1. Unit 2:3 Micro-organisms and Decay
Micro-organisms are everywhere: in the air, soil, and water. On our skin and inside our bodies.
Micro-organisms can break down organic matter (a substance that has been made from a living thing) to make it decay.
Decaying micro-organisms can be useful. They break down dead bodies and animal waste and return nutrients from the organic matter back to the soil to help plants grow.
Micro-organisms grow slowly when they are cold. That is why we keep food in the refrigerator, to slow down the decaying process of our food.
Micro-organisms grow fastest when they have:
A warm temperature
Plenty of water
Plenty of oxygen

2. Unit 2:4 Micro-organisms and Food
Making cheese and yoghurt
Micro-organisms can feed on the same things that humans eat.
Sometimes we like the changes the micro-organisms make in our food.
For example, special kinds of bacteria are used to change milk into yogurt or cheese.
These bacteria feed on the sugar in the milk and change it into a weak acid called lactic acid.
Making bread
We use yeast to make some kinds of bread.
The yeast feed on nutrients in the flour.
When the yeast respires (uses food to make energy), it makes carbon dioxide.
The gas makes bubbles in the dough, which make it rise.
Yogurt Video