Rice Properties and Processing Bienvenido O
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This article was downloaded by: [Tomsk State University of Control Systems and Radio] On: 17 February 2013, At: 04:31 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Food Reviews International Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lfri20 Rice properties and processing Bienvenido O. Juliano a a Cereal Chemistry Department, The International Rice Research Institute, Los Baños, Laguna, Philippines Version of record first published: 03 Nov 2009. To cite this article: Bienvenido O. Juliano (1985): Rice properties and processing, Food Reviews International, 1:3, 423-445 To link to this article: http://dx.doi.org/10.1080/87559128509540778 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. Food Reviews International, 1(3), 423-445 (1985-86) RICE PROPERTIES AND PROCESSING BIENVENIDO O. JULIANO Cereal Chemistry Department The International Rice Research Institute Los Baños, Laguna, Philippines RICE PROPERTIES Morphology Several reviews on rice chemistry and technology have appeared during the last two decades, with emphasis on different aspects of the subject (1-5). The present review deals with rice grain properties and their effects on rice processing. The rice grain (rough rice or paddy) consists of an outer protective covering, the hull, and the rice caryopsis (brown rice or dehulled or husked rice) (Figure 1). Brown rice consists of the outer layers of pericarp, seedcoat, and nucellus, and the germ or embryo, which are maternal tissues, and the endosperm. The endosperm consists of the aleurone layer, with the endosperm proper consisting of the subaleurone layer and the starchy or inner endosperm. The aleurone layer encloses the embryo. The aleurone layer varies from 1 to 5 cell layers, thicker at the dorsal than at the ventral side, and thicker in short-grain than in long-grain rices. The Downloaded by [Tomsk State University of Control Systems and Radio] at 04:32 17 February 2013 aleurone cells are rich in protein bodies (containing globoids or phytate bodies) and in lipid bodies (6). Phytate is mainly the potassium magnesium salt of myo- inosital hexaphosphate. 423 Copyright © 1986 by Marcel Dekker, Inc. 8755-9129/85/0103-0423$3.50/0 424 JULIANO Awn Lemma Palea Caryopsis Subaleurone —Pericarp layer _Seedcoat —Nucellus Starchy endosperm ^—Aleurone layer Embryo Rachilla Sterile lemmae Figure 1. Longitudinal structure of the rice grain. The endosperm cells are thin-walled and packed with amyloplasts contain- ing 3-9 /zni compound starch granules. The two outermost cell layers (the subaleurone layer) are rich in protein and have smaller amyloplasts and com- pound starch granules. Protein occurs mainly in the form of spherical protein bodies 1-4 fim in size throughout the endosperm (7,8). But crystalline protein bodies and small spherical protein bodies are localized in the subaleurone layer (8). Rice starch granules are polyhedral and mainly 3-9 /xm in size. Postharvest Technology Conditions during grain ripening and drying in the field affect the processing characteristics of the rice grain. Evenness of flowering affects the percentage of Downloaded by [Tomsk State University of Control Systems and Radio] at 04:32 17 February 2013 immatures in the harvest crop, with photosensitive rices having more synchron- ous anthesis than nonsensitive varieties. However, among nonsensitive rices, early-maturing (90-110 days) rices tended to have more immature grains than medium-maturing rices (130-140 days). Immature grains reduce milling quality (9). Dormancy is a desirable trait because it prevents germination of wet grain in the field, but it is a nuisance if the harvested crop is to be immediately used as RICE PROPERTIES AND PROCESSING 425 seed. Dormancy in rice is not true embryo dormancy, because removing the covering hull and bran layers breaks dormancy (10). Storing 3-4 weeks at ambient temperature breaks dormancy, probably due to a temporary barrier to oxygen and water uptake by the embryo (10,11). Stack burning occurs when wet grain, particularly unthreshed grain, is piled without provision for aeration. Microbial respiration causes the mass to heat to over 60°C (12). The resulting milled rice becomes yellow, regardless of variety, due to the heating effect rather than direct microbial infestation, since the yellowing can be simulated even for milled rice in a laboratory oven. Yellowing reduces the lysine content of rice about 10% with a resultant drop in net pro- tein utilization (NPU) in rats (10). Hull tightness is a desirable storage trait of rough rice because a tight hull protects the brown rice from infestation by insects and microorganisms. Milling and Milling Fractions Rice is consumed mainly as whole-grain cereal. Rough rice processing con- sists of dehulling followed by abrasive milling of the brown rice to produce milled or polished or white rice and bran and polish. In the Engelberg or huller- type mills, dehulling and milling are done in one step with greater grain breakage. The by-product is a mixture of hull and bran-polish, which is usually used for animal feed. Modern rice mills produce separate fractions consisting of bran, hull, and milled grain. For modern plants, hull disposal is a problem. An important factor contributing to grain breakage during milling is pre- formed fissures or cracks in brown rice resulting from moisture adsorption stress during drying, storage, and milling (13). Varietal differences in suscep- tibility to fissuring have been demonstrated for both rough rice and brown rice (13,14). Hull tightness would affect the rate of moisture adsorption of rough rice. Immature grains result in small, chalky brokens. Chalky portions in nonwaxy endosperm contribute also to grain breakage (15). Chalky portions correspond to loose arrangement of the cell contents (air spaces) (7). Yellow grains resulting from stack burning are resistant to breakage during milling. Parboiled rice also requires greater pressure to mill because of a harder endo- sperm. The bran from parboiled rice also tends to clog the sieves (16). Starch is localized mainly in the endosperm of the mature rice grain (Fig- ure 1). Starch is classified based on amylose (linear fraction) content into waxy Downloaded by [Tomsk State University of Control Systems and Radio] at 04:32 17 February 2013 (0-2% amylose) and nonwaxy-low (10-20%), intermediate (20-25%), and high (25-33%) (17). Amylose content is measured by the iodine colorimetry of alkali-dispersed milled rice with iodine-potassium iodide solution in acetate buffer of pH 4.5-4.8 (18). Rice starch granules also differ in final gelatiniza- tion temperature (GT), classified as low (<70°C), intermediate (70-74.5°C), and high (75-80°C) (17). Not all combinations of amylose content and GT are r OS Table I. Comparative Properties of Rough Rice and Its Fractions at 14% Moisture Property Rough Rice Brown Rice Milled Rice Rice Bran Rice Hull Crude protein (% N X 5.95) 5.8-7.7 7.1-8.3 6.3-7.1 11.3-14.9 2.0-2.8 Crude fat (%) 1.5-2.3 1.6-2.8 0.3-0.5 15.0-19.7 0.3-0.8 Crude fiber (%) 7.2-10.4 0.6-1.0 0.2-0.5 7.0-11.4 34.5-45.9 Crude ash (%) 2.9-5.2 1.0-1.5 0.3-0.8 6.6-9.9 13.2-21.0 Available carbohydrates (%) 64-73 73-87 77-89 34-62 22-35 Neutral detergent fiber (%) 16.4 3.9 0.7-2.3 24-29 66-74 Energy content (kJ/g) 15.8 15.2-16.1 14.6-15.6 16.7-19.9 11.1-13.9 Density (g/ml) 1.17-1.23 1.31 1.44-1.46 1.16-1.29 0.67-0.74 Bulk density (g/ml) 0.56-0.64 0.68 0.78-0.85 0.20-0.40 0.10-0.16 > Downloaded by [Tomsk State University of Control Systems and Radio] at 04:32 17 February 2013 o RICE PROPERTIES AND PROCESSING 427 known: waxy starch is mainly low GT with some high GT as with low amylose starch. High GT is rare among intermediate- and high-amylose rices. Bran is richer in minerals, vitamins, protein, fat, and fiber than milled rice (5) (Table 1). Bran proteins are rich not only in the lysine-rich proteins albumin (water-soluble) and globulin (salt-soluble), but also in antinutrition factors—trypsin inhibitor and hemagglutinin or lectin, mainly in the embryo. By contrast, milled rice (endosperm) is rich in glutelin (~80% of total), 15% albumin-globulin, and less than 5% prolamin (alcohol-soluble). Crystalline protein bodies are rich in albumin-globulin, whereas spherical protein bodies are rich in prolamin (19). Protein complexed with starch granules not removed by protease or dodecylbenzene sulfonate (0.1-0.7%) is mainly the Wx gene product (20), which is proportionate to amylose content (21).