Summary Students show what lactic acid bacteria does in a bread or cake starter mix, by mixing together baking soda and vinegar. Science content Chemistry: States of Matter, Properties of Materials (K-7) Chemistry: Chemical Changes (2, 7) Science competencies (+ questioning + manipulation + others that are in every activity) Evaluating: inferring (3 up) Lessons activity is in Chemistry of sourdough bread / traditional yeast cake Materials optional: piece of sourdough bread or Amish yeast cake small cups baking soda vinegar scoops optional for older students: sugar molecule model Procedure There is a bacteria in sourdough bread or Amish yeast cake starter mix that gives these baked goods their distinct taste. It is a kind of bacteria that makes lactic acid from sugar. The acid made by the lactic acid bacteria helps stop the growth of other bacteria and yeast (they can't grow in acid). So lactic acid bacteria starter mixes can be kept for a long time. The lactic acid bacteria also helps make holes in the bread/cake to make it light. For yeast cakes, and some sourdough breads, baking soda is added to the recipe, which combines with the lactic acid. Model what happens when the lactic acid and baking soda mix: Put a scoop of baking soda in a cup. Add some vinegar (an acid like the lactic acid). What happens? [get bubbles]. The acid from the bacteria and the baking soda make gas bubbles. In a yeast cake (or sourdough bread where baking soda is added) the bubbles get trapped in the batter and make the cake/bread rise, and are seen as holes in the finished cake/bread. Optionally use model molecules to show what happens when baking soda mixes with an acid. As well as lactic acid bacteria, sourdough and Amish yeast cake starters also have yeast in them. See the other activity in this lesson plan for the yeast action. Grades taught Gr 2 Gr 3
