Food Structure Volume 8 Number 1 Article 13 1989 Cereal Structure and Its Relationship to Nutritional Quality S. H. Yiu Follow this and additional works at: https://digitalcommons.usu.edu/foodmicrostructure Part of the Food Science Commons 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.