Guided Inquiry Activity #27
Bread Model 1: Wheat flour is ~70-­‐80% starch and 7-­‐15% protein. Surprisingly, it is that relatively small percentage of protein that makes it possible for wheat flour to turn into bread. Differences in wheat type, growing climate and soil conditions impact the amount of protein present within the wheat grains. Spring-­‐sown wheat varieties tend to have higher protein content and are often described as ‘hard’ flours. A ‘soft’ flour is usually obtained from winter wheat, which produces flours with a protein content of less than 10%. Table 27.1. Protein content for types of wheat flour1 Type of flour Grams of protein per cup Percent protein Semolina (Durum wheat) 21g 13% Wheat flours, bread, unenriched 16g 12% Wheat flour, white, all-­‐purpose 13g 10% Wheat flour, white, cake, enriched 11g 8% Over 75% of the protein in wheat flour is comprised of two types of proteins: glutenin and gliadin. When mixed with water and mechanically worked or mixed, these two proteins form a dense elastic matrix known as gluten (Fig 27.1). Wheat flour protein is essential for forming the elastic protein matrix that traps air bubbles and allows dough to rise…without the protein, there would be NO bread (see Model 3 for a lesson on “rising”)! Gluten also provides the structure and chewiness to risen breads. So, what is gluten? Gluten is a matrix of glutenin and gliadin proteins formed from the addition of water to wheat flour. Gliadins are smaller, tightly coiled proteins that fold in a compact, spherical three-­‐dimensional structure. Glutenin proteins form looser spirals that are stabilized by hydrogen bonds between the amino acids of the protein (Fig 27.2). When you have dry flour or a ‘just-­‐combined’ flour/water mixture, the gliadin and glutenin proteins are in isolated, disorganized clumps. However, once a flour and water mixture is worked -­‐ through stirring, mixing or kneading -­‐ the gliadin and glutenin proteins begin to interact with one another via non-­‐covalent hydrogen bonding and 1
Nutrition data is from the United States Department of Agriculture, Agricultural Research Service, National Nutrient Database for Standard Reference Release 27. http://ndb.nal.usda.gov/ Copyright © 2016 Wiley, Inc. Page 1 Guided Inquiry Activity #27
some covalent bonds (discussed below). The compact gliadins integrate between the spirals as the mixture is kneaded – the mechanical stretching helping to organize the glutenin spirals into longer chains. Figure 27.1. A cartoon of gliadin and glutenin proteins in wheat forming gluten. Pages Not Included in Sample Copyright © 2016 Wiley, Inc. Page 2 Guided Inquiry Activity #27
1. In order to form gluten, the glutenin and gliadin proteins must absorb water. a) A roux is the base of a flour thickened sauce or gravy. In a roux, dry flour is first cooked in fat – how would this affect the formation of gluten? b) “Cutting” or “blending” flour with fat is the first step in making dough for light and tender pastries. Why is this step advantageous when making a light and tender baked good that easily crumbles in your mouth? Copyright © 2016 Wiley, Inc. Page 3 Disulfide bonds can form between glutenin protein molecules. Together hydrogen bonds and disulfide bonds make long chain of glutenin molecules linked end to end. This network of linked and coiled protein is the matrix we call gluten. Figure 27.3. Disulfide bonds between peptides (a) link glutenin chains as shown in the cartoon in part (b) Page 4 Copyright © 2016 Wiley, Inc. Guided Inquiry Activity #27
Model 2. Oxidation of protein chains can result in a new covalent bond between two cysteine residues within a single protein or between two proteins. Guided Inquiry Activity #27
2. When a disulfide bond forms, the new covalent bond is between what two atoms? Does that explain why this bond is called a disulfide bond? Please elaborate. 3. Using your knowledge of covalent bonds (vs. non-­‐covalent bonds), please explain why forming disulfide bonds between glutenin proteins helps to make a strong gluten matrix. 4. Flour that is aged -­‐ literally, the flour is older -­‐ makes better bread. Considering that aged flour has sat out and been exposed to the air (and oxygen in the air…) for longer. How might that explain the stronger gluten matrix which forms when using aged flour. Copyright © 2016 Wiley, Inc. Page 5 Model 3. A typical bread recipe Ingredients • 1 ¼ cups water (70 to 80°F) • 2 teaspoons sugar • 1 teaspoon salt • 3 ½ cups bread flour • 1 ½ teaspoons active dry yeast • 1 tablespoon cornmeal Guided Inquiry Activity #27
What are yeast? And why are they in this recipe? Figure 27.4. The overall breakdown of starch into sugar, then ethanol and CO2 by yeast. Risen dough is like a gluten balloon filled with molecules of gas. The “balloon” is filled carbon dioxide generated by the yeast, as well as air that was introduced into the dough during mixing and kneading. The dough inflates while on the kitchen counter, but it inflates even more when it gets in the oven! Why is this? Scientific law tells us that gases Copyright © 2016 Wiley, Inc. Page 6 Guided Inquiry Activity #27
expand when heated; this is known as the ideal gas law. When the dough is placed in the oven, the gas pockets expand, and the dough rises. In addition to the expansion of the CO2 and air gas molecules, at about 140°F/60°C, more gas is generated in the rising bread due to vaporization of the yeast-­‐created ethanol and water into more gas; this expands the dough by as much as half of the initial dough volume. Table 27.2. Melting and boiling points for glucose, ethanol and carbon dioxide
C6H12O6 (the sugar glucose) C2H5OH (ethanol) CO2 (carbon dioxide) Temperature at which the substance melts (from a solid to a liquid) 332°F/155°C -­‐173°F/-­‐114°C -­‐109°F/-­‐78.5°C Temperature at which the substance boils (goes from a liquid to a gas) -­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐-­‐ 172°F/78°C -­‐70 °F/ -­‐57 °C Yeast prefer warm temperatures to grow (95°F/35°C). Yeast start dying at ~120°F/49°C and are dead by 140°F/60°C. 7. Yeast is mixed into bread dough to convert sugars to ethanol and carbon dioxide. Bread bakes at ~350°F (~175°C). Now consider Table 27.1 and the questions below: • At the temperature of baking bread, what happens to the yeast? • At the temperature of baking bread, what happened to the ethanol? To the carbon dioxide? • How might the answer to the previous questions explain the “rising” of dough? What is making the dough rise? Use the phrase physical state in your answer. Pages Not Included in Sample Copyright © 2016 Wiley, Inc. Page 7 Guided Inquiry Activity #27
Putting it all together 5. In the gluten matrix, hydrogen bonds are holding the protein spirals together and disulfide bonds are linking them end to end (like a telephone cord), but also hydrogen bonds can hold neighboring linked spirals together. Draw on the cartoon below to indicate these interconnecting hydrogen bonds. Copyright © 2016 Wiley, Inc. Page 8 Guided Inquiry Activity #27
6. Dough that is left to rise in the refrigerator will take much longer (8-­‐12 hours), compared to dough that is left to rise in a warm kitchen. Why is this? 7. An allergy to the protein gluten is the cause of the symptoms of Celiac Disease. According to Celiac.org, Celiac symptoms include abdominal cramping, intestinal gas, distention and bloating of the stomach, chronic diarrhea or constipation (or both), anemia, unexplained weight loss with large appetite or weight gain2. Recent improvements in diagnosis of Celiac disease have led to an increased awareness of the disease, and a response from food manufacturers. Celiacs can now find a variety of “gluten-­‐free” products in the regular grocery store. In order to be “gluten-­‐free” what must be true about the product? Use the words protein, primary sequence, glutenin and gliadin in your answer. 2
www.celiac.org Copyright © 2016 Wiley, Inc. Page 9