Introduction

Kendu (Diospyros melanoxylon Roxb.) fruit is consumed by the local tribes in different traditional forms such as pulp, paste, powder and fruit wine. It belongs to Ebenaceae family and Diospyros species. It is native to India and Sri Lanka.^1,2 It is also available in Philippines, Japan, China, and Thailand. Its trade name is ebony, tendu/kendu and is also called as coromandel ebony and contributes to socio-economic livelihood of tribal people in India.^3,4

Kendu is a seasonal fruit available mainly in summer during May and June.⁵ The leaf, bark and fruit of the Kendu plant have also been used for traditional medicines.^6,7 The leaves are commonly used for making bidis (an indigenous traditional cigarette, which uses the kendu leaf for rolling instead of paper).^8,9 The fruit has a pleasant and sweet taste when fully ripen. The fruits are gathered by the tribes in summer and are dried and stored for use in off-season. Kendu fruit is globose in shape with 3-4 cm diameter (Fig. 1). It usually contains 3 to 4 seeds which are brown in colour, compressed and oblong in shape.^10,11 The plant is distributed all across the Indian states of Bihar, Madhya Pradesh, Chhattisgarh, West Bengal, Odisha, and Jharkhand.^9,12,14 There are many nutritional use of various parts of this plant starting from leaves, fruits, bark and seeds. Leaves are used as styptic, in the treatment of scabies and old wounds, and as laxative and carminative medicine.⁷ Kendu fruit also helps in stomach disorders. The dried fruit powder is used as carminative and astringent agent and is useful in treating urinary, skin and blood diseases. This may be attributed to high tannin content (15-23 %) of the fruit.¹² Kendu bark also contains various steroids, alkaloids, glycosides, proteins, phenolic compounds and tannins.^4,5

However, due to lack of proper processing techniques and consumer awareness on its nutritional aspects, most of the fruits are goes wasted and are forced to be utilised in a non-economical way.^3,4 There is a dearth of scientific literatures on the chemical and nutritional aspects of Kendu fruit. The information on the physico-chemical, textural and nutritional properties of Kendu would add to the scientific knowledge base. These information can be used for developing suitable techniques and practices for post-harvest management and processing of the fruit.

Materials and Methods

Sample Preparation

Kendu fruits were procured from local market of Rourkela (22°14′57″N, 84°52′58″E), Sundergarh, Odisha, India for further analysis of various physico-chemical, textural and nutritional properties. The average initial moisture content was determined by hot air oven method following AOAC,2000 protocols.¹⁵ The fruits were weighed using an analytical balance (WENSOR, Model No: IND/09/08/558) with least count of 0.001g.

Physical Properties

Principle dimensions of fruit such as length (L) i.e. axial ends, and diameter (D) i.e. radial end, were measured using digital vernier calliper having a least count of 0.01cm. The principal dimensions were used to calculate the arithmetic (D_a) and the geometric (D_g) mean diameters. Sphericity  and surface area (SA) were computed using the following relations that are previously used for spherical fruits¹⁶;

Dg = (L D²) ^1/3         (2)

SA= π (L ^2/3 x D ^4/3)      (3)

The fruits were divided into three categories based on its size (i.e. Arithmetic mean Ẋ) satisfying the following conditions;

Small: X < Ẋ – σ                                                           (4)

Medium: Ẋ- σ < X < Ẋ + σ                                        (5)

Large: X > Ẋ + σ                                                         (6)

Where, X and σ are the experimental value and standard deviation, respectively.^17,19 The porosity (e) of bulk fruit was computed from the values of true density (pt)  and bulk density (pb)  using the relationship given as below²⁰;

Angle of repose of the whole fruit was determined using the formula²⁰;

Where, H is the height of the cone and D is the diameter of circular plate of apparatus.

Angle of inclination of the fruit was measured on three different surfaces such as plywood, aluminium and mild steel sheet .The measurements were made in two directions (radial and axial) with respect to calyx end and static coefficient of friction of whole fruit was determined from the following relationship¹⁶;

μ = tan α                (9)

Where, µ is the Static coefficient of friction and α is the angle of tilt in degree.

Colour of the fruit was measured using colorimeter (Flex EZ, Hunter Lab, USA). Fruit skin colour measurements were taken along the equatorial axis of each fruit. Readings of each colour index in the Hunter scale (L, a, b) were taken per fruit with three replications. The colour values represented whiteness or brightness/darkness (L), redness/greenness (a) and, yellowness/blueness (b).

Mechanical Properties

Mechanical properties were measured using a texture analyser (CT3, Brookfield, USA) with a 6 mm diameter stainless steel probe (TA-41) and a 10 kg load cell. Insertion of probe through the surface, was taken as the surface hardness (N) of the fruits. Average force recorded by the probe is presented as pulp firmness (N) i.e. the energy (N/mm) required to pass the probe through the sample was measured.^16,18 The operating conditions such as pre-test speed (0.5 mm/s), post-test speed (0.5 mm/s) and trigger load (0.05 N) were fixed.

Figure 1: Kendu fruit  Click here to View figure

Figure 2: Frequency distribution curve of kendu fruit dimensions at 66.17 ±1.84 %.( db.) moisture content  Click here to View figure

Figure 3: Hardness properties of fresh kendu fruit at 66.17 ±1.83 %.( db.) moisture content   Click here to View figure

Chemical Properties

Proximate Composition

Proximate parameters of the sample such as moisture, ash, crude protein and fibre, fat, ascorbic acid and, carbohydrate, were determined using AOAC, 2000.¹⁵ Total soluble solids (TSS, ^oBrix) was determined using a temperature compensating hand-held refractometer (ERMA Inc., Japan). Titratable acidity (TA) was expressed as percent citric acid. Total sugar was determined by spectrophotometric method at 630 nm using anthrone reagent.²¹ Reducing sugar was analysed by using 3, 5-Dinitrosalicylic acid (DNS) method.²² The energy value (E in KJ/100g) of the kendu pulp was measured using the following empirical formula.²³ All measurements were in triplicate using analytical grade reagents.

E = [(% CH X 17) + (% P X 17) + (% F x 37)]                                   (10)

Where, CH, P and F represents available carbohydrate, protein and fat content, respectively.

Antioxidant content

Total Phenolic content (TPC) was determined using Double Beam UV-VIS Spectrophotometer, (AU 2701, Systronics, India) by Folin-Ciocalteu reagent (FCR) method.²⁴ The carotenoid (Beta-carotene) content of fruits was measured in 85 % acetone extract of the fresh sample³⁴ 2, 2-diphenil-1-picrylhydrazyl (DPPH) radical scavenging activity was determined using DPPH with slight modifications.²⁵ Ferric reducing ability in plasma (FRAP) activity was measured by standard UV-spectrophotometry²⁶ with absorbance measured at 595 nm wavelength.

Mineral Content

The minerals (Ca, P, Mg, Fe, Zn, Na, Pb, and Cu) were analysed by atomic absorption spectrophotometry (AAS Analyst 200, Perkin Elmer, USA) by wet digestion method. Potassium (K) was analysed by the spectrophotometer method using ammonium molybdate solution.^27,28 All mineral standards used for this were of analytical grade (Sigma-Aldrich, India).

Results and Discussion

Physical Properties

Individual principle dimensions of fruit mass, length and mid-section diameter of the Kendu fruit were studied. Fruit size distribution at an average initial moisture content of 66.17±1.84 % (db.) is shown in Table 1. Fruit samples were divided into three sizes (small, medium and large) according to their arithmetic mean diameter (Da). The fruit has an average sphericity, volume and surface area of 0.97±0.02, 25.25±1.37 cm³, and 40.30±1.55 cm² respectively. Frequency distribution curves of length and width show more numbers of fruits of medium range (Fig. 2). The curves were narrow with high peaks, indicating that the seed has a dimension close to their corresponding means. About 63.4 % of the Kendu fruit samples were of medium size with average length and width of 3.61±0.13 and 3.57±0.13 respectively.

The average values of bulk density and true density were 1155.91±2.47 kg/m³ and 1280.90±3.81 kg/m³ respectively (Table 2). The average porosity of the fruit was found to be 11.51±0.90 %. These characteristics can be used to separate fruit from other foreign materials and can be helpful in air and heat flow studies. These parameters are important for determining the capacity of storage and transport.

Angle of repose for the average moisture content of 66.17±1.84 % (db.) was 25.50 ±1.22 ^o (Table I). The static co-efficient of friction was highest for plywood and lowest for aluminium sheet. The least static co-efficient of friction may be due to the smooth and polished surface of the aluminium sheet used. Due to the spherical nature of fruits, the rolling resistance was found to be low but the tendency to roll was higher when kept in radial direction than the axial direction. Similar results were found in other spherical fruit.¹⁶

Fruit colour and composition

The fruit pulp colour for fresh sample was determined on Hunter Lab. A yellowness coloration (b value) of the fruit pulp was found significant indicating the fruit pulp is yellow in colour (Table 2).  The fleshy portion of the fruit consists of pulp (44.23±0.03 %) and seed (25.53±0.59 %). Each fruit contains about three to five seeds.

Table 1: Physical dimensions on size distribution of Kendu fruit

*Values are means, ± Standard Deviation for three replicates

Table 2: Physical Properties of Kendu fruit

*Values are means, ± Standard Deviation for three replicates

Mechanical Properties

The force- deformation curve of kendu fruit shows surface or skin hardness was higher than the pulp (Fig. 3). Due to the fibrous nature of the fruit, the graph has several peaks. The non-uniform and continuous small peaks signifies the different harness value of different layers. The probe was subjected to a depth of 18 mm. The highest peak was found to be 9.50±1.85 N at a distance of 2 mm which was found to be the skin hardness. The pulp firmness was found to be 9.06±1.74 N which was higher in radial direction than the axial. This may be due to the orientation of fibre in the pulp. Similar trend has also been reported for nuts, strawberry, and mango.^28,30 Same range of fruit hardness was found oil palm fruits.¹⁸ There is an increase in load to compress the fruit as we go deeper due to its own compaction (Table 3). We can conclude that for an averaged size Kendu fruit, an average load of 17.49 ±1.58 N is required for ≥50% deformation.

Table 3: Compression test at five distances from the surface of Kendu fruit

*Values are means, ± Standard Deviation for three replicates.

Chemical Properties

The average moisture content, ash content, crude fibre, fat, carbohydrate, protein were analysed (Table 4).  Acid levels are expressed frequently in terms of pH and titratable acidity (TA) while sugar concentrations are reported as total soluble solids (TSS). The values of TA (%) and pH are 0.16±0.02 and 5.474±0.15 respectively. Similar range of values have been reported for mango, apple and medlar.^32,34

The TSS level of the pulp was 16.45±0.50 ^oBrix, which is in the same range of that of cherry and mango.³⁵ It is generally recognized earlier that the fruits have higher sugar-acid ratio that shows the fruit is of high quality.³⁶ TPC and β-carotene of fresh fruit was 125.912±0.56 mg GAE/100g and 416.04±1.08 µg/ 100g pulp respectively (Table IV). Similar values of phenolic content were found for guava, plum and star fruit,^36-37 and beta carotene content was found similar to different variety of mangoes.^38,39 The average value of DPPH scavenging activity of 83.806 ± 1.03 (%) was in the same range of orange, papaya, pineapple and apple. FRAP value of Kendu was 5876.02 ± 0.52 mg TE/100 g of DW. Similar value has been reported for kiwi, guava, red apple, banana.^40,41 The bark of the Kendu plant is reported to have total phenolic and ascorbic acid content of 44.32±0.103 (mg/g) and 5.12±0.117 (mg/g), respectively, much lower than that of the fruit.⁴²

Mineral composition of kendu fruit pulp is shown in Table 5. The fruit pulp has high content of calcium (58.42±0.50) and phosphorus (20.00±0.41). It can be a good source of calcium and phosphorus required for development and maintenance of bones. Similar trend of mineral composition can be found in mango and passion fruit.⁴¹ The potassium content of Kendu fruit is 123±0.01 mg/100g. The bark of the plant is reported to have only 0.55±0.04 mg/100g of potassium.⁴²

Table 4: Chemical and Antioxidant properties of fresh Kendu pulp

*Values are means, ± Standard Deviation for three replicates

TSS, total soluble solids; FRAP, ferric reducing antioxidant power; ORAC, oxygen radical absorbance capacity; TPC, total phenolic content; db- dry basis; KJ- kilo joule; mg GAE/100g – gallic acid equivalent per hundred gram dry weight; mg TE/100 g of DW- trolox equilvalent per hundred gram dry weight.

Table 5: Mineral composition of fresh Kendu pulp

*Values are means, ± Standard Deviation for three replicates

Conclusion

In conclusion, the physico-chemical parameters such as moisture content, ash, crude fibre, crude fat, protein, carbohydrate, total soluble solids (TSS), acidity, pH, and colour are important for further processing and value addition of Kendu fruit. A combination of a number of physico-chemical parameters are required to specify the quality of Kendu fruits. These properties were consistent with that of similar fruits. More studies on physical, chemical, textural and nutritional relationship among different varieties and maturity levels of Kendu need to be undertaken that may help in designing process and equipment for harvesting, transportation, storage and processing of fresh Kendu fruits. Proper value-addition techniques and processing facilities for this ethnic fruit can contribute to the socio-economic livelihood of tribal people.

Acknowledgements

The authors would like to thank Department of Food Process Engineering, National Institute of Technology, Rourkela, Odisha, India for providing laboratory facilities to conduct the experiments.

Conflict of Ineterest

The authors have no conflicts of interest to declare.

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