dough microstructure and dough handling and baking properties

Review

Relationships between Dough and bread are primarily composed of proteins, lipids, carbohydrates, water and air. The dough ingredients in combination with the processing conditions determine the microstructure, and the microstructure determines the appearance, texture, taste perception and stability of the final product. Microstructural studies have the potential for elucidating the phenomena underlying dough handling and baking properties.

Great structural changes take place during breadmaking. During mixing, water and flour are transformed into a viscoelastic dough. Wheat flour dough consists of two continuous and immiscible aqueous phases: the gluten phase, and the liquid phase containing water-soluble compounds and starch. During baking, the viscoelastic dough is transformed into an elastic bread. The most dramatic change at the macroscopic level is the expansion of gas cells into an open network of pores. The baking and handling properties are greatly influenced by the processing conditions (mixing, moulding, sheeting, fermentation, baking) and by the ingredients (flour quality, water content, added lipids, sugar, enzymes, bran). The various baking and handling properties can be explained by the structure of the dough at different levels of resolution: molecular, supramolecular and macroscopic. The effects of processing and of various ingredients such as lipids, emulsifiers, enzymes and bran on the microscopic and macroscopic structures of wheat dough are reviewed. We also attempt to describe how these structural levels are related to dough handling properties and baking quality. Microscopy and imaging techniques in dough and bread analysis A variety of microscopy techniques is available for studying gluten network structures, gas cell architecture, fat and cell-wall structures, and starch gelatinization. Bright-field and fluorescence microscopy techniques are frequently employed because they allow selective staining of different chemical components. Both cryo- and plastic sections have been used. However, cryo-sectioning involves freezing, and the formation of ice crystals may damage the structure. On the other hand, preparation for plastic embedding involves dehydration, which may cause shrinkage’. The most commonly used staining systems for doughs and breads in bright-field2 and fluorescence microscopy3 are presented in Tables 1 and 2. Starch and fat are birefringent, and can be examined under polarized light. Some components exhibit autofluorescence. In cereals, the maih sources of autofluorescence are lignin and femlic acid3. The use of fluorescently lahelled antibodies and lectinsl that hind to

flour/dough microstructure and dough handling and baking properties K. Autio and T. Laurikainen

specific protein or carbohydrate components of the respectively, increases the number of specific components that can be studied. Figure 1 shows a fluorescence micrograph of a wheat grain. Stains that are used in conventional light microscopy can also be applied in confocal laser scanning light microscopy (CSLM). This technique offers many advantages for studying the relationships between composition, processing and final quality of the product5m7.One of the main advantages is the minimal degree of sample preparation that is required. Because the sample is optically sectioned, it does not require prior freezing or embedding. CSLM is especially suitable for high-fat foods, which are difficult to prepare for conventional microscopy without the loss or migration of fat. CSLM yields three-dimensional images, enabling visualization of dynamic processes such as the growth of air bubbles during fermentatior-3. dough,

Table 1. Most commonly used stains for doughs and breads in bright-field microscopy’ Component

Stain

Colour

Starch

Iodine solution

Black, violet

Amylose

Iodine solution

Blue

Amylopectin

Iodine solution

Beige, brown

Protein

Light green

Green

Lipid

SudanIll and IV

Red

Oil red 0

Red

Sudan black B

Blue-black

K. Autio and T. Laurikainen are at VTT Biotechnology PO Box 1500, FIN-02044 e-mail: [email protected]’i).

W,

tspoo,

and Food Research, Finland (fax: +358-9-455-2103;

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Table 2. Most commonly used stains for cereals in fluorescence microscopy” Component

Stain

Colour

Protein

AN5

White or blue

Acid fuchsin

Red, brown, orange

Calcofluor

Blue

Congo red

Orange

Nile blue

Yellow

B-Glucan

Fat “Adapted from Ref. 3 ANS, l-Anile-8-naphthalene

sulphonic acid

Electron microscopy is required to reveal the fine details of the structure. The value of information obtained from examining the ultrastructure of gluten and dough depends on the method of sample preparations. Gluten is highly sensitive to freezing, but can be prepared for structure evaluation by rapid freezing and freeze etching’. Good results have also been obtained by critical-point drying after chemical fixation with osmium tetroxide and a low concentration of glutaraldehyde’O. Critical-point drying involves the dehydration of samples with fluids such as ethanol and acetone, followed by immersion in liquid carbon dioxide. Cryo-scanning electron microscopy (cryoSEM) is suitable for examining high-fat samples’. Effects of processing Mixing The major purposes of mixing are to blend the ingredients into a quasi-homogeneous mixture, to develop the gluten matrix in wheat dough, and to incorporate air. In an undermixed dough, starch and proteins are unevenly distributed, and compact protein masses are stretched out into sheets during mixing”J2. This is demonstrated in Fig. 2a and 2b. Some flours resist overmixing, whereas others change rapidly13. Overmixing may cause damage to the gluten network’“, increased solubility of proteins and decreased extractability of lipids. Overmixing usually

Fig. 1 Embedded section of a wheat grain. The section was stained wrth acid fuchsm and Calrofluor White M2R New (Polysciences Inc.), and then examined under a fluorescence microscope. The primary cell walls appear blue, protein brown to red, starch black, and the outer lignified layers yellow. Scale bar = 250 p,m.

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results in a sticky dough15; one reason for this is that the mechanical forces applied to the dough decrease the molecular weight of the protein. In addition to overmixing, the surface tension of water-soluble compounds and the levels of endogenous enzymes affect the stickiness of dough. After mixing, dough contains occluded gas cells whose diameters are typically in the range of 10-100ym’6. A precise determination of the size distribution of gas cells is difficult, because microscopic examination will underestimate the number of small gas cells. This is because those cells with a diameter that is smaller than the thickness of the section may not necessarily be observed. The number and size of the gas bubbles have a great effect on the final bread character”. High-speed mixers with blades that shear the dough produce large numbers of small bubbles and result in a fine-structured bread, whereas low-speed mixers, such as spiral-type mixers, occlude more air but result in an uneven pore size distribution. Dividing, moulding and sheeting The dividing and moulding of dough modify the gas cell structure by causing the small cells to burst and coalesce into larger vnes17.The sheeting of dough is an important unit operation in the production of breads, pizzas, pastry doughs and cookies, and has an important role in determining the final quality of the dough’“. Microstructural studies have shown that sheeting results in the organization of the protein network in the dough, and that excessive sheeting can break it down”. Repeated sheeting also affects the gas cell structure by reducing the bubble size”. Because repeated sheeting results in a gradual decrease in the extensibility and resistance of wheat dough, as is the case in the production of multi-layered pastries, the doughs used to make such products need to be formulated to a higher strengthL8. Fermentation Yeast fermentation generates carbon dioxide and the dough expands as a result of excess pressure in the gas cells16. The growth of gas cells depends in part on the size of the cells. Greater pressure is needed to expand a small gas cell than a larger one, and it is possible that the smallest bubbles will not expand at a1116.Bubbles that have a diameter of just a few micrometers have been observed between large bubbles in a dough matrix*l. Gas cell stabilization and gas retention are of considerable interest because they largely determine the crumb structure and volume of wheat bread. Sandwich breads contain large numbers of small bubbles of uniform size, whereas the size of the bubbles in baguettes is more random”. In the case of frozen fermented doughs, the gas cell structure significantly affects the frozen storage stability22,23.A dough that contains a large number of small bubbles with a narrow size distribution and thick walls will be more stable than a dough that contains bubbles with a less uniform size distribution and thin walls surrounding the larger bubbles. It is also known that the stability of frozen dough can be increased by the presence of lipids. Recent studies on the Trends in Food Science & Technology June 1997 [Vol. 81

roles of lipids in the stabilization of gas cells have greatly improved our understanding of the.mechanism involved24.Z. This is discussed below under ‘Lipids and emulsifiers’. It is unclear what happens to the gas cells during fermentation and baking. It is generally believed that the loss of gas is due to the rupture of the walls of the gas cells’6,26.SEM studies have shown that wheat bread doughs produced using Fig. 2 the Chorleywood Bread Process con- Cryo-sections of an undermixed dough (a) and an optimally mixed dough (b). The sections were stained tain discontinuities in the starch-gluten with iodine solution and light green. Protein appears green, and starch granules brown. matrix that surrounds the gas cells”. The Scale bars = 100 pm. authors suggested that the integrity of the gas cells is maintained by the existence of a liquid film of surface-active materials at the Effects of ingredients gas-liquid interface, and that the gas cells are stabilized Lipids and emulsifiers The incorporation of lipids into bread dough results in by the liquid film. In their view, this liquid film plays an important role in gas retentionz8. The gas cells remain a larger final loaf volume (improved ovenspring), a softer crumb, a less crisp crust and improved keeping discrete during the first stage (Fig. 3a), until discontinuities develop in the starch-protein matrix (Fig. 3b), and quality of the bread35.Recent work on the roles of lipids in the stabilization of air bubbles in the degree to which such discontinucake batters’” and bread doughs25has ities occur is largely dependent on the (a) shown that lipid crystals that origigluten proteins. The rheological propnate from shortenings move from the erties of the bulk phase determine the lipid phase to the gas-liquid interface, extensibility at this stage. With inwhere they are in direct contact with creasing fermentation time, the surthe gas in the bubble. The expansion face area of the liquid film will inof gas cells during fermentation recrease. The stability of the liquid film sults in more crystals becoming abdetermines the behaviour of the dough 6) sorbed to the bubble surface. During at this stage. Surface-active materials baking, the lipid crystals melt, allowprobably stabilize the film so that it ing the bubbles to grow without rupcan expand across a larger area withturing, giving the bread a large volout rupturing2”. ume and a fine crumb structure. The Fermentation may also cause baking performance is very much dechanges in cell-wall components, through the activities of endogenous pendent on the type of lipid. Solid fat composed of a large number of very or added enzymes. This is discussed small fat crystals with high melting in more detail below in connection points will facilitate stabilization betwith the roles of enzymes. ter than larger and fewer crystals. Emulsifiers are added to bread to Baking 0 Starch granules The major structural changes that (1) increase the dough strength and gas retention; (2) improve the prodtake place during the heating of wheat E Starch-protein matrix uct volume, particularly in the case dough are the expansion of gas cells 0 Gas cell lined with a liquid film (in the early oven stage), starch gelaof crusty and high-volume breads, where they may be an essential intinization29,30, protein crosslinking3’, melting of fat crystals and their incor- tig. 3 gredient; (3) allow weaker flours to be used, in those instances where poration into the surface of air cellsZ4.Zs, A model of dough expansion (adapted standard bread quality would not be rupture of gas cells (Fig. 3c) and some- from Ref. 28). (a), At early stages of possible without a suitable emulsifier times fragmentation of cell walls3’. fermentation, the expanding gas cells are Starch gelatinization results in de- embedded in a starch-protein matrix. (b), being included in the recipe; (4) aid watering of the gluten phase33.These At advanced stages of fermentation and dough stability and gas retention and changes are dependent on the temincrease volume in brown. mixed grain early stages of baking, the starch-protein and high-fibre breads; (5) improve perature, humidity and duration of baking. The most dramatic change at matrix fails to enclose the gas cells crumb softness and keeping quality in enriched bakery products such as the macroscopic level is the expansion completely, leaving areas with just a thin of gas cells into an open network of liquid film. (c), At the end of ovenspring, teacakes, soft rolls and fruit buns; pores’“. the liquid film will rupture. and (6) decrease staling36. Trends in Food Science & Technology June 1997 [Vol. 81

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According to Maat et al.&, the positive effect of xylanase on bread volume is due to the redistribution of water from the pentosan phase to the gluten phase. The increase in the volume of the gluten fraction increases its extensibility, which will result in better ovenspring. The results of baking were better when a-amylase was used together with xylanase than when either of the enzymes was used separately46. Bran Increased dietary fibre content has been reported to cause several changes Fig. 4 in wheat dough and bread: the dough yield increases by 3-5%, the dough Embedded sections of wheat breads: (a) with no enzymes added; (b) with added cell-wall-degrading becomes shorter and more moist, and enzymes. The sections were stained with acid fuchsin and Calcofluor White M2R New (Polysciences the fermentation tolerance decreases4’. Inc.). The primary cell walls appear blue, protein brown to red, and starch black. Scale bar = 250b.m. Other dough properties may also be affected, including kneading and handThe mechanisms by which emulsifiers strengthen gluten ling properties, proofing time, fermentation time, postand decrease the staling of bread are not fully understood. stiffening and stickiness. The proportion of soluble and Emulsifiers improve the ability of gluten to form a film insoluble fibre influences the rate of water absorption by around the gas bubbles3’. Amylopectin recrystallization the flour mixture46. The most marked changes in loaf is still believed to be the major cause of bread staling38. properties are that the bread volume decreases and the However, because emulsifiers are known to form com- bread crumb loses its elasticityJ7. plexes with amylose, but are widely believed not to form The deleterious effects of the addition of fibre on complexes with the amylopectin fraction, it is unclear dough structure have been suggested to be due to the how they affect retrogradation. More recent studies have dilution of the gluten network, which in turn impairs gas suggested that lipids do in fact interact directly with the retention rather than gas production. Microscopic examylopectin fraction, and retard retrogradation through amination revealed a major difference between the crumb the formation of amylopectin-lipid complexes39. structure of control and fibre-containing breads48. The crumb structure of wheat breads was composed of thin Enzymes sheets and filaments, which were essentially absent in Enzymes are used in baking to optimize baking prop- fibre-enriched breads. According to Gan et a1.49,the erties and improve the quality of baked products. Amylases outer layers of bran in expanded dough appear to disrupt are widely used to increase the bread volume and reduce the starch-gluten matrix and also restrict and force gas the staling rate of the crumb. According to Cauvain and cells to expand in a particular dimension. This greatly Chamberlair?, fungal a-amylase prolongs the period of distorts the gas cell structure and may contribute to the dough expansion in the oven, increasing the maximum resultant crumb morphology, which is an important dough-piece height and loaf volume. The proportion of element of crumb texture. Supplementation of baked a-amylase in an enzyme preparation must be carefully products with dietary fibre requires changes in processoptimized, because the presence of cY-amylase has been ing techniques to achieve a product that is of acceptable reported to cause sticky doughs and problems in dough quality for consumers. handling ‘5,41.Optimal levels of fungal ol-amylase have been reported to improve the crumb grain42. ol-Amylase Future trends affects the structure of starch granules by making the Elucidation of the relationships between texture and granule surface porous42, and thus amylose may leach food structure will allow us to improve existing products out of the granules. and design new ones with specific textural properties. Hemicellulases are used in baking because of their The task is not easy. The structural components need to ability to decrease water absorption by the dough, by be studied at different levels. We need to know how the hydrolysing the pentosans. Hemicellulases have been various structural elements interact to form a threereported to soften dough43,44,increase loaf volume and dimensional structure. The trend towards the conversion improve crumb structure 45,46 . Microstructural studies of of images into numerical data should help in achieving wheat dough and bread have shown that, without hemi- this goal. The quantification of structural parameters by cellulases, insoluble cell-wall fragments are dispersed in image analysis should facilitate the collection of better the gluten matrix; hemicellulases decrease the number statistical data on the structural components of dough of fragments32(Fig. 4a and 4b). and bread from a large number of sections. 184

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