Yeast is a living organism and is in the air around us. Yeast belongs to the fungi family and is a very small single cell microorganism. Like those found in humans, yeast cells are living and natural. They need a warm environment, food and water to grow and ferment. This fermentation process gives off carbon dioxide gas, alcohol, and other organic compounds. The gas is the rising agent in bread, and the other "waste" products create the subtle flavors and texture that make a good loaf.
Three Essential Needs for Yeast Fermentation:
Food: Sugar is the most important food requirement for yeast in a fermenting dough and flour is the principal source. Flour has a natural fermentable sugar content of around 1.5 percent. It provides further sugar through the production of maltose by the flour amylase enzymes acting on the damaged starch. Extra sugar or malt products may be added but when the yeast has an adequate supply of sugar, the addition of more will not persuade it to feed faster and therefore produce gas at a higher rate. These sugars will have a beneficial effect on the finished products in terms of crust colour and crumb softness. Sugar addition to a dough has little effect on gas production until it reaches about 5 percent of the flour weight. As the level increases further the rate of gas production decreases. This is why much higher yeast levels have to be used in bun doughs than in bread doughs.
Water: It is mainly found in flour and in the water added to make bread dough. Without water, there can be no fermentation. Yeast and enzymes can only work when dispersed in water and yeast can only absorb food which is in solution. In stiff dough there is little free water so the carbon dioxide production by yeast is slow. In a weak dough there is a lot of free water and therefore too little binding between the dough constituents. The carbon dioxide production is fast. A lot of water (weak dough) will give an increased enzyme activity. Less water will reduce the enzyme activity.
Environment: The medium into which yeast is introduced is important as its physical and chemical composition needs appropriate salt content and temperature ranging between 10°C and 30°C. Dough temperature directly affects the rate of yeast fermentation. As the dough temperature increases so does the rate of gas production until the optimum is reached at about 40°C, after which there is a progressive thermal killing of the yeast cells and therefore a decrease in gas production, until they are all killed at around 50°C. Conversely, as the dough temperature decreases so does the rate of gas production until it almost stops at around 4°C and this is made use of in the retardation and deep freezing of fermented goods.
Production of carbon dioxide: Carbon dioxide is generated by the yeast as a result of the breakdown of fermentable sugars in the dough. The evolution of carbon dioxide causes expansion of the dough as it is trapped within the protein matrix of the dough.
Causes dough maturation: This is accomplished by the chemical reaction of yeast produced alcohols and acids on protein of the flour and by the physical stretching of the protein by carbon dioxide gas. This results in the light, airy physical structure associated with yeast leavened products.
Development of fermentation flavor: Yeast imparts the characteristic flavor of bread and other yeast leavened products. During dough fermentation, yeast produce many secondary metabolites such as ketones, higher alcohols, organic acids, aldehydes and esters. Some of these, alcohols for example, escape during baking. Others react with each other and with other compounds found in the dough to form new and more complex flavor compounds. These reactions occur primarily in the crust and the resultant flavor diffuses into the crumb of the baked bread.
Factors affecting fermentation:
- Temperature of the dough; optimal fermentation temperature is 78 - 82 degrees F
- Temperature of the room: optimal temperature being 75 - 80 degrees F. (When the temperature exceeds 85 degrees F, off flavors result.) Dough can still rise in cooler environments, but much more slowly.
- Fermentation time; allows for the development of distinctive flavor and texture, depending on type of pre-ferment. Slower fermentation is best for the development of flavor and strength.
- Amount of yeast; the more yeast the faster the fermentation. Too much can add an undesirable yeasty flavor.
- Type of yeast; instant active dry yeast contains fast acting yeast
- Amount of salt; typical Baker's Percent is 1.8 to 2.5
- Amount of sugar; small quantities (up to 5 Baker's Percent) increases yeast activity. Above 10 Baker's Percent, slows yeast activity
- Type of sugar; sucrose, glucose and fructose are fermented rapidly; maltose is fermented slowly; lactose is not fermented at all
- pH of dough; optimal pH is acidic 4 to 6. Above, fermentation slows. As yeast ferments, it produces acids to lower the pH to that range
- Presence of antimicrobial agents; Most spices, have antimicrobial activity, such as cinnamon and can slow fermentation. Be careful how much is added to the dough directly.
There are two forms of baker's yeast are; compressed cakes (also called fresh yeast) and dehydrated granules (dry yeast).
Fresh yeast is ivory colored with a yellowish hue and is soft and moist and should easily crumble. Make sure it is fresh smelling and there are no dark or dried places on the yeast. It is highly perishable, has a short shelf life and must be refrigerated. For longer term storage it can be frozen. Compressed yeast contains about 70% moisture. It needs to be proofed before using and should have a pleasant yeasty smell and be foamy.
Dry yeast is fresh compressed yeast that has been pressed and dried until the moisture content is only about 8% which makes the yeast dormant. The granules only become active again when mixed with a warm liquid. The advantage of dry yeast is it has a much longer shelf life than fresh yeast and does not need to be refrigerated. The tiny, dehydrated, bead-shaped, sand colored granules are most often sold in convenient small foil-lined packages weighing 1/4 ounce (7 grams) that have been packaged under pressure. It is also sold in jars but once opened, the yeast needs to be stored in the refrigerator away from moisture, heat, and light because once yeast is exposed to air it deteriorates rapidly.
There are two types of dry yeast: regular active dry and rapid-rise. 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. However, you do sacrifice flavor and texture in order to save time as the yeast does not have time to develop its own flavor.
You may have noticed that in some recipes it calls for dissolving the yeast first in a warm liquid and then adding this active yeast mixture to the flour. Other recipes, however, call for the yeast first being added to the flour and then the warm liquid is added. The dissolving of the yeast first in a warm liquid is done to make sure the yeast is still fresh and active. This step really doesn't need to be done though because of how reliable the dry yeast is today. Also, the dry yeast has such a small granule size that it dissolves easily into the dough without having to be reconstituted separately. Some bakers, however, still feel that it is a good idea to test the yeast to make sure it is still active before adding it to the flour.
Remember that the more you work with yeast, the more comfortable you'll become. Baking demands that you experience occasional failure; once you accept this, you'll be on your way to a lifelong baking habit.