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

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

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

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

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

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

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

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

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’