Leavening agents, or chemical leaveners, are blends of additives that, when they react, release gas, which can be trapped by doughs, increasing the volume of the final product. They are also known as chemical yeast or baking powder. As we will see, leavening agents are used in a multitude of products, such as many batter-based recipes (muffins, cakes, etc.), cookies, or some types of bread. They can also be used in batters or other products.

Generally, leavening agents are composed of a basic substance and an acid.

Bicarbonate

The most commonly used basic substance for making a leavening agent is sodium bicarbonate (E-500ii). Sodium bicarbonate can react in two different ways:

2NaHCO₃ + Calor → Na₂CO₃ + CO₂ + H₂O

NaHCO₃ + H⁺ → Na⁺ + CO₂ + H₂O

While the first reaction requires temperatures above 120°C, the second can occur at room temperature. In the first reaction, sodium carbonate is generated, which produces unpleasant flavours. Fortunately, this reaction does not usually occur because the products to which it is added are moist, and the temperature does not rise above 100°C inside, as at that temperature, the heat is “spent” evaporating the water, which does not fully evaporate. The first reaction could occur in cookies, where the water evaporates entirely, and the dough reaches higher temperatures. But usually, the bicarbonate is consumed before reaching this point.

The second reaction occurs when acidic substances release hydrogen ions, and it is common in products like muffins or cakes and in the early stages of baking cookies.

The reaction rate of sodium bicarbonate also depends on its solubility, so it can be slowed down by using a coarser particle size. Additionally, it should be noted that sodium bicarbonate increases the pH of doughs, which can affect Maillard reactions, resulting in darker crusts. To achieve this effect, it is necessary to use a greater amount of sodium bicarbonate than the amount that will react with the acidic substances present in the dough naturally or added. Otherwise, the dough pH will decrease again when the bicarbonate is consumed, before the crust temperatures reach the values necessary for the Maillard reaction.

Doughs typically have a slightly acidic pH, so sodium bicarbonate can react with the acids normally present in the dough, but this reaction is usually insufficient for some products. Therefore, it is common to incorporate acids to enhance this reaction.

If it is necessary to replace sodium bicarbonate, it can be substituted with potassium bicarbonate (E-501ii). Generally, sodium bicarbonate is preferred because it produces more familiar flavours, is more economical, and has a greater gas-generating power. However, in cases where a low-sodium product is desired, potassium bicarbonate is a good alternative.

Ammonium bicarbonate (E-503ii) can also be used as a leavening agent, especially in cookie production, but its mode of action is very different from that of sodium bicarbonate and does not require the action of an acid, so it will be addressed in another entry, independently.

Acids

There are certain organic acids that are naturally present in foods, such as citric acid (lemon juice), lactic acid (yogurts), tartaric acid (cream of tartar), or acetic acid (vinegar), among others. These substances, although naturally present in food products, are also considered additives if added in purified form, with the following E numbers: citric acid (E-330), lactic acid (E-270), tartaric acid (E-334), acetic acid (E-260). These substances were the first to be used to react with sodium bicarbonate, but they had the disadvantage of reacting very quickly, even at room temperature, so the gases generated escaped from the dough.

The reaction rate of an acid with sodium bicarbonate depends on its solubility and its ability to ionize and release a hydrogen ion, which is what reacts with the bicarbonate. The mentioned acids are soluble at room temperature and are suitable for producing products that react at these temperatures, such as powdered soft drinks. However, to retain the gases formed, the doughs must have a certain consistency, which is achieved when the starch is gelatinizing, for which heating is necessary during baking. If the mentioned acids or the substances containing them are used, it is necessary to minimize waiting times and to quickly introduce the doughs into the oven to avoid gas loss. On the contrary, if the gases are generated once the starch has gelatinized and the structure of the cake is formed, it, not being flexible, breaks, generating cracks or breaks in the central area of the cakes, instead of smooth surfaces.

At the beginning of the 20th century, the industrial production of new acids or substances that react with sodium bicarbonate at higher temperatures was achieved. In fact, a series of substances that can react at different temperatures are discovered, allowing the generation of leavening agents with specific characteristics. Among these acids, the following stand out: monocalcium phosphate (E-341i), which reacts at lower temperatures (slightly higher temperatures if anhydrous), sodium acid pyrophosphate (E-450i), sodium aluminum phosphate (E-541), and sodium aluminum sulfate (E-521), or dicalcium phosphate (E-341ii), which reacts at higher temperatures. Monocalcium phosphate reacts at room temperature similarly to cream of tartar, so it is considered an acid with very rapid action. The rest of the acids hardly react at room temperature. Among them, dicalcium phosphate is the one that reacts at higher temperatures (maximum at 80°C), and therefore towards the end of the baking cycle. Sodium acid pyrophosphate and sodium aluminum phosphate have intermediate reaction temperatures (with maxima between 60 and 70°C), and sodium aluminum sulfate somewhat higher, but lower than dicalcium phosphate. Sodium acid pyrophosphate has the characteristic of reacting somewhat more slowly when there are calcium ions in the medium, as in preparations with milk. This characteristic has been used to produce different pyrophosphates that react at different temperatures. The reaction rate can also be reduced with a larger particle size.

Most commercial leavening agents are double-acting leavening agents, containing a fast-release acid and a slower-release acid, to cover a wider range of temperatures. Thus, those marketed are usually composed of sodium bicarbonate, along with monocalcium phosphate and sodium aluminum sulfate, together with a starch or flour to dilute the product, facilitate weighing, and prevent early reaction. On the other hand, industrial ones usually use sodium acid pyrophosphate instead of sodium aluminum sulfate.

When choosing the right acid and formulating the leavening agent, two factors must be taken into account. Firstly, its reaction rate in the oven, or reaction temperature, as we have previously discussed, and secondly, its neutralization value, or the amount of bicarbonate that can react per 100g of acidic substance. Occasionally, the equivalence value is also used, which is the inverse of the neutralization value (the amount of acidic substance needed to neutralize one gram of bicarbonate). For example, monocalcium phosphate has a neutralization value of 80 and an equivalence value of 1.25. In this case, the low neutralization value of dicalcium phosphate (35) stands out, consequently leading to a high equivalence value (2.86), requiring a greater amount of acid to neutralize the bicarbonate.

Table 1. Characteristics of the main acids used in leavening agents.

Acid	Neutralization Value	Equivalence Value	Speed
Monocalcium Phosphate (MCP)	80	1.25	Fast
Sodium Acid Pyrophosphate (SAPP)	72	1.39	Slow
Sodium Aluminum Phosphate (SalP)	100	1.00	Slow
Sodium Aluminum Sulfate (SAS)	104	0.96	Very Slow
Dicalcium Phosphate Dihydrate (DCP.Di)	35	2.86	Very Slow

In recent years, new acidic substances have been used to control the action of sodium bicarbonate in the oven. These substances are being used to replace phosphates and sulfates due to the negative perception of these acids by certain groups. On the one hand, there is glucono-delta-lactone (GDL) (E-575), which generates gluconic acid when hydrolysed once hydrated, but does so progressively over time, making its action at room temperature much slower than that of citric or tartaric acid. However, this hydrolysis accelerates with temperature and acidity, and therefore it will occur during the baking process. On the other hand, there are encapsulated acids, such as citric acid. The encapsulation protects the acid and delays its solubilization. This encapsulation breaks with temperature, releasing the acid halfway through cooking.

Choosing the leavening agent is not straightforward, as factors such as some ions present in the acids can help stabilize the dough, affect its colour, or impart different flavours. Additionally, selecting the reaction temperature allows for modifying the shape of the products obtained, and by regulating the baking temperature and the type of leavening agent, products such as muffins with or without a dome can be obtained. This effect can be achieved by combining the starch gelatinization temperature, which depends on the type of flour and the formulation, with the baking temperature and the leavening agent’s speed, as the heating of the dough is faster on the outer parts and slower in the centre of the mass.

For further information:

Russell, E.B. (2018) Chemical leavening basics. AACC International. St Paul, MN (USA)