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A simple device for dehulling seeds and grain
Animal Feed Science and Technology, 3 (1978) 109--116
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© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
A SIMPLE DEVICE F O R D E H U L L I N G SEEDS AND G R A I N
J.R. ASHES and N.J. PECK C.S.I.R.O., Division o f Animal Production, P.O.Box 239, Blacktown, N.S.W., 2148 (Australia)
(Received 14 September 1977)
ABSTRACT Ashes, J.R. and Peck, N.J., 1978. A simple device for dehulling seeds and grain. Anita. Feed Sci. Technol., 3: 109--116. The simple device described operates on the principle of bouncing the seed between a "squirrel cage" type rotor and a ripple plate, thus impacting the hull from the kernel in contrast to the conventional milling or rolling procedures generally employed. Thirteen seed and grain types were dehulled during a single pass through the mill and screening device. Safflower seed could be effectively dehulled but required two passes through the mill. The efficiency of dehulling after one pass varied, but was 90% with sunflower seed and 95% with cottonseed. The extent of the dehulling was proportional to the velocity of the rotor tips and could be readily varied. Consequently the mill was able to process a wide variety of seed and also other materials such as dry lucerne hay. Wear and maintenance were minimal as sharp knife edges or a precise gap between the rotor and the ripple plate were not required.
INTRODUCTION Seeds and grains possess an o u t e r covering c o m m o n l y referred to as a hull or husk w h i c h shields t h e inner kernel, the f r a c t i o n c o n t a i n i n g m o s t o f the lipid, p r o t e i n and readily digestible c a r b o h y d r a t e . T h e hull, w h i c h m a y a c c o u n t f o r u p to 40% o f the t o t a l seed weight, is usually largely c o m p o s e d o f fibre. Cons e q u e n t l y , r e m o v a l o f t h e hull f r a c t i o n m a y o f t e n be desirable in o r d e r t o imp r o v e e f f i c i e n c y in the s u b s e q u e n t m e c h a n i c a l or c h e m i c a l processing o f the seed, e.g. in oil e x t r a c t i o n (Norris, 1964). Similarly, hull r e m o v a l will o f t e n i m p r o v e the digestibility o f the r e m a i n i n g fractions b y p e r m i t t i n g access b y digestive e n z y m e s to n u t r i t i o n a l l y desirable c o m p o n e n t s . T h e r e m o v a l o f hulls f r o m l e g u m i n o u s seeds, such as pulses in h u m a n diets, greatly improves the a p p e a r a n c e o f the p r o d u c t while r e d u c i n g fibre ( G o p a l a n and Balasubramanian, 1 9 6 3 ) and i m p r o v i n g digestibility ( H e a t o n , 1973). In c o n v e n t i o n a l dehulling e q u i p m e n t the husks are usually p a r t e d f r o m t h e seed kernels b y a p p l y i n g to the seed compressive or shearing forces i n d u c e d b y the a c t i o n o f t w o or m o r e o p p o s e d surfaces o r edges. In practice this is achieved b y passing the w h o l e seeds t h r o u g h pairs o f rollers, or t h r o u g h a mill
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in which the seeds are carried r ound by longitudinal projections on the surface o f a rotating dram, and forced through a gap of considerably smaller width than any dimension of the whole seed (Norris, 1964). The disadvantages of e q u i p m e n t employing this principle are fairly obvious; it is o f ten difficult to separate the seed c o m p o n e n t s milled in this m anner as the husk and kernels are broken d o w n to particles of similar sizes, or, as found with certain oil seeds, the compressive and sheer forces applied tend to release the oil from the spherosomes. This m ay result in a loss of part of the oil with the husk or the f or m a t i on of an oil/husk " c a k e " which can cause blockages in the machinery. Both rolling and milling procedures require a set gap of smaller dimensions than the seed. The seed generally must be subjected to cleaning and grading before dehulling, in order to prevent small seeds passing through the e q u i p m e n t uncracked or larger seeds being broken into small fragments; similarly, the seed must be free of foreign materials to minimise the risk of damage to the fine tolerances of the sharp edges usually employed. This paper describes a machine designed to avoid m a n y of the problems associated with milling or rolling; the principle involves bouncing the seed repeatedly between a r o t o r and a baffle plate until the impact cracks the hull and releases it from the kernel. MATERIALS AND METHODS A small scale dehuller consisting of a mill and screening device was constructed. Th e mill {Fig. 1) comprises three basic parts a. Housing b. R o t o r c. Baffle plate The r o t o r is in the form of a "squirrel cage" and consists of two 3 mm X 200 m m diameter discs welded to a 40 mm diameter shaft. Sixteen evenly spaced 18 m m diameter rods, 110 m m long, are riveted between the discs at 178 mm pitch circle diameter. T he r o t o r is supported by sealed ball bearings held in the housing and powered by a 4.8 Kw petrol driven engine, pulley driven by vee belts to rotate the r o t o r in a clockwise direction at speeds o f 1000--6000 r.p.m. A baffle plate is fixed to the lower half of the housing and co n s tr u cted in the form of a ripple plate. The ripple angle is 90 ° at the centre of the plate, b u t is gradually increased at either e n d t o 120 ° , to reduce the possibility of blockages at the inlet or outlet. The r o t o r is offset 5.5 mm from the centre o f the housing away from the baffle plate, while the minimum clearance between the baffle plate and r o t o r is 6 mm. A h o p p e r with an adjustable inlet is fitted t o the top of the mill so as to feed variable quantities of seed (not shown in Fig. 1); similarly the dehulled seeds and husks are discharged on to a vibrating aspirated 3 m m screen (also n o t shown in Fig. 1). The dimensions of the screen are 1200 m m X 500 mm. The aspiration rate is adjustable with a m a x i m u m flowrate of 6 m3/min at
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Fig. 1. A cross-sectional view of the mill consisting of: (1) the housing; (2) the rotor; (3) the baffle plate; (4) the impact rods; (5) rotor support shaft. The rotor is in the form of a "squirrel cage" and the baffle plate, a ripple plate. All dimensions are in mm. The direction of rotation of the rotor is indicated.
2 5 0 0 r.p.m. B o t h the a s p ~ a t o r and v i b r a t i n g screen are driven b y t h e s a m e p o w e r s o u r c e as t h e mill. T h e mill and v i b r a t i n g screen u n i t s are c o n s t r u c t e d o f m i l d steel, e x c e p t t h e b a f f l e p l a t e w h i c h is m a d e o f I m m stainless steel. Moisture, e t h e r e x t r a c t , p r o t e i n and ash c o n t e n t s w e r e d e t e r m i n e d b y m e t h o d s o u t l i n e d b y t h e A s s o c i a t i o n o f Official A n a l y t i c a l C h e m i s t s (1970). F i b r e c o n t e n t was e s t i m a t e d as a c i d - d e t e r g e n t fibre ( A D F ) ( V a n Soest, 1963). T h e results are e x p r e s s e d o n a d r y m a t t e r basis. All seed, grain a n d s t o c k f o o d s a m p l e s w e r e o b t a i n e d f r o m local c o m m e r c i a l suppliers. RESULTS AND DISCUSSION T h e l i m i t a t i o n s o f d e c o r t i c a t i n g seeds b y a c o m p r e s s i o n and shearing a c t i o n h a v e b e e n described. I t was f o u n d t h a t b y striking a n d p r o j e c t i n g t h e seeds b y a r o t o r a n d i m p a c t i n g t h e m at high v e l o c i t y against a rigid surface, t h e h u s k c o u l d be p a r t e d f r o m t h e kernel. In Fig. 1 t h e seed enters t h r o u g h t h e t o p o f t h e mill a n d is s t r u c k b y t h e r o t o r . I t was f o u n d t h a t t h e shearing a n d c o m pression forces o n the seeds c o u l d b e r e d u c e d b y o f f - s e t t i n g t h e r o t o r a w a y
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from the baffle plate and by employing rods t hat would strike the seed with a tangential surface rather than a sharp edge. In this manner the seed was propelled against the housing of the mill, partially parting the husk from the kernel. By positioning the baffle plate inside the housing (Fig. 1) the seeds were n o t directly swept o u t of the mill but were impeded. A variety of baffle designs was tried, the most effective being the ripple plate shown in Fig. 1. This tended to deflect the seeds back into the path of the r o t o r w h e r e b y t h e y were again struck and propelled against the housing, further parting the husk and kernel. Figure 2 shows the wear patterns on the side of the mill housing made by deflected sunflower seeds after a period of operation. T o obtain these patterns, the mill was fitted with an open sided four paddle r o t o r of similar dimensions to the squirrel cage. This confirmed t hat the intact seeds, and partially dehulled seeds, were deflected by the baffle plate into the path o f the rotor, rather than being swept straight out of the mill. Although the decorticator was primarily designed for sunflower seeds, it was f o u n d th at a large variety of seeds and grains could readily be dehulled by using this equipment. The minimum gap of 6 m m between the r o t o r and baffle was substantial in relation to the diameter of the seed types employed, hence the forces applied to the seed were largely proportional to the velocity of the r o t o r tips. Consequently, by varying the r o t o r speed the degree of
Fig. 2. The wear pattern on the lower outlet section of the side of the mill housing after milling 20 tonne of sunflower seed at 3400 r.p.m, with an open sided four-paddle rotor to illustrate the deflection patterns from the ripple plate. The direction of rotation of the rotor is indicated.
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dehulling could be altered as could the throughput. T he optimal dehulling speed varied with the t y p e and physical properties of various seeds, and optimal speeds and throughputs for thirteen seed types and grain are indicated in Table I. These values are subject to some variation because each seed t y p e will vary in moisture content, age, length of storage and size, etc., and in all cases e x c e p t for safflower t h e y are the minima for com pl et e dehulling. If partial dehulling is desired, lower r o t o r speeds can be applied; these also result in the p r o d u c t i o n of fewer fines, while at higher speeds the kernel and husk are b o th reduced to smaller particles. TABLE I Optimal rotor speeds and throughputs for dehulling various seeds Seed type
Rotor speed (r.p.m.)
Throughput (Kg/h)
Cotton Clover Field peas Linseed Lupin Milo Oats Rape Rice Safflower Sorghum Soya bean Sunflower
3800 3500 2800 4000 2500 3000 3800 4000 3800 4000 3500 2500 3400
400 450 420 450 430 420 550 450 450 300 450 400 450
The mo s t difficult seed to process was safflower which has a very hard husk and soft oily kernel. By milling it at 5000 r.p.m, with a lower t h r o u g h p u t the seed could be dehulled; however, a preferable procedure was to mill at a lower r o t o r speed (4000 r.p.m.), separate the husks and kernels, and recycle the uncracked seed which was a b o u t 20% of the input. The separation achieved with safflower seed with this two-pass system of milling and screening is shown in Fig. 3 A and B. All ot he r seed types tested were easily dehulled after only a single pass through the mill. Figure 3 C and D demonstrate the separation achieved with rape seed. With this mill (200 mm diameter X 110 m m long) the rate of dehulling sunflower seeds was 450 kg/h at 3400 r.p.m., and of soya beans was 400 kg/h at 2500 r.p.m. By coupling an electric m o t o r to the mill and measuring the current drawn, the pow e r c o n s u m p t i o n at these dehulling rates was f o u n d to be 720 watts for both seed types. This gave a pow er c o n s u m p t i o n of 1.8 Kw per t o n n e o f seed, which was low compared with conventional mills such as a
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Fig. 3. T h e h u s k a n d kernel f r a c t i o n s o b t a i n e d a f t e r milling a n d s c r e e n i n g s a f f l o w e r a n d rape seed at 4 0 0 0 r.p.m. ; 20% o f the safflower was largely i n t a c t a f t e r one pass, a n d t h e p h o t o g r a p h s h o w s t h e final p r o d u c t a f t e r this f r a c t i o n was recycled. A, s a f f l o w e r kernel; B, s a f f l o w e r h u s k ; C, rape k e r n e l ; D, rape husk.
commercial cotton gin (7.4 Kw per tonne), or a hammer mill adjusted to produce a coarse pea meal (7.4 Kw per tonne) (Perry, 1963). Consequently, the 4.8 Kw petrol engine connected to the mill was more powerful than necessary but its variable speed provided extremely good flexibility. Probably an electric m o t o r of suitable power fitted with a variable speed gearbox and revolution counter would be an ideal controlled-drive system. The capacity of the mill could be increased by extending the length of the rotor and increasing the size of the drive system. Separation of the husk and kernel fractions will not be discussed in detail; the unit constructed and coupled to our dehulling mill was up to 90% efficient with one pass of sunflower seeds and 95% with cottonseeds. Many commercial separation units are available, most of which could be adapted to this mill. In Table II the analysis of the seed and seed fractions obtained from this unit are shown for six types of oil seeds. As expected the nutritionally desirable components such as the oil and protein are concentrated in the kernel, while the husk fractions contain a large proportion of fibre. The removal of much of the fibre should improve the digestibility of the protein and oil as well as improving the efficiency for subsequent mechanical processing, such as oil extraction or particle size reduction (e.g. for lupin seed).
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TABLE II Composition of seed and seed fractions Seed
Percent composition* Protein
Ether extract
ADF**
Ash
Cotton Kernel Husk
24.9 32.6 3.4
22.2 30.5 0.6
26.7 7.3 65.3
4.4 5.4 2.4
Lupin Kernel Husk
30.3 43.5 3.4
5.5 6.2 0.9
23.5 6.9 67.9
2.9 3.0 2.3
Rape Kernel Husk
20.2 23.4 15.4
41.3 47.1 9.1
14.4 9.2 48.3
4.0 4.0 4.9
Safflower Kernel Husk
16.9 24.6 5.8
31.3 53.9 5.3
33.4 11.8 60.5
2.4 3.2 2.0
Soya bean Kernel Husk
44.7 44.2 11.9
16.3 16.3 1.4
7.3 5.6 51.1
5.3 5.1 4.6
Sunflower Kernel Husk
19.1 23.7 7.0
46.1 57.4 7.6
16.0 4.0 58.8
2.7 3.0 2.5
*D.M. basis. **Acid-detergent fibre.
A p p l i c a t i o n s o f this milling principle are n o t limited to seed and grain. It was f o u n d t h a t w h e n d r y lucerne h a y c o n t a i n i n g 16.8% c r u d e p r o t e i n in the d r y m a t t e r was milled at 5 0 0 0 r.p.m, and screened w i t h o u t aspiration it yielded t w o fractions of a p p r o x i m a t e l y equal mass (Fig. 4). T h e f r a c t i o n t h a t passed t h r o u g h the screen (Fig. 4A) was largely leaves; it c o n t a i n e d 20.5% c r u d e p r o t e i n and 22.2% fibre, a n d has p o t e n t i a l value as a leaf p r o t e i n concentrate. T h e r e m a i n i n g f r a c t i o n (Fig. 4B) was c o m p o s e d o f stem material and c o n s e q u e n t l y useful as a l o w e r p r o t e i n (11.8%) higher fibre (47.2%) r o u g h a g e f o r r u m i n a n t animals. T h e same b a t c h o f lucerne h a y was milled by a c o n v e n t i o n a l h a m m e r mill fitted with a 3 m m screen (Fig. 4D) and c u t b y a c h a f f - c u t t e r (Fig. 4C), n e i t h e r o f w h i c h yielded a significant proteinenriched f r a c t i o n w h e n screened. T h e principle o f i m p a c t i n g the hull or h u s k f r o m the kernel b y passing it b e t w e e n a r o t o r and ripple plate has several advantages. A mill o f this t y p e is very versatile and can h a n d l e a wide variety o f seeds and o t h e r materials o f
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Fig. 4. Dry lucerne hay milled at 5000 r.p.m, and screened to obtain a leaf protein concentrate. A, lucerne leaf concentrate; B, lucerne stem material; C, lucerne cut by a chaff-cutter; D, lucerne milled by a conventional h a m m e r mill fitted with 3 m m screen.
differing physical properties, with a low power consumption relative to the output. The mill is readily adjusted by altering the speed of rotation, hence the life of the components should be long with minimal maintenance requirements because gaps and edges do not have to be set to fine tolerances. ACKN O W L E D G E M E N T S
We gratefully acknowledge the discussions with Dr. G.S. Sidhu and Mr. S.C. Mills during the preparation of this manuscript.
REFERENCES Association of Official Analytical Chemists, 1970. Official Methods of Analysis. 11th edn. A.O.A.C., Washington D.C., pp. 122--138. Gopalan, C. and Balasubramanian, S.C., 1963. The nutritive value of Indian foods and the planning o f satisfactory diets. Spec. Rep. Ser. Indian Counc. Med. Res. No. 42, 6th rev. edn., p. 50. Heaton, K.W., 1973. F o o d fibre as an obstacle to energy intake. Lancet, 2: 1418--1421. Norris, F.A., 1964. E x t r a c t i o n o f fats and oils. In: D. Swern (Editor), Bailey's Industrial Oil and F a t Products. 3rd edn. Interscience, New York, pp. 641--646. Perry, J.H. (Editor), 1963. Chemical Engineers' Handbook. 4th edn. McGraw Hill, New York, pp. 8--45. Van Soest, P.J., 1963. Use of detergents in the analysis o f fibrous feeds. II. A rapid m e t h o d for the d e t e r m i n a t i o n of fiber and lignin. J. Assoc. Off. Agric. Chem., 46: 829--835.
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© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
A SIMPLE DEVICE F O R D E H U L L I N G SEEDS AND G R A I N
J.R. ASHES and N.J. PECK C.S.I.R.O., Division o f Animal Production, P.O.Box 239, Blacktown, N.S.W., 2148 (Australia)
(Received 14 September 1977)
ABSTRACT Ashes, J.R. and Peck, N.J., 1978. A simple device for dehulling seeds and grain. Anita. Feed Sci. Technol., 3: 109--116. The simple device described operates on the principle of bouncing the seed between a "squirrel cage" type rotor and a ripple plate, thus impacting the hull from the kernel in contrast to the conventional milling or rolling procedures generally employed. Thirteen seed and grain types were dehulled during a single pass through the mill and screening device. Safflower seed could be effectively dehulled but required two passes through the mill. The efficiency of dehulling after one pass varied, but was 90% with sunflower seed and 95% with cottonseed. The extent of the dehulling was proportional to the velocity of the rotor tips and could be readily varied. Consequently the mill was able to process a wide variety of seed and also other materials such as dry lucerne hay. Wear and maintenance were minimal as sharp knife edges or a precise gap between the rotor and the ripple plate were not required.
INTRODUCTION Seeds and grains possess an o u t e r covering c o m m o n l y referred to as a hull or husk w h i c h shields t h e inner kernel, the f r a c t i o n c o n t a i n i n g m o s t o f the lipid, p r o t e i n and readily digestible c a r b o h y d r a t e . T h e hull, w h i c h m a y a c c o u n t f o r u p to 40% o f the t o t a l seed weight, is usually largely c o m p o s e d o f fibre. Cons e q u e n t l y , r e m o v a l o f t h e hull f r a c t i o n m a y o f t e n be desirable in o r d e r t o imp r o v e e f f i c i e n c y in the s u b s e q u e n t m e c h a n i c a l or c h e m i c a l processing o f the seed, e.g. in oil e x t r a c t i o n (Norris, 1964). Similarly, hull r e m o v a l will o f t e n i m p r o v e the digestibility o f the r e m a i n i n g fractions b y p e r m i t t i n g access b y digestive e n z y m e s to n u t r i t i o n a l l y desirable c o m p o n e n t s . T h e r e m o v a l o f hulls f r o m l e g u m i n o u s seeds, such as pulses in h u m a n diets, greatly improves the a p p e a r a n c e o f the p r o d u c t while r e d u c i n g fibre ( G o p a l a n and Balasubramanian, 1 9 6 3 ) and i m p r o v i n g digestibility ( H e a t o n , 1973). In c o n v e n t i o n a l dehulling e q u i p m e n t the husks are usually p a r t e d f r o m t h e seed kernels b y a p p l y i n g to the seed compressive or shearing forces i n d u c e d b y the a c t i o n o f t w o or m o r e o p p o s e d surfaces o r edges. In practice this is achieved b y passing the w h o l e seeds t h r o u g h pairs o f rollers, or t h r o u g h a mill
110
in which the seeds are carried r ound by longitudinal projections on the surface o f a rotating dram, and forced through a gap of considerably smaller width than any dimension of the whole seed (Norris, 1964). The disadvantages of e q u i p m e n t employing this principle are fairly obvious; it is o f ten difficult to separate the seed c o m p o n e n t s milled in this m anner as the husk and kernels are broken d o w n to particles of similar sizes, or, as found with certain oil seeds, the compressive and sheer forces applied tend to release the oil from the spherosomes. This m ay result in a loss of part of the oil with the husk or the f or m a t i on of an oil/husk " c a k e " which can cause blockages in the machinery. Both rolling and milling procedures require a set gap of smaller dimensions than the seed. The seed generally must be subjected to cleaning and grading before dehulling, in order to prevent small seeds passing through the e q u i p m e n t uncracked or larger seeds being broken into small fragments; similarly, the seed must be free of foreign materials to minimise the risk of damage to the fine tolerances of the sharp edges usually employed. This paper describes a machine designed to avoid m a n y of the problems associated with milling or rolling; the principle involves bouncing the seed repeatedly between a r o t o r and a baffle plate until the impact cracks the hull and releases it from the kernel. MATERIALS AND METHODS A small scale dehuller consisting of a mill and screening device was constructed. Th e mill {Fig. 1) comprises three basic parts a. Housing b. R o t o r c. Baffle plate The r o t o r is in the form of a "squirrel cage" and consists of two 3 mm X 200 m m diameter discs welded to a 40 mm diameter shaft. Sixteen evenly spaced 18 m m diameter rods, 110 m m long, are riveted between the discs at 178 mm pitch circle diameter. T he r o t o r is supported by sealed ball bearings held in the housing and powered by a 4.8 Kw petrol driven engine, pulley driven by vee belts to rotate the r o t o r in a clockwise direction at speeds o f 1000--6000 r.p.m. A baffle plate is fixed to the lower half of the housing and co n s tr u cted in the form of a ripple plate. The ripple angle is 90 ° at the centre of the plate, b u t is gradually increased at either e n d t o 120 ° , to reduce the possibility of blockages at the inlet or outlet. The r o t o r is offset 5.5 mm from the centre o f the housing away from the baffle plate, while the minimum clearance between the baffle plate and r o t o r is 6 mm. A h o p p e r with an adjustable inlet is fitted t o the top of the mill so as to feed variable quantities of seed (not shown in Fig. 1); similarly the dehulled seeds and husks are discharged on to a vibrating aspirated 3 m m screen (also n o t shown in Fig. 1). The dimensions of the screen are 1200 m m X 500 mm. The aspiration rate is adjustable with a m a x i m u m flowrate of 6 m3/min at
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Fig. 1. A cross-sectional view of the mill consisting of: (1) the housing; (2) the rotor; (3) the baffle plate; (4) the impact rods; (5) rotor support shaft. The rotor is in the form of a "squirrel cage" and the baffle plate, a ripple plate. All dimensions are in mm. The direction of rotation of the rotor is indicated.
2 5 0 0 r.p.m. B o t h the a s p ~ a t o r and v i b r a t i n g screen are driven b y t h e s a m e p o w e r s o u r c e as t h e mill. T h e mill and v i b r a t i n g screen u n i t s are c o n s t r u c t e d o f m i l d steel, e x c e p t t h e b a f f l e p l a t e w h i c h is m a d e o f I m m stainless steel. Moisture, e t h e r e x t r a c t , p r o t e i n and ash c o n t e n t s w e r e d e t e r m i n e d b y m e t h o d s o u t l i n e d b y t h e A s s o c i a t i o n o f Official A n a l y t i c a l C h e m i s t s (1970). F i b r e c o n t e n t was e s t i m a t e d as a c i d - d e t e r g e n t fibre ( A D F ) ( V a n Soest, 1963). T h e results are e x p r e s s e d o n a d r y m a t t e r basis. All seed, grain a n d s t o c k f o o d s a m p l e s w e r e o b t a i n e d f r o m local c o m m e r c i a l suppliers. RESULTS AND DISCUSSION T h e l i m i t a t i o n s o f d e c o r t i c a t i n g seeds b y a c o m p r e s s i o n and shearing a c t i o n h a v e b e e n described. I t was f o u n d t h a t b y striking a n d p r o j e c t i n g t h e seeds b y a r o t o r a n d i m p a c t i n g t h e m at high v e l o c i t y against a rigid surface, t h e h u s k c o u l d be p a r t e d f r o m t h e kernel. In Fig. 1 t h e seed enters t h r o u g h t h e t o p o f t h e mill a n d is s t r u c k b y t h e r o t o r . I t was f o u n d t h a t t h e shearing a n d c o m pression forces o n the seeds c o u l d b e r e d u c e d b y o f f - s e t t i n g t h e r o t o r a w a y
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from the baffle plate and by employing rods t hat would strike the seed with a tangential surface rather than a sharp edge. In this manner the seed was propelled against the housing of the mill, partially parting the husk from the kernel. By positioning the baffle plate inside the housing (Fig. 1) the seeds were n o t directly swept o u t of the mill but were impeded. A variety of baffle designs was tried, the most effective being the ripple plate shown in Fig. 1. This tended to deflect the seeds back into the path of the r o t o r w h e r e b y t h e y were again struck and propelled against the housing, further parting the husk and kernel. Figure 2 shows the wear patterns on the side of the mill housing made by deflected sunflower seeds after a period of operation. T o obtain these patterns, the mill was fitted with an open sided four paddle r o t o r of similar dimensions to the squirrel cage. This confirmed t hat the intact seeds, and partially dehulled seeds, were deflected by the baffle plate into the path o f the rotor, rather than being swept straight out of the mill. Although the decorticator was primarily designed for sunflower seeds, it was f o u n d th at a large variety of seeds and grains could readily be dehulled by using this equipment. The minimum gap of 6 m m between the r o t o r and baffle was substantial in relation to the diameter of the seed types employed, hence the forces applied to the seed were largely proportional to the velocity of the r o t o r tips. Consequently, by varying the r o t o r speed the degree of
Fig. 2. The wear pattern on the lower outlet section of the side of the mill housing after milling 20 tonne of sunflower seed at 3400 r.p.m, with an open sided four-paddle rotor to illustrate the deflection patterns from the ripple plate. The direction of rotation of the rotor is indicated.
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dehulling could be altered as could the throughput. T he optimal dehulling speed varied with the t y p e and physical properties of various seeds, and optimal speeds and throughputs for thirteen seed types and grain are indicated in Table I. These values are subject to some variation because each seed t y p e will vary in moisture content, age, length of storage and size, etc., and in all cases e x c e p t for safflower t h e y are the minima for com pl et e dehulling. If partial dehulling is desired, lower r o t o r speeds can be applied; these also result in the p r o d u c t i o n of fewer fines, while at higher speeds the kernel and husk are b o th reduced to smaller particles. TABLE I Optimal rotor speeds and throughputs for dehulling various seeds Seed type
Rotor speed (r.p.m.)
Throughput (Kg/h)
Cotton Clover Field peas Linseed Lupin Milo Oats Rape Rice Safflower Sorghum Soya bean Sunflower
3800 3500 2800 4000 2500 3000 3800 4000 3800 4000 3500 2500 3400
400 450 420 450 430 420 550 450 450 300 450 400 450
The mo s t difficult seed to process was safflower which has a very hard husk and soft oily kernel. By milling it at 5000 r.p.m, with a lower t h r o u g h p u t the seed could be dehulled; however, a preferable procedure was to mill at a lower r o t o r speed (4000 r.p.m.), separate the husks and kernels, and recycle the uncracked seed which was a b o u t 20% of the input. The separation achieved with safflower seed with this two-pass system of milling and screening is shown in Fig. 3 A and B. All ot he r seed types tested were easily dehulled after only a single pass through the mill. Figure 3 C and D demonstrate the separation achieved with rape seed. With this mill (200 mm diameter X 110 m m long) the rate of dehulling sunflower seeds was 450 kg/h at 3400 r.p.m., and of soya beans was 400 kg/h at 2500 r.p.m. By coupling an electric m o t o r to the mill and measuring the current drawn, the pow e r c o n s u m p t i o n at these dehulling rates was f o u n d to be 720 watts for both seed types. This gave a pow er c o n s u m p t i o n of 1.8 Kw per t o n n e o f seed, which was low compared with conventional mills such as a
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|_
Fig. 3. T h e h u s k a n d kernel f r a c t i o n s o b t a i n e d a f t e r milling a n d s c r e e n i n g s a f f l o w e r a n d rape seed at 4 0 0 0 r.p.m. ; 20% o f the safflower was largely i n t a c t a f t e r one pass, a n d t h e p h o t o g r a p h s h o w s t h e final p r o d u c t a f t e r this f r a c t i o n was recycled. A, s a f f l o w e r kernel; B, s a f f l o w e r h u s k ; C, rape k e r n e l ; D, rape husk.
commercial cotton gin (7.4 Kw per tonne), or a hammer mill adjusted to produce a coarse pea meal (7.4 Kw per tonne) (Perry, 1963). Consequently, the 4.8 Kw petrol engine connected to the mill was more powerful than necessary but its variable speed provided extremely good flexibility. Probably an electric m o t o r of suitable power fitted with a variable speed gearbox and revolution counter would be an ideal controlled-drive system. The capacity of the mill could be increased by extending the length of the rotor and increasing the size of the drive system. Separation of the husk and kernel fractions will not be discussed in detail; the unit constructed and coupled to our dehulling mill was up to 90% efficient with one pass of sunflower seeds and 95% with cottonseeds. Many commercial separation units are available, most of which could be adapted to this mill. In Table II the analysis of the seed and seed fractions obtained from this unit are shown for six types of oil seeds. As expected the nutritionally desirable components such as the oil and protein are concentrated in the kernel, while the husk fractions contain a large proportion of fibre. The removal of much of the fibre should improve the digestibility of the protein and oil as well as improving the efficiency for subsequent mechanical processing, such as oil extraction or particle size reduction (e.g. for lupin seed).
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TABLE II Composition of seed and seed fractions Seed
Percent composition* Protein
Ether extract
ADF**
Ash
Cotton Kernel Husk
24.9 32.6 3.4
22.2 30.5 0.6
26.7 7.3 65.3
4.4 5.4 2.4
Lupin Kernel Husk
30.3 43.5 3.4
5.5 6.2 0.9
23.5 6.9 67.9
2.9 3.0 2.3
Rape Kernel Husk
20.2 23.4 15.4
41.3 47.1 9.1
14.4 9.2 48.3
4.0 4.0 4.9
Safflower Kernel Husk
16.9 24.6 5.8
31.3 53.9 5.3
33.4 11.8 60.5
2.4 3.2 2.0
Soya bean Kernel Husk
44.7 44.2 11.9
16.3 16.3 1.4
7.3 5.6 51.1
5.3 5.1 4.6
Sunflower Kernel Husk
19.1 23.7 7.0
46.1 57.4 7.6
16.0 4.0 58.8
2.7 3.0 2.5
*D.M. basis. **Acid-detergent fibre.
A p p l i c a t i o n s o f this milling principle are n o t limited to seed and grain. It was f o u n d t h a t w h e n d r y lucerne h a y c o n t a i n i n g 16.8% c r u d e p r o t e i n in the d r y m a t t e r was milled at 5 0 0 0 r.p.m, and screened w i t h o u t aspiration it yielded t w o fractions of a p p r o x i m a t e l y equal mass (Fig. 4). T h e f r a c t i o n t h a t passed t h r o u g h the screen (Fig. 4A) was largely leaves; it c o n t a i n e d 20.5% c r u d e p r o t e i n and 22.2% fibre, a n d has p o t e n t i a l value as a leaf p r o t e i n concentrate. T h e r e m a i n i n g f r a c t i o n (Fig. 4B) was c o m p o s e d o f stem material and c o n s e q u e n t l y useful as a l o w e r p r o t e i n (11.8%) higher fibre (47.2%) r o u g h a g e f o r r u m i n a n t animals. T h e same b a t c h o f lucerne h a y was milled by a c o n v e n t i o n a l h a m m e r mill fitted with a 3 m m screen (Fig. 4D) and c u t b y a c h a f f - c u t t e r (Fig. 4C), n e i t h e r o f w h i c h yielded a significant proteinenriched f r a c t i o n w h e n screened. T h e principle o f i m p a c t i n g the hull or h u s k f r o m the kernel b y passing it b e t w e e n a r o t o r and ripple plate has several advantages. A mill o f this t y p e is very versatile and can h a n d l e a wide variety o f seeds and o t h e r materials o f
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Fig. 4. Dry lucerne hay milled at 5000 r.p.m, and screened to obtain a leaf protein concentrate. A, lucerne leaf concentrate; B, lucerne stem material; C, lucerne cut by a chaff-cutter; D, lucerne milled by a conventional h a m m e r mill fitted with 3 m m screen.
differing physical properties, with a low power consumption relative to the output. The mill is readily adjusted by altering the speed of rotation, hence the life of the components should be long with minimal maintenance requirements because gaps and edges do not have to be set to fine tolerances. ACKN O W L E D G E M E N T S
We gratefully acknowledge the discussions with Dr. G.S. Sidhu and Mr. S.C. Mills during the preparation of this manuscript.
REFERENCES Association of Official Analytical Chemists, 1970. Official Methods of Analysis. 11th edn. A.O.A.C., Washington D.C., pp. 122--138. Gopalan, C. and Balasubramanian, S.C., 1963. The nutritive value of Indian foods and the planning o f satisfactory diets. Spec. Rep. Ser. Indian Counc. Med. Res. No. 42, 6th rev. edn., p. 50. Heaton, K.W., 1973. F o o d fibre as an obstacle to energy intake. Lancet, 2: 1418--1421. Norris, F.A., 1964. E x t r a c t i o n o f fats and oils. In: D. Swern (Editor), Bailey's Industrial Oil and F a t Products. 3rd edn. Interscience, New York, pp. 641--646. Perry, J.H. (Editor), 1963. Chemical Engineers' Handbook. 4th edn. McGraw Hill, New York, pp. 8--45. Van Soest, P.J., 1963. Use of detergents in the analysis o f fibrous feeds. II. A rapid m e t h o d for the d e t e r m i n a t i o n of fiber and lignin. J. Assoc. Off. Agric. Chem., 46: 829--835.