Gluten in the Kitchen

In the introduction to gluten, we described gluten as a protein that contributes to the structure and texture of baked goods. The ideal amount of gluten depends on what we’re making, and there are several ways we can influence the extent of gluten development. We can first choose a flour based on its protein content, then work on the molecular scale of the chemical interactions between glutenin and gliadin to form a sparser gluten network or a denser one.

Choosing a flour

Flours vary in their glutenin and gliadin content because they were milled from many kinds of wheat. Wheat, like all living organisms, expresses different proteins in different ratios depending on its environment and the stage of its growth cycle, and the proteins present in the wheat at harvest become part of the flour. Factors such as climate, soil acidity, water availability, and time of harvest all influence the final protein content of the flour. In turn, the amount of glutenin and gliadin present in the flour affects the amount of gluten that ultimately develops in our kitchens—if there are fewer proteins to begin with, less gluten can form.

The amount of protein also varies the volume of water the flour can absorb. Recall from the introduction that proteins in dry flour are like critters frozen in tide pools, and the critters reach for water until they are fully thawed. If there are fewer critters, less water needs to be allocated to thawing them. Similarly, in flour, proteins reach for water until they are fully hydrated, so flours with fewer proteins do not absorb as much water as flours with more proteins. The same amount of water that makes a nice dough with a high-protein flour will make a thin batter with a low-protein flour. (I am working on a post that shows this in muffins—just swapping out flours results in batters with very different consistencies!)

Different types of flours contain different amounts of protein. Cake flour, for example, is a low-protein flour, typically with 7.5 to 8.5% protein by weight, and is so named because we minimize gluten in cakes for a fluffy texture. Bread flour is a high-protein flour, usually with 11 to 14% protein, and it is aptly named because we encourage gluten development for chewiness in breads. All-purpose flour falls in between at 9.5 to 12%, making it a versatile (or all-purpose) option for cake and bread recipes alike. Notice, however, the variation in protein content even within these types of flours. Depending on the flour the recipe was written for and the flour in your pantry, you may benefit from subtle adjustments to the liquid or flour content in the recipe. Factors in processing (for example, bleaching) also affect the chemical composition of the flour, how it interacts with other ingredients, and the taste of the final product. These are ideas I will explore in more depth in future posts devoted to flour.

Recipes specify which type of flour to use, but if you’d like to play around with substituting flours to change textures, stay tuned for future posts!

Increasing protein interactions for more gluten

Once we have chosen a flour, we can encourage the glutenin and gliadin within it to bond and form a denser gluten network.

Mixing the batter or dough to develop gluten

One of the most important factors to consider in gluten development is mixing. Mixing combines the flour with water so that gliadin and glutenin hydrate and form the chemical bonds to create gluten. Because we limit gluten development in cakes and muffins, their recipes often instruct “Mix until just combined” so we don’t overmix the batter, create excess gluten, and make a chewy cake. In fact, muffin batter is often left lumpy. The lumps are pockets of dry flour with dry proteins that cannot form gluten. On the other hand, in bread dough, we want a dense gluten network to trap the gas the yeast release, so many bread recipes require extensive kneading. The longer the batter or dough is worked, the more gluten develops. The next post will demonstrate the effects of overmixing muffin batter!

(Note: In breadmaking, expanding air during rises also strengthens gluten. No-knead breads rely on this for their gluten network. This is also the problem with overproofing: the gluten becomes overdeveloped. I will discuss this in more detail with bread-related posts in the future.)

Mixing brings gliadin and glutenin together to form gluten. The longer a batter or dough is mixed, the more gluten will develop.

Mixing method also influences gluten development. Folding with a spatula by hand, which merely brings the batter from the bottom of the bowl to the top, is the gentlest mixing method and minimizes gluten formation. Stirring encourages more gluten development because the utensil drags through the batter, lining up proteins as it goes. If the utensil has multiple tines (like a whisk), each tine will align proteins as it moves across the bowl. Similarly, when we knead dough, we position the glutenin coils to form a cohesive gluten network. Hand mixers, due to their speed, develop gluten faster than mixing by hand (although they do not have the power to mix stiff doughs), and stand mixers, with even greater power, are fastest. Whisks, hand mixers, and stand mixers also introduce air into the batter, which further strengthens gluten.

Beating in air to strengthen gluten

To understand the role of air in gluten development, we need to know more about the chemical bonds between the gliadin and glutenin. Remember that these proteins are like strings of amino acid beads with rules about the colors that can touch. Now imagine that the touching beads might be super glued, glue sticked, or taped together—they are bonded with different strengths. One of the strongest types of bonds between amino acids is called a disulfide bond, which is a general term for any bond between two sulfur atoms. In proteins, cysteine is the only amino acid that can form them. Because disulfide bonds are like super glue, the more disulfide bonds there are in a gluten network, the more strongly connected the proteins, and the stronger the gluten. Oxygen, contained in air, encourages the formation of disulfide bonds at a molecular level. Thus, when we beat air into a batter, we introduce oxygen, which super glues cysteine amino acids together to form a stronger gluten network.

Air strengthens gluten. Gluten proteins contain sulfur atoms (S) that can form disulfide bonds (S-S) with each other (left side). Oxidizing agents (OX) such as air and Vitamin C encourage the formation of disulfide bonds (right side), which strengthens the gluten.

Note: Ascorbic acid, or Vitamin C, which some bakers add to their bread doughs for strength, is another compound that encourages the formation of disulfide bonds. In this case, compounds like ascorbic acid and oxygen act as oxidizing agents, which I will discuss more in the future. The opposite of an oxidizing agent is a reducing agent. A reducing agent would break disulfide bonds apart and weaken the gluten network, but reducing agents are more often used in commercial bakeries.

Adding salt to tighten gluten

Salt strengthens gluten. If you’ve ever played with salt dough, you’ve experienced this for yourself. Salt dough is a mixture of flour, water, and lots of salt (1 cup to 2 cups of flour, about 110% by flour weight) with the consistency of modeling clay. After it’s molded, often into sculptures or holiday ornaments, it can be baked hard, painted, and varnished. Bread dough contains much less salt, about one or two teaspoons per two cups of flour, but it still stiffens the dough and makes it harder to knead. Some bakers wait to add salt to their dough to minimize kneading time and power.

To a scientist, “salt” is a general term that applies to compounds of electrically charged particles called ions. I’ll write about ions and different types of salt in a future post, but in the kitchen, the salt we use to flavor food is sodium chloride. Sodium chloride is made of positively charged sodium ions and negatively charged chloride ions that stay together because their opposite electric charges attract. This type of association due to electric charge is called an ionic bond, and the ions arrange themselves into a lattice to produce the table salt we know. When salt mixes with water, the water molecules pull the ions away from each other so they separate and the salt dissolves. In a batter or dough, these ions are now free to interact with other molecules, including gluten proteins.

Remember how we talked about proteins as strings of amino acids with different chemical properties? Some amino acids are electrically charged and either repel or attract other charged particles. These particles might be other amino acids or just other molecules in the mixing bowl ocean. In an ocean of just flour and water, glutenin strands in the gluten maintain some distance from each other because like charges on their amino acids repel each other.

When we introduce salt to our dough, the amino acids on the glutenin attract the oppositely charged ion, either sodium or chloride. The sodium and chloride ions are much smaller than the amino acids, so several ions can squeeze in around an amino acid. The salt ions help to neutralize the charge of the amino acid, so the glutenin strands no longer repel each other as strongly and they come closer together. This tightening of the glutenin strands yields a strong gluten structure that has the strength to resist gravity and expand upwards instead of outwards during rises and baking.

Salt strengthens gluten. Ionic amino acids with like charges repel each other and keep glutenin strands apart (left side). Oppositely charged salt ions mask the charges of the amino acids. As a result, the amino acids repel each other less and the glutenin strands move closer together (right side).

Water hardness affects gluten strength for the same reason. Hard water contains ions (most notably calcium and magnesium) from dissolved salts that tighten gluten strands in the same way sodium chloride does. Ideally, salts provide the dough enough ions for strength, but not so many that it becomes tight and inflexible.

Decreasing protein interactions for less gluten

We strengthen interactions between glutenin and gliadin to develop gluten, but we can also isolate the proteins and prevent them from bonding. We can break down gluten that has already developed, too.

Adding fat to prevent gluten formation

Fat and water do not mix: if we combine oil and water in a glass, we see two distinct layers of liquid. This is due to a difference in polarity, which I will discuss in a future post. In fact, fat forms a barrier against water. I like putting mayonnaise or cheese directly on my sandwich bread, then piling the other ingredients on top to prevent the bread from getting soggy.

In the kitchen, we capitalize on this property of fats to keep baked goods tender. When we add fat to flour, we form a barrier between the flour proteins and the water. It’s like laying a tarp down on our beach of frozen tide pools before the tide rises again. If the glutenin and gliadin critters cannot reach water, they cannot bond with each other to form gluten.

Fat inhibits gluten formation. Fats coat gliadin and glutenin, preventing them from hydrating and from forming bonds to create gluten.

Depending on the recipe, we can either prevent gluten development by mixing the flour with fat first, or we can encourage a little gluten formation by mixing the flour with water, then adding butter. The photo at the top of the article shows six bowls of cookie dough containing the same amounts of sugar, butter, and flour. The only difference between the bowls is the amount of water added to the flour before combining it with the butter and sugar. The more water added to the flour, the more gluten the dough contained and the more cohesive the dough. (I’ll be sharing this experiment in greater detail soon!) In cakes, on the other hand, we mix the flour with the fat first to inhibit gluten formation. A typical cake recipe beats the butter, sugar, and eggs together, then alternates adding flour and milk into the butter mixture. Crucially, the recipe adds the flour to the butter before the milk. This allows the butter to grease the flour before the majority of the water (from the milk) reaches the flour, thus reducing gluten development in the cake. (I will also be sharing an experiment in cupcakes that illustrates this!)

On the other hand, gluten is important for rise and texture in brioche, a type of yeast bread that contains a lot of butter. The top Google results for “brioche bread recipe” all combine the flour with the liquid first, then add the butter. Adding the liquid to the flour first encourages gluten development before the butter has a chance to sequester the flour proteins. In Bakewise, Shirley Corriher calls the recipe that uses this method a “Light, Airy, Bread-Like Brioche.” She also presents a second preparation method with the same ingredients that combines the flour with butter first, then adds the liquid. This one is called “Buttery, Cake-Like Brioche.” Both are “The Ultimate Brioche.” Understanding how ingredients interact can help us modify recipes to our own tastes.

Shirley’s two brioche breads illustrate another important change in greased flour: fewer proteins can reach water molecules, so the flour absorbs less water. One of the few differences in the ingredients between the two brioches is the amount of water. In the cake-like brioche, where the flour is mixed with butter first, there is 25% less water than the bread-like brioche. Furthermore, because the cake-like brioche does not develop much gluten, the dough cannot hold as much air during rises, and the rise of the bread depends on water vaporizing in the oven, not on the yeast.

We can also choose the fat depending on the strength of the barrier we want to form between the flour and water. Oil greases flour more effectively than solid butter or shortening because it can flow and coat the proteins more thoroughly. Since the oil-coated proteins absorb less water, there is more water left in the batter, and we taste a moister cake. This is not the only reason oil cakes are more moist than butter ones, but I will explore those reasons in future posts about fat!

Adding sugar to prevent gluten formation

Sugar, like fat, is a tenderizer. In a batter or dough, sugar bonds with both glutenin and gliadin. When the two building blocks of gluten are entangled with sugar, they cannot interact with each other to form gluten. If we think about the critters in the tide pools again, it’s like the water brings a precious treasure to some critters as they thaw. The critters hug the treasure tightly, and even if it sees its tide pool neighbors floating by, it will not let go of the treasure to hug its neighbor. (Like Scrat from Ice Age when you give him an acorn.)

Sugar inhibits gluten formation. Sugar binds to both gliadin and glutenin, preventing them from bonding to each other and forming gluten.

Since sugar prevents gluten formation, we see very little sugar in yeast breads. A small amount feeds the yeast, but any more would prevent the necessary gluten network from forming. On the other hand, some tart crusts and shortbread cookie doughs are so crumbly that they have to be pressed into pans, a result of the high butter and sugar content preventing a cohesive gluten network from forming.

I once had too many sweet potatoes on hand, so I decided to make a sweet potato quick bread using a recipe I remembered would yield a moist, delicate loaf. This time around, I read in the comments that some readers had cut out one-third of the recipe’s total sugar content (from 1 1/2 cups to just 1 cup) with success. I thought, “Why not give it a try?” The resulting loaf was just as flavorful as I remembered, but the texture was dry and chewy. It was still delicious, but I was disappointed until I remembered I’d left out half a cup of sugar. With less sugar to bind to glutenin and gliadin, the proteins absorbed more water, bound to each other, and formed more gluten, resulting in a drier, chewier bread.

Omitting one-third of the sugar in this sweet potato quick bread resulted in a drier, chewier loaf that nonetheless tasted delicious.

Adding acid to break down the gluten network

Acids are another tenderizer that keep gluten sparse (though they have other more significant roles in baking, which I will explore in future posts). Chemists have a few definitions of acids, but for our purposes, we can use the definition of an acid as a source of positive hydrogen ions. Remember that ions, as we briefly discussed in the section on salts, are electrically charged particles. These ions form chemical bonds with basic (alkaline) molecules. (Fun fact: When we eat, hydrogen ions from acids trigger our taste receptors to tell our brains we taste something sour!)

Like salt, which we discussed earlier, acids interact with gluten at the level of the amino acid bonds. In addition to charged amino acids, proteins also contain acidic and basic amino acids, which typically bond with each other. When we increase the acidity of the protein’s environment, however, the extra hydrogen ions from the acid replace acidic amino acids in their bonds to basic amino acids. This disrupts the interactions that kept the gluten strands long, so the acid ultimately shortens the gluten strands and tenderizes the batter or dough.

Acids shorten gluten strands. Basic amino acids (B) bond with acidic or positive amino acids (A) (left). When acid (H+) is added, the hydrogen ion replaces the acidic or positive amino acid and bonds to the basic amino acid (right).

Note that an excess of alkaline molecules will have a similar effect: the alkaline molecules compete with basic (alkaline) amino acids in the protein to bond with acidic amino acids, thus weakening the gluten. As I will discuss more in a post about proteins, gluten (and all proteins) loses strength when the acidity of its environment is outside its optimal range. However, recipes typically use acidic ingredients, not alkaline ones, to tenderize.

Baking with gluten

In this post, I highlighted some of the most common strategies recipes use to control gluten. A deeper understanding of gluten and its interactions with other ingredients helps us to follow recipes more closely and to deliberately modify recipes to our kitchens and our tastes. Recipe developers adjust ingredient ratios until the recipe consistently yields the result they desire, but we may have a different ingredient on hand or prefer a different texture. I suggest following a recipe as written the first time for a baseline, but afterwards, the kitchen is your lab for experimentation. Be on the lookout for the next few posts, which will illustrate some of these ideas in baked goods!

References

Buehler, E. Enzymes: The Little Molecules that Bake Bread. Scientific American Blog, 2012.

Corriher, S. O. Bakewise; Scribner: New York, 2008.

Figoni, P. How Baking Works, 3rd ed.; John Wiley & Sons, Inc.: Hoboken, 2011.

Gardel, E. Materials Science in the Kitchen: Basic Bread Revisited. Science in the News, 2010.

Tuhumury, H. C. D.; Small, D. M.; Day, L. The effect of sodium chloride on gluten network formation and rheology. Journal of Cereal Science, 2014, 229-237.

Water. King Arthur Flour.

Wieser, H. Chemistry of gluten proteins. Food Microbiology, 2007, 24, 115-119.



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