1. YEAST Single Celled fungus (plant)
No chlorophyll - cannot produce its own food.
Generally feeds on dead organic material.
Often seen as a grey dust on foods (like grapes and plums)
Needs food and a suitable temperature to grow
Multiplies by budding

2. Yeast – bread making Yeast can be white, brown, pink or red
All prefer different temperatures
E.g. pink prefers 20oC
All types respire to release energy to stay alive
Yeast can respire when dried – but very little
When active, release lots of CO2
Bubbles of CO2 helps bread dough to rise
Heat in the oven then kills the yeast

3. Beer Yeast converts sugar into alcohol & CO2 This is fermentation
Fermenters are used for alcohol production
Beer is made from malt, water, sugar, hops & yeast
Alcohol content depends on:
- type of yeast used
- temperature of fermenter
- fermentation time
Yeast can be top or bottom fermenting

4. Beer maturing Beer is matured in a storage container
This develops flavour
Brewery conditioned beer
- stored in large tanks
- yeast removed, and extra CO2 added before packaging
Cask-conditioned beer
- stored in wooden or steel casks
- yeast cells given more sugar, to produce more CO2
- beer often dark, with a strong flavour

5. Growing yeast - aseptic
Can be grown on agar gel
Agar gel made from agar powder, nutrients & water
Mixed in a McCartney bottle
All plates, bottles etc sterilised in an autoclave first
Agar gel mixture is then poured into a sterile petri dish
A bunsen is used to sterilise the bottle, and carry any spores upwards

6. Kefir Kefir - alcoholic (2%), fermented milk drink
- a colony of bacteria & yeast added to milk
- lactic acid gives a sour taste
- CO2 gives a slight fizz

7. Enzymes & Immobilisation
Enzymes are produced by all living cells
Each control a biochemical reaction
Can be removed from cells for industrial processes
Kefir can be made using immobilised enzymes
These are trapped in gel beads
The enzyme (lactase) reacts with the milk
The sugar in the milk (lactose) is broken down

8. Food flavouring & colouring
Yeast flavours many foods
- soups, snacks, and stock cubes
Pink yeast can also be added to fish feed
This can give fish a pink colour e.g. salmon
This is better than using dyes in food

9. Environmental impact Industries often release yeast into rivers
Acts as a food source for bacteria/algae
They grow, and use up all the oxygen in the river
Other river animals die
Yeast can be upgraded
- to make ‘cattle cake’