Molecular Gastronomy

The chemistry behind baking


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Yeast in All its Glory

Once you step into a kitchen, the baker becomes the chemist and the recipe the experiment. In the following, we will explore the relationship of yeast to the staple of baked goods, bread. The baker combines flour, yeast, liquid, and salt in a bowl, forms a dough which is then left to rise and baked in a hot oven.

Flour:
As the grain seeds used for flour are crushed, starch and proteins are released. Starch molecules are long, gangly polymers of simple sugars linked head to tail by chemical bonds. Proteins are more complex as a single protein may contain hundreds of amino acids strung together. But what causes that amazing fluffy texture one aspires to produce? Well this answer lies in the proteins gliadin and glutenin. They swell like sponges when kneaded with water and form the tough but elastic gluten. It is essential for gluten to stretch and trap the gaseous bubbles that make dough rise which in turn comes from yeast.

Yeast:
So essential to bread are those fungi, yeast. Unwitnessed by the naked eye, enzymes from the yeast cells attack starch, breaking it down into glucose. In addition, other enzymes transform glucose molecules into carbon dioxide and ethanol through glycolysis and fermentation. The carbon dioxide gas is released as a by product causing the dough to rise. Yeast can ferment or respire depending on the condition. With aerobic metabolism the yeast gain more ATP, energy, through respiration but in anaerobic metabolism,fermentation occurs.

Baking powder vs. Yeast
If baking powder is used instead of yeast do not have the same fermented molecules that give gread a great taste. When the baking powder gets wet, a chemical reaction occurs that releases only carbon dioxide, salt, and water. With yeast however, the yeast cells grow under anaerobic conditions and are not able to convert glucose molecules completely to gas. Some of the sugar molecules are converted to alcohols, acids, and esters which in turn add additional flavor.

Slower? Faster?
Water from boiling potatoes, eggs or sugar serve as a catalyst in yeast growth.
Salts and fats like butter slow down yeast growth. Salts effectively slow down the enzymes which catalyze the breakdown of proteins, strengthening gluten. But be careful, adding the perfect amount can be tricky. Too little and your dough will be tough and sticky. Too much, and water flows out of yeast cells by osmosis. If this happens, nutrients are lost and production of carbon dioxide is slower than normal.

In the Oven:
Once risen, the pockets of gas in the dough expand further in the oven. Because lactose is not fermented by the yeast, it is present in the bread to undergo a browning process. At 175 °C, lactose turns brown which is due to either caramelization 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. They compose the major flavor and aroma of bread crust. Yet again, the alcohol from fermentation add to bread flavor. The optimal temperature for yeast to ferment sugar is 32°C and if the temperature is warmer, 45 °C, the yeast cells will die.

Try this fun experiment to get a hands on understanding of what occurs.

What you will need:

sucrose
fructose
lactose
baker’s yeast
hotplate
3 beakers 100 mL
3 snap-cap vials 20 mL
glass stirring rod
watchglass d = 8 cm
thermometer

What to do:
1. Fill the three 100 mL beakers with 40 mL of water
2. 10 g of the appropriate sugar is dissolved each beaker
3. 1 g of baker’s yeast is added to each of the sugar solutions.
4.The solutions are warmed on the hotplate to a temperature around 25 to 40 °C.
       super easy (:

What you may expect:
Different amounts of visible foaming will occur in each beaker- a visible sign of the release of carbon dioxide from the reaction. The strongest will be in sucrose and slightly less in fructose while none will be seen in lactose as the yeast can no longer react with lactose.

Author: Liat Kugelmass

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