The Science of Baking: Leavening

Whether the leavening comes from air or chemical leaveners, controlling the amount of leavening present is crucial for successful high altitude baking. Reducing chemical leaveners needs to happen for a couple of reasons:

  1. The reduced air pressure at high altitude requires less leavening because the air is lighter and
  2. Water is required to activate both baking soda and powder.

Since baking uses carefully balanced formulas, and that altitude changes the boiling temperature of water, the water evaporates faster and doesn’t neutralize all of the leavening agent. Chemical leaveners need to be reduced significantly at high altitudes.

Yeast, however, is another story. Yeast provide flavor to breads through the fermentation process. Yeast breads rise more quickly at altitude, again, due to the decreased air pressure. An extra rise will give more flavor to yeast leavened breads. Or, another way to think about it is to allow the bread to rise for the time indicated in the recipe but punch down each time the dough doubles in size.

Leavening Agents:

Any agent which aerates a mixture thereby lightening it.


  • Air- introduced into the mixture through:
    • Sifting dry ingredients
    • Creaming and whipping batters
    • Incorporating whipped egg whites
    • Folding or kneading of doughs
  • Steam- produced from wet ingredients changing to steam during the baking process
    • This is enhanced by higher baking temps and is usually combined with other leavening agents like air or CO2.


  • Baking Soda
    • Alkaline substance called bicarbonate of soda.
    • Used alone, baking soda causes baked goods to spread.
    • In combination with an acid ingredient and moisture it will become a leavening agent by producing CO2.
    • used to neutralize acid components in baked goods
  • Baking Powder
    • Comprised of baking soda, solid acids and cornstarch (to absorb moisture)
    • All baking powders sold in the US to consumers are “double acting”
      • Double acting baking powders have a slow and fast dissolving acids.
      • Double acting means that CO2 is generated as soon as the baking powder comes in contact with moisture during the mixing process and then again when the batter comes in contact with the heat of the oven.
    • Baking powder does not create bubbles but enlarges the ones created during the mixing process (whipped egg whites, creaming, sifting, etc.)



3 types: compressed/fresh; active dry (milled, dry compressed yeast); instant (finer milled compressed yeast)

Forms of Yeast:

  • Fresh
    • Moist mixture of yeast and starches
    • Should have a pleasant smell, be tan in color and crumble easily
    • Must be refrigerated; be sure to wrap well
    • Has a 2-4 week shelf live
  • Dry Active Yeast (DAY)
    • Dehydrated
    • Pass through dryers that reduce the moisture content to about 8% making the cells dormant
    • Does not need refrigeration
    • Must be hydrated with warm water- 105-110 degrees- prior to using
    • To substitute DAY for Fresh Yeast cut the total amount in ???
  • Instant Yeast
    • This is an activated dry yeast
    • Can add the yeast into the dough at an early stage without any activation
    • This yeast is coated with ascorbic acid, which is a dough conditioner that speeds up fermentation
    • Fastest working yeast
    • Need only 1/3 the amount of yeast when substituting for fresh
    • 1 oz fresh = 1/3 oz instant

Leavening occurs when air and other gases are added to a batter/dough and, more importantly, when these gases expand from the heat of the oven.

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