Journal of Food Engineering 45 (2000) 43±48
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Physical properties of ackee apple (Blighia sapida) seeds T.O. Omobuwajo a,*, L.A. Sanni b, J.O. Olajide c a
c
Department of Food Science and Technology, Obafemi Awolowo University, Ile-Ife, Nigeria b Department of Agricultural Engineering, Obafemi Awolowo University, Ile-Ife, Nigeria Department of Food Science and Engineering, Ladoke Akintola University of Technology, Ogbomoso, Nigeria Received 21 May 1999; accepted 31 January 2000
Abstract Physical properties often required for the designing of hopper, dehuller and sundry grain processing machineries were determined for ackee apple seeds at a moisture content of 9.88% (wet basis). The average seed length, width and thickness were 24.30, 19.70 and 12.90 mm, respectively, while sphericity, roundness and aspect ratio were 75.50%, 74.44% and 81.15%, respectively. True density, bulk density and porosity were 888.73 kg mÿ3 , 556.91 kg mÿ3 and 37.07%, respectively. On three dierent surfaces, static coecient of friction varied from 0.38 to 0.46, while the angle of repose was essentially 25°. The suspension air velocity for the seed, kernel and hull were 9.95, 9.78 and 5.45 m sÿ1 , respectively. The speci®c heat of the seed at 80°C was 2.83 kJ kgÿ1 Kÿ1 . Ó 2000 Published by Elsevier Science Ltd. All rights reserved. Keywords: Ackee apple seeds; Size and shape indices; Coecient of static friction; Angle of repose; Suspension air velocity; Speci®c heat capacity
Nomenclature Ac
Mc Ms Mw Ra Ri Sp qb qt
area of smallest circumscribing circle when the seed is traced on paper in its natural rest position (m2 ) largest projected area when the seed is traced on paper in its natural rest position (m2 ) length of seed (mm) width of seed (mm) thickness of seed (mm) speci®c heat capacity of calorimeter (J kgÿ1 Kÿ1 ) speci®c heat capacity of seed (J kgÿ1 Kÿ1 ) speci®c heat capacity of water (J kgÿ1 Kÿ1 ) porosity (%) average slope of cooling curve for seed sample (K sÿ1 ) average slope of cooling curve for blank or water sample (K sÿ1 ) mass of calorimeter (kg) mass of seed sample (kg) mass of water in calorimeter (kg) aspect ratio (%) roundness index (%) sphericity index (%) bulk density (kg mÿ3 ) true density (kg mÿ3 )
Subscripts 1 2
including the scar or remains of style ignoring the scar
Ap a b c Cc Cs Cw e Gs Gw
*
Corresponding author. Tel.: +234-36-230192; fax: +234-36-232401. E-mail address:
[email protected] (T.O. Omobuwajo).
1. Introduction Ackee (Blighia sapida, Koenig) is an underutilized tree crop native to West Africa, and widespread in tropical and subtropical environments (Assi, 1988). The tree, which can be 10 m or more in height (Fig. 1), is highly distinctive for its heavy dark green foliage (which provides shade from the sun) and bright red fruits (Keay, Onochie & Stan®eld, 1964). The fruit is a capsule (Olorode, 1984) comprising of three ¯eshy valves which split open when mature and ripe, conspicuously displaying the three glossy black elongated seeds with cream-coloured arils at their base (Fig. 2). The arils or apples may either be eaten fresh as a vegetable, roasted and eaten as such, or used in soups as a meat substitute. Aside from the above food uses, the tree has a number of additional economic bene®ts. First, the aril gives a rich lather in water, which is used to ®x colours in locally manufactured fabrics. Second, perfumed water obtained from the fragrant ¯owers is used as a cosmetic. Third, the tree bark is used as medicament in treating skin diseases. Fourth, the light hardwood is used as timber. Fifth, the seed oil is used in soap-making. Sixth, the dried husks and seeds are ashed and used in soapmaking on account of their rich potash. Furthermore, preliminary analysis of the oil found in the aril (Oladeji, 1993; Olapade, 1997) highlight the potential of the seed
0260-8774/00/$ - see front matter Ó 2000 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 0 4 0 - 6
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T.O. Omobuwajo et al. / Journal of Food Engineering 45 (2000) 43±48
Fig. 1. Ackee apple tree.
2. Materials and methods Seeds were collected from mature and ripe fruits dropped from ackee apple trees in three locations within the metropolis of Ogbomoso (8°070 N, 4°160 E) in southwestern Nigeria, and pooled together to obtain approximately 100 kg of seed material. The material was kept in a jute bag and allowed to dry under ambient room conditions (28±34°C, 80±90% RH) to the equilibrium moisture. 2.1. Moisture and fat contents
Fig. 2. Ackee apple fruits and seeds: (A) fruit, (B) fruits with splitting valves displaying three glossy black seeds, (C) ventral side of single seed with aril, (D) dorsal side of seed.
oil as a drying oil with potential application in the paints and varnish industry. In view of the enormous potential of the crop in the food industry and also in the allied surfactant, coating, pharmaceutical, cosmetic and oleochemical industries, it is important to determine the physical properties of the seeds such as size, shape, mass, density characteristics, coecient of static friction and dynamic angle of repose. These properties are often required in the design of new processing machineries or adaptation of existing ones.
The sample was prepared for analysis by grinding about 20 g of the seeds to pass through a sieve with circular openings of 1 mm diameter and mixed thoroughly. Two grams of the comminuted material was vacuum dried at 80°C for 10 h, cooled in a desiccator and weighed. Weight loss on drying to a ®nal constant weight was recorded as moisture content. For crude fat assay (AOAC, 1984), the dried sample for the previous determination was extracted in a Soxhlet-type extractor with petroleum ether (boiling point 60±80°C). The extract was dried for 30 min at 100°C, cooled and the residual fat weighed. Reported values are means of three determinations. 2.2. Physical characteristics The seed material was divided into 5 lots and 20 seeds were selected at random from each lot to obtain 100 seeds for conducting the experiment; hence, measurement of all size and shape indices as well as the seed mass, are replicated one hundred times. The seed size, in terms of the three principal axial dimensions, that is
T.O. Omobuwajo et al. / Journal of Food Engineering 45 (2000) 43±48
length (a), breadth (b) and thickness (c), were measured using the vernier caliper (Kanon Instruments, Japan) reading to 0.01 mm. Two measurements were taken in respect of the length because of the scar or remains of the style (Fig. 3). The seed shape was determined in terms of its sphericity, roundness and aspect ratio. The sphericity index (Sp ) was computed (Mohsenin, 1978) as Sp
abc a
1=3
100:
1
To obtain the roundness, each seed was placed on a sheet of tracing paper in its natural rest position and the edges carefully traced with a sharp thin pencil. The largest inscribed circle and the smallest circumscribing circle were constructed for each of the traces. The area of the smallest circumscribing circle (Ac ) was calculated, while the largest projected area of each trace (Ap ) was measured using a planimeter. The roundness index (Ri ) was computed (Shepherd & Bhardwaj, 1986) as Ri
Ap 100: Ac
2
The aspect ratio (Ra ) was calculated (Maduako & Faborode, 1990) as Ra
b 100: a
3
The mass of individual seeds were determined in a Mettler Toledo PB 153 electronic balance (MettlerToledo GmbH, Greifensee, Switzerland) to an accuracy of 0.001 g. True density of the seed was determined by the water displacement technique (Dutta, Nema & Bhardwaj, 1988). Twenty randomly selected seeds were weighed and then coated with a thin layer of table gum and allowed to dry in order to prevent water absorption. The seeds were lowered with a metal sponge sinker into a 1 l capacity measuring cylinder containing 500 ml of distilled water, such that the seeds did not ¯oat during immersion in water. Net volumetric water displacement
Fig. 3. The principal axial dimensions of the ackee apple seed: length including the scar or remains of the style (a1 ), length ignoring the scar (a2 ), width (b) and thickness (c).
45
by the seeds was recorded. This technique was found to be suitable, as the increase in seed mass due to the coating was about 2%, causing a negligible error in the determination. Moreover, there were no changes in the net water displacement on immersion of the coated seeds in water for 3 min, indicating the functional eectiveness of the coating. The bulk density was determined using the mass/ volume relationship (Fraser, Verma & Muir, 1978), by ®lling an empty plastic container of predetermined volume and tare weight with the seeds by pouring from a constant height, striking o the top level and weighing. The density ratio was ratio of mass density to bulk density expressed as percentage, while porosity (e) was computed (Jain & Bal, 1997) as
qt ÿ qb 100: qt
4
Reported values of all density characteristics are means of 10 replications. The coecient of static friction of the seed was found with respect to three structural materials, namely, unsanded plywood, galvanized steel sheet and mild steel sheet, using the inclined plane apparatus (Norwood Instruments, Honley, UK) as described by Dutta et al. (1988). The table was gently raised and the angle of inclination to the horizontal at which the sample started sliding was read o the protractor attached to the apparatus. The tangent of the angle was reported as coecient of friction. The dynamic angle of repose or the emptying angle was determined on the same aforementioned surfaces using the method described by Maduako and Faborode (1990). A 450 450 450 mm3 paper carton was ®lled with the seeds, lifted up to a distance of about 150 mm above the ¯at surface and the bottom was opened. The gradual lifting of the carton continued until a conical heap was formed. The emptying angle was calculated from the height and base radius of the heap. Measurement of all frictional characteristics was replicated 10 times. The suspension air velocities for the seed, the kernel and the hull were determined. The experimental set-up used was similar to that described by Kepner, Bainer and Berger (1978). The upper outlet of air blown by a centrifugal fan, through a vertical hollow cylinder of transparent plastic sheet of 8 cm diameter and 25 cm length, was ®tted with an air meter (Air¯ow Development, England). The other (lower) end of the hollow cylinder was overlaid with a wire netting of approximately 1.5 mm square holes and the air current from the fan passing through the opening was protected with a hollow paper carton measuring 150 140 80 mm3 . The air ¯ow rate was controlled by varying the speed of the fan motor using a Variacâ variable speed transformer (Zenith Electronic, London, UK). Air velocity when the sample was just momentarily lifted o the
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T.O. Omobuwajo et al. / Journal of Food Engineering 45 (2000) 43±48
contacting wire netting surface was recorded as suspension velocity with the aid of the air meter. Reported values of suspension air velocities are means of 10 such determinations. The speci®c heat capacity of the seed was determined in an adiabatic drop calorimeter using the method of mixtures (McProud & Lund, 1983). About 10 g of the seed, tightly wrapped in a thin polythene foil, was dropped into the calorimeter containing about 300 g of water at an equilibrated starting temperature of 80°C. The heat loss data were collected to plot the heat loss curve. Data for the heat loss correction curve were collected using blank (water) sample, and the speci®c heat capacity was calculated from the heat balance equation as Cs
1 Mw Cw
Gw =Gs ÿ Mc Cc : Ms
5
Determination of the speci®c heat capacity was replicated ®ve times.
3. Results and discussion A summary of the results of determined physical parameters is shown in Table 1. The seed length, width and thickness were found to be 24.3, 19.7 and 12.9 mm, respectively. Corresponding values for the oilbean seed (Oje & Ugbor, 1991) were 65.4, 41.3 and 13.7 mm while 7.98, 5.95 and 5.82 mm were reported for gram (Dutta et al., 1988). The ackee seed is thus smaller than the oilbean seed but bigger than gram. Speci®cally, the oilbean seed is approximately two and half times longer than the ackee seed, approximately two times as wide, and of about the same thickness. Compared with gram, however, the ackee seed is about three times longer and wider and about twice as thick. The importance of these and other characteristic axial dimensions in determining aperture sizes and other parameters in machine design have been discussed by Mohsenin (1978) and highlighted lately by Omobuwajo, Akande and Sanni (1999).
Table 1 Some properties of ackee apple seed Physical property
Number of observations
Unit of measurement
Mean value
Minimum value
Maximum value
Standard deviation
Moisture content (wet basis) Oil content (wet basis) Length (with scar) Length (without scar) Width Thickness Geometric mean dimension (with scar) Geometric mean dimension (without scar) Sphericity (with scar) Sphericity (without scar) Roundness (with scar) Roundness (without scar) Aspect ratio (with scar) Aspect ratio (without scar) Seed mass True density Bulk density Density ratio Porosity
3 3 100 100 100 100 100 100 100 100 100 100 100 100 100 10 10 10 10
wt% wt% mm mm mm mm mm mm % % % % % % g kg mÿ3 kg mÿ3 % %
9.88 14.10 24.30 21.28 19.70 12.90 18.97 17.96 75.5 82.5 74.4 84.6 81.2 93.0 2.80 889 557 63 37
9.35 13.50 20.00 15.50 17.50 10.25 16.93 14.83 68.9 73.1 61.1 70.8 72.9 80.1 2.19 775 542 57 27
10.40 14.90 26.70 24.90 22.60 18.40 21.17 20.60 88.9 103.1 86.6 95.9 96.5 126.4 3.68 992 569 73 43
0.53 0.72 1.32 1.89 1.01 1.36 0.85 0.96 3.6 5.3 5.2 5.2 4.8 8.1 0.30 58 8 5 5
Coecient of static friction on Plywood with grains parallel Plywood with grains perpendicular Galvanized steel sheet Mild steel sheet
10 10 10 10
± ± ± ±
Dynamic angle of repose on Plywood Galvanized steel sheet Mild steel sheet
10 10 10
degrees degrees degrees
Suspension air velocity Seed Kernel Hull
10 10 10
Speci®c heat capacity at 80°C
5
0.383 0.383 0.380 0.458
0.325 0.325 0.344 0.404
0.488 0.466 0.404 0.509
0.059 0.046 0.018 0.033
25.40 25.04 25.05
23.35 23.83 23.72
29.74 26.98 27.61
1.83 0.93 1.19
m sÿ1 m sÿ1 m sÿ1
9.95 9.78 5.45
9.57 9.50 5.27
10.20 10.03 5.87
0.20 0.19 0.20
kJ kgÿ1 Kÿ1
2.83
2.43
3.47
0.43
T.O. Omobuwajo et al. / Journal of Food Engineering 45 (2000) 43±48
The seed sphericity and roundness were found to be 75.8% and 74.4%, respectively. These values are much higher than corresponding values of 60.5% and 40.0% reported for oilbean seed (Oje & Ugbor, 1991) but in the same range as the 74% and 70% reported for gram (Dutta et al., 1988). The high sphericity of the ackee seed is indicative of the tendency of the shape towards a sphere, while the high roundness indicates that the corners of the seed are round rather than sharp. Taken along with the high aspect ratio (which relates the seed width to length), it may be deduced that the ackee seeds will rather roll, like gram, than slide on their ¯at surfaces like oilbean seed. This tendency to either roll or slide is very important in the design of hoppers. Furthermore, the shape indices indicate that the ackee seed may be treated as an equivalent sphere like gram for an analytical prediction of its drying behaviour. It is important to point out that some of the size and shape characteristics of the seed can be modi®ed by trimming the scar as indicated by the physical parameters determined with the scar ignored. This trimming may be necessary especially in the adaptation of an existing processing machine and also in taking advantage of the higher sphericity and roundness values. The average seed mass was 2.8 g. As expected, this value is much bigger than 0.713 g reported for gram but smaller than the 20.2 g reported for oilbean seed. The seed true density, bulk density and porosity were 889, 557 kg mÿ3 and 37%, respectively. These values are lower than the corresponding values of 1311, 780 kg mÿ3 and 40.5% reported for gram (Dutta et al., 1988), and 1578±1623, 830±886 kg mÿ3 and 45±49% reported for the three pearl millet varieties (Jain & Bal, 1997). Thus, while gram and the three millet varieties will sink in water (density 1000 kg mÿ3 ), the ackee seed will ¯oat. This property may be useful in the separation and transportation of the seeds by hydrodynamic means. The lower porosity or percentage of volume of voids in the ackee seed may be due to the higher sphericity and roundness, which ensure a more compact arrangement of the seeds. The coecient of static friction of the ackee seed was 0.383 on plywood with grains parallel to the direction of ¯ow, 0.383 on plywood with grains perpendicular to the direction of ¯ow, 0.38 on a galvanized steel sheet and 0.453 on a mild steel sheet; while the dynamic angle of repose on plywood, galvanized steel and mild steel were 25.4°, 25.0° and 25.1°, respectively. These frictional properties present an interesting trend. For while the higher values of coecient of friction on mild steel compared with galvanized steel on one hand, and on plywood compared with galvanized steel on the other, were in agreement with the trends in results of previous studies (Dutta et al., 1988; Jain & Bal, 1997), the results obtained on plywood in the two directions were not. The coecient of friction on plywood with grains perpen-
47
dicular to the direction were expected to be higher than values obtained with the grains parallel to the direction of ¯ow. As there was no statistically signi®cant dierence between the two sets of values at a 5% probability level, the implication is that there were no statistically signi®cant dierences in the roughness at the seed/plywood interface in the two directions. An explanation for this observation could be the very smooth glossy surface of the seed, which could have moderated the minor dierences in the surface roughness in the two directions such that any observed variation between the two sets of values could have been due to chance. Interestingly too, the angles of repose on the three surfaces were practically the same, especially on the two steel surfaces. This also might have been due to the earlier mentioned moderating in¯uence of the smooth seed surface. The statistically insigni®cant dierences (at a 5% probability level) in the values of the angles of repose must have been responsible for the report of a single value of angle of repose by previous authors (Dutta et al., 1988; Oje & Ugbor, 1991; Joshi, Das & Mukherjee, 1993; Jain & Bal, 1997) who investigated frictional properties of dierent seeds on several structural material surfaces. It is nevertheless important to note that the angle of repose for the ackee seed is higher than the 17° reported for the oilbean seed (Oje & Ugbor, 1991), is in the same range as 25.5° and 23±25° reported for gram and pearl millets, respectively (Dutta et al., 1988; Jain & Bal, 1997), but lower than the 37° reported for pumpkin seeds of similar moisture content (Joshi et al., 1993). The air velocities required to suspend the whole seed, the dehulled seeds (kernel) and the hull were found to be 9.95, 9.78 and 5.45 m sÿ1 , respectively. These values were higher than the 2±7 m sÿ1 reported for the various fractions of pumpkin seeds (Joshi et al., 1993), but more importantly, the results follow the same trend with the terminal velocity of the hull being signi®cantly lower than for the seed and kernel. The appreciable dierence between the suspension air velocities for the kernel and the hull indicates that separation of these fractions by pneumatic means is feasible. The speci®c heat capacity of the seed at 80°C was found to be 2.83 kJ kgÿ1 Kÿ1 . This value is within the range of 2.14±5.32 kJ kgÿ1 Kÿ1 reported for various sizes of the oilbean seed (Oje & Ugbor, 1991). The speci®c heat will be useful in thermal processing of the seeds, and especially in predicting its thermal behaviour. References AOAC (1984). Ocial methods of analysis (14th ed.). Washington, DC: Association of Ocial Analytical Chemists. Assi, L. A. (1988). Diversity of under-utilized species in Africa. In F. Attere, H. Zedan, N. Q. Ng, & P. Perrino, Crop genetic resources of Africa, vol. I (pp. 53±88). Nairobi: International Board for Plant
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