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Bread chemistry

The usually quiet world of breadhas been disturbed recently byboth economic and technologicalchanges. Wild fluctuations in theprice of wheat in the past twoyears have made life difficult forthe milling and baking industriesand, at one point, the percentageincrease in price of bread in the UKwas greater than at any time sincederegulation in the 1950s. The creditcrunch has pushed people towardsbread as a replacement for moreexpensive foods in the UK, theannual quantity of bread sold up to

April 2009 showed the first increasefor 35 years.

The traditional English preferencefor white bread has led to at leasttwo centuries of the use of additives,or adulterants, depending on yourpoint of view. Initially this was tocheapen bread, enabling it to appearwhite in spite of the inclusion ofpotatoes, oats or worse. In morerecent decades, the goal has beena soft, springy and open crumbstructure, even with the non-ideal,protein-poor wheat that is grown inEurope.

Protein is a key constituent ofthe wheat flour used for breadproduction. An essential componentof the baked breads final structure,it is a favourite target for scientistslooking to intervene in the bakingprocess. Flour used for most modernbread-making in the UK contains atleast 12 per cent protein, almost allof this the so-called gluten proteinsthat would provide nitrogen for thegrowth of the germinating wheatseedling.

As the baker mixes flour withwater at the start of the bread-

making process, these proteinshydrate to form gluten, a viscoelasticmatrix holding the starch granulesthat constitute the bulk of flour.Gluten is not a single molecularprotein species but a mixture ofwater-insoluble proteins fallinginto two broad classes, the gliadinsand the glutenins.1,2 A typicalwheat variety may have as manyas 40 different gliadins, whosemolecular weights fall between28 000 and 55 000. In contrast, theglutenins are multi-protein macro-polymers whose overall molecular

On the riseThe ancient tradition of bread baking depends on a cascadeof chemical reactions. As scientists have unravelled thiscomplex chemistry, they have also found myriad ways tomodify the process, say Bryan Reuben and Tom Coultate

In short

A series of chemicalreactions take placethroughout the processof turning flour intobreadBread owes much ofits structure to glutenproteins a key targetfor chemists looking tointervene in the processTraditional breadmaking is a slow process,prompting scientists to

develop no time doughsVarious additives cankeep bread from goingstale by helping retain itsmoisture

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weights may exceed 2 million. Astheir insolubility suggests, all thegluten proteins are dominatedby hydrophobic amino acids,particularly glutamine. This amide-bearing amino acid has a strongtendency to form hydrogen bondsbetween protein strands a majorfactor in the physical structure andbehaviour of gluten. In addition,the glutenin protein chains of thesubunits contain thiol groups fromthe amino acid cysteine, whichform disulfide bridges that hold theglutenin macro-polymer and thegluten complex together.2 Recentresearch suggests that cross links arealso formed between the tyrosineresidues in the gluten. Like thedisulfide bridges, their formation iscatalysed by oxidising agents.3,4,5

Dough in no timeThe hydration of flour to give

dough is not just a simple mixingprocess; much mechanical work isalso needed, initially applied by themixer and then by expanding CO2bubbles within the dough. Thesebubbles form as yeast ferment thesugars liberated from hydratedstarch granules by the flours naturalcomplement of amylase enzymes.Mechanical work stretches thegluten into sheets that trap the CO2 as the dough expands, the glutenmolecules uncoil and reveal far moreinter-molecular hydrogen bondingopportunities, and the dough

becomes stronger.For the dough to stretch, thedisulfide bridges holding theglutenin components togethermust repeatedly cleave andreform as the protein chains slideover each other. These exchangereactions require the participationof oxidisingreducing systems,whose components occur naturallyin flour. This is the point at whichthe chemists have, wittingly orunwittingly, often intervened.

The traditional breadmakingprocess has a serious drawback.

After mixing, the dough must be leftto prove for at least three hours forthe bubble expansion to develop thegluten. The dough is then knockedback to remove most of the CO2 aprocess usually combined withscaling (ie getting the dough into theright size chunks) and moulding tofit the tin before being allowed toprove a second time before it goesinto the oven. Although this longfermentation develops flavour, it iscostly in time, with bakers having torise early in the morning to producefresh bread for the day. In addition,

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the mounds of fermenting doughtake up expensive floor space andpresent hygiene problems. Researchefforts were therefore directed todeveloping a no time dough: byincreasing the amount of yeast; bymixing vigorously to increase therate of bubble formation; and byadding oxidising agents to promotedisulfide bond formation.

Azodicarbonamide, potassiumiodate and potassium bromatehave been used as oxidising agents,bromate being the most cost-effective. It was preferred in theDo-Maker process (developed inthe US) and in early versions of theChorleywood bread process (CBP),which was developed in the UKby the Flour Milling and BakingResearch Association in the early1960s and which featured powerfulmechanical stirring to incorporatebubbles. Even though bromate was

permitted at the time, and was inany case decomposed to bromide atbaking temperatures, it was undersuspicion as a carcinogen and wasbanned in the UK in 1990, althoughnot in the US. Bromates ban wasfacilitated by the development ofbread-making processes dependanton the use of ascorbic acid (vitaminC). Ascorbic acid is, of course, areducing agent but, during doughmixing, the enzyme ascorbic acidoxidase, naturally present in flour,catalyses its conversion to itsoxidising form, dehydroascorbic

acid (DHAA). In a reaction catalysedby another flour enzyme, DHAAconverts glutathione (GSH), atripeptide naturally found in wheatflour, to its dimer (GSSG). GSH, butnot GSSG, can form disulfide bonds so removing GSH prevents it fromdisrupting the disulfide crosslinkingthat otherwise form between glutenproteins.

The CBP, which still dominatesthe UK bread market and hasalso been adopted in othercountries including South Africaand Australia, required large,

powerful and expensive mixers,and the smaller bakers requiredsomething less elaborate, leadingto the development of the activateddough development (ADD) process.This still used potassium bromateand ascorbic acid as the oxidisingagents, but l-cysteine was added as arapid-acting reducing agent, whichaided the fission and reformationof disulfide linkages, easing theexpansion of the dough. Extra waterwas added to compensate for lackof natural softening, plus extrayeast to maintain normal proving

times, and the resulting processcould be used by small bakers withinexpensive mixers. Bizarrely, untilJapanese food company Ajinomoto

commercialised a chemicalroute in 2001, l-cysteine was stillmanufactured from human hair. TheADD process provided 18 years ofbaking happiness until the banningof bromate in the UK in 1990 sent theindustry back to the drawing board.

Enzyme assistanceThe difficulties with chemicaladditives reawakened interest inthe use of enzyme supplements.6

Flour naturally contains both- and-amylases, which between thembreak down some of the starch inthe dough to the fermentable sugars,maltose and glucose. By artificiallyincreasing the level of amylasesin the dough, the quantities ofthese sugars available for the yeastfermentation can be enhanced,accelerating the production of CO

2 and of ethanol, the other glucosefermentation product, which is amajor contributor to the aroma ofbaking bread. Choice of amylasesis crucial. The bacterial amylaseswidely used in the manufacture ofglucose syrups are unsuitable as theyare heat stable they would survivethe baking temperatures and go on togenerate sticky dextrins that would,in a few hours, reduce the centre of aloaf to a glutinous mess.

The added enzymes have similarproperties to those naturally

occurring in dough and aredestroyed in the baking process.Consequently, in 1996, the useof enzymes in bread was totallyderegulated, to the horror of theanti-additive lobby. Their use hasproved an attractive alternative tothe CBP.

Proteases are enzymes that cleavethe peptide bond in proteins. Inbread making, they decrease mixingtimes by increasing the speed of

As mechanical workstretches the dough,more hydrogen bonds(black) can form

between chains of glutensubunits (orange)

After an initialfermentation, dough isshaped and sized beforeit goes in the oven

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water absorption, increasing doughextensibility and reducing doughconsistency. They also produce smallquantities of amino acids from theproteins in gluten, which interact viaa Maillard reaction with glucose, togive flavour components and crustcolour. These processes are slow andare the reason why artisan bread hasmore flavour then no-time breads.More than 300 flavour compoundshave been identified.7

On the shelfAs most people will have

inadvertently discovered, breadkept too long goes stale. This processcan be slowed by adding fats to thedough. Initially the linear amylosepolymers in flour about one fifth ofthe total starch are aligned, side byside, in a microcrystalline structure.This structure is destroyedduring baking, but subsequentlycrystallisation recommences, to acrystal form which contains lots ofwater of hydration. This reducesthe amount of free water in thebread, and it loses springiness andappears to dry out. Adding fats to

the dough, which are hydrophobic,slows the migration of water duringthe crystallisation, keeping thebread moist. They also improve loafvolume and crumb softness.

Animal fats and fish oils wereused in the past, but modernbaking processes demand a solidfat HPKO (hardened or partlyhydrogenated palm kernel oil) iscurrently widely used, but palmoil cultivation is destroying thehabitat of the orang-utan, and palmoil is increasingly sought after asa potential source of biofuel. One

possible alternative is to use thehigher melting compounds fromvegetable oils, which are extractedby cooling the oil until theycrystallise out.

Polar lipids with hydrophobicside chains are also used asanti-staling agents.7 Their polarcomponents interact with thesurface of amylopectin moleculesbut, in so doing, their non-polarfatty acid chains provide a degreeof waterproofing. They act,apparently, by binding to glutenproteins. Modern wholemeal bread

depends crucially on anti-stalingagents for its palatability.Salt is always added to dough

and not just for the taste. Its ionsshield glutens charges from oneanother and enable the proteinmolecules to approach more closely,giving a stronger and more stabledough.7 Governments are oftenanxious to reduce salt levels in thediet, but there is a limit as to how far

this can be carried with bread. Inthe absence of salt, dough issticky, and the resulting bread isunpalatable.

There is little doubt that slowlyfermented dough gives a moreflavoursome bread than no-timedoughs. Nonetheless, CBP bread hasmany advantages for consumers.It is soft, stales only slowly, andmakes good toast and sandwiches.It can use low protein Europeanflour and, indeed, this was the spurto the development of CBP in thefirst place. It is also cheap compared

with the sophisticated ethnic breadsand so-called morning goods (suchas rolls and croissants) currentlyoccupying the top end of the market.It has gained at their expense duringthe past year.Bryan Reuben is the co-author ofBread: a slice of history. Stroud, UK:The History Press, 2009 (see p66)Tom Coultate is the author ofFood:the chemistry of its components.Cambridge, UK: RSC Publishing,2009 (see p64)Both authors are associated with

London South Bank University.

References1 M Dnvan, Domestic economy. 1830, Lndn,

UK: Lngman et al2 H Wieser, Food Microbiol., 2007, 24, 115 (DoI:

10.1016/j.fm.2006.07.004 )3 K A Tille et al, J. Agric. Food Chem., 2001, 49,

2627 (DoI: 10.1021/jf010113h)4 E Pea et al, J. Cereal Sci., 2006, 44, 144 (DoI:

10.1016/j.jcs.2006.05.003)5 A Rdriguez-Mates et al, J. Agric. Food

Chem., 2006, 54, 2761 (DoI: 10.1021/jf052933q)

6 H Gesaert et al, in Bakery products, ed. y HHui. 2006, oxfrd, UK: Blackwell

7 E Buehler, Bread science. 2006, Carrbr NC,US: Tw Blue Bks

Bonds hold breadsprotein chains together:(left to right) hydrogenbonds between glutamineside chains; disulfidebridges; and cross-linksbetween tyrosine sidechains

Bread needs a goodcrumb structure