Yeast is a tiny, unicellular organism. It was discovered by Louis Pasteur. It belongs to the fungi family and has tiny, rounded, colourless bodies. Yeast is found in soil, in air, on plant surfaces and in the nectar of flowers. When a weak sugar solution is exposed to air, a light frothy scum appears on the surface. This is because yeast present in the air settles on the liquid and converts sugar into alcohol and carbon dioxide. Yeast is commercially used in wine, beer and bread making industry. It is a rich source of protein and is rich in vitamin B.