• Leavening Agents

Leavening agents are essential in baking as they produce the gases that cause doughs and batters to rise, creating the desired texture and volume in the final product. There are three main types of leavening agents: chemical, biological, and mechanical. Each type operates through different mechanisms and is suitable for different baked goods.

Chemical Leavening Agents

Chemical leavening rely on reaction of added chemicals to produce gas such as carbon dioxide and ammonia, causing batter to rise.

Baking soda is used to refer to sodium bicarbonate, which reacts with acid to form carbon dioxide. However acids are not always present in cake ingredients so baking powder which has acids in it is used for these recipes. Baking soda is typically used in baking cookies as sodium bicarbonate can also increase the amount of browning that occurs during baking. This is because sodium bicarbonate is a base which increases the pH of the dough, causing amino acids to lose the hydrogen atom from their carboxylic acid more easily, leaving an anion with a negatively charged oxygen that will donate its lone pair of electrons to form new bonds. This speeds up Maillard reaction causing increased browning (refer to the article on Maillard reaction).

Baking powder is a mixture of sodium bicarbonate and acid salts such as sodium aluminium sulphate, potassium hydrogen tartrate (cream of tartar), tartaric acid and calcium phosphate. Bicarbonate ions can react with acid salts to release carbon dioxide in an aqueous solution (in batters where moisture is present). This is because acid salts are soluble in water so ions could react, whereas in solid state the ions in the ionic compounds are fixed in place so they cannot react. By-products are less alkaline than sodium carbonate formed from the reaction of baking soda, so there would not be undesirable effect on the taste of the product. Baking powder is typically used in biscuits, cakes and scones.

Biological Leavening Agents

Biological leavening is when organisms release carbon dioxide as part of the fermentation process to help baked goods rise. When yeast breaks down starches in flour or simple sugars in honey, syrup and sugar, carbon dioxide and ethanoic acid are produced through anaerobic respiration. Active dry yeast is composed of tiny hydrated granules of yeast that are activated when mixed with warm liquid. Whereas instant yeast is composed of slightly smaller granules and do not have to be pre-activated. Compressed fresh yeast is when fresh cream yeast is formed into small blocks.

In sourdough starter, fermented mixture of flour and water allow sourdough bread to be fermented with naturally contained wild yeast and lactic acid bacteria. Wild yeast is less active so it takes a longer amount of time for leavening.

Mechanical Leavening Agents

Mechanical leavening involves physically incorporating air into the batter or dough, which expands upon heating. This is because during heating, the particles gain kinetic energy causing them to vibrate faster. This reduces its density, expanding the air bubbles in the dough/batter.

Whipped cream and beaten egg whites can trap air bubbles within a layer of proteins, these elastic layers then expand during heating and set in its expanded position.

Comparison of Leavening Agents

An experiment was conducted (Lee, 2021) to compare the effectiveness of different leavening agents — air, baking soda, eggs, instant yeast, and sourdough starter —  in raising bread. With sourdough starter and instant yeast being found to be the most effective, closely followed by baking soda. The results are depicted in the following images:

Reference