Agriculture and Food 7–12 · Years 11–12
Coagulating milk protein with rennet enzymes and with acid
Food Quality: functional properties of food
The idea
Milk proteins coagulate into curd when enzymes such as chymosin act on casein at about 37 °C or when acid is added, the functional property behind cheese making, and temperature changes the rate.
What you need
- whole pasteurised milk, 40 mL per tube
- rennet of bovine origin (containing chymosin and pepsin), 1 mL
- chymosin produced by fermentation, 1 mL, and a microbial (fungal) protease coagulant, 1 mL
- the same rennet boiled for 5 minutes and cooled, 1 mL (denatured control); deionised water, 1 mL
- white vinegar, 5 mL
- 10 mL plastic syringes, boiling tubes or sample bottles, glass rods
- water bath at 37 °C, an ice bath, thermometer, stop clock
How to do it
- Warm 40 mL of milk per tube to 37 °C in the water bath.
- Add 1 mL of rennet, fermentation chymosin, microbial protease, boiled rennet or water to separate tubes, stir for 5 seconds and start the clock.
- Rock each tube gently or dip a glass rod in it every 30 seconds and record the time when a precipitate first clings to the glass.
- Repeat the rennet tube at room temperature and in the ice bath.
- Add 5 mL of vinegar to a warm tube of milk and compare its curd with the rennet curd after 15 minutes.
What you should see
At 37 °C the active coagulants form a clot within about 5 to 15 minutes, and the three enzymes differ markedly in speed, as the RSC problem describes. Water and boiled rennet give no clot. Rennet clots more slowly at room temperature and little or not at all in the ice bath. Vinegar curdles the milk at once into a soft, grainy curd, while rennet gives a smooth, continuous gel. The learner knows it worked when the water and boiled-rennet controls stay liquid.
What changes
- What you change
- coagulant (and temperature for rennet)
- What you measure
- time to clot (s); curd description
- What you keep the same
- milk type and volume
- coagulant volume
- stirring
- temperature
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Rennet is a kind of bacteria.
- Enzymes always work faster the hotter they get.
- Curdled milk has always gone off.
Safety card
Hazards
- dried enzyme dust can sensitise
- hot water
- milk allergy
Controls
- avoid skin contact with enzymes and wipe spills before they dry
- eye protection
- check allergy records
- nothing is tasted in the laboratory
Note
Food handling follows the FSANZ food safety standards (Standard 3.2.2) as explained on the FSANZ food temperature and thermometers page; check class allergy and anaphylaxis records before any food is prepared; the Food Technology Years 7–10 Syllabus reminds teachers that food allergies can cause anaphylaxis. Enzyme preparations: 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.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
Sources
The pages the author read to write this activity.
- edu.rsc.org/in-search-of-more-solutions/from-milk-to-curds-and-whey-which-enzyme/571.article
- www.nsw.gov.au/education-and-training/nesa/curriculum/tas/food-technology-stage-6-2013
- www.nsw.gov.au/sites/default/files/noindex/2025-03/food-technology-st6-syl.docx
- education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/5._Volume_2_Appendices.pdf