1. There are several trillion friendly bacteria in the average human gut.
9-Bread & Brewing
Making Yoghurt
Chemicals
Equipment
Yoghurt
250ml. Beaker
Powdered Milk
Thermometer
Fresh Milk
Water Bath
Plastic Cup & Square of Foil
Fascinating Fact
There are several trillion friendly bacteria in the average human gut.
Collect your equipment – beaker, thermometer & 2 plastic cups & two squares of foil.
Carefully measure out 200ml of milk using the measurement on the beaker.
Put the beaker on a gauze mat over a Bunsen burner and “warm” to 65oC.
Maintain this temperature for 1 min.
Then remove from the heat and allow to cool to 40oC before adding the yoghurt culture
ADD
1 tbsp. of commercial yoghurt (this is your culture)
1 tbsp. of powdered milk (this is to slightly thicken your yoghurt without the aid of gelatine or starch)
Cover with Foil and place in the water 40oC for the next 12 hours (tomorrow)
Did you know?
The average adult carries around approximately 2kg of good microbes in our guts – the same weight as 2 bags of sugar!
Why is it important to heat the milk to 65oC?
Why don’t you add the yoghurt culture until the temperature is down to 40oC?
Why is it important that this temperature is kept during the fermentation process?
Why is this different to body temperature?
Why do you only need a tablespoon of live yoghurt?
Why is yoghurt good for your gut?
Why do you think it is important to kill all bacteria (sterilize) on surgical equipment before a surgeon performs an operation. 
Bacterial Reproduction
Bacteria can reproduce very quickly. If conditions are correct bacteria can divide every 20 minutes. This means a small number of bacteria can multiply very quickly.
If 100 bacteria get into your body and reproduce every 20 minutes how many will there be in 2 hours?
Time( min) Number of bacteria
0 100 20
  40
  60
  80
  100
  120

2. Background Information
With the help of microorganisms called bacteria, milk is turned into yogurt. Do not freak out though, these are not the kind of bacteria that cause you to get sick. The bacteria in yogurt are good bacteria that can actually help you! There are certain species of bacteria that are commonly used to make yogurt. If you look at the ingredients listed on the yogurt product's packaging, you can often figure out the exact species of bacteria that it contains. Some species you might find listed include: Streptococcus thermophilus (S. thermophilus); Lactobacillus bulgaricus (L. bulgaricus); L. acidophilus; L. casei; L. rhamnosus; Bifidobacterium animalis (B. animalis, or sometimes just "Bifidus"); and B. bifidum. To turn milk into yogurt, these bacteria ferment the milk. Fermentation is when a substance gets broken down and turned into another substance. During fermentation to make yogurt from milk, small sugars in the milk (specifically lactose sugars) get turned into a different chemical, specifically lactic acid. The lactic acid is what causes the milk, as it ferments, to thicken and taste tart. Because the bacteria have partially broken down the milk already, it is thought to make yogurt easier for us to digest. Additionally, eating yogurt can help restore the good bacteria that normally lives in your stomach and intestines (your gastrointestinal tract) after they have been lost from, for example, taking antibiotics or having an upset stomach. Pasteurization is a method of reducing pathogenic microorganisms in food through heat. The concept of heat treating food to prevent spoilage began hundreds of years ago, before refrigeration, when people were looking for ways to extend the shelf life of beer and wine. It wasn't until French scientist Louis Pasteur began experimenting with the time and temperatures needed to effectively reduce microorganism that an effective method was developed that adequately also preserved the flavour and texture qualities in food. Pasteurization is the process of heating liquids or foods to kill microorganisms (such as Brucella, Campylobacter, E. coli O157:H7, Listeria, Mycobacterium bovis, Salmonela, and Yersinia) that can cause disease.