How Much Energy is on my Plate?

Contributed by Lane Seeley, Seattle Pacific University and Karin Kirk, SERC

This activity addresses leads students through a sequence of activities that highlight the embodied energy that is necessary to produce various types of food.

Context

This activity can be used in many types of courses, from high school through college level. The topic of the energy requirements of food production could fit into a course on energy, environmental science, policy, or human health. The activity contains several parts which can be used in combination or separately.

Goals

After completing this activity students should be able to:

• complete simple calculations and unit conversions
• determine which source of protein requires the lowest energy input
• explain why different sources of food require different inputs of energy
• explain how variables like farming methods, processing methods and transportation can change the energy input of a given food.

 

Activity Description

Step 1. Concept Sketch 
(Learn more about Concept Sketching from the Starting Point project)

1. In a whole class discussion, have students describe a meal with several items on the plate. The meal can contain some meat or seafood, some grains and some vegetables or fruits.
2. Break the class into groups of 3-4 students. Assign each group one food item on the plate.
3. Have each group create a concept sketch of the steps required to produce the food item and get it on the plate. Students should trace the food production as far back as possible and should include as many inputs as they can think of. Arrows and labels can be used to accompany the drawings. Each group will need a sheet of poster paper and markers.
4. After each group has completed their concept sketch (which can take from 15-30 minutes), conduct a gallery walk through the meal, with each group describing their sketches to the entire class. 
   (Learn more about Gallery Walks from the Pedagogy in Action project)
5. In a whole class discussion, ask students for their responses to the sketches. What surprised them? What elements of food production had they not considered prior to this activity?

Step 2. Movie: King Corn (optional)

Have students watch the movie, keeping an eye out for more details about aspects on their concept sketches. Have they left any parts out? Did they learn more about parts of their concept sketches?

Step 3. Energy to Produce Various Foods

Students return to their teams and work as a group to determine how much energy goes into producing a given type of food. Each team will pick a source of protein for their diet. We'll assume that a person needs at least 30 grams of protein per day. Your team's goal is to estimate the amount of energy that is required to produce your 30 grams of protein per day.

Along the way, each team should answer the following three questions:

A. What quantity of your food in grams is required to provide 30 grams of protein?

B. How many food calories of your food is in the amount that provides 30 grams of protein?

C. How much energy is required to produce the amount of your food that will provide 30 grams of protein?

Start by doing soybeans together as an example:

• 100 grams of fresh (green, raw) soy beans has about 13 grams of protein and and contains 147 food calories or kcal of energy. (This value was found using the USDA National Nutrient Database)
• So we would need to eat about 230 grams (about 1 ½ cups) of soybeans to get 30 grams of protein.
• By this estimate it takes 7,800,000 kcal of energy to grow 5,556 kg of soybeans. ([https://biodieseleducation.org/Literature/Journal/2005_Van_Gerpen_Biodiesel_energy_bal.pdf])
• So ... to grow 1 kg of soybeans takes about 1400 kcal (1.6 kWh) of energy.
• Therefore to grow 230 grams of soybeans would take about 320 kcal (0.4 kWh) of energy.

To simplify things we will make two big assumptions:

• We will assume that chickens, cows and pigs only eat soybeans.
• We will also assume that the only energy that goes into producing animal based protein is the energy that goes into the food the animals eat.

Obviously these assumptions are pretty unrealistic but they we allow us to make some meaningful comparisons between protein sources. Later on we will talk about these assumptions and discuss a variety of food production issues that they might cause us to overlook. 

To constrain the activity somewhat, students should select from the following list of foods:

• beef
• cheese
• chicken
• eggs
• milk
• soybeans
• pork
• corn
• apples

Depending on the type of course, students can be left to search for answers to the question on their own or they can be guided toward the answer by providing some key numbers and letting them do the math from there. Here is one way to scaffold the activity so that the different teams provide estimations that can be reasonably compared with one another.

At this point we could either turn them loose to do research or provide more guidance. I would anticipate a lot of questions and discussions within the group. Some of these questions could be raised by the instructor if a group quickly reaches a conclusion.

For example:

• How much milk does a cow make in a day?
• How much does a cow eat in a day?
• Do we need to worry about feeding the cow before it is old enough to be milked?
• Do we need to worry about how many years a cow gives milk?

When the groups finish making their estimate they can present their results to their classmates using whiteboards or a more formal slideshow.

Follow-up activities could include:

• Comparison of the energy content in the food to the energy required to produce the food. Students may not at first realize that a food calories can be quantitatively compared to other type of energy. They also may not recognize the the energy used to produce the food does not include the light energy that is necessary for the soybeans to grow.
• Comparison with the values given here: http://www.theoildrum.com/node/6252
• Discussion of the different methods of raising livestock (grazing, feedlot, etc...) There seems to be a wide variety of opinions and conclusions here.
• Estimates of the energy in transportation required to produce the protein products. Is transportation a big part of the energy budget? This could be done first as a large group brainstorming session with individual components delegated to different teams.
• How important is methane in the greenhouse gas production associated with livestock?

Teaching Materials

some solutions in excel (Excel 2007 (.xlsx) 40kB Jun27 11)

Graph showing the energy inputs needed to create a can of sweet corn

Assessment

Assessment of this activity can be done via the calculations students do to determine the most efficient type of food, from their responses in class discussions, and their performance on similar questions used in class exams. This activity can be used as part of The Lifestyle Project.

References

• Energy Inputs for US Food System (Acrobat (PDF) 139kB Apr18 11)
• List of Foods by Environmental Impact and Energy Efficiency
• Protein in common foods from the USDA National Nutrient Database for Standard Reference (pdf file)
• USDA National Nutrient Database - for looking up protein and other nutrient values in various foods
• Where does methane come from (EPA)
• Life cycle assessment of farming systems
• The Greenhouse Hamburger
• Greener Pastures by Judith D. Schwartz, Conservation Magazine. This article discusses how properly managed cattle grazing can improve grasslands and reduce the loss of organic carbon from topsoils.
• Food, Energy, and Society, Third Edition, by David Pimentel and Marcia H. Pimentel
• Life Cycle-Based Sustainability Indicators for Assessment of the U.S. Food System, by M.C. Heller and G.A. Keoleian. 2000. University of Michigan: Ann Arbor.
• Learn more about Concept Sketching from the Starting Point project.
• Learn more about Gallery Walks from the Pedagogy in Action project.