Grades 3–5 · Everyday Life
How do tiny bacteria turn milk into thick yogurt?
Yogurt begins with milk and selected kinds of bacteria. These bacteria are living things too small to see without a microscope. They use milk sugar as a food source and produce lactic acid. This change is a kind of fermentation.
In the diagramTiny bacteria change milk into yogurtThe bacteria use milk sugar and release lactic acidWhat bacteria doYogurt gelMilk sugarBacteriumLactic acid accumulatesMilk proteins join a networkThe protein network makes yogurt thick; bubbles do not.
The acid changes the milk. Milk contains proteins called caseins, normally dispersed in tiny groups. As the milk becomes more acidic, those groups can join together into a network. The network holds water, so the milk becomes a thick gel instead of a freely flowing liquid. Lactic acid also contributes yogurt's tangy taste.
The bacteria do not build the yogurt by inflating it with gas. Its thickness mainly comes from the changed protein network. The amount of water, the milk's treatment and added ingredients can also affect texture. Different food cultures do different jobs: bread-making yeast can produce gas, while yogurt cultures are selected for their acid-making work.
About this explanation
Starting knowledge: Know that living things can be too small to see and that milk contains protein.
Sources
- University of Guelph — Unit 5a. Lactic Cultures in Yogurt and Kefir