Cookbook:Canning

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Canning is a method of preservation that sterilizes and seals food in airtight containers.

General principles

Preventing spoilage

The ultimate purpose of canning is to preserve the safety and quality of foods for an extended period of time at ambient temperature. Several types of spoilage can happen to foods over time. The first—simple discoloration and the development of off-flavors—is relatively harmless but typically still undesirable. These issues are commonly caused by enzymatic activity and exposure to ambient oxygen in the environment. A more concerning type of spoilage involves the growth of microbes that, if consumed, can cause significant illness. These include 'bad' molds and bacteria, the former of which is generally visibly apparent, and the latter of which can be harder to detect. Mold takes a little while to grow, but bacteria can grow very rapidly—on the order of hours—and colonize contaminated food. Conditions favorable to this kind of spoilage include moist, nutrient-rich environments at ambient temperature.

One of the most dangerous types of spoilage, especially with respect to canning, is botulism. Spores of Clostridium botulinum bacteria are widely present in the environment, including in agricultural products, and they normally pose little to no danger to healthy adults. However, when the spores are left in a moist, low-oxygen, low-acid environment around ambient temperature (all conditions sometimes found in canned or other foods), they quickly multiply and produce a very potent and dangerous neurotoxin that can be rapidly fatal if ingested.

Heat and acid

To prevent moisture loss and off-flavors, it is simple enough to seal foods in a jar with little to no air, but this doesn't address microbial spoilage. Currently, the two components used in canning to prevent such microbial spoilage are heat and acidity. Boiling food for a prescribed amount of time (specific to a given recipe and altitude) is usually sufficient to kill most spoilage bacteria and molds (e.g. Campylobacter, Listeria, Salmonella, yeast etc). However, it is very difficult to kill botulism spores at the temperatures achieved by boiling—it would take hours and hours of continuous boiling to do so, which is not feasible for reasons of both practicality final food quality. If you want to destroy botulism spores, you must use a pressure canner to achieve high enough temperatures (upwards of 240–250°F/115–121°C) that will eliminate them in a reasonable time frame.

Now, with acidic-enough foods, it is possible to eliminate the danger of botulism with just boiling water and not a pressure canner. Because the botulism bacteria cannot grow in a high-acid environment (i.e. uniformly and well below a pH of 4.6 throughout the food), it does not matter if the spores are not destroyed; the bacteria cannot grow and produce the botulinum toxin. Note, still, that other spoilage microbes aside from botulism bacteria are not inhibited by the same acidic conditions alone, which is why acidic foods must still be processed in boiling water. And, heating the jars can help drive acidity into the individual components of a mixture, ensuring even acidity throughout. Furthermore, growth of some other microbes can reduce the acidity of their environment and make a food vulnerable to botulism even if it started off acidic enough to confer protection. Some unreliable sources may say that botulism bacteria growth can be prevented with sufficient sugar or salt, but this is incorrect with the concentrations of sugar in salt used in canned prepared foods. While high sugar and salt can suppress some microbial growth at the concentrations used in some preparations, they do not protect against all microbial growth, and least of all botulism.

Generally, foods to be canned are classified as low-acid or high-acid. Examples of low-acid foods are most fresh vegetables, dairy, meats, and seafood, as well as products made with these. Examples of high-acid foods, which can usually be boiling water-canned, include many (but not all) fruits, fermented pickles, and fruit preserves (e.g. jams and jellies). In some low-acid foods, recipes call for them to be acidified—that is, they have acid added to them (often in the form of lemon juice, citric acid, or vinegar) to bring them to safe levels. Common examples include figs, tomatoes, tomato sauces, and salsas. Interestingly, though tomatoes are commonly thought of as acidic, there is enough variation among them to require acidification for ensured safety.

According to the USDA, one way to destroy existing botulism toxin and make it less dangerous is to boil food for 10 minutes below 1,000 ft of altitude, adding an additional minute per 1,000 ft above that point; however, this should not be construed as a recommendation to consume foods you suspect to be contaminated with botulism. Rather, it is for harm reduction purposes only.

Key Temperatures in Canning

Temperature	Description
170–190°F (76–87ºC)	Most enzymes are inactivated.
180–212°F (82–100ºC)	Many microbes are killed (with sufficient time and/or acidity).
212°F (100ºC)	Boiling point of water at sea level
240°F (115ºC)	Required temperature to destroy microbial spores in low-acid foods; achievable with a pressure canner only

Proper sealing

In addition to sterilizing the contents, heat processing of jars helps create a vacuum seal when used with the correct lids. As the jars heat up, the contents expand, forcing air out. Then, as the jars cool, the contents contract, pulling the lid to the jar and creating a tight vacuum seal. This reduces the available oxygen in the jar that could react to produce off-flavors, and the seal prevents any microbes from entering the jar.

Ingredient prep

It's important in canning to prepare the ingredients exactly as described in a recipe, because the proportions and sizes are often important for precise acid balance and heat penetration. For example, ingredients should be cut up into the exact shape called for in the recipe—if a recipe says to cut something in slices, do not cut into spears. Now, good recipes from reputable canning sources will have some room for error, so you don't have to worry about whether your chop size differs slightly from the chop size of the recipe designer, and it's okay if the recipe uses volumetric measurements, which are inherently imprecise for some solids. Still, for safety, be as careful and precise as you can when measuring.

Jars and lids

The most tried and true containers for modern home canning are sturdy glass mason jars with two-piece metal lids, and these are the recommended jars by the USDA. The jars come in a variety of sizes, with both regular and wide openings. The lid consists of the lid/disc itself, which is the part that makes contact with the jar mouth, and the band, which hold the lid against the jar. The lids have a sealing compound around the edge, which forms a gasket and is what helps create an airtight seal upon cooling. Most modern lids should also have a raised "button" in the center, which should stay depressed once you have a proper seal and pop up when the seal breaks. The jars and bands may be reused if they are free from rust, nicks, and cracks; the disc component is single-use since the sealant will not form as good a seal when reused. Old discs may have degraded sealant and may not seal properly.

In some regions, commercial-style jars with single-piece lids and/or lug lids may be available. These are not currently recommended by the USDA for a few reasons. First, mason jars are typically thicker and sturdier than the commercial jars, which makes them better able to stand up to repeated use and the physical stress of canning (especially under pressure). Second, it is easier to assess the integrity of the lid seal with two-piece lids, since the bands should be removed for storage, and it's very evident when the seal has failed. With single-piece lids, there is no reliable way to truly assess the lid seal without unscrewing and breaking it.

Some manufacturers sell removable-gasket lids in two- or three-piece styles. In these styles, the rubber gasket is not physically bonded to the lid, instead being carefully applied underneath it. Tattler lids function in three pieces: the flat (made of plastic), the gasket ring, and the screw band—they function largely like two-piece metal lids and are accepted for use. Weck jars and lids also have a removable gasket, but they do not screw down. Instead, the lid is held on during processing by clips, and these clips are removed after processing in order to check for a seal and store. Note that Weck-style jars are neither officially recommended for nor against by the USDA.

Wire bail jars may also be available, but these are not currently recommended by reputable authorities such as the USDA. The commonly stated reason is that there is no good way to check for a seal. Novelty jars with non-standard sizes and shapes are also not recommended due to their unusual heat distribution and potential deviation from safe processing.

Before using canning jars and lids, check them carefully for defects. Jars should have no cracks or nicks, especially around their sealing surfaces. Bands and discs/flats should have no rusting, denting, warping, or other bending. Gaskets should be smooth and flexible, with no indentations, tearing, or crumbling. Make sure the jars, lids, discs, and gaskets are all compatible with each other.

• 
  Two-piece lid mason jars. Wide-mouth style on the left and regular mouth to the right.
• 
  Single-piece jar (lug thread)
• 
  Weck-type jars and lids
• 
  Bail-style jar (not recommended)

Pre-cleaning and sterilization

Before use, canning jars and lids must be cleaned and, in some cases, sterilized. Always wash jars and lids to make sure there is no dust or residue on them. Some sources state that the jars must then always be pre-sterilized in boiling water, but this is not necessary if the jars will be boiling water-processed for 10+ minutes or processed in a pressure canner. Regardless, the jars should be preheated and kept hot to reduce the chance of thermal shock and breakage when filling with hot food; this can be done in the dishwasher or in simmering water. Check with the manufacturer's directions regarding prehating the lids. Ball, for example, says not to preheat their lids.

Filling

Tool for measuring headspace

Using a specialized funnel to fill jars

Magnetic tool for lifting lids

Once the food to be canned is prepared, it is packed into the cleaned and warmed jars, either using a raw pack or a hot pack. In raw packing, the completely unheated food is placed in the jar before the liquid component is added. Conversely, in hot packing, the food is brought to a boil, simmered briefly, and packed while still hot. Hot packing forces some air out of the food, which helps prevent it from floating and reduces oxidative damage after storage. It also shrinks the food somewhat, allowing more to fit in the jar. Raw packing is generally better suited to pressure canning, while hot-pack is best for boiling water canning.

When filling jars, it is important to leave enough space between the surface of the food and the bottom of the lid—this is called headspace. Without sufficient headspace, the contents of the jar may expand and bubble out from under the lid, leaving residue and preventing a proper seal. On the other hand, having excessive headspace can result in too much remaining air (and resulting discoloration), an insufficient vacuum, or an excessive vacuum causing gasket damage, the latter two of which can both cause failure to seal properly. Typical headspace is ¼ inch (6 mm) for juices and fruit preserves; ½ inch (13 mm) for fruits, tomatoes, and pickles in liquid; and 1–1½ inches (2.5–3.8 cm) for meats and vegetables, but consult your recipe for specifics.

Once filled, solid-liquid mixtures like vegetables or fruits in water or syrup must be de-bubbled to release any air trapped among the solid pieces. To do this, simply insert a flat, nonmetal tool like a small spatula into the jar, and gently move it up and down while turning the jar. Sometimes, if significant air is released, the headspace may need to be adjusted to bring it back up to the required level—this is done with the same liquid used to fill the jar initially.

After finishing all adjustments, the rim of the jar is wiped with a damp towel to remove any residue, and the lid is applied. For disc-and-band lids, place the disc, gasket-side down, flat onto the mouth of the jar, then screw the band over top of it. With this type of lid, make sure to only screw the band down fingertip-tight—this means to only hold the band with the tips of your fingers when tightening the band, which should prevent you from over-tightening it. Overly tight lids will prevent air from successfully escaping during the processing stage, which may negatively impact the ability to form a seal. Overly loose lids will allow contents to leak or water to enter the jars when water bath processing.

Processing

Once jars are filled and closed, they must be processed for the correct amount of time in the appropriate medium (see below). The type of processing and the exact duration are determined based on the specific characteristics of a given recipe and the size of the jar. It's important to make sure that the processing is long enough to allow sufficient heat to fully penetrate through the jar and eliminate any microbes of concern, and this amount of time is different for every recipe. Examples of things that can affect this time are viscosity, size of food particles, altitude, water circulation, jar shape and proportions, headspace, and acidity—regarding this latter point, it may take more or less heat/time to neutralize certain microbes under varying acidities.

Specialized jar lifter

For boiling water bath processing, make sure the water is preheated to 140°F/60°C for raw-packed foods and 180°F/82°C for hot-packed foods. Use a jar lifter to transfer the jars in an out of your canning setup, making sure not to tilt them beyond upright on the way in or out—the contents can interfere with the sealing surface if this happens and prevent a seal. Once the directed processing stage is complete and the jars are removed from the canner, allow them to cool, undisturbed and an inch or two apart from each other, at room temperature for 12–24 hours so that a seal may form. Do not open the jars to add additional contents, even if some was lost during processing. Do not retighten the bands or invert the jars while allowing them to cool—doing so can cut through some types of gasket, leading to seal failure, or can force a false seal.

Altitude

One common factor that will require individuals to make processing time adjustments is altitude. Canning relies on using the heat of boiling water to destroy spoilage microorganisms; however, as altitude increases, the temperature of boiling water decreases. This means that processing at higher altitudes takes longer and/or requires more pressure (in the case of pressure canning). Reputable canning recipes should give the required processing time for different altitudes above sea level and/or different pressures. Make sure you check the elevation of the location where you will be doing canning, and note that it can vary significantly even in the same municipality or district depending on the terrain.

Checking seal

Once jars have fully cooled and been given the opportunity to seal for 12–24 hours, the quality of the seal should be inspected. Remove the bands or clips holding the lids onto the jar, depending on the variety of jar you are working with. The following are all ways to check for the presence of a seal, and meeting multiple criteria is a good indicator:

1. For lids with a central button, the button should be fully depressed and not move up and down. The lid should certainly not be bulging.
2. Metal lids should make a pinging sound when tapped with a metal spoon; note, however, that a dull sound can result either from an improper seal or simply from food touching the underside of the lid.
3. The vacuum seal should be strong enough for you to pick the entire jar up by holding onto the edges of the lid.

Remember that the presence of a seal by itself does not guarantee a safe canned product if the processing was done incorrectly—a seal can form even if the processing was insufficient. However, if the processing was done correctly, the seal should prevent recontamination of the contents.

Reprocessing

In some cases, a properly-processed jar that simply failed to vacuum seal within 24 hours may be reprocessed. To do so, empty the contents into a pot, bring back to boiling, and repeat the entire canning and processing with a fresh jar and lid. Note that this can only be done for hot-pack recipes, and it may result in a quality decrease in the final product.

Storage

If jars were properly processed but failed to seal, they may be transfered to the fridge or freezer within 24 hours and consumed from that state. Jars that have sealed correctly should be washed to remove any food residue, then labeled with the date and batch if applicable. Do not store the jars with their bands or clamps on, since you want it to be easy to detect any later seal failures, and bands can sometimes rust to their lids. The ideal environment for storage of canned foods is a dark, dry space from 50–70°F (10–21°C), but anything under 95°F (35°C) should be fine as long as the seal holds. Moisture may corrode the containers and cause seal failure. Freezing in itself will not cause the contents to spoil, but temperature fluctuations can cause the seal to fail. The recommendation is to use canned products within 12–18 months.

Processing

This section details the currently accepted safe processing methods, as well as some methods that are not accepted or otherwise considered safe.

Boiling water bath

Water bath canning

Boiling water bath canners can be very simple setups. The minimum requirements are a stovetop, a metal rack, and a lidded pot large enough to hold the rack, jars, and enough water to cover by 1–2 inches (2.5–5 cm). If you don't have a dedicated canning rack, you can use a small cooling rack or fashion one by tying lid bands together—the important thing is to elevate the jars from the bottom, protecting them from the heat source while allowing hot water to circulate around them.

While you are preparing the recipe, fill the canner halfway with water, and preheat it to 140°F (60°C) for raw-pack and 180°F (82°C) for hot-pack. Place your filled jars on the rack in the canner, making sure they are covered by 1–2 inches (2.5–5 cm) and adding additional hot water as necessary. Cover the canner, and turn the heat up to get a rolling boil—you can lower it slightly to get a gentle boil once you hit this point. Once the water is boiling, start the timer for the processing time indicated by the recipes; if the boiling stops at any point during this process, you'll have to return to a boil and start the timer over from the top. Once finished, turn off the heat, remove the lid, and let the jars rest for 5 minutes in the hot water to prevent siphoning.

Atmospheric steam

Steam canner

Though not initially recommended, atmospheric steam canners were approved by the NCHFP around 2015 for use whenever boiling water baths may be used. These are not the same as pressure canners, which use pressurized and not atmospheric steam. Their chief benefit is the lower water requirement for operation. The principle is the same as the above, since the steam at atmospheric pressure has the same temperature as boiling water, and the processing time will the the same as for a boiling water canner, with adjustments for altitude as appropriate. Due to the risk of the water boiling dry, steam canning is not recommended for processing times greater than 45 minutes.

To can with a reputable atmospheric steam canner, follow the manufacturer directions. Fill the base with water, add the rack, and heat the water to 140°F/60°C (raw pack) or 180°F/82°C (hot pack). Place filled jars on the rack, cover the canner, and raise the heat to bring the water to a boil. Wait until a 6-inch (15 cm) column of steam is continuously escaping from the vent hole, which indicates that the canner is properly full of steam. Once you have reached this point, start the processing timer, making sure the steam column continues through the entire process. Turn the heat off, remove the canner lid, and let the jars rest for 5 minutes as with a boiling water bath.

Note that, per the NCHFP, jars must be preheated before filling and not allowed to cool before processing. Do not force cooling faster than would take place at ambient room temperature.

Pressure canning

Dial-style pressure canner

Pressure canning uses a specialized pressure canner to raise the temperature of boiling water steam high enough (>240°F/115°C) to kill particularly resistant microorganisms like the botulism bacteria. A pressure canner is not the same thing as a pressure cooker—it is a specialized piece of equipment that cannot be replicated with regular kitchen equipment. Additionally, even when pressure canning, adjustments must be made at higher altitudes. This is because the overall pressure (atmospheric + canner) still decreases with increased elevation.

To use a reputable pressure canner for pint jars or smaller, begin by filling with at least 2–3 inches (5–7.5 cm) of water, or more if called for by a reputable recipe. Place the filled jars on the rack, and cover with the lid. Do not close the vent or apply the weight or pressure regulator at this point. Raise the heat to bring the water to a boil, and wait until the vent is releasing a steady column of steam for at least 10 minutes straight—this is critical to ensuring the canner is completely full of steam, and failing to achieve this step can result in insufficient temperature and underprocessing. Follow the manufacturer's directions to close the vent and apply the pressure gauge or weight as applicable, and make sure the canner reaches the pressure required by the recipe. With dial gauges, the readout should indicate the desired pressure or greater; with weighted gauges, the gauge will rock or jiggle continuously when the correct pressure is attained. Once correct pressure is reached, begin timing the required processing time. If the pressure dips below that specified, restart the timer. Turn off the heat, and let the entire canner cool at ambient room temperature until the pressure returns to zero, plus two minutes, before removing the weight or pressure regulator. Do not force depressurization. Let the entire setup cool for another 10 minutes before removing the lid to prevent siphoning.

If using a dial gauge, be aware that you need to monitor the canner during processing to adjust the heat as necessary and ensure that the pressure stays continuous and in the correct range, without excessive pressure or jumps If using a weighted gauge, it will naturally release steam periodically and maintain the precise pressure required—if it's jiggling for the whole time, it is working correctly, but check the manual for specifics. Note that a disadvantage of weighted gauges is that they cannot correct well for higher altitudes and must be operated at higher pressures than indicated at altitude.

Because pressure gauges that read too high can result in dangerous underprocessing, make sure that your pressure gauge is properly tested for accuracy on an annual basis. In the United States, many state university extensions will offer this service. Adjustments can be made if the gauge is 2 pounds off, but anything beyond that should be replaced. Make sure you properly clean and handle the gaskets of your canner to ensure proper functioning. Do not tamper with the safety release fuses.

Be aware that pressure canning with smoked fish and with quart jars follows slightly different procedure from that described above. Follow the exact directions specified by the USDA in their Complete Guide to Home Canning.

Low-temperature pasteurization

While the three processing methods described above are the most commonly used, low-temperature pasteurization may be used in occasional cases where it has been tested to be safe. Typically, the reason for its use is to avoid negative textural changes that could occur with boiling water processing. In this method, jars are placed in a water bath preheated to 120–140°F, much like in boiling water processing. The water is then brought to a steady and continuous 180–185°F (82–85°C), which must be maintained for the entire processing time. It is essential to use an accurate thermometer when doing this, since precise temperature control is required to ensure adequate processing.

Unapproved processes

Before rigorous, scientifically-based canning procedures were developed, dangerous canning techniques were often employed. Unfortunately, some of these still persist today despite their known safety concerns.

Non-processing

One historical practice used for fruit preserves was pouring melted wax over the surface of the food in the jar. Purportedly, this created a seal. However, these seals are not sufficiently resilient or durable to actually protect the preserves from the environment, and contamination is common. Additionally, you cannot process jars when wax is used. As such, this method is not recommended for any product, even if you may know other people who have used it. A similar—and unfortunately still widespread—method is "open-kettle canning". Here, jars are pre-sterilized, filled with hot food, and sealed without any further processing. Sometimes, the jars are inverted to try and force a seal. This is recommended against (even for high-acid foods) for the following reasons:

1. During the filling stage, airborne microorganisms like mold spores can generally contaminate jars, which may not be hot enough for long enough to inactivate them.
2. Specifically, a given food may require a longer heating time to properly inactivate microorganisms.
3. Residual air from a lack of processing can speed the deterioration of the food.
4. The inversion process carries a greater risk of spills and burns.
5. Interruptions can cause delays in filling and sealing, which cannot be corrected if not processing.
6. The food may not be hot enough to form a strong seal.

Dry heat

Some people promote "oven-canning", where jars are either preheated or "processed" in a hot oven. This is generally regarded as an unsafe practice due to the risk of jars exploding and uncertainties about proper heat circulation, distribution, and penetration for jars in an oven. Never heat canning jars in an oven.

Another dry heat method sometimes seen is so-called "dry canning". This is the practice of either a) filling and processing jars with dehydrated foods that are normally safe to store at room temperature or b) processing perishable products in jars without filling the empty space with liquid. This is recommended against for the following reasons:

1. Heat penetrates differently through air than it does in liquid. Leaving excess air in the jar may therefore result in insufficient heating and sterilization of the product.
2. Many microorganisms are more susceptible to moist heat than to dry heat. Leaving excess air in the jar may therefore result in the failure to destroy dangerous microorganisms.
3. In foods that are already uniformly dry enough and packaged to store at room temperature, jarring, heating, and cooling can force moisture out of the food and cause it to condense, creating a moist spoilage risk area that did not exist before. Using a dry vacuum sealer is a better choice for things of this type like dried nuts and grains.

Industrial canning

Home and otherwise small-batch canning techniques tend to be quite different from specialty industrial canning due to the technological limitations. Just because a type of canned product is available commercially does not mean a safe procedure has been developed for canning it at home.

Troubleshooting

The following table outlines some potential issues that may occur when canning:

Problem	Potential causes	Prevention
Apparent loss of liquid from jars during processing	Canner pressure is suddenly decreased.	Do not forcibly cool the canner or leave in a drafty area; do not prematurely open the vent.
	Pressure fluctates too much during processing.	Keep constant temperature and pressure during processing.
	Jar is not properly de-bubbled before processing.	Follow de-bubbling procedure as described above.
	Imperfections in the jar opening or sealing surface	Use fresh lids and bands without warping or rusting; clean the sealing surface before putting the lid on.
	Bands or clips are not tight enough to hold the lid down.	Tighten bands slightly more, remaining within the realm of fingertip-tight.
	Insufficient water coverage of jar	Make sure the jars are covered with 1–2 inches of water through the entire processing period.
	Starchy foods absorbing more liquid than expected	Fully rehydrate dried beans before filling and canning; use hot pack method.
	Over-filling or over-packing causes expansion or boiling over from the jars.	Increase the headspace slightly.
Failure to seal	Chips or cracks in jar sealing surface	Check sealing surface visually and by touch.
	Food residue on sealing surface	Wipe sealing surface before applying lid; trim fat from meats and add no extra fat; use the recommended headspace.
	Bands in poor condition	Use fresh lids and bands without warping or rusting; clean the sealing surface before putting the lid on.
	Bands not sufficiently tightened	Tighten bands slightly more, remaining within the realm of fingertip-tight.
	Inverting jars or lifting jars by lids when still hot	Use a specialized jar lifter, and don't touch the lids; keep jars upright.
Product darkening at surface	Too much remaining air in jar causes oxidation.	De-bubble correctly and use recommended headspace.
	Insufficient liquid to cover all food in jar; oxidation as above	Cover all solids with the packing liquid (e.g. water, syrup, brine)
	Food not processed, causing air retention; oxidation as above	Process as recommended in reputable recipe.
Other undesired color changes	Minerals in utensils or water	Use softer water and/or different, nonreactive utensils.
	Overprocessing	Follow stated directions for canners and processing times.
	Immature or overmature ingredients	Select ingredients at the correct stage of ripeness.
	Light exposure	Store jars in a dark place, such as a cupboard or box.
	Natural compounds in produce (e.g. pink or blue in apples/cauliflower/peaches/pears)	None
	Spoilage	See below.
Cloudy liquid	Starch from ingredients	Select ingredients at the correct stage of ripeness. Use fresh boiling water to cover potatoes instead of cooking liquid.
	Minerals in water	Use soft water.
	Minerals in salt additives	Use pure refined salt.
	Spoilage	See below.
Sediment in jars	Starch from ingredients	See above.
	Minerals in water	See above.
	Minerals in salt additives	See above.
	Natural yellow sediment in green vegetables or onions	None
	Natural white crystals in spinach	None
	Spoilage	See below.
Spoilage	Poor produce quality	Select ingredients at the correct stage of ripeness; can as soon as possible after harvest.
	Incorrect processing temperature	Pressure can low-acid foods; follow reputable recipe instructions.
	Incorrect process time for product	Follow timing specified in reputable recipes; do not overfill jars.
	Incorrect pressure	Have your dial pressure gauge checked for accuracy annually; follow proper operation procedure for pressure canners.
	Poor seal	See above.
Floating pieces	Fruit is lighter than the packing syrup.	Select firm, ripe fruit; use a lighter syrup (i.e. lower sugar content)
	Air trapped in food pieces.	Use hot pack method instead of raw to expel air.
	Improper packing.	Pack fruit as closely as possible without crushing; de-bubble properly.

Safety

Because canned food is stored at conditions that typically cause spoilage, and because particularly dangerous types of spoilage such as botulism can occur, safety is absolutely paramount when canning.

Safe sources

Only use canning techniques and recipes that have been scientifically tested by a reputable authority or that are safe modifications to such a recipe. Recipes shared by non-experts should list the original source and any modifications they made. Examples of reputable recipe publishers include the USDA, the National Center for Home Food Preservation, most US state university extensions, Bernardin, and Ball Canning in their modern forms. Know, however, that there can be minor differences in their advice—when possible, try to understand the evidence behind their particular guidance. Be aware that not all sources generally thought to be canning experts are actually using the latest evidence-based methods (e.g. the British Women’s Institute). Stick to the latest evidence-based guidance and recipes from these sources, since some of them have previously issued information that was recanted after later research. Avoid old canning books and family traditions—even if someone has been using an unverified recipe and hasn't gotten sick yet, they may have just gotten lucky.

Process authorities are another reliable resource—these are specialized organizations that use laboratory techniques to test canning recipes for a variety of safety parameters like pH, heat penetration, water activity, etc. Never can a recipe that you or another layperson have developed unless it has been submitted to and approved by a qualified processing authority.

Detecting spoilage

Mold growing on fig jam

In many—but not all—cases, it is possible to detect spoilage of canned goods after it has occurred. When opening a new jar, use your senses of sight and smell to check for the following:

• Broken seals
• Mold (fuzzy growths)
• Swelling or bulging
• Bubbles or spurting liquid
• Unusual odors
• Unexpected sliminess
• Streaks of food beyond the lid

Historically, it was thought that scooping off mold and consuming the "unaffected" portion of the food was safe. However, later research has revealed that this can still leave behind mycotoxins produced by the mold, rendering the entire food product unsafe. Moreover, mold growth can sometimes raise the pH, turning a high-acid food into a low-acid one and potentially allowing for botulism growth. Be aware that botulism spoilage of food will often have no telltale signs like the above, so if there is even a small doubt about safety of the product, simply discard it. Low-acid foods should be treated with particular caution.

Jars that are suspected to be contaminated can be detoxified for safety. To do so, completely submerge them in multiple inches of water, bring to a boil, and boil for at least 30 minutes. Remember that botulinum toxin is dangerous via ingestion and skin contact. Household bleach can be used to clean contaminated surfaces.

Modifying recipes

To reiterate, the best way to ensure a safe canned product is to follow an unmodified recipe developed and tested by a reputable authority as described above. However, there are some modifications you can make to a recipe without compromising safety; and, high-acid foods like jams and jellies are usually safer to tweak than low-acid foods. If you ever change something about a canning recipe, it's important that you understand exactly how the change can impact the acidity or processing time/temperature in order to fully ascertain the risk.

Acidity

One of the most dangerous things to change is acidity due to how it can impact botulism growth. For recipes that have an added acid like vinegar or lemon/lime juice, do not reduce or eliminate the acid. If it seems too sour to you, try adding a very small amount of sugar to offset the flavor. It's also important to use the right kind of juice or vinegar. Tested recipes typically use vinegar with ≥5% (50 grain) acidity, so you must use a food-grade vinegar with at least that amount of acid. Do not use vinegars of unknown acidity, and do not dilute vinegars. Bottled lemon/lime juice is typically used instead of fresh because bottled juice has a standardized acidity, while freshly-squeezed juice can vary significantly. If a reputable recipe says to use fresh lemon/lime juice, that is acceptable, and it is also fine to use bottled juice instead of fresh. Because bottled lemon/lime juice is more acidic than 5% acid vinegar, you may safely substitute an equal volume of lemon/lime juice for vinegar but not vice versa.

Citric acid (also called sour salt) is another acid added to products to safely acidify them. It can be sold as either a coarse or fine crystal—if you purchase the coarse form, it's important to finely crush it (to the consistency of table salt or table sugar) in order to make sure you measure out enough for safety. According to the authors of Putting Food By, 1 tablespoon bottled lemon juice is equivalent to ¼ teaspoon fine citric acid.

It is typically safe to add more of a given acid. Note that just adding some acid to a recipe does not necessarily make it safe to change any of the approved processing techniques—you cannot be certain that you have achieved safe and consistent acidity without professional evaluation. You still have to pressure can recipes that originally call for it, and you cannot reduce the processing time.

Be aware that boiling vinegar mixtures for too long can be dangerous because acetic acid evaporates faster than water.

Sugar

In addition to its sweetening and thickening abilities, sugar is claimed by many sources to act as a preservative in canned foods. While this does have some truth to it, the "preservative" label can be somewhat misleading. It is true that when used in recipes for canning whole fruits, sugar helps preserve their color and texture. And, it is true that by sequestering a large amount of water in some products, sugar can make it a little harder for some microorganisms to grow—products with less sugar tend to have a shorter shelf life once opened than their equivalents with more sugar. However, sugar as used in canning does not actually prevent microbial growth outright, and especially not botulism growth. Heat and acidity are the factors used to prevent dangerous microorganism growth, so sugar can in theory be decreased in a canning recipe.

When canning whole fruit in syrup, it is relatively trivial to decrease the amount of sugar; in fact, these fruits can be safely canned in lighter syrup, water, or juice so long as all the other parameters of a reputable recipe are followed. When canning fruit preserves, you can also typically safely reduce the sugar, but this will change the safe processing time—fruit preserves that have their sugar reduced must be processed the same way as if processing the constituent fruit whole in syrup/water. Note that reduced-sugar recipes must be hot-packed. Recall as well that sugar is critical for setting certain types of pectin, so if the final concentration of sugar is decreased, you may have trouble with the setting of jams and jellies.

It is also possible to substitute in other sugar-based sweeteners. Light corn syrup, brown sugar, and mild honey can all be used, and one recommendation is to replace up to half of the sugar called for in a recipe with one of them. If you are fine with intense flavor and color, molasses and sorghum molasses are also acceptable.

Thickeners

Because the thickness of a food to be canned impacts the heat penetration during processing, altering it can be dangerous. You may not add a thickener to a canning recipe unless it is specifically called for by a reputable recipe. Common thickeners include cornstarch, flour, pectin, clearjel, and thermflo. Clearjel is a modified cornstarch generally preferred by the USDA, but Thermflo is a similar product accepted by Penn State Extension due to its heat stability and similar viscosity. Do not use instant clearjel, which has different thickening characteristics. When flour is called for, use all-purpose flour only. Pectin of various types can be used, but modified pectin requires longer processing.

Salt

In the majority of canning recipes, salt (sodium chloride) is added in small amounts and only for flavor, not safety. This is the case with sauces, salsas, meats, soups, vegetables, etc; for all of these recipes, it is perfectly safe to reduce or omit the salt. In a small handful of cases, however, salt is a critical component because it favors the growth of specific desirable bacteria, which are critical to the safety of the food. Examples of this type of food include fermented vegetables and brined pickles, and you may not reduce the amount of salt called for by the recipe.

The recommended type of salt for most recipes is canning or pickling salt. Some table salts have anti-caking agents added, and these can cause harmless but sometimes undesirable clouding in canned foods. Avoid flaked salt, since the density is too variable to ensure safe volumetric measurement. Reduced-sodium salt blends containing potassium chloride exist, and these can be used in recipes for which it is considered safe to reduce the salt.

Other seasonings

Generally speaking, it is safe to include small amounts of dried spices and herbs in canning recipes, since they are used in such small amounts that they do not affect the acidity or viscosity of the food. The accepted safe amount to add is 1 teaspoon of dried ground herbs/spices per pint. You can also add whole spices during the cooking process and remove them before filling, which can help reduce the cloudiness of ground spices. Adding fresh herbs to a recipe yourself is not recommended due to potential safety issues from the extra water and bulk. It is also usually safe to omit an herb/spice when called for.

Fat

Fat can pose challenges in canning for two reasons. First, fat that makes its way to the sealing surface of a jar can interfere with the lid seal. The second reason is that excess fat can change the heat penetration into the food, making it necessary to use a different processing time for safety. For these reasons, adding additional fat to canning recipes is not recommended.

General addition and substitution

In general, you have to be extremely careful when doing addition, subtraction, or substitution in canning. Adding significant quantities of low-acid ingredients—especially when not in the original recipe—can change both the acidity and the heat penetration of a food, so you should never add things like meat, pasta, rice, most vegetables, etc. beyond what's called for. You can, however, typically decrease low-acid vegetables like onions, peppers, and celery (but never decrease tomatoes). Even substituting low-acid ingredients for each other can be dangerous due to differences in their composition and therefore heat penetration. A handful of substitutions are considered safe:

• Globe onions (e.g. red, white, yellow) are interchangeable with each other, as long as you do not increase the total volume or weight of onion in the recipe (onion is a low-acid ingredient).
• English or other grocery store cucumbers may be safely substituted for pickling cucumbers.
• All tomato varieties are interchangeable with each other, as long as you use the same total volume/weight called for. Similarly, green tomatoes and tomatillos can be safely substituted for tomatoes.
• Fresh chile peppers and bell peppers can all be substituted for each other; again, as long as you use the same total volume/weight called for originally. You cannot substitute the same count of larger peppers for smaller ones, since the total amount will go up.
• Peaches and nectarines can be substituted for each other.
• Apples and pears can be substituted for each other (with the exception of Asian pears).
• Frozen unsweetened fruit and fresh fruit are interchangeable with each other in jam and jelly recipes. Make sure to measure the frozen fruit before it thaws, since the volume will decrease after thawing.

Jar and processing

The size and shape of a jar affect the heat penetration and circulation into the food. It is harder to heat up larger jars than smaller ones; for this reason, never use a larger jar than called for in a recipe. You may use a smaller jar, but do not guess at the processing time—use the processing time for the larger jar in order to ensure safety. Be aware also that novelty jars with unusual shapes may have trouble heating properly, so it is safest to stick with standard jar shapes and sizes when canning.

Never guess at the processing type or time for a recipe. In the same vein, never reduce the processing time. If a recipe says to pressure can, you may not water bath it, and vice versa.

Cooking

Certain cooking steps can be fiddly when canning, including batch work and heating limitations. You may be tempted to change the batch size when making jams and jellies, but this is not typically advised, since the gelling of the pectin can be negatively impacted by the longer cooking required by larger batches. You may also think it's perfectly safe to boil pickling liquid for longer than specified, but doing so can actually cause over-evaporation of the vinegar and unsafely decrease the acidity in the food.

Never do

Never do any of the following:

• Use a boiling water bath when a recipe says to use a pressure canner (for safety);
• Use a pressure canner when a recipe says to use a water bath (for quality);
• Shorten processing time;
• Use a pressure cooker instead of a pressure canner;
• Add a thickener unless called for;
• Add a different thickener than the one called for;
• Use fresh herbs if dry are called for;
• Can foods for which there are no modern tested recommendations.

Recipes

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