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Flour, yeast, water and salt makes bread

school-breadBaking bread is an experiment in biochemistry.  The baker combines flour, yeast, water and salt in a bowl, shapes it all into a dough, lets it ferment  and bakes it in a hot oven.  The fermentation makes the bread chewy, fluffy and tasty.

The main ingredient in bread is flour.  It is made by milling grains such as wheat so that they release their starch and proteins.  Whole wheat flour contains about 70% starch, 14% proteins, 11% fiber, 2% fat, 1% complex sugars and 0.5% simple sugars.

Protein and a little salt make the bread chewy

Proteins are molecules build from hundreds of amino acids.  When flour is added to water and kneaded, two of the proteins  (gliadin and glutenin) swell up like sponges and form a tough elastic substance called gluten. Gluten can stretch and trap the bubbles of gas that make dough rise.

Salt strengthens gluten by slowing down the enzymes which catalyze the breakdown of proteins. If you add too little salt, the dough is tough and sticky. If you add too much, water flows out of yeast cells by osmosis. Then nutrients are lost and production of carbon dioxide slows down.

Yeast breaks down starch to make bread fluffy and tasty

The microscopic yeast cell is a tiny form of fungi or plant-like microorganism.  It lacks chlorophyll, and gets its energy from fermenting simple sugars.  Like other cells, yeast uses enzymes to carry out chemical reactions.  For a great introduction into cells and their enzymes, refer to How cells work.

The starch molecule is a long polymers of simple sugars linked head to tail by chemical bonds.  There are two main stages during the fermentation of a dough:

  1. Enzymes released by yeast break down the starch into simple sugars that can pass the membrane of the yeast cell:
    • Starch is a very complex sugar.  The yeast cells release enzymes that break down the starch  into complex sugars. The flour naturally contains the enzymes:
      • α-amylase, degrades the starch Amylopectin into dextrin, after which the dextrin is degraded by the β-amylase into maltose.
      • β-amylase, degrades the starch Amylose  into maltose.
    • Maltrose is a complex sugar and mainly the result of broken down starch.  Enzymes release by the yeast convert these in simple sugars.  The enzyme:
      • saccharase, transforms saccharose into glucose and fructose
      • maltase, transforms maltose into glucose.
        :
  2. The simple sugars, glucose and fructose,  can directly pass the cell membrane.  simple sugars into alcohol, carbon dioxide and energy.  Inside the cell:
    • Another enzyme takes over
      • zymase, in turns splits the simple sugars to ethanol and carbon dioxide.(C6H12O6 →  2C2H5OH + 2CO2 + energy).  Yeast cells use the energy that it gains to reproduce.  In reality, the chemistry of fermentation is actually much more complex than this, with the glucose being incorporated into other by-products such as pyruvic acid, acetaldehyde and glycerol.
      • In breads leavened with yeast, the cells grow under anaerobic conditions and cannot convert glucose molecules completely to gas. Some sugar molecules get sidetracked and are converted into alcohols, acids, and esters–substances which add to bread’s flavor.

As the dough bakes into bread, the heat causes the alcohol to evaporate and the carbon dioxide bubbles to break.  This leaves the tiny air pockets in the baked bread that make it light and fluffy.

Ratios

In 100 grams of flour, 1 to 10 million micro-organisms live among which only 30,000 are the so-called wild yeasts. A standard amount of 2.5 grams of baker’s yeast for 100g of flour provides 25 billion yeast cells. This is the proof that baker’s yeast is predominant in the bakery fermentation.

Sources

http://www.cofalec.com/Default.aspx?lid=1&rid=122&rvid=150

http://science.howstuffworks.com/environmental/life/cellular-microscopic/cell2.htm

http://www.bigsiteofamazingfacts.com/how-does-yeast-make-dough-rise

http://www.bigsiteofamazingfacts.com/who-discovered-that-yeast-makes-breads-rise

http://www.tpt.org/newtons/TeacherGuide.php?id=1285

http://www.truevisionmicroscopes.com/experimenting-with-yeast-under-the-microscope.html

Coert Vonk

Coert Vonk

Independent Firmware Engineer at Los Altos, CA
Welcome to the things that I couldn’t find.This blog shares some of the notes that I took while deep diving into various fields.Many such endeavors were triggered by curious inquiries from students. Even though the notes often cover a broader area, the key goal is to help the them adopt, flourish and inspire them to invent new technology.
Coert Vonk

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