Recently I got an email from a reader who had read one of my posts comparing the energy usage and general environmental impact of an electric bicycle and electric scooter. The reader wanted to know which was the greener option: Buying frozen vegetables or buying them in a can?
A quick glance at the book Food, Energy & Security is an illuminating and concise answer—a rarity when dealing with these sorts of questions: Frozen Packaging Requires Less Energy to Make
Assume about 450g of corn, the amount that will fit into a normal can. To make the packaging for it, if its going to be frozen requires about 722 kcal of energy, that's about 840 watt-hours if you prefer to think in those terms. To make the can that corn will go in requires about 1006 kcal of energy.
Canning Less Energy Intensive Than Freezing
Then comes the differences in processing: The amount of energy required to process the corn for each storage method. Processing and freezing that 450g of corn requires about 1550 kcal of energy; processing it for canning requires about 1300 kcal.
Canning & Freezing Nearly Equal in Energy Usage
All told, for 450g of corn, the can totals out to be 2,306 kcal, while freezing requires 2,272 kcal. Pretty much a dead heat...Except when you consider that you can store the can in the cupboard without any additional input of energy.
Food, Energy & Security assumes that it's going to take about 120 kcal/month of energy to store each package of frozen corn. That means that if that corn sits around in a freezer for more than about 100 days, the very slight advantage it has over canned corn is gone.
The verdict: In terms of energy usage in packaging and processing, freezing and canning come out pretty even.
Which then leaves us with variables well beyond the scope of this comparison: Nutritional value of canned versus frozen, how long you plan on storing them, if you're going to be doing so for emergency usage when you can't count on electricity being available, the fact that pretty much everyplace will recycle those cans but not necessarily the frozen packaging, etc, etc, etc...
Not to mention a third option: Canning your own.
Frozen veg photo: Paul Roth; tomato cans photo: Stephen Pierzchala, both via flickr
Strawberry Fields Forever: 5 Reasons Why Preserving Your Own Food Is Green
Frugal Green Living: Preserving the Harvest
How Can We Eat Local All Year Long?