 5 pounds per pint, depending on the kind of greens involved. Um, yum?

Other ideas?

Why is canned food shelf-stable?

When you can something, you’re doing two things. First, you’re killing bacteria, molds, and yeasts through the addition of heat. Second, by creating a seal, you’re preventing new bacteria from getting in.

What’s the difference between pressure canning and water-bath canning?

For the purposes of canning, all foods are either high-acid or low-acid foods. The kinds of food-born pathogens that can live in a high-acid environment can be killed fairly easily with moderate heat, so you can process them in a boiling water bath. Low-acid foods, on the other hand, can harbor botulism spores that are not destroyed at 212°F. Low-acid foods must be processed in a steam pressure canner than can reach much higher temperatures (usually 240ºF).

How do I know if a food is high or low acid?

All fruits except for figs and tomatoes are acidic enough to can in a water-bath canner. Figs and tomatoes can be safely canned this way with the addition of a small amount of lemon juice. Everything else—including vegetables, meats, fish, and mixtures of high acid and low acid foods (for instance, salsa)—either has to be pressure canned or made more acidic. The short answer to this question is to follow the recipe. The longer answer, recommended only for experienced canners, involves comparing fruit/vegetable/acid ratios and densities from trusted sources. Please be cautious when using internet canning recipes, and consider comparing unfamiliar instructions to published guidelines, such as those of the National Center for Home Food Preservation and those listed in the Ball Blue Book of Preserving.

What’s the processing time?

The term “processing time” refers to the amount of time that you either boil or pressure can your jars. The amount of time depends on the food—check your recipe. In a boiling water bath, you start timing when the water returns to a boil. In a pressure canner, you start timing when the canner reaches pressure.

I have a pressure cooker. Can I use it as a canner?

That depends. Small pressure cookers are not necessarily guaranteed to reach and maintain the appropriate temperatures necessary to kill botulism spores. Most pressure canners are 23 quarts or larger—large enough to hold 7 quart jars on a rack, or 14 jelly jars, stacked. If you’re not sure, contact the manufacturer. My attempt to explain the science of pressure cookers is here.

Do I have to sterilize the jars?

If the recipe calls for a processing time of more than 10 minutes, no. Otherwise, yes. In either case, manufacturers generally recommend that you pre-heat your jars to reduce the chance of breakage, particularly before processing in a water-bath canner.

Can I use a dishwasher to sterilize my jars? What about to process them?

If your dishwasher has a sterilization setting, you may use it to sterilize your jars. You may not, however, process your jars in the dishwasher. You need to use a boiling water bath or a steam pressure canner, as per the recipe.

If I have a low-acid food, but I just want to keep it for a few weeks, can I water-bath can it?

No. Food is either safe for water-bath canning, or not. If you do not have a pressure canner but have a food that needs to be pressure canned, you either need to refrigerate it or find some other way to preserve it.

How long will canned food keep once it’s opened?

Once the jars are opened, canned food is just like regular food, with similar keeping times. Something pickled might last months; a highly sweetened jam might last several weeks; and a tomato sauce might last only a few days.

Can I reduce the sugar or salt in a recipe? What about sugar substitutes?

YES. Contrary to what you may have heard, it is perfectly safe to reduce the sugar in a canning recipe so long as you are using an appropriate processing time. (The highly liability conscious company that sells Ball jars backs me up on this.) Sugar is a preservative, which means that food (particularly fruit) with sugar added will retain more of its original color, taste, and texture. It also means that the jars will keep longer once you’ve opened them. You may not get the texture you’re expecting if you reduce or substitute sugar—for instance, your jams might not set. But since sugar does not affect acidity, it doesn’t affect safety. Same thing with salt: the amount of salt in canning recipes is not enough to act as a preservative. It’s there for flavor. If you need to reduce your salt, just leave it out.

How will I know if a jar has gone bad?

A broken seal, a bulging lid, moving bubbles, mold, foam, bad smells, funky texture, and sliminess are all signs that you should not eat the contents of a jar.

Why do you store jars without their rings?

See previous question. If you’ve got an active bacterial population in your jar, they will produce various gasses. If the rings are removed, the pressure inside the jars can eventually build up to the point that the lid pops off. This is your signal, months later when you find the jar in the basement, to not eat it. If the ring is attached, the lid might not pop off. In rare cases, the jar might even explode. On a more mundane level, you should remove the rings because moisture trapped between the jar and the lid will cause them to rust.

How long can you store your jars?

That’s a good question. The USDA generally says 1 year. Many experienced canners will tell you that they fairly regularly keep their canned goods for longer than that, and just as many will tell you that food begins to lose its flavor much sooner (say, 6 months). Since the whole point of preserving foods is to hold you over until the next year’s harvest, shoot for a year.

Got more questions? Bring ‘em on!

Here’s my logic. Onions are a low-acid food. To make them safe for water-bath canning, you have to add sufficient vinegar to drive the pH below 4.6. The question is whether there is enough vinegar in this recipe. If you have pH strips handy, then you could just find out, and proceed accordingly. If you don’t, however, you have to do some sleuthing.

Two things made me nervous. First, while Joy of Cooking includes a similar recipe for what they call “red onion marmalade,” it’s not included in their list of condiments safe for canning. But it was what I found in the Ball Blue Book that really convinced me. The Blue Book includes two different recipes for preserved onions: one for water-bath canning, and one for pressure canning. Their red onion marmalade, safe for water-bath canning, has a ratio of 1 1/2 c. onions to 1/4 c. vinegar, plus some orange peel and a whole lot (4 cups!) of sugar. The red onions with honey recipe, on the other hand, has 2 1/2 quarts onions, 1/2 c. wine, and 1 c. honey, and must be pressure cooked. Serious Eats’ confiturra has about 10 cups of onions, 3/4 c. vinegar, and 1/2 c. wine, but by the Blue Book logic, you would need 1 2/3 c. vinegar for 10 cups of onions. Given that the Blue Book is known to be conservative, it strikes me as within the realm of the possible for 3/4 c. of vinegar to be enough, but since I’m giving these as gifts, I wanted to play it safe. Fortunately, since this is a heavily cooked product anyway, there’s no harm in pressure canning the jars.

All of which is a long way of saying: if you want to preserve a low- to moderate-acid onion confit, pressure can the jars at 10 pounds for 15 minutes. If you don’t have a pressure canner, they’ll keep in the refrigerator for at least a month. As a reminder, I’ve got step-by-step pressure canning instructions here.