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This is called multiplication. The carbon dioxide makes the dough rise which gives the bread a light and spongy texture. Yeast also works on the gluten network. The by-products of “fermentation”, or rising, give bread it’s characteristic flavour and aroma. The yeast continues to grow and ferment until the dough reaches around 46°C at which temperature yeast dies.
Yeast uses sugars by breaking them down into carbon dioxide and water. The yeast needs lots of oxygen in order to complete this type of fermentation.
In a bread dough, oxygen supply is limited and the yeast can only achieve partial fermentation and instead of carbon dioxide and water being given off, carbon dioxide and alcohol are produced. This is called alcoholic fermentation.
Production, Growth and Reproduction
To live and grow, yeast needs moisture, warmth, food and nutrients. Commercial yeast is manufactured on an aerated suspension of molasses. Molasses, a form of sugar, provides the food for the yeast so it can reproduce. The molasses is mixed with water and sterilised to kill off unwanted bacteria, clarified by removal of sludge and then held in vats. Once it has been through this process it is called wort.
Yeast has a phenomenal growth rate and can duplicate itself every 90 minutes by a process called budding. During budding, a mature yeast cell puts out one or more buds, each bud growing bigger and bigger until it finally leaves the mother cell to start a new life on its own as a separate cell.
When conditions are unfavourable for the yeast, for example when no food is or very dry conditions, it doesn’t die but goes through a process called sporulation. The yeast spores can then withstand long periods of drought, cold and high temperatures until conditions are right for reproduction and it starts to bud all over again.