More on the Energetics of Food Distribution
In The Energetics of Food Distribution I explored the energy demand associated with transporting food based on estimates of gallons of fuel burned per ton of food moved. This is a perfectly reasonable unit, but a departure from calories of input energy per calorie of edible food, the unit I use in The Energy Cost of Food, The Energy Basis of Food Security and much of my other analytical work in food system energetics. Calorie input/output is a useful framework for studying energy flow in food systems because it affords an intuitive means of gauging the efficiency of transforming input energies - commonly from nonrenewable resources - into edible food energy. In this post I'll revisit my earlier analysis of food distribution through this calorie input/output lens.
It's common to talk about food as if it were a homogenous commodity. It's not. There are many types of foods, and some pack more bioavailable energy into a given weight than others. Take bacon for instance: according to US Department of Agriculture data there's about 420 kilocalories of food energy in 100 grams (3.5 ounces) of bacon . While there are certainly foods that are more calorie dense - butter contains over 700 kilocalories per 100 grams while lard, which is nearly pure fat, packs over 900 - most foods are much less so. Carrots, a staple in veggie trays and at summer farmer's markets, yields only 40 kilocalories per 1oo grams, and lettuce yields fewer than 20.
Given this wide variation in the calorie density of food, it should come as no surprise that the energy input/output ratio - or perhaps more accurately the energy input/food throughput ratio - of food distribution depends not just on how food is moved and how far, but also on what type of food is being moved. An energy intensive mode of distribution that moves a very calorie dense food might yield an energy input/throughput ratio comparable or even superior to a far more efficient mode of transport that moves a less energy dense food. The devil, as they say, is in the details.
Say we want to move comparable weights of bacon and carrots 100 miles via semi truck and pickup truck. Which is more efficient? Assuming the semi carries 40,000 pounds of food at 6 mpg and the pickup carries 1,000 pounds at 18 mpg, it's clear that by far the most efficient option is to move the calorie dense bacon via semi, and by far the worst is moving carrots via pickup. Moving carrots via semi enjoys a slight advantage energetically over moving bacon via pickup, but the high calorie density of bacon cancels out much of the efficiencies of scale that come with moving larger quantities of carrots with a much larger diesel engine. In general it's easier to make a case for transporting calorie dense foods such as meat, cheese and even eggs regardless of mode of transport, since less calorie dense vegetables and fruit require more fuel energy input per unit of food energy moved.
Much of the discourse in the local food movement revolves around the assertion that by buying food produced nearby we reduce the amount of fuel burned in the service of food transport. As I note in The Energetics of Food Distribution this can be true, but given the efficiencies of scale seen in moving larger quantities of food it isn't always true. Another wrinkle worth adding to food distribution discussions, as I've hopefully demonstrated here, revolves around what type of food is being transported, particularly whether we're choosing between shipping bacon and other calorie dense, animal-derived foods or more calorie diffuse foods such as vegetables and fruit. As long as fuel remains cheap the practical differences between these foods are small, but rising energy prices may compel us to be more discerning about what types of food we ship.
- 'Calories' are simply units of heat, and for those unfamiliar with metric prefixes 'kilo-' means 1,000. In popular literature it's common to speak of nutritional calories, and a nutritional calorie is actually a kilocalorie, which is 1,000 thermal calories.
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