Video transcript: raising agents

So when you bake or eat bread or muffins you are appreciating the work of gas & air expanding within the baked goods.

So you sample the effects of gases every time you eat a slice of bread, or you open a bottle of fizzy water.

The process of adding air to food is called ‘aeration’ & there are a couple of different processes.

Aeration method 1: mechanical aeration

So mechanical aeration can be demonstrated by a mechanical process such as whisking or beating to create a foam. Now you can use a hand whisk, you could use a hand powered whisk or you could use an electric whisk.

The foam itself is unstable but it can be made stable by chilling it or cooking it in such ways as creating a soufflé or a roulade.

What we’ve done there is we’ve mechanically aerated the egg white we have added small bubbles of air & created extra volume within the mixture which would be perfect when you bake up your roulade or you’re making your soufflĂ©.

Aeration method 2: physical change

When you heat up water to boiling point it produces steam which expands & is used as a raising agent for products such as Yorkshire puddings or profiteroles.

The texture steam produces as a raising agent is very open and uneven. Pockets of air are left after the steam has escaped.

Aeration method 3: chemical reactions

The gas, carbon dioxide can also be added to our food by chemical reactions, using an acid and an alkali or as you might know it bicarbonate of soda or baking powder & cream of tartar.

Science activity: chemical aeration in action - volcanic eruptions

So we can demonstrate the acid & alkalis working together with a quick little experiment we like to call volcanic eruptions.

We are going to get the flour, and we’re going to mix in the water & we are going to create ourselves a real basic little dough on here.

Then you’re going to place the dough around the yoghurt pot to create a volcanic shape.

Once you’ve done that we’re going to add to your yoghurt pot a little bit of the alkali, we’re using the bicarbonate of soda here, about 2 tablespoons.

We’re going to add a little bit of red food colouring & then we’re going to add our acid, but don’ be worried this is white vinegar, otherwise known as acetic acid, I’m going to pour that in ……and watch our volcano erupt...

…through fizzing or effervescence, which is releasing CO2, in the same way that CO2 would be released into your baked items.

Aeration method 4: biological fermentation

Aeration can also occur through biological reactions, such as Yeast Fermentation. But what exactly is yeast & what does it need to ferment?

A closer look at yeast

Yeast, is a live, single-celled fungus activated by warm liquid ideally about body temperature, fed by sugars in flour, yeast produces C02 and alcohol.

The yeast are microscopic fermenting machines used for fermenting alcoholic drinks such as beers or lager! And in fact – the froth on top sometimes called the barm – was used hundreds of years ago to make the bread (although quite a bitter bread!).

And how microscopic is microscopic? Well if I was just to get 1 of my hairs & hold it under this tiny microscope, I would be able to see 15 yeast lined up end to end on the end of 1 of my hairs.

So how many different types of yeast are there?

Well there are over 160 species of yeast out there and they are all around us …in the air on our skins on grain and on fruit...

In ‘quick breads’ which have no yeast (and these breads don't require hours and hours for dough to rise), an instant leavening agent like baking powder or bicarbonate of soda is used, which relies on chemical reactions between the acid and alkali compounds to produce the CO2.

So in baking there are 3 common types of yeast that we use – the first is a fresh yeast, the 2nd is a dry active yeast both requiring fermentation prior to adding to the ingredients…

…and the 3rd is a fast action yeast which can be added directly to the dry ingredients.

Science Activity: Wake the Yeast Beast

So you might not be able to see the yeast but you can see the reactions of the yeast.

We’re going to use some plastic bottles, some yeast & some sugar – but different amounts at different temperatures to see what the reactions will be…

  • In bottle 1 we have control – yeast warm water & sugar
  • In bottle 2 we have yeast cold water & sugar
  • In bottle 3 we have yeast, boiling water & sugar
  • In bottle 4 we have yeast, warm water & no sugar
  • And in bottle 5 we have yeast, sugar & no water


  • For this experiment add 2 teaspoons of dried active yeast to each bottle
  • Add 1 teaspoon of sugar to every mix that requires sugar
  • Use 200 ml of water at the required temperature for every mix that requires water
  • For the mix that requires cold water – use crushed ice

On a cautionary note be careful with boiling water, pour the boiling water into a flask with the yeast first, otherwise you’re going to end up with a melted plastic bottle.

We’ll check back on these in 20 minutes.

20 minutes later

Ok so we’ve come back to our yeast beast experiment & you can see the results here.

It demonstrates what conditions are required for the yeast to perform & ferment our food…

...they need warmth, they need moisture, they need food & ultimately they need time to produce the carbon dioxide which has inflated this balloon.

Take away 1 of these conditions & you’re not going to have full fermentation or a production of carbon dioxide.

So with cold water, it is too cold for the yeast to ferment

With the boiling water we’ve killed the yeast

With the yeast, warm water & no sugar, we’ve not provided any food for the yeast

& with the 5th bottle we’ve not provided any moisture for the yeast.

If one of the conditions that yeast needs to reproduce is unavailable fermentation does not take place.

To demonstrate how chemical aeration can cause foods to rise we are going to carry out an activity called loo roll rockets.

Science activity: loo roll rockets

What you’re going to need for this experiment is the alkali (bicarbonate of soda), some single ply toilet tissue & some old film canisters.

In addition to some acid, …don’t be worried, it’s just lemon juice

Open up the canister & place about a cm worth of lemon juice into the bottle

Then cover the top of the canister with a sheet of toilet tissue …..this will stop the acid & alkali from mixing until you are ready for the reaction

Onto that add between half a teaspoon or a teaspoon of bicarbonate of soda

Finally add the top and press it down until it clips.

Cautionary note – for this experiment you must wear safety glasses and gloves
Anyone watching must stand at a safe distance

What I’m going to do now is I am going to flip the tray over & once I’ve done that the rockets should launch into the air, to show you the trust that can be created through chemical aeration & the combination of acid & alkali.

Stand back and wait for the chemical aeration to take place

[Rockets go off] ...the acid in the lemon juice and the alkaline in the bicarbonate of soda quickly react together to produce carbon dioxide gas, this is the same gas that aerates your food.


  • Gases in our food add variety to our diet.
  • Gas may be added to foods in different ways, for example mechanically by whisking egg whites, physically – by water turning into steam, biologically by fermenting yeast and chemically by adding baking powder to recipes.
  • Yeast is a living organism which grows best when it has food, warmth, moisture and time to grow.
  • Yeast is available in many different forms.
  • Both yeast and baking powder release the gas carbon dioxide which makes our food lighter and improves the texture.
  • Baking powder is made from an acid (cream of tartar) and an alkali (bicarbonate of soda) when mixed together with a liquid they produce carbon dioxide gas.