Q.Sprinkle a small spoonful of yeast pellets/powder over warm water and then add sugar. Cover it and wait for 10 minutes. Yeast becomes bubbly over the water proving that it is still active.

This activity shows that dried yeast is alive: given warmth, water and sugar it starts respiring and multiplying by budding. …

What to do. Take a cup of warm (not hot) water, sprinkle a small spoonful of dried yeast on it, add a spoon of sugar, cover the cup and leave it for about 10 minutes.

What you see. A layer of froth or bubbles builds up on the surface and the mixture smells faintly of bread or beer.

Why it happens. Dried yeast pellets are dormant, living cells of a unicellular fungus. Warm water rehydrates them and the sugar is food they can respire. As they respire they release carbon dioxide gas, which cannot escape through the liquid quickly and so collects as bubbles, and with the food supply they also begin to reproduce asexually by budding, each cell putting out one or more small buds that grow and separate. Under a microscope a drop of the froth shows mother cells with buds, and sometimes short chains of buds, exactly the picture of Fig. 1.1. …