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Cooking with Chemistry: From bacteria to baking, the unique reactions that beget bread

(Muling Shi/Daily Bruin)

By Jack Tulyag

Jan. 30, 2019 6:31 p.m.

Chemistry as a science sounds like something that should strictly be confined to a laboratory with poisonous toxins and exploding reactions, but as it turns out, most labs aren’t quite like that. In fact, the most important laboratory is found in every home: the kitchen. Understanding the complex processes that go on in the kitchen allow us to go further than the recipe to improve our cooking.

Bread has risen to a nearly sacred status within society. Its importance can hardly be understated considering the fact that it has been staple food for thousands of years. The word “companion” literally means “with bread.

From a scientific perspective, the bread-making process is a fascinating chemical phenomenon. Bread is essentially a type of foam, or a phase mixture of gas and solid that forms a rigid structure with the proteins found in flour.

Is this gluten-free?

Before thinking about the science, let’s understand what exactly goes on in the kitchen. The basic process is going to be the same for all types of bread: mixing flour, yeast and warm water. Bakers add in other ingredients, like salt or oil, to change the texture, flavor and vitality of the bread. Mixing the components forms dough, which a baker will typically knead before throwing it into an oven.

Now that we know the process, let’s take the chemistry step by step.

Flour, which is just powdered wheat, is composed of several crucial ingredients to the bread-making process. Starch, a huge molecule that is constructed from simple glucose molecules, is a sugar that makes up about 70 percent of the flour. The remaining 30 percent – the various proteins that constitute the rest of the flour – is vitally important for bread-making, especially two proteins known as gliadin and glutenin. Proteins have numerous unique properties, but the ones we care about in cooking are their sensitive structures and their interaction with water.

It is the mixing of flour and water that creates the basic architecture that we know as bread. Instead of dissolving in water, gliadin and glutenin absorb it, unfolding in a rather complicated manner. The interconnected mixture that they form is what we know as gluten, which forms the structural framework for bread. Kneading the dough continues to cause the proteins to unfold, eventually turning from long fibers into wide sheets.

If we wanted to get technical, we can say that we formed a viscoelastic solid, or a material that flows like liquid but is also stretchy like a rubber band. Otherwise, we call it dough. We can attribute its mechanical properties to the gluten network that formed while kneading. The more we knead, the more proteins tighten with each other, improving the resistance of our solid. Knead too much, and the gluten tightens so much that the dough tears easily.

On the rise

The gluten in dough absorbs water and forms a tough solid that looks like sheets on the molecular level. In doing this, we are slowly but surely building the structure and consistency of the bread. This process includes one other ingredient that we haven’t discussed yet – yeast.

Yeast is quite literally bacteria used as a cooking ingredient. Its purpose is to produce gas – specifically carbon dioxide – as a waste product from eating starch. In particular, damaged starch crystals serve as an energy source for yeast to feast on due to the fact that damaged starch is smaller and mixes better with water than large granules. Too much damaged starch and the dough has a tendency to get sticky and gassy due to too much water being absorbed.

The process of producing gas in bread is known as leavening, but what actually prevents the gas from escaping? The answer is the gluten sheets that formed from kneading the bread, effectively forming a trap for any gases that might be forming. Hence, dough is a type of foam, which is a combination of gas and solid phases. The gas bubbles end up forming the pores that we see in bread.

 

Thermal treatment of a viscoelastic solid

Once all the gas is built up and the dough is nice and tough, the last thing left to do is bake it. In the oven, gluten will release the absorbed water and transfer it to the starch, leaving a rigid gluten structure behind.

The brown color of bread is due to a particular chemical reaction known as the Maillard reaction. This is a particular interaction between amino acids on a protein and the sugars that are present in many foods including steak, dumplings, cookies and, of course, bread.

The starch that absorbs the water from gluten degrades. In particular, the granules leech out a chain of glucose known as amylose. This particular molecule is interesting because when baked bread cools down, amylose begins to crystallize in a process known as retrodegradation. This is what causes bread to become stale – not a lack of water as one might think, but instead crystals growing on bread.

Adjusting bread parameters

There are a variety of different variables we can adjust to produce different types of breadlike products. We can replace wheat flour with different types of flour and look for different sources of gas.

Instead of relying on gas production from bacteria, a baker can use chemical leavening agents such as sodium bicarbonate or baking soda. Baking soda is a crystal that dissolves particularly well in the presence of an acid, releasing carbon dioxide in the process. Typical acids include lemons or vinegar. Utilizing chemical leavening agents is faster than waiting around for bacteria to eat up sugars, crucial for desserts such as cookies and pancakes.

If we replace our usual bacteria with a different species, we may produce sourdough bread. Typical bread uses baker’s yeast, a particular species that leavens bread fairly quickly. Sourdough relies on wild yeast and lactobacilli, both species that take a bit longer to leaven. With careful preparation to leaven the bread, the result is the production of lactic acid, which leads to the sour taste.

What happens if we don’t leaven the bread at all? Well, the bread just won’t rise. In fact, this is exactly what tortillas are: unleavened bread made from either wheat or corn flour.

For our gluten-free friends, we need some sort of substitute to replicate the same structural purpose of gluten. Since gluten is a combination of proteins unique to wheat, the solution is to replace wheat flour with flour from another type of grain such as corn or rice.

Bread is a miraculous feat of human ingenuity. While there may be many types of flour products we encounter, the basic chemistry is the same. Understanding the purpose of each ingredient and their interactions is important to creating the best breads.

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