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Yeast & bread making

Public summary: 

Like all the best biology experiments, this is hands-on and messy! See what happens when you add living yeast to a bread mix, and don't forget to come back a bit later to see what's happened to your dough!

The idea is to look at how yeast grows and how it relates to bread making. It involves getting the kids to make bread dough.
Useful information
Kit List: 

Baker's yeast (a fair a amount - we got through 3 of the tins of it last year)
Flour 80g per child (lots!)
Sugar (a couple of bags)
water (warm - will have to keep getting this from tap)
beakers/cups for cultures
500 ml plastic drinks bottles
Trays to do experiment over to contain the mess
Possibly somewhere for kids to wash hands as it gets a bit messy

Frequency of use: 

Key messages to get across about this experiment:
• Yeast is alive
• It gives off gas (carbon dioxide) when bread rises giving you bubbles in the bread

Suggested demonstration:
• Start with bread – ask if they have ever eaten bread (hopefully a ‘yes’ but this should get them engaged with you as it’s a simple question that they don’t have to struggle to think about).
• Ask if they know what goes into bread; you’ll probably get told that it is made from flour. Some may tell you about water and yeast.
• Ask them whether they’ve ever seen bread dough rise. It rises because of bubbles of gas. This gas is made by yeast.
• Show them some yeast and add about half a teaspoonful to some water and sugar in a plastic cup.
• Yeast is alive (it’s a fungus) and when you add water to the dried yeast it starts growing again. Yeast are little round creatures (if the kids are old enough to know what a cell is, they have one cell) and they grow, getting bigger, and then a new yeast buds off the side of the old one.
• Inside bread there isn’t much oxygen so the yeast can’t breathe oxygen to allow them to produce energy like we do. Instead they use ‘fermentation’ of the sugar that you gave them. This produces a gas called carbon dioxide (and ethanol/alcohol). This gives the bubbles that you get in bread.
• Add flour to the water and yeast mix and get the child to mix them together to form dough.
• Did they notice how the dough went stringy as they kneaded/mixed it? Why is that? Flour contains some proteins that make gluten when water is added, and as you mix the dough these all line up becoming more stretchy. This traps the bubbles of gas made by the yeast so the dough rises.
• Mark the level of the dough on the plastic cup. Tell them to come back and look at it later, but also show them some from earlier that have risen. If parents want to take dough then they can (but don’t encourage it), but stress that it’s not edible because it was made in a lab.
• Ask if they want to see something else? If yes, have a look at the balloon demonstration.
• Put some warm water, sugar and yeast in a 500 ml drinks bottle
• The yeast will use the oxygen in the bottle first, and then start fermenting the sugar to produce carbon dioxide
• This will increase the volume of gas in the bottle and inflate the balloon
• You may need these on a ‘conveyor-belt’ system where you set one up with a group but show them the previous group’s one which is hopefully partially inflated.

Use trays as 'workstations' to contain the mess. Throw well-risen dough out after a few hours again to reduce mess.

Risk Assessment
Date risk assesment last checked: 
Wed, 08/01/2020
Risk assesment checked by: 
Matt Worssam
Date risk assesment double checked: 
Sat, 25/01/2020
Risk assesment double-checked by: 
Beatrix Huissoon
Risk Assessment: 

Mixing dried yeast with sugar and water to inflate balloon over neck of bottle. Mixing dried yeast with sugar, water and flour to make bread dough.

Hazard Risk Likelihood Severity Overall Mitigation Likelihood Severity Overall
Dough Kids may eat the dough, which is not good, as the kids' hands will not be clean and it is in a lab. 3 3 9 Stop kids eating the dough, and tell parents of small kids it isn't clean.
If dough ingested inform parents and advise them to take child to GP if they become ill.
2 3 6
Wet/floury surfaces Slip hazard. 3 3 9 Clear up any spills immediately. Use containment trays for the experiment. Use wet floor sign if necessary.
In case of injury call first aider.
2 3 6
Flour/yeast Possible risk of allergy to flour or yeast. 2 3 6 Demonstrator to ask about allergies to flour or yeast before commencing experiment.
In case of a reaction, call a first aider.
1 3 3
Plastic, tins etc. Risk of cuts from broken plastic, yeast tin lids etc. 2 3 6 Supervised use only. Demonstrator to dispense yeast and dispose of any broken plastic cups/spoons.
In case of injury call first aider.
1 3 3
Balloons, flour Possible choking hazard if child tries to eat balloon or inhales flour. 2 5 10 Keep only the balloons in use on display and make sure that children don’t eat them, don't let children eat dry flour either and be careful not to knock it over and cause a flurry of flour dust.
If child is choking, get parent to slap firmly on back and find first aider.
1 4 4
Publicity photo: 
Experiment photos: