YOGURT ANYONE?

(Testing the pH of Yogurt)

by Paulette M. Ashworth and Bobbie K. Keller

Louisiana Curriculum Framework Content Strand: / Science as Inquiry and Life Science / Grade
Level / 10 - 12
Objectives: The students will be able to:
  • Use a CBL and pH probe to measure the pH of a milk and yogurt solution over a period of 20 hours.
  • Construct and correctly label a table and line graph of the data collected during experimentation.

Teacher Information

Benchmarks
SI-H-A1, SI-H-A2, SI-H-A3, SI-H-A4, SI-H-A7, SI-H-B2, SI-H-B3, SI-H-B4, LS-H-D3, LS-H-F2, LS-H-F3, LS-H-G2 / Time Frame
50 minutes Set-up (Day 1)
20 hours Data Collections (overnight)
50 minutes Results and Analysis (Day 2)
Curriculum Integration
Math and English / Materials
CBL system (power adapter recommended), TI-83+, link cable, pH probe, temperature probe or thermometer, buffer calibration solutions (pH = 4.0 and 7.0), distilled water, 50 ml beaker, 600 ml beaker, 1000 ml beaker, 400 ml whole milk, one teaspoon of plain yogurt containing active cultures, hot plate, ring stand and clamp, beaker tongs, aluminum foil, stirring rod.
Applications
Real life situations: Students test yogurt. / Student Groupings
Groups of 3 – 4.
Possible Obstacles to Student Learning
  • Lack of knowledge and use of equipment.
  • Graphing and labeling errors.
  • Lack of knowledge and understanding of pH.

Opportunities for Assessment
  • Group or individual lab reports. (Lab reports should include data tables, graph of results, and analysis questions.
  • Practicum. (Students demonstrate use of CBL, calculator, pH probe, and temperature probe or thermometer.

Lesson Procedure
Experiment Set Up
  1. Connect the CBL to the TI 83+ using the link cable.
  2. Connect the pH probe to the Channel 1 port (CH1) located at the top edge of the unit.
  3. Keep the pH electrode in its storage solution until you are ready to begin the experiment.
Performing the Experiment
  1. Pour 400 ml of whole milk into the 600 ml beaker. Label the beaker “YOGURT”.
  2. Pour a small amount of water into the 1000 ml beaker (the water bath) and place the beaker labeled yogurt into the 1000 ml beaker.
  3. Place the water bath on a hot plate and heat the milk at 81 degrees Celsius for 5 minutes. Use a thermometer or temperature probe to monitor the milk temperature. Stir the milk to heat it evenly but do not let it boil. This process kills all the non-spore forming bacteria present in the milk.
  4. Using the beaker tongs, carefully remove the beaker containing the milk from the water bath and let the milk cool to 39 degrees Celsius.
  5. Add one teaspoon of yogurt to the milk and stir to mix. CAUTION: DO NOT INSERT pH ELECTRODE INTO THE HOT SOLUTION.
Collecting the Data
  1. Turn on the CBL unit and the calculator. Start the CHEMBIO program and proceed to the MAIN MENU.
  2. Select SET UP PROBES from the MAIN MENU.
  3. Enter “1” as the number of probes.
  4. Select pH from the SELECT PROBE menu.
  5. Enter “1” for the channel number.
  6. Prepare the CBL and calculator for calibration. (Follow procedure for calibration. See attachment “1”.)
  7. Collect the pH data for yogurt.
  8. Select COLLECT DATA from the MAIN MENU.
  9. Select TIME GRAPH from the DATA COLLECTION menu. (System will warm up for 30 seconds.)
  10. Enter “3600” for time (in seconds) between samples. (3600 seconds = 1 hour)
  11. Enter “20” for number of samples. (20 samples will be collected in 20 hours.)
  12. Screen should reflect 72000 as total experiment length.
  13. Enter USE TIME SETUP if no errors have been made in setting up time.
  14. Choose a location that will not be disturbed during the experiment and place pH probe into yogurt/milk solution. Clamp the electrode to the ring stand and cover the solution with aluminum foil.
  15. When you are ready to collect data, press ENTER.
  16. The CBL is programmed to collect pH data for 20 hours. The CBL is finished collecting data when DONE appears at the left of the CBL viewing screen. Do not remove the electrode until all of the data has been collected.
Retrieving the Data
  1. Make sure the CBL is finished collecting data. Then with the CBL and the TI 83 + on enter RETRIEVE DATA.
  2. Time will show in L1 and data (pH) will show in L2.
  3. Press ENTER to display graph.
Recording the Results
  1. The collected data is stored in lists L1 and L2. The time, in seconds, is stored in L1 and the pH data is stored in L2. The graph obtained in the experiment represents the pH of the milk/yogurt solution with respect to time. Construct and correctly label a table and line graph of the data collected during experimentation.
  2. Use the graph to verbally describe how the pH of the milk/yogurt solution changed during the experiment.
Analyzing the Results
  1. Use the data collected in the experiment to describe any changes that occurred in the milk and yogurt solution. Use specific pH readings in your description.
  2. What did the bacteria produce during the experiment? What evidence do you have to support your answer?
  3. Explain how to use the data collected in the experiment to describe the population growth of the bacteria in the milk/yogurt solution.
  4. Explain why the pH of the milk/yogurt solution stabilized towards the end of the experiment. Support your explanation with evidence.
  5. Limiting factors exists in every environment. These factors limit the growth and population of the organisms in the environment. List several factors present in the experiment.

Attachments
Attachment 1. pH Calibration
Attachment 2. Resources
Attachment 3. Rubric
Attachment 4. Sample data
Exploration and Extension
Repeat the experiment again except this time do one or more of the following:
  • Change the amount of milk used in the experiment.
  • Change the amount or type of yogurt used in experiment.
  • Use skim, 2%, or lactose-free milk.
After repeating the experiment, write any results (including graphs) and conclusions on a separate sheet of paper. Compare your results and conclusions to those obtained in the original experiment.

Attachment 1

pH Calibration
First Calibration Point
  1. Select PERFORM NEW from the CALIBRATION menu. Follow the directions on the calculator screen to allow the system to warm up, then press ENTER.
  2. Remove the electrode from the bottle by loosening the lid, and then rinse the electrode with distilled water.
  3. Place the electrode tip into pH-7 buffer. Wait for the readings displayed on the CBL screen to stabilize, press TRIGGER on the CBL.
  4. Enter “7” (the pH value of the buffer) on the calculator.

Second Calibration Point

  1. Rinse the pH Electrode with distilled water and place it in the pH-10 buffer solution.
  2. Wait for the readings displayed on the CBL to stabilize and press TRIGGER.
  3. Enter “10” (the pH value of the buffer) on the calculator and press ENTER to return to the MAIN MENU.

Attachment 2

Resources
  • Clemson University. (7-15-01)
  • Cobb, Vickie. Science Experiments you can ear, JB Lippincott co, 1972
  • Danone World Newsletter #5 Lactic Acid Bacteria. (7-14-01)
  • Aolman, Scott and Masterman, David. Biology with CBL, 1998.

Attachment 3

Science Lab Report Evaluation Rubric assignment # ______
Student Name:
Group Members:
/ Self-evaluation Score:
This analytic rubric is used to verify specific tasks performed when producing a lab report. If the task has been completed, all points are awarded. No points are awarded if the task is not complete.
Category / Scoring Criteria / Points / Student
Evaluation / Teacher
Evaluation
Lab Introduction
15 points / The question to be answered during the lab is stated. / 5
The hypothesis clearly shows it is based on research. / 5
Research references used to prepare the lab are listed. / 5
Procedures
15 points / Procedures are written during pre-lab preparation and clearly state what is planned. / 5
There are no "understood" procedures. / 5
Specific formulas or equations for reactions during the lab are shown. / 5
Observations
15 points / "Results" of a procedure are clearly recorded. / 5
Measurements, when required, show proper units. / 5
Calculations, when required, are clearly shown. / 5
Conclusion
20 points / Summarize the essential lab data. / 5
Show how the essential data answers the lab question. / 5
Identify the one area of the lab most likely responsible for measurable experimental error. / 10
Presentation
25 points / Report is neatly printed in ink, with no visible corrections. / 10
A diagram of the essential apparatus used in the experiment is drawn in the largest available white space on the front of the lab report. / 10
Report is written in such a way that others could accurately duplicate the experiment. / 5
Lab Safety
10 points / No group members were sited for safety violations. / 10
Score / Total Points / 100
Self-evaluation / Students are expected to honestly evaluate their own work. If the difference between the student evaluation and the teacher evaluation is more than 10 points, 5 points will be deducted from the teacher's score when the grade is recorded.
Deadline / Lab reports are due at the beginning of class the day after lab. Reports will be accepted at the beginning of class the second day after lab for 3/4 credit. No credit will be given after this time.
Comments

Attachment 4. Sample Data