Food preservation

Process in Canning – What Does It Mean, How To Do It, and Why?

As you delve into the world of food preservation, you will often read the term ‘process.’ But what does it mean to ‘process’ in canning?

The word ‘process’ in canning refers to the steps that we take to safely preserve food for later use. The goal in this canning stage is to heat jars to the required temperatures and for the correct period of time to kill spoiling or pathogenic micro-organisms, inactivate enzymes, and vent excess air.

The term ‘process’ is sometimes used to refer to the entire procedure of canning from beginning to end. But it is also often used to refer only to the water canning, steam canning, or pressure canning once the jars have been prepared and filled.

In this article, we’ll take a closer look at processing in canning in this narrower context. We’ll talk about what it is, how we do it and delve a little deeper into why we do it too.

The canning process overall is a relatively simple one. But there are many intricacies and complexities that mean that learning how to process in canning is not something you can learn all at once.

What Does it Mean to Process in Canning?

It is essential first to understand what the canning is to understand what it means to process in canning. And to explore briefly the different types of canning that are commonly undertaken at home.

Home canning is a way to prevent food waste and stop food products from spoiling. It involves preserving foods such as fruits, vegetables, and other perishable goods by packing them into glass jars.

In the UK and in other places worldwide, most traditional home canning (also called bottling) usually involves a method called ‘open kettle’ processing. In this process, jams and other preserves are made, then placed into sterilized jars. No further process is undertaken.

But the scientific consensus is that the techniques which involve further processing of the bottles are the best and safest way to preserve food. Food safety is paramount when preserving food at home.

A better scientific understanding of food preservation and food safety means that home canning (especially in the US) now usually involves putting jars through a further process. This involves heating the jars to create a vacuum seal and to kill the organisms that would cause spoilage or pose a threat to human health.

Home bottling or canning can be an effective way to preserve fruits, vegetables, and other foods at home. But it is essential to make sure that it is done correctly, using the right techniques for the different types of food that are being preserved.

Why Process in Canning?

There is a lot to learn about effective and safe home canning. And it is vitally important to always consult a reliable source for best practice, timings, and recipes if you plan on utilizing these food processing techniques to preserve your own seasonal produce. Fail to do things properly, and botulinum toxin and other pathogens may be a concern. Make sure that you educate yourself properly before taking the plunge.

Many old-timers may have been making jams and jellies using the ‘open kettle’ technique – just sticking their jam in sterilized jars – for years. They may swear by their techniques and eschew further processing. But if you do not process filled jars, you need to do so. It is essential to learn that if you’ve never got sick from your preserves – you’ve been lucky!

Heat processing of all filled jars of home-canned foods is essential. It is 100% required to create an adequate seal for food safety. Failure to adequately heat process jars can result in seal failure, food spoilage, and substantial health risks. There is a very real danger of botulism with low-acid foods or acid foods that experience mold growth.

We have a wealth of scientific knowledge in this area now. And science simply cannot be ignored. It is essential to process correctly in canning. If you do not, you are taking an immense risk with your own health and the health of those you love.

How To Process in Canning: Step By Step

Depending on which produce is being preserved, modern methods often discussed for canning or bottling include:

  • Hot Water Canning
  • Steam Canning
  • Pressure Canning
  • Oven Canning

All of these involve putting the canning jars through an additional process once the jars have been filled. Below, we will explore the process for each of these types of canning, step by step. And explain why oven canning is most definitely no longer recommended.

Hot Water Canning

You can preserve many foods safely by using approved hot water canning techniques. In hot water canning, filled jars of food are placed into a hot water bath and boiled, completely covered in vigorously boiling water, for a prescribed time. 

The process in hot water canning is all about the time it takes to heat the contents of the jars to the required temperature and keep them at that temperature long enough to make the foods safe for storage and later consumption.

Processing time will vary depending on the recipe you are working with and the size of your jars. You should also alter the processing time, depending on your altitude. This is also true for the other types of canning described below.

At higher altitudes, water boils at temperatures lower than 100 C (212 F). Lower temperatures are less effective in destroying micro-organisms. Therefore adjustments must be made to heat processing instructions to extend exposure to adequate heat to kill micro-organisms.

Processing Time Adjustment for Hot Water and Steam Canning:

Altitude (in Ft)Altitude (in M)Time to Add To Processing Time (in minutes)
0 -10000 – 3050
1,001 – 3,000306 – 9155
3,001 – 6,000916 – 1,83010
6,001 – 8,0001,831 – 2,44015
8,000 – 10,0002,441 – 3,05020

In water bath canning you:

  • Take the filled jars and put on their lids.
  • Ready your water canner (or another large pan).
  • Place a trivet or rack in its base to keep jars off the base.
  • Place your jars upright on this rack or trivet. Don’t lay jars on their sides, or they are likely to expel their contents rather than air, and jars may break.
  • Add water (it is quicker if you use water pre-boiled in a kettle first). Make sure the jars are covered by 1-2 inches (3-5cm) of water.
  • Put a lid on your pot and transfer it to the heat.
  • Start timing for the processing time for your canning recipe only when the water reaches a full, rolling boil. You’ll have to lift the lid regularly to keep checking.
  • Once the water is boiling, let it continue to do so for the prescribed process time, with the lid on.
  • When the full processing time has elapsed, turn off the heat and take off the lid, but leave the jars where they are in the canner.
  • After five minutes have passed, you can remove the jars from the canner. Place them on a towel or wire rack, away from cold drafts.
  • Don’t cover the jars or fiddle with the rings, tilt, tip, or wide your jars. Leave them alone.
  • After 12-24 hours, you can remove the bands and check that the flat lids have sealed properly. Use any that have not sealed properly right away.

Steam Canning

For any processing times under 45 minutes, only for high acid foods, you can also consider steam canning. This is approved as an alternative to water bath processing for high acid foods. You will steam can for the same process time called for in a water canning recipe. Take your altitude into account. 

Steam canning was ‘authorized’ in 2015 after research was undertaken that showed that it could be used in place of water canning with the caveats mentioned above. 

To can using steam, you will need to invest in a steam canner (a vessel that will fill with steam and heat the food in the jars with that rather than with water). A steam canner could be a good investment. It uses less water than a conventional water bath canner, of course. And since you won’t have to boil as much water, it can also save fuel since you will reduce the pre-processing time.

It is a great idea to invest in a steam canner with a gauge on top, so you can rest assured that the right temperatures have been reached. 

In the steam bath canning you:

  • Take your filled jars and place them on the bottom rack that came with the steam canner.
  • Add the amount of water recommended for the steam canner you have bought.
  • Put the steam canner on the heat, and vent steam until you see a solid, unbroken column of steam that is 6-8 inches (15-20cm long) coming out. 
  • Once you see that solid column of steam, start timing your process time.
  • Make sure heat is maintained and regulate heat to maintain that column of steam throughout the process. Don’t lift the lid off.
  • If you lose the column of steam for whatever reason, bring it back, then restart processing time from the beginning. 
  • At the end of the process, follow the steps as described above for water bath canning. 

Pressure Canning 

Another method of canning is pressure canning. This method of preservation uses a pressure canner to superheat steam. This allows heating the jar contents much to a much higher temperature than you can achieve with any direct heating of their contents.

This type of canning can be used to preserve a much wider range of produce and other foods than the other canning methods mentioned above. You can preserve jars of low-acid vegetables, soups, stews, pasta sauces, and more, as long as you get two things right with it comes to the process.

In pressure canning, pressure and processing time must be set calculated for different recipes and different foods. You must follow directions to the letter.

Determining Pressure: Altitude and Pressure Chart (Most Foods)

Altitude feet / metresWeighted
Gauge lb
Gauge lb
Gauge kPa
Gauge kPa
0 – 1000 / 0 – 30510116976
1001 – 2000 / 306 – 609151110376
2001 – 4000 / 610 – 1219151210383
4001 – 6000 / 1220 – 1828151310390
6001 – 8000 / 1829 – 2438151410397
8001 – 10000 / 2439 – 30481515103103

To pressure can, you will need to invest in a pressure canner. Note: this is not the same thing as a pressure cooker. You cannot use a pressure cooker to process in canning.

In pressure canning you:

  • Check and triple check the pressure and processing time required for your recipe. Make sure you do take your altitude and also the size of your jars into account.
  • Add the amount of water recommended for your pressure canner. You can also add a few squirts of vinegar so your cans and canner stay sparkling clean.
  • Load the filled jars. For hot pack recipes, have the water ready simmering around 180F (80C). For the raw back, have the water starting to steam at around 140F (60C).
  • Put the lid on and lock it down.
  • If you have a modern canner, leave the weighted gauge off. In older canners, leave the petcock open.
  • Crank up the heat and get the water boiling.
  • Keep boiling until you start to see steam.
  • Once you see steam coming out of the canner steadily, allow it to vent for 10 minutes. Note: venting time may be longer for some recipes, so be sure to follow instructions.
  • Once the venting is done, close up and bring the canner up to pressure.
  • Only start timing the actual processing time when you reach the required pressure.
  • Lower the heat to maintain pressure and regulate it to keep the pressure where it is needed.
  • At the end of the processing time required, turn off the heat.
  • Let the pressure drop naturally, still fully closed up. Do not try to rush the cooldown. Natural cooling is vital for food safety and the quality of your canned food. Bringing the pressure back down too quickly may also cause lids to buckle or seals to breach.
  • Once the pressure is back at zero, wait 10 minutes before removing the lid. Then you may remove your jars.

Oven Dry Pack Canning – A Process in Canning No-No

Let me begin by saying that dry pack canning is not recommended for several reasons. Most notably, because there are no guarantees that the food can be heated to the required temperatures. While you can set an oven dial to 250-300F (120-150C), this is not the temperature that the inside of your jars will reach.

Even if you can get something to boil inside a jar in the oven, the jar contents will not get hotter than 212 F (100 C) as there is no pressure to allow them to reach higher temperatures. That temperature is not high enough to kill off botulism spores and other pathogens that may be present in low-acid foods. 

Dry heat is very slow to penetrate the jars, and temperatures inside ovens vary considerably. So it is impossible to have standard processing times. No reliable, research-based safe process times have been developed for oven canning. 

Another very worrying concern is that jars heated in an oven may explode! No canning jar manufacturer makes jars suitable for canning in this way. You are risking not only your internal health but also life and limb by placing glass jars in a hot oven. 

Can You Overprocess in Canning?

Yes, you can overprocess. Under-processing is more dangerous because the food may not have been heated sufficiently for food safety. But processing for too long can also be problematic – mainly because it can reduce the quality and spoil the texture of canned goods. That is why, whatever sort of canning you are doing, it is vitally important that you follow the recipe and process your jars precisely as it says you should do.

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