Yeast and Sugar - The Chemistry must be right

Demonstration Experiment on Video

Yeast and Sugar - The Chemistry must be right

Objective: Alcoholic Fermentation

Peter Keusch

German version

Supermarket products:
sucrose
fructose
lactose
baker's yeast

Apparatus and glass wares:
hotplate
3 beakers 100 mL
3 snap-cap vials 20 mL
glass stirring rod
watchglass d = 8 cm
thermometer

Experimental procedure:

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.

Results:

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.

Video clip (Download RealPlayer .rm file)

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.

Naker'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.

The released carbon dioxide causes dough to rise and to hold it high. The produced alcohol contributes to the bread's flavor. The optimal temperature for yeast to ferment sugar is 32°C. In warmer temperature (45 °C) the yeast cells will die.

Also fructose and sucrose care used by the yeast as fermentation substrates. Sucrose is immediately transformed by a yeast enzyme, the invertase, into glucose and fructose. Sucrose is a good substrate for fermentation. When sucrose or glucose is added to the dough, they are faster fermented than maltose.

Variety of bread

Since lactose is not fermented by bakers yeast, it remains available for the browning process. Lactose, when heated to 175 °C turns brown. The browning process may be caused by carmelization reactions (formation of burnt sugar) during baking and storage or the Maillard reaction. The Maillard reaction occurs between a reducing sugar such as lactose, and the amino groups of proteins. Melanoidins are generated on the surface of the loaf. They are the major flavor and aromatic compounds of bread crust.

Literatur:
G. Schwedt: Experimente mit Supermarktprodukten, Wiley-VCH-Verlag, Weinheim (Germany) 2001

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