Alice McCubbin-Howell, of Waikari School, asks :-

While making bread I wondered how yeast works. How does it?

Ron Hooker, a food technology consultant with the Country Fare Bakery, responded.

Yeast is a small living organism, a "micro-organism", just slightly larger than bacteria and they can be seen through a strong microscope. As it is living it requires warmth, food and water to grow and this is why bread dough is warmed in a prover. Yeast multiplies by budding.

Bread basically is made from flour, water, yeast and salt. During either intensive mixing (mechanical dough development) or long standing with gentle mixing for about 4 hours (conventional development), the dough changes as the flour absorbs the water and the structure of the mixture develops into a mass like bubble gum or play dough. That is it can be worked by hand and moulded and it will stretch like a balloon.

The purpose of the yeast is to expand the dough - when warm the yeast feeds on the food (nutrients) in the flour or on added sugar or other yeast foods in the mix. As it feeds and multiplies it produces carbon dioxide gas which is trapped inside the dough. When the dough goes into the oven the gas expands because of the high oven temperatures and the dough blows up into the shape of a loaf of bread and then at the end of the baking time the yeast is killed and the bread firms up. The yeast also produces some alcohol and other pleasant tasting compounds which add to the flavour of the bread and this is the cause of the lovely smells found in a bakery. Yeast is also used in wine making and in other processes in the food industry.