That dry ice topic and Aviator's post that didn't develop got me thinking... and googling:
Apparatus and glass wares:
3 beakers 100 mL
3 snap-cap vials 20 mL
glass stirring rod
watchglass d = 8 cm
40 mL of water are placed in each of three 100 mL beakers. 10 g of sugar are dissolved - in beaker 1 lactose, in beaker 2 fructose and in beaker 3 sucrose. 1 g of baker' yeast is adedd to each of the sugar solutions. The solutions are warmed to 25 to 40 °C.
An unequal strong foaming can be observed in the three beakers. The strongest foaming occurs in the beaker containing glucose. A moderate foaming takes place in the beaker with fructose. Evidently lactose does not react with yeast.
Discussion and background:
Baker's yeast enzymes convert sugar (glucose, fructose) to ethanol and carbon dioxide. The fermentation process is accompanied by the release of carbon dioxide which causes foaming.
Baker's yeast is cultivated from the strain Saccharomyces cerevisiae because of its superior fermentation abilities. The yeast propagates in pure culture using special culture media comprised of melasse and other ingredients. With respect to their metabolism baker' yeasts are facultative anaerobe. They can ferment or respire depending upon environmental conditions. In the presence of oxygen respiration takes place, without oxygen present, fermentation occurs.
Baker's yeast plays a key role in bread dough fermentation. Amylases present in flour, break down starch into a smaller sugar, maltose. The reaction starts as soon as water is added to the flour and stops during baking. The action of the flour amylases is completed by an enzyme of yeast, the maltase, which splits maltose into two glucose molecules. Glucose is fermented by the yeast to ethanol and carbon dioxide.