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Leavening Systems:
Making Products Rise and Shine
March 1995 -- Applications
By: Lynn A. Kuntz
Associate Editor
Tortillas, matzo bread and other unleavened products are fine, but it’s hard
to imagine a world without flaky crackers, tender cakes or springy bread.
To produce the textures, flavors and appearance of many grain-based
products, food designers must employ leavening systems.
Leavening systems incorporate small gas cells into a product, affectingstructure and texture. As gas forms, the cells expand. While air and steamprovide some leavening action, most products require a system thatgenerates carbon dioxide. In baked goods, two types of leavening systems-- yeast and chemical -- provide the appropriate result. Most often each isused independently, but occasionally a combined system provides thedesired result.
These ingredients affect the pH of a food, altering flavors and colors. Theyoften produce characteristic flavors, some desirable, some undesirable.
Because of the wide-ranging effect of these ingredients, it is critical toselect an appropriate leavening system for a product.
Saccharomyces situation
The original leavening system consisted of bakers’ yeast, Saccharomycescerevisae, and a source of nutrients that allows the yeast to produce CO2.
Gluten in the flour forms a structure that traps the CO2 in the dough, whichincreases the volume and forms a characteristic cellular structure. Otheryeasts have been used for specialty products, such as sourdough bread,but S. cerevisae remains the type used in most yeast raised products.
"Hard wheat flour, as is used in bread, is leavened with yeast because ofthe protein content and gluten formation," says Mark Shubert, marketingmanager, Gist-brocades Food Ingredients, King of Prussia, PA. "Soft wheatflours, used in products like cakes, do not contain the same level of glutenand cannot contain the gases generated by yeast." Thousands of strains of this organism are commercially available. Theyexhibit variable properties, especially in terms of their ability to fermentsugars, speed of fermentation, sugar and osmotic tolerance, andsusceptibility to antimicrobials, especially calcium propionate. Yeast alsocomes in several different forms, both fresh and dried. The characteristicsand applications of the different types of yeast were reviewed in theJanuary 1994 issue of Food Product Design.
Determining the correct yeast level is difficult because of variations inyeast activity and in the process. For example, pizza crust can containanywhere from 0.5% to 6.0% yeast, depending on the processing methodand the end product required.
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"There are conversion rates between the fresh yeasts and the dry yeasts,but they are only guidelines," Shubert says. "Sometimes you can use moreor less. That’s a matter of the dry matter versus the viability. Fresh yeastthat has been around for a while can lose a lot of its activity." Frozen doughs require yeast that retains its activity through extendedfrozen storage conditions. While freezing yeast by itself will not adverselyaffect its performance, freezing it along with the components of a doughwill. Ice crystals can form in the product, disrupting cell walls. Also, as thedough freezes, the amount of available water decreases and theconcentration of the solutes increases. The increase in osmotic pressurecan destroy cells. Therefore, yeast with a high osmotic tolerance isrequired.
Using yeast in bakery products is somewhat challenging, but it creates twomain advantages: a distinctive flavor and dough conditioning. As yeastferments, it forms a number of compounds including organic acids, alcohols,aldehydes, esters and ketones. Some of these are volatilized during baking,some undergo further reaction, and most contribute to the flavor and odor ofthe product.
Some of the compounds generated during fermentation act as doughconditioners and increase dough extensibility by relaxing the gluten. This is afunction of time and pH; the longer the ferment, the more pronounced theeffect.
The chemical connection
While using yeast for leavening has advantages, it is not suitable for allproducts. In some cases, chemical leavening may be used to form carbondioxide gas. Chemical leavening is a faster, more convenient, and oftenmore consistent method than yeast.
Chemical leavening systems produce CO2 by one of two means: chemicaldecomposition through the application of heat, or a reaction of an acid witha base. Gas can form during all phases of the operation, including mixing,forming and baking. The point at which gas formation occurs is controlledlargely by the compounds used. Both the rate and point at which the gas isreleased affect the finished product. The leavening system influences theappearance, texture, color and even the flavor of the finished product.
"Chemical leavening gives you a high level of control of the reaction at thedesirable stage, whether it’s during the mixing stage, at the bench or in theoven," says Frank Chung, senior technical manager, leavening systems,Rhône Poulenc Food Ingredients, Cranbury, NJ. "If you look at the historicaldevelopment of chemical leavening, the major development area has beenhow to control the reaction and its speed." Ammonium bicarbonate decomposes into ammonia and carbon dioxidewhen exposed to temperatures above 104°F. Some gas is released atroom temperature, but most is generated during baking. Acidic conditionsaccelerate the reaction at lower temperatures. The release of gas is rapid,resulting in a fairly large cells. Ammonium bicarbonate not only increasesthe volume or height, but also tends to increase the spread in cookies.
"Ammonia results in the lowest pH in the finished product, "says PeterGoldstein, senior food chemist, Church & Dwight Co., Princeton, NJ. "Afterdecomposition, it forms ammonia gas, water and carbon dioxide, andthere’s not any residual salt. As it decomposes, it generates basicsubstances, so it does go through a temporary pH spike which dissipates." Ammonium bicarbonate dissipates in low-moisture products that have aporous structure, such as cookies and crackers. In higher moisture file:///F:/old_rumen_05_12_2008/e/doc/New%20Folder%20(2)/baki.
products, water retains the ammonia, giving the product an undesirableflavor and odor.
Acid basics
Acid/base leavening systems produce carbon dioxide in the presence ofheat and water. An alkaline product, usually sodium bicarbonate combinedwith an acid or acidic salt, reacts in the presence of moisture and heat.
Potassium bicarbonate can be used in place of sodium bicarbonate,especially for reduced-sodium products. However, because it has a greatermolecular weight than sodium bicarbonate, it requires approximately 20%more for the same leavening action. Potassium bicarbonate results in ahigher crumb pH than sodium and may contribute a sharp aftertaste toproducts.
Sodium bicarbonate is available in several grades differentiated by particlesize. Treated types contain anti-caking agents. Grade 1 is the standardpowder grade. Use of the coarser grades, 2 through 4, can reduce thetendency for pre-reaction and increase the system’s stability.
Sodium bicarbonate also can be encapsulated with fat-based coatings toincrease stability, particularly for refrigerated doughs. "These aremoderately successful, but the bicarbonate load is not very high," saysGoldstein. "The cost can be a factor, too, so they are typically only used invalue-added products." If the level of soda is too high in the finished product, it creates soapyoff-notes. If the level is too low, it will allow the acidic flavors to comethrough. Excess levels also result in over-browning.
A number of leavening acids can be incorporated into the leaveningsystem. Natural acids such as lemon juice or sour milk can be used, buttypically in a commercial operation, chemical leavening acids are added toprovide consistent, controlled gas production. Choosing the right acidulantsdepends on the effect on the finished product and on two other majorcharacteristics: the reaction rate and the neutralizing value.
The reaction rate is the rate at which the carbon dioxide is released inresponse to moisture or heat.
This can be measured in several ways. The Dough Rate of Reaction(DRR) measures the rate of gas released during mixing and holding. Thedough is held at a constant temperature, and the amount of gas ismeasured, converted to the percentage of sodium bicarbonate and plottedagainst time.
"There’s no such thing as a leavening acid that will stay completely inactivefrom the very beginning," Chung says. "Most react as soon as they contactwater, but some to a much greater degree than others." Increasing temperature usually increases the reaction rate. A test calledthe Batter Rate of Reaction measures the time necessary to complete acertain percentage of the reaction over a range of temperatures. Thisindicates how a particular leavening system reacts during baking.
"It’s important that the majority of leavening takes place before theproduct’s structure is set," notes Chung. "This helps determine the type ofleavening you use. With a fried doughnut, there is a direct correlationbetween how fast the chemical leavening reacts and the amount of fat itabsorbs. You have to start the chemical leavening immediately so there ispositive pressure from inside the dough." The neutralizing value (NV) is the amount of sodium bicarbonate needed tocompletely neutralize 100 pounds of that acid. In most applications, the goal file:///F:/old_rumen_05_12_2008/e/doc/New%20Folder%20(2)/baki.
is to retain little or no sodium bicarbonate or leavening acid in the finishedproduct. However, sometimes an excess is required to provide a specific pHrelated effect, such as color or flavor modification.
Other ingredients affect the pH of a product, so adding neutral proportionsof leavening acids and bases does not guarantee a neutral pH in the finishedproduct. Changing the pH can affect the speed and reactivity of theleavening system. The degree of flour bleaching can cause the dough tobecome puffy, for instance, according to Chung. Adding ingredients likefruit, buttermilk or even high-fructose corn syrup and cocoa powder cansignificantly change the pH of the finished product and reduce its volume.
"After formulation, it’s important to check that the pH of the product iscorrect," says Chung. "It may be necessary to adjust the pH to get the rightresult in your finished product." Says Goldstein: "Many ingredients containorganic acids which will react very quickly with the bicarbonates. We wouldlike to quantify the effect of the acidic ingredients in balancing formulas, butit’s a major project so we currently don’t have much information." Baking powders consist of one or more of these organic acids incombination with baking soda and a carrier such as starch. The starchphysically separates the acid and base and prevents them from reactingduring storage. Combinations of acids can be used to create double-actingbaking powders, or leavening systems that release a small amount of gasduring mixing and the majority when heated. The initial gas release providessmall gas cells that promote uniform expansion during baking. This gas cellnucleation also can occur by incorporating air during mixing. The better thedispersion of these nucleating cells, the finer the grain in the finishedproduct.
Assorted acids
Several commercial products serve as leavening acids. They vary inneutralizing value, reaction rate and effect on the finished product.
Monocalcium phosphate (MCP) reacts quickly with the soda when
dissolved, releasing approximately 60% to 70% of the CO2 in the first twominutes. It is often combined with a slower acting acid in products requiringa double-acting leavener, such as pancake batter. Coated anhydrousmonocalcium phosphate is used in applications where the initial gas releasemust be slowed. The initial reaction releases about 20% of the CO2, withapproximately 40% to 50% released after 10 to 15 minutes. This type ofproduct is used in cake mixes and self rising flour or cornmeal. MCP oftenacts as a nucleating acid.
Sodium aluminum sulfate (SAS) was used in the first double acting
baking powders in combination with MCP. It reacts too slowly to be usedextensively in commercial applications, although sometimes it is used inretail cake mixes.
Sodium acid pyrophosphate (SAPP) has a slow rate of reaction,
especially under cold conditions. It releases most of the CO2 during the heatof baking. Several types show various rates of reaction. These differ interms of processing, porosity and granulation, but they are chemically thesame. Typically SAPP releases from 22% to 40% of the gas in the first twominutes of mixing, little or none during holding, and the balance duringbaking. High levels can promote a slightly bitter aftertaste in the finishedproduct. SAPPs often are used in cake doughnuts and refrigerated andfrozen doughs.
"SAPP often gives a scratchy sensation at the back of the throat," notesChung. "High levels of sweetness tend to mask this taste." Sodium aluminum phosphate (SALP) reacts slowly and does not result
in off-flavors in the finished product. It is often used in place of SAPP in file:///F:/old_rumen_05_12_2008/e/doc/New%20Folder%20(2)/baki.
products like cake and muffin mixes. It initially releases about 22% of thecarbon dioxide, with the rest generated during baking. SALP tenderizesbaked products.
Dicalcium phosphate dihydrate (DPD) is used in special applications
that require very slow gas release. It shows no activity during mixing or onthe bench, and only reacts when the temperature exceeds 135°F. Thismeans that it can be used only in products that have extended bake times.
Glucono delta lactone (GDL) has a slow but continual reaction rate.
The main gas release occurs during baking as the ingredient is slowlyhydrolyzed. GDL is used in pizza doughs, as well as some refrigerated andfrozen doughs.
Other acids. Organic acids -- especially adipic and fumaric acids -- often
act as nucleating acids for baking powders, and they can lower the pH forspecial applications. Cream of tartar or tartaric acid have been usedtraditionally in baking applications, but their use in commercial applications islimited. These have very fast reaction rates.
Raising results
Besides reacting with baking soda, leavening acids affect the structure,color, flavor and pH of the finished product. Each acid produces a slightlydifferent texture. Ions making up the acid salts influence the crumb structureand texture. Calcium and aluminum ions contribute to a fine grain and alsostrengthen the structure of the batter and produce a spongy texture.
The pH resulting from an imbalance in the leavening system can be used toadvantage in some products. Chocolate products often require a higher pHto enhance color and flavor, while the same pH in a lightly colored andflavored product would give soapy flavors and unacceptable color. A low pHfurnishes a tart flavor, suitable for lemon but unappealing for vanilla.
"In most cases you want to balance the leavening system to achieve aneutral pH," says Chung. "But sometimes you want to adjust the pHintentionally. In a chocolate product, you might want to bring the pH up ashigh as 8.5. For a light-colored product, you must bring the product toneutral or slightly below. Some bakers find that if the pH is slightly alkaline,it improves volume and texture." Some common ingredients affect leavening action. Calcium ions addedwith dairy ingredients or via hard water can alter the reaction rate of SAPP,for example.
The end use influences the choice of leavening acids. For example, Chungrecommends SAPP for an institutional dry mix because of its stability. Aretail mix has faster turnover, and typically uses an MCP and SALP system.
Sometimes retail mixes contain SAS, but that has been associated withaccelerated spoilage of fat.
Chemical leaveners and yeast usually are not combined, but there areexceptions. Saltine crackers and pretzels undergo yeast fermentation, butthe primary reason is to generate flavor and dough conditioning. Thesubsequent sheeting action removes much of the gas that is generated, andchemical leaveners are required to provide lift during the bake. In somefrozen or refrigerated doughs, yeast and chemical leavenings complementeach other.
"Chemical leaveners don’t give you the flavor characteristics needed,"Shubert points out. "But yeast doesn’t hold up well in frozen products. Youneed one-and-a-half times the amount of yeast that you would in anunfrozen dough." Chemical leaveners for prepared doughs require high stability to maintaintheir activity, according to Chung. "You often must add from 10% to 15%more than in a regular application to maintain the activity, but adding file:///F:/old_rumen_05_12_2008/e/doc/New%20Folder%20(2)/baki.
chemical leavening can provide some improvements in texture and structuralstrength. One of the areas we’re examining is when to add chemicalleavenings to enhance the performance of yeast raised products." Whether chemical or biological, the leavening system must be tailored to aspecific product and process so the product doesn’t fall flat.
"You have to consider the food system you are working with," saysGoldstein. "There are major differences among the finished products.
Moistures are dramatically different, the composition is different, and thestorage and release requirements are different. All these influence how wella particular system works." Neutralizing Value
(Grams of Sodium Bicarbonate needed to neutralize 100 grams of leavening acid) Leavening Acid
Neutralizing
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