Physico-fracture Characteristics of Makhana (Euryale ferox) Seeds

ABSTRACT

The fracture characteristics of Makhana (Euryale ferox) seed kernels were determined at moisture contents varying from 10.28 to 15.2% (dry basis), as a function of compressive force, deformation, and energy absorbed. The force required for kernel rupture decreased as the temperature and time of roasting increased while moisture content of the seed decreased. The deformation of the seed at the point of rupture was found to be in the range of 1.69 to 2.95 mm. Energy absorbed per unit volume, sphericity and angle of repose increased linearly with increasing moisture content and were found to be in the range of 310 to 822 mJ, 0.97 to 0.98, and 46 to 50°, respectively.

KEYWORDS:

• Multi-spouted bed roasting
• Makhana kernel
• Fracture
• Deformation
• Rupture force

Introduction

Makhana (Euryale ferox) also known as Gorgon nut, Fox nut is an aquatic crop belonging to the family of Nympheaceae and it is grown in stagnant perennial water bodies like ponds and swamps. It is characterized by its hard seed coat, black color, and round shape with a diameter ranging from 4.5 to 14.5 mm.^[1] The edible part of the nut is its starchy kernel which cannot be separated easily from the raw nut. It is, therefore, popped manually by the traditional method.^[2,3] Makhana contains 9.7% easily digestible protein, 76% carbohydrate, 12.8% moisture, 0.1% fat, 0.5% total minerals, 0.9% phosphorus, and 1.4% mg iron/100 g. These seeds are low in saturated fats, sodium, and cholesterol and are high in magnesium, potassium, and phosphorus. The amino acid index (89–93%) and arginine+lysine/proline ratio (4.74–7.6) of Makhana is high and superior when compared to many cereals. Popped Makhana is used in preparation of a number of Indian sweet dishes, pudding, and milk-based sweets. The seed is analgesic and aphrodisiac, hence, used in the preparations of a number of Ayurvedic medicines.^[4–6]

Understanding fracture properties of Makhana seeds play a crucial role in improvement of postharvest handling and processing in addition to development of better cultivars. There are many studies pertaining to physical and engineering properties of plant material such as locust bean,^[7] pigeon pea,^[8] cotton,^[9] chick pea,^[10] calabash nutmeg,^[11] fenugreek seeds,^[12] coriander seeds,^[13] arigo seeds,^[14] and bay laurel seeds.^[15] The moisture dependent fracture properties of Makhana seeds grown in India are not available in literature. Hence, this study was undertaken to determine fracture properties of Makhana seeds at various moisture contents.

Materials and methods

Material

The Makhana seeds were procured from agricultural experimental station, Darbhanga, Bihar, India. The seeds were cleaned to remove extraneous material and dried to protect the seeds from infestation. The diameter of the seeds was measured using vernier caliper (Mityoto, Japan; measurement accuracy 0.01 mm) and was found to be 11.5 mm (average of 30 numbers). The moisture content of the Makhana seeds was determined by drying the samples in an air-ventilated oven at 105 ± 1°C temperature for 24 h. Dried Makhana seed kernels were conditioned by adding an estimated quantity of water, mixing thoroughly, and then sealing in individual polyethylene bags. All the physical and fracture properties were determined at the moisture contents of 10.28, 11.89, 13.32, 14.22, and 15.15% (dry basis).

Physical properties

To determine the size of Makhana seed kernels, a sample of 30 kernels was randomly selected. The three major perpendicular dimensions of the Makhana seed kernels namely length (L), width (W), and thickness (T) were measured using a vernier caliper (Mityoto, Japan; least count, 0.02). The geometric mean diameter D_g, arithmetic mean diameter D_a, and the equivalent mean diameter D_p of the seed was calculated by using the following relationship.^[16,17]

(1)

(2)

(3)

where, L is the length, W is the width, and T is the thickness in mm, D_g is the geometric mean diameter, D_a is the arithmetic mean diameter, D_p is the equivalent mean diameter.^[18,19] The sphericity was computed using the following expression:^[18]

(4)

To determine the angle of repose (θ), a cuboidal glass container of 125 × 106 × 100 mm provided with a removable front panel was used. Makhana seeds were filled into the container up to the top and allowed it to fall freely by opening the front panel and thus, the slope formed was measured for its perpendicular height and base. The angle of repose (θ) of Makhana seed kernels was calculated using the following formula.^[20]

(5)

where, P is the height of slope, B is the base of slope. The static coefficient of friction (µ) of Makhana seeds was determined on three structural surfaces: mild steel, stainless steel, and glass, the regular materials used for storage and processing of grains using a laboratory set up according to Jha.^[20] The bulk density, the ratio of the mass sample of the kernels to its total volume, was determined by filling a 1000 mL container with kernels from a height of about 15 cm, striking the top level and then weighing the contents. The bulk density of kernel was computed from the values of kernel apparent density using the following relationship given by Omobuwajo et al.^[21]

(6)

where, ρ_b is the bulk density, M is the total mass of grain, and V is the total volume of grain. The apparent density defined as the ratio of mass of the sample to its kernel volume was calculated by toluene displacement method as:

(7)

where, _t is the apparent density, M_s is the mass of solid particle, and V_s is the volume of solid particle. All the experiments were replicated three times for each Makhana seed samples, and the average values were reported.

Force-deformation characteristics

For fracture studies, the conditioned samples were visually inspected prior to loading, and those with cracks on the hull or kernel were discarded so that the maximum force and the deformation that the seeds can withstand prior to rupture can be considered. Quasi-static compression tests were performed with a food texture measuring instrument (LLOYD-LR-5K) equipped with a 5000 N compressive load cell and an integrator. Each individual seed was loaded between two parallel plates and compressed at a fixed cross-head speed of 10 mm/min^[22] until the hull ruptures. It was assumed that rupture occurred at the bio-yield point, which is the point in the force-deformation curve where there is a sudden drop in force. As soon as the bio-yield point was detected, the compression test was stopped. Twenty seeds were tested at roasting temperatures of 150, 175, 200, 225, and 250°C each for a duration of 2 and 4 mins, respectively, giving a total of 200 test samples. The energy absorbed during the loading up to rupture was calculated from the area under the load-deformation curve^[18] computed as:

(8)

where, E_a is the rupture energy, F is the force, D is deformation.

Results and discussion

Physical properties of Makhana

Tables 1 and 2 show the physical properties of Makhana at different moisture content. The Makhana seeds differed physically in sizes and other properties as indicated by significant mean difference (α < 0.05) and sample size (n = 5). The angle of repose and the static coefficient of friction values of Makhana seeds determined on three structural surfaces: mild steel, stainless steel, and glass are given in Table 2. It can be seen that with increase in moisture content the angle of repose increased. This may be attributed to the fact that an increase in moisture content increased the cohesion between the seeds, thus increasing the friction the seed experiences during its movement. The static coefficient of friction increased with increase in moisture content. This may be due to the fact that an increase in moisture content increased the cohesion between the seeds, thus increasing the friction the seed experiences during its flow/movement on the respective surfaces.^[23] Similar observations were made by Akcali et al.^[24] for pea nuts, Karababa and Coşkuner^[25] for dry sweet corn kernels. They found that the friction coefficient was influenced by the materials in contact to a great extent and in the angle of repose increases linearly with the increase in moisture content.

Table 1. Axial dimensions of Makhana seeds as influenced by moisture content.

Moisture content (% d.b.)	Length (mm)	Breadth (mm)	Thickness (mm)	Da (mm)*	Dp (mm)*	Dg (mm)*
14.5	12.06	11.88	11.88	11.82 ± 0.98	11.88 ± 0.95	11.80 ± 0.98
14.3	11.6	11.6	11.18	11.46 ± 0.97	11.4 ± 0.96	11.43 ± 0.99
13.8	11. 29	11.07	10.71	11.02 ± 0.95	11.02 ± 0.97	11.02 ± 0.98

*Values are represented as means ± standard deviation (n = 5).

Table 2. Physical properties of Makhana.

Sl No	Moisture (% d.b.)	Sphericity	Angle of repose (°)	Static coefficient of friction			Bulk density (kg/m^3)	True density (kg/m3)
				Mild Steel	Stainless Steel	Glass
1	14.5	0.975	50.19	0.29	0.27	0.29	638.46	1106.66
2	14.3	0.985	48.24	0.27	0.25	0.26	587.12	1090.10
3	13.8	0.978	46.12	0.25	0.23	0.24	515.38	957.142

Force-deformation characteristics

Force-deformation characteristics of Makhana seeds at different moisture content under compressive loading and the energy absorbed are shown in Figs. 1 and 2 Table 3 gives the average values of rupture force, deformation at different moisture contents, roasting temperatures, and time. It was observed that within the range of roasting time and temperature, the compressive force on the seeds increased with an increase in deformation. There was a decrease in the force after rupture occurred in the specimen and the point at which this rupture occurs is normally referred to as the bio-yield point.^[22] At higher moisture content, the yield point was the point at which cracks developed at the surface of the kernel, whereas at lower moisture content it was the point where the hull split in halves without disturbing the kernel. Deformation at hull rupture decreased as the moisture content decreased and it ranged between 3.14 to 1.98 mm.

Table 3. Effect of temperature on moisture content, rupture force, and deformation at rupture for makhana seed hull.

Sample	Temperature °C	Moisture content (% d.b.)	Rupture force (N)*		Deformation (mm)*		Rupture energy (mJ)
			2 min	4 min	2 min	4 min	2 min	4 min
1	150	15.15	588.59 ± 2.3	557.81 ± 3.2	3.66 ± 0.6	2.95 ± 0.4	1077.12	822.77
2	175	14.22	542.36 ± 3.1	525.68 ± 2.8	3.39 ± 0.4	2.86 ± 0.5	919.30	751.72
3	200	13.32	510.45 ± 1.9	489.29 ± 3.5	3.20 ± 0.5	2.31 ± 0.2	816.72	565.13
4	225	11.89	495.29 ± 2.5	449.03 ± 2.4	2.81 ± 0.3	2.01 ± 0.3	695.88	451.27
5	250	10.28	433.45 ± 2.8	369.42 ± 2.6	2.71 ± 0.5	1.69 ± 0.4	587.32	312.16

*Values are mean ± standard deviation (n = 5).

Figure 1. Force-deformation characteristics of Makhana seed under different period of roasting.

Figure 2. Effect of moisture content of Makhana seed on energy absorbed per unit volume at rupture.

The force required to rupture the hull of Makhana seeds decreased as the roasting temperature and time increased. It was found to be 369.4 N at 250ºC and 4 ± 0.5 min as compared to seeds treated without any roasting requiring 682.2 N indicating the energy absorbed at the rupture per unit volume of seed also decreased as the moisture content decreased which varied between 95 and 40 mJ. This decreased in rupture force with increase in roasting temperature and time, might be due to the brittle characteristics of the hull. Koya et al.^[26] presented the compressive strength properties of sponge gourd (Luffa aegyptica) seeds to facilitate the design or adaptation of an appropriate dehuller and found compressive strength of the seed varied with moisture content and geometric properties which are in close agreement with the present study.

Size distribution and morphology

The size and morphological changes of Makhana seeds during the roasting process can be seen in Fig. 3. The Makhana seeds treated at 150°C are unevenly cracked. When subjected to increased temperature of 250°C, the seeds appear to be fractured and swollen, agglomerating into larger more amorphous particles.

Figure 3. Microscopic view of ruptured Makhana seed roasted for 4±0.5 min at (a) 150°C and (b) 250°C.

Conclusion

The compressive force needed to initiate rupture of Makhana seed decreased with a decrease in moisture content of the seed while the deformation at rupture decreased. Energy absorbed per unit volume increases with an increase in moisture content of the seed. At moisture content of 15.2% db, the seed hull requires more force, which damages the starchy kernel as well. With lower moisture content, kernel gets separated from the hull and the hull becomes more brittle, which decreases the load required to break the hull. Studies on fracture behavior at different load conditions such as impact loading, quasi-static loading, and energy consumption will be very useful in rational design of efficient process optimization and post harvesting systems of Makahan seeds and similar food materials. The seed is a good source of protein and fat. After frying, the seeds are used as snacks as well as in the preparation of vegetable dishes and can be used to develop into a snack food at the industrial scale. Proximate values of the protein, oil, and carbohydrate contents of the seeds suggest that they may be adequate for the formulation of animal feeds, subject to knowledge of the levels of possible toxic substances.