Published by the MIT News Office at the Massachusetts Institute of Technology, Cambridge, Mass.

IAP Presents a Lesson in Leavening

EGG WHITES OR BAKING POWDER
IAP Presents a Lesson in Leavening

Yeast, egg whites and baking powder--how do these baking products make 
bread dough, angel food batter, and muffin mix rise?

At a recent session of Kitchen Chemistry, an IAP event sponsored by the 
Experimental Studies Group, ESG lecturer Melanie Holland gave 
explanations for all three phenomena, complete with equations on the 
blackboard and a follow-up open house in the ESG kitchen for 
participants to experiment with their own concoctions.

Leavening agents work in a variety of ways, Ms. Holland said. For 
example, the egg whites in angel food batter create that cake's familiar 
consistency because when whipped, "the protein in the whites forms a 
matrix that traps air," Ms. Holland said. When heated, the air bubbles 
expand and the cake rises. 

Other leavening agents liberate carbon dioxide. As the gas is released, 
the batter or dough rises. For example yeast, which is actually made up 
of living plant cells, works when those cells "metabolize into your 
bread," Ms. Holland said. "As long as you're nice to them, and don't do 
something like pour boiling water over them, they'll live and make CO2." 

Baking soda--that product often packaged in a small orange box--
liberates CO2 if you let it react with an acid, Ms. Holland said. An 
acid? Try sour milk, buttermilk, lemon juice or molasses, she suggested. 
"All of those will donate a proton to the sodium bicarbonate [baking 
soda] and you'll get CO2 and water."

Ms. Holland pointed out that if you don't add an acid, baking soda will 
still give out carbon dioxide in the oven--it reacts with itself when 
heated--but in addition to CO2 you get sodium carbonate, which gives "an 
awful, bitter taste," she said.

Baking powder--packaged in a can with a plastic cover--works on the same 
principles as soda, but doesn't need the addition of an acid. "It 
already has in it an acid and base ready to react," Ms. Holland said. 
However, unlike baking soda, baking powder can go bad. "If any water 
gets in [the can], it will react," Ms. Holland said. As a result, a can 
of baking powder also contains lots of starch to absorb water. 

Before opening up the kitchen for "wild experimentation," Ms. Holland 
suggested some possible concoctions. "What would happen if you used sour 
milk or coca cola in a cake? They're all acidic," she reminded. "And how 
about egg whites versus baking powder in muffins?" She didn't say who 
would taste the results.