Pique the Geek 20100307: How Canning Food Works

(10PM EST – promoted by Nightprowlkitty)

Hello, all.  I did not have research time to finish up the next installment about nuclear fusion in stars, so we will have to do with this.  I began planting my garden last week, so the subject of canning food came to mind.

Most people do not realize that canned foods are relatively recent developments, not counting wine and beer, which are at least technically, canned in many cases.

Personally, I have always been fascinated by all the different elements involved in food processing, and I am particularly interested by all the latest developments in machinery and equipment.

One of my friends works in the food processing industry, and just the other day he was telling me all about how his company has recently purchased a brand new reindeer flour mill. Flour mills are essentially used to break solid particles into smaller pieces or fine powder by grinding or crushing them.

However, today, we are going to be talking about the canning process.

Napoleon (“An army marches on its stomach.”) offered a cash prize for an inventor coming up with a way to preserve foods other than by drying, salting, or brining.  It was a Frenchman that came up with the concept of canning, but old Boney was out of power at the time.

First, let us consider the cause of food decay.  There are three major ones, and several minor ones.  The three major ones are 1) bacterial attack, 2) fungal attack, and 3) oxygen damage.  Canning solves all three of those, and also deactivated decomposition enzymes in fresh food that cause loss of flavor and color, as well as nutrients.

The essential process of canning is simply taking food, placing it in a container that can be sealed from the atmosphere, and heating it long enough and at a high enough temperature to sterilize it.  That is it.

Originally, canning was done in hand crafted tin plate metal containers.  Seals were not very good, and without modern synthetic liners the interiors of the cans deteriorated relatively rapidly in many cases.  With the advent of mechanized can forming machinery and better sealing and lining compounds, canned goods became cheap and of high quality in the late 1800’s.

Glass was also used in canning, and still is, and almost all home canning is now done in glass containers.  There are several advantages to glass, but it is heavy and does break, so steel cans are used commercially for the most part.  The equipment to seal steel cans, whilst excellent in high speed industrial settings, is not very practical for home canning, but some folks still home can in steel.

In a typical industrial setting, cans (and lids) are bought from a vendor and transported to the cannery.  These cans are electroplated with a very thin layer of tin to prevent rust (tin also has a good deal of antimicrobial activity and is relatively nontoxic to higher lifeforms), then coated with an interior layer of a polymer to increase the resistance of the tin plate to corrosion.

Incidentally, there is growing controversy about the nature of the polymer coating. since many cans are lines with a material based on bisphenol-A, a known estrogen mimic.  I look for the can industry to reformulate to use materials of lesser adverse health potential.

The cans are then packed with whatever food that the factory is packing, then run through a lid seamer.  This device attaches the lid to the can (the lid has a rubbery sealing compound on the rim), and the seamer produces a double seam, twisting the metal into a seam twice, locking it.

Then the cans are run through an autoclave to sterilize the contents.  These autoclaves are run at 240 degrees F, or higher, to inactivate spores of the bacterium Clostridium botulinum, the bacterium that causes the most deadly form of food poisoning, botulism.  As a matter of fact, in the United States there has been only a single outbreak of botulism in the past 50 years, and defective processing equipment was the cause.

If kept in relatively cool and dry environments, commercially processed canned goods stay safe to eat for decades, but obviously quality deteriorates after a while.  I recommend that you eat canned goods within two years of canning, but I have eaten some much older than that.

Canning is glass, commercially, is very similar, but home canning is quite another thing.  Instead of high speed, high volume, home canning is a tortuous process.  After saying that, I must allow that I love to can food at home, and plan many, many jars of tomatoes, purple hull peas, and other material this coming year.  As a matter of fact, I just canned six jars of cushaw pulp for pies a couple of weeks ago.

The trick in home canning is to know how to do it.  I learnt from my grandmother, and still use her water bath canner, many of her jars, and her canning funnel.  Very sweet things, like jelly, jam, etc. can be canned using rather simple materials (the open kettle method), whilst high acid foods like tomatoes and pickles can be canned using the water bath method.  Both high sugar content and high acidity inhibit the growth of microorganisms, especially C. botulinum, so those foods are safe to eat after such relatively low processing temperatures.

However, if one wishes to can things like beans, meat, fish, poultry, and anything else that is not acidic, it is essential to use the pressure canner method.  This device is essentially an autoclave that is headed by a kitchen range, with a pressure gauge or weighted rocker to control the pressure, and thus the temperature, internally.  Proper use of well maintained equipment allow the safe canning of all kinds of foods that will be much cheaper than the commercially canned ones, after the cost of the equipment and jars are offset.  By the way, there is no energy cost for canning in winter, as the heat from the range just makes your heating system run less.

I shall update you on what I am canning hither and yon throughout the season, either here or when it is my turn to write What’s for Dinner? with pictures.  I solicit your canning tips and stories for tonight.

Well, you have done it again.  You have wasted another perfectly good stream of photons activating your retinae in reading this.  And although Eric Massa (what a doggone disappointment, by the way) says he is really guilty of harassment when he reads me say it, I always learn much more than I could possibly hope to teach writing this series.  Please keep those comments, questions, corrections, and other items coming.  Remember, no scientific or technical subject is off topic in comments here.

Warmest regards,


Crossposted at Dailykos.com.com

UPDATE:  thanks, Kitty, for the front page!  I very much appreciate it.

Even more warm regards,



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  1. good home canned stuff?

    Warmest regards,


    • RiaD on March 8, 2010 at 3:33 am


    thanks Doc!

  2. …. that story is still developing.  Anybody with half a lick of sense could see that his actual social issues liberal viewpoint was going to be seen as a threat by the WH and the DOD.  Sounds like the staffer thought it was fun to play dirty smear game at the behest of somebody else.

  3. I am hoping to get some tomatoes, jalapennos, onions and garlic in this year and, if the timing works out, can some salsa for the winter. Could also just can jalapennos. I grew two types of tomatoes last year in two gallon pots. Kept us in fresh slicing tomatoes and salsa tomatoes. Might repeat that and try putting more in the ground. I found that my plants quit bearing about August, so I might try starting some fresh ones around the first of July, hoping for tomatoes till frost.

    We will see. My back and knees are the real limit and there is a lot of other work on the property that needs doing. Plus, my online activities have swallowed much of my time, 🙂  but I have over an acre with several areas that get plenty of sun. Might try icebox watermelons and cantaloupe as well. Worse come to worst I could try selling them at the twice weekly farmers market on the square by the courthouse–but I would have to get up early. Ugh.

  4. My wife and I have been working on growing vegetables in our back yard over the last few years, and we have some great fruit trees, but we haven’t tried canning yet. Usually we just eat everything fresh.

    Thanks Again

    • melvin on March 9, 2010 at 8:08 am

    One of the beauties of canning is the chance to personalize. I love pears. Each jar in a bath can be different if you like; ginger in this one, cinammon or anise in that, whatever flavoring or liqueur you want in another. They are beautiful just sitting on the shelf.

    Opening each one brings back the memory of canning, maybe of picking them at the u pick. Somehow it’s just more satisfying than saving a few pennies on the – always too sweet – Costco version.

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