Tuesday, August 6, 2019

Efficiency matters

A typical canning kettle that can hold seven quart jars typically holds 21 quarts of water
Suppose you wanted to can an large amount of food. The direct approach would be to load the food into jars. Put the jars into the kettle and fill the kettle with water. Place kettle on the heat and bring the water to boiling for the required amount of time. Remove the jars and place in a safe place to cool down.

Repeat as many times as necessary.

Further, let's assume this is not your first rodeo. You didn't use 59 degree tap water, you filled buckets, or maybe your bathtub and let it warm up to the dew point...which just happens to be 69 degrees in Eaton Rapids at the time of this writing.

If the food going into the jars starts at 80F then seven quarts (14 pounds) requires 1400 BTU to bring it up to 180F, the minimum temperature needed to kill MOST spoilage organisms. The 28 pounds of water starting at 69F requires 4000 BTU to bring it to 212 F.

About 25% of the heat goes into heating the food (the payload) and 75% of the heat goes into heating the water (a necessary evil).

Assuming 10000 BTU/hr for the large burner on a typical home stove and a 50% heat loss rate, it will take sixty-five minutes to bring the kettle up to heat, not counting convective losses from the kettle.

I know there are two kettle drawn, but assume there is a single kettle. Also assume it holds ten jars to make the numbering work out.
Commercial canning is much more efficient. Assume the flow of jars/cans is from bottom-right (apologies) to upper-left. A commercial canner will have cooling liquid, water or air, moving in the other direction. Cooling water will enter the upper-left and flow in the opposite direction of the jars. It warms up as it pulls heat from jars.

Then, it bypasses the canning kettle and preheats the incoming jars. That is, the heat that was removed from the jars that were cooling was used to heat the jars heading toward the kettle.

The system is designed so there is never a large temperature difference between the jars being heated/cooled and the water they are in. That minimizes the entropy in the system. The total heat used is the difference between the warm jars leaving the process and the cold jars entering, presuming the fluid is recycled.

In an austere environment like the one described in Kates Store, one way to approximate the commercial process is to have multiple cooling/warming stages as shown in the conceptual picture above.

Jars 1-through-12 are warming up. Jars 34-through-23 are cooling off.

The water stays in the tubs and does not move. The operator moves incoming from right-to-left and finished, cooling product from left-to-right.

The product that is cooling down is always warmer than the water in the tub. The product that is pre-heating is always cooler than the water in the tub.

The fact that you are recovering most of the heat that was invested in the jars means you don't need to burn the fuel or take the clock-time to burn the fuel.

The fact that the heated water is not tossed out means that water only needs to be heated from 69F once...except for what needs to be added to make up for evaporative losses.

Austere environments
There will never be enough time or labor.

There will never be enough fuel.

But I repeat myself. Every chunk of wood required that a person go into a swamp in the winter, cut down a tree, buck it into manageable bolts, drag it out of the swamp, cut to 16" lengths, split it, stack for drying, then move it to the site where it will be burned.

Being able to recycle the hot water decreases the fuel requirements (and the time to burn it) by 75% compared to the first system described above. Salvaging half of the heat from the jars coming out of the kettle and using it to pre-heat the jars going into the kettle would reduce the total BTU required per jar to 12% of the original system.

That is why efficiency matters. A 88% reduction in fuel and time-per-jar is a very big deal when nothing is easy.


  1. I must say I continue to learn things from your writing. Thanks!

  2. Now look at heat recovery ventilators for the house to allow for air exchange without sky-high heating or cooling bills.

    The general term for this setup is counter-current heat exchange.

  3. I see your point if canning foods that are generally started cold, like saurkraut (which by the way can be crock stored if it is used regularly) Your greens example, even with added vinegar, would normaly be wilted (cooked) before packing. You get alot more greens in a jar that way. Tomato sauce is generally cooked down to reduce volume so it can be packed hot and put directly in a boiling water bath. The only commercial canning I have seen were canning pink salmon and the pulled the racks of cans out of the steam canners hot and put the next one in(no breakage with tin). Fruit and pickles seem to be the main foods such a system works for.

    1. Thanks for writing. It is always great to have somebody who laid eyes on the real thing pipe-up.

      Forgive the pun but steam canning is an entirely different kettle of fish.

      Fish, meat and other non-acidic or non-nitrite treated foods are usually pressure canned because the 180 degrees internal is not sufficient to kill Clostridium spores. Botulism and Tetanus belong to the genus Clostridia, for instance.

      My educated guess is that the steam cabinets had doors that could be pressurized. They probably had a purge cycle where the incoming steam displaced air. Then they allowed the space to pressurize with fifteen or twenty PSIG steam. That is more than what most non-commercial canners want to attempt.

      I don't want to get too emphatic, but many of the niceties of canning like peeling fruit and boiling down sauce are high labor, high fuel operations. Consider peeling peaches or tomatoes. My SWAG is that it triples the labor involved in moving the peaches/tomatoes from the basket on the floor to the jars on the pantry shelf. My gut feel is that the people living in an austere environment would be thrilled to quarter the fruit, remove the pits, if any, stuff them in a jar and process. If they had fruit juice they might pour that in to take out air bubbles.

      On February 15 any skins would be overlooked if there were no other options.

      Again, that is just my opinion.

    2. I'm pretty certain that the srean cabinets were pressurized. On a personal note I have three pressure canners and if we luck into an August moose I'l end up putting what good cuts I can fit in the freezer, brine a five gal bucket for corned meat (I keep a supply of nitetrite cure) and be canning until the rest is done. I have also hard salted both salmon fillets and herring to later be freshened and used. I understand very early settlers in the Catskills of N.Y. did that with spring run suckers. Just some thoughts on feeding people. I wouldn't mind the peach skins at all but consider that some of the high intensity work (like stringing the green beans I mentioned earlier can be done by preteen who nolonger have an Iphone to keep them busy.

  4. Skins/peelings, cores and trimmed bits (nothing good enough for pie filling or fruit as a dessert) are cooked down into fruit butters. Then, run the cooked scraps through a food mill. I tried this the first time out of curiosity. My mother talked about her mother doing this but I never saw Mom do it. This will make a good amount of fruit butter with very little waste. The chickens didn't get much!

    And Howard is right, anybody over the age of 5 is going to be put to work weeding and prepping produce so there is something to eat during winter and early spring.

    1. One other observation, tomatoes were cooked down in my family to conserve jars and lids. They are expensive and you only have a finite amount of jars.

      By the way, Mom saved any jar that a regular or wide-mouth lid-n-ring would screw down snugly on. She usually used those jars for pickling or anything that was water-bath canned since they were suppose to be single-use jars.


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