Yeast is a single-celled living organism that digests starches and gives off carbon dioxide gas in the process. Bread can be made light and airy by mixing yeast into the flour and water that make up bread dough. The dough is then left to rise in a warm place. The warmth encourages the yeast to give off tiny bubbles of carbon dioxide, which are trapped within the elastic dough. When the dough is put into the oven, some water evaporates from the flour mixture, and the dough becomes firmer, with the tiny bubbles trapped within it.
Yeast is a tiny plant-like microorganism that exists all around us – in soil, on plants and even in the air. It has existed for so long, it is referred to as the oldest plant cultivated by man.
Yeast works by serving as one of the leavening agents in the process of fermentation, which is essential in the making of bread. The purpose of any leavened is to produce the gas that makes bread rise. Yeast does this by feeding on the sugars in flour, and expelling carbon dioxide in the process. As the yeast feeds on the sugar, it produces carbon dioxide. With no place to go but up, this gas slowly fills the balloon. A very similar process happens as bread rises. Carbon dioxide from yeast fills thousands of balloon-like bubbles in the dough. Once the bread has baked, this is what gives the loaf its airy texture.
There are two types of dry yeast: Regular Active Dry and Instant Yeast (also known as Fast-Rising, Rapid-Rise, Quick Rise, and/or Bread Machine Yeast). The two types of dry yeast can be used interchangeably. The advantage of the rapid-rise is the rising time is half that of the active dry and it only needs one rising.
You can speed up standard yeast bread recipes by changing the yeast in the recipe. Substitute one package Instant or fast-acting yeast for one package regular active dry yeast. Instant yeast is more finely ground and thus absorbs moisture faster, rapidly converting starch and sugars to carbon dioxide, the tiny bubbles that make the dough expand and stretch.
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