Your Guide to Preferments
by Didier Rossada, baking consultant
Modern Baking's Bread and Rolls Handbook
Prefermented dough and poolish, sponge and biga preferments can improve bread quality naturally and traditionally.  Learn how they differ and how to incorporate them into your production.
For bread bakers, an understanding of the fermentation process in yeast-risen baked products is vital.  Depending on the conditions under which the dough is mixed and handled, proper fermentation can contribute to many of the desirable characteristics expected in good bread, including aroma and shelf life.  Preferments, when properly used and understood, can naturally improve bread quality.
A preferment is a dough or batter prepared prior to mixing the final dough and composed of a portion of the total formula's water, yeast (natural or commercial) and sometimes salt.  The dough (or batter) is allowed to ferment for a controlled period of time and then added to the final dough.
Depending on the type of product you are baking, your production schedule and available equipment, you have a number of options to consider when deciding what type of preferment to use.  Considerable confusion an misinformation exists abut the differences among preferments, including prefermented dough, poolish, sponge and biga.
Prefermented Dough
Prefermented dough (or old dough) is a fairly new method.  Originally, this preferment had been developed to compensate for the mediocre quality of bread produced by the straight dough process with a short first fermentation.  Prefermented dough allows baker to produce a better  quality product even when, due to production scheduling or mechanization, the first fermentation has to be shortened.
The process is fairly simple.  A piece of regular dough (made with whiter flour, water, yeast, and salt) is allowed to ferment for a period of time before incorporating it back in the final mix.  In order for the baker to get the most benefit from this process, the prefermentation should last at least three hours at room temperature.  Prefermented dough can ferment up to six hours at room temperature.  If you do not want to use the preferment right away, let the dough ferment one or two hours at room temperature and then refrigerate the preferment until it is incorporated in the final dough.
Store prefermented dough  at a low temperature (35-40F) as long as 48 hours. If you use this procedure, remove the prefermented dough from storage one of two hours before incorporation into the final dough.  If this is impractical, adjust the water temperature in the final dough to compensate for the cold preferment.
Prefermented dough  also could be a piece of dough saved from a previous mix. For example, a piece of whole-wheat dough can be used as preferment for the next day's whole-wheat production.  Baguette dough, however, is traditionally used for most preferments.  Baguette dough, which is composed of only four ingredients, offers more versatility and can be used in any kind of final mix.
The most convenient way to achieve the necessary quantity of prefermented dough needed for the next production is to remove the dough to be used as a preferment just after the first fermentation, and store it in the refrigerator.  Formulas can call for as little as 10% and as much as 180% prefermented dough (based on the flour of the final mix), but 40% to 50% is the most commonly used proportion.
Another alternative is to mix the dough to be used for the preferment as separate dough the day before, or at lest three hours prior to incorporation in the final dough.  In this case, usually about 20% to 30% of the flour from the total formula is used in the preferment. he absorption should b adjusted to obtain a medium consistency (generally 54% to 66%).  Salt is 2%, and yeast (fresh) is 1% to 1.5%. These percentages are calculated based on the four in the preferment.
Product Uses: Prefermented dough is a very versatile preferment and can be used in many different  products, from viennoiseries( croissant, brioche, Danish, etc.)
Tip: The biggest drawback to using prefermented dough is overnight storage, so be prepared to use a large amount of refrigerated space.
Poolish was one of the first preferments elaborated with commercial yeast.  Polish bakers, where the name originated, are credited with inventing this preferment in Poland at the end of the 19th century.  The process then was adapted in Austria and later in France.  Bread made with a poolish was lighter and less acidic than the sourdough bread commonly baked during that time, and it started to gain popularity.  With the availability of commercial yeast, more bakers began using the poolish process, and the sourdough process declined.  Technically, we could classify the poolish as a transitional preferment between baking using sourdough and baking with commercial yeast using a straight process. Even in Paris today, some windows of older bakeries display two sighs.  One reads "pain Viennois" - bread from Vienna (made with commercial yeast), and the other reads "pain Francais" - bread from France (made with sourdough.)
Traditionally, the size of the poolish is calculated based on the water involved in the total formula.  Use from 20% to 80% of the water to prepare the poolish. Then, elaborate the poolish using the same amount of flour as water (this 100% hydration, provides a liquid consistency); no salt is usually incorporated in the poolish.  It is important to note that the poolish is allowed to ferment at room temperature.  Therefore, the quantity of yeast  is calculated depending on the fermentation time of the poolish. Although it is difficult to give precise numbers, Chart A provides some guidelines to calculate the quantity of yeast to use in the poolish.
Chart A
How to determine yeast quantity for poolish
Fermentation time 3 hours 7 to 8 hours 12 to 15 hours
Quantity of yeast* 1.5% .7% .1%
(*Percentages of fresh yeast based on the flour involved in the poolish)
These guidelines are applicable for a bakery temperature of 80F to 85F and a water temperature of 60F.  If the temperature of the bakery is warmer, the yeast quantity or the water temperature should be decreased. The goal is to obtain a poolish that is perfectly matured at the time of the final dough mixing. The full maturation of the poolish can be recognized when it has domed slightly on the top and just begins to recede, creating on the surface some areas a little more concave. (See photos)
A poolish  that has not matured adequately does not provide the benefit of lower acidity; one that has over-matured can create other types of acidity which might affect the flavor of the final product.
It is better for the baker to opt for an overnight poolish if production and storage are adequate for two main reasons.  A longer poolish produces more favorable aromas, and it requires less yeast, increasing the amount of available time to use the poolish ( as many as 2 1/2 hours) without the poolish over-maturing.
Product Uses:  Poolish can be used in many different bread or sweet products but generally, poolish is the preferment of choice for baguette dough.
Tips: If you need large amounts of poolish for a variety of dough, divide the poolish into containers for each dough right after mixing the poolish, instead of measuring the poolish after its maturation phase.
Originally, sponge was used as preferment in pan bread production in England.  Unfortunately today the sponge process has been replaced by the straight dough method with dough conditioners replacing the sponge.  Sponges were, and still are, also used in the  production of sweet dough.
The sponge process is similar to the poolish process; thy differ primarily in dough hydration.  While the poolish has a liquid consistency, the absorption of the sponges around 60% to 63% (stiff dough).  The sponge usually does not contain salt, and the quantity of yeast is calculated depending on the length of the fermentation.  The same yeast guidelines for a poolish (Chart A) could be applicable for a sponge process.
A sponge also should be used after it has reached full maturation.  As with the poolish, the surface of the sponge contains vital clues to help bakers determine its readiness.  When many bubbles are evident and  some cracks start to form, crating some collapsing, the sponge is ready  for incorporation into the final dough.  An under-mature sponge would not be as beneficial because of inadequate acid development;' an over-mature sponge could negatively affect the strength of the dough due to an increase in the acidity level.  It also would affect the flavor of the bread. due to the formation of other acids.
A sponge using minimal yeast and overnight fermentation offers bakers a longer period of time between under-maturation and over-maturation. Because the longer fermentation time generates more acidity, the final product also will get better flavor and longer shelf life..
Product Uses: A sponge can be used in many products.  Sweet dough, in particular, will get the most benefit from the sponge method..  Because of its stiffer consistency, the sponge will improve the strength of the dough.  This increase in strength is usually enough to compensate for the potential weakening of the gluten generated by the sugar and fat frequently found in sweet bread formulas.
Tip: The stiffer consistency of the sponge process makes it easier to handle than a poolish.  Flavor-wise, sponge and poolish generate similar aromas.
Many Italian bread formulas start with a "biga" as preferment.  A biga, even if the basic ingredients are the same (flour, water, and yeast), could have different characteristics.  Some are liquid or stiff, and some are sour.  Some are fermented at room temperature, while others are fermented in a cold environment.
For Italian bakers, biga is more a generic term for preferment than a specific process.  In the United States, occasionally the word biga is used instead of prefermented dough, poolish, or sponge to add a touch of "Italian authenticity" to the bread.
Biga originally was a very stiff preferment used by Italian bakers to reinforce the strength of the dough.  A traditional biga is prepared using flour, water and yeast.  The hydration is around 50% to 55% (very stiff).  Unlike the poolish and the sponge process, the biga's yeast quantity, fermentation temperature and fermentation time are constant.  Usually, 0.8% to 1% of fresh commercial yeast is used.  The biga is then held at around 60% for about 18 hours.
Because of the very stiff consistency and the cooler fermentation, biga provides a lot of strength to the dough, which was its original purpose.  Today with stronger flour, bakers must be careful to use the biga properly, or the added strength could penalize extensibility in the dough.  The advantages of a properly fermented biga are similar to other methods: better flavor and extended shelf life.
Product Uses: True biga can be used for products requiring stronger dough characteristics, such as brioche or stollen.  It also is a good choice in dough with high hydration.
Tip: If the biga is causing an excess of strength to the final dough, higher hydration or autolyse will help regain a better balance in elasticity and extensibility.
Subtle Flavor and Aroma Differences
Prefermented dough, poolish, sponge and biga are the primary types of commercial yeast raised preferments available to bakers.  It is possible for bakers to develop a unique preferment (between an sponge and a poolish, for example), but the concept stays the same.
The use of preferments is a simple and inexpensive way to improve bread quality.  Each preferment generates different aromas depending on its characteristics.  Aromas and final bread flavors are influenced by the preferments' liquid or stiff consistency, fermentation temperature, salt including or exclusion and the use of commercial yeast or wild yeast.
Although it is difficult to describe all the flavors of each preferment, the poolish is generally described as having a nutty flavor, the sponge is sweeter with more acidity, and the prefermented dough is a little bit more acetic without being sour.
The main factors to take into consideration when opting for a specific type of preferment are production and space requirements, flour characteristics and flavor.  Knowing all those parameters, you should be able to decide what kind of preferment is best for your production.  Once the choice is made, it is better to limit the type of preferment to two or three kinds. 
Using preferments is one more example of how the baking process can be simple and complex at the same time.  But, when you understand how to work with preferments, they can provide a natural and traditional technique to improve bread quality.