Gas Cell Stabilisation and Gas Retention in Wheat Bread Dough

Journal of Cereal Science 21 (1995) 215–230

Mini ReviewGas Cell Stabilisation and Gas Retention in Wheat

Bread Dough

Z. Gan∗†, P. R. Ellis† and J. D. Schofield∗†

∗The University of Reading, Department of Food Science and Technology, Whiteknights, PO Box 226,Reading RG6 6AP, UK and †King’s College London, Division of Life Sciences, Campden Hill Road,

London W8 7AH, UK

Received 1 May 1994

DedicationThis paper is dedicated to the memory of Professor Terry Galliard, a good friend andcolleague, who made invaluable contributions to the work carried out in our laboratory,which is described in this review, and to Cereal Science in general.

ABSTRACTGas cell stabilisation and gas retention are of considerable interest because of their technological sig-nificance in bread making. We review recent studies in relation to the stabilisation of gas cells and themechanisms of gas retention, and discuss how these may be affected by the liquid phase of dough. Thepossibility is discussed of the involvement of surface active materials, such as proteins and pentosansdissolved in the dough aqueous phase, and, perhaps more importantly, non-starch polar lipids in theformation and stabilisation of gas cells. There is accumulating evidence for the hypothesis that liquidfilms play a critical role in the mechanisms of gas retention in dough. The hypothesis proposes that twoclosely related, consecutive stages are involved in dough expansion. During the first stage, the expandinggas cells remain discrete until discontinuities develop in the starch–protein matrix, leaving areas con-taining only a liquid film. The timing and the degree to which such discontinuities occur is largelydependent on gluten proteins. The second stage involves an increase in the surface area of the liquidfilm as discontinuities become increasingly frequent during expansion. Failure of the lamellar film tomaintain the rate at which new surface area is generated leads to the rupture of this film and, consequently,the loss of gas retention. Consideration is also given to the role of bakery fat in gas retention and toadditional factors that affect gas retention in wholemeal doughs, in particular the physical disruption ofthe foam structure of such doughs by components of the outer layers of the grain.

Keywords: gas cells, gas retention, bread dough, liquid film

INTRODUCTION : PoL=polar lipids; NPoL=non- The unique position of wheat, compared withpolar lipids; NSL=non-starch lipids; ESP=equi-

other cereals, in bread making is due to the abilitylibrium-spreading pressure; FA=free fatty acid;of wheat flour dough to retain gas on expansion.PUFA=polyunsaturated fatty acid; LC–L=liquid–To produce a loaf of bread with a light and evencrystalline; SEM=scanning electron microscope;crumb texture, the dough must be able to retainCBP=Chorleywood Bread Process; SSL=sodium

stearoyl lactylate. the gases produced by yeast fermentation as dis-Corresponding author: J. D. Schofield, The University of crete gas cells for a sufficiently long period1,2. The

Reading. gas cells play a crucial role in gas retention, buthow they are formed, and especially what happensto them during bread dough processing, is poorly

0733–5210/95/030215+16 $08.00/0 1995 Academic Press Limited

215

Z. Gan et al.216

understood. Although work during the late 1930s Under normal baking conditions, the initial lossof gas is slow, but a sharp increase in the rate ofand early 1940s produced much useful in-

formation3,4,5,6, and considerable effort has since loss occurs towards the end of oven spring14,15. Theslow initial loss of gas can be explained by itsbeen devoted to the elucidation of gas cell structure

and the mechanisms of gas retention, present diffusion to the external surface of the dough fol-lowed by evaporation, whereas the rapid loss hasknowledge still remains fragmentary.

The foam structure of a fermenting dough has been attributed to the rupture of the starch–proteinmatrix surrounding gas cells. The latter is thoughtlong been assumed to be a dispersion of discrete

gas cells in a continuous starch–protein matrix2,3,4. to result in the interconnection of adjacent gas cells,converting the foam structure of dough into an openThe matrix ruptures during baking, leading to

the establishment of a continuous gas phase and, sponge and enabling direct escape of the gas (Fig.1). The rupture of the matrix has, in turn, beenconsequently, the rapid loss of gas. Mechanisms

involving different factors that control gas retention attributed to the sharp increase in dough viscositythat occurs mainly as a result of starch gel-have been proposed on this basis, emphasising

largely the role of the gluten proteins7,8 and non- atinisation16. The improving effects on loaf qualityof oxidising improvers, which increase the resistancestarch polar lipids9,10. Results from this laboratory,

however, have indicated the involvement of in- of dough to deformation17,18, and, of fat and water,which decrease the resistance of dough to de-terfacial films at the gas/liquid interfaces in

fermenting bread dough11,12. The components re- formation18,19,20 are difficult to reconcile on this basis,however. Furthermore, proteins are known to playsponsible for stabilising the films are not yet

known, but are envisaged to be non-starch polar a crucial role in determining the gas-holding cap-acity of a dough, but the above description doeslipids and/or proteins and possibly pentosans

dissolved in the aqueous phase of dough. not explain why proteins are so important in breadmaking. The polymerisation of glutenin moleculesStudies with specific systems or individual flour

component have tempted cereal scientists frequently as a result of sulphydryl–disulphide interchange21

may contribute to the sharp increase in tensile stressto attribute the central role to a single componentand to discount the involvement of the others in that occurs in starch–protein matrix on heating,

thus leading to the rupture of the bulk phase andbread making. The present review aims to providea critical discussion of this area, making particular to the loss of gas retention.

Others have suggested that gluten contributes toreference to the role of surface active materials ingas retention. gas retention by slowing the diffusion of gas through

the dough phase8,22. CO2 is produced by yeastfermentation in the aqueous phase of dough. It thenGAS CELLS AND DOUGH EXPANSION diffuses through the aqueous phase to the gas cells,where it evaporates to generate within them anThe gas phase of a proving dough exists as a

dispersion of discrete gas cells in a semi-solid bulk excess pressure that provides the driving force fordough expansion. The CO2 cannot diffuse out ofdough phase comprising starch, gluten and other

minor components. The gas cell nuclei, which the gas cells because the aqueous phase surroundingthem is saturated. The saturation of the aqueouslater expand as carbon dioxide (CO2) produced

by yeast fermentation is transported to them, are phase is maintained because the yeast continues toproduce more CO2. A small proportion of the gasincorporated by the occlusion of air during mixing5,6.

Yeast is incapable of producing new gas cells in does diffuse to the surface of the dough piece andevaporates into the surrounding atmosphere givingthe dough5, although subdivision of existing cells

occurs during punching and moulding operations. a slow release of gas from the dough. Towards theend of baking, however, a sharp increase in the rateThe technological significance of the gas cells im-

mediately after mixing is that a sufficiently large of loss of gas occurs. Although some researchershave attributed it to an increase in the rate ofnumber, estimated13 to be between 1011 and 1013/

m3, is required. At advanced stages of bread dough diffusion through the dough aqueous phase8,22, oth-ers have argued that this sharp increase is due toprocessing, the survival of intact gas cells is of great

importance in order to prevent the loss of gas, the rupture of the starch–protein matrix1,2. Breaddough undergoes a structural transformation fromthereby producing bread with a light and even

crumb texture (i.e. with large specific volume andgas cells of relatively uniform size).

Gas cell stabilisation and gas retention 217

Foam

Sponge

= Gas cell = Starch–protein matrix

Loss of gas

Rupture of the matrix

Starch gelatinisation at above 60°C

Increase in tensile stressin starch–protein matrix

END OF BAKING

END OF MIXING END OF PROVING

Figure 1 The structural transformation of dough as explained by the conventional starch–protein matrix hypothesis. Soonafter mixing, the structure of dough is represented by small gas cells dispersed in a continuous starch–protein matrix. Eachdiscrete gas cell expands in response to CO2 production during fermentation, and the foam structure is maintained by thinmembranes separating adjacent cells at the end of proof. During baking, starch gelatinisation induces a dramatic increase indough viscosity, resulting in a rapid increase in tensile strength in the membrane. This initiates membrane rupture, convertingthe foam into a sponge.

a foam into an open sponge during baking asindicated by previous studies3 and confirmed by

energy required to produce a given deformationand the rupture stress, for example, was shown to

recent observations (Fig. 2) made in our laboratory11,12. decrease with increasing water level20. In fact, atAlthough diffusion controlled processes will cer- very small deformations, the rheological propertiestainly be important for gas retention during fer- of an experimental dough can be reproduced frommentation, it seems more likely that the physical any flour by the control of water addition24.rupture of the bulk dough phase and/or liquid films A wheat flour dough contains typically 0·6–0·8 gsurrounding gas cells is more relevant to the sudden

water/g of dry flour, of which approximately halfloss of gas during baking than an increased diffusionis thought to be ‘bound’ or unfreezable25,26. Therate through the dough phase.presence of ‘free’ or freezable water can be de-tected only when the water content exceeds about

WATER AND THE LIQUID PHASE OF DOUGH 30–35% by weight. Further addition of waterforms a second aqueous phase that may be sep-Irrespective of the process involved, the primaryarated by centrifuging the dough in very highevent in bread making is the addition of water tocentrifugal fields23,27. The aqueous phase is neces-a dry flour at the onset of mixing, forming asary in dissolving soluble flour components and incohesive, viscoelastic dough. The mechanical orproviding the medium for reactions to take placerheological behaviour of the dough is profoundly

influenced by the amount of water added23. The in the dough28.

Z. Gan et al.218

Figure 2 SEM micrographs of wheat bread dough and bread prepared by the Chorleywood Bread Process (CBP), showing(A) the apparent interconnections in bread doughs at advanced stages of proving, and (B) the open sponge structure of breadcrumb at the end of baking.

Beneficial effects on loaf volume and crust colour used to furnish the required water in bread mak-ing28. It has been shown also that effectively nowere observed when the liquid phase (dough li-

quor) centrifuged from wheat flour doughs was gas is retained by dough below a water content of


about 35% of the total dough mass27, but gas extensibility. The second stage involves primarilyan increase in the surface area of the liquid film,retention improves almost linearly when the water

content is increased above this value up to about which maintains the integrity of the gas cells asdiscontinuities in the starch–protein matrix be-44%. Electrical conductivity data further indicated

that the aqueous phase is continuous rather than come increasingly frequent during expansion. Thebehaviour of the dough at this stage is determinedcomprising dispersed droplets (MacRitchie, 1976).

MacRitchie27 envisaged a structure of dough in primarily by the stability of the liquid film. Surfaceactive materials, such as endogenous flour polarwhich gas cells are embedded in a continuous

starch–protein matrix and are enveloped by a lipids, and proteins and pentosans dissolved in thedough aqueous phase may contribute positively tocontinuous thin liquid film. Although it is difficult

to assess the technological significance of the liquid gas retention by stabilising the film so that it canexpand to a larger surface area without rupturing.film in gas retention in relation to that of the

mechanically strong solid dough phase, the hy- It is the rupture of the liquid film, rather than thatof the starch–protein matrix, that is thought to leadpothesis offers a new way of thinking in the search

for a better understanding of the mechanisms of to the rapid loss of gas and to the end of oven spring11,12.Gluten proteins undoubtedly play a major rolegas retention.

in determining the baking performance of wheatflours through their effects on dough rheologicalproperties, though the important role of the liquidTHE LIQUID FILM HYPOTHESISfilm in gas cell stabilisation and gas retention should

Recent studies using scanning electron microscopy not be overlooked. The change in free surface energy(SEM) have highlighted the significance of a con- has been calculated to account for about 80%tinuous lamellar film in gas cell stabilisation and gas of the elastic energy involved in the deformationretention11,12. It has been shown that a substantial (expansion) of dough29. Although this calculationnumber of gas cells are incompletely enclosed has been disputed13, the conclusion remains that aby a continuous starch–protein matrix in bread greater pressure in gas cells is needed to overcomedoughs at advanced stages of proving; rather, surface tension than viscous resistance of the dough.there are discontinuities in the matrix, with ‘holes’ Nevertheless, the significance in gas cell stabilisationostensibly forming inter-connections between ad- of the liquid film relative to that of the starch–proteinjacent cells (Fig. 2A)11. It is clear that the structures matrix during bread making may vary amongstobserved cannot be present as such in the dough flours. A stable liquid film could compensate for ain situ since that would imply an open sponge starch–protein matrix of relatively poor extensibility,typical of that of the bread crumb (Fig. 2B), and as may occur when surfactants are added to improveincapable of retaining gas. Hypothetically, the gas gas retention, or vice versa, as may occur when glutencells remain separated by a fragile liquid lamella is added to improve baking performance. If bothstretching across the holes (Fig. 3)11,12. This film, phases are unsatisfactory, a dough with poor gasstabilised by surface active materials, is supported retention will result.initially by a continuous starch–protein matrix, Cryo-stage SEM and SEM of doughs preparedbut that matrix fails to enclose the gas cells com- by gradual freezing have provided indirect evidencepletely at advanced stages of expansion, leaving for a liquid film partly separating contiguous gasareas between them containing only the lamellar cells11. In particular, a fibrillar structure was noted,film11,12. which resembled hydrated protein in excess water,

Thus, two consecutive stages are envisaged as and it was concluded that the presence of a liquidoccurring during dough expansion. Firstly, the film between adjacent gas cells may be necessaryexpanding gas cells are embedded in a continuous for the formation of the observed fibrils11.starch–protein matrix, which develops into thin, Furthermore, cutting a dough rapidly across thecontinuous membranes between adjacent cells surface using a sharp razor blade at the end ofuntil its tensile stress increases to the point that proving (50 min) had little effect on its volume, i.e.rupture occurs or until there is simply not enough the dough did not collapse as would be expected ifmaterial to maintain the continuity of the mem- gases, which support the structure of dough, arebranes. The behaviour of the dough prior to this retained only by a top ‘skin’. Interestingly, however,is determined primarily by the rheological prop- doughs collapse rapidly, losing an estimated 25–30%

in height within 2–3 s, when the tins containingerties of the bulk dough phase, particularly its

Z. Gan et al.220

= Starch granules

Liquid lamellae

End of oven spring or baking

Early stages of fermentation Advanced stages of fermentationto

early stages of baking

= Starch–protein matrix

= Gas cell lined with a liquid film

Figure 3 A revised model of dough expansion. Soon after mixing, the dough consists of discrete gas cells lined with liquidfilms and embedded in a continuous starch–protein matrix. The matrix fails to enclose the gas cells completely at advancedstages of fermentation, leaving areas that contain only a thin liquid lamella. Baking increases the rate of expansion until thelamellar film is incapable of meeting the demand for new surface area generation, thus converting the foam structure of doughinto an open sponge. The loss of gas retention is caused, therefore, by the rupture of the liquid film, not that of thestarch–protein matrix.

them are struck sharply on one end (L. S. Besford, flour, and starch lysophospholipids, which do not32.RHM Research and Engineering Ltd, personal com- Using reconstitution methods, a general, al-munication). Further mechanical shocks produce no though unusual, relationship was found betweenfurther reductions in dough height. The internal the content of the natural flour lipid and loafstructure of the dough, therefore, does not allow volume measured in a baking test, in which exo-free movement of gas over extended distances, but genous lipid was omitted from the formulation33.the structure is mechanically fragile as for most The loaf volume was high for a flour from whichliquid foams. the NSL had been removed (Fig. 4). As this lipid

was added back, volume decreased to a minimumat a lipid content intermediate between the de-

THE ROLE OF SURFACE ACTIVE LIPIDS fatted and untreated flours. After this point, loafvolume increased, approaching a constant valueThe lipid reserves of wheat are triacylglycerols.at lipid contents higher than that of the originalSmall quantities of diacylglycerols, monoacyl-flour. The NPoL fraction produced a progressiveglycerols and free fatty acids (FA) may be in-decline in loaf volume, however, and the FA oftermediates in triacylglycerol biosynthesis30, butthe NPoL fraction have been shown to be thosethey may also be degradation products producedwhich are mainly responsible for the depressionby lipase action31. These lipids are collectivelyin loaf volume34. Loaf volumes were also found totermed non-polar (apolar) lipids (NPoL), although,be highly correlated with parameters such as PoLin fact, some are moderately polar. The othercontent, PoL/NPoL ratio and galactolipid contentlipids in wheat are structural lipids in variousof petroleum ether-extracted (free) lipid, but notmembranes and organelles, comprising numerouswith the water-saturated butanol-extracted (freeglycolipids and phospholipids, collectively termedand bound) lipid35,36. Similar relationships havepolar lipids (PoL). The endosperm lipids are alsobeen reported between loaf volume and free lipid37.conveniently divided into non-starch lipids (NSL),

which affect the baking performance of wheat Although the results from the above studies have


Wheat lipids, which are found to be associatedclosely with the liquid phase of dough (about 0·3%)27,28,probably represent the most significantly surface-active components in wheat flour dough. Theselipids, and particularly the polar fraction, arethought to assist in the foamability of dough byforming a lipid monolayer at the gas/liquid in-terface10,40. Because doughs prepared from defattedflours can expand33, it must be assumed that surfaceactive lipids are not essential for gas retention andthat proteins and/or pentosans dissolved in thedough aqueous phase can take the place of lipidsin defatted flour. There is no doubt, however, thatendogenous PoL lipids (e.g. phospho- and galacto-1000

200

Added lipid (mg)

Loa

f vo

lum

e (m

l)

800

180

160

140

0 200 400 600

NPoL

PoL + NPoL

PoL

lipids), as well as surface active synthetic emulsifiers,such as DATA esters, improve gas retention.Figure 4 Loaf volume (ml) as a function of lipid content.

The responses to small changes in lipid content,PoL, polar lipids; NPoL, non-polar lipids. Dashed line rep-resents volume at end of proving. Additions were made to a which do not affect the rheological properties ofdry flour weight of 30·2 g (reproduced with permission from dough, can be rationalised on the basis that theMacRitchie and Gras33). lipid exerts its effects at the gas/liquid interface,

thereby influencing the formation and stability ofthe gas cells. Wheat lipids are capable of orienting

all agreed in showing the PoL fraction to be themselves at the gas/liquid interface to form afavourable and the NPoL fraction to be detri- lipid monolayer. The spreading pressure of the filmmental in bread making, a number of experiments provides a force that counteracts the interfacialhave been unsuccessful in establishing correlations be- tension between the gas and the liquid phases, and,tween loaf volume and free lipid38,39. thus, stabilises the gas cells. Unlike the hexagonal

A characteristic feature of lipids is their poly- phase, which behaves like a plastic fat and is notmorphic behaviour. PoL may form various types of spontaneously dispersible, the LC–L phase spon-phase when wheat flour is hydrated to form dough, taneously forms small, bilayer aggregates (liposomes)depending on factors such as the proportions of the in an aqueous environment on mixing. The rapidPoL and water and the structure of the PoL class. formation of condensed lipid monolayers at in-The phase relationships, corresponding to the terfaces, such as gas/liquid interfaces in wheat breadthermodynamic equilibrium between various pro- doughs during fermentation and the early stagesportions of PoL and NPoL in water, are usually of baking, requires lipid depots to be availabledescribed by so-called ternary phase diagrams40. The throughout the process in the dough structure. Thelamellar, liquid–crystalline (LC–L) phase consists of LC–L phase in the form of liposomes is ideal forlipid bilayers with the hydrocarbon chains in a this purpose. A liposomal dispersion of lecithin, forliquid-like disorder forming the core and the polar example, when added (about 2%, w/w) to a 1:1head groups forming the two outside surfaces. PoL mixture of a wheat and rice flour, produced breadin the LC–L form can transform spontaneously in of the same volume as that produced when wheatan aqueous environment on mixing into lamellar flour alone was used40. A ‘lecithin sludge’ obtaineddispersions, comprising small, bilayer aggregates as a by-product of soya oil refining produced similar(liposomes). When such a transformation occurs, results in a wheat/sorghum flour blend42.virtually all PoL and most NPoL are likely to bedetected in the bound form41. In a fermenting doughthe PoL may provide surface-active molecules at THE ROLE OF SURFACE ACTIVE PROTEINSthe gas/liquid interface to stabilise the dispersed gas

Proteins, being amphipathic in nature, may alsophase10. The lack of appreciation of the differentexert a surface effect at air/water interfaces inphysical states of lipids may well have contributeddough by forming a continuous film. Proteins differto the often contradictory conclusions that are foundsubstantially in their surface activities, and thesein the literature regarding the effects of lipids on

baking performance. differences are not attributable simply to unspecific

Z. Gan et al.222

variation in amphipathicity, since most proteins process of transport and adsorption of proteinsexhibit similar distributions of hydrophobic and from the bulk phase, they may have limited sig-hydrophilic residues43. The variation is believed to nificance in terms of understanding the mech-arise from differences in conformational structure, anisms and molecular processes involved in gasstability and/or flexibility of the conformation, cell stabilisation in wheat bread dough. Manysymmetric or asymmetric distribution of polar and functional properties of proteins, including theirapolar patches on the surface of the molecule, and ability to form gels and to stabilise emulsions andin molecular size and shape43. foams, depend on their solubility52. Gluten proteins

Alternative pathways for folding protein are well known for their insolubility in salt so-molecules to minimise free energy become possible lutions, and yet the calculated salt concentrationwhen proteins are adsorbed at an air/water in- in the aqueous phase of a bread dough is aboutterface. In addition to folding into the interior of 0·5 , assuming that all the salt added as a dougha globule in the aqueous phase, apolar side chains ingredient remains in the aqueous phase. Howcan be located in the air phase44. The rate of the gluten proteins might be transported to andadsorption is diffusion-controlled in the initial adsorbed into the interface in dough under suchstages of adsorption (i.e. at low surface coverage), conditions is difficult to envisage. Moreover, im-whereas there is an energy barrier to adsorption munolocation studies have failed to show the pres-at high surface coverages45. Although the exact ence of gliadin proteins at gas cell surfaces innature of the energy barrier is not fully understood, bread, although these proteins were readily locatedit may be related to pA, where p is the surface at the cut surfaces of the bulk bread crumb53.pressure and A is the molecular area required to Proteins represent about 2–3% by weight of thebe cleared for the molecule to adsorb at the liquid phase of dough27,28. If they do have a rolesurface45,46. The basic premise is that, for a protein to play in interfacial film stabilisation, it is themolecule to clear and occupy an area A at the water and salt extractable fractions, which accountsurface against the surface pressure, it should pos- fof 15–20% of total flour protein, that seem likelysess a molecular energy equal to or greater than to fulfil this role. These proteins have been shownpA. The energy barrier may actually be related to to be beneficial in bread making54–56. The ad-surface denaturation of the protein, which in turn is sorption of such proteins at the gas/liquid interfacedependent on surface water activity47. The surface could be significant in this respect.denaturation of proteins is thought to be caused

Also of interest in relation to the possible roleby the reaction of high-energy surface waterof protein in stabilising interfacial films in breadmolecules with the internal polar groups of thedoughs is the recent discovery of an amphiphilicproteins, which facilitates their unfolding at theprotein, puroindoline, in wheat flour57,58. Puro-surface (Fig. 5).indoline was isolated from the detergent rich phaseIn addition to lowering interfacial tension, pro-of a 4% (w/v) Triton X-114 extract of flour aftertein molecules can form a continuous film at thephase partitioning by condensation of the de-interface via complex intermolecular interactions.tergent at 35°C. Friabilin, a protein identifiedThese are thus able to impart structural rigiditypreviously as a starch granule surface associatedto the interface48. The development of such mech-protein59, is also extracted from flour by Tritonanical strength may not be possible in the case of aX-114 and partitions in the detergent rich phase;simple surfactant film, in which the intermolecularit shows extensive homology with puroindoline57,58,60

interactions are relatively weak.(also D. Marion, INRA, Nantes, France andMuch of the earlier work on the surface be-P. Greenwell, FMBRA, Chorleywood, UK, un-haviour of wheat proteins was limited to examiningpublished results). The phase partitioning be-effects of protein monolayers spread at the surfaceshaviour of puroindoline suggests that it may haveof aqueous media49–51. Both gliadin and gluteninstrong polar lipid binding properties and that itsfractions have been shown to be surface active asamphiphilic nature may make it strongly surfacedetermined by surface balance techniques. Theactive. Indeed, evidence has been presented thatgliadin fraction spreads more quickly at the gas/puroindoline is surface active and has the abilityliqud interface, achieving much higher surfaceto stabilise interfacial films/foams61. Defining thepressure per unit area concentration than therole of puroindoline and related proteins in gasglutenin fraction51.

Since such studies do not represent the dynamic retention will be an important goal in the future.


= High energy water molecules

Gas

= Low energy water molecules

Liquid

Train

LoopTail

(a)

(c)

(b)

Gas

Liquid

Figure 5 Schematic representation of protein adsorption at a gas/liquid interface in dough. (a) native protein immersed ina liquid phase; (b) protein in contact with high energy water molecules at the interface; (c) adsorbed, unfolded, and hydratedprotein molecules with trains in direct contact with the interface, loops between the trains projecting into the two bulk phases,and tails at the N- and C-terminal ends of the polypeptide. The tails are expected to be present in the aqueous phase sincethe N- and C-terminals of a polypeptide are charged at around neutral pH (adapted and reproduced with permission fromTer-Minassian-Saraga47).

terfaces (Fig. 6). Interesting, puroindoline, whichFILM STABILITY WITH CO-EXISTING LIPIDS,is thought to be a lipid binding protein, can actPROTEINScooperatively with polar lipids in interfacial films

Although the mechanical stability of a film with under some circumstances resulting in increasedtwo or more components is believed to be some- foam stability61.what lower than for any of the highly condensedstates of the single components, especially if theindividual component molecules do not interact THE ROLE OF PENTOSANSwith each other48, lipids and proteins can co-existin interfacial films and are generally envisaged as Pentosans are the major non-starch poly-

saccharides of wheat flour. They originate fromacting in a competitive manner. This may explainthe unusual relationship observed between PoL the endosperm cell walls of wheat grains, and

comprise mainly arabinoxylans and arbino-content and loaf volume when PoL is added backincrementally to defatted flour (Fig. 4)33. Gas cells galactans, which are partly extractable with water.

The high water-binding capacity of pentosans62,63,are probably stabilised by surface active proteinswhen NSL have been removed, resulting in the together with their high solution viscosities64–66 and

their ability to undergo oxidative gelation67, haveinitial high volume. As PoL is added back, theprotein films are destabilised, reaching minimum been considered potentially important in in-

fluencing flour water absorption, dough mixingstability at approximately 100 mg PoL, probablyby formation of a mixed film, with reduced in- and dough rheological properties62,63,66,68–70.

Despite considerable research, however, thetermolecular interactions of proteins at the in-terface. Further additions of PoL may result in functional role of pentosans in bread making is

still not well defined. This is due largely to a lackthe formation of PoL-dominated films by gradualdisplacement of proteins from the gas/liquid in- of insight until relatively recently into the structural

Z. Gan et al.224

= Protein

0

Added polar lipid (mg)

(c)

= PoL

= Protein-PoL complex

(a)

(b)

400

Loa

f vo

lum

e (m

l)

aqueous phase

Figure 6 Speculative mechanisms for liquid film stabilisation as PoL is added incrementally to defatted flour. (a) filmstabilisation by surface active proteins, resulting in the initial high loaf volume when NSL have been removed; (b) as PoL isadded, the protein films are destabilised, probably by formation of a mixed film, with reduced intermolecular interactions; (c)further additions of PoL gradually displace the proteins from the gas/liquid interfaces, forming a stable, single componentPoL film.

properties and heterogeneity of arabinoxylans, ilarly, whereas detrimental effects on bread makingperformance of adding water-unextractable ara-thus limiting the design of meaningful experiments

and the interpretation of experimental results. binoxylans were noted by some69,78, others ob-served positive effects79,80. It is difficult to be sure,Studies to examine the role of pentosans in bread

making have been of three types in the main— however, that the functional effect of a componentadded to a dough will be the same as that presentreconstitution, correlation, and enzymic modi-

fication—and the results of such studies have been endogenously in a flour. Indeed, different methodsof addition of pentosans to dough have been shownto some extent contradictory.

Beneficial effects on loaf volume of adding wheat to produce different functional effects81.Correlation studies showed a positive re-and/or rye water-extractable pentosans have been

observed in a number of studies68,71–75 using relationship between the water-extractable pentosancontent of 58 hard wheats and bread texturalconstitution methods. However, other groups had

observed that addition of extra pentosan did not characteristics82. On the other hand, a slight neg-ative effect of water-extractable pentosans on loafincrease loaf volume76,77, although water-ex-

tractable pentosans were found to be responsible volume was also observed83. Analysis of six Euro-pean wheat flours with varying bread makingfor the reduction in loaf volume when water-

solubles were removed from wheat flour76. Sim- potentials indicated an inverse relationship be-


tween the handling properties of a dough and the viscosity films are less stable. It is likely, therefore,that the increase in the amount of high Mr ara-flour water-extractable pentosan content84. Pre-

dictive modelling studies of the baking properties binoxylan, and, consequently, an increase in theviscosity of the dough aqueous phase as a resultof wheat85 showed that inclusion of non-starch

polysaccharide variables helped to explain some of the limited solubilisation of arabioxylan fromwater-unextractable pentosans at the optimumof the variation in dough rheological properties

and contributed significantly to variation in flour level of enzyme addition90, improved the foamstability of the dough. This is consistent with thewater absorption, but did not explain variation in

loaf volume. finding that arabinoxylan of high intrinsic viscositywere more effective in stabilising an aqueous pro-Although earlier studies with purified en-

doxylanase enzymes showed adverse effects on loaf tein foam during thermal expansion91. The ad-dition of arabinoxylan was also shown to produceproperties70, more recent work has shown that

substantial improvements in bread making per- a substantial reduction in the surface tension ofwater, and that, at a concentration of 0·5%, allformance can be produced by such enzymes86,87.

The use of an endoxylanase preparation in a pentosan preparations from eight wheat cultivarswere surface active91. Thus, apart from playing aChorleywood Bread Process (CBP) formulation

resulted in prolonged expansion of dough during positive role in dough rheology, water-extractablethe early stages of baking87 indicating that gas pentosans may contribute to the stabilisation ofretention by the dough was improved. The mech- gas cells by improving the mechanical strength ofanism of this effect has not been defined, but the liquid films in the dough. Overall, the studiesanalysis of fractionated polysaccharides from summarised above indicate a positive effect ofdoughs at different stages of mixing and proof water-extractable arabinoxylans on gas retention.indicated that the enzyme caused a rapid and Firm conclusions about the functional role(s) ofspecific degradation of water-unextractable ara- pentosans in bread making will only be possiblebinoxylans and thus a substantial increase in the when the structure and properties of wheat flourwater-extractable fraction88. Kulp89 had also shown arabinoxylans and their variation, especially thoseearlier that the bread making properties of waterof the water-extractable fraction, are more fullyunextractable pentosans were improved by di- understood.gestion with a crude fungal pentosanase.

The release of arabinoxylan from water-un-extractable pentosans by a crude pentosanase- THE INVOLVEMENT OF BAKERY FATcontaining enzyme preparation increased the spe-

It is outside the scope of this review to considercific viscosities of aqueous extracts of dough,in detail the role of added bakery fat or shorteningthough the increase was not proportional to thein gas retention in bread doughs. Nevertheless,extent of solubilisation of the unextractable pen-some mention should be made of the possibletosans90. An improvement in dough quality wasinvolvement at the gas/liquid interface of fat crys-evident when a greater viscosity was achieved attals and their influence on gas retention.the optimum level of enzyme addition, whereas at

Fat is an optional ingredient for ‘traditional’excessive enzyme levels, viscosity decreased andlong or bulk fermentation bread making processes.dough quality deteriorated90. There is a clear needFor ‘no time’ mechanical dough development pro-to define the mechanism of bread improvementcess, such as the CBP, however, it is an obligatoryby xylanases more precisely, and this may throwingredient for production of bread of acceptablefurther light on the role of pentosans themselves.quality. Furthermore, not only must fat be in-Aqueous solutions of water-extractable pentosanscluded, but a proportion (5%) of that fat must beare highly viscous and it is possible for suchsolid (i.e. in crystalline form) at proof temperature.solutions to be converted into gels via oxidative

Fat crystals per se are not surface active, andcovalent cross-linking67,91; the arabinoxylan frac-there has been no direct demonstration to date oftion has been identified as the major componentthe involvement at the gas/liquid interface of fatresponsible for the high viscosity91.crystals and their influence on gas retention inThe stability of a lamellar film, and thus thebread doughs. The association of fat crystals withstability of a foam itself, depends on several factors,the surfaces of gas cells in cake batters has beensuch as film viscosity, shear resistance and elast-

icity. Low elasticity, low shear resistance and low described recently, however, and is thought to be

Z. Gan et al.226

a secondary process92. Initially, the gas cells in finer textured loaf produced by using improvers,emulsifiers and added wheat gluten. The non-cake batters are stabilised by an interfacial film of

egg proteins. Fat globules containing both liquid endosperm components occurring naturally in thewheat caryopsis are known to be responsible forfat and fat crystals are also surrounded by a

proteinaceous interfacial film, and, indeed, ejec- producing the low specific volume and densecrumb structure of the traditional wholemealtion of fat crystals from liquid fat may be aided

by this surrounding film. As mixing proceeds, gas bread32,94. This effect cannot be explained solelyby these components diluting the gluten-formingcells and fat crystals come into contact, and the

protein layers fuse so as to form a single continuous proteins.The interactions of white flour fractions withfilm surrounding both the gas cells and the fat

crystals. It has been shown that fat crystals become the degradation products of the lipids of germ andbran contribute to the depression in loaf volumealigned tangentially at the gas cell surface in cake

batters and that this improves gas retention92,93. In of bread made from stored wholemeal flour95,96.Lipid hydrolysis, due mainly to enzymic activitygeneral, small fat crystals of the b′ polymorph

have a greater ability to stabilise gas cells than the in the bran, liberates free fatty acid, the poly-unsaturated fraction (PUFA) of which (ca. 60%much larger b crystals.

It has been speculated that similar events may of the total) is oxidised by lipoxygenase activityconcentrated mainly in the germ95. This oxidation,occur in bread doughs containing fat and may

help to explain the role of fat in improving gas which occurs when the dough is formed as wellas in the flour itself, competes with the yeast forretention in bread doughs92. Such fat crystals are

also thought to be important in another respect, available dissolved oxygen in the dough and withother oxygen-requiring reactions, such as the ox-i.e. in gas retention during the baking stage. As

the temperature of the dough or batter increases, idation of ascorbic acid, which is a necessaryfirst step in its bread improving reaction. This isthe fat crystals melt and the liquid oil then flows

over the inner surface of the gas cells to form a particularly important in wholemeal bread doughs,in which ascorbic acid is added as the sole breadhybrid interface comprising the oil layer in ad-

dition to the protein and/or polar lipid layer92. improver in countries such as the U.K.94. The freefatty acids and other lipid degradation productsThe layer of oil helps to maintain the continuity

of the gas/liquid interface in the expanding dough may also act as foam destabilisers and depress loafvolume. The effect of enzymic degradation of lipidspiece and thus aids gas retention during oven

spring. A further advantage of the b′ fat crystal on baking performance is much more significant inwholemeal than white flours because of the muchpolymorph over the b polymorph in this respect

may be that the former has a lower melting tem- greater rate at which it occurs in wholemealflours96.perature and thus may be more readily available

for contributing to gas cell surface integrity during The addition to wheat flour of small amounts(1–2%) of pearlings, obtained by dry abrasionbaking than the latter.

In addition to its effect on the rheological be- (pearling) of the outer pericarp layers of the wheatcaryopsis, has been shown to have a marked detri-haviour of the bulk dough phase, the possibilitymental effect on loaf volume97. Since lipase isthat bakery fat/shortening also participates in gasconcentrated in the bran fraction98, it is possibleretention through events at the gas/liquid interfacethat the effect of pearlings on loaf volume couldadds another level of complexity. It will be neces-be due, in part, to increased levels of FA containingsary in future work to determine the relative con-PUFA. The differences in FA levels between whitetributions of the different factors that have beenflour, wholemeal and flour from pearled grain areproposed as being of importance to gas cell sta-small99, however, and are unlikely to account forbility, gas retention and final bread quality.the observed differences in baking performance.

Moreover, heat treatment (autoclaving) does notalleviate the adverse effect of the outermost branGAS RETENTION IN WHOLEMEAL BREADfraction on loaf volume99, indicating that the ob-DOUGHSserved effect is due largely to a physical rather

Wholemeal bread is produced in various forms, than a biochemical mechanism. In other words,ranging from products with low specific volume the result is inconsistent with the possibility that

heat-sensitive components (e.g. enzymes) are re-and a dense crumb texture to a more expanded and


sponsible for the adverse effect of this fraction on expansion, the precise architecture of the gas cellsis still incompletely understood. Nevertheless,loaf volume. It is quite plausible that particulate

components, of the kind that have been identified recent studies have provided accumulating evid-ence for the involvement of interfacial phenomenain wholemeal bread, could create areas of weakness

in an expanding wholemeal dough97,99. SEM mi- in gas cell stabilisation and gas retention. Hypo-thetically, the dough consists of discrete gas cellscrographs revealed that the bran materials are

incorporated into the gas cell walls of doughs lined with liquid films stabilised by surface activematerials and embedded in a continuous starch–produced from flours of both control and pearled

wheat97,99. These materials, notably the epicarp protein matrix soon after mixing. As the volumefraction of the gas increases, the expanding gashairs, were found to impede the normal formation

and development of the gas cell structure of whole- cells become polyhedral, with discontinuities de-veloping in the matrix, leaving areas that containmeal dough, and perhaps more significantly, to

restrict and force gas cells to expand in a particular only a thin, lamellar film between neighbouringcells. The degree to which such discontinuitiesdimension. This distorts greatly the gas cell struc-

ture, and may contribute to the resultant crumb occur is dependent on the extensibility of thestarch–protein matrix, which, in turn, is dependentmorphology (e.g. the size, the shape and the dis-

tribution of the gas cells), which is an important largely on the gluten proteins. The surface areaof the lamellar film increases as discontinuities inelement of crumb texture99.the starch–protein matrix develop further duringThe outer branny materials may also providethe second stage of expansion. Failure of thean extremely effective capillary system, which maylamellar film to maintain the rate of new surfaceremove water from the dough aqueous phase andarea generation towards the end of oven springthus reduce its availability for formation of lamellarleads to the rupture of this film and, consequently,films. This was indicated in an experiment wherethe loss of gas. Thus, the end of dough expansion2% additional water was found to have a beneficialis marked by the rupture of the film, and not thateffect on the volume of loaves made from whiteof the starch–protein matrix. Additional factors,flour to which bran had been added100. Furthersuch as the presence of the outer branny layers ofaddition of water produced a marked improvementthe grain, which may alter the continuity andonly in the presence of sodium stearoyl lactylaterheological properties of the starch–protein matrix(SSL); loaves with 6% additional water and 2%and the availability of water for lamellar filmSSL had volumes almost equal to that of theformation, are involved in gas cell stabilisationcontrol without bran100. The latter observationand gas retention in wholemeal doughs.may be explained by the combined contribution

Further research is needed to determine theof water and surfactants to the formation andvalidity of the liquid film hypothesis and to definestabilisation of lamellar films and thus to gas cellthe molecular species involved in surface filmstabilisation and gas retention.stabilisation in wheat bread doughs. The individualThe technological significance of gluten proteinsand joint contributions of the starch–proteinin bread making depends on their contribution tomatrix and of the liquid film to gas retention alsothe extensibility and cohesive strength of the bulkneed to be clarified. Once the specific roles of thedough phase. The addition of vital wheat glutenstarch–protein matrix and the lamellar films arehas beneficial effects on the microstructural in-understood, differences between cultivars could betegrity of wholemeal bread97. The gas cell wallsinvestigated in order to improve efficiencies in theappear to be ‘strengthened’ by the extra glutenuse of materials and ingredients, and to facilitatein the starch–protein matrix. This may result ingreater control over processing performances andimproved mechanical properties of the starch–product quality.protein matrix so that its rupture is delayed, lead-

ing to improved gas retention.

AcknowledgementsThe research described here that was carried out inCONCLUSIONSour laboratory was funded in part by a research stu-

Although efforts have been made over decades to dentship to ZG from RHM Research and Engineeringinvestigate the mechanism(s) of gas retention in Ltd, The Lord Rank Centre, High Wycombe, UK and

by an Overseas Research Student Award also to ZGdough and the events that occur during dough

Z. Gan et al.228

time, temperature, and water absorption. Rheologica Actafrom the Committee of Vice-Chancellors and Principals9 (1970) 223–238.of UK Universities, to both of whom thanks are ex-

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Gas Cell Stabilisation and Gas Retention in Wheat Bread Dough

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