Compositional, Mass-Volume-Area Related and Mechanical Properties of Sponge Gourd (Luffa aegyptiaca) Seeds

Abstract

Some physical, mechanical and compositional properties of sponge gourd seeds were determined according to standard methods. The length, width, thickness and equivalent diameter were 9.6, 6.17, 1.59 and 4.53 mm for whole seeds and 6.99, 4.31, 0.79 and 2.84 mm for the dehulled seeds respectively. Sphericity, surface area, seed mass, bulk density, apparent density and porosity were 0.47, 0.57 cm², 0.079 g, 0.43, 0.50 g/cm³ and 0.14 for whole seeds and 0.41, 0.33 cm², 0.044 g, 0.57, 0.93 g/cm³ and 0.39 for dehulled seeds, respectively. Coefficient of friction against wood, aluminum, galvanized steel and mild steel surfaces were 0.50, 0.59, 0.64 and 0.70 for the whole seeds and 0.55, 0.66, 0.67, and 0.76 for the dehulled seeds, respectively. Sponge gourd seeds contain 33.38 and 39.11 g of protein and fat per 100 g of sample, respectively, the fat being largely made up of 50.1% linoleic and 31.03 % stearic acids.

Keywords:

• Sponge gourd seeds
• Proximate composition
• Mass-volume-area related and mechanical properties

INTRODUCTION

Sponge gourd (Luffa aegyptiaca) is of the cucurbitaceae family of plants, which are generally found in the warmer regions of the world.[1] Immature cucurbitaceae fruits such as pumpkin, cucumber and some gourd plants are generally eaten as fresh fruits or cooked vegetables in different parts of Africa and Asia.[2] In Nigeria, cucurbitaceae seeds are generally known as egusi based to their broadly similar physical characteristics and common use of some members of the family in native food preparations. The seeds are roasted and eaten as a food snack or powdered and prepared as curries, but the use of sponge gourd seeds for such food applications have not being reported.[3,1] Egusi is powdered and cooked with leafy vegetables, fish, meat and seasoning into a popular soup otherwise called obe osiki among the Yorubas. obe osiki is widely eaten with yam or cassava based staples like pounded yam (iyan), amala, foofo or eba as choice delicacies. In Nigeria, no part of sponge gourd plant is eaten and it is not cultivated, but it grows richly in the wild in its season. At maturity, the fruit dries up, turns brown and afterward, breaks at the tip creating an aperture thorough which the loose seeds get discharged (Fig. 1a). The name, sponge gourd, originated from the spongy and fibrous nature of its seed-bearing vascular bundles which is useful as a biodegradable domestic sponge in the rural areas of Africa and Asia (Fig. 1b).[4] Usually, the seeds get wasted everywhere,[2,4] but the sponge is used industrially as a material for packing mattresses and upholstery; thermal insulator for containers; raw material for water and oil filters and as sound deadener for acoustic materials.[5,6] The whole seeds are usually black, structurally similar to watermelon (Citrullus lanatus) seeds but have flanges along the edge (Fig. 2). The dehulled seeds have a very thin, green outer seed coat speckled with dark grey colour.

Figure 1 (a) Some typical Nigeria grown sponge gourd fruits. (b) De-skinned sponge gourd fruits.

Figure 2 Sponge gourd seeds (a) whole seeds; and (b) dehulled seeds.

The properties and composition of cucurbitaceae seeds such as: egusi ibara (Citrullus colocynthis), egusi itooro (Cucumeropsis mannii), egusi elegede (Cucurbita pepo), egusi igba (Lagenaria siceraria), watermelon (Citrullus lanatus), snake gourd (Trichosanthes cucumerina), bitter gourd (Momordica charantia), and so on, have been widely studied.[7–20] They contain about 50 and 35% of crude fat and protein respectively[2] and the presence of significant amount of minerals and vitamins, which are vital in human, and livestock nutrition have been reported. They are rich sources of essential amino acids such as arginine, tryptophan, and methionine. Their oil is largely made up of unsaturated fatty acids, thus of high nutritional value. Their fatty acid profile generally show a high concentration (60–70%) of linoleic acid and the presence of oleic, stearic and palmitic acids.[3] The presence of conjugated fatty acids among some cucurbit oils make them highly suitable for frying.[2] Borne out of the need to harness the nutrient potentials of some of the lesser-known oilseeds, their use as sources of vegetable oil and protein has been reviewed.[4,8,9,20] Some of these seeds are relatively rich in protein and can be used to supplement protein intake especially in the developing countries where the supply of animal protein is inadequate.[8,20] However, there is very negligible information on the properties, processing and exploitation of sponge gourd seeds either for its oil or protein.[3] Knowledge of the properties of such underutilized species provides essential data for their handling and processing and opens a new frontier for developing new food products for the benefit of man or livestock. This work focuses on the determination of the compositional, mass-volume-area related and mechanical properties of sponge gourd seeds.

MATERIALS AND METHOD

Bulk quantities of mature and dry sponge gourd fruits (Luffa aegyptiaca) were gathered from the bush in Obafemi Awolowo University campus, Ile Ife, Nigeria in February 2006. The fruits were sorted and the damaged ones discarded. The length, minor and major diameter (Fig. 3) and mass of 50 randomly selected fruits were determined using a venier caliper and precision electronic balance with 0.01 accuracy (Model GT 2100 Germany), respectively. The fruits were deskinned, and the seeds carefully extracted by hand to avoid mechanical damage. In some cases, the spongy mesh of the fruit was torn loose to enhance total removal of seeds. The seed count per fruit was determined. Some of the seeds were dehulled and cleaned manually to remove shell remnants, impurities, and broken seeds.

Figure 3 Physical dimension of Sponge gourd pods (l = length; a = minor diameter and b = major diameter).

The moisture content of the dehulled seeds was determined using standard methods.[21] Three sub-samples weighing approximately 10 g from each of the two parts were obtained and oven dried at 130°C for six hours. The sub-samples were then cooled in desiccators for few minutes and weighed. The weight difference was calculated as the percentage moisture in the samples and this was the moisture content at which other investigation on the samples were carried out. Samples of the dehulled seeds were analyzed for proximate composition. The ash, crude fibre, protein, and ether extract content were analyzed by standard AOAC procedures.[22] The Fatty acid profile was determined by methylation of the total lipids of oil sample in three replicates and the fatty acid methyl ester (FAME) prepared from the samples were separated in a gas chromatograph 14-A (Shimadzu, Japan). Fatty acids were identified by comparing retention times with those of standard methyl esters.

The length, width, thickness and mass of 100 seeds (randomly picked one after the other from the lot) were determined (Fig. 4) using a vernier caliper and an electronic balance, respectively. Similar measurements were carried out for 100 dehulled seeds and the equivalent diameter, aspect ratio, and sphericity were determined by the following relationships.[15 19]

(1)

(2)

(3)

where φ is the sphericity; and De, L, W, T are the equivalent diameter, length, width, and thickness in mm, respectively. The volume of the whole seed was determined by water displacement method using a sinker.[18,23–25] Known weight of seeds sampled from the lot was allowed to float on water inside a graduated cylinder, the sample was lowered with a sinker, and the mass of water displaced was recorded. This was repeated for the dehulled seeds and there were 10 replicates each. The apparent density (ρ_t) in both cases was calculated as:

(4)

where ρ_w is the density of water; and m_a and m_w are mass of seed or kernel in air and water, respectively.[26,27] The bulk density of the whole seeds was determined by filling an empty plastic container of known volume and mass with the sample,[21,25,28] and the weighed was determined again. This was carried out in five replicates and same was repeated for the dehulled seeds. Bulk density was calculated as:

(5)

where ρ_b = bulk density, M_s = bulk mass of sample and V_c = volume of plastic container.[28,29] Porosity was calculated as the ratio of the differences in the fruit and bulk densities to the fruit density value and expressed as a percentage.[25,28]

(6)

where P is porosity; ρ_b is bulk density; ρ_t is true density. [23] The surface area was determined by selecting 20 whole seeds randomly from a bulk sample and tracing each with a sharp pencil on a graph sheet. The area of each trace was estimated by calculating the area of the approximate number of squares it occupies. The same procedure was repeated for the dehulled seeds.

Figure 4 Linear measurement of whole and dehulled sponge gourd seeds, where, L: length; W: width; T: thickness of the whole seed; l: Length, w: width; and t: thickness of the dehulled seed.

Coefficient of friction (CF) was determined using a 480 mm × 360 mm × 260 mm bottomless wooden frame box filled with the sample and mounted on an apparatus with adjustable tilting surface. The angle was measured against four surfaces: polished wood, mild steel, galvanized steel and aluminum. The surface was raised gradually until the box just began to slide. The angle (θ) made by the inclined surface with the horizontal just before the sample began to slide down was measured. For each surface, this was repeated five times and coefficient of friction was calculated as:

(7)

The angle of repose(π) was determined using a cylindrical container opened at both ends and placed on a flat surface. The cylinder was filled to the brim with the sample, and the excess was scraped off with a knife's edge. The cylinder was then lifted up gradually allowing the sample to flow and form a pile. The angle of repose was calculated from the measurements of the vertical depth and base radius of spread of the sample. This was replicated five times.

RESULTS AND DISCUSSION

The average length, minor and major diameter, mass of a mature sponge gourd fruit and number of seeds per fruit were 178.60, 40.76, 52.11 mm, 26.42 g, and 140 seeds respectively (Table 1). The proximate composition of dehulled sponge gourd seeds is presented in Table 2. The results showed that the dehulled seed contains 39.11% of crude fat, and 33.38% of crude protein. This compares reasonably with 42.34% crude fat, and 37.57% crude protein reported earlier by Dairo et al.[4]. The differences can be attributed to agro-ecological variation in the places where the seeds were grown and oil extraction methods and conditions. Other popular cucurbitaceae seeds such as Cucumeropsis mannii, Citrullus colocynthis, Citrullus lanatus and Cucumeropsis pepo contain 26.2, 28.0, and 45.9% of crude protein and 47.3, 53.0, and 32.5% of crude fat respectively.[30,31] Subject to further investigation, critical safety evaluation, and depending on the bioavailability and quality of the protein, sponge gourd seeds like other cucurbitaceae could qualify as a good source of vegetable protein for man and livestock.

Table 1 Linear dimension, mass and seed count of sponge gourd fruit

Serial no	l (mm)	a (mm)	b (mm)	m (g)	Number of seeds per fruit
1	168	47.00	53.50	24.51	93
2	194	49.50	60.34	47.42	108
3	163	52.50	55.74	22.57	53
4	223	54.00	67.74	46.53	202
5	136	29.88	46.84	9.65	27
6	190	48.78	56.00	30.68	193
7	135	41.61	49.53	17.29	140
8	165	43.76	46.38	20.36	174
9	197	40.22	50.68	20.38	165
10	108	37.38	40.82	8.39	60
11	149	34.16	49.76	15.66	84
12	196	48.28	57.00	34.79	162
13	175	40.50	50.10	26.10	208
14	153	45.50	59.54	21.60	204
15	224	54.84	43.00	48.89	265
16	123	37.80	46.20	14.10	103
17	152	44.50	48.86	18.52	133
18	140	40.00	51.20	14.53	78
19	139	41.20	45.50	21.80	149
20	178	36.14	47.20	25.20	190
21	130	30.00	59.62	14.84	54
22	264	45.62	58.50	44.72	183
23	189	36.80	51.74	17.11	53
24	167	49.92	59.48	27.36	106
25	216	45.00	55.96	44.58	213
26	203	50.00	46.34	42.27	214
27	150	38.20	50.00	23.69	50
28	160	42.38	58.26	20.91	186
29	220	45.18	54.00	35.78	170
30	119	22.64	40.02	22.50	197
31	155	39.78	44.14	25.52	200
32	130	37.68	39.50	14.45	120
33	213	38.50	53.26	31.00	184
34	206	36.18	55.48	30.54	162
35	146	34.50	50.50	17.06	54
36	166	40.00	51.80	21.91	101
37	118	33.15	49.35	6.80	30
38	189	33.65	51.50	22.97	90
39	233	40.65	58.10	35.41	168
40	170	39.62	52.65	22.60	113
41	212	29.65	52.80	22.67	78
42	215	36.85	55.10	31.35	210
43	265	49.60	59.00	49.28	236
44	247	38.75	59.40	31.17	183
45	198	44.50	50.25	28.20	188
46	151	38.55	40.75	13.48	140
47	205	35.15	63.30	34.04	200
48	164	33.45	46.00	13.31	148
49	183	40.10	54.10	33.70	106
50	238	44.30	58.90	52.61	99
Mean±sd	178.60 ± 38.88	40.76 ± 6.73	52.11 ± 6.31	26.42 ± 11.54	140.54 ± 60.17
Min.	108	22.64	39.50	6.80	27
Max.	265	54.84	67.74	52.61	265
l: length, a: minor diameter, b: major diameter, m: mass; and sd: standard deviation.

Table 2 Proximate composition of sponge gourd kernels (g/100 g of sample)

Crude protein	Ether extract	Crude fiber	CHO	Ash	Moisture content
33.38 ± 0.62	39.11 ± 0.20	2.65 ± 0.32	13.50 ± 0.38	5.07 ± 0.27	9.83 ± 0.43
Mean of three replicates ± standard deviations.

The crude fat content places sponge gourd seeds in the range of other oil seeds such as sunflower (35–45%), safflower (30–35%), rapeseed (40–45%), and groundnut (45–50%).[20,32] The fatty acid profile of the oil (Table 3) shows that it contains 57.51% of unsaturated fatty acids of which 50.1% is linoleic and 42.48% saturated fatty acids (made up of stearic, lauric, myristic, and palmitic acids). Others were not detected. Linoleic acid concentration in other cucurbitaceae seed oils are: 68.7% for Citrullus lanatus, 65% for Citrullus colocynthis, 53% for Cucurbita moschata, and 43% for Cucurbita pepo.[35,36] The oil is liquid both at ambient temperatures and refrigerated temperatures but may break down to form carcinogenic substances during high temperature frying.

Table 3 Fatty acid composition of oil expressed from sponge gourd kernels

Saturated fats	% Composition
Lauric	0.92
Stearic	31.03
Palmitic	3.95
Myristic	6.59
Arachidic	negligible
Behemic	negligible
Lignocenic	negligible
Total	42.49
Unsaturated fats per gram of oil	% composition
Oleic	0.50
Linolenic	6.90
Linoleic	50.1
Palmitoleic	negligible
Total	57.51

The mass-volume-area and mechanical properties of sponge gourd whole seeds and kernels are presented in Table 4. Being a smaller seed, length, width and thickness were 9.6, 6.17, and 1.59 mm which are lower than those of Citrullus lanatus, C. colocynthis and Cucurbita pepo.[15,19,37] From the size distribution (Table 5), 53% of the whole seeds had lengths ranging from 9 to 10 mm, while 17% and 30% had lengths ≤9 mm and >10 mm respectively. The frequency distribution of the whole seeds based on dimension and mass shows a trend towards normal distribution (Fig. 5). The principal dimension of the whole seed can be described by the relationship:

Figure 5 Frequency distribution of sponge gourd seeds dimensions.

Table 4 Mass-Volume-Area related properties of sponge gourd whole and dehulled seeds

Property	Number of replicates	Whole seeds at 8.44% moisture (wb)	Dehulled seeds at 9.83% moisture (w.b.)
Axial dimension, mm
Length, a	100	9.60 ± 0.62	6.99 ± 0.38
Width, b	100	6.17 ± 0.43	4.31 ± 0.32
Thickness, l	100	1.59 ± 0.20	0.79 ± 0.27
Equivalent Diameter, mm	100	4.53 ± 0.27	2.84 ± 0.39
Aspect ratio	100	0.64 ± 0.05	0.62 ± 0.05
Sphericity	100	0.47 ± 0.03	0.41 ± 0.06
Roundness	20	0.79 ± 0.07	0.80 ± 0.11
Surface area, cm^2	20	0.59 ± 0.08	0.33 ± 0.02
Mass, g	100	0.08 ± 0.01	0.04 ± 0.01
True density, g/cm^3	5	0.50 ± 0.02	0.93 ± 0.07
Bulk density, g/cm^3	5	0.43 ± 0.02	0.57 ± 0.012
Porosity,%		13.98	38.50
mean ± standard deviations.

Table 5 Distribution of sponge gourd seeds by size and mass

	Size distribution
Particulars	Ungraded	Small	Medium	Large
Length of seed, mm	8.05–11.2	L≤9	9<L≤10	L>10
Percent of sample
by number,%	100	17	53	30
by mass,%	100	16.95	52.33	30.72
Average dimensions
Seed
Length (L), mm	9.60 ± 0.62	8.70 ± 0.27	9.48 ± 0.23	10.32 ± 0.31
Width (W), mm	6.17 ± 0.43	5.99 ± 0.42	6.09 ± 0.39	6.41 ± 0.41
Thickness(T),mm	1.59 ± 0.2	1.56 ± 0.22	1.58 ± 0.21	1.61 ± 0.18
Mass (M), g	0.08 ± 0.01	0.08 ± 0.01	0.08 ± 0.01	0.08 ± 0.01
Kernel
Length (l), mm	6.99 ± 0.40	6.49 ± 0.39	6.94 ± 0.07	7.40 ± 0.16
Width (w), mm	4.31 ± 0.32	4.12 ± 0.29	4.37 ± 0.32	4.30 ± 0.31
Thickness (t), mm	0.79 ± 0.27	0.70 ± 0.29	0.78 ± 0.31	0.84 ± 0.17
Mass (m), g	0.04 ± 0.01	0.04 ± 0.01	0.04 ± 0.01	0.04 ± 0.01
Mean ± standard deviations.

(9)

(10)

From Table 6, the following general expressions can also be used to describe the relationship between the dimensions.

Table 6 Correlation of sponge gourd whole and dehulled seeds dimension

Ratio.	Mean	Max	Min	Correlation
L/W	1.56 ± 0.12	1.83	1.21	0.38
L/T	6.16 ± 0.88	9.27	4.15	0.03
L/M	122.7 ± 15.61	176.91	88.57	0.09
l/w	1.63 ± 0.14	2.2	1.32	0.14
l/t	10.11 ± 4.5	27.88	3.7	0.01
l/m	164.69 ± 31.47	255.02	105.6	−0.01
L/l	1.37 ± 0.12	1.66	1.01	0.97
W/w	1.44 ± 0.16	1.86	1.09	0.06
T/t	2.28 ± 1.02	5.62	0.82	0.04
M/m	1.85 ± 0.37	2.89	1.2	0.03
Mean ± standard deviations.

(11)

(12)

(13)

(14)

where, L = length of seed; W = width of seed; T = thickness of seed; M = mass of seed; l = length of kernel; w = width of kernel; t = thickness of kernel and m = mass of kernel. In Table 6, the coefficients of correlation of the linear dimensions of seed and kernel show that L/W ratio is higher than L/T and L/M ratios; meaning that the width of the seed is closely related to its length, while the thickness and mass has less association. Similar behaviour was reported by for Cucurbita pepo and Citrullus lanatus seeds respectively.[15,19] The sphericity, equivalent diameter, surface area, seed mass, bulk density, true density, and porosity for the whole seeds were 0.47, 4.53 mm, 0.57 cm², 0.08 g, 0.43 g/cm³, 0.50 g/cm³, and 0.14; while the corresponding values for the dehulled seeds were 0.41, 0.33 cm², 0.044 g, 0.57 g/cm³, 0.93 g/cm³, and 0.39 respectively (Table 4). These low values confirm that there is a high variation between the principal dimensions of both the shelled and unshelled materials hence; they cannot be classified as spherical in shape. The coefficients of friction against four surfaces: wood, aluminum, galvanized steel and mild steel were obtained as 0.50, 0.59, 0.64, and 0.70 for the whole seeds and 0.55, 0.66, 0.67, and 0.76 for the dehulled seeds respectively. The angle of repose was 23.8° for the whole seeds and 32.8° for the dehulled seeds.

There is less variability in the surface area of the dehulled seeds when compared to that of the whole seeds. This is applies generally to most cucurbitaceae seeds. The bulk density of the whole seeds compare with that of Citrullus colocynthis seeds but much lower than that of other cucurbitaceae seeds.[2] The low values of porosity of the whole and dehulled seeds may be due to the flat nature of the seeds with few voids when packed in a container. The frictional properties are shown in Table 7. The coefficient of friction of sponge gourd seeds is similar to that of sunflower seeds in which the forces of solid friction are greater on mild steel than on galvanized steel.[34] Due to the lower forces of solid friction on aluminum and polished wood surfaces the coefficients of friction of sponge gourd seeds was low. The coefficient of friction of the kernel were higher than the values obtained for the seeds on all the surfaces employed indicating that the dehulled seeds are less smooth when compared to the seeds. This behaviour is similar to sunflower seeds, [34] but a different observation was reported for dehulled Cucurbita pepo and Citrullus lanatus seeds, which are reportedly smoother than the whole seeds.[15,19] The angle of repose of whole sponge gourd seeds (23.8°) was also found to be lower than that of the dehulled seeds (32.8°) indicating that the whole seeds are smoother than the dehulled seeds. These are lower than the corresponding values reported for sorrel, sunflower, and Cucurbita pepo [15, 36, 33] but dehulled Cucurbita pepo and Citrullus lanatus seeds were relatively smoother than dehulled sponge gourd seeds.[15,19]

Table 7 Frictional properties of sponge gourd whole and dehulled seeds

Property	Seed	Kernel
Coefficient. of Friction
Wood	0.50 ± 0.01	0.55 ± 0.02
Aluminum	0.59 ± 0.03	0.66 ± 0.04
Galvanized steel	0.64 ± 0.01	0.68 ± 0.03
Mild steel	0.70 ± 0.03	0.76 ± 0.02
Angle of repose (°)	23.8 ± 2.09	32.8 ± 2.59
Mean ± standard deviations.

CONCLUSION

Sponge gourd kernel oil contained 57.51 and 42.49% of unsaturated acids and saturated acids respectively with linoleic acid as its major constituent. The oil is stable at ambient temperatures and when refrigerated but may break down to form carcinogenic substances at high temperatures due to its unsaturation. The seed dimensions show a high variation between the principal dimension and this provides valuable information for the design of dehulling machine. Being a slender seed, the seeds live few voids when packed inside a container, porosity is therefore low. The coefficient of friction of the dehulled seeds was higher than those obtained for the whole seeds on all the surfaces employed. These properties of sponge gourd seeds provides essential data and baseline information on handling and processing and opens frontier for developing value added and protein-fortified food products for the benefit of man or livestock.