1. Making Cheese, Bread and Gingerbeer
With the help of Micro-organisms

2. Respiration
If we want to make cheese, bread or gingerbeer – we need micro-organisms.
To make these things, humans rely on a life process that micro-organisms carry out.
The process of respiration

3. Respiration Respiration is the process of releasing energy from food.
Humans need to breathe in oxygen from the air for our cells to carry out respiration.

4. Anaerobic respiration
But some bacteria and fungi can carry out respiration without oxygen.
Respiration in the absence of oxygen is called anaerobic respiration.

5. By-products
One big difference between aerobic (with oxygen) and anaerobic (without oxygen) respiration is their by-products
When humans carry out aerobic respiration, we produce carbon-dioxide and water as by-products.
Aerobic respiration
Glucose + Oxygen  Carbon dioxide + Water

6. By-products
But when lactic acid bacteria carry out anaerobic respiration, all they produce is lactic acid!
Glucose  Lactic acid

7. By-products
And when yeast carries out anaerobic respiration it produces carbon dioxide and ethanol.
Anaerobic Respiration
Glucose  Carbon dioxide + Ethanol
Sometimes we call this process fermentation.

8. Cheese There are many different types of cheese.
But they all come from milk.
It could be cows milk or goats milk.
But definitely milk

9. Cheese When acid is added to milk, it separates into curds and whey.
The curds are the solid clumpy bits. The whey is the watery liquid.

10. Cheese
In class, we used vinegar as our acid for separating the curds and whey.
But there is a better method!

11. If you put lactic acid bacteria into milk they carry out anaerobic respiration.
They use all the lactose and glucose for energy and produce lactic acid as a by-product.

12. Cheese
This lactic acid does a great job of lowering the pH of the milk and separating the curds and whey.
It works even better than vinegar.

13. Cheese
Remember, the lactic acid is a by-product of the bacteria carrying out anaerobic respiration.

14. Bread

15. Bread People have been making bread since 9000BC.
It is made by cooking a dough of flour and water.

16. Bread
Most of the time we want our bread to rise and to be full of little holes.
But how do we make it light?
How do we get these little holes?

17. Bread The answer is with yeast. Yeast are unicellular fungi.
So when you buy yeast at the supermarket, you are buying a living thing!

18. Bread
The yeast we use for making bread is a species called Saccharomyces cerevisiae.
But there are more than 1500 other species too.

19. Glucose  Carbon dioxide + Ethanol
Bread
When yeast is put with flour and water, it uses the sugars in the flour to carry out anaerobic respiration.
When yeast carries out anaerobic respiration, it produces carbon dioxide and ethanol as by-products.
Glucose  Carbon dioxide + Ethanol

20. Bread The ethanol evaporates during baking (giving a nice smell)
The carbon dioxide forms bubbles which makes the bread light, helps it to rise and gives us all the little holes.

21. Ginger-beer

22. Ginger-beer
To make ginger-beer, again we use Saccharomyces cerevisiae.
This time we give the yeast some ordinary sugar for food.

23. Glucose  Carbon dioxide + Ethanol
Ginger-beer
Remember, respiration is all about getting energy from the glucose in food.
The yeast will get energy from the sugar by carrying out anaerobic respiration.
Glucose  Carbon dioxide + Ethanol

24. Ginger-beer
The carbon dioxide is useful again, because this time it forms all the bubbles that make our ginger beer fizzy.
Anaerobic respiration is very useful.

25. We know that anaerobic respiration produces carbon dioxide and ethanol.
And we know that carbon dioxide is useful for making bread and gingerbeer.

26. Ethanol?
But what’s ethanol?
Is ethanol useful for anything?

27. Ethanol
Ethanol is an organic compound with a hydoxyl functional group.
This means it’s an alcohol. And this particular alcohol is the sort we use to make our drinks alcoholic!

28. Ethanol
Without micro-organisms and the life process of anaerobic respiration, there would be no alcohol of any sort.

29. Recap
Some bacteria and fungi carry out anaerobic respiration to obtain their energy from food.
Lactic acid bacteria produce lactic acid from the lactose in milk.
This acid helps to separate the curds and the whey for making cheese.
When yeast carries out anaerobic respiration it produces carbon dioxide which is useful for making bread and gingerbeer…
And ethanol which is useful for making alcoholic drinks.