YEAST : is an essential ingredient in bread baking, particularly for baking yeast breads, as it acts as the leavening agent which is responsible in the volume of the bread as well as it also contributes to the creation of the taste of the bread.
Yeast is a single cell living microorganism – a fungus, which multiplies by a process known as budding.. Yeast is used in bread baking as a leavening agent. That is its main function. Under the right conditions of water, starches, warmth, and dissolved minerals, yeast causes fermentation. During the fermentation process, the yeast utilizes the sugar in the dough to produces carbon dioxide thereby leavening or causing the dough to rise. And during fermentation the temperature increases, pH falls, alcohols produced and ultimately dough gets mellowed (extensible). During fermentation, the yeast feed on the carbohydrates that are present in the starches and sugars (from both the added sugar to the dough and the sugar produced from the starch in the flour), it ferments them, converting the sugar to carbon dioxide and ethanol (an alcohol), and various flavours. As the dough is heated during baking, the carbon dioxide is trapped within the dough (by the gluten network in the dough) which causes the bread to rise (that makes the volume of the bread). The alcohol on the other hand evaporates during baking. Longer storage of yeast, dough temperature, pH, and availability of water and food supply affect the yeast activity. The temperature has great influence on the propagation of yeast and quality of bread.
Below is the table for the yeasts activities at certain temperature
Temperature for the yeast development
|34 oF (2 oC)||Inactive|
|60-70 oF (16-21 oC)||Slow Action|
|75-95 oF (24-35 oC)||Best Temperature for yeast activity|
|85-100 oF (29-38 oC)||Best Temperature for hydrating instant yeast|
|100-110 oF (38-43 oC)||Best Temperature for hydrating active dry yeast|
|138 oF (59 oC)||yeast dies|
Aside from temperature, the amount of sugar and salt also affects the proper functioning of the yeast. The minimum quantity of sugar required for proper fermentation is 3-3.5%. Higher levels of sugar provide for darker crust colour, sweeter taste and helped to retain moisture. Bread Manufacturers to improve colour and taste use higher levels of sugar. There are several types of sugars used in the formulations like, sucrose, fructose, and dextrose, molasses, brownsugar, cornsurp etc. The starch hydrolysis produces sugars as monosaccharides, disaccharides, maltose, dextrins, etc. (see BAKING INGREDIENTS – SUGAR) By the way, not all of the sugar will be used up by the yeast during the fermentation process though, some of it is used in crust colour and the remaining sugar is for taste.
Too little or too much salt will affect the activities of the yeast and thereby affecting the outcome result of the bread not only in taste but also in volume, texture, etc. For salt, please see next post on Baking Ingredients: SALT
Properties of yeast:
To better understand how yeast functions, let’s discuss its properties:
Enzymes in yeast consist of invertase, maltase, zymaseand protease. There are others, but they are not important in bread production. The protease enzyme in yeast is only activeif the cell wall of the yeast is damaged in some way. That is the only time it can penetrate the cell wall of the yeast. There are always a few damaged yeast cells especially in dry yeast where some cells are damaged during the drying process, or in yeast that has gotten a little old. If the protease enzyme does penetrate the cell wall of the yeast, it will weaken the gluten. (source: http://www.bakingandbakeryscience.net/ by Willie Prejean)
Invertase enzyme is an entercelluar enzyme. Sucrose, (cane or beet sugar) when dissolved enter the cell wall and are changed to dextrose and fructose, which are later changed by the enzyme zymase (below). Maltase attacks malt sugar. The malt sugar is changed to two molecules of the simple sugars dextrose. (source: http://www.bakingandbakeryscience.net/ by Willie Prejean)
Zymase is the enzyme which changes the simple sugars, dextrose and fructose into carbon dioxide gas and alcohol, and several esters which result in producing the unique aroma flavor and taste of breads. Also, the gas produced causes the dough to rise.. Fermentation will be discussed in more detail in a later paragraph on bread production procedures. (source: http://www.bakingandbakeryscience.net/ by Willie Prejean)
Types of Yeast and Their Usage in Bread Making
Usage of yeast in bread making ranges from 2-5% depending on the kind of yeast used. Here are the 3 types of yeast used in baking that are available in the market
Fresh Yeast or compressed yeast– is a mixture of yeast and starch and contains 70% moisture. Fresh yeast should be creamy white or ivory colored with a yellowish hue and is soft and moist and with a cakey consistency that crumbles easily It should have a fresh, yeasty smell with no dark or dried places on the yeast. Do not use fresh yeast that has a sour odor, brown color, or with slimy film. This is mainly used by large industrial bread manufacturers and not home-bakers like us, since fresh yeast are highly perishable and must be used within a the same day of purchase or the following day. It needs to be refrigerated or frozen for storage. However, frozen fresh yeast will lose 5% of its activity when thawed.
Usage: Fresh yeast needed to be softened in twice its weight in warm water (100oF / 38oC) before adding to the bread.
Active Dry Yeast – active dry yeast differs from compressed yeast in that virtually all the moisture has been removed by hot air.. The absence of moisture renders the organism dormant and allows the yeast to be stored without refrigeration for several months. When used in preparing doughs, dry yeast is generally rehydrated in a lukewarm (110 deg F / 43 deg C) liquid before being added to the other ingredients. (source : On Baking: A textbook of baking and pastry fundamental by Sarah Labensky )
Instant Dry Yeast – it has gained popularity because of its ease of use; it is added directly to the dry ingredients in a bread formula without rehydrating. The water in the formula activates it. Like all yeasts, instant dry yeast is a living micro-organism and will be destroyes ate temperature above 138 deg F / 59 deg C (refer to table above) While instant yeast can be added to flour without hydration, some bakers still prefer to hydrate instant yeast before using it in certain types of formulas. When doughs are mixed briefly or are very firm, such as bagel or croissant dough, instant dry yeast may not fully dissolve during mixing. In such cases the yeast us moistened in four to five times its weight of water. Deduct this amount of water from the total water for in the formula. (source : On Baking: A textbook of baking and pastry fundamental by Sarah Labensky)
The flavors of dry and compressed yeasts are virtually indistinguishable, but dry yeast are at least twice as strong. Because too much yeast can ruin a bread, always remember to reduce the specified weight for compressed yeast when substituting dry yeast or active dry yeast formula. likewise, if a formula specifies dry or active dry yeast, increase the quantity specified when substituting compressed yeast. For yeast substitution, use the formulas in the table below (source : On Baking: A textbook of baking and pastry fundamental by Sarah Labensky )
|YEAST SUBSTITUTIONS||(source: On Baking: A Textbook of Baking and Pastry Fundamentals)|
|Compressed (fresh) yeast||x||0.5||=||Active dry yeast|
|Compressed (fresh) yeast||x||0.33||=||Instant yeast|
|Active dry yeast||x||2||=||Compressed (fresh) yeast|
|Active dry yeast||x||0.75||=||Instant yeast|
|Instant dry yeast||x||3||=||Compressed (fresh) yeast|
|Instant dry yeast||x||1.33||=||Active dry yeast|
The Principal Qualities of Good Yeast from the Bakers’ Standpoint
(source : Yeast: A Treatise on Baking – http://abrfaq.info/treatise/192)
1. Strength of Yeast : this refers to the longevity or the power of yeast to raise and condition the dough batch throughout the entire dough stage. The word strength when applied to yeast means. It also refers to the power to initiate and sustain the entire process of fermentation of the dough until this action is terminated by the heat of the oven. Strong yeast is absolutely necessary for the production of the highest quality yeast raised baked goods.
One very important characteristic of yeast strength is its thermal death point or the temperature at which its fermentative activity in the dough is killed by the heat of the oven. A strong yeast is one which will function vigorously in the dough from start to finish,—not merely exhibiting- a flashy start with subsequent weakening.
2. Uniformity and Dependability : Uniform yeast is yeast which can be depended upon to raise and condition the dough batch in the same way and at the same rate day in and day out,—all other conditions remaining the same. This means absolutely dependable yeast. The uniformity of yeast is of fundamental importance for a better bakeshop production operation as bakers are able to adhere to a daily standard schedule of bakeshop operation with the assurance of the production of uniform quality yeast raised products each and every day. Without uniform yeast, a uniform loaf is practically impossible.
3. Purity of Yeast : refers to the absence of foreign wild yeasts or undesirable bacteria from bakers’ yeast. As explained previously, bakers’ yeast represents one strain of yeast selected and grown for commercial use because of its particular properties which make it ideally adapted for bakers’ use. There are of course, many other kinds of yeasts and bacteria floating in the air, which may not be ordinarily harmful, but nevertheless, are highly undesirable in bakers’ yeast. Their action in the dough causes what is termed wild fermentation resulting in the production of undesirable flavors and, in general, inhibits and interferes with the normal healthy fermentation of the dough, thereby impairing the quality of the baked goods produced.
The modern yeast manufacturer exercises extreme care to avoid the invasion of foreign organisms in bakers’ yeast for he realizes that purity of yeast is a factor of utmost importance to the baker.
4.Hardiness : refers to the ability of yeast to retain its baking strength throughout shipping and handling until used. Like other living plants yeast is naturally perishable and in order to maintain its maximum leavening power, it is necessary that care be exercised so as to insure proper shipping and delivery conditions. The compressed yeast supplied to bakers in this country and
represents only yeast cells which have been grown to proper maturity so as to develop extreme hardiness. Canada
5. Consistency, feel, and fracture : Bakers’ compressed yeast should be firm and “springy” to the touch, fairly brittle and should break with a clean, sharp fracture without undue crumbling and should not have the consistency of butter. Yeast should exhibit a very slight moistness.
6. Taste and odor : It is difficult to describe the taste and odor of yeast in any other way than “yeasty.” This flavor is clean and pleasant, somewhat resembling that of apples.
7. Appearance : color of yeast may range from a light cream color, with a slight yellowish or grayish hue to a darker yellow or almost pale brown shade. The variations in color are usually caused by unavoidable changes in the water or slight variations in the color of the raw material used in the preparation of the food extract on which the yeast was grown.
Sometimes, though rarely, the outside of a cake of yeast may be whitish, while the inside possesses a more normal yellowish tinge. This thin white coating is usually due to a slight drying out on the surface which is of no material importance.
Within a considerable range, these differences in yeast color are not significant of the quality of the yeast.
© Fresha-licious (16December 2011)
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Sources / References :
1. Previous training notes provided by Puratos
(puratos.com ) Philippines
3. On Baking: A Textbook of Baking and Pastry Fundamentals by Sarah R. Labensky
2. Yeast: A Treatise on Baking by The Fleischman Division of Standard Brands, Inc – http://abrfaq.info/treatise/192
4. The Professional Pastry chef : Fundamentals of Baking and Pastry (4th Ed) by Bo Friberg.
5. My Bread by Jim Lahey
6. The Essential Baker by Carole Bloom
7. Baking Ingredient Science By Linda Larsen, About.com Guide (http://busycooks.about.com/od/howtobake/a/bakingingredien.htm )
8. Yeast dough – http://www.four-h.purdue.edu/foods/Yeast%20dough.htm
9. www.bakingandbakeryscience.net by Willie Prejean