Agriculture and Food 7–12 · Years 7–8
Yeast and the rise of bread dough at 20, 30 and 37 °C
Food and agricultural practices
The idea
Yeast ferments sugar to carbon dioxide, and the gas trapped by gluten makes dough rise faster in warm conditions than in cool ones.
Safety card
Hazards
- scalds from kettle water
- glass cylinders tipping in a water bath
Controls
- teacher tops up baths from the kettle
- clamp or wedge cylinders upright
- no tasting in the laboratory
Note
Hot water: NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1 (general information for all staff) and Volume 2 Appendix D (generic risk assessment advice for each chemical). Complete a CSIS or RiskAssess site-specific risk assessment before the lesson.
What you need
- plain flour, 25 g, and sugar, 1 g, per group
- yeast suspension, 30 mL per group (7 g dried yeast stirred into 450 mL warm water)
- 100 mL beaker, 50 mL and 250 mL measuring cylinders, spatula or glass rod
- a wide funnel made by cutting the stem off a plastic funnel
- water baths at 20, 30 and 37 °C, or large beakers topped up from a kettle
- thermometer, stop clock, graph paper
How to do it
- Weigh 25 g of flour and 1 g of sugar into the beaker.
- Add 30 mL of yeast suspension and stir to a smooth paste that can be poured.
- Pour the paste through the wide funnel into the 250 mL cylinder without letting it touch the sides, and read the starting volume.
- Stand the cylinder in the water bath assigned to the group and record the bath temperature.
- Read the dough volume every 2 minutes for 30 minutes, then keep reading every 5 minutes to 45 minutes.
- Plot volume against time, and put every group's curve on one class graph by temperature.
What you should see
The dough rises fastest at 37 °C and 30 °C and slowest at 20 °C. The RSC notes report that after 35 to 45 minutes the gluten breaks and carbon dioxide escapes, so the volume stops rising or falls. The 1 g of added sugar can give at most 0.514 g of carbon dioxide (about 291 mL at 30 °C and 101.3 kPa); flour's own sugars can add some gas and some escapes or dissolves, so this is a reference figure to compare with the largest volume gain, not an exact ceiling. The learner knows it worked when the class curves are steeper at the higher temperatures.
What changes
- What you change
- water bath temperature (20, 30 or 37 °C)
- What you measure
- dough volume (mL) every 2 minutes
- What you keep the same
- 25 g flour
- 1 g sugar
- 30 mL of the same yeast suspension
- cylinder size
- starting paste temperature
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Yeast is a chemical, not a living organism.
- Hotter water always makes yeast work faster.
- Dough rises because wet flour swells.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Food Technology Years 7–10 Syllabus (2019)FT4-4FT4-10
- Technology 7–8 Syllabus (2023), focus area Food and agricultural practicesTE4-SAF-01
- Australian Curriculum v9AC9TDE8K05AC9S8I02
Sources
The pages the author read to write this activity.
- edu.rsc.org/experiments/yeast-and-the-expansion-of-bread-dough/1748.article
- edu.rsc.org/magnificent-molecules/gluten/2000021.article
- www.nsw.gov.au/education-and-training/nesa/curriculum/tas/food-technology-7-10-2019
- www.nsw.gov.au/sites/default/files/noindex/2025-09/food-technology-years-7-10-syllabus-2019.docx
- curriculum.nsw.edu.au/learning-areas/tas/technology-7-8-2023/outcomes
- education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/5._Volume_2_Appendices.pdf