Summary Observe yeast making carbon dioxide gas as it breaks down sugar. Model the chemical reaction. Use to model why bread/yeast cake rises. Science content Biology: Features, Adaptations of Living Things (K, 1, 3, 7) Chemistry: States of Matter, Properties of Materials (K-7) Chemistry: Atoms, Molecules (3-7) Chemistry: Chemical Changes (2, 7) Science competencies (+ questioning + manipulation + others that are in every activity) Planning/conducting: planning investigations (3 up) Evaluating: inferring (3 up) Lessons activity is in Bread (and butter) with the chemistry Chemistry of sourdough bread / traditional yeast cake Materials tall thin tubes e.g. 15ml tubes and rack scoops (1/8 teaspoon) yeast sugar warm water in squeeze bottles coffee stir stick sugar molecule model. One molecule has 6 carbons, 12 hydrogens, 6 oxygens, plus 24 bonds. Sets can be plastic modelling sets (e.g. Molymod) or home-made set with modelling clay and toothpicks Procedure Put a scoop of yeast and a scoop of sugar in at tube, then add warm water to half way up the tube. Take a wooden stick and stir until the yeast and sugar have dissolved in the water. Optionally pull a balloon over the top of the tube. Leave the mixture for a few minutes for the chemical reaction to happen. After a few minutes gas bubbles appear. If there is a balloon on the tube, it will slowly fill with the gas and inflate. The yeast is a living thing. It is dry or dormant in the jar - it can live like that for a long time. When you add water and food, it starts to eat the sugar and breaks it down into carbon dioxide and ethanol. If you bake at home, you might have seen yeast and sugar at the start of a recipe making the bubbles of gas. With older students, use molecule models to show the chemistry of the sugar breakdown into a gas, and figure out what the gas is: Give each student/student group a sugar molecule (C6H12O6). Tell them that one of the products as yeast breaks glucose apart is ethanol, and show them how to build it (CH3CH2OH). They should make two ethanol molecules. Ask them to make two identical molecules with the atoms that remain. (Two CO2 molecules). It might be a challenge to figure out that there are two double bonds in CO2. Once they have all made the CO2 molecules, spell out the name while pointing at the atoms "C-O-2", and some students may recognize the name and know it is carbon dioxide. Ask what state of matter CO2 is (gas), and ask what might happen if this gas is stuck in bread dough (it will make it rise). When yeast is ued to make wine or beer, the gas makes it slightly fizzy and the ethanol stays in the drink. Non-alcoholic drinks remove this ethanol afterwards and/or use a yeast that makes much less ethanol. Attached documents bread_booklet.pdf Build bread booklet.pdf bread_and_cakes_take_home.pdf Notes Yeast is a fungus, not a plant. Grades taught Gr K Gr 1 Gr 2 Gr 3 Gr 4 Gr 5 Gr 6 Gr 7
