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.
|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|
|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:
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.