Microbes in the kitchen and the field

Yeast and dough (Activity 2.8). Take two bowls, each with 200 g of flour and a pinch of sugar. Mix yeast powder into bowl A only, so you can compare. Knead both with warm water, cover them with a damp cloth and keep them somewhere warm. After 4 to 5 hours the dough in bowl A has risen, gone fluffy and smells different. Bowl B has not changed.

Yeast is a microorganism, a kind of fungus, and it grows well when it is warm. Like every living thing it respires, breaking down food (here, the sugar) to get energy. As it does, it gives off carbon dioxide, which forms bubbles that make the dough soft and fluffy. It also makes a little alcohol, which gives the dough its smell. That is why we add sugar (food for the yeast) and warm water (the warmth it likes).

Yeast is used to make bread, cakes and more. Some bacteria, such as Lactobacillus, ferment idli and dosa batter, and bhatura dough.

Milk and curd (Activity 2.9). Put lukewarm milk in bowl A and cold milk in bowl B, and stir a small spoon of curd into each. Keep A warm and B somewhere cool, such as a fridge.

Bowl A turns into curd and tastes a little sour. Bowl B stays milk. Curd contains bacteria, among them Lactobacillus. It feeds on the sugar in milk, called lactose, multiplies, and turns the milk into curd. Instead of alcohol it makes lactic acid, and that makes curd sour. It grows well in warmth, which is why A set and B did not.

Out in the fields there is a fair deal. Bacteria such as Rhizobium live in swellings called nodules on the roots of legumes such as beans, peas and lentils. The plant gives the bacteria a home and food. In return, they trap nitrogen from the air and make it useful for the plant, so it grows without chemical fertilisers.

That is why farmers grow legumes in rotation with other crops: it naturally adds nitrogen to the soil and keeps it healthy for the next crop.