Post #G22: Energy consumption required for home-canned pickles

Posted on August 22, 2020

I ended my just-prior post with some speculation on the energy (in)efficiency of home canning.  In this post, I work up the numbers and confirm that home-canned pickles require quite a bit of energy.  My calculation is that I use 17 fossil fuel calories for every edible pickle calorie preserved. 

As a way of contrast, I calculate that commercially-canned diced tomatoes require just 2 fossil fuel calories for every edible calorie preserved.  (That’s only for the canning, not for the transportation, but despite what you may read, the energy used in transporting canned goods to the store is minimal. I may need to do a separate post on that.)

Much of that difference is due to the energy density of the foods (canned tomatoes have about 5 times as many calories per volume as canned pickles).  Factoring that out, it appears that my home canning is maybe half as energy-efficient as commercial canning.

You may read blog postings and such suggesting that home-canning is a net energy saver, because you save the transportation costs for the food, and so on.  I’m not sure sure about that.  It’s entirely possible that the relative inefficiency of home canning offsets the fossil fuels used in transportation.  But that’s part of a different calculation.

This is the energy used in canning (preserving) pickles.  You can make pickles with no direct energy use, as in lacto-fermentation.  But if you want to put your pickles on a shelf, to eat some time next year, you have to can them.  That’s what we’re talking about.

Details follow.


Venn Diagram of the Research Literature:

I can, occasionally.  Mostly pickles, as in this year’s canning so far, pictured above.  And I have always been struck by what seems to be the amount of energy required.  You boil a lot of water, for a lot of time, to produce (e.g.) five quarts of pickles.

In fact, canning produces so much heat that the manufacturer of my stove says to move the pot around on the stove if you’re doing more than one batch.  That’s because my stove has electronic controls, and prolonged heavy use can fry the electronics.  It’s not like it’s a great stove or anything.  I just thought it was telling that canning is specifically mentioned as a hazard.

I tried to find any systematic analysis of the energy requirements for home canning, and found just one article, close to 30 years old, behind a paywall.  Basically, zip.

The Venn Diagram of the research literature (above) makes perfect sense.   Home canning had its heyday about half a century before people got concerned about fossil fuel use.  And so, in the grand scheme of home energy use, energy used in home canning just doesn’t register.  This analysis is focused on someting that is trivial within the big picture of household carbon footprint.

Two calculations from the internet, and one done from scratch.

I found just two things of value, using modern data, and not behind a paywall.

First:  “A Baseline Life Cycle Assessment of California Tomato Cultivation and Processing“, by Dr. Sonja Brodt, Dr. Alissa Kendall, Dr. Kiara Winans, and Emma Bell, all from University California at Davis.  Their method was fairly bulletproof.  They measured the natural gas and electricity used by a canning factory, and the quantity of canned tomatoes that came out.

I figure that has to put a lower bound on canning energy use, because a commercial facility is going to be vastly more efficient than home canning.  (Although home-canned tomatoes need to be processed for a considerably longer time than home-canned pickles).

Using Table 6 from that reference, I come up with about two fossil fuel calories for every edible calorie, for commercially-canned diced tomatoes.  Like so:

I also found an excellent blog post where the author went through the calculation in a seemingly competent manner.  She figured out, from scratch, the energy required to heat the water and keep it boiling.  For home-canned salsa, she comes up with a vastly smaller number — just about 0.2 fossil fuel kilocalories per edible kilocalorie.   I’m going to refer to this blog as LFL (Living the Frugal Life).

I don’t think it’s credible to suggest that home canning is an order-of-magnitude more energy-efficient than commercial canning operations.  So while I agree with the outline of her calculation, I think there were some important omissions in that blog post calculation.

I’m going to redo the LFL calculation, fixing what I see as those important omissions.  And modifying it to match my circumstances.  I should point out that, with the exception of the number of gallons of water that need to be heated, virtually all of the changes I make correspond to various comments on that blog.  So this mostly just quantifies the points that various commenters raised.

My data.  The pot I typically used for water-bath canning holds three gallons of water.  I can process five quart jars in that pot.  And I’m processing pickles, which are a relatively low-energy food.  I’m not counting any time spent (e.g.) sterilizing the jars, because you do that as part of heating up the water for the canning operation itself.  And I’m doing this in my air-conditioned home.  On a gas stove.  Change any of those assumptions, and your results will differ from mine.

The first big difference between my calculation and the LFL calculation is that LFL assumes heating of one gallon of water, while I assume that I’m heating up three gallons.  My point being that I have to heat not just the water bath, but all the food, to the boiling point.  So, while I have less than three gallons of water in the pot, all told, yeah, I’m heating up three gallons of something — either water, or something that’s very nearly 100% water (cucumbers in brine).

The second big difference is that I factor in gas stove inefficiency.  Based on a single citation, it appears that only about 40% of the energy from the natural gas burner actually makes it into the cook pot.  The rest is vented into the room.  (Which, honestly, sure is how it feels when I’m canning.)  So that ups the energy input considerably.

The third big difference is that I’m doing this in an air-conditioned house, and, rule-of-thumb, it takes as much fossil fuel to remove the cooking heat from the home, as it did to generate that heat in the first place.  I’ll go through that in detail, but when I work the numbers, that’s just about exactly what I get.

Finally, I’m processing a smaller quantity (five quarts vs 19 pints) of a lower-calorie-density food (64 calories per quart for pickles, something like 260 calories per quart for salsa).

This is all by way of quantifying the main reasons why my numbers end up so different from hers.  A minor final factor is that she starts with water that is somewhat warmer than the 50F that I assume for mine.

So without further ado, here’s my calculation, in full.

When I redo the calculation, accounting for the changes, I find that my home canning, of a small batch of pickles is about 9 times less efficient than commercial tomato canning.  That’s in terms of fossil fuel calories used to can an edible calorie of food.

But most of that difference comes from the energy density of the food.  A quart of canned tomatoes has at about five times the food energy of a quart of pickles (depending on your data source).  If I factor that out, my home canning is only about 2 times less efficient than commercial canning.  (This ignores the longer processing times required for tomato products versus pickles).

And that — that home canning is maybe half as efficient as commercial canning — strikes me as optimistic-but-plausible.  Half my energy use was in the air conditioning, and it’s a pretty fair bet that commercial canneries aren’t air conditioned.  My stove is only 40% efficient.  And so on.  And yet, there is some energy expended in the cannery for moving the items through the production line, and so on.  When you factor all that in, it’s surprising that home canning is only half as efficient as commercial canning.

And so, in the end, it all boils down to boiling.  How much energy does it take to raise water to the boiling point, and keep it there.  That’s physics, and that’s going to be the same for the commercial canner or the home canner.  Beyond that, it’s about how efficiently you can come up with and get rid of that required amount of energy.

I think the moral of the story is that pickles are going to be an energy-intensive food, no matter what, in terms of fossil fuel kcalories per edible kcalorie.  A quart of cucumbers has just slightly more calories than a quart of water.  Canned water requires an infinite number of fossil fuel calories per edible calorie.  Unsurprising, then, that canned cucumbers would require a high, but not infinite, number of fossil fuel calories per edible calorie.

The other interesting factoid is that it does take as much energy to remove cooking heat from the house as it does to produce that heat in the first place.  If your house is air conditioned.  I vaguely recall having read that at some time in the past.  And here, I validated that by direction calculation.