Our Daily Bread — Part 3
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Our Daily Bread — Part 3

Author: Klaus Roth

A host of regional bread specialties has long marked the German-speaking realm as a sort of bread paradise, one truly unique in the world. Let us therefore pursue creation of a loaf of German bread, from rye and wheat field all the way to the table, taking time as we go to note perhaps with some surprise the numerous chemical processes that unfold before us.

9. The Chemistry of Baking

9.1 Wheat Bread

50 °C
At the start of the baking process, yeast cells, viable up to 50 °C, multiply first through aerobic and then anaerobic degradation of maltose, leading to the formation of carbon dioxide and water or ethanol. Yeast cells cleave maltose initially with their own maltase into two glucose molecules, which are then degraded in the usual way via glycolysis.


65 °C
Beginning at ca. 65 °C, things really get underway in the dough—from a chemical standpoint—since this is when the heart of the bread, the crumb, starts to form. The gluten proteins, readily deformable at lower temperatures, have created, as a consequence of the expansion of tiny carbon dioxide bubbles, an extensive three-dimensional framework in which grains of starch are embedded (Part 2, Section 4.1). Around 65 °C the gluten proteins are denatured, thereby losing their plasticity. A plastic dough is thus transformed into an elastic one.

Not only the gluten proteins, but also the protein skins surrounding the starch grains, are denatured at ca. 65 °C, suddenly becoming permeable to water. Water released by the gluten proteins is completely absorbed by starch grains, causing the starch to swell. This water transfer in itself is impressive, since a dough that looks to us moist, feels completely dry after baking, even though, considering the high temperature, the loss of water during baking is rather insignificant (roughly 10 %). In other words, the crumb in freshly baked bread contains almost as much water as raw bread dough. Most of that water remains tightly bound: to gluten proteins in the original dough, and to swollen starch in the resulting bread [9]. Starch grains increase in volume by ca. 40 % through water absorption, causing the surrounding protein skins to burst. This swollen starch is now attacked by β-amylases (optimum temperature: 65 °C), and cleaved into large fragments (dextrins). At the same time, the α-amylases attack (optimum temperature: 50 °C) these fragments from their chain ends, to form maltose (Fig. 10) [10].

Relationship between amylases and starch in the production of bread

Figure 10. Decomposition of starch by α- and β-amylase. α-Amylase splits the starch molecule in the middle. β-Amylase cuts off maltose units from the end of the starch molecule.

Due to the high pressure exerted by the swelling starch grains, all crystalline starch structure is destroyed, and the starch becomes gelatinized with its own decomposition products. In this manner the desired solid crumb forms. How sensitive the choreography is for the various chemical processes occurring here in parallel is clearly illustrated by breads that have failed (Fig. 9).

Bread mistakes, and their causes

Figure 9. Bread mistakes, and their causes. © UNIFERM GmbH & Co. KG, Werne, Germany.

A) Enzyme-deficient wheat flour: too little starch is decomposed in the course of baking, so the dough is too firm, and the crumb leaves an impression when chewed of being too firm and dry. Moreover, it tastes bland and doesn’t seem fresh.

B) Wheat flour rich in enzymes: Too much decomposition of starch causes the dough to be too soft, and the bread shows too little cohesion. Due to insufficient capacity for gas retention by the soft dough, the pores are very irregular, and it is difficult to apply a spread to the slices.

C) Enzyme-deficient rye flour: With rye flour, if a sufficient supply of enzymes is lacking, or the dough is too acidic, very little starch is decomposed. The dough becomes too firm, and the resulting bread seems soggy and offers little evidence of freshness.

D) Rye flour rich in enzymes: If too much starch is decomposed due either to an excess of enzymes in the rye flour or too little acidification, too much carbon dioxide is released in a dough that is too soft. Cavities form in the bread, and the bread itself lacks elasticity and tends toward formation of soggy masses.

A well-baked bread has unique material characteristics: Such a bread does not lend itself to shape distortion, but is supple, and elastic when subjected to pressure. The crumb is not elastic under tension, and thus readily chewable. It is firm, and lends itself readily to cutting; such a slice of bread can easily be buttered or covered with an alternative spread. These material characteristics may appear here to be overemphasized, but this would be a false judgment, since it is essential that a bread fulfills its function. Above all, this means that one can cut off a single slice, the slice can be buttered, and a piece can then be bitten off and chewed.

110 °C
For many epicures, the crust of the bread, with its seductive aroma, is the best part of all. Due to a low water content, here only minimal gelatinization of starch grains has transpired. At temperatures above 110 °C there occurs a non-enzymatic cascade of browning steps on the upper surface of the crust (the Maillard reaction [11]), involving the degradation products of proteins reacting with a wide variety of saccharides. Above all, arabinose and the maltose derived from α-amylase contribute to an intensive browning, with formation of important aroma-producing substances.

140 °C
Caramelization of the sugars present begins at 140 °C, followed by typical “roasting reactions” above 150 °C. Baking leads to literally hundreds of aromatic substances (Fig.11), though only a few are crucial to the typical aroma of bread’s crumb and crust.

Figure 11. The most important aromatic substances associated with white bread. The first six compounds are characteristic for the aroma of white bread crust. White bread crumb, however, smells of methional, (E)-2-nonenal, and the three last compounds [12].

9.2 Rye Bread

In the course of baking a rye bread, different chemical processes are involved than in the case of wheat bread. Rye flour contains very few gluten proteins, but instead many pentosans. The factors that are decisive in differences in the production process for rye bread as opposed to wheat bread lie in the fact that the starch grains in rye already begin to swell above 55 °C; i.e., 10 °C lower than in the case of wheat [13]. This actually has fatal consequences for the dough, because α-amylase has its maximum catalytic activity at 65 °C, and after swelling the starch finds itself defenseless with respect to α-amylase, with the result that it is severely decomposed. Moreover, rye contains much more of this enzyme than does wheat. Were one not to intervene somehow, the starch would be essentially completely decomposed, and a resulting rye bread would be greasy, nonelastic, and no longer enjoyable (Fig. 9, D).

The only way around this dough-disaster is necessarily chemical in nature. Thus, the pH of the dough must be reduced to the point (<5) that the activity of the α-amylase is choked off. This is especially true in moist harvest years, in which the rye obtained is especially rich in enzymes. The necessary acidification can be achieved with a nutritionally harmless acidulant such as lactic or citric acid, or through addition of sourdough. Europeans should not be dismayed, by the way, to find that a bread is suddenly reported to contain “E 270” or “E 330”: these are the official designations for lactic acid and citric acid, respectively, which must be indicated in the European Union or Switzerland whenever they are added to a food item. Both substances are also present naturally in milk and lemons, but in this case they of course need not be reported, since they were there originally, and thus would not be classed as “additives”. The situation is especially curious, by the way, when it comes to ordinary salt. Salt is not naturally present in flour, so must therefore be added, but it still need not be declared, presumably because it has for centuries been added to improve the flavor of bread dough.

Acidifying dough also improves the degree of swelling of the proteins. Moreover, at low pH values and higher temperatures, arabinose molecules are cleaved hydrolytically from pentosan chains; the resulting insoluble pentosans contribute significantly to formation of a stable crumb. The pentosans bind water much more strongly than do the swollen starch grains in wheat flour. This is the reason why rye breads stay fresh for much longer, and dry out much more slowly than wheat breads.

10. The Daily Bread, As We Understand It Today

For the baking of bread, the only true necessities are flour, water, salt, and yeast or sourdough. Our ancestors made bread from these ingredients alone, so why do we add other things to the dough? The answer is simple: we consumers today insist on a tasty bread of a consistent, highest quality, always available freshly baked, but at the same time as inexpensive as possible.

What the consumer forgets in the process, however, is that grain quality sits at the very origin of the bread, and this in turn is heavily dependent upon the weather during growth and harvest. Other factors are the soil condition, fertilization, variety of grain, etc. Several of these parameters cannot even be controlled. Therefore, every grain shipment arriving at the mill will necessarily have a different composition, leading to different baking characteristics. But as consumers we insist that our bread always be the same, coming from the baker’s oven with consistent, high quality. This will be assured only if the miller and the baker are permitted to add special ingredients [14] that will compensate for deficits caused by varying properties of the raw materials. Due to their importance, these particular additives are among the most thoroughly studied of all food additives from a technical and toxicological point of view. In what follows we offer a few examples of the nature of such supplements.

10.1 Bread Improvers

Malt is one of the most ancient of the bread improvers. It is made from either barley or wheat. The grain is moistened, and germination is interrupted after a few days by drying.

Malt is rich in amylases, and is introduced into wheat flour displaying a shortage of enzymes. This ensures that starches will be subjected to more extensive enzymatic degradation. Maltose present in the malt provides a nutritional basis for the yeast cells, to support their growth, causing a loosening of the dough. In addition, maltose is degraded during baking to give desired aroma-producing agents, and is caramelized on the surface as an aid to browning of the crust.

10.2 (l)-Ascorbic Acid (3)

Kernels of grain contain no ascorbic acid (vitamin C) [15], so adding it to bread dough would appear to lead to a welcome improvement to a basic foodstuff. In truth, the story is a bit more complicated, because vitamin C serves only to improve the quality of the bread, it being completely destroyed thermally during the baking process.

The corresponding improvement in the dough is a consequence of intervention of ascorbic acid in the redox chemistry of disulfide bridges. In the course of flour maturation, as well as during the kneading of dough, free SH groups in cysteine residues of the gluten proteins are oxidatively joined via disulfide bridges.

This cross-linking stabilizes the glutens, makes them more elastic and ductile, and better maintains the gas pressure from the little carbon dioxide bubbles. This in turn increases the baking volume (Part 2, Section 5.1).

Wheat flour may contain as much as 60 mg/kg of natural glutathione (1), which has a negative influence on the baking characteristics of the flour. This is because glutathione interferes with the formation of disulfide bridges, in that in the presence of oxygen it reacts with the HS-Cys bonds of the gluten protein chains.

As a consequence, cross-linking of the gluten proteins is inhibited, gluten adhesion remains weak, and gas bubbles escape. Doughs of this sort lead only to low-volume, flat baked goods, which does not exactly correspond to our vision of a plump, round loaf of bread!

The use of ascorbic acid as a baking agent is derived from an accidental discovery by the Danish chemist Holger Jörgensen [16]. The effect came as a complete surprise, since until then, comparable improvements could be achieved only with strong oxidizing agents, like potassium bromate, persulfate, or periodate. Today, addition of the latter compounds is forbidden throughout the EU.

Ascorbic acid, on the other hand is a strong reducing agent! The reductive properties of vitamin C are utilized quite commonly in food technology, as for example in the preservation of apple juice [17].

The apparent contradiction was ultimately clarified, however [18–20]. Ascorbic acid (3) added to dough undergoes reaction during the kneading process, in which it is oxidized within minutes by embedded oxygen to give dehydroascorbic acid (4).

To be precise, it is this oxidation product that improves the flour characteristics, not ascorbic acid itself. Clarification of the mechanism of action for the ascorbic acid can be described as a stereochemical gem.

Initially, within a few minutes of kneading, the (l)-ascorbic acid (3) is oxidized, as noted above, to dehydroascorbic acid (4) by oxygen already embedded in the dough. This reaction is catalyzed by ascorbic acid oxidase. Next, glutathione (1) naturally present in dough is transformed into oxidized glutathione (2), whereby the oxidizing agent is the dehydroascorbic acid, and the reaction itself is catalyzed by glutathione dehydrogenase. As a consequence, the glutathione is no longer in a position to inhibit cross-linking of cysteine-SH bonds in the gluten proteins.

The three stereoisomers on the ascorbic acid, the unnatural (d)-ascorbic acid and the enantiomeric iso-ascorbic acid show no or only slight improvement of the bread.

Figure 12. Bread baked with different amounts of (l)-ascorbic acid. Left: No ascorbic acid; middle: 20 mg; right: 60 mg.

Overall, preparation of a bread dough and its subsequent baking amounts to the occurrence of innumerable sequential chemical reactions. The severance and reconnection of disulfide bridges represents a series of redox reactions; decomposition of starch molecules and pentosans corresponds to hydrolysis; alternating bonding of water to gluten proteins and to starches is based on the formation of hydrogen bonds; and, finally, formation of aromatic components in the crumb and crust is the consequence of thermally induced reactions between sugars and proteins.

With a freshly baked and fragrant loaf of bread we have before us not only a healthy treat, but also a lovely piece of chemistry! Let us resolve to enjoy both.

Acknowledgements
I wish to express my gratitude to the Verband Deutscher Mühlen and the Verband der Backmittel- und Backgrundstoffhersteller for their invitation to the seminar series “Mythos und Wissenschaft – Getreideverarbeitung im Spannungsfeld“ (“Myth and Science – Grain Processing Between Conflicting Priorities”). I wish also to thank the following colleagues for their help in researching and processing information from this complicated field of food chemistry.

References

[9] S. Ablett et al., in Chemistry and Physics of Baking (Eds. J. M. V. Blanshard, P. J. Frazier, T. Galliard), The Royal Society of Chemistry, London, UK, 1986. ISBN: 978-0851869957
[10] B. Meyer, PdN-BioS 2007, 56, 10. Link
[11] M. Angrick, D. Rewicki, Chem. Unserer Zeit 1980, 14, 149. DOI: 10.1002/ciuz.19800140503
[12] From a lecture by P. Köhler “Chemistry of Bread “, 2005, Kulmbach, Germany.
[13] B. Meyer, PdN-BioS 2007, 56, 4. Link
[14] B. Meyer, Getreidetechnologie 2006, 20, 1.
[15] The Composition of Foods, B. Holland et al., The Royal Society of Chemistry, Cambridge, UK, 1992.
[16] H. Jörgensen, Biochem. Z. 1935, 280, 1, and ibid., 1935, 283, 134.
[17] A. Deifel, Chem. Unserer Zeit 1993, 27, 198. DOI: 10.1002/ciuz.19930270405
[18] P. Maltha, Getreide&Mehl 1953, 3, 65.
[19] W. Grosch, Getreide, Mehl und Brot 1975, 29, 273.
[20] P. Köhler, PdN-BioS 2007, 56, 8. Link

Prof. Klaus Roth

Freie Universität Berlin, Germany.

The article has been published in German in:

and was translated by W. E. Russey.

Our Daily Bread — Part 1

Transformation of ripe ears of grain into a fragrant, aromatic bread borders on the miraculous and behind such a miracle lies chemistry

Our Daily Bread — Part 2

Fragrant, aromatic bread, whether its wheat- or rye-based, undergoes a host of chemical reactions during the kneading of the dough


Other articles by Klaus Roth published by ChemViews magazine:

 

See all articles published by Klaus Roth in ChemViews Magazine

 

 

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