What is Yeast?
Yeast belongs to the fungi family. It is a very small single cell micro-organism. Like all other fungi it doesn't have the power to produce food by photosynthesis. Instead it ferments carbohydrates (sugars) to produce carbon dioxide and alcohol which gives bread it's texture, colour and aroma.
There are several types of yeasts but the important ones for the baking industry are those belonging to the genus Saccharomyces cerevisiae, which means "sugar eating yeast".
Yeast has been used by man to make bread and alcohol for thousands of years. Evidence of this has been found in ancient Babylonian wall carvings and Egyptian hieroglyphics dating back to 2000 B.C.
The leavening of bread was considered an art form because the ancient peoples didn't understand the process of fermentation. They probably stumbled across leavened bread when a piece of old over-fermented dough full of yeast cells was mixed in with fresh dough and the resulting bread was more palatable than the unleavened bread they had been used to.
In 1676 Anton van Leeuwenhoek, when looking through a microscope, identified that yeast was a cell and that different types of yeast cells could be used for brewing beer or making wine.
In the early days of bread production a piece of dough from yesterday's bake was kept and added to the new day's dough because it was found that the resulting dough was more consistent and fermented faster. The old piece of dough is called the starter or "leaven".
In the 1800's an understanding of the leavening process was fully developed through the work of Louis Pasteur. He discovered that yeast was the orgasnism that caused fermentation.
Since then, many strains of yeast have been isolated and produced. Research and development on yeast is still continuing in an effort to produce strains of yeast that will improve the bread baking process.
Yeast in bread making
Yeast is used for the leavening of bread. Yeast uses the sugars and oxygen in dough to produce more yeast cells and carbon dioxide gas. 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 ths 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.