In the process of fermentation, yeast produces carbon dioxide, alcohol and other compounds which enable dough to rise and modify its physical properties. When the fermentation is correctly achieved, depending on the quality of flour among other things, the baker will obtain the proper external and internal characteristics (grain and texture) suitable for a determined finished product.
Together with alcohol, a small amount of other volatile compounds are released. These are involved in the original taste and flavours of baked leavened products. Most of these compounds, and all of the alcohol, evaporate when baked.
There are two stages during the fermentation of a dough made from flour, water, salt and yeast, and with no extra sugar added.
First of all, yeast ferments sugars naturally present in flour, which can be directly and easily assimilated. These sugars represent about 1.5% of the flour weight. At the end of this first stage, gaseous releases more or less slow down.
The second stage corresponds to the fermentation of a sugar contained in flour called maltose. Maltose comes from the action of some enzymes, the amylases, on the starch granules of the flour, damaged during the milling process. Amylases which are naturally present in flour, split starch into small fractions of a much simpler sugar, the maltose. The action of amylases starts as soon as water is added to the flour and stops during baking.
The action of the flour amylases is completed by that of another enzyme of yeast, the maltase which, in its turn, splits maltose to give the most simple sugar, glucose. The glucose is transformed by the yeast into carbon dioxide and alcohol.
Maltose formed from starch must be present in sufficient quantity so that the production of carbon dioxide makes dough rise correctly until it is put in the oven.
Since 1960, the bakery industry has benefited from better adapted yeast strains, allowing a rapid process as they ferment the maltose sugar earlier. Effectively, the maltose is fermented only when there are no or little pre-existing sugars or added simple sugars left in the dough.
The yeast industry has studied to a great extent baker’s yeast strains, not only with a view to making them more rapid, but also to adapt them as much as possible to different types of baking processes.
If sucrose has been incorporated into the dough, it is immediately transformed by a yeast enzyme, the invertase, into glucose and fructose (the two basic links composing sucrose are thus separated).
When sucrose or glucose are added to the dough, they are directly fermented before maltose. This means that in such a dough, mainly sucrose or glucose are consumed by the yeast. These sugars do not contribute to giving the bread a sweet taste since they are partly consumed.