How does yeast work?

 YEAST in dough

Yeast is a microorganism it's proper name is Saccharomyces Cerevisiae which literally means “sugar-eating fungus”. The eukaryotic (the cells have a nucleus or nuclei) yeast fungus respires to release energy. It is known as a "sugar-eating fungus" because when yeast respires aerobically glucose (a form of sugar) is used up and converted to energy. The energy allows the yeast cells to grow, repair and function.

The aerobic respiration equation is Glucose + Oxygen --> Carbon dioxide + Water + Energy.

However, in flour the sugar is starch (a polysaccharide) and the sugar in the white granulated stuff you put in your tea exists as sucrose (a disaccharide).
Yeast can break down sucrose into glucose and fructose by producing an enzyme called invertase which catalyses the hydrolysis of the disaccharide. To break down the starch in flour malted barley can be added to the flour for enzymatic balance. This means the flour has more active enzymes and this is usually in the form of more alpha amylase enzymes. These enzymes which can also be found in saliva break down the complex sugars and starch into simple sugars for example maltose. The yeast can produce the enzyme maltase which is able to break down maltose into two glucose molecules.

All of this glucose is then digested by the yeast and used in respiration and fermentation. Fermentation can also be known as anaerobic respiration and the fermentation equation is Glucose --> Ethanol + Carbon dioxide. Fermentation happens when the yeast is in an oxygen-less environment and is an equation catalysed by the enzyme zymase. As you can see alcohol is produced however this evaporates during the baking process as ethanol has a very low boiling point.

Both of these reactions of yeast produce carbon dioxide these bubbles of gas get trapped in the sticky dough and when the dough is baked in the oven the bubbles of carbon dioxide expand causing the dough to rise.

Snail Cinnamon Rolls:

I found this recipe on Rosanna Pansino's YouTube Channel, she made a YouTube video in which she made extremely cute snail rolls explaining the steps of the process. I have tried to note down and summarise this to make it slightly easier if you would like to recreate her amazing idea. I suggest you watch her video here !

Ingredients :

For the Rolls

(4) Cups Flour:
(1/2) Cup of Sugar:
(1) Packet of Yeast:
(1) Tsp Salt:
(1) Cup Warm Milk
(1/3) Cup of Butter (76g)
(2) Eggs

For the Filling
(1) Cup Light Brown Sugar:
(2-1/2) Tbsp Cinnamon:
(2) Tbsp Flour:
(1/3) Cup Butter (76g)


Method :

1. Warm the milk slightly in a pan and then dissolve the yeast in the warmed up milk and mix together.
2. Add the sugar and 1 cup of flour and mix again.
3. Cover the mixing bowl with cling film and leave to rise for 30 minutes.
4. When half an hour has passed add the salt, the 2 eggs, the butter and the remaining flour then mix together.
5. Flour your work surface and knead the dough for 5 minutes.
6. Use a bit of olive oil to grease a bowl and put your lump of dough into the bowl, cover with cling film and leave the dough to rise for 1 hour.
7. While the dough is proving make your filling – mix the sugar, cinnamon and flour together in a bowl.
8. When the dough has finished proving punch it down in the bowl and then roll it out to be around 21 inches by 16 inches (this is what Rosanna Pansino suggests).
9. Butter the top of the dough and then sprinkle the cinnamon filling so it is covering the whole surface of the rolled out dough.
10. Choose a long side and start rolling the dough back up into a swiss-roll like structure.
11. When the dough is all rolled cut the cinnamon rolls around 2 inches thick. I suggest you refer to Rosanna's video to fold them into a snail like shape.
12. Then bake at 180 degrees Celsius for 25 minutes until they are golden brown.