What is Yeast?
Yeast is a micro-organism, a single-celled fungus. It has a nucleus, cytoplasm, a vacuole, a cell membrane and a cell wall.
Yeast can respire with oxygen (aerobic):
glucose + oxygen => carbon dioxide + water + energy
Or without oxygen (anaerobic):
glucose => ethanol + carbon dioxide + energy
When yeast respires anaerobically, it produces less energy.
To make bread rise, you use yeast.
1) Yeast is put into the dough and the dough is left to rise in a warm environment.
2) The yeast uses the sugar in the dough to respire producing carbon dioxide.
3) The carbon dioxide expands and gets trapped in the dough, which is made lighter.
4) The heat of the oven kills the yeast, so that you are not eating live yeast.
To produce the alcohol in beer, you use yeast, which must respire anaerobically.
1) Barley grains are allowed to germinate. Their starch is broken down into sugars using enzymes.
2) The grains are dried in a kiln. (malting)
3) The malted grain is mashed up with water to produce a sugary solution.
4) The solution is sieved to remove the pieces of grain.
5) Hops are added to the mixture to give beer its flavour.
6) Yeast is added and the beer is kept covered so that the yeast respires anaerobically and produces ethanol from the sugar.
-- When making wine, the yeast uses the natural sugars in the grapes.
Most cheese is made using bacteria.
1) A culture of bacteria is added to milk.
2) The bacteria produce solid curds in the milk.
3) The cheese is drained to remove the solid curds from the liquid whey.
4) More bacteria are added to the curds.
5) The whole thing is left to ripen for a while.
5) Moulds are added to give blue cheese its colour and taste.
Bacteria are often used to make yoghurt.
1) Milk is heat treated to kill off bacteria, and then cooled.
2) A starter culture of bacteria are added. Bacteria turn the lactose sugar in the milk into lactic acid.
3) The acid causes the milk to clot and solidify.
4) Colours and flavours are added.