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Expression of oil from oilseeds—A review
J. agric. Engng Res. (1983) 28, 495-503
REVIEW PAPER
Expression
of Oil from Oilseeds-A
Review
L. M. KHAN*; M. A. HANNA*
Generally, the recommended pre-pressing operations for oil expression include grinding or flaking and then cooking pre-cleaned oilseeds. The literature indicates that pressure, temperature, pressing time and moisture content are the factors which affect oil yield during expression processing of oilseeds. Nearly all of the yield data reported correspond to hydraulic presses while the current technology, at least in the U.S.A., for expression processing is the screw press. Research is still process in the same way and to the needed to determine if these factors affect the screw-pressing same extent as they do in a static pressing operation.
1. Introduction Expression is the process of mechanically pressing liquid out of liquid-containing solids. Screw presses, roll presses and mills, collapsible-plate and frame-filter presses, disc mills, interlocking-finger juice extractors, juice reamers, rack and cloth presses and hydraulic presses are examples of the wide variety of equipment available for expression processing. The efficiency of a mechanical-expression process cannot be equal to unity and, in actual operations, it seldom exceeds 90%. The advantages of expression over extraction processing is that it gives endproducts free of dissolved chemicals and is inherently a safer process. Extraction is the process of separating a liquid from a liquid-solid system with the use of a solvent. This process gives a higher recovery of oil and a drier cake than expression. Cake is the de-oiled material and foots are the solid particles expelled with the oil. Extraction processing, or solvent extraction, is capable of removing nearly all of the available oil from oilseed meal or flakes. In addition to the higher yield obtained by a more complete removal of the oil, better preservation qualities and a higher protein meal is produced. Data on the range of oil contents occurring naturally in a variety of the more common oilseeds and the approximate oil yield per hectare (generally international averages) are given in Table 1, assuming all of the oil is expressed or extracted. ’ If processing yields are less than lOOO/” then the yields in Table 1 must be reduced accordingly. TABLE
I
Available oil in selected common seeds’
-
Oilseed
Oil content, y0 -
Castor bean Coconut Cottonseed Flax Maize (corn) Mustard Peanut Rapeseed Sesame Soybean Sunflower
--
Yield, kg/ha
35-55
_-
Oil yield, kg/ha
950 1412 560 650 5720
40-45 24-26 5-40 6-10 25-60 43-45 33-43 50-57 13-25 4@-42
485 600 140 230 286 468 447 456 220 319 519
1100 1016 1200 330 1790 1266
*Department of Agricultural Engineering. University of Nebraska, Received IS October 1982; accepted in revised form 4 June 1983
Lincoln,
Nebraska,
U.S.A.
495 0021~8634/83/060495
+09 $03.00/O
0 1983 The British Society for Research in Agricultural
Engineering
496
EXPRESSION
2.
OF
OIL
FROM
OILSEEDS
Expression techniques
An ancient Chinese wedge press as shown in Fig. 1 was reported by Boatwright. Bags of ground oilseeds were placed on a wooden plank which was firmly held by two angle-braced logs as end supports. A trough was placed beneath the plank to permit the oil to flow the entire length and to drain into a container. Wooden wedges were then pressed, one after another, between the two end bags and the end supports to increase the pressure uniformly for the expulsion of oil. Wedges
I ,
I
Trough
CUP
Beam
, ,____;
I ,I , I II II III L____.d
Fig. I. Wedge press
Hurst3 described three kinds of press in use at the beginning of the 20th Century. The first was a “stamper and wedge” press which was an updated version of the Chinese wedge press. It had an estimated capacity of 550 kg/d. The second, a screw press, found little use because of design limitations. It consisted of a cylinder into which seed was placed and a piston which was driven up and down by a screw turned by a lever. The third, a hydraulic press, virtually superseded all other forms of oil presses. The hydraulic press, as presented by Hurst3 and illustrated in Fig. 2, consisted of a series of horizontal corrugated iron plates which separated four to 14 premoulded cakes of oilseed. Pressing was accomplished in two stages. Initially, the samples were pressed at approximately 5 MPa for 15-20 min. Then a pressure of 28 MPa was applied for 5-10 min to complete the expression process. The output of this press varied depending on the sizes and the seed being pressed. The maximum capacity of the presses described was 175-200 kg/h of seed cake.
Hydraulic
pumps
L-_.ll Fig. 2. Hydraulic
oil press
L. M. KHAN;
497
M. A. HANNA
replaced by “screw” presses. This is not to be More recently, hydraulic presses have be confused with the screw press used in the 19th ‘Qe, tury. An oilseed screw press, as illustrated in Fig. 3, has a horizontal main shaft carrying the screw assembly which is formed integrally with the shaft. The screw rotates within a cage or barrel which is lined with case-hardened, tool-steel bars. Spacers are used between the lining bars to permit drainage of the oil as the pressure on the At the discharge end, a movable cone or choke controls the operating feed material is increased. pressure by changing the width of the annular space through which the press cake must pass. The choke is typically adjusted by a hand-wheel on the opposite end of the screw. The heat generated as a result of friction can be dissipated by cooling the cage and shaft with water. Feed
oil
hopper
receiver
Press
Cake
bow
cake
outlet
Choke
-
Fig. 3. Screw press
The screw is designed so that the volume displacement at the feed end of the press is considerably greater than at the discharge end. As a result, when the material is conveyed from the feed end to the discharge end, the pressure increases and oil is expelled through the slots between the cage lining bars. The compression ratio of a press is the volume displaced per revolution at the feed end of the screw divided by the volume per revolution at the discharge end. A typical compression curve of a screw press is shown in Fig. 4. The compression curve is typically split into feed, ram and plug sections. The radial pressure along a screw-press barrel is shown in Fig. 5. The maximum radial pressure is generated at the feed end of the ram section, The axial pressure follows the radial pressure up to the beginning of the plug section and then the fall-off in axial pressure toward the discharge end is less marked. A pressure gradient exists toward both ends of the press. Ward4 indicated that the feed end of the press handling a high oil content seed must be designed to dissipate the back pressure, move a sufficient volume of meal forward with minimum rotation and provide drainage of the expelled oil and air. Tests using radioactive isotopes have confirmed that the greatest part of the slip and rotation occurs at the feed end of the shaft. Purely axial flow of material along the screw is highly recommended under ideal circumstances. To achieve suitable operation of a press, it is essential to have a screw assembly and cage-lining-bar material with a low-friction coefficient. A low-friction shaft should be used in conjunction with higher friction bars for softer seeds. Tindale and Hill-Haa? reviewed the screw presses currently available to processors, This review included a discussion of the technical features of each machine. The use of expellers (screw presses) and research associated with the expression process diminished with the development of the solvent extraction process. Now, as in the past, a two-step, expression and then
498
EXPRESSION
<
Feed section
Ram section
OF
OIL
FROM
OILSEEDS
>
Plug section
I\ “5
V
i
I
pomt
Distance
along barrel
-A
Discharge point
FiK. 4. Compression curve (compression ratio = vl/v2)
Rodiol
Feed point
pressure
Distance
along
barrel
-+
Discharge pofnt
Fig. 5. Pressures in screw press barrel
extraction process is being implemented and to reduce solvent requirements. 3.
for high-oil-content
materials
to improve
overall yields
Pre-pressing operations
BredesorP enumerated three steps for full expression of vegetable oils. The first was to roll decorticated oilseed thoroughly and completely to rupture the greatest number of oil cells and to provide a homogeneous flake. The second step was a leisurely complete cooking to ensure no scorching or burning and to provide a minimum amount of agitation to rupture the remaining unruptured oil cells and to coagulate the protein in the meal. The third step was to have an efficient screw-press operation. Ward4 indicated that it is important to understand that a screw-pressing operation is selfdefeating. The oil in the seed is contained in sacs or in fibrous capillaries. When pressure is applied, the volume of the capillaries is reduced to expel the oil. However, at the same time, the capillaries are narrowed, sheared, and eventually sealed by the increasing pressure. Ward4 further reported that preparing and reducing the seed is to break down or weaken the oil-cell walls so that the oil is available to be expelled. Cooking is essential because it completes the breaking down of oil cells, lowers the viscosity of oil to be expelled, coagulates the protein in the meal, and adjusts the moisture content of the meal to the optimum level for pressing. In addition, cooking
L. M. KHAN;
499
M. A. HANNA
sterilizes the high-oil-content seeds and tires certain phosphatides in the cake to lower the subsequent refining loss. Steinbock’ reported that a combination of mechanical pressing and solvent extraction occasionally gives better results than either process used separately. The seed is first pressed to reduce the oil content to about 20% and the remaining cake is then solvent-extracted. Hexane is an excellent solvent because of its narrow evaporation range, which is important in distilling the solvent from the oil. He further explained that both oil yield and process costs depend on seed preparation prior to pressing. With the exception ot very small-sized seeds like sesame, reduction of the seed to flakes by rolling is essential. The size and hardness of the seed determines the number of stages for the flaking operations, without which the oil yield is reduced. The size of the flakes, the temperature of the seed prior to pressing and the moisture content must be carefully controlled to achieve the best results. Seed cleaning is important for the expression of vegetable oils, as the efficient removal of foreign material will increase the pressing efficiency. Steinbock’ further indicated that, in the case of pressing, care must be taken to establish correct conditions for any type of seed. Galloway’ described techniques and equipment for cleaning, dehulling, decorticating and flaking of oilbearing materials. He emphasized, as did Steinbock,’ the fact that the best preparation is somewhat different for each oil-bearing seed material. The suggested processing operations for several oilseeds are shown in Table 2. TABLE 2
Process sequence for different kinds of seeds’ Clean
Delint
Dehull
Crack
Flake
Cook
___Cottonseed Palm kernels Peanuts Flax seed Sesame soybeans
‘R
stands
R” R R R R R
R
R R
Press -__-
R R R
R
R R R R
R R R R R
R R R R R R
for recommended process sequence
Woolrich and Carpenter9 indicated that the preconditioning of cottonseeds is essential to get the maximum oil yield during expression. The most important operation in the preparation for cooking was rolling. Rolling gave an oil yield equivalent to grinding the oilseeds to pass through a 50-mesh sieve. The real function of rolling appeared to be that it exposed a greater area of oilbearing cells to the moisture and heat during cooking. If the seeds are properly processed, the cell walls become more porous and give a better outlet for the oil. Othmer and Agarwal’O reported that the inability to get oil from whole and half soybeans clearly indicated that cell walls must be broken by a flaking operation to allow the oil to be removed from otherwise impervious cells. Woolrich and Carpenter9 also indicated that if moisture was added just before cooking and the seed temperature was raised to 99°C with steam, the time required in the cooker could be reduced to a fraction of the time required when the moisture was added while the cottonseeds were still in storage. Taylor” found that cooking cottonseed at pressures above atmospheric, with temperatures over 130°C and with meal moisture contents between 7% and 8%, reduced the cooking time, increased the oil yield and improved the quality and uniformity of the finished products. Williams and Rathod’* reported that a moisture content of 7-8% gave the best oil yields from soybeans. Their work was done on a modified screw press developed for the production of soy-flour in India. In a triple-pass expelling process, they were able to remove over 80% of the oil from soybeans.
500
EXPRESSION
OF
OIL
FROM
OILSEEDS
Gumham and Masson13 studied the effect of sample size on the pressure applied on fibrous materials. They concluded that the effect of sample size was negligible at pressures greater than 4-7 MPa in a 28.6 mm dia. cylinder. At lower pressures, small samples were denser than large samples. The effect of liquid on the pressure-volume relations depended on whether the fibre was affected chemically or physically by the liquid. Their work, however, only involved the measurement of equilibrium pressures and volume changes and did not take into account the dynamic stresses which are considered to be important in expression procedures. Kormendy14 reported that for expression of apple juice the pressing time required for the percent yield of fluid was proportional to the square of the initial thickness of the material. 4. Pressing operations Oil quality and yield, as affected by the conditions of expression, were studied by Smith and Kraybill.15 Fine-ground soybean meal which passed through a 2 mm sieve was used. The samples were dried in a vacuum oven at 48-50°C. A desiccator produced 0,4, 6 and Sy:, moisture content on a wet basis. The samples were then pressed in the laboratory hydraulic press with hot plates above and below the cylindrical press to control the temperature. Test temperatures ranged from approximately 25°C to 100°C. The time required for pressing was 2 h. The pressure was increased by approximately 35 MPa every 30 min until 138 MPa was reached. The yield was found indirectly by determining the oil content of the cake after pressing according to the method of the American Oil Chemists Society. l6 According to their results, better yields were obtained at higher temperatures and lower moisture contents. Beisler” found that in the cold-press processing of tung nuts, the oil yield was reduced when the moisture content exceeded 6%. Jamieson la stated that S-9o/o moisture on a wet basis in the meal gave good results. Koo19 reported that pressing temperature, pressure, time and moisture content affected the yield of vegetable oils. To study the effect of these factors on oil yield, he used a laboratory hydraulic press similar to the Carver model (Fisher Scientific Co., Pittsburg, Pennsylvania, U.S.A.) for the expression of cottonseed oil. The press had a pressing face of 3870 mm2 between which a maximum pressure of 34.5 MPa could be generated by an oil pump which was fixed at the bottom of the hydraulic press. On the bottom, the pressing face was attached to a hollow cylinder, 88.9 mm i.d. and 190.5 mm high, with fine holes to let the oil flow out during the pressing. Fitted inside the cylinder was a plunger. Electric hot plates were used to provide heat. In all of his studies on the expression of vegetable oils, Koo’~-** varied one of the four factors at a time, holding the other three factors constant. Tables 3,4, 5 and 6 are included to show the format and the results of his experiments on cottonseed oil. It was found, as shown in Table 3, that the oil yield was directly proportional to the square root of the pressure. Table 4 shows that the oil yield was inversely proportional to the square root of the kinematic viscosity, which was a function of temperature only. The effect of pressing time on oil yield appeared to be of little importance, since the square of the oil yield was proportional to the cube root of pressing time (Table 5). TABLE 3
Effect of pressure oncottonseedoil yield (temperature = WC, time = 4 h)” Pressure (P), MPa 13.8 17.2 20.7 241 27%
Oil yield ( W), wt %
Wp-‘I2
10.0 11.3 12.2 13.1 14.0
2.69 2.72 2.68 2.67 2.66
--
L.
M.
KHAN:
M.
A.
501
HANNA
TABLE4 Effect of temperature on cottonseed oil yield (pressure = 24.1 MPa, time = 3 h)lp
Temp., “C
Oil yield(W),
Kinematic viscosity (v), 10e5 m”js
wt %
13.0 13.6 16.1 21.3 23.8 24.2
18 25 50 75 loo 125
w
VW
0.12 0.11 0.08 0.08 0.06 0.05
8.40 5.98 2.55 1.28 0.70 0.50
TABLE 5
Effect of time of pressing on cottonseed oil yield (pressure = 24.1 MPa, temperature = 18”C)‘9
I
I Passing
time (t), h
Oil yield(W),
0.5 1.0 1.5 2.0 3.0 4.0
Jp
wt %
t-w 106 117 103 110 116 108
9.14 10.8 10.8 11.8 13.0 13.1
TABLE6 Effect of moisture content on cottonseed oil yield (pressure = 24.1 MPa, time = 3 h)” At 18°C
___-
Moisture, % (w.b.)
2.94 3.62 5.67 6.97 8.20 11.2
At 103°C
I
Oil yield, wt %
Moisture, % (w.b.)
0 0 12.8 12.7
3.10 5.35 8.20
13.0 12.7
11.0 13.2 14.20 20.6 22.0
Oil yield, wt %
14.9 23.8 21.6 21.0 32.1* 19.5* 20.7* 19.4*
‘The oil expressed was cloudy due to water contamination
The results of the effect of moisture content (Table 6) on oil yield showed that at a temperature of 18°C and below 4% moisture in the cottonseed, oil cannot be pressed out; while from 5.67 o/0to 11.2% moisture content, about 12.5% of oil yield was expressed. When the temperature was raised to lOO”C, oil was pressed out even at a moisture content of 3.1 O/$. Again, in the moisture range of 5.35-l l*O”/& oil yields were approximately the same. At higher moisture contents, the oil was contaminated with meal containing water such that the output appeared cloudy and
502
EXPRESSION
OF
OIL
FROM
OILSEEDS
became difficult to separate. Koo concluded that with cottonseed, the optimum range of moisture content was from 5% to 11y0 for the temperature range of 18-100°C. To estimate the expression of oil from cottonseed, soybean, rapeseed, peanut, sesame, tung nut and caster bean, KOOKY developed the following general equation : w z cw,
pV t1/6v- z/2
where W is the oil yield (in wt %), C is a constant for the kind of oil seed (units consistent with unit analysis), W, is the oil content of the seed (in wt %), P the pressure (in MPa), t the pressing time (in h), v the kinematic viscosity of the oil at press temperature (in m2/s) and z is an exponent of kinematic viscosity varying from l/6 to l/2. A summary of the C, W,, and z values is given in Table 7. The experimental data revealed that for any one oilseed, there was an optimum range of moisture content for maximum oil yield. Of the various seeds studied, this range was from 5% to 13% (dry basis). TABLE 7
Constants and exponents for general oilseed expression equation= Oilseed
Soybean Cottonseed Rapeseed Peanut Tung nut Sesame seed Castor bean
z
cx 103
W0
l/2 l/2 l/3 l/3 l/3 l/6 l/6
5.40 6.42 15.0 19.4 23.4 46.5 51.3
19.5 34.7 42.2 51.9 64.5 53.0 64.2
-
The relative efficiency of a pressing operation was dependent on the kind of oilseed being pressed. Koo2’ reported press efficiencies, for a hydraulic press using the same operating conditions, from a minimum of 62.1 y0 for soybeans to a maximum of 91.2% for tung nut. REFERENCES
Duke, J. A.; Bagby, M. 0.
Comparison of oilseed yields: A preliminary review. Proc. Int. Conf. on Plant and Vegetable Oils as Fuels. Am. Sot. agric. Engrs, 1982 1 Boatwright, J. H. A wedge press for oil extraction. Approp. Technol. 1979 6 (2) 24-25 ’ Hurst, G. H. Lubricating oils, fats and greases. (Their origin, preparation, properties, uses and analysis.) A publication by Scott, Greenwood and Son, London, 1911 ’ Ward, J. A. High oil content seeds in continuous screw presses. J. Am. Oil Chem. Sot., 1976 53 261-264 5 Tindale, L. H.; Hill-Haas, S. R. Current equipment for mechanical oil extraction. J. Am. Oil Chem. Sot., 1976 53 265-270 J. Am. Oil Chem. Sot., 1977 54 489-490 L Bredeson, D. K. Mechanicalpressing. ’ Steinbock, S. R. Vegetable oilprocessing. Can. Chem. Process Ind., 1948 32 910-915 * Galloway, G. P. Cleaning, cracking, dehulling, decorticating and flaking of oil-bearing materials. J. Am. Oil Chem. Sot., 1976 53 271-274 ?? Woolrich, W. R.; Carpenter, E. L. Many problems beset vegetable oilproducers. Fd Inds, 1933 6 260 lo Othmer, D. F.; Agarwal, J. C. Extraction of soybean, theory and mechanism. Chem. Engng Prog., 1955 51 372-378 ” Taylor, R. B. Pressure cooking contributes increased cottonseedprocessingprojits. Chem. metall. Engng, 1937 44 978-981 la Williams, S. W.; Rathod, K. L. A case study of expeller production of soybean flour in India. College of Agriculture, University of Illinois at Urbana-Champaign. Int. Agric. Publ. INTSOY SERIES No. 3 ‘I Gurnham, F. C.; Masson, H. J. Expression of liquids from fibrous materials. Ind. Engng Chem., 1946 381309-1315 ’
L.
M.
KHAN:
M.
A.
HANNA
503
” Kormendy, I. A pressing theory with validating experiments on apples. J. Fd Sci., 1964 29 631-636 ‘5 Smith, R. L.; Kraybill, H. R. Soybean oil. Quality and yield as affected by conditions of expression. Ind. Engng Chem., 1933 25 334-336 ‘& American Oil Chemists Society. Official methods of chemical analysis, 1929 ” Beisler, W. H. Recovering tung oil from nuts grown in Florida. Chem. metall. Engng, 1930 37 614 ‘(I Jamieson, G. S. Vegetablefats andoils. A.C.S. Monogr. New York: Chemical Catalog Co., 1932 58 20 ‘* Koo, E. C. Studies on expression of vegetable oils I. Expression of cottonseed oil. J. Chem. Engng China, 1937 4 15-20 ‘O Koo, E. C. Studies on expression of vegetable oils II. Expression of soya bean oil. J. Chem. Engng China, 1937 4 207-211 ” Koo, E. C. Studies on expression of vegetable oils 111. Expression of tung oil. J. Chem. Engng China, 1938 5 47-52 ” Koo, E. C. Studies on expression of vegetable oils IV. Expression of rapeseed oil. J. Chem. Engng China, 1938 5 69-73 a’ KOO, E. C. Expression of vegetable oil. A general equation of oil expression. J. Chem. Engng China, 1942 34 342-345
REVIEW PAPER
Expression
of Oil from Oilseeds-A
Review
L. M. KHAN*; M. A. HANNA*
Generally, the recommended pre-pressing operations for oil expression include grinding or flaking and then cooking pre-cleaned oilseeds. The literature indicates that pressure, temperature, pressing time and moisture content are the factors which affect oil yield during expression processing of oilseeds. Nearly all of the yield data reported correspond to hydraulic presses while the current technology, at least in the U.S.A., for expression processing is the screw press. Research is still process in the same way and to the needed to determine if these factors affect the screw-pressing same extent as they do in a static pressing operation.
1. Introduction Expression is the process of mechanically pressing liquid out of liquid-containing solids. Screw presses, roll presses and mills, collapsible-plate and frame-filter presses, disc mills, interlocking-finger juice extractors, juice reamers, rack and cloth presses and hydraulic presses are examples of the wide variety of equipment available for expression processing. The efficiency of a mechanical-expression process cannot be equal to unity and, in actual operations, it seldom exceeds 90%. The advantages of expression over extraction processing is that it gives endproducts free of dissolved chemicals and is inherently a safer process. Extraction is the process of separating a liquid from a liquid-solid system with the use of a solvent. This process gives a higher recovery of oil and a drier cake than expression. Cake is the de-oiled material and foots are the solid particles expelled with the oil. Extraction processing, or solvent extraction, is capable of removing nearly all of the available oil from oilseed meal or flakes. In addition to the higher yield obtained by a more complete removal of the oil, better preservation qualities and a higher protein meal is produced. Data on the range of oil contents occurring naturally in a variety of the more common oilseeds and the approximate oil yield per hectare (generally international averages) are given in Table 1, assuming all of the oil is expressed or extracted. ’ If processing yields are less than lOOO/” then the yields in Table 1 must be reduced accordingly. TABLE
I
Available oil in selected common seeds’
-
Oilseed
Oil content, y0 -
Castor bean Coconut Cottonseed Flax Maize (corn) Mustard Peanut Rapeseed Sesame Soybean Sunflower
--
Yield, kg/ha
35-55
_-
Oil yield, kg/ha
950 1412 560 650 5720
40-45 24-26 5-40 6-10 25-60 43-45 33-43 50-57 13-25 4@-42
485 600 140 230 286 468 447 456 220 319 519
1100 1016 1200 330 1790 1266
*Department of Agricultural Engineering. University of Nebraska, Received IS October 1982; accepted in revised form 4 June 1983
Lincoln,
Nebraska,
U.S.A.
495 0021~8634/83/060495
+09 $03.00/O
0 1983 The British Society for Research in Agricultural
Engineering
496
EXPRESSION
2.
OF
OIL
FROM
OILSEEDS
Expression techniques
An ancient Chinese wedge press as shown in Fig. 1 was reported by Boatwright. Bags of ground oilseeds were placed on a wooden plank which was firmly held by two angle-braced logs as end supports. A trough was placed beneath the plank to permit the oil to flow the entire length and to drain into a container. Wooden wedges were then pressed, one after another, between the two end bags and the end supports to increase the pressure uniformly for the expulsion of oil. Wedges
I ,
I
Trough
CUP
Beam
, ,____;
I ,I , I II II III L____.d
Fig. I. Wedge press
Hurst3 described three kinds of press in use at the beginning of the 20th Century. The first was a “stamper and wedge” press which was an updated version of the Chinese wedge press. It had an estimated capacity of 550 kg/d. The second, a screw press, found little use because of design limitations. It consisted of a cylinder into which seed was placed and a piston which was driven up and down by a screw turned by a lever. The third, a hydraulic press, virtually superseded all other forms of oil presses. The hydraulic press, as presented by Hurst3 and illustrated in Fig. 2, consisted of a series of horizontal corrugated iron plates which separated four to 14 premoulded cakes of oilseed. Pressing was accomplished in two stages. Initially, the samples were pressed at approximately 5 MPa for 15-20 min. Then a pressure of 28 MPa was applied for 5-10 min to complete the expression process. The output of this press varied depending on the sizes and the seed being pressed. The maximum capacity of the presses described was 175-200 kg/h of seed cake.
Hydraulic
pumps
L-_.ll Fig. 2. Hydraulic
oil press
L. M. KHAN;
497
M. A. HANNA
replaced by “screw” presses. This is not to be More recently, hydraulic presses have be confused with the screw press used in the 19th ‘Qe, tury. An oilseed screw press, as illustrated in Fig. 3, has a horizontal main shaft carrying the screw assembly which is formed integrally with the shaft. The screw rotates within a cage or barrel which is lined with case-hardened, tool-steel bars. Spacers are used between the lining bars to permit drainage of the oil as the pressure on the At the discharge end, a movable cone or choke controls the operating feed material is increased. pressure by changing the width of the annular space through which the press cake must pass. The choke is typically adjusted by a hand-wheel on the opposite end of the screw. The heat generated as a result of friction can be dissipated by cooling the cage and shaft with water. Feed
oil
hopper
receiver
Press
Cake
bow
cake
outlet
Choke
-
Fig. 3. Screw press
The screw is designed so that the volume displacement at the feed end of the press is considerably greater than at the discharge end. As a result, when the material is conveyed from the feed end to the discharge end, the pressure increases and oil is expelled through the slots between the cage lining bars. The compression ratio of a press is the volume displaced per revolution at the feed end of the screw divided by the volume per revolution at the discharge end. A typical compression curve of a screw press is shown in Fig. 4. The compression curve is typically split into feed, ram and plug sections. The radial pressure along a screw-press barrel is shown in Fig. 5. The maximum radial pressure is generated at the feed end of the ram section, The axial pressure follows the radial pressure up to the beginning of the plug section and then the fall-off in axial pressure toward the discharge end is less marked. A pressure gradient exists toward both ends of the press. Ward4 indicated that the feed end of the press handling a high oil content seed must be designed to dissipate the back pressure, move a sufficient volume of meal forward with minimum rotation and provide drainage of the expelled oil and air. Tests using radioactive isotopes have confirmed that the greatest part of the slip and rotation occurs at the feed end of the shaft. Purely axial flow of material along the screw is highly recommended under ideal circumstances. To achieve suitable operation of a press, it is essential to have a screw assembly and cage-lining-bar material with a low-friction coefficient. A low-friction shaft should be used in conjunction with higher friction bars for softer seeds. Tindale and Hill-Haa? reviewed the screw presses currently available to processors, This review included a discussion of the technical features of each machine. The use of expellers (screw presses) and research associated with the expression process diminished with the development of the solvent extraction process. Now, as in the past, a two-step, expression and then
498
EXPRESSION
<
Feed section
Ram section
OF
OIL
FROM
OILSEEDS
>
Plug section
I\ “5
V
i
I
pomt
Distance
along barrel
-A
Discharge point
FiK. 4. Compression curve (compression ratio = vl/v2)
Rodiol
Feed point
pressure
Distance
along
barrel
-+
Discharge pofnt
Fig. 5. Pressures in screw press barrel
extraction process is being implemented and to reduce solvent requirements. 3.
for high-oil-content
materials
to improve
overall yields
Pre-pressing operations
BredesorP enumerated three steps for full expression of vegetable oils. The first was to roll decorticated oilseed thoroughly and completely to rupture the greatest number of oil cells and to provide a homogeneous flake. The second step was a leisurely complete cooking to ensure no scorching or burning and to provide a minimum amount of agitation to rupture the remaining unruptured oil cells and to coagulate the protein in the meal. The third step was to have an efficient screw-press operation. Ward4 indicated that it is important to understand that a screw-pressing operation is selfdefeating. The oil in the seed is contained in sacs or in fibrous capillaries. When pressure is applied, the volume of the capillaries is reduced to expel the oil. However, at the same time, the capillaries are narrowed, sheared, and eventually sealed by the increasing pressure. Ward4 further reported that preparing and reducing the seed is to break down or weaken the oil-cell walls so that the oil is available to be expelled. Cooking is essential because it completes the breaking down of oil cells, lowers the viscosity of oil to be expelled, coagulates the protein in the meal, and adjusts the moisture content of the meal to the optimum level for pressing. In addition, cooking
L. M. KHAN;
499
M. A. HANNA
sterilizes the high-oil-content seeds and tires certain phosphatides in the cake to lower the subsequent refining loss. Steinbock’ reported that a combination of mechanical pressing and solvent extraction occasionally gives better results than either process used separately. The seed is first pressed to reduce the oil content to about 20% and the remaining cake is then solvent-extracted. Hexane is an excellent solvent because of its narrow evaporation range, which is important in distilling the solvent from the oil. He further explained that both oil yield and process costs depend on seed preparation prior to pressing. With the exception ot very small-sized seeds like sesame, reduction of the seed to flakes by rolling is essential. The size and hardness of the seed determines the number of stages for the flaking operations, without which the oil yield is reduced. The size of the flakes, the temperature of the seed prior to pressing and the moisture content must be carefully controlled to achieve the best results. Seed cleaning is important for the expression of vegetable oils, as the efficient removal of foreign material will increase the pressing efficiency. Steinbock’ further indicated that, in the case of pressing, care must be taken to establish correct conditions for any type of seed. Galloway’ described techniques and equipment for cleaning, dehulling, decorticating and flaking of oilbearing materials. He emphasized, as did Steinbock,’ the fact that the best preparation is somewhat different for each oil-bearing seed material. The suggested processing operations for several oilseeds are shown in Table 2. TABLE 2
Process sequence for different kinds of seeds’ Clean
Delint
Dehull
Crack
Flake
Cook
___Cottonseed Palm kernels Peanuts Flax seed Sesame soybeans
‘R
stands
R” R R R R R
R
R R
Press -__-
R R R
R
R R R R
R R R R R
R R R R R R
for recommended process sequence
Woolrich and Carpenter9 indicated that the preconditioning of cottonseeds is essential to get the maximum oil yield during expression. The most important operation in the preparation for cooking was rolling. Rolling gave an oil yield equivalent to grinding the oilseeds to pass through a 50-mesh sieve. The real function of rolling appeared to be that it exposed a greater area of oilbearing cells to the moisture and heat during cooking. If the seeds are properly processed, the cell walls become more porous and give a better outlet for the oil. Othmer and Agarwal’O reported that the inability to get oil from whole and half soybeans clearly indicated that cell walls must be broken by a flaking operation to allow the oil to be removed from otherwise impervious cells. Woolrich and Carpenter9 also indicated that if moisture was added just before cooking and the seed temperature was raised to 99°C with steam, the time required in the cooker could be reduced to a fraction of the time required when the moisture was added while the cottonseeds were still in storage. Taylor” found that cooking cottonseed at pressures above atmospheric, with temperatures over 130°C and with meal moisture contents between 7% and 8%, reduced the cooking time, increased the oil yield and improved the quality and uniformity of the finished products. Williams and Rathod’* reported that a moisture content of 7-8% gave the best oil yields from soybeans. Their work was done on a modified screw press developed for the production of soy-flour in India. In a triple-pass expelling process, they were able to remove over 80% of the oil from soybeans.
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Gumham and Masson13 studied the effect of sample size on the pressure applied on fibrous materials. They concluded that the effect of sample size was negligible at pressures greater than 4-7 MPa in a 28.6 mm dia. cylinder. At lower pressures, small samples were denser than large samples. The effect of liquid on the pressure-volume relations depended on whether the fibre was affected chemically or physically by the liquid. Their work, however, only involved the measurement of equilibrium pressures and volume changes and did not take into account the dynamic stresses which are considered to be important in expression procedures. Kormendy14 reported that for expression of apple juice the pressing time required for the percent yield of fluid was proportional to the square of the initial thickness of the material. 4. Pressing operations Oil quality and yield, as affected by the conditions of expression, were studied by Smith and Kraybill.15 Fine-ground soybean meal which passed through a 2 mm sieve was used. The samples were dried in a vacuum oven at 48-50°C. A desiccator produced 0,4, 6 and Sy:, moisture content on a wet basis. The samples were then pressed in the laboratory hydraulic press with hot plates above and below the cylindrical press to control the temperature. Test temperatures ranged from approximately 25°C to 100°C. The time required for pressing was 2 h. The pressure was increased by approximately 35 MPa every 30 min until 138 MPa was reached. The yield was found indirectly by determining the oil content of the cake after pressing according to the method of the American Oil Chemists Society. l6 According to their results, better yields were obtained at higher temperatures and lower moisture contents. Beisler” found that in the cold-press processing of tung nuts, the oil yield was reduced when the moisture content exceeded 6%. Jamieson la stated that S-9o/o moisture on a wet basis in the meal gave good results. Koo19 reported that pressing temperature, pressure, time and moisture content affected the yield of vegetable oils. To study the effect of these factors on oil yield, he used a laboratory hydraulic press similar to the Carver model (Fisher Scientific Co., Pittsburg, Pennsylvania, U.S.A.) for the expression of cottonseed oil. The press had a pressing face of 3870 mm2 between which a maximum pressure of 34.5 MPa could be generated by an oil pump which was fixed at the bottom of the hydraulic press. On the bottom, the pressing face was attached to a hollow cylinder, 88.9 mm i.d. and 190.5 mm high, with fine holes to let the oil flow out during the pressing. Fitted inside the cylinder was a plunger. Electric hot plates were used to provide heat. In all of his studies on the expression of vegetable oils, Koo’~-** varied one of the four factors at a time, holding the other three factors constant. Tables 3,4, 5 and 6 are included to show the format and the results of his experiments on cottonseed oil. It was found, as shown in Table 3, that the oil yield was directly proportional to the square root of the pressure. Table 4 shows that the oil yield was inversely proportional to the square root of the kinematic viscosity, which was a function of temperature only. The effect of pressing time on oil yield appeared to be of little importance, since the square of the oil yield was proportional to the cube root of pressing time (Table 5). TABLE 3
Effect of pressure oncottonseedoil yield (temperature = WC, time = 4 h)” Pressure (P), MPa 13.8 17.2 20.7 241 27%
Oil yield ( W), wt %
Wp-‘I2
10.0 11.3 12.2 13.1 14.0
2.69 2.72 2.68 2.67 2.66
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TABLE4 Effect of temperature on cottonseed oil yield (pressure = 24.1 MPa, time = 3 h)lp
Temp., “C
Oil yield(W),
Kinematic viscosity (v), 10e5 m”js
wt %
13.0 13.6 16.1 21.3 23.8 24.2
18 25 50 75 loo 125
w
VW
0.12 0.11 0.08 0.08 0.06 0.05
8.40 5.98 2.55 1.28 0.70 0.50
TABLE 5
Effect of time of pressing on cottonseed oil yield (pressure = 24.1 MPa, temperature = 18”C)‘9
I
I Passing
time (t), h
Oil yield(W),
0.5 1.0 1.5 2.0 3.0 4.0
Jp
wt %
t-w 106 117 103 110 116 108
9.14 10.8 10.8 11.8 13.0 13.1
TABLE6 Effect of moisture content on cottonseed oil yield (pressure = 24.1 MPa, time = 3 h)” At 18°C
___-
Moisture, % (w.b.)
2.94 3.62 5.67 6.97 8.20 11.2
At 103°C
I
Oil yield, wt %
Moisture, % (w.b.)
0 0 12.8 12.7
3.10 5.35 8.20
13.0 12.7
11.0 13.2 14.20 20.6 22.0
Oil yield, wt %
14.9 23.8 21.6 21.0 32.1* 19.5* 20.7* 19.4*
‘The oil expressed was cloudy due to water contamination
The results of the effect of moisture content (Table 6) on oil yield showed that at a temperature of 18°C and below 4% moisture in the cottonseed, oil cannot be pressed out; while from 5.67 o/0to 11.2% moisture content, about 12.5% of oil yield was expressed. When the temperature was raised to lOO”C, oil was pressed out even at a moisture content of 3.1 O/$. Again, in the moisture range of 5.35-l l*O”/& oil yields were approximately the same. At higher moisture contents, the oil was contaminated with meal containing water such that the output appeared cloudy and
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EXPRESSION
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FROM
OILSEEDS
became difficult to separate. Koo concluded that with cottonseed, the optimum range of moisture content was from 5% to 11y0 for the temperature range of 18-100°C. To estimate the expression of oil from cottonseed, soybean, rapeseed, peanut, sesame, tung nut and caster bean, KOOKY developed the following general equation : w z cw,
pV t1/6v- z/2
where W is the oil yield (in wt %), C is a constant for the kind of oil seed (units consistent with unit analysis), W, is the oil content of the seed (in wt %), P the pressure (in MPa), t the pressing time (in h), v the kinematic viscosity of the oil at press temperature (in m2/s) and z is an exponent of kinematic viscosity varying from l/6 to l/2. A summary of the C, W,, and z values is given in Table 7. The experimental data revealed that for any one oilseed, there was an optimum range of moisture content for maximum oil yield. Of the various seeds studied, this range was from 5% to 13% (dry basis). TABLE 7
Constants and exponents for general oilseed expression equation= Oilseed
Soybean Cottonseed Rapeseed Peanut Tung nut Sesame seed Castor bean
z
cx 103
W0
l/2 l/2 l/3 l/3 l/3 l/6 l/6
5.40 6.42 15.0 19.4 23.4 46.5 51.3
19.5 34.7 42.2 51.9 64.5 53.0 64.2
-
The relative efficiency of a pressing operation was dependent on the kind of oilseed being pressed. Koo2’ reported press efficiencies, for a hydraulic press using the same operating conditions, from a minimum of 62.1 y0 for soybeans to a maximum of 91.2% for tung nut. REFERENCES
Duke, J. A.; Bagby, M. 0.
Comparison of oilseed yields: A preliminary review. Proc. Int. Conf. on Plant and Vegetable Oils as Fuels. Am. Sot. agric. Engrs, 1982 1 Boatwright, J. H. A wedge press for oil extraction. Approp. Technol. 1979 6 (2) 24-25 ’ Hurst, G. H. Lubricating oils, fats and greases. (Their origin, preparation, properties, uses and analysis.) A publication by Scott, Greenwood and Son, London, 1911 ’ Ward, J. A. High oil content seeds in continuous screw presses. J. Am. Oil Chem. Sot., 1976 53 261-264 5 Tindale, L. H.; Hill-Haas, S. R. Current equipment for mechanical oil extraction. J. Am. Oil Chem. Sot., 1976 53 265-270 J. Am. Oil Chem. Sot., 1977 54 489-490 L Bredeson, D. K. Mechanicalpressing. ’ Steinbock, S. R. Vegetable oilprocessing. Can. Chem. Process Ind., 1948 32 910-915 * Galloway, G. P. Cleaning, cracking, dehulling, decorticating and flaking of oil-bearing materials. J. Am. Oil Chem. Sot., 1976 53 271-274 ?? Woolrich, W. R.; Carpenter, E. L. Many problems beset vegetable oilproducers. Fd Inds, 1933 6 260 lo Othmer, D. F.; Agarwal, J. C. Extraction of soybean, theory and mechanism. Chem. Engng Prog., 1955 51 372-378 ” Taylor, R. B. Pressure cooking contributes increased cottonseedprocessingprojits. Chem. metall. Engng, 1937 44 978-981 la Williams, S. W.; Rathod, K. L. A case study of expeller production of soybean flour in India. College of Agriculture, University of Illinois at Urbana-Champaign. Int. Agric. Publ. INTSOY SERIES No. 3 ‘I Gurnham, F. C.; Masson, H. J. Expression of liquids from fibrous materials. Ind. Engng Chem., 1946 381309-1315 ’
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” Kormendy, I. A pressing theory with validating experiments on apples. J. Fd Sci., 1964 29 631-636 ‘5 Smith, R. L.; Kraybill, H. R. Soybean oil. Quality and yield as affected by conditions of expression. Ind. Engng Chem., 1933 25 334-336 ‘& American Oil Chemists Society. Official methods of chemical analysis, 1929 ” Beisler, W. H. Recovering tung oil from nuts grown in Florida. Chem. metall. Engng, 1930 37 614 ‘(I Jamieson, G. S. Vegetablefats andoils. A.C.S. Monogr. New York: Chemical Catalog Co., 1932 58 20 ‘* Koo, E. C. Studies on expression of vegetable oils I. Expression of cottonseed oil. J. Chem. Engng China, 1937 4 15-20 ‘O Koo, E. C. Studies on expression of vegetable oils II. Expression of soya bean oil. J. Chem. Engng China, 1937 4 207-211 ” Koo, E. C. Studies on expression of vegetable oils 111. Expression of tung oil. J. Chem. Engng China, 1938 5 47-52 ” Koo, E. C. Studies on expression of vegetable oils IV. Expression of rapeseed oil. J. Chem. Engng China, 1938 5 69-73 a’ KOO, E. C. Expression of vegetable oil. A general equation of oil expression. J. Chem. Engng China, 1942 34 342-345