Leavening in Bread Making

Adding yeasts to the flour and ingredient mixtures and leaving the mixture in the right temperature and moisture for some time has been a key action in bread making.  It is well observed that, with time, the mixture, or called the dough, will rise to double its original size and is about right for further shaping and proofing.

What gives rise to the doubling of the size is gas filled cavities inside the dough.  The process induced by the yeast is called fermentation - the yeast take action using the fermentable sugars contained in the mixture to give carbon dioxide and ethanol.

This process is also referred as leavening - a foaming action indued in doughs so that the final products will be softened and lightened up.

There are two main ways to introduce yeasts to the dough.

The traditional way is to use a starter - which is essentially a dough used in previous bread making kept to allow for the yeast to continue to grow inside the dough.  Since the ongoing fermentation gives an acidic property, the dough kept for such purpose is also called a sourdough.  The main food safety concern about keeping a sourdough is to ensure it is clean and not contaminated with molds.  The latter will give rise to terrible colour and smell and render the starter useless and even dangerous.

A more convenient way nowadays is to add prepared dried yeast obtained from grocery store and add directly, or priorly dissolved in water, to the flour mixture.   This saves the time and effort in keeping the dough.

Yeast may also be present in ingredients like unpasteurized beer, buttermilk etc.  But these are probably not very convenient for urban people like us.

The main issue about using yeast for leavening is time.  The yeast will time to act.   So for an ordinary loaf of bread with 500g of flour and 7 to 10g of dried yeast, the time required for rising and proofing may range from 1 to 3 hours, depending on the moisture and temperature of the environment.

To save time, one may also use chemical leavening agents to induce the foaming actions.  Bicarbonates is a common used chemical, usually in the form of commercially available baking soda.  Baking powder is a mixture with bicarbonates (baking soda) with flour.   The texture and tastes of the product may be different from that of the yeast-risen bread.

For completeness sake, I include here the physical method of leavening, primarily by the process of vigorous mixing or whipping - but the pre-requisite is the presence of the appropriate ingredients and/or physical condition.  For instance, mixing of steam and air in high temperature may hold the steam inside the food.  Mixing of solid butter and sugar crystal gives rise to creamy texture.  Vigorous whipping of cream and egg whites can also results in foaming action.  As they require specific ingredients (eggs, sugar, butter or other mild products), they seem to be less used in ordinary bread-making.