98 (2007) 304 – 309
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Research Paper: PH—Postharvest Technology
Aerodynamic properties of tef grain and straw material
Food Science & Postharvest Technology, Alemaya University, P.O. Box 49, Ethiopia
ar t ic l e i n f o
Article history: Received 15 January 2007 Accepted 1 August 2007 Available online 20 September 2007
Terminal velocities were measured for both tef (Eragrostis tef (Zucc.) Trotter) grains and straws using the suspension velocity method. The terminal velocity of tef grains increased linearly from 3.08 to 3.96 m sÀ1 with increase in moisture content from 6.5% to 30.1% wet basis (w.b.). Straws with different lengths, node-free, with a node in the middle, and with a node at the end were tested for their terminal velocities. Straws with nodes had higher terminal velocities compared with node free. Straws with a node at the end had the highest terminal velocity (5.4 m sÀ1 for 10-cm-long straws). Terminal velocity decreased from 3.69 to 2.13 and 3.08 to 1.70 m sÀ1 for middle-node and node-free straws, respectively, but for end-node straws the terminal velocity showed gradual increase from 3.32 to 5.40 m sÀ1 as the length of straw increased from 1 to 10 cm. The drag coefﬁcients for grain and resistance tally obtained terminal velocities. Tef grains drag coefﬁcient decreased from 0.83 to 0.65 with increase in moisture content from 6.5% to 30.1%. Node-free straws exhibited higher resistance coefﬁcients and end-node straws the lowest. & 2007 IAgrE. Published by Elsevier Ltd. All rights reserved. coefﬁcient (drag coefﬁcient Â frontal area) for straw were calculated from the experimen-
Tef (Eragrostis tef (Zucc.) Trotter) is an important cereal crop indigenous to Ethiopia, constituting about 20% of cereal production of the country (Bultosa & Taylor, 2004). Countries such as USA, Canada, Australia, South Africa, and Kenya have produced tef for different purposes such as a forage crop and a thickener for soups, stews, and gravies (Ketema, 1997). Tef threshing is carried out in Ethiopia by trampling over the cut crop collected on a ﬂat surface with oxen. Separation of tef grain is carried out by throwing the grain and material out of grain mix in air using the difference in aerodynamic properties. Cleaning is performed by manually wafting air over the grain chaff mix with a dried hard leather strap. Bultosa and Taylor (2004) commented on the possibility of using a combine harvester for harvesting of tef, but they added that tef grain losses can be high due to the very small size and light mass of the grain. The equivalent diameter of
tef grain was reported to vary between 0.71 and 0.87 mm and thousand grain mass 0.257–0.421 g (Zewdu & Solomon, 2007). Tef was said to have a prolate spheroid shape, which results in its dimensions being represented by a length and diameter. The sphericity of tef varies between 0.6 and 0.7 (Zewdu & Solomon, 2007). In order to harvest tef with combine harvester, or perhaps a stationary thresher and cleaning unit, it is important to know the physical and aerodynamic property of tef grain and straw. By deﬁning the terminal velocity of different threshed materials, it is possible to determine and set the maximum possible air velocity in which material out of grain (MOG) can be removed without loss of grain (Zewdu, 2004; Freye, 1980) or the principle can be applied to classify grain into different size groups (Wu et al., 1999). The difference in terminal velocity between damaged and undamaged grains was utilised by Bueermann (1991) to separate them in a vertical wind tunnel. In addition, agricultural materials and food
ÃCorresponding author. Current Address: Adama University, P.O. Box 1888, Adama, Ethiopia.
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