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Our Food and Microorganisms

Christopher Soldat

Type of Activity:

  • Hands-on
  • Inquiry lab
  • Group/
    cooperative learning
  • Community outreach

Target Audience:

  • Integrated Science (level 2)
  • Biology
  • Biotechnology

This series of activities help students answer questions about how microorganisms can affect our food supply and how they can be controlled.


Notes for teachers:

Baking yeast is used as an exemplar microorganism. It is safe and easy to use. This activity has been drawn from a larger collection of materials from an unit on food processing and science for ninth grade students.

Notes for students:

Students work in cooperative lab groups. The first set of inquiry activities are directed by the teacher. Subsequent activities are based on students generating their own questions and seeking answers by designing their own experiments.


It is crucial that a fresh yeast suspension be prepared for a class. Allow 10 - 15 minutes for the yeast mixture to sit prior to classroom usage.

Class time:

This series of activities can last from three to six instructional periods.



Students have first spent instructional time investigating common food preservation techniques, i.e. drying, freezing, canning, techniques, etc. Students are challenged to design single variable experiments which will measure yeast's growth based on temperature, sugar concentration and other variables. They present their findings to the class while discovering the key concepts of optimal growth, limits and thresholds of yeast growth. Using these key ideas, students are again challenged to design experiments which they think will limit the growth of yeast.


  • Plastic cups
  • yeast
  • sugar
  • stir rods
  • warm water
  • salt
  • ascorbic acid solution
  • citric acid solution
  • vinegar
  • sodium carbonate solution
  • sodium benzoate solution


Students investigate the necessary ingredients for microorganism growth , including water, food and the correct temperature. Students are asked to design an experiment which will measure the growth of yeast with varying amounts of food (sugar). Students are asked to first try mixing yeast solution and sugar so they can observe yeast growth and gas bubbling. As a class, they decide on a rating scale for measuring the growth of yeast using rising and bubbling as an indirect indicator of growth. Students are asked why it is important to measure carefully and to make their observations on a regular and periodic basis. Students graph and share their results. They repeat these labs in the next class periods using temperature with an optimal sugar concentration to yeast solution ratio. Again they are asked to graph and share their results. Students are asked to observe that after optimal growing conditions occur that inhibition of growth occurs with both increasing sugar concentrations and temperature.

Students are asked to survey ingredients labels from food containers and suggest which chemicals might be used to inhibit the growth of microorganisms. Within cooperative lab groups students are asked to choose a substance to test which might inhibit the growth of yeast solutions. Students may choose from such substances as ascorbic acid, citric acid, sodium carbonate or sodium benzoate solutions, vinegar, salt, sugar or other viable test solutions. Students are then asked to design a standardized single variable experiment. They meet as a large group and give each other feedback about their plans. The next class period students are asked to conduct their original investigation, gather their data, clean up and prepare a report for the large group.


Each group is responsible to present a poster session which summarizes their question, procedure and results.


Students take their data and survey food packaging and identify current food preservative techniques such as irradiation and genetic engineering.

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