Thursday, November 6, 2008

Is it (energetically) cheaper to can or freeze?

I know some of you had a garden this summer. And you were probably thinking to yourself “Man, what the hell am I going to do with all these zucchini?” I’m grateful that so many people overplant, because lots of people gave me excess zucchini. However, I was eventually faced with a quandary about the best method of preservation.

I was thinking about the carbon footprint of different foods, and how the processing affects that carbon footprint.

For instance, if I walk out to my garden and pluck two tomatoes, and just slice one up and eat it, I’m not really increasing the carbon footprint of the tomato. If I take the other one and fry it, I’m obviously using more energy to cook it, and therefore I’m increasing the carbon footprint of my food just by the way I process it.

Thinking about this is actually pretty huge, and I’m going to focus in on one particular aspect of processing: Preservation.

I’m going to focus on peaches, because I preserved 25 lbs of them this summer using a variety of methods. Peaches can be canned relatively easily (without a pressure boiler), and can be frozen equally easily (adding a little anti-browning agent keeps them from turning dark). Figuring out the carbon footprint between different energy sources can be difficult, so to simplify things, I’m just going to compare electrical usage.

The equation should be pretty simple really. We want to estimate the energy used per day of storage via these different methods: The electricity used divided by the duration of storage. For canning, almost all of the energy use will be up-front, and the longer you store the cans, the better overall energy use rate you get. On the other hand, foods stored in the freezer will continue to cost energy as long as you preserve them, so long storage times will lead to bad energy efficiency.

So here's the data I collected for this little exercise. Basically, I needed to figure out the energy cost to keep a pint of peaches frozen. I used dedicated freezers because A) its a hell of a lot easier and B) I've been thinking about getting one. I've cut some of the significant digits off to make this a little more legible, and I'm not going to give you the brands of the freezers, because I don't think it really matters much.

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Energy per year 274 279 442 582
Energy per day 0.731 0.744 1.179 1.552
Capacity in cubic feet 6.8 7.2 14.2 15.8
Peach pints per cubic foot 59 59 59 59
Peach pint capacity 401.2 424.8 837.8 932.2
energy per day per peach pint 0.0018 0.0017 0.0014 0.0016

Ok, so now we've got an estimate of the energy used per day to store peaches in these freezers. Now we've got to figure out what the energy costs of canning the peaches are on a per-pint basis. This becomes a little bit of a problem. The cooking phase has a limited capacity: My boiler only holds 5 pints at a time (for instance). So really, I'm going to assume you're canning in the appropriate increments. I had a hard time actually finding data on my stove, so I just used the data from a 2600 watt replacement burner. Remember, for most canned foods you have to first sterilize the jars and lids (for ten minutes), then heat the food, then process the cans.
So I opted against figuring out the units of energy that would actually be required to do this work, because I've found that I usually just have to crank my burner on high or close to high for the entire time I'm doing the sterilizing, cooking or processing. Therefore, I'm making a worst case scenario here for the canning. Although I'm making a best case scenario in terms of getting 'even' numbers of cans per process. Regardless, here are my estimates of the energy used to can:
Burners 8-inch burner
Energy consumption 2.6
Sterilizing jars 0.166
boiling peaches 0.333
processing jars 0.333
kilowatt hours 2.1632
Peach pints per process 5
kW/h per pint 0.43264
# of Pints 15
Total energy for canning 6.4896

Ok? So what does this mean? Like I said, the critical part here is how long you actually store the food. Let's take a look at a graph demonstrating this:

What we're seeing here is the estimate of energy use through time. Obviously, the canning doesn't change. Once you've done the canning, you're done. On the other hand, the freezer gets progressively worse through time. However, it takes a surprisingly long time for the freezer to get more energetically expensive. Between 8-10 months depending on the brand.

Obviously, there's a lot of wiggle in these numbers. My estimates on the freezer assume that the entire freezer is in use (if not for peaches, then for something) and if this isn't the case, then your energy cost per unit goes up. Let's look, for example, at a scenario where the freezers are operating at 80% capacity:

Now we're talking about a 6-8 month time-period where freezing is more effective. However, we've still got a lot of assumptions built in here. For one, each time you remove a can, but do not turn off the freezer, you've got to replace that can with something, or you'll be incrementally be increasing the cost per unit to keep the other ones cold. The decision to use a freezer is committing you to an unknown proportion of energy going towards keeping empty space very cold, and unless you're willing to turn off the machine and eat the remainder at some point, you could end up with some really bad efficiency. On the other hand canning uses only the resources necessary to preserve the food you've got.

The other set of assumptions revolve around manufacturing costs and cleaning costs being approximately equal.

So now that I've identified all these problems, I'm a little less sure about my result. I think that if you already own the jars and the freezer, this analysis gives you an idea of how best to utilize them, but I think in terms of the original question, I'm going to have to dig deeper.

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