Food Structure

Volume 8 Number 1 Article 13

1989

Cereal Structure and Its Relationship to Nutritional Quality

S. H. Yiu

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Recommended Citation Yiu, S. H. (1989) "Cereal Structure and Its Relationship to Nutritional Quality," Food Structure: Vol. 8 : No. 1 , Article 13. Available at: https://digitalcommons.usu.edu/foodmicrostructure/vol8/iss1/13

This Article is brought to you for free and open access by the Western Dairy Center at DigitalCommons@USU. It has been accepted for inclusion in Food Structure by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. FOOD MICROSTRUCTURE, Vol. 8 (1989), pp. 99- 113 0730- 5419/89$3 . 00+. 00 scanning Microscopy International, Chicago (AMF O'Hare) , IL 60666 USA

CEREAL STRUCTURE AND ITS RELATIONSHIP TO NUTRITIONAL QUALITY

S. H. Yiu

Food Research Centre, Agriculture Canada, Ottawa, OntarIo, Canada KlA OC6

Abstract Introduction

Factors that determine the digest1b11Hy of Cerea 1 s are good sources of carbohydrates carbohydrates and mi nera 1s 1n cerea 1 s are exa­ and minerals important for sustaining the energy mined . Most carbohydrates and minerals in ce­ and growth requirements of humans and animals. reals are structurally bound, either surrounded Cereals also contain dietary fiber. Increased by or associated with cell wall components not consumption of dietary fiber has been associated easily digested by non-ruminant animals and hu­ with various health benefits (Trowell , 1976; mans. Treatments such as mechanical grinding and Anderson and Chen, 197g). heat improve the digestibility of nutrients . Information concerning factors that affect Further processing and cooking result in struc­ the nutritional quality of cereal food derives tura 1 and phys 1cochemi ca 1 changes of cerea 1 from studies conducted in humans, or animals such starch, phytate, and dietary fiber. Such changes as rats, fed diets containing various cereal greatly Influence the physiological and metabolic products (McCance and Wi ddowson, 1935: Jenkins et effects in animals and humans. The digestive al., 1975; Ismall-Beigi et al., 1977; Simpson et breakdown of most nutrient components is also al., 1981; Navert et al., 1985; Heaton et al., dependent on the activities of enzymes in cereals 1988). In vitro conditions, simulating gastro­ and in the marnnalian digestive system. However, intestinal environments, have also been used starch, phytate, and dietary fiber are not en­ (Snow and O'Dea, 1981; Holm et al., 1985, 1988; tirely and readily degraded by enzymes. Unde­ Platt and Clydesdale, 1984, 1987). Oat and wheat graded components reduce both the ca 1ori c va 1ue brans are corrmercially available products and of the food and the availabilities of other nu­ most frequently studied because of interests in trients by interacting with them in the gastroi n­ the physiological and metabolic effects of phytic testinal tract. Studies on avallabil Hies of acid and dietary fiber (Prornare and Heaton, 1973; carbohydrates and minerals in cereal foods are Reinhold et al., 1975, I981; Davies et al., 1977; conducted in humans and rats or under in vitro Anderson et a I., 1984; Moak et a I., 1987; Shl n­ conditions, using various analytical methods nlck et al., I988) . Results obtained from many Including microscopy. The advantage of applying nutritional studies indicate that the structures 1 ight microscopy and scanning electron microscopy and physical forms of cereal food components coupled with energy dispersive X-ray microanaly­ greatly affect the availability and uti I lzatlon sis to study the digestive breakdown of structur­ of the nutrients (Snow and O'Dea, 1981; Van al components in cereal foods is highlighted by Soest, 1984; Jenkins et al., 1986; Heaton et al. , demonstrating the capabilities of the techniques 1988; Holm et al., 1988). to reveal both structural and microchemical in­ Most cereal carbohydrates and minerals are formation. associated with microscopically distinct struc­ tures (McMasters et al., 1971; Fulcher and Wong, 1980) . Microscopic studies reveal much detail concerning the morphological organization and nutrient composition of oat and wheat grains Initial paper received February 6, 1989 before and after cereal processing {Pomeranz and Manuscript received June 5, 1989 Shellenberger, 1961; 8uttrose, 1978; Fulcher and Direct inquiries to S.H. Yiu Wong, 1980; Fu lcher, 1g86; Lockhart et al., 1986; Telephone number: 613 995 3722 x7703 Ylu , I986; Yiu et al., 1987). Using oat and wheat products as examples, the present review demonstrates how microscopy, particularly fluo­ rescence and other light microscopy, has contri­ buted to the understanding of relationships Key Words: Oats, Wheat, Bran structures, Process­ between cereal structures and the availability of ing effects, Nutrient availability, Dietary fi­ carbohydrates and minerals 1n cereal foods. More ber, Starch, Phytate specifically, the review examines factors such as

99 S. H. Yiu processing, cooking, and enzymes that i nfluence Figure Captions the structures and digestibilities of starch, phytate, and dietary fiber. Some of the effects Unless otherwise stated, all micrographs of undigested fiber and phytate on the absorption show 3~ glutaraldehyde-fixed, glycol methacry­ of other nutrients in the rnarrmal ian gastrointes­ late-embedded sections of oat or wheat grain tinal tract are also discussed. tissues. Numbers at scale bars are 1n lJm. Pho­ tographed using fluorescence excHer/barrier filters set for maximum transmission at 365 nm/ >418 nm (FCI) or 490 nm/>520 nm (FCII). Structure and Distributi on Starch is located i ns ide the cells of the ..E..i.9.:.._.l A sect ion of wheat kernel showing str uc­ endosperm and is rarel y f oun d in the germ and tures of sta r ch gr an ul es (arrows) after stai ning aleurone tissues in mat ure grains. Starch occurs with F-LCA (fluorescein-labelled Lens culinaris in the form of colorless translucent bodies, agglutinin, 1.2 mg/ml in O.OIM sodium phosphate identified as starch granules. Reviews by Evers buffer, pH 7). FC!l. (1979), Hood and liboff (198n and Fulcher (1986) gave detailed descriptions of the structures of f..!.g_,____f A section of oat kernel showing the cereal starches. Cereal starch granules vary in structures of compound starch granules (*) and s 1ze and morpho 1 og 1ca 1 appea ranee, depending on cell walls {arrows) after staining with F-LCA and the species of the cereal grain. For example, 0.01'1: Congo Red. FCI l. wheat starch consists of both small (2 - 10 IJIII) spherical and large {20 - 40 IJIII) lenticular ~ An unstained, frozen section of oat ker­ granules (Fig. 1) . Unlike wheat starch, oat nel viewed under polarized light to reveal the starch occurs chiefly as compound granules which pattern of birefringent starch granules (arrows). are aggregates Of sub-granules (Fig. 2). The oat Photographed using polarizing optics. starch granules range from 20 to 100 lJffi in s i ze. Various techniques of 1 ight microscopy are f..!.9.,__1 A section of quick-cooking rolled oats suitable for studying the distribution of starch stained with F-Con A (fluorescein-labelled Con­ in cereal grains . For instance, staining proce­ canavalin A, 1.2 mg/ml in O.OIM sodium phosphate dures using iodine - potassium iodide, the pe­ buffer, pH 7) showing broken compound starch riodic acid-Schiff reagent, or fluorescein­ granules (a rrows). FC I I. (Yiu, 1986). coupled plant lectins such as Lens culinari s agglutinin and Concanavalin A, are appropriate f..!.g_,__2 A section of cooked rolled oats stai ned for revealing the location and structural organi­ with F-Con A, showing the structure of cooked zation of starch in a variety of cereals and ce­ starch (arrows). FCJI. (Yiu, 1986). real foods (Jensen, 1962; Fulcher and Wong, 1980; Miller et al., 1984; Yiu, 1986). The staining ~ A section of digesta removed from the procedures provide both convenience and speed for small intestine of a rat fed a diet containing detecting starch content in cereals. Scanning wheat bran, showing structures of wheat starch and transm1ss ion e 1ectron microscopy are a 1 so granules (arrows) . Photographed using bright­ useful for investigating the complex structure of field optics. starch {Gallant and Gui lbot, 196g; Yamaguchi et al., 1979). Most cereal starches exhibit bire­ f..1.9.:.___l A section of rat digesta prepared and fringence under polarized 1 ight {Wivinis and May­ stained the same way as in Fig. 6, demonstrating wald, 1967; Greenwood, lg79). 8oth oat and wheat the presence of partially digested corn starch starches show the characteristic 'maltese cross ' granules (arrows} . pattern under a polarized 1 ight microscope (Fig. J). Birefringence is lost when a starch granule f..!.g_,__J! An elemental profile of oat phyt1n glo­ undergoes physical changes associated w1th gela­ boid. A 2 ~m thick, glycol methacrylate-embed­ tinization (Sandstedt , 1961; lineback and Wongs­ ded, carbon coated section of rat colonic digesta rikasem, 1980; Varriano-Marston, 1982) . examined under a scanning electron microscope at P races s 1 ng a nd Coo k; ng 20 k.V, a nd a nalyzed w1th a n energy dispers i ve During the milling of wheat, endospermtc X-ray microprobe f or 100 s/site. Probe current : cells of the subaleurone layer are more resistant 5 x 10 - 9 A. Probe size: 180 nm. K: potassium, to the force of grinding, and are reduced in size Mg: magne s ium, P: phosphorus . less readily than cells of the inner starchy en­ dosperm (Kent, 1966; Pomeranz, 1982). The coarse ~ A section of wheat kernel stained with fraction of wheat flour, which derives primarily 0.1~ Acriflavine HCl to show the distribution of from the centre of the endosperm, has higher phytin globoids (small arrows) within the aleu­ starch content than the finer flour fraction, rone layer with cell walls (large arrows) of high which contains more fragments of the protein-rich phenolic contents. FCI. suba 1eurone layer (Pomeranz, 1982). Endosperm cells in soft wheat varieties contain starch granules embedded in a friable protein matrix continuous protein matrix, resulting in breakage which is susceptible to the grinding force, re­ of both starch and the protein matrix (Moss et sulting in the release of intact starch granules al., 1980; Pomeranz , lg82). Starch damage in the with little damage (Kent, 1969). On the other flour increases the water binding capacity and hand, endosperm cells of hard wheat varieties susceptibility to a-amylase degradation (Jones, tend to shatter rather than powder due to the 1940; Pomeranz, 1982).

100 7 I..,. t ~ ~') (1. ,?l ~ (7 4 ' ..'()

101 102 Cereal Structure And Nutritional Quality

Figure Captions Mechanical grinding, e . g. , rolling and flak- 1ng, tends to induce the breakdown of compound starch granules in oats {Lockhart et al . , 1986; Unless otherwise stated, all micrographs Y1u, 1986) . The thinner the oat flake, such as show 3% glutaraldehyde-fixed, glycol methacry­ those of quick-cooking rolled oats, the more the late-embedded sections of oat or wheat grain breakdown occurs {F ig. 4). However, the struc­ tissues. Numbers at scale bars a re in )lm. Pho­ tural integrity of starch sub-granules remains tographed using fluorescence exciter/barrier unchanged (Yiu, 1986) . filters set fo r maximum t ransmi s sion at 365 nm/ \olhe n sta rch is heated 1 n the presence of >418 nm (FC1) or 4go nm/>520 nm ( FCI1) . water , the gra nule swells as a resu l t of water abso rpt1on. The swelling 1s 1n1t1a ll y hampered by the r1 g1d1ty of the ce ll wa ll , r esul t 1ng in .E..i.9..:..... A section of whea t bran st ai ned with many d1 s t a rted and convol uted starch structures 0. 1% Acridine Orange , showi ng the presence of (F1g. 5) . The integ r i ty of the granule is lost phytin globoids (arrows) . FCI. when starch becomes compl e tely gelatinized . The expanded structure of starch provides greater E.!.9..,_!! A sect1on of puffed whea t stained with access1b111ty to enzymes, resulting in an in­ 0.1% Acr1flavl ne HCl , demonstrating changes 1n creased rate of sta rch digestion {Wursh et al . , the aleurone cell structure {arrows). FCIJ. 1g86; Y1u et a l. , 1g87) . Furthermore, the rate of increase of g lucose and insulin concentrat1ons .E.1.9..,__!1 A sect1on of 1leo d1ges t a removed from a rat fed a diet containing oat bran , and stained 1n blood 1s d1rectly related to the percentage of starch gelat1nized (Holm et al ., 1g95, 1g88; Ross with 0.1% Acridine Orange , showing the presence etal. . 1g87) . of phyt1n globo1ds (arrows) within the aleurone cells. FCI!. (Yiu and Mongeau, 1g87). Processing methods such as extrusion cook- 1ng, explosion puffing, and 1nstant1zat1on hy­ drate the starch granules or disrupt the native ~ A section of rat colonic digesta stained with 0.1% Acriflavine HCl to show the presence of structures, similar to but surpassing the result und1gested phyt1n globo1ds (small arrows) and of convent1onal heating (Brand et al., 1g85). cell wall fragments of high phenolic contents Such processing co nditions appear to change (large arrows) . FC I. (Y1u an d Mong ea u , 1g87). starch d1ges t i bil i ty and e l icH different glycem- 1c responses (Ho lm et al., 1g85 ; Jenkins et al. , f..i.!l..:__ll An elementa l prof 1le of an undi gested 1g86; Ross et al. , 1g87). phyt1n gl obo1 d . A 2 ~m thi ck, gl ycol methacry- Di ges t1b11 i ty 1ate-embedded, carbon-coated sect ion of rat co­ Cerea l starches are readily digested by hu­ lonic dtgesta examined under a scanning electron mans and animals. Starch-specific hydrolytic en­ microscope at 20 kV, and analyzed with an energy zymes are abundant in most cereal grains , micro­ dispersive X-ray microprobe for 100 s/sHe. Probe organisms, and marrmalian salivary and pancreatic current : 5 x to-9 A. Probe size: 180 nm. Ca: secretions {Jones , 1940; Marshall and Whelan, calcium, J<: : potassium, Mg : magnesium, P: 1979). Detailed mechanisms involving the enzy­ phosphorus. matic hydrolysis of starch are described by Man­ ners {1985). Briefly , a-amylase randomly hydro­ lyzes amylose and amylopectin to mal tosaccharides ~ A section of oat kernel stained with 0.01% Calcofluor WhHe in 50% ethanol , showing which are degraded by a-glucosidases to glucose. The present review mainly examines factors that the distribution of ~-glucan-rich aleurone (A) and sub-aleurone(*) cell walls . FC!. influence the digestive breakdown of cereal starch. Particle size reduction of the starch­ ~ A section of instant rolled oats stained with fluorescein-labelled Lens culinaris agglu­ bearing matrix increases the digestibility of tinin (1.2 mg/ml 1n 0. 01M sodium phosphate starch . Grea ter accessibility to enzymatic re­ buffer, pH 7) and 0.01% Congo Red to show the actions, makes s t a rch of fine l y ground flour more extent of ce ll f ractu re (arrows) af ter readi ly digested than starch of unp rocessed ce­ processi ng. FCII. rea l gra in s (Snow and O'Dea, 198 1; Heato n e t al. , 1g88) . Damaged starch is more susceptib l e to amy lase degradation than intact granules (Jones, .E..i9..=..___! A section of regular rolled oats stained 1g4D). and photographed the same way as in fig. 16 to The presence of a-amylase inhibitors tn demonstrate the intact cell wall structures (arrows). cereals reduces sta rch d1gest1bil1ty (Sha1nkin and 81rk , 1g7D; Rea et al., 1985) . Alpha-amylase inhibitors can be removed through milling and ~ A section of rat ilea digesta stained cook1ng (Snow and O'Dea, 1g81; Rea et al . , 1g85). with 0. 01% Cellufluor in 50% ethanol, showing the partially digested sub-aleurone (large arrows) Results of in vitro and in vivo studies demon­ strate that interactions can take place between and relatively intact aleurone (small arrows) layers. FC!. (Y1u and Mongeau , 1g87). starch and other f ood components like 1 ip1ds (Larsson and Mi ez i s , 1g79; Holm et al., 1983), prote1ns (Anderson et al. , 1g81 ; Jenk1ns et al., ~ A section of rat colonic digesta stained with 0 .11 Acridine Orange to show the partially 1987), polyphenols (Thompson et al., 1g84; Bjorck d1gested aleurone cell walls (arrows) . FCI!. (Y1u and Nyman, 1g87 ; Knudsen et al., 1g88). or phyt1c and Mongeau, 1g87). ac1d (Yoon et al., 1g83; Thompson, 1986),

103 S. H. Yiu resulting in the formation of complexes that referred to as phytin globoid crystals or phytin resist enzymatic degradation. globoids (Lott and Spitzer, 1980). Ranging from A fraction of starch ingested from processed 1 to 2 ~m in diameter, phytin g l oboid crystals cereals has been identified as resistant to are mostly spherical in shape and contain high breakdown by a-amylase both in vitro and in the concentrations of phosphorus (P), potassium (K), small intestine of man (Levine and Levitt, 1981; and magnesium (Mg) (Lott and Ockenden, 1986). Englyst and Cunmings, 1985 ). Processing proce­ When su bjected to EOX microanalysis, the globoid dures, particularly freezing and thawing, cause crystals emit X-rays character1st1c of their ele­ retrogradation of the starch and increase resis­ mental composition. A typical EDX spe ctral pro­ tance to amylolytic action (Englyst et al., file of oat phytin globoids is composed of three 1983). Starches that resist digestion are avail­ major element peaks, P, K,and Mg (Fig. 8). A able for microbial fermentation in the lower gut, small quantity of other elements is al so present but the generated energy is reduced (Was l ien, (Buttrose, 1978). The co ncentrations vary de­ 1988). Resistant starches can cause inaccuracy pending on grain varieties and locations of in quantifying the amount of dietary fiber in growth (8uttrose, 1978; Batten and Lott, 1986). food products. Methods which rely on gelatiniza­ Rapid detection of the distribution of phy­ tion in water and enzymatic removal of starch tin glaboids in cereals and cereal foods can be prior to the quantification of dietary fiber are achieved using optical 1 ight microscopy. Pola­ affected by the presence of resistant starches. rized 1 ight microscopy effectively locates the A method has been developed to determine the birefringent structures of phytin globoids in amount of starch in processed cereals resistant hand-prepared or glycol-methacrylate embedded ma­ to amylolytic enzymes used for dietary fiber terials without any stalning (Fulcher, 1982; Yiu determination (Englyst et al . , 1983). However et al., 1982). Fo r confirmation, other types of resistant starches constitute only a small light microscopy are often used. Cationic stains fraction of starch that escapes in viva digestion such as Acriflavine HCl, Ac r idine Orange, and (Englyst and Cummings, 1985). Hence, alternative Toluidine Blue a re suitable microscopic markers add1tional methods are required to assess the far phytin inclusions (Yiu et al., 1982; Fulcher. content and digestibi 1 ity of starch in cereal 1982; Yiu, 1986; Yiu and Mongeau, 1987). Fig. 9 foods. illustrates the distribution of phytin glabaids Microscopy serves as a practical tool for in the aleurone cells of wheat as revealed by detecting and analyzing the digestive breakdown fluorescence microscopy. of starch in cereal food s. For example , light Processi ng and Cooking microscopy using iodine - potassium iodide as a Mi 11 ing reduces the phytate co ntent in wheat staining reagent, can be used to detect starch in (Nayini and Markakis, 1983) by removing the bran the small intestine of the rat (Fig. 6). The and germ fractions from the flour, but milling structural appearance of starch present in rat does not dissociate the structural attachment of digesta reflects the extent of starch breakdown phytin globoids from bran and germ fractions (Fig. 7). According to Sandstedt (1955) and (Fig. 10). Vigorous processing methods 1 ike ex­ Evers et a l . (1971), a-amylase-digested starch trusion cooking and puffing induce structural has a hollow centre 1 inked to the surface by a changes in cereals to such an extent that compo­ few radial channels , whereas amyloglucosidase­ nents within the aleurone cells are no longer digested starch has a surface covered with identifiable by phytin-speciflc staining (Fig. shallow pits. 11). Extrusion cooking alters the physicochemi­ cal properties of phytate. reduces phytate de­ gradation in the intestine (Sandberg et a l., 1986) and eliminates endogenous activities of Structure and Distribution phytate-specific enzymes (phytase) in cereals Phytate (!!!:i.Q inositol hexaphosphate) ac­ (Sandberg et al., 1987). The decrease in phytate counts for 70-90% of the total phosphorus reserve degradation is associated w1th decreased absorp­ in most mature cereal grains {Ashton and Wil­ tion of zinc, phosphorus, and magnesium in the liams, 1958; O'Dell et al., 1972; Lolas et al. , human small intestine (Kivisto et al., 1986). 1976; Frolich and Nyman, 1988). Chemical data Milder heat treatment like domestic cooking re­ (O'Dell et al., 1972) indicate that the majority duces the phytate content in cereals such as of the phosphorus reserve is contained in the wheat and rye, but not oats (Sandstrom et al., bran and germ fractions of cereal grains like 1987). During bread making, the presence of wheat. Microscopic studies contribute to knowl­ additional phytases from yeast and the baking edge of the occurrence and distribution of phy­ process significantly reduce the phytate content tate in cereal grains. Phytate-containing parti­ in bread (de Lange et al., 1961; Nayini and cles can be identified and l ocated in the aleu­ Markakis, 1983). Phytate-reduced bread has less rone and scutellum tissues of mast cereal grain effect an in vitro and in vivo absorption of kerne 1 s by e 1ec tron microprobe X- ray ana 1ys is minerals than phytate-containing bread {Reinhold coupled with scanning electron microscopy (Tanaka et al., 1974; Navert et al., 1985). et al., 1974), transmission electron microscopy Oigestibll ity of Cereal Phytate and Nutritional (Ogawa et al., 1975), and energy dispersive X-ray Imp lications (EDX) microanalysis (Liu and Pomeranz, 1g75; 8ut­ Early metabolic studies indicated that phy­ trose, 1978). The phytate-containing particles tate phosphorus is not readily available far di­ are electron-dense inclusions embedded in the gestive absorption by humans and anima 1 s (McCance protein matrix of the aleurone grains, and are and Widdowson, 1935; Mellanby, 1949). Dietary

104 Cereal Structure And Nutritional Quality

deficiency of phosphorus is unlikely since phos­ Szafranska, lgB7; Sandberg and Andersson, 1gBB), phorus is readily available from other dietary and certain processings and baking (de Lange et sources . However, when cereals constitute a al., 1961 ; Reinhold et al., 1g74; Nayini and large portion of the diet, the degree to which Markakis, lgB3; Navert et al., 19B5) . humans can utilize phytate may become important. Microscopic exam ination of dietary phytate The degradation of dietary phytate chiefly tn colonic contents of rats revealed intact oat depends on the hydrolytic activiti es of phytases bran phytin globoids (Yiu and Mongeau, lgB7). (Nayini and Markakis, rgB6). Phytases, or !!!19,­ The undigested phytin globoids not only retained inositol hexaphosphate phosphohydrolases, are en­ morphological and staini ng characteristics (Fig. zymes that break down phytic acid to !!!19_-inositol 13) but also the elemental contents as revealed and inorganic pho sp hate via intermediate !!11Q-1no­ by EDX-microanalysis (Fig. 14), Undigested s i tol phosphates (penta- to mono-phosphates). phyttn is a cause of concern since the cation­ Phytase activities exist in the end osperm of binding activities may impair mineral b1oavaila­ wheat (Peers, lg53), and in the aleurone cells of bility (Mellanby, 1g4g; Erdman, 1g7g; Oberleas rice (Yoshida et al ., 1g75), barley (Tronier et and Harland, lgBI). al., rg71), sorghum (Adams and Novell ie, rg75), Mineral deficiencies have been noted in and corn (Chang, 1g67). Phytase activity in­ humans and monogas tric animals whose diets con­ creases during germination {McCance and Widdow­ sist predominantly of whole grains of high phy­ son, 1944; Bartnik and Szafranska, 19B7) result­ tate content (Mellanby, 1g49; Erdman, 1g7g). The ing in a decrease in the phytate content of the presence of six ortho-phosphate moieties in the grain (Nayini and Markakis, 1gB6) . By comparison phytic acid molecule provides the compound w1th a with wheat and rye, oats have lower phytase acti­ potential for complexing cations such as zinc , vities both before and after germination (McCance iron, magnesium, and calcium (O'Dell , 1g6g; and Widdowson, 1g44; Bartnik and Szafranska, Morris, 1gB6). Interactions between phytic acid 1987). Recent results based on 31 P-nuclear mag­ and cations result in the formation of insoluble net 1c resonance spectroscopy confi rrn that oat complexes, thereby reducing the availability of phytase is inactivated by the heat treatment re­ minerals (Erdman, 1g79; Platt and Clydesdale, ceived during corrmercial oat processing (Frolich 1987) . In addition, interactions can occur be­ et al . , lgBB). Other studies conclude that pro­ tween protein and phytic acid, protein-cation and cessing such as extrusion cooking impairs phytase phytic acid , or starch and phytic acid (Cheryan, activity in wheat bran (Sandberg et al., 1gB6 & lgBo; O'Dell and de Boland, rg76; Thompson, 1gB6; 1gB7) . Hence, l ow or reduced activities of phy­ Wise, rgB6). However, the deleterious effect of tases account for the relatively high phytate phytate on mineral metabolism in humans and ani­ contents and low phytate digestibi 1ities in cer­ mals is avoidable when diets are well balanced, tain oat and extruded wheat products (Mellanby, especially in mineral contents (Morris, 1906; 1g4g; Sandberg, et al.. 1gB7; Yiu and Mongeau, Moak et al., 1gB7). 1gB7). Phytase activities are located in the muco­ Dietary Fiber sal tissues of most manmalian intestines (Nayini and Markakis, 1gB6). Some of the phytate present The definition of dietary fiber is still a in the small intestine is likely hydrolyzed by debatable subject (Trowell, rg76; Cummings, 1g76; mucosal phytase. The extent of the enzymatic re­ Southgate, 197B; Selvendran, lgBJ; Englyst et action depends on the presence and concentration al., rgB7; Asp et al., !9BB). However, it is of dietary minerals like Ca and Zn (Wise, 1gB6). generally accepted that indigestible plant mate­ The sign1ficance of the involvement of mucosal rials are the main constituents (Trowell, 1988). phytase in phytate degradation is not clear. One This review examines only cereal brans that are feeding study shows that mucosal phytases and known to be associated with the major physiologi ­ enzymes such as alkaline phosphatase do not play cal effects of dietary fiber and are of major important roles in phytate digestion, as close to cof1111erc1al interest (Anderson and Chen, 1979; g5% of the ingested phytate from phytase-deacti­ Anderson, rgB5; Schneeman, lgB7) . Cereal bran is vated wheat bran can be recovered in human ileos­ a product of corrrnercial processing, the outer tomy contents (Sandberg and Andersson, 1gBB). part of a grain kernel isolated through mechani­ Another study reports that much of the ingested cal grinding and sieving (Deane and Corrrners, phytate, present in oat bran fed to rats, remains 1986) . Bran is composed of several layers of undigested in the small intestine (Yiu and Mon­ fibrous tissues, including the per1carp and seed geau, 19B7). Microscopic examination of the rat coat, and parts of the endosperm which include digesta shows that many of the ingested phytin the aleurone and subaleurone cells (Figs . 10 & globoids are structurally associated with the 15). aleurone tissues (Fig. 12). Microscopic observa­ Physiological and Metabo lic Effects of Oat and tions also provide direct evidence that the Wheat Brans majority of the phytate breakdown takes place in Oat and wheat brans attract public interest the lower gut of the animal (Yiu and Mongeau, because of known physiological and metabolic 1gB7). Phytases originating from the microflora, effects believed to be beneficial. Oat and wheat which usually populate the large intestines of brans can b1nd water, bile salts. and other sub­ animals and humans, seem to play a key role in stances in the intestinal tract (Promare and phyta te degradation. However, despite the pre­ Heaton, 1973; Eastwood and Mowbray, 1976; East­ sence of microbial phytase, phytate degradation wood et al., 19BO; Spiller et al., 1gB6; Anderson is significantly influenced by phytase activi­ and Chen, I9B6), resulting in various potential ties endogenous to most cereals (Bartnik and health benefits (Trowell, lg76; Anderson and

105 S. H. Yiu

Chen, 1979; Anderson, 1985; Anderson and Tietyen­ components of the bran, including the outer peri­ Clark, 19B6; Schneeman, 19B7; Burkitt, 19BB). carp and seed coat layers, which have h1gh lignin However, oat and wheat brans dlffer 1n colonic and cutin contents (Ring and Selvendran, rgBO; and metabolic functions. Oat bran 1s effective Schwarz et al., !9BB) as well as the subaleurone in reducing serum cholesterol levels and slowing starch granules, can also be revealed with fluo­ glycemic responses, and wheat bran in increasing rescence microscopy (Fig. 9). fecal weight and decreasing translt time, thereby Precess; ng and Cooking decreasing the incidence of diverticulosis and Mechanical processing breaks down the endo­ colorectal cancer (Kritchevsky et al., 19B4; sperm cell walls of oats and wheat, reducing par­ Anderson, 19B5; Anderson and Chen, 19B6). ticle size (Schultze and MacMasters, 1962; Moss Fiber Composition of Oat and Wh eat Brans et al., 1980; Yiu, 19B6) . The extent of process­ Dietary fiber has been divided into two ca­ ing affects the degree of cell wall breakdown. tegories, soluble and insoluble (Anderson and For example, instant rolled oats have consider­ Chen, 1979). The rationale for this division is ably more cell wall fractures (Fig. 16) than based on solubility in hot water. Soluble cereal regular rolled oats (Fig. 17), as the former are fiber includes polysaccharides referred to as subjected to more processing steps than the lat­ gums and some hemicelluloses, whereas insoluble ter (Deane and Commers, 19B6). Particle size fiber includes cellulose, some hemicellulosic reduction through grinding collapses the physical polysaccharides and lignin (Southgate, 197B; An­ structure of wheat bran and alters its physico­ derson and Chen, 1979; Southgate and Kritchevsky, chemical properties to such an extent that the 19BI). Physiologically based definitions, such water-holding (Van Soest, 19B4; Cadden, 19B7) and as adopted by the Canadian Expert Conmi ttee on bile salt-binding capacities of wheat bran (Mon ­ Dietary Fiber (Health and Welfare Canada, 19B5), geau and Brassard, 19B2) are reduced. On the refer to plant materials not digestible by man . other hand, processing wheat bran of low moisture Such materials consist of nonstarch polysaccha­ content by extrusion cooking at high temperature rides and lignin, and may include associated and pressure and short duration of time increases substances . not only the soluble fiber content but also the Oat and wheat brans differ in their fiber digestibility of wheat bran In the rat (Bjorck et contents and compositions . According to Frolich a 1 •• 19B4) . and Nyman (1988), corrrnercial oat bran has less Domestic cooking reduces the water-holding than half the amount of total dietary fiber of capacity of wheat bran (Wyman et al., 1976). wheat bran, but its soluble fiber content is Extensive heating and chemica 1 treatments, such g reate r, approximate 1y 35%, as compared to wheat as isolation and purification of fiber components bran which has about 2%. Furthermore, the major­ like delignified cellulose, decrease the hydra­ ity of the soluble oat fiber is present in the tion capacity of the f1bers, slow down the rate form of (1~3)(1~4)-~-D-glucan, generally known as of degradation in the colon (Van Soest, 19B4), oat gum (Wood, 1986), whereas arabinoxylans are and modify mineral-binding activity (Frol ich et the major soluble fibers in wheat (Selvendran, al., 1984}. In rolled oats, cooking facilitates 19B3). the release of ~-D-glucan from the cell wall (Yiu Structures of Oat and Wheat Brans et al., rgB7). The amount of the ~-D-glucan re­ Cereal cell walls, particularly cell wall s leased is greater in porridge prepared by cooking present in the bran, are major sources of dietary rolled oats gradually from room temperature than fiber (Cummings, 1976; Southgate, 197B; Selven­ by cooking rolled oats rapidly in boiling water dran, 19B3). Chemical studies of isolated cell (Fig. 20). Microscopic examination indicated wall fractions provide detailed information on the chem1 ca 1 compos it ion of cerea 1 ce 11 wa 11 s. Most cereal cell walls are composed of cellulose 1J m1crofibr1ls embedded in a matrix of hemicellu­ 12 loses, some of which are cross-linked by lignin and phenolic esters, and/or proteins (Mares and Stone, 1g73; Bacic and Stone, 19BI; Se lvendran, 1983). ! Differences in fiber composition between oat 0.9 and wheat brans are best revealed by fluorescence "j 08 microscopy which provides both structural and 0 07 microchemical i nformation . When stained with dyes such as Congo Red or Calcofluor White (Wood " 06 et al., 19B3), oat bran is characterized by Its 05 ~-D-glucan content located chiefly in the Inner 04 aleurone and subaleurone cell walls (Fig. 15). Wheat bran, on the other hand, does not have the 0.3 same histochemistry; its aleurone cell walls are dominated by the relatively high phenolic content 15 best revea 1ed by fluorescence microscopy when Coolcmglm\e viewed under short wavelength (<365nm) excitation f..!.g_,__1Q Effect of different preparation methods (Fig. 9). Intense autofluorescence is detected on P-glucan release from rolled oats. Gradual in the wheat aleurone cell wall because of its cooking: ( ... ), Rapid cooking: ( ... ), soaking: feru11c and g-coumarlc acid contents (Fulcher et (o-o). Cooking time = simmering time. (Yiu et al. , 1972; Fulcher, 19B2). Other structural a l. , 19B7).

106 Cereal Structure And Nutritional Quality

that gradually cooked rolled oats have consider­ Many cell walls remain structurally intact after ably more cell wall disruption (Fig. 21) than passing through the colons of humans or rats rapidly cooked rolled oats (Fig. 22) . (Olntzls et al., 1979; Ylu and Mongeau, 1987) . Fluorescence microscopy is a useful tool to study the digestive breakdown of cereal cell walls by animals (Fulcher and Wood, 1983; Ylu and Mongeau, 1987}. Microscopic observation provides direct evidence of differences in d1gestibil1ty among the various structural components of cereal bran subjected to digestive processes (Viu and Mongeau, 1987). For example, the ~-0-gl ucan- rl ch subaleu rone cell wall of oat bran is susceptible to the digestive environment of the smal 1 intes­ tine of the rat (Fig . 18), whereas degradation of the aleurone cell wall does not take place unt11 the bran reaches the colon (Fig. 1g). On the other hand. most of the pertcarp and seed coat layers (Fig . 13) as well as the trlchomes re­ mained undigested (Yiu and Mongeau, 1987). Trl­ chomes, which are hair-like tubular structures found on the surface of most oat grains, have a high sll ica content. Figs. 21 22 Glycol methacrylate-embedded The detection of undigested cell wall compo­ sections of rolled oats prepared by gradual nents in the colonic digesta indicates unavail­ cooking and rapid cooking, respectively, and able food materials. Not only are carbohydrates stained with 0.01~ Calcofluor in 50~ ethanol to of cereal cell walls unavailable, but structural­ reveal d1fferences tn cell wall breakdown in the ly associated trace minerals , such as silicon, Inner oat endosperm. (Ylu et al., 1987). chromium, manganese. and cobalt likewise are not absorbed (Jones, 1978) . Furthermore, certain fiber components of oat and wheat brans, such as lignin, ce llulose,and hemic ellul ose, have affini­ ties for minera ls includi ng ca l cium, iron, zinc, copper, and magnesium (Reinhold et al. , 1975; Jones, 1978; James et al.. 1978; James, 1980; Gillooly et al., 1984; Platt and Clydesdale, 1984; Moak et al., 1987) . The main functional groups in the organic components of cereal cell walls that may be involved in mineral binding include the carboxyl and hydroxyl groups of phe­ nolic compounds, lignin,and certain polysaccha­ rides (Jones, 1978; James et al., 1978). Results of several metabolic studies indicate that long­ term intake of high-fiber food increases fecal mineral excretion. However, the excretion has no deleterious effect on mineral balance in humans due to abilities of the human body to adapt to changes in dietary conditions (Isma1l-Beig1 et Degradation and Nutritional Implications al., 1977; Van Ookkum et al., 1982; Morris and The degradation of cereal fibers ts depen­ Ellis, 1985). dent on the enzymatic activities provided by the Both phytate and fiber present in the bran microflora normally present in the colon where are tn close proximity to one another. Hence, 1t absorption of the products of fiber fermentation is often difficult to assess the individual ef­ occurs (Hungate, 1976; Cummings, 1976; Cummings fect of the bran components on mineral binding by and Englyst, 1987). The products are mostly analytical methods involving chemical extraction volatile fatty ac1ds including acetic, butyric, and determination (Davies et al . , 1977; Platt and and propionic acids and carbon dioxide, hydrogen, Clydesdale, 1987) . Microscopy, however, can pro­ and methane gases. Activities of cellulases, vide such information. Microscopic evidence sug­ p-glucosidases, cellobiase, and 13-glucanases are gests that Indigestible remnants of wheat bran, reported in the human colonic microflora (Salyers mostly pericarp tissues, are associated w1th in­ et al., 1976; Bacon, 1978). Chemical and meta­ creases in the excretion of calcium and iron in bolic studies of individual fiber components humans (Olntzis et al, 1985). The structural reveal that about 56%-87% of hemicelluloses are (Fig. 23) and elemental (Fig. 24) contents of digestible, and about 40% of ingested cellulose some of the minerals defecated by rats fed a dtet ts degradable (Cunmtngs, 1976; Anderson and Chen, rich in oat bran can be analyzed using fluores­ 1979; Nyman et al., 1986) . However, highly llg­ cence microscopy and scanning electron microscopy nHied cell walls and the presence of phenolics, coupled w1th EOX-microanalysts. Structural asso­ as well as substances such as cutin and silica ciation between the minerals and oat bran compo­ greatly reduce the digestibility of cereal fibers nents 1s not evident. However , the microscopic (Van Soest and Jones, 1968; Cummings , 1976).

107 S. H. Yiu

Sumnary Examples given in this review demonstrate how microscopy can be used to study cereal microstructures. The nutritional quality of cereal food is affected by the organization of structural components associated with starch, phytate, and dietary fiber . Processing and cook­ ing, as well as spec1f1c enzymes are factors which can alter cereal structures to such an ex­ tent that nutrient ava 11 ab11ites are affected. The availabilities of most cerea l carbohy­ drates and minerals for digestive absorption are affected by biological structures associated wlth cereal foods. Some structures are not readily accessible to the digestive enzymes present in the gastrointestinal tract. Mechanical grinding and heat are used to improve their digestibili­ ties by reducing particle size, breaking down cell walls, inducing starch gelatinization, destroying a-amylase inhib1tors and activating phytase in cerea 1s. Excess 1ve process 1ng and high heat, on the other hand, may alter the morpholog ical and physicochemical properties of ; starch, phytate, or cereal f1ber to such an ex­ 24 tent that digestib11ity is reduced. Undigested phytate and dietary fiber have the potential to adversely influence the b1oavailab111ty of min­ r i I erals in humans and animals . However, repeated -~ T -- -- metabolic studies demonstrate that, with suffi­ -: ' cient intake of dietary minerals, mineral balance : ' ! can be maintained on diets high 1n phytate and . i fiber contents. ,i i i ' ' Used in conjunction w1th other analytical I methods, particularly EDX-microanalysis, micro­ . . scopy is an important tool which has the abll ity to obtain not only structural but also microchem­ ical infonnation pertinent to the nutrient compo­ i sltion of cereal foods. - ~ ; Acknowledgement The author wishes to thank Ms. Gill ian Coo­ /~~ per for technical assistance and Ms. Candy Obas for typing the manuscript.

References Figs. 23 & 24 A glycol methacrylate-embedded, carbon-coated section of rat colonic digesta Adams CA, Novellle L. (1975). Acid hydrolases exam1ned under a scanning electron microscope at and acetolytic properties of protein bodies 20 kV , and analyzed with an energy dispersive and spherosomes isolated from ungerminated X-ray microprobe for 100 s/site, showing (Fig . seeds of Sorghum bicolor. Plant Physiol. 23) the structures of unabsorbed minerals and 55:7-11. (Fig. 24) an elemental profile of one of the Anderson IH, Levine AS, Levitt, MO . (1981) . In­ structures (identified by an * in Fig. 23). complete absorption of the carbohydrate in Probe current: 5 x J0-9 A. Probe size: 180 nm. all-purpose wheat flour. New Eng . J. Med. 304: Ca: calcium, P: phosphorus . 891-892. Anderson JW (1985). Health Implications of wheat fiber . Am J . Clln. Nutr . 41:1103-1112. Anderson JW, Chen WJL. (1979}:" Plant fiber carbo­ study on excreted materials demonstrates the hydrate and lipid metabolism. Am. J . Clin. analytical capability of microscopy combined with Nutr . 32:346-363. other detection methods such as EDX-m1croanalys- Anderson Jw, Chen WJL. (1g86). Cholesterol-lower­ 1s. EOX-m1croanalysis in conjunction wHh mi­ ing properties of oat products. In "Oats: croscopy may serve as useful techniques for Chemistry and Technology" . FH Webster (ed). studying direct association between minerals and Am . Assoc. Cereal Chern., Inc., St. Paul . , p. individual bran components. 309-334.

10 8 Cereal Structure And Nutritional Quality

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E Graf (ed.), P1latus Reinhold JG, Garcia JS,-Garzon P. (1981). Binding Press, Minneapolis. p. 57-76. of iron by fiber of wheat and maize. Am . J. Morris ER, Ellis R. (1985) . Bioavailability of Cl in. Nutr. 34:1384-1391. dietary calcium, effect of phytate on adult Reinhold JG, Ismai1-Bei9i F, Farakji B. (1975). men consuming non-vegetarian diets. In: "Nu­ Fibre vs phytate as determinant of the avall­ tritional Bioava11ab111ty of Calciumw . Am. ab111ty of calcium, zinc, and 1ron of bread­ Chern. Soc. Symposium Series No. 275 , C Kies stuffs. Nutr. Rept. Int. 12:75-85. (ed.), Am. Chern . Soc . , Washington, D.C . p. Reinhold JG, Parsa A, Karimiari' N, Hamnick JW, Is­ 63-72. ma11-Beigi F. (1974). Availability of zinc in Moss R, Stenvert NL, K1ngswood K, Pointing G. 1eavened and un 1eavened whol emea 1 wheaten (1980). The relationship between wheat micro­ breads as measured by solubility and uptake by structure and flour milling. Scanning Electron rat intestine in vitro. J. Nut r . 104:976-982. Microsc. 1980; III:613-62D. Ring SC, Selvendran RR. {1980) . IsOlation and Navert B, Sandstrom B, Cederblad A. (1985) . Re­ analysis of cell wall materials from beeswing duction of the phytate content of bran by wheat bran (Trlti cum aestivum). Phytochem . .!.J!: leavening in bread and its effect on zinc 1723-1730. absorption in man . Br . J . Nutr. 53:47-53. Ross SW, Brand J, Thorburn AW, Truswell AS. Nayini NR, Markaki s P. (1983). Effect of mill ing {1987) . Glycemic index of processed wheat extraction on the inositol phosphates of wheat products. Am. J. Clin. Nutr. 46:631-635. flour and bread. J . Food Sci . 48:1384-1389. Salyers AA, Palmer JH, BalascioJR. (1976). Di­ Nayini NR, Markakis P. (1986).- Phytases. In: gestion of plant cell wall polysaccharides by "Phytic Acid: Chemistry and Application". E bacteria from the human colon. In: "Dietary Graf (ed.), P1latus Press, Minneapolis. p. Fibers: Chemistry and Nutrition". GE Inglett, 101-118. SI Falkehag (eds. ), Acad . Press, New York . Nyman M, Asp N-G, Cummings J, Wiggins H. (lg86) . p. 193-201. Fermentation of dietary f i bre in the intesti­ Sandberg AS , Andersson H. {1988). Effect of di­ nal tract: comparison between man and rat. etary phytase on the digestion of phytate in Br. J. Nutr. 55:487-496. the stomach and small intestine of humans. J. Oberleas D, Harland B. (1981). Phytate content of Nutr. 118:469-473. foods: effect on dietary zinc bioavailability. Sandberg K, Andersson H. Carlsson NG, Sandstrom J . Am . Diet. Assoc. 79:433-436. B. {1987) . Degradation products of bran phy­ O'Dell BL. (1969). Effect of dietary components tate formed during digestion in the human upon zinc ava11ab1lity . A review with original small intestine: effect of extrusion cooking data . Am. J. Clin. Nutr. ££:1315-1322 . on digestibility. J. Nutr. !.!2:2061-2065.

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Sandberg AS, Andersson J, Kivisto B, Sandstrom Tronier B, Cry RL. Henningsen KW. (1971). Charac­ B. (lgB6). Extrusion cooking of a high-fibre terization of the fine structure and proteins cereal product. I. Effect on digestibility and i~~-~~~~~ey protein bodies. Phytochem. !.Q: absorption of protein, fat, starch, dietary fibre and phytate in the small intestine. Br. Trowell HC . (1976) . Definition of dietary fiber J. Nut r . 55:245-254. and hypothesis that it is a protective factor Sandstedt RM:'" (1955) . Photomicrographic studies in certain diseases. Atn. J. Cl1n. Nutr. 29 : of wheat starch . III. Enzymat ic digestion and 417-427. - granule structure. Cereal Chem. E: (Suppl . ) 17. Trowe ll HC. (1988). Dietary fiber definitions . Sandstedt RM. (lg61). The function of starch in Am. J. Clin Nutr. 48:1079-1080. the baking of bread. Bakers Digest. 35 :36-39. Van Dokkum w, Wesstra A, Sc hippers FA . (1982). Sandstrom B, Almgren A, Kivisto B. Cederblad A. Physiological effects of fibre-rich types of (1987). Zinc absorption in humans from meals bread. I. The effect of dietary fibre from based on rye, barley, oatmeal, triticale and bread on the mineral balance of young men . whole wheat. J. Nutr. 117:1891-1902. Br. J . Nutr. 47: 451-460. Schneeman BO. (1987) . Dietary fiber and gastro­ Van Soest PJ . (i9a4). Some physical characteris­ intestinal function . Nutr. Rev. 45:129-132. tics of dietary fibres and their influence on Schultze WE. MacMasters MM. (1962)-:-ereakage of the microbial ecology of the human colon . endospenn cell walls in flour mill i ng. Cereal Proc. Nutr. Soc. 43:25-33. Chem. 39 : 204-209. Van Soest PJ, Jones DiP. (1968). Effect of silica Schwarz Pif, Kunerth WH, Youngs VL. (1986) . The in forages upon digestibility. J. Dairy Sci. distribution of 1 ignin and other fiber compo­ 51:1644-1649. nents within hard red spring wheat bran. Ce­ Vari'iano-Marston E. (1982). Polarization micros­ real Chem . 65:59-64 . copy: applications in cereal science. In: •New Selvendran RR.-(1983) . The plant cell wall as a Frontiers in Food Micro structure". DB Bechtel source of dietary fiber: chemistry and struc­ (ed.), Am. Assoc . Cereal Chem., Inc., St. ture. Am. J. Clin . Nutr. 36 : 320-337. Paul. p. 71-108. Shainkin R, Birk Y. (1970). a-Amylase inhibitors Waslien CI. (1988). What is dietary fiber and from wheat isolation and characterization. what is starch. Cereal Foods World 33:312. Biochem. Biophys. Acta. 221:502- 513. Wise A. (1986). Influence of calcium or1trace me­ Shinni ck FL, Longacre MJ, ---rr.k SL, Marlett JA . tal-phytate interactions . In: "Phytic Acid: (1988) . Oat fiber: composition versus physio­ Chemistry and Application". E Graf (ed.), Pi­ logical function in rats. J. Nutr. 118:144- latus Press, Minneapolis. p . 151-I60. 151. - Wivinis GP, Maywald EC . (1967). Photographs of Simpson KM, Eugene BS, Morris R, Cook JD. starches . In: "Starch: Chemistry & Technology (1981). The inhibitory effect of bran on iron II. Industrial Aspects. RL Whistler, EF Pas­ absorption in man. Am. J. Clin. Nutr. 34:1469- chall (eds . ) . Acad. Press, New York. p. 649- 1479. - 685 . Snow P, O'Dea K. (1981). Factors affec ting the Wood PJ . (1986). Oat ~-glucan: structure, loca­ rate of hydrolysis of starch 1n food. Atn. J . tion. and properties. In: 11 0ats: Chemistry and Clin. Nutr. 34:2721-2727. Technology" . Asn. Assoc. Cereal Chern., Inc., Southgate OAT. (1978). The definition, analysis Minnesota. p. I21-152. and properties of dietary fibre. J. Plant Food Wood PJ, Fulcher RG, Stone SA . (1983). Studies on 3:9-19. the specificity of interaction of cereal cell So uth9a te OAT. Kritchevsky D. (1981). Tenni nol ogy wall components with Congo Red and Calcofluor. of dietary fibre . In: Symposia from the XII Specific detection and histochemistry of (1•3) (1•4)-~-D-glucan . J. Cereal Sci . 1:95- International Congress of Nutrition. Alan R. 110 . - Liss, Inc., New York, p. 219-222. Spiller GA, Story JA, Wong LG. (1986). Effect of Wursch P, Del Vedovo S, Koellreutter B. (1986) . increasing levels of hard wheat fiber on fecal Cell structure and starch nature as key deter­ wei ght , minerals and steroids and gastrointes­ minan ts of the digestion rate of starch in tina l transit time i n healthy young women. J. legume. Am. J. Clin. Nu tr. 43:25-29. Nutr. 116:778-785. Wyman JB , Heaton KW, Manni ng AP, Wicks ACB. Tanaka K,Yoshida T, Kasai , z. (1974). Distribu­ (1976) . The effect on intestinal transit an d tion of mineral elements in the outer layer of the f eces of raw and cooked bran in different rice and wheat grains, using electron micro­ doses. Am. J . Clin. Nutr. 29: 1474- I481. probe X-ray analysis. Soil Sci. Plant Nutr. Yamaguchi M, Kainuma K, FrenCh D. (1979). Elec­ 20:87-91. tron microscopic observations of waxy maize Thompson LU. (1986) . Phytic acid: a factor influ­ starch. J. Ultrastruct. Res. 69:249-261. encing starch digestibi 1 ity and blood glucose Yiu SH. (1986) . Effects of processing and cooking response. In: "Phytic Acid: Chemistry and Ap­ on the structural and microchemical composi­ plications". E Graf (ed.), Pilatus Press , tion of oats. Food Microstruc . 5:219-225. Minneapolis. p. 173-194. Yiu SH, Mongeau R. (1987). Fluorescence and light Thompson LU, Yoon JH, Jenkins DJA, Wolever TMS. microscopic analysis of digested oat bran. Jenkins AL. (1984) . Relationship between Food Microstruc. 6:143-150. polyphenol intake and blood glucose response Yiu SH , Poon H, Fulcher RG, Altosaar I. (1982). of normal and diabetic individuals. Am . J. The microscopic structure and chemistry of Clin. Nutr. ;)2:745- 751. rapeseed and its products. Food M1crostruc. l= 135-143.

112 Cereal Structure And Nutritional Quality

Yiu SH, Wood PJ, Weisz J . (1987). Effects of B.G . Swanson: Can you compare observation of cooking on starch and a-glucan of rolled dietary fiber by fluorescence and scanning elec­ oats. Cereal Chern . 64:373-379. tron microscopy? Yoon JH, Thompson LU,-Jenkins OJA. (1983) . The Author: While scanning electron microscopy has effect of phytic acid on in vitro rate of better resolving power than fluorescence micros­ starch digest1bil1ty and blood glucose res­ copy, it does not reveal any chemical information ponse. Am. J . Clin. Nutr. ~ : 835-842. of a fiber structure. The distribut1ons of fi ­ Yoshida T, Tanada K, Kasai Z. (1975) . Phytase ber-associated substances, such as phenolic acids activity associated with isolated aleurone and components such as a-O-glucans tn cereal cell particles of rice grains . Agr. Bioi. Chern . walls, can be easi ly detected using fluorescence ;l2 : 289-290 . microscopy.

L.U. Th ompson: Will you please further clarify Discussion with Reviewers how the appearance a nd compost tton of the crys­ talline minerals illustrated in Figures 23 and 24 B.G . Swanson: What evidence can you provide to may reveal nutrient interactions? suggest that microscopy can serve as an accurate Author: Figures 23 and 24 are included to de­ analyt1cal tool? monstrate the analytical capability of SEM and Author: With the aid of specific staining rea­ EOX microanalysis. Such techniques have the ge nts such as iodine - potassium iodide and acid ability to provide both structural and elemental Fuchsin, microscopy can be used to accurately information . Hence, any changes in the elemental differentiate starch granules from protein bodies compos1tion of phytin globoids after they have in most cereal grains. passed through the gastrointestinal tract can be detected using the above techniques. Differences B. G. Swanson : How do you perceive microscopy can in the composition should reflect the mineral­ quantitatively determine starch digest1bil1ty? binding act1v1ty of the globoids, provided that Author: Microscopic observation can reveal artifacts such as the migration of soluble ele­ structural changes of starch granules subjected ments in and out of the globoid structures during to enzymatic digestion, but cannot quantify how sample preparation are tak.en into account or much starch 1 s digested. eliminated .

B.G. Swa nso n: What is you r co nclusion regarding L. U. Th ompson: Si nce phytase i n cereal foods may importance of phytate to digestibil1ty, mineral affect the breakdown of phyt1c acid in the gas­ absorption and the nutritional quality of trointestina l t ract , have you or others tried cerea I s·t estimating its location and concentration by Author: The major concern of phytate in relation microscopic techniques? How? to the nutritional quality of cereals is the min­ Author: Tronier et al. (1971) localized phytase era 1-bi ndi ng property. However , phytate contents activity in the aleurone cell of barley using in most cereal grains are reduced as a result of transm1 ss ion electron microscopy. Other mi ere­ processing, baking, and mild heat treatments. scopic techniques such as inmunofluorescence or Feeding studies indicated that the deleterious enzyme-linked inmuno-staining are suitable for effect of phytate on mineral metabolism can be detecting and quantitating enzyme activities in avoided by maintaining diets that are well animal or plant tissues. balanced in mineral contents . Contribution No . 804

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