Rennet is an extract from the stomach of ruminants containing a number of enzymes. Its main constituent is the enzyme rennin (also known as chymosin). Rennin coagulates the protein casein in milk and is used in the production of most cheeses.
Rennet is available in both liquid and tablet form from some health food stores, science supply companies, and from Australian web-based companies selling cheese- and yoghurt-making products.
The enzyme rennin is also commercially available for cooking as junket. Junket is available in tablet or powder form from some supermarkets, grocery stores, health food stores and science supply companies.
Science ASSIST has developed a list of recommended science suppliers for schools to use, see School science suppliers. For cheese- and yoghurt-making products there are several Australian suppliers you can access on-line.
Tips for using Rennin (as Junket or Rennet)
- For enzyme experiments on milk protein, rennin works best at mammalian body temperature of 37° C. If the milk is too cold, the reaction is very slow, and if the milk is too hot, the heat will denature the rennin, rendering it inactive.
- The optimum pH range of the substrate milk should be between 5.8—6.2.
- It works best with whole milk. You can use dry powdered milk, but allow more time for it to coagulate.
- Do not use evaporated milk as the gums and stabilisers in the milk will not allow the protein to coagulate. The same applies to ultra-heat treated milk (UHT), which is treated above 90° C and so reduces the milk’s ability to coagulate.
- Dissolved in water, junket powder forms a solution that coagulates milk. It is considered more convenient because the powder has a longer shelf life than liquid rennet.
- Store dry powder or tablets at room temperature out of direct sunlight.
- Vegetarian Rennet1 in liquid form, is guaranteed to be active for at least 12 months if kept in the dark and refrigerated at 4° C.
- Always follow the manufacturer's instructions for preparation and use.
- Rennet or junket solutions are best freshly made just before use.
- A recommended working strength of 1% solutions of rennin is generally suitable for enzyme digestion activities. Use 1 mL of 1% working solution with every 10 mL of milk. After mixing the enzyme and milk substrate together, leave to stand without stirring or bumping the container. Disturbing the mixture will disrupt the coagulation process. Setting usually takes 10–15 minutes.
- Being a food additive, junket powder is considered quite safe to work with. However, as a school science activity, it is recommended to follow safe work practices when working with enzymes. See 'Working with Enzymes and General Safety Precautions' below.
- Rennet in powder form is a hazardous substance2, but in low aqueous concentrations it is considered to be a low hazard.
- Never allow students to consume any food products made in the science laboratory, see link on the Science ASSIST website for Eating in Labs?
Working with Enzymes and General Safety Precautions:
- Enzymes are biologically active proteins that should be handled with care. It is advised to avoid inhalation of enzyme dust or aerosols, which can lead to sensitisation and allergic reactions. Enzymes may cause asthma and are irritating to the eyes, respiratory system, mucous membranes and skin. When working with powdered enzymes, always wear safety glasses, gloves, a dust mask or work in a fume cupboard that is not turned on to minimise exposure to any dust. Always use practices that do not generate dust or aerosols.
- Enzymes in powder form are considered hazardous substances3. However, in dilute aqueous concentrations are considered to be a low hazard.
- Enzymes when dissolved into solution are much less stable than in powder form and lose their activity quickly. Therefore, it is best to prepare only what is required just before use.
- Enzymes in powder form should be stored in the fridge (4° C), unless otherwise specified. Diluted solutions can be stored in the fridge but should be used within of an hour or two of preparation and be kept on ice during an experiment.
- It is always advisable to check the enzyme reaction is working as required and make adjustments to the conditions and concentrations if needed before any practical class.
- Minor spills should be cleaned up immediately without generating any dust. Place waste into a designated, labelled container for disposal via a waste contractor. Do not discharge into sewer or waterways
Science Assist recommends you conduct a site-specific risk assessment to assess and control the risks. We have developed a risk assessment template for schools to use, see Risk Assessment Template.
Rennin acts on casein in milk to cause clotting or coagulation causing the milk to separate into “curds” (solids) and “whey” (liquid).
milk + rennin → curds + whey
These days, rennin is usually produced from microbial or plant sources rather than by extraction from the digestive tract of calves.
An enzyme is a protein molecule that acts as a biological catalyst. A catalyst increases the speed of a chemical reaction but does not form part of the final product. Enzyme activity is affected by concentration, temperature, pH, substrate concentration and can be affected by the age of the reagents. Enzymes act on substrates to make products in a chemical reaction and they are highly specific to the reactions they catalyse (the lock and key model).
It is important to keep enzymes stable and prevent them from denaturing. Never froth an enzyme solution, as it can denature. It is best to use the lowest concentration and smallest amount possible.
Science ASSIST has previously answered a number of chemical disposal questions see:
The website link below contains more information on enzyme experiments in food chemistry:
Institute of Food Technologists. n.d. Experiments in Food Science Series – Food Chemistry Experiments, IFT website: https://web.archive.org/web/20180219050706/http://www.ift.org/~/media/Kn... (Link updated June 2021. This resource courtesy of the Internet Archive.)
1'Vegetarian Rennet', Cheeselinks Website, http://www.cheeselinks.com.au/products/category/Calcium-and-Rennet (Accessed February 2016).
2Chemwatch Gold March 2013.Safety Data Sheet: Rennin
Chemwatch Gold September 2012.Safety Data Sheet: Sigma-Aldrich Rennet
3‘MC23.9 – Rennet’, Southern Biological website https://www.southernbiological.com/chemicals/enzymes/mc23-9-rennet/ (Accessed October 2015)
Science ASSIST. 2018. Chemical Management Handbook for Australian Schools – Edition 3, Science ASSIST website, https://assist.asta.edu.au/resource/4193/chemical-management-handbook-au... (See Laboratory notes on Enzymes)
Enzyme Technical Association (ETA). Nd. ‘Working Safely With Enzymes’, Enzyme Technical Association (ETA) website, https://www.enzymetechnicalassociation.org/wp-content/uploads/2017/11/Wo... (Accessed via https://www.enzymetechnicalassociation.org/documents/ )
‘SAFETY - Working with enzymes’, National Centre for Biotechnology Education University of Reading website, http://www.ncbe.reading.ac.uk/SAFETY/enzymesafety.html (2017)
'enzymes', Science ASSIST's Chemical Management Handbook for Australian Schools – Edition 3, pp 135–143, https://assist.asta.edu.au/resource/4193/chemical-management-handbook-au... (Accessed May 2019)
‘Enzymes’, Royal Society of Chemistry website https://www.rsb.org.uk/education/teaching-resources/secondary-schools/chemistry-for-biologists (Accessed October 2015)