Food Structure Volume 1 Number 2 Article 2 1982 Grain Structure and End-Use Properties Y. Pomeranz Follow this and additional works at: https://digitalcommons.usu.edu/foodmicrostructure Part of the Food Science Commons Recommended Citation Pomeranz, Y. (1982) "Grain Structure and End-Use Properties," Food Structure: Vol. 1 : No. 2 , Article 2. Available at: https://digitalcommons.usu.edu/foodmicrostructure/vol1/iss2/2 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. I (1982), pp. 107-124 SEM Inc., AMF O'Hare (Chicago), IL 60666 U.S.A. GRAIN STRUCTURE AND END-USE PROPERTIES Y. Pomeranz U.S . Department of Agriculture, Agricultural Research Service, No rth Central Region, U. S. Grain Narketing Research Center, ~1anhattan , Kansas 66502, USA Abstract Introduction Practical implications o ~ grain structure Grain structure is an expression of grain relate to every step from grain development and composition as it reflect s properties from the production through market ing to processing, util iza­ standpo i nt of plant physiology. The pl ant does tion, and consumption . The struct ure and adherence not synthesize or incorporate components into of the hulls may contribute to protection of grain structures unless they have a specific functi on in during germination or malting and protection against preservati on or propagation of the species. insect infestations . Germ retention during t hresh­ Cereal chemis t s and technologists, on the other i ng and separation during processing depend on the hand, are interested in another set of properties germ structure and location in the ke rnel. The -- the function the grain or its fractions can per­ subal eurone and central endosperm layers differ in form in the production of nutritious foods , that cell si ze , shape, and structure and in composition, have good shelf life, and are acceptable to the especially with regard to protein contents and consumer. quality. The mai n factors in grain hardness are the Thus, in a way, grain structure forms the intri nsic hardness of t he main components , the link between compos i t ion that i s the source ofour strength of i nteraction within t he cel l, and the basic knowledge of bi olog ical systems and utili­ interaction of i ndividual cell s to produce overall zation of those components in food production . grain structure. For optimum utilizatirin of cereal grains, know­ Endosperm struct ure and hardness i s related to ledge of t hei r structures and compositions is wheat co nditioning, to breakage in milltng, and to req uired. The practical impl ications of kernel the structure and composition of the mi l led flour structure are numerous. They relate to the var­ particles. Milling qual ity is governed by morpho­ ious steps of grain producti on, harvest, storage, logi cal characteristics of t he wheat kernel and its marketing, and utilization. Some of t he implica­ mechano-phys ical properties and by the methods of ti ons are listed in Table 1. Discussed here, in grinding and separation. Reducing changes in det ail, are primari ly studies that deal with wheat texture and structure during drying of maize and and barley. Other cereal grains are reviewed rice are important in minimi zing breakage du ring briefly. hand ling, storage , and transportation, dust forma ­ tion, and infestation. Differences in grain Kernel Structure- General structure are expressed in differences i n composi­ tion, gradients of components in grain tissues, The cereal grain is a one-seeded fruitcalled and end-u se properties . Those differences have a caryopsi s, i n which t he fruit coat i s ad herent important nutriti onal implications . New micro­ to the seed. As the fruit ripens, the pericarp scopic methods to determi ne grai n structure, compo­ (fruit wall) becomes firmly att ached to the wall sition, and end -use properties have the potential of the seed proper. The pericarp, seed coats, of contributing to improved nutritional quality nucellus, and al eurone cells form the bran. The and utilization of cereals by modifying-restructur­ embryo occ upies only a small part of the seed . ing grain morphology t hrough classi cal plant The bulk of the seed i s taken up by the endo sperm , breeding and geneti c engi neering. which constitut es a food reservoir. The floral envelopes (modified l eaves known Initial paper received April 10, 1982. as l emma and palea), or chaffy pa rts , within Fina l manusc ript received September 16, 1982. which the caryopsis develops, persist to maturity Direct inquiries to Y. Pomeranz . in t he grass family (MacMasters, 1962). If the Te lephone number: 913-539-9141 . chaffy st ructures envelope the caryopsi s so cl osely t hat t hey remai n attached t o it when the KEY WORDS: Gra in Structure, Cerea l Endosperm , grain is threshed (as in rice an d most varieties Al eurone Layer , Hull and Bran Layers, Germ , Grai n of oats and barley), the grain i s considered to be Processing, Wheat Hard ne ss and Mil l ing, Germi na­ covered . However, if the caryopsi s readily tion and Ma lting , Grain Storage, Grain End- Use separates from the floral envel opes on thresh ing, Properti es 107 Y. Pomeranz Table 1. Some Implications of Kernel Structure Significance in Parameter Effect Commodity Threshing Germ damage or Reduced germin­ All cereal grains skinning abi 1 i ty, impaired storabil ity Drying Cracks, fissures, and Reduced commercial Mainly corn and breakage; hardening value; lowered grade, rice impaired storability, dust formation,re­ duced starch y~eld Discoloration Reduced commercial ~1ai nly rice value, lowered grade r~arketing Breakage Reduced commercial Mainly corn and value in food rice processing General use High husk: caryopsis Reduced nutritional All cereal grains ratio or high pericarp: value--as food or endosperm ratio feed General use Kernel shape and dimen­ Yield of food prod­ All cereal grains sions,proportions of ucts; nutritional tissues in the kernel, value of cereal (or distribution of nutri­ cereal products) as ents in the tissues food or feed Malting Germ damage, skinning, Reduced germinability, t~ai nly barley or inadequate husk uneven malting adherence Milling Uneven surface, deep Reduced milling Ma.i nly wheat and crease or uneven aleurone yield rice Milling Steely texture Increased power re­ Wheat and malt quirements, starch milling damage, high water absorption, diffi­ c u1 ty i n a i r­ classification Germination-Malting Starch granule size Uneven degradation All cereal grains Consumption­ Distribution and com­ Change in nutritional All cereal grains Nutrition position of protein value as with common wheats, rye, hull-less barleys, to remain attached to the outer coats during and the common varieties of corn, the grain is wheat milling, it is considered by millers as the considered to be naked. innermost bran layer. The structure of the wheat kernel is shown in The embryonic axis consists of the plumule Fig. 1. The dorsal side of the wheat grain is and radicle, which are connected by the mesocotyl. rounded, while the ventral side has a deep groove The scutellum serves as an organ for food stor­ or crease along the entire longitudinal axis. At age. The outer layer of the scutellum, the the apex or small end (stigmatic end) of the grain epithelium, may function either as a secretory or is a cluster of short, fine hairs known as brush as an absorption organ. In a well-filled wheat hairs. The pericarp, or dry fruit coat, consists kernel, the germ comprises about 2-3% of the of four layers: the epidermis, hypodermis, cross kernel, the bran 13-17%, and the starchy endosperm cells, and tube cells. The remaining tissues of the remainder. The inner bran layer (the aleurone) the grain are the inner bran (seed coat and is high in protein, fat, and minerals, whereas, nucellar tissue), endosperm, and embryo (germ). the outer bran layers (pericarp, seed coats and The aleurone layer consists of large rectangular, nucellus) are high in cellulose, hemicelluloses, heavy-walled, cells. Botanically, the aleurone is and minerals. The germ is high in proteins, the outer layer of the endosperm, but as it tends lipids, sugars (chiefly sucrose), and minerals; 108 Grain Structure and End-Use Properties certain floral envelopes. In light thin oats, hulls may comprise as much as 45 % of the grain; in very heavy or plump oats, they may represent only 20%. The hull normally makes up ~ 30 % of the grain. Rice is a covered cereal; in the threshed grain (or rough rice), the kernel is enclosed in a tough siliceous hull, which renders it unsuit­ able for human consumption. When this hull is removed during milling, the kernel (or caryopsis), Endosperm comprised of the pericarp (outer bran) and the seed proper (inner bran, endosperm, and germ), is it\~1~,~;~~~~~-- ~t~~:~~~~~a':i:l:s known as brown rice or sometimes as unpolished in Protein Matr-ix rice. Brown rice is in little demand as a food. Unless stored under very favorable conditions, it Cellulose Walls of Cells tends to become rancid and is more subject to insect infestation than the various forms of milled white rice. When brown rice is subjected f',j_~rfR1fpJfiTi~~~ ~~~:2;..:,~?.~~~7 ~~~~~or;a;;~~. £l endosperm b~o~t to further milling processes, the bran and germ separated with bran) are removed and the purified endosperms are mar­ keted as white rice or polished rice.