Pressure Canning Guide & FAQ: put ’em up!

Image Adapted From USDA Complete Guide To Home Canning

Image Adapted From USDA Complete Guide To Home Canning

Although pressure cooking and pressure canning both use the power of pressure –  the equipment and procedures are quite different. With a little investment in time, you can use top-quality ingredients to make your own convenience foods. In this article we will answer these questions:

What is canning? What is pressure canning?

tempsCanning, also known as putting up,  is a way to preserve fruits and vegetables beyond their season and conserving meat without the use of refrigeration.  Canning removes air and destroys enzymes to stop the growth of undesirable bacteria, yeasts, and molds.  The ph-level (acidity) of a food dictates whether it should be canned with the water bath method or using pressure.

Acidic foods (low ph) contain enough acid to destroy or block  bacteria growth. Acid foods include fruits, pickles, sauerkraut, jams, jellies, marmalades, and fruit butters. These products can be conserved with water bath canning (non-pressure) for the recommended time following tested recipes (to ensure acidity).

Low-acid foods (high ph) are not acidic enough to prevent the growth of bacteria – the airless environment inside a jar of low-acid food is the perfect host for a deadly bacteria which is odor-free, taste-less and otherwise invisible: Botulinum. Low-acid foods include meats, seafood, milk, grains and all fresh vegetables except for most tomatoes.  Tomatoes are tricky, the USDA wrote a special canning guide just for them. These foods can only be conserved safely with pressure canning carefully following USDA- tested processing times and temperatures1.

For both  water bath and pressure canning the exact processing time, temperature and/or pressure depends on the kind of food being canned, the way it is packed into jars, and the size of the jars – adjustments must also be made for altitude.

hip info: food poisoning

Botulism spores grow only in the absence of air (like inside an improperly canned food). They can only be neutralized following the USDA research-based processing times which ensure the destruction of heat-resistant microorganisms in home-canned foods.

Improperly canned foods that either used the wrong canning method, processing time, or with jars that have lost their seal can cause Botulism food poisoning.

Any case of foodborne botulism is considered to be a public health emergency because of the potential for that others will consume the toxin-containing food. State and local public health officials by law must be informed immediately whenever botulism is suspected.

The incubation period is usually 12 – 36 hours but symptoms may appear up to 10 days after exposure. The first symptoms include nausea, vomiting, and diarrhea that precede the onset of neurologic illness or growing paralysis without fever.

More info:  Botulism Report,  National Organization for Rare Disorders

 

Is there any way to can low-acid foods without using pressure?

According to the USDA Complete Guide to Home Canning1.

 The time needed to safely process low-acid foods in a boiling-water canner ranges from 7 to 11 hours...

USDA Complete Guide to Home Canning, ( click image to download)

USDA Complete Guide to Home Canning

While  pressure canning low-acid foods ranges from 15 to 100 minutes. If you are inclined to boil something for 11 hours, keep in mind that the boil has to be maintained the entire time.  Any interruption to the heat source (electricity goes out, gas flashes off, flame blown by wind) means you need to start start all over again.

That’s a lot of time, fuel, water and worry that can easily be solved with a pressure canner – which will get the job done in minutes not hours.

And, no, the USDA has not published non-pressure canning processing times for low-acid foods. So, if you insist on processing these foods without pressure – you’re on your own!

See Also: Pressure Canning Processing Times Chart, hip pressure cooking

 

What’s the difference between a pressure cooker and a pressure canner?

Although they both use pressure to get the job done, pressure cookers and pressure canners are made and used quite differently.  The most obvious difference is in size but they are also made of different materials, and valves.

The USDA Complete Guide to Home Canning1 states that a pressure canner is..

A specifically designed metal kettle with a lockable lid used for heat processing low-acid food. These canners have jar racks, one or more safety devices, systems for exhausting air, and a way to measure or control pressure. Canners with 16- to 23- quart capacity are common. The minimum volume of a canner that can be used is one that, will contain 4 quart jars. Use of pressure saucepans with smaller capacities is not recommended.

The story goes that researchers did test smaller pressure cookers (saucepans) but they got inconsistent results.  Because these cookers come in different sizes, shapes and volumes, the USDA couldn’t come up with a reliable rule of thumb to use with smaller cookers3. So, in 1980’s  they declared the minimum volume of a pressure cooker to be used for canning be the volume that can house at least 4 quart jars (this usually corresponds with  10 or 12 quart/liter pressure cooker).  Without doing further testing, the USDA cannot guarantee that a smaller pressure cooker will yield perfectly safe food in all jar size and food combinations tested for larger canners.

Pressure Cooker vs. Pressure Canner Table

Pressure Cooker


pressure_cooker
Pressure Cooker/Canner
fagor_canner
Pressure Canner
aa-canner
1 to 12 quarts/liters
(cooks for 1-12 people)
10-12 quarts/liters
(cooks for 5-12 people)
16 to 41 quarts
(not recommended for cooking)
mainly stainless steel (some economy aluminum models)mainly stainless steel (some economy aluminum models)mainly aluminum, some hard-anodized aluminum (no stainless steel)
Can cook all food types directly in cooker base.Can cook all food types directly in cooker base.Food should not come into contact with the base of this cooker, only for cooking foods in containers, processing jars.
Can be used on electric, gas, induction, halogen, ceramic, camping or portable cooktops.Can be used on electric, gas, induction, halogen, ceramic, camping or portable cooktops.Can be used on electric or gas cooktops only.
No pressure canning.yes pressure canning
(max 4 quart jars)
Yes pressure canning
(max jars depend on size)
Minimum liquid 1-2 cupsMinimum liquid 1-2 cupsMinimum liquid 12+ cups

canning_tableOur pressure cooker recommendation for materials and sizes remain the same: stainless steel body, reasonable size, easy to use. So, even though there are 16 quart aluminum “pressure cooker/canners” on the market, we cannot in good conscience recommend them to also be used for cooking.  The aluminum is reactive so you won’t be able to use tomatoes, wine, lemon or anything else acidic directly in the pot.  These aluminum vessels are really designed to process food in jars.  The minimum liquid requirement is really high (3 quarts – 12 cups) compared to the average pressure cooker (1-2 cups). This greatly affects the minimum amount of food you can cook in it.  Finally, the size is unwieldy to use and wash for everyday cooking.

If you want to pressure cook and pressure can: consider the Fagor 10L Pressure Cooker/Canner . If that’s too small for your pressure canning needs, purchase a separate pressure cooker and pressure canner.

See Also: Recommended Pressure Cooker/Canners, hip pressure cooking

So, why does my pressure cooker have pressure canning instructions?

Confusingly, some manufacturers include pressure canning instructions with their pressure cooker –  even if their cookers are not large enough to be considered pressure canners according to the USDA (see previous answer), or their technologies have not been fully vetted (see next answer).

For example, one German pressure cooker manufacturer includes instructions for preserving meat, see below. The instructions do not state how the meat should be prepared and in what size jar it should be.  The USDA recommends meat be processed anywhere from 75-90 minutes, depending on the cut,  but this manufacturer’s instructions only require 20 minutes.

So, without information at how a manufacturer arrived at their recommended processing times, we recommend  not conserving vegetables, meat, seafood, grains, milk and tomato products in a pressure cooker with vague pressure canning instructions.

Some manufacturers may include pressure canning instructions – even though their cooker is not compatible with USDA guidelines for pressure canners.

 

Can I use my electric pressure cooker to pressure can?

Unfortunately, there are some electric pressure cooker manufacturers advertising their products as safe for pressure canning. One, even said in their infomercial that their electric multi-cooker “Meets USDA standards for pressure canning” prompting NCHFP (an offshoot of USDA) to post an alert on their website warning consumers against these products3.

This multi-cooker infomercial shows an actress putting jars in the cooker with the words "Meets USDA Standards for Canning." The National Center for Home Food Preservation says it DOES NOT!

This multi-cooker infomercial shows an actress putting jars in the cooker with the words “Meets USDA Standards for Canning.” The National Center for Home Food Preservation says it does not.

The NCHFP warning said..

Even if there are referrals to the National Center for HFP in the instructions for canning in the manufacturer’s directions, we do not currently support the use of the USDA canning processes in electric, multi-cooker appliances.

The manufacturer of that product claim that their product reaches the right temperatures for pressure canning under 2,000 feet and fits four quart jars – so  what’s the problem? Doesn’t that meet the USDA standards?

The reason electric pressure  and multi-cookers  are unsafe for pressure canning are:

  • on/off heat cycles  electric pressure cookers are heated with a ceramic element that turns off when the target temperature, or pressure, are reached and back on when the temperature drops  below a certain threshold.  Comparable pressure cookers can vary by about 1 psi during this process (see chart) – these limits can vary and be programmed by the manufacturer.  It is not clear what the minimum temperature during the “off “cycle will be.
  • no altitude adjustments processing times for foods above 2,000 feet require an increase in pressure – but the construction electric pressure and multi-cookers currently on the market cannot sustain  pressure above 11 psi for extended periods of time.
  • no venting –  instructions for pressure canning for all of the electric pressure and multi-cookers we evaluated do not include specific instructions to thoroughly vent the canner for USDA recommended time of 10 minutes  before processing1 – also see the answer about exhausting pressure canners.
  • not stovetop – The USDA process standards were developed with stovetop pressure canners, there is no data to either prove or disprove pressure canning is safe in electric pressure cookers. However, there is plenty of data about the dangers of botulism in improperly canned foods.

See Also: Consumer Alert! No Pressure Canning in Electric & Multi-cookers says NCHFP, hip pressure cooking

How does altitude affect pressure canning?

boiling_pointWhile in pressure cooking, time is increased for higher altitudes, with pressure canning the process time remains the same and, instead, the adjustment is made to the pressure (or weight).

When the pressure canner is used at higher altitudes the temperature it can achieve is lower.  That’s because both a pressure canner and pressure cooker only add pressure to the current atmospheric (ambient) pressure – as altitude increases, air pressure decreases.  This means that water comes to a boil at lower temperatures as altitude increases.  This phenomena has a direct, negative  impact on the temperatures inside the pressure canner.

To compensate for lower temperatures at high altitudes the USDA requires an increase of processing pressure by 1 psi for every 2,000 feet of altitude.  So, let’s say you want to pressure can pints of ground meat – their recommendation is to process for 75 minutes at 11psi.  BUT, if you’re at 5,000 feet altitude that 11psi is equivalent to just 9 psi at sea level- and that’s a lower pressure and temperature than is safe for pressure canning.  So, they recommend processing that same pint for 75 minutes at the higher pressure of  13psi.

Don’t worry if this is starting to sound confusing – all you really need to know is your altitude, and the USDA has already figured out all of the adjustments.

Here’s a handy little chart that sums up the USDA pressure canner adjustments.

Pressure Canner Altitude Adjustments Table

Do I need to follow a specific, tested,  recipe for pressure canning?

Tested recipes are  only necessary for hot water-bath canning (non-pressure) preservation.  When the food is conserved using the fruit’s natural or added acid (pickles) it is very important to follow tested recipes to ensure that the end result has a low enough ph to remain bacteria-free (see first answer).

hip info: don’t can me

The USDA warns against adding any of the following ingredients to canned foods:

  • pasta or noodles
  • rice
  • flour
  • cream
  • milk
  • any other thickening agents

Other than pasta and rice becoming mushy, the reason you shouldn’t add these ingredients into the jar is because they slow the penetration of heat through the food – so processing these foods at the right time and pressure might still generate insufficient heat to kill bacteria (you know, deadly botulism).

For the same reason, you should not pressure can mashed potatoes or pumpkin pulp – but you can cube them for canning and mash them after opening.

Pressure canning does not require specific recipes.  The process raises the temperature of the food inside the jars to a point that is high enough to kill bacteria and inactivate the spores that can grow into deadly Botulism. The key to pressure canning is not the recipe but the processing time and consistency (thickness) of the food. With a few exceptions (see “don’t can me” sidebar), you can safely pressure can your own, or our, recipes.  To figure out the processing time, look up each ingredient in the pressure canning processing chart, and use the time for the longest-cooking ingredient. Overly-thick or dense recipes cannot be properly processed- the box below outlines which ingredients to exclude.

So, let’s say you want to make a monster batch of the classic Bolognese pasta  sauce to store away  in pint jars.  The recipe calls for ground meat and pancetta (75 minutes processing time) and veggies like carrots, onions and celery (25 minutes) and tomato sauce (35 minutes).  The longest processing time is for the meat so that is how long you should process the sauce (75 minutes).  We used a “trick” sample recipe because it also calls for cream – leave it out of the jars.  Splash it into the sauce when you’re reheating it before serving!

More Info: Complete Guide to Home Canning – Selecting, Preparing, and Canning Vegetables and Vegetable Products (Guide 4), USDA

Why not just process everything for the longest time possible?

The problem with pressure canning everything more than the recommended time is that the contents of the jars could be impacted negatively in color, texture and taste. This is called over-processing.  Let’s say you’re pressure canning green beans, and the USDA says that they only need to process 20 minutes, but you want to make sure those green beans are safe so you process them for 100 minutes, instead. You’ll end up with are jars of tasteless gray pulp, instead of delicious green beans.  It’s ok to go a little over the processing time, it’s not ok to double or triple it.

What size jar(s) can I use for pressure canning?

Most pressure canning processing times are published for pint (2 cups or about 500ml) and quart (4 cups or about 1000ml) jars – a few are published for half-pints.

You may want to pressure can smaller jars to give as gifts to friends and family.  There are no published processing times for all foods in smaller jars,  the safe way to go is to process the jars following the timing for pints.  However, keep in mind that this might over-process the food (causing a degradation in texture and color) so before you pressure can 100’s of these smaller jars do a trial run!

Ball Canning jars

Can I process jars of different size in the pressure canner?

If all the jars contain the same recipe, but they have a different size, follow the processing time for the largest jar in the canner.  Even if there is just one quart jar and the rest are pints – you should follow the processing time for the larger quart jar, if pressure canning another batch is not possible.

However, the foods in the smaller jars might over-process so it is always best to do separate batches of each size.

Can I process jars with different contents (some with veggie and some with meat) in the pressure canner?

It’s best to do one batch of each kind of food.  But if you’re pressed for time, it’s possible to process jars with different contents in the pressure canner.  To do this, follow the processing time for the jar with the ingredient that requires the longest processing time.  For example, if you’re pressure canning three jars of veggie stock (15 minutes) and one jar of beef stock (25 minutes) you will need to process the entire contents of the canner for the longest time (25 minutes).

Be aware that  the foods that need less processing time might-over process.

Do I really have to exhaust the pressure canner before processing?

USDA Complete Guide to Home Canning: exhausting a pressure canner

USDA Complete Guide to Home Canning: exhausting a pressure canner

Exhausting, or venting, a pressure canner is done before processing to ensure that all of the air is out of the canner.  This is an important step because the presence of air can affect the temperature inside the canner – causing false readings of the pressure gauge or dial.

There is a direct, and expected, correlation between pressure and temperature – and it’s those temperatures are used to calculate processing times to kill bacteria.

Trapping air in a pressure canner will display the “correct” pressure on the dial or gauge but since that pressure is achieved with a mixture of air and steam, and not steam alone,  the actual temperature inside are affected.  Here’s a chart that illustrates how much the internal temperature changes in the presence of air.

Temperature inside Pressure Canner with presence of Air

Assumes cooker or canner has been vented or exhausted.

Assumes cooker or canner has been vented or exhausted.

The pressure of the steam alone is a known value if steam is the only gas inside the pressure canner.  But if the canner contains a combination of steam and air the total pressure inside is a combination of the partial pressures of various gasses 4.

How to do it

In the USDA’s illustrated step-by-step instructions for using a pressure canner, they instruct bringing the canner on the highest heat setting and to let the steam flow continuously for 10 minutes.  After that time, the weight or petcock should be closed or placed on the vent1.

Exhausting can also be done with a spring valve pressure cooker/canner by setting the valve to the open, or exhaust, position – and then, instead of adding the weight after the time is up,  the dial can be simply set to the desired pressure selection.

Bottom line: You should still exhaust your pressure canner or pressure cooker/canner regardless of whether the manufacturer recommends doing so in their instructions (some don’t)!

Should I throw away jars that didn’t seal properly?

USDA Complete Guide to Home Canning: checking jars for seal

USDA Complete Guide to Home Canning: checking jars for seal

Improperly sealed jars can be re-processed or refrigerated to use within a week.  If those jars have remained in the “Danger Zone” (see chart in first answer) for more than two hours just boil the contents for 10 minutes  before consuming.

Where can I find reliable information on pressure canning?

ball_canning

Pressure canning requires specialized equipment, processes and procedures – to ensure the stability and safety of preserved food.  With the proper attention to detail and reliable equipment and processes it can be done safely at home.

If you have more questions or comments please post them below in the comments section or come visit the pressure canning forum and we’ll do our best to answer them – or find someone who can.



1. “USDA Complete Guide To Home Canning” National Center for Home Food Preservation website
2.  “Burning Issue: Canning in Pressure Cookers” National Center for Home Food Preservation website
3. “Can I can in a multi-cooker?” National Center for Home Food Preservation website
4. Myhrvold, Nathan, Chris Young, Maxime Bilet, and Ryan Matthew. Smith. Modernist Cuisine: The Art and Science of Cooking. Vol. 2. Bellevue, WA: Cooking Lab, 2011. Print.