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Science and Our Food Supply - Middle School Guide: Module 2 - Farm

Science and Our Food SupplyTeacher's Guide for
Middle Level Science Classrooms

2007 Edition

Return to Table of Contents



Module 2 - Farm - Who's responsible for food safety? It's everyone's responsibility, from the farmers who grow the food to the people who place the food on your table.

Hand holding a pencil in a green circle background Chain of Food - explores the path food takes along the Farm-to-Table Continuum.



Video tape in a green circle background This section explains the specific science concepts presented in Module 2 of the video/DVD, including fascinating facts relative to the module. Read this section before watching the video module or conducting the activities and experiments.

man in white coat in a laboratory, talking

Dr. Theodore Elsasser
explains competitive


There are many places on a farm that can be contaminated by harmful bacteria, so farmers have to make sure that the areas where food is handled are kept clean and at the right temperature. There are many innovations on the farm that help prevent the growth of bacteria - like special areas for washing vegetables, refrigerated storage areas for milk and eggs, and portable sanitation in fields.

Competitive Exclusion

Salmonella is a foodborne pathogen sometimes found in the intestines of chickens. It can be passed on in the meat and also in the chicken's eggs. The best way to reduce the risk of foodborne illness from eating contaminated chicken is to prevent the Salmonella from living in the animal in the first place.

Using a process called competitive exclusion, chickens ingest a blend of good bacteria, which ultimately

There are chickens in an open container and a spray container aimed at them

Chickens being sprayed
with good bacteria

shields them from pathogenic Salmonella microbes.

Young mammals are born with undeveloped gastrointestinal tracts. It's fertile ground for both good and bad bacteria. Whichever organisms get introduced to their systems first will take over.

Scientists developed mixtures of beneficial bacteria to prevent bad bacteria, like Salmonella, from colonizing and infecting the chickens. To make it work, scientists use a blend of nonpathogenic bacteria naturally found in the gastrointestinal tract of mature chickens and spray it on day-old chicks. Through the natural interactions of the chickens grooming each other, the bacteria enter their intestinal tracks.

Competitive Exclusion results in naturally disease-resistant, mature, healthy birds - making it virtually impossible for Salmonella to multiply. It also reduces Salmonella in the environment because there are fewer infected birds to contaminate the farm.

Compost field with a tractor

Compost fields at the
USDA Agricultural Research
Service in Beltsville, MD

Composting to Kill E. Coli:

Another way farmers keep down the spread of bacteria is through composting.

Compost is actually made up of the decomposed parts of all the residuals that come from the farm operation - the waste from the animals, leftover food the animals didn't eat, hay/straw, etc. It all gets mixed together and heaped up so that the microbes can eat it and create compost, which the farmers use to fertilize their crops.

The microbes are basically getting a workout from eating all of the organic materials. As the microbes work at digesting the wastes in the compost, the temperature of the compost rises. The heat plays an important role because E. coli O157:H7 can't survive in temperatures above 131° F (55° C).

E. coli may be found in the manure that is used in the compost. So farmers have to be very careful

A woman in the field holding a temperature gauge

Dr. Patricia Millner
discusses composting

about cross-contamination when the compost is used on any crops, but the risk may be greatest for low-growing crops, such as lettuce and strawberries. Scientists are working to develop ways for farmers to assure that their compost reaches high enough temperatures to kill pathogens and make the compost safe for their crops. Note: This is still in the research stage.


Microbes eating the organic materials in the compost heat up so much that they actually cook themselves.


Hand holding a pencil in a green circle background (large)


Time: One 45-minute class period


Students will explore the path food takes along the Farm-to-Table Continuum. They will begin on the farm, and throughout the unit they will investigate food safety issues during processing, transportation, at restaurants and supermarkets, and finally, in their own homes. Teams will identify how food can become contaminated along the continuum and develop and present strategies for preventing contamination at each step.


Everyone along the Farm-to-Table Continuum plays a major role in keeping our food safe. If a link in this continuum is broken, the safety and integrity of our nation's food supply can be threatened.




  • Dr. X and the Quest for Food Safety video/DVD, Module 2 - Farm
  • Food Safety A to Z Reference Guide ( [Farm-to-Table Continuum], [Farm-to- Table Initiative]), and Food Safety Farm-to-Table Illustration
  • Cooked hot dog on a bun
  • Grated cheese
  • Relish
  • Banana
  • Paper plate
  • Poster board
  • Markers


Put the grated cheese and relish on top of the cooked hot dog in the bun. Place the hot dog and the banana on a paper plate and set the plate where the students will see it when they enter the room.


Use food specific to your region or to the tastes of your students. Just make sure that a variety of food groups and types are represented - meat, dairy, fruits or vegetables, fresh, processed, cooked, local products, imported foods, etc.




Look surprised when someone mentions the hot dog or banana. Then go over, pick up the hot dog and banana and ask: Does anyone know where these foods came from? Let the students speculate for a few minutes. Then comment: I confess, I put them there, but let's look at who else played a part in getting the hot dog, bun, cheese, relish, and banana to us. Allow the students to review the Farm-to-Table Continuum steps (farm, processing, transportation, retail, and home) they learned in The Big Picture activity (Module 1).

  • Tell the students: In the next few activities, you'll learn about people you never dreamed had a role in getting this food to you.
  • What does science have to do with the farm? Give the students time to make a few suggestions.
  • Then ask: What do you think could happen to food along the Farm-to-Table Continuum that could affect the safety of our food supply? List their answers on the board.
  • Food doesn't start at the supermarket or restaurant. Today, we'll trace the path of food along the Farm-to-Table Continuum and discover some of the ways it can become contaminated. Then we'll develop and present strategies for preventing contamination at each step.

Video tape in a green circle background TIME TO TUNE IN . . .Module 2 - Farm

Let's tune in to the first step on the Farm-to-Table Continuum. While watching this module, keep these questions in mind:

  •  Would you feed a baby chick bacteria? Why or why not?
  •  What's compost all about, and how is it relevant to food safety on the farm?
    Tune in, and take notes. Show video/DVD Module 2 - Farm (Time: 4 minutes).

Video tape in a green circle background INSTANT REPLAY Time to review and summarize.

  1. Why did Dr. Elsasser feed a baby chick bacteria? (Good bacteria are fed to baby chicks, so there is no room left for the bad bacteria to grow.)
  2. What did you find interesting about Dr. Elsasser's job?
  3. We also met Dr. Patricia Millner, another scientist who conducts research for keeping our food safe on the farm. What did she say about compost, and how is it relevant to food safety on the farm? (It's heat again. If enough heat can be generated from the compost, it will kill harmful bacteria, especially E. coli O157:H7. The compost is then safe to use on crops that we will eat.)
  4. How does Dr. Millner's research benefit us? (It will help keep our food safe.)


  1. Divide the class into 5 groups. Assign a food to each group (hot dog, bun, cheese, relish, and banana).
  2. Have students begin researching their food. Using poster board, let each team trace their food from the farm to the table. This will serve as the "first draft" of their food journey chart. Remind students that some foods are imported from other countries, so be sure to trace them from their origin. (Students can find out where a variety of foods come from by visiting the Economic Research site.)
  3. Post the charts around the classroom, and keep them up during the unit. As the teams learn more about the continuum, they can add to or change the information.
  4. Challenge the students to include all the people involved at each step (e.g., farmers, produce pickers, milkers, truckers, grocery workers, shelf stockers, restaurant workers, etc.). Create a competition that focuses on which team can identify the most people. This challenge comes in #5.
  5. For each person the team identifies, they must include what that person does to help control the spread of bacteria. Students should label all the places where contamination of their food may occur, then write a strategy for preventing that particular contamination. Use the 4 Cs to help develop the strategy. For example, in the video they learned about the potential contamination of crops at the farm - the compost must reach at least 131° F (55° C) to ensure that the compost doesn't contaminate the crops. One suggestion could be to develop ways for compost to reach high enough temperatures to kill pathogenic bacteria and to make the compost safe.
  6. At the end of Module 4, have each team share their food journey chart with the class. The team that traces the banana should also address the global issue. Ask students: What do these foods have in common? Where do the similarities and differences occur along the Farm-to-Table Continuum?
  7. Have each team add up the number of people they identified. Which food had the most people involved in the Farm-to-Table Continuum? Why?


  • Food Safety A to Z Reference Guide
    (See the following terms - Bacteria, Competitive Exclusion, Composting, Contamination, Cross-Contamination, Escherichia coli O157:H7, Farm-to-Table Continuum, Farm-to-Table Initiative, Foodborne Illness, Food Safety, and Pathogen.) Also see the Farm-to-Table illustration and the 4 Cs section.
  • Dr. X and the Quest for Food Safety video/ DVD Module 2 - Farm
  • Web site
    Economic Research Service/USDA


Everyone along the Farm-to-Table Continuum plays a role in keeping our food safe from harmful bacteria. If a link in this continuum is broken, the safety of our nation's food supply is at risk. There are food safety precautions, including the 4 Cs of Food Safety, that help prevent contamination of food at each step.


  • Visit the Economic Research Service Web site, find your favorite food, and see how many different countries it comes from. Or, select a country and see how many foods we get from that country.
  • Using the Web site above, look on a map and calculate how many miles your favorite food traveled from one of the countries to your state. For example, how many miles did the banana travel from where it was grown to your state?
  • Relate your pathogen to this experiment and record the information in your food safety portfolio.


Career Connection icon with lab coat (large)

See real-life scientists in action!

  • Food Safety A to Z Reference Guide


UP NEXT . . . Ever heard of methylene blue?
Well, it's a clue to a very important concept in pasteurization technology. You'll discover the clue in the next lab experiment as you, along with Dr. X and Tracy, explore Processing - the next step along the Farm-to-Table Continuum.