Students will learn the role of bacterial fermentation and evaluate the effect of fat content, sugar content (lactose), and temperature in bacterial fermentation as they make yogurt. Grades 6-8 

active cultures: also called a live culture; a colony of living microorganisms (e.g. bacteria), growing within a substance such as yogurt

bacteria: a group of single-celled living things that cannot be seen without a microscope that reproduce rapidly and sometimes cause diseases

fermentation: a food-based reaction caused by the action of enzymes that breaks compounds into simpler substances; used for food preservation and preparation

yogurt: a milk-based food product that is prepared from the bacteria of fermented milk

FoodMASTER (Food, Math and Science Teaching Enhancement Resource) is a compilation of programs aimed at using food as a tool to teach mathematics and science. For more information see the Background and Introduction to FoodMaster Middle document. This lesson is one in a series of lessons designed for middle school. 

Milk, cheese, yogurt, and ice cream are all part of the dairy group. Drinking milk helps build strong bones. Unfortunately, most Americans do not drink or eat enough foods from the dairy group every day. For this reason, it is important to learn about the science and nutrition of dairy foods. In this lesson, students will explore milk by learning about the role of bacteria in yogurt production and the many health benefits associated with consuming dairy products.

When milk is fermented with bacteria, the sugar in milk, lactose, will turn into lactic acid. The lactic acid will cause the milk to curdle and thicken. This process is used to make certain dairy products, like yogurt. Yogurt is made my mixing two types of bacteria, Lactobacillus bulgaricus and Streptococcus thermophilous. In mass yogurt production, bacteria are added to milk and the mixture is heated. The heating process promotes fermentation and helps the yogurt develop the desired consistency, flavor and acidity. Once the desired consistency has been reached, the fermentation process is stopped. The yogurt can then be chilled or heated. Chilling allows the bacterial culture or active culture to stay alive. If the yogurt is heated, the cultures are destroyed. Consuming these products will help maintain and/ or restore normal intestinal bacteria.

1. Tell students that you are thinking of a food that requires bacteria and fermentation to make. Allow students time to think, then ask them if they can name the food you are thinking of (yogurt).
2. Provide further clues until students can identify yogurt as the food you are thinking of. Examples include:
   • This food is considered "live" because it has active bacteria cultures.
   • This food contains "good" bacteria that aides in digestion.
   • This food is sweet, tangy and creamy.
   • It comes in many forms including low fat, fat free, and Greek.
3. Ask if students know how yogurt is made. Show the video clip, How It's Made: Yogurt. 

Lab: Magnificent Microbes

Teacher Preparation:

1. Review information found in the Background Agricultural Connections section of the lesson, lesson Procedures, and the attached Essential Files.
2. Note that this lab investigation will take two days to complete. Students will set up the lab investigation on day 1 and will complete the investigation on day 2.
3. Prepare materials for each group. It is recommended that you plan approximately one-week ahead to allow time to gather needed materials. You will need to have coolers and ice on hand the day the investigation begins. Additional coolers may be needed if you plan to complete the lesson with multiple classes.
4. Assign each group a milk type. During the investigation, each group should receive 1 cup of their assigned milk type. For example, if you have six groups, two groups will receive 1 cup of  heated lactose-free milk, two groups will receive 1 cup of heated skim milk, and two groups will receive 1 cup of heated whole milk (each heated to 175° F).
5. You will also need to heat an additional 2 cups of whole milk for student observations of heated whole milk placed in a cold environment. After heating the milk to 175°F, allow it to cool to 120°F. Pour 1 cup of milk into two separate styrofoam cups. Then stir in 1 Tbsp. of plain yogurt.
6. Toward the end of the investigation:
   • Boil enough hot water to fill the four heavy-duty plastic bottles with very hot water and place them in one of the coolers. This will act as an incubator for the warm environment.
   • Fill the second cooler with enough ice to fill half way. This cooler will act as the cold environment. If accessible, a refrigerator can be used as an alternative to the cooler.
   • Each group will leave 1 cup of their assigned milk type (whole milk, lactose-free milk, or skim milk) in the warm cooler. For the class to observe, you should also place 2 cups, filled with heated whole milk, in a cold cooler (ice-filled) overnight.
   • Students will record observations the following day.

Lab Procedures:

1. Distribute lab materials. It is recommended that materials are organized into stations for easier distribution. Students should be arranged in small groups of 4-5. Each group should receive the lab supplies outlined in the Materials section as well as 1 copy of the Magnificent Microbes lab sheet.
2. Before beginning the lab investigation:
   • Require students to wash their hands.
   • For food safety reasons, DO NOT allow students to taste any of the yogurt samples. It is not safe to taste sample due to the inability to completely control contamination from bacteria present in the surrounding environments. If you would like your students to taste different types of yogurt, purchase pre-packaged yogurt from a store. Packaged yogurt should be refrigerated until tasting.
   • Emphasize the importance of practicing good food safety behaviors by not consuming substances used as part of the lab investigation.
3. Launch Day 1 of the lab by asking students to make a prediction about which milk type will produce the most yogurt and why (see top of page 2 of the lab sheet.
4. You (teacher) should prepare the milk (see Teacher Pre-Lab Preparation). Students will allow milk to cool and then mix in plain yogurt. Yogurt is produced from bacterial fermentation of milk. The lactose in milk is fermented by the bacteria. Bacteria or “yogurt cultures” used to make the yogurt in this lab are obtained from the addition of the plain yogurt. The bacteria will ferment the lactose present in the milk and produce lactic acid. The lactic acid will act on the milk protein, providing yogurt with its characteristic texture and tang.
5. Instruct students to make visual observations of their assigned milk type. Students should specifically observe the texture, color, and odor. Groups should then be allowed to share their assigned milk types to allow students to complete Table A on the 4th page of their lab sheet.
6. Begin Day 2 of the lab by showing students the provided video lab demonstration, Lab: Magnificent Microbes. The video will help students understand what they should see in their milk mixtures on day two of the lab. 
7. Instruct students to gather the milk samples they stored on day 1. Students should make observations of their assigned milk type. Students should specifically observe differences in texture, color, and odor. Do not forget to pass out the cold storage whole milk samples for students to observe. Observations should be recorded on Table A of the lab sheet.
   1. Lactose-free Milk (Warm Storage): Students should observe a small amount of liquid on top of the milk. When stirring, the milk should have a thick texture; however, compared to skim and whole milk the texture will be less thick. This milk type will be less thick compared to skim and whole milk due to the absence of lactose. Typically in yogurt production, lactose is fermented by the bacteria producing the lactic acid needed to produce yogurt.
   2. Skim Milk (Warm Storage): Students should observe very little liquid on top and the thickness should between Lactose-Free and Whole Milk-Warm.
   3. Whole Milk (Cold Storage): Students should observe no formation of yogurt, however, a small amount of liquid may be produced.
   4. Whole Milk (Warm Storage): Students should observe formation of a thick yogurt with little to no liquid produced on top. The comparison between the cold and warm storage will demonstrate the role of temperature in yogurt production. Warm temperatures are needed to encourage bacterial growth. Temperatures that are too hot or cold will negatively impact bacterial growth and may cause denaturation of the milk proteins.
8. Allow students to work in small groups on the Magnificent Microbes lab sheet and respond to lab questions. Encourage students to refer to the table in the reading to help support their conclusions. Answers to the questions can be found in the attached Teacher Key.
   • (Optional) Launch the lab extension by allowing students to observe an active yogurt culture under a microscope.

At the conclusion of this activity, review and summarize the following key concepts:

• Processing some whole foods, such as milk into yogurt, can add value to foods by increasing shelf life, altering the nutrients per serving, and adding variety to food.
• Scientific processes are used to create many of the foods we eat on a daily basis.
• Microbes, or tiny organisms, are used to make yogurt.

1. http://www.huffingtonpost.com/2014/03/26/yogurt-fun-facts_n_4952223.htm
2. https://www.wsj.com/news/articles/SB10001424127887324886704579052974211438870?utm_source=huffingtonpost.com&utm_medium=referral&utm_campaign=pubexchange_article
3. http://mentalfloss.com/article/63073/12-tangy-facts-about-yogurt

This lesson was partnered with East Carolina University. The FoodMASTER program was supported by the Science Education Partnership Award (SEPA) which is funded from the National Center for Research Resources, a component of the National Institutes of Health.

• Primary Authors:
  • Virginia Stage, PhD, RDN, LDN
  • Mary White
  • Ashley Roseno, MAEd, MS, RDN, LDN
  • Melani W. Duffrin, PhD, RDN, LDN
• Graphic Design: Cara Cairns Design, LLC

• Brittlelactica: Planet in Need
• Dairy in the Mountain West: Our Family of Farmers
• Eat Happy Project video series
• The American Dairy Industry