What is Gluten?

Bread is traditionally made using 100% wheat flour. This is because wheat flour contains gliadin and glutenin proteins that work together to impart a unique visco-elastic property when wheat flour is mixed with water in a 3:1 ratio. The combination of these two proteins is what we refer to as gluten. So, I guess if you want to be technical, wheat does not really have gluten. What it has are proteins that have the potential to produce gluten.

Gliadins Versus Glutenins

Gliadins are globular (round) single-chained polypeptides with a molecular weight ranging between 30000 – 80000 Da. In contrast, glutenins are polymeric proteins having up to 19 protein subunits. Their molecular weight ranges from 12000 – 130000 Da (Damodaran, 2008). You may imagine glutenins as long ropes and gliadins as tiny ball bearings that the glutenins “glide” on. The gluten structure that they form is both strong and elastic. This is important in bread fermentation and baking since the dough must stretch and hold air bubbles without collapsing on itself.

Image from: elisilvernd.com

What Makes Gluten So Strong?

The nature of the internal bonds within the gluten network is important in explaining its behavior. Gliadins and glutenins are connected by inter-chain disulfide bonds due to the presence of cysteine amino acids. Although cysteine concentration is small in gluten (2%/mol of amino acids), they are responsible for forming very strong disulfide bonds (Wieser, 2006).  

Image source: bio.miami.edu

Secondly, gluten has a very high concentration of glutamines, making up 35% of its total amino acids. Glutamine forms numerous inter-chain hydrogen bonds with other glutamine and hydroxyl amino acids (serine and tyrosine). These individual bonds are weak, but in combination they are very strong, contributing to the high cohesiveness of gluten (Delcour and Hoseney, 2010; Kieffer, 2006).

Interchain hydrogen bonding between glutamine residues (Kieffer)

Thirdly, gluten is high in proline, an amino acid that makes up 14% of its total amino acids. Proline creates kinks (bends) in the gluten protein chain, contributing to increased folding and elasticity (Delcour and Hoseney, 2010).

Finally, gluten proteins are generally very hydrophobic. This is because 35% of its amino acids are of the hydrophobic type (glycine, alanine, valine, leucine, isoleucine, phenylalanine, methionine, lysine, and tryptophan). The numerous hydrophobic interactions result in strong cohesion in the dough (Delcour and Hoseney, 2010).    

How to Substitute Gluten in Bread

Given gluten’s unique chemistry, it is difficult to achieve the same dough functionality when wheat flour is substituted for non-gluten flours. However non-gluten flours may improve nutrition by increasing dietary fiber, protein, and micronutrients. They can also reduce the cost of wheat imports in countries that do not grow wheat. Non-gluten flours that have been used in bread making include cereals (corn, rice, sorghum, millet, barley), pseudo-cereals (amaranth, buckwheat, quinoa), tubers (yam, cassava, potato), oilseeds (coconut, flax, sunflower, peanut), legumes (beans, peas and lentils), and fruit crops (plantain, banana, breadfruit) (Houben et al, 2012).

Depending on how much is substituted, gluten development may be poor and hence the dough unable to effectively retain fermentation gases. Changes such as reduction in oven-spring, reduction in bread volume, denser crumb, and lower consumer acceptability should be expected.

Dobraszyzyk (2001) indicated that bread of acceptable quality can only be made with wheat flour containing 11% protein. Therefore, you should aim to have at least 11% wheat proteins in your composite bread flour mixes. If you know the protein content of your wheat flour, you can work out the proportion of wheat flour to non-wheat flour that you will need by using either a mass balance equation or Pearson Square. Here is a scenario.   

A baker wants to make a 100 kg batch of composite flour consisting of wheat flour (14% protein) and potato flour. How much potato flour must he add to the bread flour to get 11% wheat protein in the final batch?

 A. Mass Balance Method

Step 1: Draw the mass balance diagram showing protein input and final output

Step 2: Write the mass balance equation: x + y = z

Step 3: Determine equation for y (potato flour): y = z – x

Step 4: Write the protein balance equation: 0.14 (x) + 0 (y) = 0.11(100)

Step 5: Solve for x (wheat flour): 0.14 (x) = 0.11(100); x = 11/0.14 =  78.6 kg

Step 6: Calculate weight of potato flour: y = z – x; y = 100 – 78.6; y = 21.4 kg

Therefore you will need to add 21.4 kg of potato flour to 78.6 kg of bread flour to make the composite bread without significantly compromising overall quality.

B. Pearson Square Method

Step 1: Draw the Pearson Square as shown below. Put the target protein content in the center of the square and the protein content of the wheat flour and the potato flour at the top left and bottom left of the square. 

Step 2: Subtract diagonally to get a positive number on opposite sides: 14 – 11 = 3; 11 – 0 = 11

This means that in order to finish with a final percentage of 11%, you must add 3 parts potato flour to 11 parts wheat flour (3:11). 

Step 3: Add the ratios: 3 + 11 = 14

Step 4: Divide the weight of the composite flour (100 kg) by 14 and multiply by the corresponding ratio: 

  • (100 kg/14) x 3 = 21.4 kg potato flour
  • (100 kg/14) x 11 = 78.6 kg wheat flour

Hence, you will need to add 21.4 kg of potato flour to 78.6 kg of bread flour to make the composite bread without significantly compromising overall quality. 

Challenges to Consider in Gluten-Free Baking

Substituting only part of the gluten content in bread and other bakery products is not enough to meet the needs of people suffering from gluten intolerance or celiac disease. They must avoid gluten-containing products all together such as wheat in all its forms (wheat varieties, durum, spelt, and kamut), barley, triticale, and rye.

Failure to avoid these grains will result in gastrointestinal problems for the gluten-intolerant e.g. diarrhea, abdominal pain, and bloating. For individuals with celiac disease, there may be additional complications such as malnutrition, weight loss, slow growth, and fatigue. This is because the immune system of these individuals attack the lining of their small intestine where nutrients are absorbed.  

Preparing 100% gluten-free products, which is necessary to avoid these health problems presents some challenges. For example, the lack of visco-elasticity in gluten-free flours creates a pasty and sticky batter with low gas-retention and low water-binding properties. Therefore you generally end up with bread having a low volume and a dry crumb. Another problem is that gluten-free products usually have high starch content causing them to stale faster. When it comes to eating quality, gluten-free products lack the level of satisfaction that is generally experienced from eating wheat-based products.

Of note also is that many gluten-free bakery products are less nutritious than perceived to be. They tend to have lower protein content and protein quality, high saturated fats, and lower micronutrient content including lack of B-vitamins, iron, and folate when compared to wheat-based products. They have also been found to have a higher glycemic index compared to wheat-based products (Naqash et al. 2017). Therefore, choosing gluten-free may be done out of necessity to avoid the pains of gluten-sensitivity. However, if you have no gluten sensitivity, you are unlikely to see any real benefit from going gluten-free. 

Currently, various additives are used to enhance the quality of gluten-free bakery products including starch, hydrocolloids (xanthan gum, guar gum, alginates, agar, and carboxymethylcellulose), protein cross-linking enzymes, milk, egg, surimi (fish protein), and legumes. No single additive seems to do the trick. Rather, more success can be obtained from using a combination of additives. Improvements may include: increased gelation and viscosity; softening of crumb; improved foaming capacity; increased gas-retention; increased water-binding; decreased stickiness; improved dough strength and crumb firmness; and improved nutrition (Houben et al., 2012).

In one of my research projects, I used texturized high protein flour (THPF) from pinto beans (44% protein) to determine the effect on bread and dough functionality. The addition of 5% THPF was found to be optimal. It resulted in a significant increase in both protein content and protein quality, and increased water retention leading to significantly higher product yield. These benefits came with no significant loss in bread volume (Simons et al., 2014). There’s a lot more work to be done to improve both the quality and nutritional aspects of gluten-free products. This no doubt provides an interesting and potentially lucrative opportunity for bakery scientists.

References

  1. Damodaran, S. (2008). Amino acids, peptides and proteins, In: Fennema’s Food Chemistry (4th edition). S. Damodaran, K. L. Parkin & O. R. Fennema, Boca Raton, (Eds.), 217 -330, CRC Press.
  2. Delcour, J. A., & Hoseney, RC. (2010). Principles of cereal science and technology. AACC International, St. Paul, MN.
  3. Dobraszcyk, B. J. 2001. Wheat flour, In: Cereals and cereal products chemistry and technology. D. A. V. Dendy & B. J. Dobraszcyk (Eds.), 100 – 124. Aspen Publishers Inc. New Dehli. 
  4. Houben, A., Höchstötter, A., & Becker, T. (2012). Possibilities to increase the quality in gluten-free bread production: an overview. European Food Research & Technology235(2), 195-208. 
  5. Kieffer, R. (2006). The role of gluten elasticity in the baking quality of wheat, In: Future of flour – A compendium of flour improvement, L. Popper, W. Schäfer & W. Freund, (Eds.), 169-178, Verlag Agrimedia, Clenze, Germany.
  6. Naqash, F., Gani, A., Gani, A., & Masoodi, F. (2017). Review: Gluten-free baking: Combating the challenges – A review. Trends In Food Science & Technology6698-107. 
  7. Nwanekezi, E. (2013). Composite Flours for Baked Products and Possible Challenges – A Review. Nigerian Food Journal318-17.
  8. Simons, C. W., Hunt-Schmidt, E., Simsek, S., Hall, C., & Biswas, A. (2014). Texturized Pinto Bean Protein Fortification in Straight Dough Bread Formulation. Plant Foods For Human Nutrition69(3), 235-240.
  9. Wieser, H. (2007). Chemistry of gluten proteins. Food Microbiology24(3rd International Symposium on Sourdough), 115-119.

Author

  • Courtney Simons

    Dr. Courtney Simons has served as a food science researcher and educator for over a decade. He holds a Bachelor of Science in Food Science and a Ph.D. in Cereal Science from North Dakota State University.