Tomatoes, Acid, and Heat: The Science of Canning

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[Photographs: Vicky Wasik]

Editor's Note: Please welcome Christina Ward, a master food preserver for Milwaukee County in Wisconsin and author of Preservation: The Art and Science of Canning, Fermentation and Dehydration. In this series, she'll break down the science of and principles behind food preservation, with accompanying recipes so you can put your newfound knowledge to work.

In this series on the science of food preservation, we've thus far discussed the chemistry of preservation, namely, the reduction of water activity and the power of acidification. Sure, I've tossed around the word "canning" in those articles, but I haven't really explained it. Well, here we go, friends. The most powerful tool in your preservation toolbox, the one that gives you the ability to defy the known limits of physics:* water-bath or pressure canning. Even if, like me, you just squeaked by in Mr. Klabunde's junior-year physics class, I promise, the power of temperature and pressure is at your fingertips.

* And by "defy the known limits of physics," what I mean is "use physics as we know it to your humble advantage." But that doesn't sound half as cool, does it?

All preservation techniques are, in scientific parlance, "hurdle technologies," which refers to the barriers we can erect to prevent microbial growth in our foods. The more hurdles a microbe has to clear in order to survive, the less likely it is that any microbes will.

The first two hurdles we looked at in this series—reducing water activity and raising acidity—are chemical ones. Adding the physical hurdle of high temperature allows us to extend the usefulness of our foods even further. Something that's pickled will last for a few months, but when you combine pickling with the high heat of the canning process, that period of time jumps up to 18 months. In fact, heat can be so effective that, depending on what method of canning you've used, it can work even if you don't erect those chemical hurdles, like acidification.

In order to survive and thrive, most microbes need a handful of things: food, water, oxygen, and an environment that's in a hospitable temperature and pH range. Canning methods perform a couple of specific tasks. First, they raise the internal temperature of the contents of a jar, killing microbes in the process. Second, they force out air and gases from the jar, which creates a vacuum that causes the lid to seal tight, preventing air from getting back in. This deprives those microbes of the air they need to survive. (There's one important exception to this: the notorious anaerobic bacteria, which thrive in oxygen-free environments. We'll get to that in a bit.)

The tools used for canning, including immersive boiling-water baths, atmospheric-steam canners, and pressure canners, all do the same basic thing, but with differences in degree and effectiveness. A pressure canner is like a sledgehammer, while boiling-water and atmospheric-steam canners are more like teeny-tiny tack hammers. You must choose your tool wisely: Some foods require a great wallop to render them safe for longer storage, while others do not.

How do these tools work? I've got two answers for you: a simple summary, and a deep scientific dive.

How Do Canners Work? Take 1: The Simple Summary

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When you set a lidded jar into a canner (whether it's a boiling-water bath, a steam canner, or a pressure canner), the contents of the jar are brought to the boiling point, creating pressure that drives out air. When removed from the canner, the jar cools to room temperature, causing the pressure inside the jar to drop and a vacuum to form. That vacuum pulls the lid down more tightly, creating an airtight seal between the lid's rubber gasket and the jar. Boiling-water and atmospheric-steam canners do that job at whatever the atmospheric boiling point is; at sea level, that would be good old 212°F (100°C). A pressure canner uses a pressurized environment to raise the boiling point higher than the atmospheric boiling point, reaching temperatures upwards of 250°F (121°C). Even the toughest microbes can't survive that.

How Do Canners Work? Take 2: The Dissertation (Feel Free to Skip This Part)

Want to know what's actually happening in those canners? Let's ask a rocket scientist (and my cousin), Dan Heil. He emailed me this recently:

At room temperature and sea level, the pressure outside the pot is 1 atmosphere (14.7 psi, or 1 bar). The pressure inside the pot is the same, 1 atm. The pressure in the jar is 1 atm, because your jar was open to the room before putting the lid on. That's all pretty straightforward—no "Delta P," as we engineers would say; no difference in pressures.

When a lidded pot of water is boiling (i.e., a boiling-water canner), the air pressure outside the pot is still 1 atm. The pressure inside the pot is slightly higher due to the weight of the lid. You could calculate the pressure differential as pounds per square inch (psi), if you knew the weight of the lid (in pounds) and the area of the pot opening (in square inches). A jar submerged in the lidded pot of boiling water, once boiling, will be at the same pressure (actually, minusculely higher due to the water depth) as the air/water in the pot, because it's venting into the pot. What we need to do is refer to the ideal gas law. PV = nRT: Pressure x Volume = n (quantity of gas) x R (gas constant) x Temperature.

Both sides of an equation have to be equal. But that doesn't mean they have to stay constant. When you raise the temperature of the water, and the volume (in the pot or in the jar) stays constant, the pressure has to increase. Which it does slightly in our boiling-water canner, but then it vents to the outside, so the pressure never gets above 1 atm (plus the pressure due to the lid). Therefore, the pressure is a known constant. So the volume is constant, the pressure is constant (but slightly higher), but the temperature is higher, so something has to give. Which means "n" changes—the quantity of gas changes. We know this because we see it escape from the pot as water vapor, and we see it as air boiling out of the jar. So "n" decreases due to the pressure of the gases in the jar.

Our hot-water system has a constant V; a constant P; a constant, higher T; and smaller "n." Now we take the jar out of the pot, and it cools. V is the same constant volume in the jar; T has gone back down to room temperature; "n," as we just established, is now smaller. Therefore, P (the pressure) in the jar has to decrease to make the formula equal. So P-room is 1 atm, and P-jar has now become less than 1 atm due to the lack of gas inside it (i.e., the vacuum), and that difference in pressure is what holds the lid on, making it airtight.

The atmospheric-steam canner is basically the same as a hot-water canner, except it uses steam to heat the jars. Steam carries more energy than hot water due to the change from water to vapor—the energy to go from water to steam is big compared to raising the temperature of the water from cold to hot. The steam condenses on the cooler jar, giving its heat to the jar; drips down; and gets reheated. But the jar gets hot, the contents boil, the gas in the jar vents, and we have the same basic process as in the boiling-water canner.

As for the pressure canner, at room temperature and sea level, it too starts out at a pressure of 1 atm (14.7 psi). When the pressure canner is heated, the pressure inside increases to about 2 atm (about 30 psi, which the canner gives as a gauge pressure of 15 psi). The pressure canner vessel is strong and holds that pressure inside the pot. The inside of the jar is at 2 atm because it's venting into the inside of the pressure canner; they must be equal, or the jar would break.

Going back to PV=nRT. The pressure (P) is now 2 atm (30 psi). The volumes (V) are constant. The temperature (T), like before, is also higher. But it's gone past the 212°F we'd expect at sea level because we've increased the pressure. Remember, both sides of the equation must be equal, but that doesn't mean they have to be constant. At 2 atm, the temperature is able to climb to around 240–250°F. That's high, but it's not twice as high as the pressure is. So how does that work? The volumes in the pressure canner and jar are constant, so something has to change, and, just like in the boiling-water bath, it's the "n," the amount of gas.

When the jar is hot, it's at the same 2 atm as the pressure canner. As the pressure canner cools and depressurizes, so does the jar, but they depressurize at different rates, which causes the jar to vent into the pot. Eventually, the canner and the jar are cooled. When you open the canner, it returns to the ambient pressure of 1 atm. But because you don't depressurize the jar, it stays at < 1 atm (less than 1 atm). VoilĂ ! Vacuum-sealed jar.

Due to the higher temperatures enabled by the high-pressure process, 240–250°F, you not only have a sealed jar, but you have a sealed jar with a wasteland of dead pathogens.

Thanks, Cousin Dan!

Choosing the Right Canner for the Right Job

How do you know which method of canning to use? Foods that are high-acid, with a pH of less than 4.6, can be processed in a boiling-water-bath canner. That canner is less effective at obliterating microbial life, but the acidity is an added hurdle that will make the food safe to eat. Low-acid foods must be processed in a pressure canner, though, because they're at much higher risk of infestation by the deadliest of bacteria, the anaerobic family.**

** One thing to keep in mind is that a pressure cooker is not necessarily a pressure canner. The ability to manually regulate the pressure is the defining element of a pressure canner. If your pressure cooker has a weighted regulator (more on that below), then it can be used for canning. I promise you that regardless of what the button says on the Instant Pot, it is most definitely not a canner. In a nutshell, there's no way to regulate the pressure on a multi-cooker, so don't try it. The National Center for Home Food Preservation was so alarmed by the popularity of using these for canning that it put out an alert.

The reverse is also generally true; that is, it's best to use pressure canners for canning only and not cooking. Canning folks tend to abide by this rule, in part because cleaning chicken cacciatore out of a vent hole is no fun.

Anaerobic bacteria are a class of pathogens that don't require oxygen. In fact, they thrive in low-oxygen environments. Unfortunately, they also produce the deadliest of toxins. Though I haven't yet mentioned the "B" word, you can see it coming...botulism! (What, you thought I was going to jump from a closet and yell "Boo!"?) The Clostridium botulinum spore survives the standard boiling temperature of 212°F, and it doesn't need oxygen. What else is in the anaerobic clan? Multiple iterations of E. coli and staphylococcal bacteria. Most people I know get truly scared when a trickster leaps from a closet yelling "Boo!", but if our rational minds were more in control, we'd be way more freaked out about the damage these bugs can do.

(Remember, it isn't the bacterial spore that kills you but the toxin produced by those bacteria. Luckily for humankind, the toxins are killed at 212°F. Ever wonder why so many recipes prior to the advent of refrigeration required long cooking or boiling times? It's because our forebears knew that you needed to cook the hell out of all your food to ensure it was detoxified. Let's put it this way: Casseroles and stews saved many, many lives.)

So far, what we've established is that high-acid foods can be safely canned using a basic hot-water canner or atmospheric-steam canner, and low-acid ones are rendered safe only with the added high-temp power of a pressure canner. But what if you're unsure of where the food you're dealing with lands on the acidity spectrum? What if...you want to can tomatoes?

You Say Tomato, I Say, How Acidic?

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Love apples, wolf peaches, 'maters; whatever people call them and however they eat them, people have feelings about tomatoes. They're one of the most commonly preserved foods, yet cooks are often confused as to the correct and safe methods for preserving them. Are they acidic enough to prevent microbial invasion? Do you need to add acidity? If they're low in acid, should they be pressure-canned? Should I just freeze them? Halp!

Looking into the development of the modern tomato helps understand our confusion and gives us a path forward. The earliest wild tomatoes grew throughout Central and South America, and ranged from smallish yellow globes to the larger, redder fruit native to the foothills of the Andes Mountains. Traders spread tomatoes throughout the region, but it was the Aztec alliance, which ruled Mexico and Central America between the 13th and 15th centuries, that began hybridizing wild plants to bring us closer to our modern tomatoes. When the Spanish conquistadors brought the treasures of the New World back to Europe, tomatoes were among the looted prizes.

The conquistadors kept on conquering, and spread tomato seeds wherever they roamed. At first, many assumed tomatoes were poisonous. It took about 200 years (and a number of inedible decorative varieties) before tomatoes were finally accepted as a desirable food. By the mid-1800s, nearly the entire world was in on the tomato craze. In Italy, the noble houses grew tomatoes and began to select for preferred traits of intense flavor and dense flesh. Elsewhere, other varieties were created.

In the span of a few hundred years, the tomato of the Aztecs morphed into thousands of varieties literally all over the world. Today, we have over 7,000 types of tomatoes, with a vast array of distinguishing characteristics. There are tomatoes that are fleshy, juicy, big, tiny, red, yellow, green, and even purple. Each and every variety has a different amount of naturally occurring acidity, which means that tomatoes span a range of pH measurements. Wait! It gets more complicated. Tomato acidity also varies depending on how ripe the fruit was when it was picked, and how much more it has ripened since. So what is the pH of tomatoes?

Because science is based on researching crazy questions like this, food scientists have investigated and come up with a deeply detailed, highly accurate, and very scientific answer: They're kinda acidic, but still not quite enough, especially if you're using heirlooms and mixing a bunch of them together. Very frustrating!

The pH scale ranges from 0 to 14, with 7 being neutral. Below 7, things become increasingly acidic; above 7, they grow increasingly basic, or alkaline. Any food with a pH below 4.6 is considered to be safe from a food-preservation standpoint. The tricky thing with tomatoes is that it's hard to know which side of that pH 4.6 line they're on.

In a 2010 study from Utah State University, scientists tested a locally grown mix of common commercial and heirloom varieties of tomatoes. This "market basket" of tomatoes ranged in pH from 3.7 to 4.3; those numbers indicate that the tomatoes were acidic enough to be safe for canning, even without a pressure canner.

But an earlier, 2004 study from the North Dakota State University extension services office tested multiple iterations of a mixed pulp of 15 tomato varieties, and reported a pH range of 4.8 to 5.2—decidedly not acidic enough for safe water-bath canning.

Out in internetland, there are lots of self-proclaimed food preservation experts who claim tomatoes do not need to be acidified before canning, or that they don't need to be pressure-canned. They cite folk wisdom and even cherry-pick supporting data from studies like the ones I just referenced. Again, here's what the research says: Tomatoes are sometimes acidic enough to resist microbes, and sometimes they're not. And, unless you test every single tomato for acidity prior to use, you can't know how acidic it is.

Here's the phrase that makes my eye twitch and stare off into the distance like Clint Eastwood: "My mom/grandma/Aunt Bertha/Uncle Stinky didn't add acid or pressure-can their tomatoes, and we never got sick." Great. Good. I'm glad you're still with us among the living. But what if the unthinkable happens? What if, the next time you can whole tomatoes like Granddad did, someone gets deadly botulism? Is it likely? Probably not, but just the risk of it happening should be enough to give you pause and do it the correct way. To quote the 80-something-year-old master food preserver from Oconto County, Wisconsin, who assisted in my training: "One day, God is going to get tired of watching over you."

Every step we take in preserving our food is about making and keeping food safe from harm, so that we can joyfully feed our friends and families. Is adding a splash of lemon juice or a dose of citric acid—two things that will boost the acidity of your tomatoes to a safe level—really so offensive that people are willing to risk poisoning their families? Pardon me while I get a little shouty: You have to disrupt the chemical composition and physical environment of your food to extend its edibility and prevent pathogen infestation!

Now that we've put the myths of tomatoes' acidity firmly to rest, we can get on with the good part: how to preserve them.

How to Preserve Your Own Tomatoes, Without Killing Yourself (Yay!)

Preserved tomatoes are a pantry staple, but you don't have to buy them at the supermarket; you can preserve them yourself. The benefits are easy to see: cost savings, flavor and ingredient control, and, of course, the pleasure of eating your home- and locally grown tomatoes long after the garden has finished producing. Heck, with store-bought canned San Marzanos going for nearly $4 a quart at the supermarket, growing and canning your own makes sense.

Today's recipe and methods focus on canning whole tomatoes using two different techniques: hot-water-bath canning and pressure canning. Both methods have their advantages and disadvantages for the home preservationist.

Any type of tomato will work for whole preserving, but plum varieties work best. In the garden or at the farmers market, you're looking for tomatoes that are ripe and unblemished. Fungus, molds, and other microbes that have invaded a damaged tomato can reduce its already-borderline acidity, putting you at even greater risk. Late tomato blight is the scourge of home growers; if your crop has it, cut off the diseased portion and use the remainder for other tomato projects. I save those tomatoes for sauce and use only the best-looking ones for whole canning.

Speaking of sauce, you can and should make and preserve your own tomato sauces, juices, and even Bloody Mary mix! Recipes for those vary, depending on what's in the sauce. Juices and very basic sauces can be acidified and then processed in a hot-water bath, while complex sauces with other vegetables often require pressure canning. This is why you don't really want to can using any old sauce recipe: Each is different, which means the pH is different, which means the acidity and heat-processing requirements will be different. Following a sauce recipe that's been calibrated for safe canning is therefore always your best bet.

For now, canning whole tomatoes makes a good and useful introduction to both longer, boiling-water processing and pressure canning.

Step 1: Peel Tomatoes

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The preparation for the tomatoes is the same regardless of which method you choose (and we'll talk about those in just a sec). You've got to peel those tomatoes. I know—everyone hates doing this, but you have to do it. The skins tend to have a bitterness and toughness that affect the final taste of your product. Prize-winning tomatoes are always skinned.

The easiest way is to slice an X at one end of each tomato, then drop them into boiling water for about one minute. Remove and transfer to an ice bath to cool. When they've cooled enough for handling, slide the skins off and plop the naked tomatoes into a bowl. (Save those skins! Dehydrate them until they're totally dry, then grind them into tomato powder for use in other dishes.)

Step 2: Add Acid

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Starting with clean quart jars (these should be washed with hot soapy water and rinsed, or run through the dishwasher), place two tablespoons of lemon juice or half a teaspoon of citric acid in each. This is the magic quantity—two tablespoons of lemon juice or half a teaspoon of citric acid per quart of tomatoes—that will guarantee that even the lowest-acid tomato is safe for canning. (The Center for Home Food Preservation recommends adding a touch of acid to tomatoes regardless of the canning method, so, even if you're pressure-canning, go ahead and give those tomatoes an acid boost.)

Step 3: Add Salt (Optional)

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Add a teaspoon of 100% pure sea salt or kosher salt; this is optional and for flavor only. Some people like sweeter tomatoes and so will add a teaspoon of sugar. Those people are monsters.

Step 4: Pack the Jars

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Gently place and pack the tomatoes into the jars. You want them packed tight, but not squished, so that they retain their shape.

Some tomatoes may split, which is perfectly okay from a safety perspective; aesthetically, we're striving for a jar of beautiful red ovaline tomatoes floating in their own juice. Remove any air pockets, and use the remaining liquid from the tomatoes (with added water, if needed) to cover the tomatoes in the jar to half an inch of headspace.

Step 5: Close Jars

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Wipe the rims of the jars, place the lids on top, then screw on the bands just until they catch. This is called "finger-tightening," the idea being that if you tighten the lids too much, air can't escape, defeating the entire purpose of all this work! Not to mention that pressure can build in the jar and cause it to crack or "blow out" the bottom, which makes a huge mess in the canner. So resist the urge to make the lids extra tight! It's the eventual vacuum that will seal your jar against air and pathogens, not your superhuman grip strength.

Step 6: Canning Time

Atmospheric-steam canners aren't an option here, because they have to be refilled with water periodically. This makes them useful only for high-acid canned foods that require a minimal processing time, less than 20 minutes. Tomatoes take longer than 20 minutes, so a steam canner is out. You're left with two choices: a boiling-water canner or a pressure canner.

For boiling-water canners, the processing time is 85 minutes. In a pressure canner, the processing time is 25 minutes at 10 psi. Sure, the venting time adds another 10 minutes, but it's still far less than nearly an hour and a half. Aside from the fact that pressure canning saves valuable time, the long processing time for boiling-water canning may result in a mushier final product. Your mileage may vary.

Here's another benefit of using a pressure canner: You can add things to those tomatoes. If you'd like to do Italian-style whole tomatoes with a garlic clove, onion, oregano, and basil leaves—go for it! By using the pressure canner, you're already processing the food as a low-acid item, regardless of the added acidification. (Always double-check the processing time of any additions, and process the entire contents of the jar according to the needs of the longest-processing item.) Use of a boiling-water canner is predicated on the food being acidified, so anything that lowers the acidity pushes the tomatoes into the danger zone.

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Boiling-water canners, though, are the easiest to use. Any stockpot can be a boiling-water canner. It just needs a lid, and a rack at the bottom so the jars don't touch the metal (allowing the heated water to circulate), and it needs to be tall enough to cover the jars by two inches. That's it. (Ball makes a nifty thing called the Canning Discovery Kit, which includes a silicone basket, a jar lifter, and some other doodads. It turns any pot into a canning pot.)

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Fill the pot halfway with water, cover, and turn on the heat. Bring the water to a vigorous boil. When it's rollicking and rolling, take off the lid, and carefully lower the jars into the boiling water (use a jar-lifting tool for a secure grip). The jars will displace the water in the pot, raising its level, which is why you don't start with a full pot of water. Your goal is for the jars to be covered by two inches of water; if they're not, add a bit more boiling water from a kettle to top up the pot.

Put the lid back on, and start your timer when the pot is back to a hard boil. After the prescribed time is up, carefully remove the jars and place them on a counter or table to cool. Jars may tip over as you try to fish them out—don't panic and stick your hand into the boiling water! I've had to stop people in my classes from doing that. Just pick up the jar with the lifter, and set it down to cool. A jar that's tipped over may not seal properly, but all is not lost! Stick it in the fridge, and use it within a week.

Always think about safety and move with purpose. No shorts. No flip-flops. Boiling water will easily cause second- and third-degree burns. No kitchen project should ever end in the emergency room.

Pressure canners have an unfair reputation for being dangerous and difficult to use. All lies. They're just power tools, albeit kitchen power tools, and need to be treated with the same respect as a chainsaw or MIG welder. Which is to say, if you use it foolishly, you can cause damage to the machine, your stove, your kitchen, and maybe your body. If you use some common sense and follow the rules, all will be fine.

Fill the pressure canner with three to four inches of water, and place the jars on the canner's rack. The water won't cover the jars, and that's perfectly okay, since the pressurized steam will be doing the work. The temperature isn't random; it's calculated by determining the type of food and the temperature required for killing microbes. Depending on the pressure, you can reach anywhere from 230 to 250°F (110 to 121°C)—these are temperatures that can kill any microbial life in under a minute. Totally obliterate it. Processing time is important, as the microbes will not be wholly annihilated unless the center of the jar reaches that elevated temperature. That's why pressure-canning instructions are explicit in giving two critical pieces of information required to render a home-canned product safe from microbial invaders.

Let's talk for a moment about the two styles of pressure canners here: weighted-gauge and dial-gauge canners. Most pressure canners have a weighted gauge. This is a heavy metal disk, two inches in diameter by half an inch wide, with three equidistant holes drilled into the edge. Each hole has a slightly different size bore and is marked with the number 5, 10, or 15. Those numbers describe the psi the canner will reach above atmospheric pressure. So if you are at sea level, where the atmospheric pressure is about 15 psi (1 atm), then setting your canner to 15 will take its internal pressure up to about 30 psi (2 atm).

When placed onto the valve stem, the weighted gauge controls the amount of pressure generated in the canner. These doohickeys are called by a number of names; "jiggler" and "regulator" are the most common. When the canner reaches the desired pressure, the jiggler rhythmically rattles back and forth.

Dial-gauge canners have a pressure gauge in the form of a dial that the operator (you!) has to monitor, adjusting the heat to keep the pressure consistent. Clearly, there's room for error there. Because of this, pressure-canning instructions will state explicitly that if you're using a weighted gauge (jiggler), you need five pounds of pressure, but if you're using a dial-gauge canner, you need six pounds of pressure. This one-pound increase is to account for human and dial-gauge inaccuracies.

High-quality canners have both: a jiggler to regulate the pressure and a dial gauge to read it. The canner still needs vigilance. Pressure canning is not a set-it-and-forget-it activity. Slight heat adjustments may be needed. Generally, once the canner has been vented and reaches its final pressure, you can do other kitchen tasks while it does its thing.

Did you catch the lingo word, "vented"? A step in the process that's specific to pressure canning requires that you bring the canner, with its lid locked in place, to a boiling temperature—enough to see the steam come out of the vent in a steady stream for 10 whole minutes. People are inherently lazy and hate doing this, as it adds another 10 minutes to the process. But do it you must! The goal is to get the center of the jar of tomatoes to the necessary temperature. Without those specific steps of venting and then pressurizing, there will be air pockets in the canner that have not reached the final temperature.

Back to the two critical pieces of information given in a pressure-canner recipe. First is the amount of pressure required, which will change based on the type of food. Broadly, fruits and vegetables are processed at five pounds (six with a dial-gauge canner), while meats are processed at 10 pounds (11 with a dial-gauge canner). Canners can be pressurized to 15 pounds, but this is usually done to account for increases in elevation; one pound of pressure is added for every 1,000 feet of elevation. (Pressure-canning tomatoes in PotosĂ­, Bolivia, would require 14 pounds of pressure.)

The second critical piece is the processing time, which varies based on the size of jar used. A pint of tomatoes needs less time than a quart to get the contents in the center to 240°F (116°C). Do not fudge these processing times! It makes a difference in the safety of your final product.

When the timer goes off and processing time is complete, turn off the heat and remove the weighted gauge/regulator. Use a hot pad, and move with speed and purpose—that's 230°F+ steam shooting out that hole! I've had a couple of steam burns, and they are not pleasant. Patience is a virtue, because the pressure in the canner must reach zero before you try to open it. If you impatiently try to pry that lid off, you're in for a heap of trouble. Remember: It's pressurized, so as soon as the clamps are off, the lid becomes a force projectile. That's how kitchens are destroyed and feet broken. Any other clever idea for accelerating the cooling of a pressure canner is a bad idea. Just let it do its thing.

The contents of the jars will be visibly boiling when you do finally open it. Let these rest in the canner for about 15 minutes before transferring them to a towel on your counter or table. The high temperatures and change in pressurization can siphon the liquid out of the jar and into the pot, and you may notice the jarred tomatoes are not fully covered by their liquid. This is okay and safe, based on the fact that you raised the temperature, drove out the air, and sealed the jar.

Step 7: Do Your Final Inspection

Do the final jar inspection after 24 hours: Are the lids sealed? Check this by pressing gently on them. (This is different from the tightness of the screw-cap ring, which is not what holds the lid on tight; what you're confirming is the presence of a vacuum.) They should be taut, with zero give. Is the jar more than halfway filled with liquid? Good. You did it! These canned tomatoes will be viable for 12 to 18 months on your pantry shelf.

Storage Tips

If at any time you notice black or white growth inside the jar on the food or at the seal, consider the tomatoes bad, and don't eat them. You can either toss the entire jar into a plastic bag and throw it away, or boil the heck out of it for 10 minutes to detoxify before putting it into the compost. Do not flush it, put it down the garbage disposal, or even feed it to raccoons without detoxifying it. You don't know what's growing in that jar—you can't see, smell, or taste botulism—and you don't want to poison the water supply. The best way to prevent botulism from developing is to correctly follow the procedures.

None of this sounds very much like cooking, because it's not—it's machine operations. Here's the secret: The pressure canner works in this very same way regardless of what you're preserving. The reward: all the best food at its seasonal peak, any time of the year you want it. Even if you stick with a boiling-water-bath canner, I promise you that once you've sampled your home-canned tomatoes during winter's long bite, you will become a preserved-foods convert.