Baking soda and baking powder

Whenever we make those lovely and luxurious cakes, with a few exceptions, we use baking powder to give the cake more rise. But not many of us appreciate the chemical reactions that take place during this vital process in cake making. But to understand baking powder, we must first know and understand one of its components, baking soda, also known as sodium bicarbonate (NaHCO₃).

NaHCO₃ is relatively cheap, very easy to store and is unstable at high temperature, resulting in the release of carbon dioxide (CO₂) when heated (equation below).

2NaHCO₃ → Na₂CO₃ + H₂O + CO₂

In an aqueous solution, NaHCO₃ starts to release CO₂ at about 100°C and is almost completely converted to Na₂CO₃ by 200°C.  However, from the above equation, we can see not all of the CO₂ is released; there are residual CO₂ that can be released from Na₂CO₃.

Na₂CO₃  → CO₂ + Na₂O

The problem is Na₂CO₃ does not decompose and release CO₂ until it is heated to 226-626°C, which is well outside the temperature range of most baked goods. Furthermore, Na₂CO₃  produced from NaHCO₃ is also a strong alkaline, i.e. it has pH above neutral, which is 7. When used in excess, the alkaline produced in this reaction makes the baked goods a bit yellow and tastes of soap. Another problem is that Na₂CO₃ reacts with hydrochloric acid (HCl) produced by the parietal cells in the stomach (production of HCl by stomach will not be covered in this blog, but the information should be present in most basic biochemistry and human physiology text books if you are interested) to release the remaining CO₂ gas, which can in turn lead to some awkward and embarrassing moments when you are with others.

Na₂CO₃ + 2HCl → 2NaCl + H₂O +CO₂

This simple biological reaction provides a fundamental clue to solve the problem. Heating NaHCO₃ at temperature required in baking alone does not release all of the CO₂ from the compound, and the product of NaHCO₃ reacts with an acid at normal core body temperature to release the remaining CO₂. That is exactly what we have in baking powder. It is simply baking soda with a weak solid acid /acid salt added.

NaHCO₃ + H⁺ → Na⁺ + H₂O + CO₂

H⁺ ions come from acids and it is the level of free H⁺ that determines the acidity of a solution. There are many acid salts used commercially to produce baking powder. The acid neutralizes the alkaline produced by heating NaHCO₃ and removes the problems in taste, colour and gas in the digestive tracts.

Cream of tartar and tartaric acid are two commonly used acid salts in baking powder, but they have different neutralizing values, i.e. efficiency at neutralizing the baking soda. For example, tartaric acid has a neutralizing value of 100 whilst cream of tartar has neutralizing value of 200; this means tartaric acid is twice as efficient than cream of tartar at neutralizing baking soda. However, this does not mean higher neutralizing efficiency is better. With high efficiency, it means more CO₂ is produced initially and as the substrate (NaCO₃) becomes depleted in the reaction, the production of CO₂ ceases, this will cause the cake to “drop” when removed from the oven and gives a much closer and doughy texture. Some bakes on the other hand, prefer a rapid rate of CO₂ production, such as in doughnuts, which allow fast aeration within the dough and hence make them float in oil and give a crisp texture. Some baking powders contain a combination of acid salts to allow fast initial CO₂ production and subsequent continuous production of the gas. There are other acid salts used in the production of baking powder, these include sodium acid pyrophosphate and acid calcium phosphate.

References

Czernohorsky JH and Hooker R (2005), New Zealand Institute of Chemistry, food and beverages, chemistry of baking. http://nzic.org.nz/ChemProcesses/food. Accessed on 01/01/2013