Practical 1: Making yoghurt from milk
Yoghurt is made by adding a starter culture to milk. The starter is a group of fermenting bacteria that break down the sugar (a carbohydrate) in milk into lactic acid. Lactic acid sets the milk and gives it a sour taste. Bacteria multiply well in warmth, so the milk is warmed to a comfortably warm temperature (about 40 °C); milk that is too hot kills the bacteria, while in cold milk they work slowly. Method: an adult boils the milk and cools it to warm; it is poured into a clean container and a spoonful of ready yoghurt is added; the container is wrapped in a towel and put in a warm place; once set, the yoghurt goes into the refrigerator (cold slows the bacteria down). Yoghurt helps digestion and contains calcium and protein. The container and spoon must be clean, and the yoghurt must smell pleasant; if it smells or looks odd, do not eat it.
Practical work at home with a parent: an adult boils the milk and cools it to warm; add a spoonful of yoghurt, wrap in a towel and keep warm. When set, put it in the refrigerator and, with your parents’ permission, taste it. Write in your notebook: how many hours did the yoghurt take to set? How do its smell, taste and thickness differ from milk? Safety: only adults handle hot milk; children allergic to milk or dairy products do not taste it; keep the container and hands clean; do not eat anything that smells spoiled.