Yeast is a microscopic (very, very small) type of Fungus. It is a living organism. (Microorganism)
It is made up of only one cell (unicellular).
What does it look like?
It has an ovoid (egg-shaped) or spherical shape, and a yellowish color.
How Does it Work?
All living things need to breath to live, but yeast can breath without oxygen (Anaerobic Respiration). Yeast cells thrive on simple sugars. Under the right circumstances when Yeast comes into contact with sugar, the sugar feeds the yeast. The yeast produces CO2 (Carbon Dioxide) and alcohol.
By fermentation, the yeast species Saccharomyces cerevisiae converts carbohydrates to carbon dioxide and alcohols – for thousands of years the carbon dioxide has been used in baking and the alcohol in alcoholic beverages. Quoted here.
What Do You Use Yeast For?
Bread-making, alcohol drinks (wine, beer, etc), nutrition and bio-fuels. Learn More.
Yeast is the driving force behind fermentation, the magical process that allows a dense mass of dough to become a well-risen loaf of bread. Quoted here.
What are the different types of yeast?
Some forms of yeast are Active Dry Yeast (alive), Instant Dry Yeast (Dead) and Nutritional Yeast. Here are more.
Active dry yeast and instant (or rapid-rise) yeast are the two most common yeasts available to us as home bakers. The two yeasts can be used interchangeably in recipes, but active dry yeast needs to be dissolved in water before using while instant yeast can be mixed right into the dough. Quoted here.
Brewer’s yeast (or nutritional yeast) is used as a food supplement. It is dehydrated at high or low temperature. Quoted here.
In the case of dead yeasts (or inactive, i.e. beyond 40°C), the yeast keeps its vitamins and minerals but it cannot be used to make bread: it is an inactive yeast! Quoted here.
Read here to learn-
Kneading the dough-
When you stir together flour and water, two proteins in the flour—glutenin and gliadin—grab water and each other to form a bubblegum-like, elastic mass of molecules that we call gluten. In bread making, we want to develop as much gluten as we can because it strengthens the dough and holds in gases that will make the bread rise. Quoted here.
To knead the dough is to use your hands to squeeze, pound, work, mold, shape, and manipulate the dough, causing the gluten strands to warm and stretch, creating a springy and elastic dough.
You can also knead with a a mixer equipped with a dough hook, or with a bread machine.
If bread dough is not kneaded enough, it will not be able to hold the tiny pockets of gas (CO2) created by the leavening agent (such as yeast or baking powder), and will collapse, leaving a heavy and dense loaf. Quoted here.
Visit this “Bread Baking Clinic,” to find out if you are over kneading or under kneading.
Here are some more resources to go even deeper into learning about yeast.