The Value Of Raising and Leavening in Baked Foods

The purpose of raising and leavening agents in baking is to create a light, airy texture in the finished product. Leavening agents are ingredients that cause the dough or batter to rise by releasing gases, such as carbon dioxide, which get trapped in the dough and create small air pockets. When the baked good is heated, the air pockets expand and the dough or batter rises, resulting in a fluffy, tender texture. Without them, most baked goods would produce flat, rather chewy foods. Many bakers fail to think about using leavening agents when they are producing bread and cookies and then try to work out why their cookies are so flat.

Typical raising agents can be grouped into three main types: these include the chemical agents such as baking powder and baking soda, the biological which is always yeast, and the physical sources such as steam and air. These are especially useful in producing light, airy textures in baked foods. 

Leavening Agents

Leavening is particularly important in bread-making, as it helps the dough to rise and gives the bread its characteristic texture and flavour. In both chemical and yeast based leavening, the production of carbon dioxide gas is the main agent in producing a rise or air pockets in the dough as it proves.

Yeast is a common leavening agent in bread-making, as it produces carbon dioxide gas during fermentation, which causes the bread dough to rise. Other leavening agents used in baking include baking powder and baking soda, which release carbon dioxide gas when they come into contact with acidic ingredients, such as buttermilk, tartaric acid, lactic acid or vinegar.

Sodium Bicarbonate/Soda

One of the most important leavening agents is soda which is sodium bicarbonate. It is extremely benign as a food ingredient and to be treated as an ally in baking. Soda is bought at the grocers in the baking products aisle as a white crystalline powder. It has a molecular weight of 84.

Sodium bicarbonate is often used when the formulation already contains a certain amount of acid in its make-up. Brown sugar for example is acidic because it contains molasses which has an acidic pH.

Heat during the baking process will also encourage carbon dioxide formation which produces small gas bubbles causing the baking goods to rise.  When heated up to 50ºC, it starts to produce gas which produces the aerated texture in bread for example. By the time of reaching 100ºC, much of it has been converted to sodium carbonate. It is popular because it is so cheap, easily handled and harmless. It doesn’t have a flavour either and can be purchased extremely pure.

Baking powders used in Germany for example contain between 2.35 and 3.00 g of bound, effective carbon dioxide in the quantity determined for 500 g of flour. This occurs when carbon dioxide is the leavening gas.

The physics and chemistry of leavening has been explored by Bellido (2007) in his Ph.D. Thesis. The thermodynamic description of chemical leavening as opposed to that by yeast is described by Van Der Sman (2021). 

Baking Powder

Baking soda only contains a chemical leavening agent which is most often sodium bicarbonate. A baking powder on the other hand contains a leavening agent, a powdered acid with a neutral, inert buffer material.

The use of baking powder incorporates acids such as monocalcium phosphate, sodium aluminum phosphate and the sulphate version, or tartaric acid as well as sodium bicarbonate in some instances. Tartaric acid is very commonly used. The neutral buffer is another flour, usually cornstarch which serves as a bulking agent.

Potassium Bicarbonate

A leavening agent also used for low-sodium products. It is used as a total or partial replacement.

Ammonium Bicarbonate

Ammonium bicarbonate breaks down on heating to release carbon dioxide and ammonia. Both gases are used as the main agent. The reaction is complete and rapid at 60ºC. There is no reaction with any leavening acid. Sodium bicarbonate leaves a residue  but ammonium bicarbonate does not when used in baking. There are no residual flavours from this salt. It does not affect the pH of bread and so no leavening acid is needed. It is not suitable for high moisture foods where the moisture content is over 5% w/w. Ammonia is soluble in water leaving an ammoniacal flavour which makes it inedible.

It is best used in low moisture foods such as dry bread, cookies, dry biscuits, wafers and crackers. Typical formulations include TUC and Ritz biscuits. No salts are produced either so it does not change the rheology of the dough.

A vast amount of research has been directed towards chemical leavening in the intellectual property arena (Gelinas, 2021). The use of carbonate-free gas releasing agents is well represented. Ammonium chloride. 

Novel Leavening Agents

A solid polymorph of (+)-catechin with mucic acid or tartaric acid has been tried with the added benefit of incorporating polyphenols into the formulation. There was a general view these leavening agents could be incorporated into muffins (Carullo et al., 2020).

Pulque, which is a fermented white sap from Mexican agave has been tested with a wheat bread product. It was claimed that the increase in volume compared to the non-leavened control was about 50% v/v (Vernon-Carter et al., 2017). The fermented sap acts similarly to a sourdough starter culture. It’s not recognised yet as a potentially commercial alternative to other sourdough-like cultures.

Chickpea derived yeast is used in leavening. It is a traditional agent derived from the natural fermentation of ground chickpea in water. A ‘sweetdough’ is produced which alters the structure of the dough. It has a lower acidity than other types of sourdough. Chickpea-based leavening extract (CLE) has also been tried with the preparation of white bread and is considered an alternative to commercial baker’s yeast. It is however not widely available (Gul et al., 2018).

The Neutralizing Value (NV)

The neutralizing value (NV) is the amount of leavening acid needed to react completely with a defined amount of baking soda. When all the leavening acid reacts with the soda, the pH will be very close to neutral which is ideal for baked foods of any sort.

Different leavening acids are available with different NVs. Those with a low NV must be used in greater amounts than those with a higher NV.

  The levels are:-

• sodium aluminium sulphate….104NV
• sodium aluminium phosphate…100NV
• anhydrous monocalcium phosphate…83NV
• monocalcium phosphate monohydrate…80NV
• sodium acid pyrophosphate….72-74NV
• cream of tartar…45NV
• glucono-delta-lactone…45-50NV
• dimagnesium phosphate….40NV
• dicalcium phosphate dehydrate…33NV

The quantity of CO₂ released during mixing depends on the rate of reaction (ROR). The leavening acids can be classified as fast-acting acids that dissolve rapidly during batter mixing, or as slow-acting acids (Ellinger, 1972). So choice of acid or its corresponding neutralization value is responsible for the timing of the release of carbon dioxide.

Leavening agents produce both a pre- and a post-leavening effect. Combinations of different acid carriers are frequently used so that the time of release can be precisely matched to the product.

When leavening acids such as monocalcium phosphate react with sodium bicarbonate they release between 60 and 70% of the total carbon dioxide. These are classified as nucleating or fast-acting leavening acids. Residual carbon dioxide gas is released thermally because it is an exothermic reaction which produces calcium phosphate Ca₃(HPO₄)₂ . The calcium phosphate also starts to breakdown and reacts with remaining sodium bicarbonate as the baking temperature rises (Brodie & Godber, 2007).

SAPP (sodium acid pyrophosphate) releases carbon dioxide only with  sodium bicarbonate as the temperature has risen above a certain point (De Leyn, 2014). Glucono-delta-lactone on the other hand is an ester of gluconic acid which spontaneously hydrolyses in water. It reacts with sodium bicarbonate producing a sustained and gradual release of CO2 over the time period of baking. Hydrolysis in water is temperature dependent so the higher the baking temperature the faster the rate of reaction (De Leyn, 2014).

Bartek Ingredients (Stoney Creek, Ontario, Canada) launched Upscale™ and Uphold™ which are patent pending technologies based on fumaric acid. These are intended for dough. These agents modify dough properties enabling increased dough absorption and reduced mixing times. The upshot is reduced use of ingredients. Upscale™ works with preferment or straight-dough processes by improving porosity and flavour. Uphold is used for tortillas. Both agents help with the use of preservatives such as calcium propionate. The sourness from fumaric acid disappears above pH 4.5.

Gas Hydrates

Gas hydrates (GH) are formed when water and low molecular weight gases are subjected to low temperature and high pressure conditions. Suitable sized guest molecules are caged in hydrogen-bonded water molecules without chemical reactions to stabilize the structure (Srivastava et al., 2022; Srivastava, 2023; Fruhling et al., 2023). The cages are composed of pentagons and hexagons, depending on the gas molecule, forming one of three preferred structural forms (sI, sII, sH). In sI, the guest molecules are enclosed in cages of 12 pentagons (5¹²) or 12 pentagons and two hexagons (5¹² 6²). The most common hydrate structure is this latter form. Very small molecules (e.g. hydrogen) cannot, therefore, stabilize gas hydrates, while large molecules rather form larger cages (sII or sH).

In this case carbon dioxide for leavening as a gas is introduced into the dough or batter as a hydrate, a solid carrier material. Carbon dioxide has the E number 290 (E290). Unfortunately, it does not generate sufficient leavening or loosening as an aqueous solution or as dry ice. The stability of these solutions is lower than that of the CO2 hydrate.

Gas hydrates are added as particulates into the dough. The leavening agent is released through physically controlled decomposition. Gas hydrates can be used in combination with gelling agents and flour improvers. Ascorbic acid is added to improve bread structure, boost dough strength and the loaf volume. It is converted to dehydroascorbic acid which is a good oxidising agent which promotes disulphide bridges between cysteine amino acids in gluten. the dough does not rupture when leavening agents begin to start working as gas bubbles are generated (Skendi et al., 2021). 

The main advantages over other leavening agents are many fold:-

• Only water and carbon dioxide are released as the hydrate breaks down.
• There are no flavour or texture changes in the final leavened product
• The process of dough leavening is decoupled from any aromatic dough ripening as in sourdough production.
• Gas release is in a few minutes compared to biological leavening.
• The use of freezing to control the breakdown of the the gas hydrate is possible. It is a process that could be viewed as a ‘cold process’ and therefore more green because energy requirements are not so high

It is also possible to see supercritical carbon dioxide being added in a similar manner. It is a plasticizer that can function similarly to a gas hydrate and is being regularly used in extrusion technologies to reduce the viscosity of incoming polymer feeds. It would be versatile enough to be a leavening agent in 3D printed foods (Sauceau et al., 2011).

Coatings

Particular leavening agents, commercial baking powders and leavening acids will use coated particles to minimise premature reactions between carbonates and acids. Coatings and release agents come in many forms. The release agents used include starch from various cereals, various phosphates, magnesium carbonate, cellulose fumed silica, silicon dioxide, gums, fats, waxes or mixtures of the substances mentioned above. Coatings consist of lipid layers or sparingly soluble calcium phosphate (Kuhnert et al., 2011).

Baking soda particles are most commonly encapsulated in hard fat which maintains and even improves their keeping and storage properties. It prevents moisture absorption and reduces premature reaction with neutralizing agents especially citric acid (Brose et al., 2001). The coatings must not melt too late into the baking process.

Baker’s Yeast And Active Dry Yeast

Baker’s yeast is the key leavening and bulking agent needed for baking (Ando et al., 2006). It is the oldest leavening method in use and has been with us for millenia. Just remember though, unleavened breads do not use yeast and so are flatter and denser because no carbon dioxide is produced.

The addition of dry yeast (Saccharomyces cerevisiae) to bread mixes is one way of introducing an agent that produces carbon dioxide. Yeast produce ‘zymase’ which breaks down sugars to produce readily usable energy as well as ethanol and carbon dioxide. The alcohol produced disappears in water vapour during the baking process. The carbon dioxide gas produced causes the dough to rise.  The presence of yeast also gives baked goods especially bread part of its characteristic flavour and aroma. It is the classic and well established biological approach to a rise in dough. Some bakers have tried both soda and dried yeast to obtain a good rise (Ali et al., 2012).

Yeast is very highly stressed during baking. Live yeast must survive a number of stresses such as air-drying. high osmotic pressures, the freeze-thaw cycle and sustained high temperatures. As a result of these stressors, the yeast suffers alteration to its normal physiological processes. The main stressor which is high temperature reduces the yeast’s viability. Optimal leavening capability is also impaired when baker’s yeast encounters high sucrose concentrations in the dough. The high external osmotic temperature causes internal cellular structures to be damaged and compromised. This is claimed to be through the generation of reactive oxygen species (ROS). 

Freezing affects yeast performance. Being able to use frozen dough is of high significance because it is more versatile for a bakery. It also means that dough production is separated from dough baking rather than having to do an immediate sequential process. It also means that baking of any product could be conducted on a larger scale because the distribution of frozen dough is now independent of the baking process (Lin et al., 2015).

Freezing also upsets yeasts by destroying cellular structures which reduce cell viability and survival. Frozen dough containing yeast which is then allowed to thaw and resume leavening is severely compromised because the metabolic activity is so severely disrupted. Yeasts for baking have been selected naturally to do their best at survival with different stress conditions and with it produce good quality bakes. However, the question now is whether they are good enough especially with freezing as a key part of the process.

Recombinant produced yeast have been explored and may well become acceptable as pressure mounts on supplying a burgeoning population. It is probably more acceptable to explore gene-edited yeast because less intervention occurs in the genome even though it has been deliberately modified. Nonetheless, public concern about ‘GM food’ and the need for rigorous safety testing to satisfy regulatory approval is required. No product produced using recombinant yeast can even be tasted until approval is granted.  

One research group has explored small nucleolar RNAs (snoRNA) by transforming Baker’s yeast. They constructed a yeast that overexpressed the SNR84 gene. This type of yeast construct containing multi-copy SNR84 had a high-temperature, high-sucrose and freeze-thaw tolerance with improved leavening (Lin et al., 2015).

To illustrate the point that modified yeast may become more acceptable, an undergroup at John Hopkins University produced VitaYeast (Nutraingredients, 2022). This is a yeast where a synthetic DNA plasmid ring is incorporated into the yeast cell and programmed to produce beta-carotene. It is not genetic modification ‘per se’ but it is incorporated into bread for baking and intended to alleviate vitamin A shortages. 

Modified forms of yeast have been produced which have indirectly been used to produce both protease and lipase as well as produce carbon dioxide. In some cases these modified yeasts have advanced metabolic machinery which produces more quantities of enzyme for metabolizing sugar as well as enzymes with greater amylase activity and with enhanced sugar transport systems. Whilst the leavening capability of the bread remained unchanged there was a recombinant yeast that produced a lipase that helped stabilise the starch granules and thus improved bread volume (Paciello et al., 2015).

Yeast rich in proline have a freeze-thaw stress tolerance, show better fermentation in high-sucrose sweet doughs, have better cell viability and a lower intracellular oxidation level. On that basis, a research group (Sasano et al., 2012) set about creating a self-cloning yeast strain that could accumulate more proline than usual. Proline, like glycerol and trehalose are osmoprotecting agents and are accumulated by yeasts (Luo et al., 2018).

 Baker’s yeast has also been modified by adjusting the production of the enzyme sucrase which alters leavening ability. Its activity reduces leavening action when yeast are producing baked goods with sweet dough. Suppressing the production of the enzyme enhanced leavening capability (Zhang et al., 2016).

One approach to speeding up yeast based leavening is to improve the metabolism of maltose. Maltose use relies on a maltase and a permease which are induced in the presence of maltose. A major factor limiting the dough fermentation rate is the repression of the synthesis of maltose-utilizing enzymes and the inactivation of the maltase enzyme by glucose (Needleman, 1991). The presence of any other sugars other than maltose repress its metabolism which is viewed as a type of catabolite repression. There is then a lag phase in carbon dioxide production. To overcome this lag phase, the native promoters for maltase and maltose permease have been replaced with constitutive promoters (Osinga et al., 1988).

Encapsulated yeast has been investigated for its leavening ability.  Hydrogels have been explored as a way of protecting yeast in frozen dough. More specifically, free yeast cells have been replaced with calcium alginate-yeast cells (CAY) or calcium alginate–starch yeast cells (CASY) hydrogels (Cozmuta et al., 2021). It appears to allow for leavening of the dough once it has reached an appropriate temperature following freezing. The implication is significant because it is an example of how to extend the processing options possible.

Yeast produces bread with greater hardness compared to bread produced using baking powder. It may be due to yeast producing a more consolidated starch structure. The resistant starch content of bread is also claimed to be higher and it increases with leavening time which is not possible when chemical leavening is used (Garcia-Hernandez et al., 2022).

Saccharomyces cerevisiae is not the only yeast of interest. Various types of Pichia and Candida have also been assessed. C. tropicalis can produce a similar bread to S. cerevisae without significant sensory differences (Condessa et al., 2022). It would appear that yeast types and cultures depend on the geographical region and artisanal approach to baking (Suo et al.,  2021).

Bellarise (Pasadena, CA.) produce yeasts that are deemed clean-label and non-GMO being produced by beet molasses rather than high corn fructose syrup. One of the drivers is the increasing use of frozen dough. This business has produced a semi-dry yeast for this purpose which has a long shelf-life. They claim to have consistent performance in terms of leavening power with a uniformly predictable rise in all baked products. There are Red and Gold versions and they work with well with lean or sweet doughs. The product has a two year shelf-life.

Lesaffre also produce leavening agents for tortillas. They launched Coolsmart Command which is a yeast also for fresh but refrigerated doughs. They also have Red Star® which is a double-acting chemical leavener that is aluminium free and does not use Cream of Tartar for batters. The amount used generally is 5% of flour weight.

 Particular baked foods lend themselves well to yeast fermentation. It is a much slower process compared to the use of chemical leaveners and it produces a much more bread like structure. That makes it less suitable for particular foods such as biscuits.  

Bacteria and Sourdough

Bacteria can also be successfully used to leaven bread and this is part of the process of making sourdough. The main bacteria used is Lactobacillus sanfrancisco which is often used in conjunction with Saccharomyces exiguus which is a nonbakers’ yeast. The bacteria metabolises maltose in the bread producing acetic acid, lactic acid and carbon dioxide gas.

In all cases, whether yeast or bacteria is used, a fermentation is only as effective as the substrate being metabolised. It needs to be significantly long enough for reactions to occur. The conditions of fermentation must also be controlled at ambient such as temperature and humidity.   

The Use Of Steam

One of the simplest yet seemingly unexciting processes for leavening bread is to use steam. For any one interested in this, it is just water vapour which is produced when dough is baked and reaches 212ºF (100ºC) and the liquid water trapped in the dough becomes steam.

The volume of steam is 1,500 times that of liquid water so with that type of expansion when a phase change like that occurs we are going to see some serious dough leavening. The effect is magnified too by baking temperatures. I only know of two baked products that rely on steam: choux and puff. As you can tell by steam leavening, the baked dough is mostly flaky and extremely light.

The use of steam comes into its own when generating flaky pastry as in puff pastry. It relies on incorporating butter into the dough and then rolling it into many folds like the leaves of a book. There are then many tens to hundreds of layers. These form separated flaky layers because of the steam produced in the dough from water in both it and the butter.

The production of choux is different. These are eclairs and beignets. In the baking process, the gluten protein is partly denatured. The elasticity of the dough falls. The starch however is gelatinized which helps structure the choux. It also helps that when steam is formed the pastry just inflates without relaxing to its original physical state and so the shape is held. Any air pockets in the pastry remain intact.

The Dough Aging Approach

A few baked goods rely on dough resting where natural enzymes including proteases, lipases, etc.  breakdown proteins and starches. It helps generate different textures and produces a slightly lighter rise. Applicable to cookie recipes.

Mechanical Leavening

Mechanical leavening is a process used in baking that physically incorporates air into doughs and batters, thereby creating a light and airy texture in the finished product. This method relies on physical actions, such as mixing, beating, or whisking, rather than the chemical leavening agents (like baking powder or baking soda) or biological leavening agents (like yeast) that we will have encountered in both industrial and home-based bakeries. .

Mechanical leavening is achieved by the process of creaming. In this technique, sugar crystals are beaten with solid fat such as butter with a spoon or in a mixer. Air is incorporated into the mixture as tiny bubbles. The sugar crystals effectively cut up the butter during what can best be described as vigorous mixing. A chemical leavening agent such as baking soda is added to help with further raising.  

Leavening can also be achieved through  other mechanical means, such as beating eggs as well as creaming butter and sugar together, which incorporate air into the dough or batter. In addition, steam can also be used to leaven baked goods, as the steam causes the dough or batter to expand and rise.

To elaborate further on the key techniques are these different physical actions:

• Whisking: Rapidly whisking ingredients, such as eggs or egg whites, incorporates air into them, creating a foam. This foam structure helps to leaven baked goods like soufflés, meringues, and sponge cakes.
• Beating: Beating ingredients together, especially butter and sugar, traps air in the mixture. This creaming method is commonly used in making cakes and cookies, where the air trapped during beating contributes to the rise and light texture of the final product.
• Folding: Gently folding whipped egg whites or whipped cream into a batter can add air without deflating the mixture. This technique is often used in delicate recipes like mousse and angel food cake.
• Kneading: In bread making, kneading dough helps to develop the gluten network, which can trap air bubbles created during mixing. While not primarily a mechanical leavening method, it contributes to the structure and texture of the bread.

What examples do we have of mechanically leavened products?

• Sponge Cake: Made by whipping eggs (whole or separated) and folding in flour and other ingredients, relying on the trapped air for leavening.
• Angel Food Cake: Uses whipped egg whites as the sole leavening agent, folded into the batter to create a light and airy cake.
• Chiffon Cake: Combines the richness of a butter cake with the lightness of a sponge cake, incorporating whipped egg whites for leavening.
• Soufflés: Made by folding whipped egg whites into a flavoured base, which puff up during baking due to the trapped air expanding.

Overpressure Mixing (OM)

The use of overpressure mixing (OM) to improve the expansion of baked foods has been tried. It produces an increase in the gas volume fraction in the dough which invariably produces a higher final volume. It is not clear if the bubble number and bubble size increases or just one aspect of this following the application of OM.

The overpressure is generated using gases such as CO₂ and N₂ at different pressures. Air can also be used -bread dough mixed at 0.5 bar above atmospheric pressure produces higher dough volume with a higher expansion ratio of 3.5 following over an hour’s fermentation (Jha et al., 2022).

Massey et al., (2001) states that the bubble size increases because of two phenomena. The first is bubble expansion when the overpressure is removed and there is a return to atmospheric pressure. The second part is desorption where solubilised gas as a result of added extra gas pressure then comes out of solution. Sadot et al., (2017) considers the increase in overpressure to cause an increase in the apparent density of the incorporated air which and that additional air is incorporated adding to the total volume of air incorporated. Palier et al., (2022) examined overpressure mixing (OM) on cake properties. They also noticed that carbon dioxide gas production during mixing produced a better cake volume. Increasing the batter void fraction did not generate any improvement in actual volume however.

The Chorleywood bread process (CBP), based on the patent GB 1044616 A (Collins, 1992) exploits the application of  a pressure – vacuum sequence to encourage rapid air uptake from the atmosphere followed by the equally rapid application of partial vacuum for bubble structure control.

Biscuits have been produced using this approach (Brijwani et al., 2008). Sheeting produces degassing but the extent to which degassing occurs depends on how the sheeting (kneading) is performed. Enough gas is retained by sheeting to produce softer biscuits which spread and lifted more during baking.

Direct Gas Injection Methods 

Direct injection of carbon dioxide is feasible and well patented. It is part of the aerated bread process. It is an exceptionally old method dating to the mid 1850s. It required injection with  carbon dioxide or, exceptionally, oxygen, air, water steam, or nitrogen as NO₂ or N₂. More advanced methods use aerosol methods which employ halogen gases as found in aerosols.

A recent approach used the mixing of a cake batter under gas pressure with no baking powder present. The idea was to force the aeration of the batter and thus solubilise any gas into solution. In this system, a mixture of air, carbon dioxide and nitrogen was applied (Palier et al., 2022). A CO₂ pressure of 0.3 and 0.5 bar produced the best results in terms of specific volume of cake with 2.5 mL/g. This was 89% of the specific volume of control cake with baking powder.

Nitrogen gas has been tried and tested. In brewing, smaller but more profuse gas bubbles are generated which form a denser but smoother foam. The same idea is feasible with bubble production in dough. Unfortunately, results to date are not promising because nitrogen gas is poorly soluble in water. It’s also highly unlikely that this gas can be effectively sparged through a dough. One study showed it was effective at increasing the gas fraction in the batter during OM but due to its low solubility didn’t contribute to oven rise during baking through the process of gas dilatation by heating (Brijwani et al., 2008).

The Chorleywood process which is covered by patent GB 1044616 A involves the application of a pressure/vacuum sequence to provide adequate air uptake from the atmosphere followed by rapid application of partial vacuum for bubble structure control. It is certainly not now a novel process.

Ultrasound Technology

The use of ultrasound waves is claimed to enhance the activity of leavening agents by promoting better gas dispersion and increasing the efficiency of gas production. This technology can also help in achieving finer and more uniform crumb structures.

Ultrasound or ultrasonication reduces both internal and external resistance to mass transfer in doughs through the process of cavitation as well as generating microscopic channels via convection.

How Leavening Affects The Baking Process

Baking as we know is a complex process that involves mass and heat transfer. The type of reactions occurring are protein coagulation, starch gelatinization, release of carbon dioxide from the leavening agent and various browning reactions. There is also some mechanical stress produced on the baked product as gas pressures change. An oven usually supplies heat in the form of conduction, convection and radiation heat transfer to the dough or batter. As the temperature of the dough increases, a phase change of liquid to vapour both at the surface and inside the product. There is a pressure generated in the pores of the matrix which rises and causes pocket deformation. The level of deformation depends on the degree of gas release and any structural changes that may occur through protein coagulation and precipitation, and through starch gelatinization.

Most of the studies on leavening are based on experimental studies with a few theoretical studies (Ploteau et al., 2015).

Proving/Proofing

A critical process step in generating a light aerated texture is the need for proving before baking. It is much more significant as a step than baking itself because mechanical or physical handling of the dough after proving can cause carbon dioxide to leave the dough.

The generation of bubbles and their growth during proofing and baking happens because of diffusion of carbon dioxide generated either from fermentation or from decomposition of chemical leavening. Bubble expansion also occurs during heating in the baking phase.

Any bubbles must work against the forces inherent in a viscoelastic dough. This viscoelastic medium, the dough, provides all the resistance to bubble expansion. Bubble growth involves coupled momentum, heat and mass transfer leading to a highly non-linear moving boundary problem. For simplification, surface tension was the main resistance to bubble growth. It was possible to couple mass transfer and momentum equations together.

When creating a dough, the proofing stage is usually between 15 and 25 minutes for the initial step and then another hour for the final proofing when kneading has been performed.

Overproofing is a persistent issue in the baking sector. Maltose produced by damaged starch hydrolysis is usually meabolised by yeast cells during over-proofing. Overproofing has a deleterious influence on the dough’s rheological and organoleptic properties, resulting in low-quality final loaves (Palier et al., 2022). 

Flavour Creation

As well as leavening gas producing the rise, various flavour precursors are generated during proofing that contribute to the flavour and aroma profile. These develop further during baking and are part of a series of Maillard reactions. Key components include 3-methyl-1-butanol (alcoholic flavor), 2-phenylethanol (floral), 2,3-butanedione (buttery), phenylacetaldehyde (honey-like aroma), 3-methylbutanal (malty).

Maillard Browning Reactions

Maillard reactions occur in foods when proteins react with reducing sugars. They produce colour, flavour and aromas. The Maillard reaction is preferred in foods with a high protein and carbohydrate content and intermediate moisture content at temperatures above 50ºC and at a pH of 4–7. The other reaction is caramelization which becomes an extension of these Maillard reactions but requires more drastic reaction conditions. Leavening agents influence pH and thus the degree of browning (Gokmen et al., 2008).

Acrylamide

Leavening agents affect acrylamide formation.

Analysis of Leavening Performance

The aeration of dough during proving and mixing is best measured using measurements of dough density immediately following mixing (Chin et al., 2005).

Considerations & Issues 

Biological leavening is not always suitable for particular doughs. If the dough has a very high sugar or fat content then the activity of yeast in particular is suppressed. This implies that the moisture content of the dough is low.  In some cases proving is conducted at much higher temperatures as in oven leavening. 

If the moisture content of the dough is very low, only chemical leavening is possible. The amount of chemical leavener used must be chosen carefully to avoid producing undesirable aromas.

We have already mentioned the undesirable flavours from chemical leavening agents. The pyrophosphates such as acid sodium pyrophosphate leave a distinct unpleasant flavour note. 

Staghorn salt promotes the formation of acrylamide in gingerbread (Komprda et al., 2017). 

Summary

To encapsulate then on all these possibilities, the purpose of raising and leavening in baking is to create a light, airy texture in the finished product. Leavening agents, such as yeast, baking powder, and baking soda, release gases that get trapped in the dough or batter and create small air pockets that expand when the baked good is heated. Leavening is an essential step in bread-making and is used in a variety of other baked goods to achieve the desired texture and flavour.

References

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