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3 Chemical Components Carbohydrates The terminology surrounding carbohydrates frequently serves to confuse rather than to clarify. Archaic and modern conventions are often inter- mixed and definitions of some terms are incon- sistent with their use. Even the term carbohydrate itself is not entirely valid. It originated in the belief that naturally occurring compounds of this class could be represented formally as hydrates of carbon. i.e. C,(H,O),,. This definition is too rigid however as the important deoxy sugars like rhamnose, the uronic acids and compounds such importance, both in their contribution to the structural and storage components of the grain, and to the behaviour of grains and their pro- ducts during processing. In this context the most important monosaccharide, because of its abund- ance, is the six-carbon polyhydroxyaldehyde: do HbH, HOCH, 4TQHHt HO H@H 1 tQoH CH,OH HfJcH,oH H:”;” HOCH, o HBOH I as ascorbic acid would be excluded and acetic ! ?H a, acid and phloroglucinol would qualify for inclu- (1) (2) sion. Nevertheless the term carbohydrate remains to describe those polyhydroxy compounds which reduce Fehlings solution either before or after It is customary to classify carbohydrates Thus: monosaccharides (1 unit), oligosaccharides (2-20 units) and polysaccharides (>20 units). Monosaccharides are the simplest carbohy- b HO,CHz 0 4 hydrolysis with mineral acids (Percival, 1962). ‘;1 YH OH H according to their degree of polymerization. (3) (4) HOCH, o drates; most of them are sugars. Monosaccharides H OH OH OH may have 3-8 carbon atoms but only those with (5) (6) 5 carbons (pentoses) and 6 carbons (hexoses) are common. Both pentoses and hexoses exist in a number of isomeric forms, they may be polyhydroxyaldehydes or polyhydroxyketones. OH H HH H wy HO Structurally, they occur in ring form which (7) (8) may be six-membered (pyranose form) Or five-membered (furanose form). In mature cereal grains the monomers are of components of polymers, they are of extreme FIG 3.1 Structural representations of (1) xylose (beta-D- xylopyranose), (2) arabinose (alpha-L-arabinofuranose), (3) glucose (beta-D-glucopyranose), (4) fructose (beta-D-fructo- furanose, (5) D-galacturonic acid, (6) ribose (beta+ (8) mannose (alpha-D-mannopyranose). little importance in their Own right but, as ribofuranose), (7) deoxyribose (beta-D-deoxyribofurose), and 53 54 TECHNOLOGY OF CEREALS Oligosaccharides The smallest oligosaccharide, the disaccharide, comprises two sugar molecules joined by a glycosidic link. Although this may appear to be a simple association it is capable of considerable variation according to the configuration of the glycosidic link and the position of the hydroxyl group involved in the bonding. Three important D-glucose. It is the monomeric unit of starch, variants among disaccharides involving only cellulose and beta-D-ghcans. a-D-glucopyranose are shown in Fig. 3.3. The most important pentoses are the poly- In these compounds the reducing group of only hydroxyaldehydes D-xylose and L-arabinose, one of the monosaccharide molecules is involved because of their contribution to cell wall polymers. in the glycosidic link and the reducing group of The structures of these compounds and of some the other remains functional. other monosaccharides found in cereals are shown In sucrose, another important disaccharide in Fig. 3.1. found in plants, fructose and glucose residues are The most abundant derivatives of monosaccha- joined through the reducing groups of both; rides are those in which the reducing group forms hence their reducing properties are lost. Sucrose a glycosidic link with the hydroxyl group of is readily hydrolyzed under mildly acid condi- another organic compound (as in Fig. 3.2), fre- tions, or enzymically, to yield its component quently another molecule of the same species monomers which of course again behave as reduc- or another monosaccharide. Sugar molecules ing sugars. Sucrose is the main carbon compound may be joined to form short or long chains involved in translocating photosynthate to the by a series of glycosidic links, thus producing grain. It features prominently during develop- oligosaccharides or polysaccharides. ment rather than in the mature grain because it -0 H -y + HO-I3 T A-I3 OH H20 FIG 3.2 Formation of the glycosidic link. CH20H HO HQOGw HO He0 Ho0 H OH H OH CHZOH H OH (1) (2) HO H OH I HO HQH H OH (3) FIG 3.3 Structural conformation of (1) maltose (a-D-ghcopyronosyl-( 1-+4)-a-D-glucopyranose), (2) cellobiose (~-~-glucopyranosyl-( 1+4)-a-~-glucopyranose), (3) isomaltose (a-D-glucopyranosyl- ( 1+6)-P-D-glucopyranose). CHEMICAL COMPONENTS 55 is converted during maturation, to structural and longer-term storage carbohydrates such as starch. In sweet corn the sucrose content is higher by a factor of 2-4 throughout grain development than in other types of maize at a similar stage, as the rate of conversion is slower (Boyer and Shannon, Milled 0.22-0.45 1983). Hull 0.6 TABLE 3.3 Proportions of Soluble Sugars in Mill Fractions of Rice* % of dry matter ill fraction Rough 0.5-1-2 Brown Bran 5.5-6.9 Embryo 0.7-1.3 8-12 Literature values for sugars in cereals vary with methods of analysis and with varieties examined and in consequence tables which bring together results of different authors can be misleading. Henry recently analyzed two varieties of each of Polysaccharides six cereal species. All results were obtained by the same methods and are thus comparable. Oligomers and polymers in which glucose Values for free glucose and total (including that residues are linked by glycosidic bonds are known in sucrose and trisaccharides) are given in as glucans. The starch polymers, amylose and Table 3.1. amylopectin, are glucans in which the ~~(1-4)- link, as in maltose (Fig. 3.2), features. Addition- ally, in amylopectin the a-(1+6)-linkY as in isomaltose (Fig. 3.3) OCCUrS, giving rise to branch points. The same linkages are present in the other main storage carbohydrate found in sweet corn. The product is known as phytoglycogen, it is Glucose 0.17 0.12 0.14 0.21 0.25 0.11 highly branched with a-( 1-4) unit chain lengths 0.09 0.13 0.19 0.29 o.21 O.ll of 10-14 glucose residues and outer chains of Fructose 2.31 1.01 0.84 5.79 3.22 1.73 1.98 1.00 0.75 5.11 3.05 2.46 6-30 units (Marshall and Whelan, 1974). Unlike the true starch polymers phytoglycogen is largely soluble in water and as a result the soluble saccharides of sweet corn contribute about 12% are discussed at greater length in a later section of this chapter. In cellulose the P-(1+4) form of linkage, as present in cellobiose (Fig- 3.3) occurs. P-Links are also involved in the other important cell wall components, ( 1-3, 1+4)-P-~-glucan. These poly- mers contribute about a quarter of the cell walls of wheat aleurone but they are particularly TABLE 3.2 important in oat and barley grains, in the starchy Proportions of Free Sugars in the Anatomical Fractions of the Maize Grain* endosperm of which they may contribute as much as 70% (Fincher and Stone, 1986). With water Grain part yoof dry matter they form viscous gums and contribute sigmficantly to dietary fibre. Both the ratio of (1-3) to (1-4) Endosperm 0.5-0.8 Embryo 10.0-12.5 links and the number of similar bonds in an un- Pericarp 0.2-0.4 interrupted sequence differ between the species. Extraction and analysis of the mixed linkage com- Tip cap 1.6 pounds are particularly difficult in the presence of such large excesses of a-glucan (Wood, 1986). * Data from Juliana and Bechtel, 1985. TABLE 3.1 Total Soluble Glucose and Fmctose in TWO Varieties of Each of Six Cereals* Barley Oat Rice Rye Triticale Wheat * Data from Henry, 1985. Free sugars are not distributed uniformly ofthe total grain dry weight. The starch Polymers throughout the grain. The distribution in the maize grain is shown in Table 3.2. The embryo has the highest concentration of free sugars in other cereals also. This is reflected in the distribution among mill fractions, as illustrated with respect to rice in Table 3.3. Whole grain 1.61-2.22 * Data from Watson, 1987. 56 TECHNOLOGY OF CEREALS -4)-B-D-XYL(p)-(l-4)-~-D-XYL(p)-(I-4)-~-D-XYL(p)-(I-4)-~-D-XYL (p)- (I- 3 3 I I I I a-L-ARA(f) a-L-ARA(f) FIG 3.4 Structure of arabinoxylan of wheat aleurone and starchy endosperm cell walls. p, represents the pyranose or &membered ring form; f, represents the furanose or 5-membered ring form. Pentosans which appear white when seen as a bulk powder because of light scattering at the starch-air inter- Whi1e glucans are po1ymers Of a sing1e face. They have a refractive index of about 1.5. sugar species the common pentosans (polymers Specific gravity depends upon moisture content of pentose sugars) comprise two or more different but it is about 1.5. The mysteries of granule species, each in a different isomeric form. Thus structure, development and behaviour have arabinoxylans, found in endosperm walls of wheat exercized the minds of scientists for hundreds of and other cereals, have a xylanopyranosyl back- years and continue to do so. Granules from bone to which are attached single arabinofuranosyl different species differ in their properties and residues (Fig. 3.4). there is even variation in form among granules from the same storage organ. Shape is determined Starch in part by the way that new starch is added to existing granules, in part by physicochemical Starch is the most abundant carbohydrate in conditions existing during the period of growth all cereal grains, constituting about 64% of the and in part by composition. dry matter of the entire wheat grain (about 70% Composition of the endosperm), about 73% of the dry matter of the dent maize grain and 62% of the proso millet grain. It occurs as discrete granules of up The main way in which composition varies is to 30 pm diameter and characteristic of the species in the relative proportions of the two macro- in shape. molecular species of which granules consist Starch granules are solid, optically clear bodies (Fig. 3.5). CH20H CH20H CH,OH CH20H ---o p&oQoQoQ O--- H OH H OH (i) H OH H OH CH 20H CH20H --.oJQ0q (Ii) 0 I CH2OH CH20H ---o ~o~o&oJF& 0 --- H OH H OH H OH H OH FIG 3.5 Structural representation of amylose (i) and amylopectin (ii). CHEMICAL COMPONENTS 57 Amylose comprises linear chains of (144) B chains - those to which A chains are attached. linked a-D-glucopyranosyl residues. Amylopectin C chains - chains which carry the only has, in addition, (1-6) tri-0-substituted residues reducing group of the molecule. acting as branch points. Amylose has between The amylose contents of most cereal starches 1000 and 4400 residues, giving it a molecular lie between 20 and 35%, but mutants have been weight between 1.6 x lo5 and 7.1 x lo5. In used in breeding programmes to produce culti- solution amylose molecules adopt a helical form vars with abnormally high or low amylose con- and may associate with organic acids, alcohols or, tents. It is in diploid species such as maize and more importantly, lipids to form complexes in barley that such breeding has been most success- which a saturated fatty acid chain occupies the ful as polyploid species are more conservative, core of the helix. Binding of polyiodide ions in with single mutations having less chance of the core in the same way is responsible for the expression (cf. Ch. 2). High amylopectin types characteristic blue coloration of starch by iodine. are generally described as waxy as the appearance The average length of amylopectin branches is of the endosperms of the first mutants discovered 17-26 residues. As their reducing groups are had suggested a waxy composition. Waxy maize involved in bonding, only one is exposed. The cultivars have up to 98% amylopectin (100% molecule is generally considered to consist of 3 according to some references). High amylose types of chain (Fig. 3.6): maize starches consist of up to 80% amylose. A chains - side chains linked only via their Granular form reducing ends to the rest of the molecule. Although some variation exists within species, there are many characteristic features by which TABLE 3.4 Characteristics of Starch Granules of Cereals* 0 _- - - - - - - - - - - - - - - - Cereal Shape and diameter Features Wheat Large, lenticular: 15-30 Characteristic -____ _-__-_ (Pm) equatorial groove Small, spherical: 1-10 Angular where closely packed Triticale Large, lenticular, 1-30 As wheat Small, spherical: 1-10 Large lenticular: 10-40 As wheat, often displaying radial cracks. Visible hilum Barley Small, spherical: 2-10 As wheat Large, lenticular: 10-30 Small, spherical: 1-5 Compound, ovoid: Simple, angular: 2-10 Comprising up to 80 - - __-- up to 60 granuli Rice Compound granules comprising up to 150 angular granuli: 2-10 p Maize Spherical: In floury endosperm Angular: In flinty endosperm Both types 2-30; average 10 As maize FIG 3.6 Structure of (potato) amylopectin proposed by Sorghum Spherica"analar: 16-20; average 15 Robin et al. (1974). Bands marked 1 are considered to be Spherical/angular: 4-12; As maize average 7 crystalline while alternating 2 bands are amorphous. Reproduced by courtesy of American Association of Cereal Chemists. - - - - - -. - - - - - - - - - - - - - - - - - Millet, pearl * Based on Kent, 1983. 58 TECHNOLOGY OF CEREALS FIG 3.7 Scanning electon micrograph of one large starch granule and numerous small starch granules of wheat. The large granule shows the equatorial groove. From A.D. Evers, Stiirke, 1971, 23: 157. Copyright by Leica U.K., Reproduced with permission of the Editor of Die Stiirke. cereals are similar in shape to the smaller popula- tion of Triticeae granules, but rice and oats have some compound granules in which many granuli fit together to produce large ovoid wholes. Shapes of high-amylose granules are varied and may be related to their individual composition. The later developers tend to be filamentous, some resembling strings of beads. Characteristics of starch granules from cereals are shown in Table 3.4. Within the endosperm of a species small differ- ences in granule shape may arise as a result of packing conditions. These can be seen in grains as mealy and vitreous (or horny) regions. In mealy regions, packing is loose and granules adopt what appears to be their natural form. In horny regions close packing causes granules to become multi-faceted as a result of mutual pres- sure. Small indentations can also arise from other an experienced microscopist can identify the source, either from observation of an aqueous suspension at room temperature or with the additional help of observed changes when the suspension is heated, leading to gelatinization at a temperature characteristic of the species and type (Snyder, 1984). The characteristic blue staining reaction with iodine/potassium iodide solution does not occur with waxy granules, which contain virtually 'no amylose, they stain brownish red to yellow. It is characteristic for amylose percentage to increase during endosperm development, consequently staining reactions change during growth. Granules of cereals from the Triticeae tribe (see Ch. 2) are of two distinct types. The larger ones are biconvex while the smaller ones are nearly spherical (Fig. 3.7). Granules from most other CHEMICAL COMPONENTS 59 FiG 3.8 Scanning electron micrograph of maize starch granules of spherical and angular types. Some angular granules show indentations due to pressure from protein bodies. endosperm constituents such as protein bodies. (Fig. 3.8). Pitting on the surface can be caused by enzymic hydrolysis and it is possible to find such granules in some cereal grains in which germination has begun or in which insect damage has occurred. There is no evidence that these two physical modifications to granule form change the chemical properties of the granules. As granules are transparent some manifesta- tions of internal structure can be detected, even if their significance cannot be fully appreciated. One such internal feature is the hilum exhibited by granules of some species. It is a small air- space, considered to represent the point of initia- tion around which growth occurred (Hall and Sayre, 1969). This assumes that granules grow by deposition of new starch material on the outer surface of existing granules, and indeed this has . been demonstrated by detection of radioactively labelled precursors incorporated into growing granules (Badenhuizen, 1969). Such a system of growth allows for the change in shape that occurs in starches of the Triticeae, by preferential deposition on some parts of the surface. As a result they change from tiny spheres to larger lentil shaped granules (Evers, 1971). Some structures not evident in untreated granules can be revealed or exaggerated by treat- ment with weak acid or amylolytic enzymes. In cereal starches a lamellate structure results from removal of more susceptible layers and persistence of more resistant layers. Layers may be spaced progressively more closely towards the outside. The number of rings appears to coincide with the number of days for which a granule grows (Buttrose, 1962). Lamellae cannot be revealed in granules from plants grown under conditions of continuous illumination (Evers, 1979). 60 TECHNOLOGY OF CEREALS Size distributions less than half the total starch present. Some 70% is amorphous; this comprises all the amylose but must also include much of the amylopectin. The evidence of biochemical studies and electron microscopy has pointed to the existence of struc- tures with a periodicity of 5-10 nm, reflecting the alternating crystalline and amorphous zones of amylopectin. Granule surface and minor components The distribution of amylose and amylopectin molecules in one starch granule was estimated by French (1984): for one spherical granule 15 pm in diameter, with a mass of 2.65 x lO-9 g there would be about 2.5 x lo9 molecules of amylose (D.P = 1000, 25% of total starch) and 7.4 x lo7 molecules of amylopectin (D.P. = 100,000, 75% of starch). If the molecular chains are perpend- icular to the surface of the granule there would be about 14 x 10' molecular chains terminating at the surface. Of these, 3.5 x 10' would be amylose molecules and the rest would be Surface characteristics of granules are also affected by the minor components of starches. Bowler et al. (1985) reviewed developments in work on these although they point out that this is an under-researched area. Non-starch materials present in commercial starch granules can arise from two sources. They may be inherent com- ponents of the granules in their natural condition or they may arise as deposits of material solubilized or dispersed during the process by which the starch is separated. The main non-starch components of starch granules are protein and lipid. Amounts vary with starch type: in maize 0.35% of protein (N x 6.25) is present on average. Slightly more is present in wheat starch (0.4%). The most significant proteins in terms of their recognized effects on starch behaviour are amylolytic enzymes bound to the surface. Even traces of alpha-amylase can have drastic effects on pasting properties through hydrolyzing starch polymers at temperatures up to the enzymes' inactivation temperatures. SDS PAGE (sodium dodecyl sulphate, poly- acrylamide gel elecrophoresis) showed surface The literature contains many tables of granule size ranges and size distributions of granules from different botanical sources. While such tables are useful guides they do not all accord in detail and some fail to indicate the nature of the distribution. For example the bimodal distribution of the Triticeae is not always indicated although this is an important characteristic by which the source of a starch may be recognized. In wheats the proportional relationship between large biconvex and small spherical granules is fairly constant (approx 70% large granules w/w), and this is the same for rye and triticale. In barley there is a wider variation, in part due to the existence of more mutant types (Goering et al., 1973). Among 29 cultivars, small granules accounted for between 6% and 30% of the total starch mass. Granule organization exhibit birefringence in the form of a maltese cross. This indicates a high degree of order within the structure. The positive sign of the birefringence suggests that molecules are organized in a radial direction (French, 1984). Amylomaize starch exhibits only weak birefringence of an unusual type (Gallant and Bouchet, 1986). Starch granules exhibit X-ray patterns, indicat- ing a degree of crystallinity. Cereal starches give an A pattern, tuber, stem and amylomaize starches give a B pattern and bean and root starches a C pattern. The C pattern is considered to be a mixture of A and B. It is accepted that the crystallinity is due to the amylopectin as it is shown by waxy granules. Furthermore, amylose can be leached from normal granules without affecting the X-ray pattern. The A and B patterns are thought to indicate crystals formed by double helices in amylopectin. The double helices occur in the outer chains of amylopectin molecules, where they form regions or clusters. The crystal- line parts of starch granules are responsible for many of the physical characteristics of the granules' structure and behaviour. Nevertheless they involve Under crossed polarizers starch granules amylopectin chains. CHEMICAL COMPONENTS 61 proteins of wheat starch to have molecular masses of water available during cooking. Digestibility of under 50 K while integral proteins were over in the intestines of single-stomached animals is 50 K. Altogether ten polypeptides have been also increased by gelatinization. separated between 5 K and 149 K. The major Gelatinization 59 K polypeptide is probably the enzyme respon- sible for amylose synthesis. It has been shown to be concentrated in concentric shells within This is a phenomenon manifested as several granuies. Two other polypeptides of 77 K and changes in properties, including granule swelling 86 K are likely to be involved in amylopectin and progressive loss of organized structure synthesis. Perhaps the most interesting of the (detected as loss of birefringence and crystallinity), surface proteins is that in the 15 K band. increased permeability to water and dissolved This has been found in greater concentration on substances (including dyes), increased leaching starches from cereals with soft endosperm than of starch components, increased viscosity of the on those from cereals with hard endosperm. The aqueous suspension and increased susceptibility protein has been called 'friabilin', because of its to enzymic digestion. association with a friable endosperm (cf Ch. 4) At room temperature starch granules are not (Greenwell and Schofield, 1989). totally impermeable to water, in fact water uptake Phosphorus is another important minor con- can be detected microscopically by a small increase stituent of cereal starches. It occurs as a com- in diameter. The swelling is reversible and the ponent of lysophospholipids. They consist of 70% wetting and drying can be cycled repeatedly lysophosphatidyl choline, 20% lysophosphatidyl without permanent change. If the temperature of ethanolamine and 10% lysophosphatidyl glycerol. a suspension of starch in excess water is raised The proportion of lysophospholipids to free fatty progressively, a condition is reached, around acids varies with species: in wheat, rye, triticale 60"C, at which irreversible swelling begins, and and barley over 90% occurs as lysophospholipids, continues with increasing temperature. The in rice and oats 70% and in millets and sorghum change is endothermic and can be quantified by 55%. In maize 60% occurs as free fatty acids thermal analysis techniques. (Morrison, 1985). Typical heats of gelatinization in J per g of dry Removal of lipids from cereal starches reduces starch are: wheat 19.7, maize 18.0, waxy maize the temperatures of gelatinization-related changes 19.7 and high amylose maize 31.79 (Maurice et and increases peak viscosity of pastes. In other al., 1983). Swelling involves increased uptake of words they become more like the lipid-free potato water and can thus lead to increased viscosity by starch. reducing the mobile phase surrounding the gran- ules; accompanying leaching of starch polymers into this phase can further increase viscosity. The swelling behaviour of starch heated in water is Technological importance of starch Much of the considerable importance of starch often followed using a continuous automatic in foods depends upon its nutritional properties; viscometer, such as the Brabender Amylograph it is a major source of energy for humans and for (Shuey and Tipples, 1980). Upon heating a slurry domestic herbivorous and omnivorous animals. of 7-10% starch w/w in water at a constant rate In the human diet it is usually consumed in a of 1°-5"C per min, starch eventually gelatinizes cooked form wherein it confers attractive textural and begins to thicken the mixture. The tempera- qualities to recipe formulations. These can vary ture at which a rise in consistency is shown is from those of gravies and sauces, custards and called the pasting temperature. The curve then pie fillings to pasta, breads, cakes and biscuits generally rises to a peak, called the peak viscosity. (cookies). Much of the variation in texture depends When the temperature reaches 95"C, that tem- upon the degree of gelatinization, which in turn perature is maintained for 10-30 min and stirring depends upon the temperature, and the amount is continued to determine the shear stability of 62 TECHNOLOGY OF CEREALS Temperature, "C recently been found that is resistant to enzyme SVSU I Porta I Hold I cool attack. Known as resistant starch, it behaves as dietary fibre and is most abundant in autoclaved amylomaize starch suspensions (Berry, 1988). 30 55 95 ~ 95 53 Setback viscosity, C Peak viscosity 0 v) > c - Starch damage (see Chs 6 and 8) x f 4J Granule damage of a particular type alters the properties of starch in some ways similar to gelatinization. Defining the exact type of damage is difficult and this accounts for the continued use of the general term. The essential characteris- tics associated with damaged starch are somewhat similar to gelatinized granules, but there are differences also. Thus mechanical damage results in: 1. increased capacity to absorb water, from 0.5-fold starch dry mass when intact to 34fold when damaged (gelatinized granules absorb as much as 20-fold); E a 2 40 60 90 Time (mid FIG 3.9 Chart showing characteristics recorded by the Brabender Amylograph. the starch. Finally the paste is cooled to 30°C and the increase in consistency is called set-back. (Fig. 3.9) Retrogradation (see also Ch. 8) 2. increased susceptibility to amylolysis; 3. loss of organized structure manifested as loss Suspensions of gelatinized granules containing of X-ray pattern, birefringence, differential more than 3% starch form a viscous or semi-solid scanning calorimetry gelatinization endotherm; starch paste which, on cooling, sets to a gel. Three 4. reduced paste viscosity; dimensional gel networks are formed from the 5. increased solubility, leading to leaching of amylose-containing starches by a mechanism mainly amylopectin. (In gelatinized granules, known as 'entanglement'. The relatively long amylose is preferentially leached (Craig and Stark, 1984).) amylose molecules that escape from the swollen granules into the continuous phase become en- tangled at a concentration of 1-1.5% in water. At a molecular level the disorganization of On cooling the entangled molecules lose transla- granules appears to be accompanied by fragmenta- tional motion, and the water is trapped in the tion of amylopectin molecules during damage network. Crystallites begin to form eventually at whereas gelatinization achieves loss of organization junction zones in the swollen discontinuous phase, without either polymer being reduced in size. causing the gel slowly to increase in rigidity Controlling starch damage level during milling (Osman, 1967). When starch gels are held for of wheat flour is important as it affects the amount prolonged periods, retrogradation sets in. As of water needed to make a dough of the required applied to starch this means a return from a consistency (see Ch. 7) (Evers and Stevens, 1985). solvated, dispersed, amorphous state to an Cell walls insoluble, aggregated or crystalline condition. Retrogradation is due largely to crystallization of amylose, which is much more rapid than that The older literature describes the components of amylopectin. It is responsible for hardening of of cereal grain cell walls as pentosans and hemi- cooked rice and shrinkage and syneresis of starch celluloses. Pentosans are defined earlier in this gels and possibly firming of bread. Although chapter, but hemicelluloses are more difficult to regarded as crystalline, retrograded gels are define and indeed the term is even now only used susceptible to amylolysis, however a fraction has loosely. Hemicelluloses were originally assumed 63 ccJ CHEMICAL C to be low molecular weight (and therefore more soluble) precursors of cellulose. Coultate (1989) writes that the name as applied now covers the xylans, the mannans and the glucomannans, and the galactans and the arabinoxylans, however others use the name to include B-glucans also (Hoseney, 1986). Cell walls are important in several contexts. 1. As a structural framework with which the grain is organized. 2. As a physical boundary to access by enzymes produced outside the cell. 3. As as source of energy in ruminants and of dietary fibre in single stomached animals, including man. 4. They or their derivatives affect processing of raw or cooked cereal products. Cell walls of different cereals have some com- mon components but composition is not con- sistent among species. Cellulose is one component present in all cell walls, it is the material of the simplest and the youngest structures. In most cases additional carbohydrates of varying com- plexity are deposited as a matrix, and some protein also becomes included. Lignin is a com- mon component of secondary thickening in the pericarp of all cereal grains. It is found in the pales but this is relevant to processing only in those grains of which they remain a part after threshing (i.e. oats, barley and rice). The walls of nucellus and seedcoat (see Ch. 2) are generally unlignified; they may contain some corky material. The pigment strand, which is continuous with the seedcoat in grains where a crease is present, is lignified and later becomes encrusted with a material of unknown composition. Similar unidentified material encrusts testa cell walls on their inner surfaces (Zee and O'Brien, 1970). The more precise composition of cell walls has been reviewed by Fincher and Stone (1986). Walls of cereal endosperm (aleurone and starchy- endosperm) consist predominantly of arabino- xylans and (1~3,1~4)-B-glucans, with smaller amounts of cellulose, heteromannans, protein and esterified phenolic acids. They are unlignified and contain little, if any, pectin and xyloglucan, or hydroxyproline-rich glycoprotein, all of which FiG 3.10 Cell walls of barley endosperm fluorescing as a result of staining with calcofluor White MR. The bright fluorescence of the starchy endosperm cell walls contrasts with that of the walls in the triple aleurone layer (cf. Ch 2) (Previously unpublished photograph kindly supplied by Dr S. Shea Miller, Centre for Food and Animal Research, Agriculture Canada, Ottawa.) )MPONENTS are common components of other primary cell walls. Walls with high 13-D-glucan content, such as endosperm cell walls in oats and barley, can be identified under the fluorescence microscope, because of a specific precipitation reaction with Calcofluor White MR (new) (Fig. 3.10). The blue fluorescence is intense and excitation by a wide range of wavelengths is possible (Fulcher and Wong, 1980). Rice is exceptional in containing significant proportions of pectin and xyloglucan, together with small amounts of hydoxyproline-rich protein. The cellulose content of rice cell walls is also CHEMICAL COMPONENTS CHEMICAL COMPONENTS CHEMICAL COMPONENTS 64 TECHNOLOGY OF CEREALS unusually high (25-30%), and mannose-contain- The term is frequently qualified to reflect the ing sugars in some may contribute as much as method of analysis employed because different 15%. methods produce different values (it should also A significant non-carbohydrate molecule inti- be noted that some methods are themselves mately associated with arabinoxylans in wheat inconsistent). The following types of fibre may and other cereal cell walls is ferulic acid, a phenol be encountered, the definitions are based on a carbonic acid very abundant in plant products. glossary by Southgate et al. (1986). It is esterified to the primary alcoholic group of Crude fibre - The residue left after boiling the the arabinose side chain (Amado and Neukom, defatted food in dilute alkali and then in dilute 1985). The formation of diferulic acid cross-links acid. The method recovers 50-80% of cellulose, is at least partly responsible for the gelation of 10-50% of lignin and 20% of hemicellulose. aqueous flour extracts or solutions of cereal Results are inconsistent. pentosans in the presence of oxidizing agents. Acid detergent fibre (ADF) - The cellulose Ferulic acid exhibits intensive blue fluorescence plus lignin in a sample; it is measured as the when irradiated with light of 365 nm. The residue after extracting the food with a hot reaction is particularly marked in aleurone cell dilute H2S04 solution of the detergent cetyl walls and thus can be used for identifying these trimethylammonium bromide (CTAB). in ground cereal products under the fluorescence Neutral detergent fibre (NDF) - The residue microscope. left after extraction with a hot neutral solution of Fractionation into water soluble and insoluble sodium dodecyl sulphate (SDS) also known as pentosans by various protocols is common analy- sodium lauryl sulphate. It is designed to divide tical practice as it has been found to distinguish the dry matter of feeds very nearly into those different functional properties. Thus it is the which are nutritionally available by the normal water-soluble pentosans (mainly arabinoxylans) digestive process and those which depend on of wheat that have a very high water absorbing microbial fermentation for their availability. capacity. They are linear molecules while those Dietary fibre - All the polymers of plants that of the insoluble fraction are highly branched. The cannot be digested by the endogenous secretions backbone of arabinoxylans consists of D-glucan of the human digestive tract. units linked by 0-( 1-4) glycosidic bonds. Single The last definition differs from those that a-L-arabinofuranose residues are attached ran- precede it in that it is not based on the method domly to the xylan and cause the water solubility by which it is determined. It represents the value of the arabinoxylans. As much as 23% of water that the analytical methods seek to achieve. in a bread dough may be associated with pento- Other terms are also in use, such as ‘unavailable sans (Bushuk, 1966). It has been suggested carbohydrate’ and ‘plantzx’, which depart from (Hoseney, 1984) that pentosans reduce the rate the indication that only fibrous material (i.e. of COz diffusion through the dough, behaving in occurring as fibres) is included. Instead it this way similarly to gluten. suggests a matrix of plant materials. It will probably be some time before a consensus is achieved because of lack of agreement on whether a functional or compositional definition is more Fibre Extraction of individual cell wall components appropriate. In the meantime methods that is complex and unsuitable for routine analysis. distinguish several classes of indigestible material Nevertheless an estimate of cell wall content will be the most useful. That of Southgate et al. is often required, particularly in relation to nutri- (1986) distinguishes among cellulose, non-cellu- tional attributes of a product. Analytical procedures losic polysaccharides and lignin. That of Asp have been devized to determine undigestible et al. (1983) distinguishes soluble and insoluble material as ‘fibre’, but not all experts are agreed fibre. Insoluble components include galacto- and as to which chemical entities should be included. gluco-mannans, cellulose and lignin, and the CHEMICAL COMPONENTS 65 A sequence of a large number of units linked by peptide bonds is called a polypeptide. The differences among amino acids lie in the side-chains attached to the carbon atom lying between their carboxyl and amino groups. Side chains may be classified according to their capa- city for interacting with other amino acids by different mechanisms. The types of interaction and the amino acids capable of engaging in them are listed in Table 3.5. soluble class includes galacturonans (pectins), (1+3,1+4)-P-glucans and arabinoxylans. Proteins Although an enormous range of proteins exists in nature they are all composed of the same relatively simple units: amino acids. The diversity of proteins comes about because the amino acids are arranged in different sequences and those sequences are of different lengths. There are only twenty amino acids commonly found in proteins. cereal proteins are important in human and animal nutrition, they provide the unique gas- retaining qualities in wheat flour doughs and bread, but in all organisms proteins are present which function as enzymes. Within the growing plant the genetic code is interpreted through the synthesis and activation of enzymes, providing the means by which characteristics of individual species are expressed. When seen in the context Serine of this function it is perhaps easier to appreciate Cysteine the subtlety of the differences in behaviour that Tyrosine can be achieved among what, at first sight, appear Tryptophan Phenylalanine to be molecules of relatively simple construction. Proline The subtle functional differences are possible chain fatty acids Methionine because of the diversity of the properties of the Leucine Isoleucine amino acids and the relationships in which they Valine are capable of engaging with other amino acids Alanine* or even with lipid, carbohydrate and other mole- Glycine* Aspartic acid cules. Additional variation comes about as a result Lysine of the environment in which a protein finds itself. Arginine A change in pH, temperature or ionic strength Histidine can lead to a single protein species behaving in different ways. TABLE 3.5 Grouping ofAmino Acid Residues According to their Capacity for Interacting Within and Between Protein Chains Type of interaction (1) Covalent - disulphide bonding Dissociated by oxidizing and reducing agents, e.g. performic acid; 2-mercaptoethanol (2) Neutral - hydrogen bonding Asparagine Dissociated by strong H-bonding Glutamine agents, e.g. urea, dimethyl formamide Threonine Amino acid Cysteinelcy stine (3) Neutral - hydrophobic interaction Dissociated by ionic and non-ionic detergents, e.g. sodium salts of long (4) Electrostatic - acid hydrophilic - basic hydrophilic Dissociated by acid, alkali, or salt solutions Glutamic acid * Amino acids with short, aliphatic side-chains show very little hydrophobicity. Both glycine and alanine are readily soluble in water. Table from Simmonds, 1989. Structure All amino acids have in common the presence of an alpha-amino group (NH,) and a carboxyl group (-COOH). It is through the condensation of these groups that neighbouring amino acids are joined by a peptide bond, as in Fig. 3.11. The order in which amino acids occur in a polypeptide defines its ‘pm’may structure’. Because of the range of interactions that can occur among the side-chains, different sequences are capable of different interactions giving rise to a seconday structure. The units of secondary structure in turn react to give rise to the tertiary structure which defines the three-dimensional conformation adopted as a result of side-chain interactions. The 0 OH C-OH + Had - C- N- I II FIG 3.11 The peptide bond. 66 TECHNOLOGY OF CEREALS secondary and tertiary structures of a protein ably. The distinction formalized by Osborne that change in response to the environment but the remains unquestionably valid is that between primary structure remains unaltered unless its albumins and globulins on the one hand and length is reduced by hydrolysis. prolamins and glutelins on the other. In composi- All the interactions listed in Table 3.5 can tion there is a marked difference due mainly to contribute to tertiary structure but the most the extremely high content of proline and gluta- stable types are the covalent disulphide bonds mine in the less soluble fractions (the name formed by oxidation of sulphydryl group on ‘prolamine’ reflects this characteristic). An interacting cysteinekystine residues. extremely low lysine content is also characteristic Such bonds also occur between cysteinekystine of insoluble cereal proteins. residues on different polypeptides giving rise to Soluble proteins a stable structure involving more than one poly- peptide. Inter-peptide links can thus produce in a protein a fourth or quaternary level of structure. These are found in starchy endosperm, Disulphide bonds are stronger than non-cova- aleurone and embryo tissues of cereals. They lent bonds but they are nevertheless capable of account for approximately 20% of the total pro- entering into interchange reactions with sub- tein of the grain. Albumins are usually more stances containing free sulphydryl groups. prevalent than globulins. The amino acid com- Such reactions are of great importance in dough position of soluble proteins is similar to that of formation. proteins found in most unspecialized plant cells suggesting that they include those that constitute the cytoplasm found in most cells. They are a complex mixture including: Cereal proteins The complexity of cereal proteins is enormous 1. metabolic enzymes; 2. hydrolytic enzymes; and the determination of the structure of gluten -the protein complex responsible for the dough- 3. enzyme inhibitors; forming capacity of wheat flour - has been 4. phytohaemaglutenins (proteins that clot red described as one of the most formidable problems blood cells). ever faced by the protein chemist (Wrigley and Bietz, 1988). To simplify their studies cereal Globulins may also arise as storage proteins, chemists have sought to separate the proteins occurring in protein bodies, particularly in oat into fractions that have similarities in behaviour, and rice endosperm. In other cereals, storage composition and structure. As protein studies proteins arising in protein bodies are exclusively have proceeded and knowledge has accumulated, of the insoluble types (Payne and Rhodes, 1982). the validity of earlier criteria of classification has The number of individual proteins in the been, and continues to be, challenged. soluble categories is large. By two-dimensional One of the most significant means of classifying electrophoresis 160 components have been sepa- plant proteins is that which Osborne (1907) made rated in aqueous extracts from wheat endosperm on the basis of solubility. Water soluble proteins and a different pattern of 140 components have were described as ‘albumins’, saline soluble as been separated from the 0.5 M NaCl extracts (Lei ‘globulins’, aqueous alcohol soluble as ‘prolamins’ and Reeck, 1986). and those that remained insoluble as ‘glutelins’. There are differences in amino acid composi- Enzymes tion between proteins in the Osborne classes (see Ch. 14) but there is also heterogeneity within each Enzymes may be considered in the context of class and this may be as significant as between the stage of the grain’s life cycle. Thus, most class differences. Newer analytical methods have enzyme activity during maturation is concerned shown that the solubility classes overlap consider- with synthesis, particularly the synthesis of storage CHEMICAL COMPONENTS 67 A m y I o s e 1 0 f products. Some hydrolytic enzymes involved in breakdown of starch and protein stored in the pericarp are found before maturity and may persist (Fretzdorf and Weipert, 1990). In the mature grain the enzyme levels are relatively low if the grain is sound and dry. If damaged, as in milling, lipids become exposed to lipase. This is particularly relevant to oats; and to germ and bran fractions of other grains. On adequate damping, germination begins and enzymes concerned with solubilization are produced. Cell walls are hydrolyzed, permitting greater access to storage products by enzymes that catalyze hydrolysis of starch and protein (see Ch. 2). Technologically the highest profile enzyme is alpha-amylase, as large quantities are essential in capacity of the solution increases rapidly. When successful malting and brewing and small quanti- the substrate is amylose the iodine staining reaction ties are necessary in breadmaking. Excessive is reduced only slowly as the chain lengths, on alpha-amylase in milling wheats is disastrous, which iodine binding depends, are slowly reduced. leading to dextrin production during baking, By contrast endo-action of alpha-amylolysis making the crumb sticky. Polyphenol oxidases through random fragmentation reduces iodine can lead to production of dark specks in stored staining relatively quickly in relation to the flour products. Other classes of enzymes of tech- increase in reducing power. A further conse- nological importance, found in cereals are quence of the rapid reduction in molecular size beta-amylases, proteases, beta-glucanases, lipases, resulting from alpha-amylolysis is a marked lipoxygenase and phytase. reduction in viscosity of a starch suspension. This is exploited in laboratory tests for the enzyme. Assaying for beta-amylase is more difficult because the rate of maltose production is influenced by the Amylases Both alpha- and beta-amylases are a-( 1+4)-~- presence of alpha-amylase, and the enzymes are glucanases; by definition catalyzing the hydro- almost always present together. Even in well lysis of the same bonds within starch molecules. washed starch they are absorbed on the granule Their action is synergistic because beta-amylase surface (Bowler et al. , 1985). gains greater access to the substrate through the Grain quality is more influenced by the alpha- activity of alpha-amylase. As this last observation enzyme ss its amount is more variable according implies, their modes of action are quite different: to the condition of the grain. beta-Amylase is alpha-amylase is endo-acting while beta-amylase present in resting grain and increases only a few is exo-acting (Fig. 3.12). fold on germination through release of a bound Exo-enzymes catalyze removal of successive form. low molecular weight products from the non- Alpha-amylase is actually synthesized during reducing chain-end, the product removed through germination and activity increases progressively, beta-amylase activity is maltose due to the hydro- as germination proceeds, by several hundred fold. lysis of alternate a-( 1-+4)-glycosidic bonds. beta- In different cereals the site of synthesis of alpha- Amylase is inactive on granular starch but is capable amylase varies; in wheat, rye and barley it occurs of rapid action when the substrate is in solution. first in the scutellum and later in the aleurone, As the exo-action produces a large number of in maize only the scutellum is involved. Several small sugars with reducing power, the reducing isoenzymes of the alpha-amylase type exist in ttt Amylopectin x--) alpha-amylase + befa-amylase T w amyloglucosidase FIG 3.12 Diagrammatic representation of hydrolytic cleavage catalyzed by alpha-amylase, beta-amylase amd amylogluco- sidase respectively. From D. H. Simmonds (1989). Reproduced by courtesy of CSIRO. 68 TECHNOLOGY OF CEREALS most cereals, they fall into two groups depending Proteolysis increases access by amylases to upon their isoelectic points. The Triticeae cereals starch granules as well as producing nitrogenous contain two groups while sorghum, millet, maize, nutrients, for the growing embryo in nature oats and rice have only one (Kruger and Reed, and for yeast during fermentation for beer 1988). production. Even the combined action of alpha- and beta- amylases cannot completely digest solubilized Lip jd modifyjng enzymes starch. Neither of them can catalyze hydrolysis of a-( 1+6)-bonds and hence branch points remain Enzymes of two types are important in catalyz- intact. Also, those a-( 1-4) bonds close to branch ing breakdown of lipids: lipase and lipoxidase. points resist hydrolysis. Hence only about 85% Both are capable of causing rancidity in cereals; of starch is converted to sugars. In order to thus both hydrolytic and oxidative rancidity increase yield of sugars in commercial processes, are recognized. Lipoxidase can only catalyze debranching enzymes may be used. Amylogluco- degradation of free fatty acids and monoglyce- sidase from fungal sources is a popular expedient, rides and therefore follows lipolysis. Lipolysis it catalyzes hydrolysis of both a-( 1-4)- and a- proceeds slowly in the dry state; enzymic oxidation (1+6)-bonds leaving glucose as the ultimate occurs rapidly on wetting. product. Some brewing processes permit the use The problem of rancidity is potentially greatest of this enzyme and sake (see Ch. 9) production in oats which have a high oil content (4-1l0h, is dependent upon it. average 7%). Maize also has a relatively high oil content because of its large embryo (about 4.4%), brown rice contains about 3% but other cereals contain only 1.5-2%. Problems caused by hydro- p- Glucanases These enzymes assume greatest importance in lysis catalyzed by lipase are prevented in the case processing of barley in which p-glucans contri- of processed oats by ‘stabilization’, a steaming bute 70% of cell walls. There are two endo-0- process which inactivates the enzyme (cf. Ch. 6). glucanases in barley malt, both synthesized In other cereals that are milled, potential storage during germination. Each catalyzes hydrolysis of problems can be avoided in starchy endosperm, p-( 1-4) linkages adjacent to p-( 1-3) links, ulti- if it is separated from other grain parts where mately producing a mixture of oligosaccharides enzyme and substrate are concentrated. This is containing three or four glucosyl units (Woodward common practice in the cases of sorghum and and Fincher, 1982). The two isoenzymes are maize grits, in which the embryo presents the synthesized in different sites, I in the scutellum greatest hazard, and in wheat and rice, in which and I1 in the aleurone. Before being susceptible the aleurone layer also has a high lipid content. In to these enzymes it is thought that another wheat, lipase activities in the embryo and aleurone enzyme, 0-glucan solubilase renders the substrate layer are 10-20-fold that of the endosperm (Kruger soluble (Bamforth and Quain, 1989). and Reed, 1988). The storage lives of bran, germ and wholemeal flour are considerably less than that of white flour for this reason (see Ch. 7). As well as true lipases, esterases are also present Proteolytic enzymes Although proteolytic enzymes may be import- in cereals and in most studies the two classes have ant technologically in baking, their significance not been distinguished. Like other hydrolases, is usually masked by the greater effects of alpha- they are synthesized during the early stages of amylase. In brewing their role is better under- germination, although oats are exceptional in stood. Both endo-peptidases and exo-enzymes having a high lipase activity in resting grain. (the carboxypeptidases which catalyze cleavage Lipases catalyze hydrolysis of triglycerides to of single amino acids from the carboxyl terminus) produce diglycerides and free fatty acids, diglyce- are present. rides to give monoglycerides and free fatty acids; CHEMICAL COMPONENTS 69 and monoglycerides to give glycerol and free fatty physiological function is not understood but it acids. The unsaturated fatty acids can be con- increases during germination (Kruger and Reed, verted to hydroperoxides which, in turn, are 1988). changed to hydroxy acids by lipoxygenase, lipo- Insoluble proteins peroxidase and other enzymes, as well as by non- enzymic processes (Youngs, 1986). Lipoxygenase is an effective bleaching agent; The state of knowledge of many insoluble a coupled oxidation destroys the yellow pigments cereal proteins has now advanced even to com- in wheat endosperm. Cosmetically, this is bene- plete sequencing of their amino acids. This is true ficial in bread doughs but undesirable in pasta of prolamins of maize which are known as zeins, products in which the yellow colour is valued since they come from Zea. Barley prolamins are (Hoseney , 1986). hordeins, rye prolamins are secalins and oat prolamins are avelins. A different basis for nomenclature is applied to the naming of wheat prolamins which are called gliadins. Phytase Phytase catalyzes hydrolysis of phytic acid The cereal prolamins have been reviewed by (inositol hexaphosphoric acid) to inositol and free Shewry and Tatham (1990). On the basis of orthophosphate. In wheat its activity increased sequencing, four major groups of zeins have been six-fold on germination and more activity was defined. The groups differ in their amino acid found in hard wheats than in soft (Kruger and content as well as the sequence in which they Reed, 1988). In oats the activity is much lower occur. As prolamins they are by definition rich than in wheat, rye and triticale (Lockhart and in proline and glutamine, and low in lysine and Hurt, 1986). tryptophan. The groups are designated a, p, y In rice the phytin level dropped from 2.67 to and 6. The p- and 6-groups are relatively rich in 1.48 mg/g of dry mass after one day of germina- methionine and the &-group is also rich in cysteine tion, then to 0.44 mg/g after five days. Phytase and histidine. activity levelled off after seven days (Juliano, a-Ze in s 1985). The predominant group is the a-group, contri- buting 75-80% of the total insoluble fraction of Phenol oxidases In the mature wheat grain several polyphenol zein. By electrophoresis the apparent molecular oxidases are present in the starchy endosperm, weights of the two major a-zeins are 19,800 and they are more concentrated in the bran. On 22,000. They can be separated by isoelectric germination an increase, including new iso- focussing into a series of monomers and oligomers, enzyme synthesis, occurs, mainly in the coleoptile though some of the latter can be extracted only and roots. Monophenolase also increases. Durum after reduction of the S-S bonds by which the wheat has less activity than other wheat types monomers are held together. It is frequently (Kruger and Reed, 1988). found in peptide sequences that the domain in the centre is quite different from those at the C- and N-terminal parts. In a-zeins the C-terminal domain consists of 10 amino acids in a unique Catalase and peroxidase Catalase and peroxidase are haemoproteins. sequence, similarly the N-terminus has a unique Peroxidase is involved in the degradation of sequence of 36-37 residues in which one or aromatic amines and phenols by hydrogen per- two cysteine residues are present. The central oxidase. Its activity is greater in wheat than in domain comprises repetitive blocks of 20 resi- other cereals. Catalase catalyzes degradation of dues that are rich in leucine and alanine. The hydrogen peroxide to water and oxygen. Its tertiary structure of a-zeins, divined from circular 70 TECHNOLOGY OF CEREALS dichroism, suggests a high content of a-helix and low P-sheet content. p-Zeins p-Zeins contribute 10-15% of total prolamin. They are rich in methionine and cysteine and can agent indicating mutual association through di- sulphide bonds' No sequences are repeated and all differ from those in a-zeins. The tertiary struc- structure (p-turns and random coil). y and &Zeins y-zeins account for 5-10% of the total pro- lamin. Like p-zeins, they require the presence of a reducing agent for extraction. Eight hexa- peptide sequences in the central domain are flanked by unique N- and C-terminal regions. The repeat sequences are very hydrophilic, rendering the proteins very soluble when reduced. &-Zeins also require reduction before extraction, no sequences are repeated in the central regon but between 17 and 29 methiofie residues occuT here. Other tropical cereals Although only the prolamins of maize among the tropical cereals have been studied extensively, avail- able evidence indicates that sorghum, pearl millet and Job's tears contain essentially similar groups. Temperate cereals Under the classical nomenclature the necessity to reduce disulphide bonds would define p-, y- and 6-zeins as glutelins. Indeed to regard all insoluble cereal proteins as prolamins is not universally accepted among protein chemists. Such a classification can be extended to tempe- tional Osborne classification is more widespread especially in wheat proteins where the functional aspects are particularly important. From two established that up to 20 different polypeptides are found in glutenins (the glutelins of wheat - see p. 69). An even greater number - up to 50 - may be found in gliadins. One argument advanced for distinguishing between wheat Pro- lamins and glutelins is the different physical Properties of the two classes when hydrated: gliadins behave as a viscous liquid and glutenins as a cohesive solid. Although both influence gluten behaviour, it is the larger polymeric glute- nins that wield the greater influence- One of the most attractive theories concerning the relationship between glutenin structure and function is the linear gluten hypothesis (Fig. 3.13). It envisages a series of polymeric subunits joined head to tail by interchain disulphide bonds. The essential features of the subunits are terminal a-helices and central regions of many p-turns (p- turns also occur in the body tissue protein elastin, they are capable of much extension under tension and can return to their former folded condition on be extracted Only in the presence Of a reducing rate cereals but at the present tirne the tradi- ture consists main1y Of P-sheet, and aperiodic dimensional electrophoresis studies it has been Stretching Relaxat ion w- L - 5 aad 0-helix flb 8- turn region region FIG 3.13 Schematic representation of a polypeptide subunit of glutenin within a linear concatenation. The subunits are joined head-to-tail via S-S bonds to form polymers with molecular weights of up to several million. The subunits are considered to have a conformation that may be stretched when tension is applied to the polymers, but when the tension is released the native conformation is regained through elastic recoil. The N- and C- terminal ends of some high molecular weight subunits, where interchain S-S bonds are located, are now thought to be alpha-helix rich domains, whereas the central domains are thought to be rich in repetitive beta-turn structures. The presence of repetitive beta-turn structures may result in a beta-spiral structure, which may confer elasticity. From D. J. Schofield (1986). Reproduced by courtesy of The Royal Society of Chemistry, London. CHEMICAL C subsequent relaxation of the tension). In general the sulphur-containing cysteine residues occur in the a-helical regions, so that the disulphide bonds form between these regions in adjacent polypep- tides. f3- Turns thus remain unencumbered by interchain bonds that might otherwise restrict their extension. Molecular weights of glutenins are upward of 105. The unusually high content of the amino acids asparagine and glutamine found in gluten pro- teins may be significant in providing stability of gluten, through their tendency to become involved in hydrogen bonding. Hydrophobic and electrostatic reactions associated with other amino acid side chains also contribute. The relative importance of glutenins and gliadins varies in wheats from different parts of the world. In Australian and Italian wheats gliadin variations have the strongest association with bread quality. In European wheats high molecular weight glutenin subunits with apparent molecular weight of 90-150 K are paramount in determining quality. Each wheat possesses a complement of 3-5 types, and a variety of individual subunits (allelic forms) may represent each type, giving rise to variation in baking properties. Gliadins are thought to behave as plasticizers, the proportional relation- ship between them and glutenins is an important Beaver I Ri band I Hereward I Mercia FiG 3.14 PAGE electrophoretogram showing distinctive gliadin patterns of four U.K. wheat varieties. Courtesy of FMBRA, Chorleywood, England. Lipids Lipids have been defined as those substances which are: 1. insoluble in water , 2. soluble in organic solvents such as chloroform, ether or benzene, 3. contain long chain hydrocarbon groups in their molecules, and 4. are present in or derived from living organisms (Kates, 1972). OMPONENTS 71 factor. Too Iowa gliadin content leads to inhibi- tion of bubble expansion while the reverse results in excessive expansion and collapse. Gliadin complements are characteristic of individual cultivars and these, revealed through polyacrylamide gel electrophoresis (PAGE), are exploited in establishing the varietal identity of wheat cultivars (Fig. 3.14) and for detecting adulteration of T. dwum products with T. aestivum additions. While this technique may be useful in other species also, it has not been developed to the same degree as in wheat. An even more sensitive method of identifying protein components is high performance liquid chromatography (HPLC). It is faster, and capable of greater resolution than PAGE. Its widespread use is limited by its greater expense and demands for technical expertise. High lysine mutants To produce cereals with better balanced pro- teins, from a nutritional point of view, breeders have exploited mutants with high lysine and high arginine contents. It is the storage proteins that are deficient in these amino acids so the mutants selected frequently achieve the improved balance through a deficiency in storage proteins (Hoseney and Variano-Marston, 1980). In the 'opaque' varieties of maize high lysine content is associated with 'opaque' ( oz) and 'floury' (flz) genes being double recessive, and the consequent inhibition of zein synthesis (Watson, 1987) (cf. Ch. 4). Thus the 'high lysine' varieties of maize, barley, sorghum and pearl millets have lower yields than their conventional counterparts. 72 TECHNOLOGY OF CEREALS This covers a wide range of compounds includ- A fatty acid in which all bonds are single is ing long-chain hydrocarbons, alcohols, aldehydes said to be saturated. In the absence of two and fatty acids, and derivatives such as glycerides, adjacent hydrogens, a double bond is formed wax esters, phospholipids, glycolipids and sulpho- and the resultant fatty acid is described as lipids. Also included are substances which are unsaturated. Where more than one double bond usually considered as belonging to other classes is present the term polyunsaturated is applied. of compounds, for example the ‘fat soluble’ The systematic description of the compound vitamins A, D, E and K, and their derivatives, depends on where double bonds are substituted. as well as carotenoids and sterols and their fatty If the remaining hydrogens are on the same side acid esters (Kates, 1972). of the chain, the conformation is called ‘cis-’. If The terms lipid, fat and oil are often used on different sides a ‘trans-’ double bond exists loosely, but, applied strictly ‘lipids’ include (Fig. 3.16). all the above while only triglycerides (triacyl- glycerols) are described as fats and oils. Fats are solid at room temperature while oils are liquid. Although many fats and oils originate in living organisms (where they function as a means of definition as it is for lipids (see (d) above). Nomenclature As well as a systematic nomenclature, a short- With many series of compounds several con- hand way of descibing the fatty acid may be used. ventions by which they are named, coexist. The Thus cis-9-octadecenoic acid has a shorthand earlier ‘trivial’ names may have been chosen to description C18: 1.9cis, indicating 18 carbons reflect the original source or other arbitrary (octadec-), a double bond (-en-) in the cis form connection. They provide no indication of the between the ninth and tenth position, counting structure of the molecules. As knowledge increases from the functional-group-carbon. and more compounds of the series are identified, the need for a systematic system of names, and Ac ylgl ycerols (91 ycerides) the means of achieving it increase. Such is the case with lipids and a systematic convention for Glycerides are compounds formed by ester- their nomenclature was recommended by the ification of the tertiary alcohol glycerol, and International Union of Pure and Applied Chemists one to three fatty acids. Esterification involves (IUPAC) (Sober, 1968). removal of the elements of water and replacing the hydrogen of hydroxy groups of glycerol with the acyl group RCO. The residue of a fatty acid forming the ester is an acyl group (acyl = Fatty acids Fatty acids present in cereal lipids mainly carboxylic radicle RCO where R is aliphatic). consist of a long hydrocarbon chain covalently Hence the systematic name for glycerides is linked to a carboxylic acid group (Fig. 3.15). acylglycerols. Glycerol has three hydroxyl groups capable of ester formation and, depending on the number esterified, the resulting compounds may HHHHHHHHHHHHHHHHH be mono-acylglycerols, di-acylglycerols or tri- acylglycerols. Plants usually store lipids as tri- I I I I I I I I I I I I I I 11 I I‘OH acylglycerols, and cereal grains conform to this plant characteristic. The highest tri-acylglycerol levels occur in aleurone and scutellar tissue, but HH -c=c- H I -c=c- II I H storing energy) this is not a feature of their ClS- Trons- FIG 3.16 Cis- and trans-configurations. I I I I I I I I I I I I I I I I I,O H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C HHHHHHHHHHHHHHHHH FIG 3.15 Generalized structure of a fatty acid. CHEMICAL COMPONENTS 73 there are appreciable quantities in cereal embryonic mono- and diglycosyl-monoglycerides. Triglycosyl- axes and in the endosperm of oats (Morrison, diglyceride and tetraglycosyl-glycerides have also 1983). They are the main lipid stored in all cereal been reported. In wheat and most other cereals, endosperm, and in wheat the endosperm contri- the sugar is mainly galactose, sometimes with butes about 12% of the total in the grain. small amounts of glucose or none. Other minor Mono- and di-acylglycerols occur only in small glycolipids include sterylglycosides (Morrison quantities as intermediates in the biosynthesis of 1983). tri-acylglycerols or products of their breakdown. The structure of gycosyl-diglycerides is shown in Fig. 3.18, where R and R ' are acyl groups and Ph osp h og I yce rides (Phospholipids) S is a sugar (mono-saccharide-tetra-saccharide). The principal phosphoglycerides in cereal grains are phosphatidyl-choline, phosphatidyl-ethanola- mine, phosphatidyl-inositol, N-acylphosphatidyl- ethanolamine and its monoacyl (lyso) derivative. CH,OCOR The monoacylphospholipids: lysophosphatidyl- I choline, lysophosphatidyl-ethanolamine and lyso- I phosphatidyl-glycerol are the major internal starch R'OCOCH CH,O-s lipids. Monoacylphospholipids are also formed from diacylphospholipids by enzymic hydrolysis (Morrison, 1983). The structures of diacylphosphoglycerides are shown in Fig. 3.17, in which R,R' and R are acyl groups. Mineral matter FIG 3.18 General formula of glycosyl-&glycerides. In glycosyl- monoglycerides R or R! = H. About 95% of the minerals in the actual fruits of cereals (i.e. the grain without adherent pales I R ' ococ H CH,OP-O-X I in the case of husked types) consists of phosphates I1 and sulphates of potassium, magnesium and 0 calcium. The potassium phosphate is probably present in wheat mainly in the form of KHZP04 and K2HP04. Some of the phosphorus is present as phytic acid. Important minor elements are iron, manganese and zinc, present at a level of 1-5 mg/100g, and copper, about 0.5 mg/100g. Besides these, a large number of other elements are present in trace quantities. Representative data from the literature are collected in Table 3.6. H OH H The content of mineral matter in the husk of barley, oats and rice is higher than that in the caryopses, and the ash is particularly rich in silica (Table 3.7). Also see Ch. 14. CH,OCOR In phosphatidyl-choline X = CH,CH2N(CH3I3 In phosphatidyl-ethanolamine X = CH,CH,NH, In phosphatidyl-inositol X = $fH$ OH In N-acylphosphatidyl-ethanolamine X = CH,CH,NHCOR" In lyso-phospholipids R or R' = H FIG 3.17 General formulae of diacylphosphoglycerides. Vitamins The distribution and nutritional signific- Monoglycosyl-diglyceride and diglycosyl- ance of vitamins in cereals are discussed in Ch. Glycos yl-glycerides (glycolipids) diglyceride are the major glycolipids, with some 14. 74 TECHNOLOGY OF CEREALS TABLE 3.6 Mineral Composition of Cereal Grains (mgllOOg d. b.) Oats Rice Element Wheat Barley Whole Groat Rye Triticale Paddy Brown White grain Main Ca 48 52 94 58 49 37 15 22 12 c1 61 137 82 73 36 15 - 19 K 441 534 450 376 524 485 216 257 100 Mg 152 145 138 118 138 147 118 187 31 Na 4 49 28 24 10 9 30 8 6 P 387 356 385 414 428 487 260 315 116 88 S 176 240 178 200 165 Si 10 420 639 28 6 - 2047 70 10 cu 0.6 0.7 0.5 0.4 0.7 0.8 0.4 0.4 0.2 Fe 4.6 4.6 6.2 4.3 4.4 6.5 2.8 1.9 0.9 Mn 4.0 2.0 4.9 4.0 2.5 4.2 2.2 2.4 1.2 Zn 3.3 3.1 3.0 5.1 2.0 3.3 1.8 1.8 1.0 Trace 0.05 0.005 - - - - 0.02 - - A1 0.4 0.67 0.6 0.6 0.56 - 0.9 Ag AS 0.01 0.01 0.03 - 0.01 - 0.007 - - B 0.4 0.2 0.16 0.08 0.3 - 0.14 - - Ba 0.7 0.5 0.4 0.008 - - 1.2 Br 0.4 0.55 0.3 - 0.19 3.3 0.1 Cd 0.01 0.009 0.02 - 0.001 - co 0.005 0.004 0.006 0.02 0.01 - Cr 0.01 0.01 0.01 - F 0.11 0.15 0.04 0.04 0.1 - 0.07 - 0.04 I 0.008 0.007 0.007 0.06 0.004 - - 0.002 0.002 Hg Li 0.05 - 0.05 - 0.017 - 0.5 Mo 0.04 0.04 0.04 - 0.03 - 0.07 - - Ni 0.03 0.02 0.15 - 0.18 - 0.08 0.1 0.02 Pb 0.08 0.07 0.08 - 0.02 - 0.003 - - Rb Sb sc Se 0.05* 0.21 0.2 0.01 0.23 - 0.01 0.04 0.03 Sn 0.3 0.065 0.21 - 0.19 - 0.03 - 0.03 Sr 0.1 0.2 0.21 - - 0.5 0.02 Ti 0.15 0.1 0.2 - 0.08 - 1.4 - 0.01 V 0.007 0.005 0.1 Zr - - - - Minor - - - - - - - - 0.005 0.007 0.007 0.006 - - 0.06 - 0.003 - 0.005 0.003 - - - 0.001 - - - - - - - - - 0.3 0.4 - - - - - 0.05 - - - - - - - - - - - - 0.005 - - - - - - - - - - - - - - 0.007 - - Ash% 1.9 3.1 2.9 2.1 2.2 2.1 7.2 1.8 0.6 Millets Element Maize Sorghum Pearl Foxtail Proso Kodo Finger Main Ca 20 30 36 29 13 37 352 c1 55 52 32 42 21 13 51 K 342 277 454 273 177 165 400 Mg 143 148 149 131 101 128 180 Na 40 11 11 5 7 5 16 P 294 305 379 320 22 1 245 323 S 145 116 168 192 178 156 184 Si - 200 - - - - - CHEMICAL COMPONENTS 75 TABLE 3.6 Continued Millets Element Maize Sorghum Pearl Foxtail Proso Kodo Finger Minor cu 0.4 1 .o 0.5 0.7 0.5 1 .o 0.6 Fe 3.1 7.0 11.0 9.0 9.0 3.0 4.5 Mn 0.6 2.6 1.5 2.0 2.0 - 1.9 1.5 Zn 2.0 3.0 2.5 2.0 2.0 - - 0.4 Ag A1 0.057 1.8 1.7 As 0.03 - 0.01 B 0.3 0.13 0.19 Ba 3.0 0.08 0.04 Br 0.26 0.14 0.38 Cd 0.012 - - co 0.008 <0.05 0.05 - Cr 0.004 0.05 0.03 F 0.04 I 0.2 - 0.0016 - - Li 0.005 0.07 0.01 Mo 0.03 0.2 0.07 - Ni 0.04 0.3 0.11 Pb 0.01 0.11 0.02 Rb 0.3 0.12 0.34 sc 0.01 - - Sn 0.01 0.07 0.004 - Sr 0.02 0.18 0.03 - Ti 0.17 0.1 0.02 - V 0.01 0.05 <0.01 Zr 0.02 Trace - <0.005 <0.005 - - - - - - - - - <0.05 - - - 2.2 - - - - - - - - - - - <0.01 - - 0.02 - - - - - - - - - - - 0.2 - 0.2 - 0.6 - 0.2 - - - - 0.02 - - - - - - - - - - - 0.006 - - 3.3 - - 0.03 - - 0.04 - - - - - - - - - Ash Oh 1.7 1.7 2.4 3.7 2.2 3.0 2.2 N.B. 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