What’s Happening in Bread?

by Deb on October 18, 2009

Bread is one of the world’s most important foods.  There are all sorts of different types of breads, made from different grains and rising (or not) in different ways.  The main grains used are wheat and rye, and the ways of getting it to rise are based on yeast or sodium bicarbonate (baking soda) but there are also lots of flat breads.  There are two main chemical processes going on during bread making.

Rising –

Yeast is a fungus.  Like many life forms it respires – it burns sugar and oxygen for energy and produces water and carbon dioxide.  So in a bread mix it is ‘eating’ the sugar and producing bubbles of carbon dioxide, which make the bread rise.  That’s why even savoury breads need sugar in them and why they need to be kept warm to rise, the yeast need warm conditions to grow.

In a sour dough bread much the same thing is happening, but you don’t get your yeast from a packet.  The starter dough is left to ferment and it picks up yeast from the air.  Yeast is one of those micro-organisms that is around us all the time, you provide them with a nice warm food source and they land and start growing.  That’s why it’s called ‘sour’ dough.

Breads made with sodium bicarbonate are often called ‘Soda Breads.’  They also rise because of carbon dioxide bubbles, but this time they are formed either through reaction with an acid or breakdown because of heat.  The acid can come from phosphates, tartaric acid (cream of tartar), citric acid (lemon juice), acetic acid (vinegar), yoghurt, buttermilk or cocoa.  Baking powder is usually a mix of sodium bicarbonate and some form of acid.

The acid reaction produces a salt and carbonic acid, which breaks down into water and carbon dioxide:

NaHCO3 + HCl -> NaCl + H2CO3
H2CO3 -> H2O + CO2 (g)

When heated above 70 degrees, so when cooking, sodium bicarbonate breaks down into sodium carbonate, water and carbon dioxide.

2 NaHCO3 -> Na2CO3 + H2O + CO2

Trapping the bubbles

It’s one thing to produce gas, but you also have to trap the bubbles.  This is done by long chains of proteins that stick together.  In many cakes or biscuits the egg provides the protein, but in breads it usually comes from the gluten in wheat and some other cereals.  Gluten is made from the proteins glutenin and gliadin and is found in the outer coverings of the cereal seeds.  It is an important source of vegetable protein for people around the world.

At the molecular level, when dough made from flour containing gluten is kneaded, the long molecules form cross links.  This makes a ‘net’ which gives bread dough it’s elasticity and traps the bubbles of gas from the rising agent.  When protein is heated it is denatured – this is what changes the texture of things like meat, eggs and dough -and the network becomes stable.

So there you have the chemistry of bread.

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{ 1 comment… read it below or add one }

Jessica October 21, 2009 at 10:08 pm

Wow! That was really interesting! I knew why bread does what it does, but I didn’t know how!


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