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Chemistry of bacteria in cake

Students show what lactic acid bacteria does in a cake starter mix, by mixing together baking soda and vinegar.
Science content (2016 curriculum): 
Chemistry: States of Matter, Properties of Materials (K-7)
Chemistry: Chemical Changes (2, 7)
Science topic (2005 curriculum connection): 
Physical Science: Chemistry (grade 7)
Lessons activity is in: 
  • Sugar molecule model
  • Small cups
  • Baking soda
  • Vinegar
  • Scoop (1/2 teaspoon)
  • Piece of Amish cake. Also other bread and cake (not essential)

What about the bacteria in the starter mix?
It is a kind of bacteria that makes lactic acid from sugar. They eat sugar and make lactic acid.
Here are model molecules to show what happens. The bacteria eats the sugar and breaks it into this.
The acid made by the bacteria helps keep other bacteria and yeast away as they don’t like acid.
It also helps make more holes in the cake.

Lets do an activity to see what the lactic acid made by the bacteria does.
Put a scoop of baking soda in a cup.
One of the ingredients of the cake mixture was baking soda.
Pick up a cup of vinegar.
This is a acid like the acid made by the bacteria in the starter mixture.
Pick up your cup of baking soda, and a cup of vinegar. Keep them separate and take them back to your desk.
Once we are all sat down.... Just as in the cake recipe, with acid from the bacteria baking soda, pour the acid into the baking soda.
What happens (get bubbles).
The acid from the bacteria (which might make it smell as though the starter is off) and the baking soda we add to the recipe make gas bubbles. These get trapped in the batter and make the holes in the cake.

Grades tested: 
Gr 2
Gr 3
Julie Johnson