Leavening baked goods requires the incorporation of gas to achieve a crumb structure which is light and airy. Leavening can be accomplished mechanically (example: creaming or lamination), biologically (example: yeast or bacteria like sourdough starters), or chemically. For this article we will focus on chemical leavening.
Carbon dioxide is the most common leavening gas, but leavening can also occur by ammonia gas, steam or even air. Leavening gas expands with heat and the resulting volume is captured in the rigid structure of the baked dough.
Leavening requires enough gas to be produced at the right time and expand before the gluten/starch network become rigid. Too much gas can collapse a weak gluten structure, not enough and the finished product will have a very dense network. Gas produced too quickly will result in the carbon dioxide being released and dissipated before the dough reaches the oven and the gluten/starch network sets. Gas produced after the network sets is lost gas because it does not change the texture.
Chemical leavening agents are predictable in creating reactions. However, chemical leavening reactions, if not properly neutralized or completed, can negatively change the final product.
Carbon Dioxide Sources:
Carbon dioxide is produced by either decomposition (break down with heat and moisture) or as a reaction product with a weak acid with heat and moisture as components to reaction rate.
1. Sodium bicarbonate: Also known as Soda, this ingredient is soluble, easy to handle and is main source of carbon dioxide seen commercially. Decomposition can occur with heat, but more typically sodium bicarbonate is mixed with a leavening acid. The acid chosen will change the reaction rate of carbon dioxide release. Generally:
Sodium bicarbonate + Acid → Salt + Carbon dioxide + Water
2. Potassium carbonate/bicarbonate: Is an alternative to sodium bicarbonate for low sodium formulas. Similar to sodium bicarbonate in that decomposition can occur at high temperature, this ingredient also requires pH balancing with leavening acids for an optimum finished product. This ingredient can have a slower release than sodium bicarbonate.
Potassium carbonate + Acid → Salt + Carbon dioxide + Water
3. Ammonium carbonate/bicarbonate: At room temperature this sodium-free leavener is unreactive which allows for robust doughs during processing. At temperatures above 59°C, ammonium bicarbonate undergoes decomposition to enable leavening. Some considerations in using this ingredient is that ammonia gas can leave an odor and potentially an off-color in high moisture products.
Ammonium bicarbonate + Heat → Ammonia gas + Carbon dioxide + Water
Leavening acids are weak, with a pH range 4-6. As mentioned earlier, the leavening acid and sodium bicarbonate need to be balanced so all of the substrates are used in the reaction and the end pH is neutral - if this does not occur, finished products may have a soapy or bitter off-flavor. Using molecular weights, calculations can identify the formula requirements.
Neutralizing value (NV) has two main functions in leavening formulation.
- Calculate the expected carbon dioxide release.
- Quickly calculate the correct formulation levels.
NV = (g NaHCO3 *100)/ g acid
g acid = (g NaHCO3*100)/NV
The reactions often require heat to facilitate carbon dioxide release often as a result of increased solubility at warm temperatures. Fast acting means the reaction is driven at room temperature often during mixing. Slow acting means the reaction is driven at warmer temperatures like those seen in baking.
- Monocalcium phosphate (MCP)
• Fast acting
• Large particle size can result in the formation of black spots
- Sodium acid pyrophosphate (SAPP)
- Offered in many grades based on reaction rate
- Slow acting
- Soluble in hot water
- High in sodium
- Sodium aluminum phosphate (SALP)
- Slow acting
- Clean flavor
- Sodium aluminum sulfate
- Very slow acting
- May provide an off flavor in some formulas
- Cream of tartar (potassium bitartrate)
- Slow acting
- Solubility and ability to react to create carbon dioxide increase with temperature.
- Tartaric acid
- Fast acting
- Not temperature dependent for solubility
- Slow release
- Constant release
- Good replacement for yeast
With all the moving parts of chemical leavening, baking powders are often used because they are balanced to neutralize completely and to provide carbon dioxide over a longer period of time. These are typically mixtures of sodium bicarbonate, more than one leavening acid, and often starch. The solubility of the ingredients at room temperature indicates whether the baking powder is slow release (gas releasing in baking), or fast release (gas starting to release in mixing), or double acting which contain ingredients to leaven during both mixing and baking.
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