Source: Wayfair
The vacuum sealer is that rare device that serves as both a kitchen appliance and a source of entertainment. Every time I run my new Nesco VS-09, I practically want to applaud when it finishes.
I don’t normally give much thought to air. Until it’s all gone. Then the arithmetic of 15 pounds per square inch leads to the realization that this goofy little countertop appliance generates a literal half-ton of crushing force on a 6″ x 10″ pint-sized bag.
But I digress. I actually bought this for the serious purpose of preserving garden produce. The fact that I find the process and results to be so entertaining is just icing on the (perfectly flat half-inch thick piece of) cake.
In any event, there is a serious purpose to this post. And that is to show that if you have a freezer that’s already running, then freezing your tomatoes is by far the most energy-efficient way to preserve them. The only method that would beat that is solar drying, and I haven’t quite figured out how to do that well in my humid Virginia climate.
Tomatoes as freezer free-riders.
The last thing I need is another kitchen appliance.
But I bought this vacuum sealer anyway, after thinking through all the food preservation I did last year. Of all the pickling, canning, drying, and freezing, by far, the tastiest, most garden-fresh results came from freezing. With drying (dried tomatoes) a close second, due to the intense flavors that produces.
And so, purely from a quality standpoint, for tomatoes to be used in soups and stews, my wife and I agree that freezing is the best option. It preserves that fresh tomato taste. But how does it stack up in terms of energy use?
Freezing gets a bad rap, as a means of home food preservation, for its relatively high energy use. But I think that’s not entirely correct.
If you run a freezer expressly for the purpose of preserving garden produce, then, sure, I’d bet that freezing has a fairly high energy cost. In that case, you’d have to pro-rate the annual electricity use of that freezer over the pounds of produce preserved. (Because, by assumption, you wouldn’t be running that freezer if you weren’t using it to preserve your garden produce.)
Just tossing out some round numbers, based on past experience, I’d bet that a typical 15-cubic-foot chest freezer has enough space to store 300 pounds of produce, and consumes about 300 kilowatt-hours (KWH) of electricity per year.
So, roughly speaking, if you run that freezer because you use it to preserve your produce, you’d consume about 1 KWH of energy for every pound of produce preserved.
By contrast, if you are already running a freezer, and will continue to run it regardless, and you have the space, then freezing your produce only costs you the energy needed to freeze it in the first place. The cost of running the filled freezer doesn’t count, because you’d bear that cost in any case.
My fridge comes with a big freezer. It’s not like I’m planning to unplug that any time soon. And so, I’m perfectly happy to let my frozen garden produce be a free rider here — taking advantage of the fact that the freezer is running, but not being asked to “pay” for it.
In that case, the only additional energy cost is the cost of getting the room-temperature produce down to the 0 F temperature of the freezer. Given that (e.g.) tomatoes are 94% water, that’s more or less the energy required to bring one pound of room temperature water down to 0 F. Including the one BTU per pound required to cool the water, and the 144 BTUs per pound required to convert to ice, that works out to (70 + 144 =) 214 BTUs, or (at 3.4 BTUs per watt-hour) 63 watt-hours. So, if you are just tossing your produce into a freezer that is going to be running in any case, freezing it takes 0.063 KWH for every pound of produce preserved.
You might think that’s a bit of a cheat, because one way or the other, you’ll want to peel those tomatoes before you use them. The most typical methods for peeling them involve heat (either boiling water, or holding them in the flame of a gas stove). But — surprise — it’s actually a snap to peel them after they’ve been frozen, per this YouTube video.
Take a look around 47 seconds into that video. My jaw dropped just after the tomato did. I know the term life-changing is overused, so let’s just say this was a tomato-life changing revelation for me. As in, I’m never going blanch and peel a tomato ever again. Arguably, it may actually take less energy to freeze-and-peel than to blanch-and-peel, what with the energy costs required to boil the water and cool the tomato afterwards.
Other preservation methods
I have already tracked the energy costs of preserving by canning or drying, in various earlier posts. Let me bring all of that together in one place, below.
Drying tomatoes in my four-tray Nesco dehydrator consumed 8 KWH of electricity (per Post G21-049). That was in the humid outdoor Virginia summer. I am fairly sure that each tray can hold less than a pound of quarter-inch-thick tomato slices,, but a) I could stack up to 12 trays at a time for drying, and b) those were very “wet” slicing tomatoes, not the paste tomatoes that are normally used for drying. That said, for illustration, let me just assume one pound per tray, four trays, yield 2 KWH for every pound of produce preserved.
Canning tomatoes in a water-bath canner consumes a considerable amount of energy as well. I did the full workup on the energy cost of home canning two years ago, in Post #G22. I had to do that because, as far as I can see, the rigorous research literature on this crucial topic looks like this:
In any case, the all-in energy cost for canning five quarts of pickles, on a gas stove, in an air-conditioned house, was 5528 kilocalories (kcal).
Source: Post #G22.
Per the USDA guide to home canning, quarts of pickles require a much shorter processing (boiling) ,time (15 minutes) compared to quarts of tomatoes (45 minutes) in a water-bath canner.
Based on my prior calculation (shown above), I need to add another 800 Kcal to account for that, bringing the total up to 5300 Kcal for 5 quarts (= 10 pounds) of tomatoes. At 1.16 watt-hours per kilocalorie, that works out to be 0.6 KWH for every pound of produce preserved.
I should note that this is a little conservative, because you have to peel the tomatoes first. That’s going to involve a little additional boiling time. But with all the boiling that’s taking place with the canning, I figured that was more-or-less rounding error.
Finally, I can take a rough guess at the energy cost of my crock-pot spaghetti sauce. Crock-pot spaghetti sauce (Post #G21-048) absolutely minimizes the labor input, and is idiot-proof to boot. But it requires processing tomatoes in both a pressure cooker (briefly) and a crock-pot (overnight). For four quarts (eight pounds), the crock-pot portion uses about 4 KWH. But the pressure-cooker portion (20 minutes at pressure) likely used almost as much energy as canning, so for four quarts I need to add one-third of my pickle canning estimate above, which, by the time all the arithmetical dust has settled, adds another 2 KWH. Or a total of 6 KWH for 8 pounds of tomatoes, or 0.75 KWH for every pound of produce preserved.
Edit, fall 2024: In hindsight, that’s much more energy-intensive than a more traditional reduce-it-on-the-stove approach to making tomato sauce. A crock pot is, in fact, a terrible (but idiot-proof) choice if you want to evaporate water out of a sauce. I’ve gone back to making my spaghetti sauce by boiling down tomatoes on the stove, like a normal person. I still briefly pressure-cook, dump in a strainer to remove the liquid, pass the solids though a Foley mill to remove the skins, then reduce. This allows me to use all types of tomatoes, including salad and cherry tomatoes, without having to peel or seed them first.
There’s no additional energy cost for peeling in this method, because the entire batch of tomatoes is run through a Foley mill after pressure-cooking. That takes out the peels and (most of) the seeds.
Let me now produce the nice neat table of energy required for food preservation, all of it expressed in terms of KWH of energy per pound of produce preserved.
All of that comes with some caveats. The canning was done on a gas stove in an air-conditioned house. The drying was done outside, in humid air. I could dry up to twelve trays at once, instead of the four that I already owned. Maybe there’s a little more energy required for the blanch-and-peel step in some methods. And so on.
Nevertheless, the results are so clear as to be undeniable. (So clear that I double-checked that freezer math a couple of times). If you have space in your freezer, and you’re going to run that freezer anyway, by far the most energy-efficient way to preserve tomatoes is to toss them in the freezer. And, per that YouTube video above, peel them as you thaw and use them.
I surely need to mention the one common method that isn’t on the list, solar (or open-air) drying. Plausibly that has zero energy cost, but I have not (yet) figured out how to do that in my humid Virginia climate. I’m already working on how I’m going to improve my simple $18 plastic-tote food dryer (Post #G21-049). The solution might be as easy as “don’t overload it”.
Two minor caveats: COP and GHG sold separately.
Two minor factors make this conclusion somewhat less that complete. Those are coefficient of performance (COP) of a freezer, and the different rate of greenhouse gas (GHG) emissions for natural gas and electricity used in the home. Near as I can tell, neither of these results in any material change in the relative efficiency of the various preservation methods.
First, this calculation isn’t complete because it doesn’t factor in the energy conversion efficiency or coefficient of performance (COP) of refrigerators or freezers. The coefficient of performance for a heat pump is the amount of heat energy it can move, for a given amount of electricity supplied to it. Almost all commercially-used heat pumps have a COP greater than 1.0. That is, they can move more than 1 KWH of heat energy for every KWH of electricity they consume. COPs for modern AC or heat pump units typically run around 2.5 to 3.5 (per the link above).
The estimate above — 0.063 KWH — is the amount of heat that needs to be (re)moved from the interior of the freezer. It will actually take less than 0.063 KWH to do that, because fridges and freezers are just another form of heat pump with a COP greater than one. While Wikipedia (cited above) assures me that they have a COP greater than 1.0, I have yet to find a source that will pin that down further. The best I’ve found is a passing reference to a COP of around 1.0 for a deep freeze unit (per this reference).
The bottom line is that a typical home freezer might use somewhat less than 0.063 KWH to remove 0.063 KWH of heat energy from its interior. But how much less, I can’t find the source that will let me pin that down. I suspect that, given the large temperature differential between interior and exterior, the COP of most freezers isn’t much higher than 1.0 or so.
Finally, KWH is not the same as GHG (greenhouse gases). This only measures energy consumed within the home, and does not differentiate between natural gas and electricity. Fossil-fuel based electrical generation is far from 100% efficient, so the actual amount of fuel consumed (to generate the electricity) is a low multiple of the energy actually delivered to the house. But in addition, electrical generation consists of a mix of generation sources, some of which create greenhouse gases, some of which do not. If the ultimate question is one of carbon footprint, we’d have to modify this calculation, treat electricity and natural gas separately, and then redo it for some assumed electrical generation mix.
That said, when I take a rough cut at the difference between natural gas (burned in a stove) and electricity (produced with a typical U.S. generating mix), I’m not sure that adjusting for each fuel type separately would make much difference.
Natural gas releases 100% of its energy within the home. But a typical natural gas stove is only about 40% efficient. That’s the energy that goes into whatever you are trying to cook, with the rest simply serving to heat up the kitchen. Basically, for every 100 units of C02 produced, you get 40 units of usable energy from your gas stove (Whatever units might mean, in this case).
For electricity, by contrast, the amount of fuel burned at the generating plant is far more than the amount that makes it into your home. But once it gets to your home, I’ve either directly measured 100% of what was consumed, or the theoretical calculation (for freezing) should be close to that. And so, as with natural gas, for every 100 units of C02 produced in generating electricity, you get X units of usable energy in the home.
The problem is that X depends on the generating mix that feeds your particular section of the grid. Even so, let me do the arithmetic for Virginia’s electrical grid.
Last time I checked, Virginia’s electrical grid released 0.7 pounds of CO2 per KWH of electricity delivered. Starting from that, I’m going to compare C02/KWH of usable energy for the Virginia grid versus a 40 percent efficient gas stove.
The EPA shows that burning a therm of natural gas releases an average of 0.0053 metric tons of C02. At 2204 pounds per metric ton, that’s 11.7 pounds of C02 per therm. A therm is 100,000 BTUs, and there are 3.4 BTUs per watt-hour.
Slapping that all together, burning a therm of natural gas produces 11.7 pounds of C02 and 29.4 KWH of (heat) energy, or 0.4 lbs C02 per KWH.
But a natural gas stove is only 40% efficient. A stove has to use (1/.40 =) 2.5x as much natural gas to deliver that usable KWH of heat. The bottom line is that a 40 percent efficient natural gas stove releases 1.0 pounds C02 for every usable KWH of heat delivered in the home.
And so, per KWH of usable energy, in terms of GHG emissions, electricity (in Virginia, at 0.7 lbs C02 per usable KWH) is slightly cleaner than natural gas burned in a (typical) 40 percent efficient stove. But only slightly. So the electrical options actually perform a little bit better than shown in the table above, relative to the gas-stove-intensive canning.
There’s nothing in any of that to change the conclusion that tossing your tomatoes into a freezer that would be running in any case is by far the most energy-efficient way to preserve them.
So, what about that vacuum sealer?
All of the above brings me back to my new toy, the vacuum sealer. If I’m going to freeze my tomatoes, the binding constraint is now the space they take up in the freezer, and secondarily, the length of time they’ll last once frozen. Both of which will be best addressed by vacuum-sealing them.
Most sources suggest that you freeze the tomatoes before vacuum-sealing. But at least one source shows tomato chunks that were vacuum-sealed and then frozen. That’s what I’m now aiming to do, only using whole tomatoes, not chunks. Given the literal tons of force that one of these sealers can generate, I’ll have to use the setting that allows the strength of the vacuum to be controlled manually. In the end, I’m aiming for a freezer stocked with nice, flat, well-preserved packages of energy-efficient frozen tomatoes.
With any luck, we’ll see how that all plays out in a few months.
