Yoghurt is made using bacteria including Lactobacilus acidophilus. The box on the right are the ingredients in a yoghurt - the live cultures are the bacteria.

You can make your own yoghurt by adding a small amount of yoghurt to warm milk and leaving it in a warm place for a few hours. Over this time the bacteria reproduce, carry out respiration and digest food.

The bacteria get their food from the lactose (a sugar) which is in the milk that yoghurt is made from. They use the lactose during respiration to release energy for them to carry out MRS C GREN. Lactic acid is also made. The chemical equation is: lactose → lactic acid + energy.

Lactic acid is an acid and has a pH of around 3.5. This makes the protein in the milk change, turning the milk thick and sour - we call this yoghurt. If you eat “natural unsweetened” yoghurt it is very sour! Most yoghurts have sugar and fruit added to them which makes them taste much less sour.

The bacteria do this process fastest at warm temperatures of between 30 and 40°C. So when yoghurt is being made the milk and bacteria mixture is kept between these temperatures.

Yoghurt is also known as a probiotic because it contains bacteria that are thought to restore the helpful bacteria in your stomach and small intestine after you have been sick or on antibiotics (antibiotics are medicines that kill bacteria). There is not a lot of medical research into how effective probiotics are, but they are not known to cause any harm to the body.

Yogurt: A food made using bacteria and milk. The bacteria change the lactose in milk into lactic acid.

Lactose: The sugar found in milk.

Lactic acid: Made by bacteria when they break down lactose. Lactic acid is an acid which makes it sour.

Probiotic: "Good" bacteria used as a medicine which may help improve digestive health.

Antibiotic: A medicine used to kill bacteria.