Journal of Food and Nutrition

Abstract

Quality evaluation of nutrient and phytochemical properties of melon, water melon and cucumber seeds were carried out. The seeds were respectively extracted from the fruits and subjected to proximate, minerals, vitamins and phytochemical analysis while the flour produced from the seeds were evaluated for their respective functional properties. All the analysis were done following standard methods. The proximate composition of the flour samples was found in the range of 3.75 - 12.27% (moisture), 0.81 – 50.96% (fat), 2.48 – 9.48% (Ash), 2.37 – 9.44% (fibre), 23.32 – 29.08% (protein) and 14.77 – 50.51% (carbohydrate). The proximate parameters of the samples showed significant (p < 0.05) differences from one another. The iron, zinc, potassium, calcium and magnesium contents of the melon, water melon and cucumber varied from 9.52 – 77.29mg/100g, 5.16 – 46.59mg/100g, 391.92 – 738.98mg/100g, 8.76 – 166.25mg/100g and 68.36 – 538.05mg/100g respectively. Vitamin C was found to be more concentrated in cucumber seed while vitamin B1, B2 and B3 were higher in melon seeds. The results of the functional properties revealed that oil absorption capacity, swelling index and bulk density of melon seed were significantly (p < 0.05) higher than those of water melon and cucumber while the foaming and water absorption capacities were significantly (p < 0.05) higher in cucumber seeds. Evaluation of the phytochemical contents revealed that water melon seed has the highest concentration of phenol (0.81mg/100g), phytate (0.49 mg/100g), tannin (0.17 mg/100g), saponin (0.89 mg/100g) and flavonoid (1.37 mg/100g). The results clearly showed that melon seed, water melon seed and cucumber seeds have high nutritional significance and could be incorporated in the formulation of other foods especially where micronutrients and phytochemicals are needed for human consumption as well as feeds, for animals.

Introduction

Fresh fruits and vegetable juices are important parts of modern day diet in many parts of the world. They are rich sources of nutrients such as vitamins, minerals and other naturally occurring phytochemicals which are of health and therapeutic benefits (Ukwo et al., 2011; Aderiye et al., 2020). Consumption of food rich in phytochemicals such as phenolics found in citrus fruits and lycopene found in tomato and watermelon have long-term benefits to human health (Elumalai et al., 2013). Phytochemicals are bioactive compounds of plant origin and are regarded as secondary metabolites (Wang et al., 2013). They have antioxidant, anti-inflammatory, hypotensive properties and may also enhance the immune system function and inhibit tumor progression in some cancers (Sivudu et al., 2014; Aderiye et al., 2020).

The significance of fresh fruits and vegetables in our daily food intake cannot be overestimated (Pamplona-Roger, 2008). Consumption of fruits and vegetables at least 400g daily as well as whole-grains, cereals and legumes at least 30g daily has been recommended as optimum diet for everyone (WHO/FAO, 2003). This recommendation supports the daily or regular consumption of foods like melon, water melon, cucumber, etc.  Melon (Curcubita citrullus L.) and water melon (Citrullus lanatus) belong to the family of Cucurbitaceae (Abbah et al., 2014; Amadi et al., 2018). Melon is generally used as soup thickener and fermented soup condiment. The seeds are good source of protein, oil and they possess antioxidant, analgesic and anti-inflammatory properties (Chen et al., 2014; Amadi et al., 2018).

On the other hand, watermelon (Citrullus lanatus) is a pleasant-tasting fruit and one of the most economically important fruit in the Curcurbitaceae family. The fruit has both nutritional and medicinal values (Gwana et al., 2014). The plant contains a significant amount of citrulline for improvement of erectile dysfunction. It possesses high level of antioxidant which decreases the risk of kidney stone and bone loss due to old age. It is a powerful diuretic diet with sufficient amino acid and beta-carotene which prevents ailment such as heart diseases (Olayinka and Etejere, 2018). Cucumber (Cucumis sativus) on its own is also an important vegetable and a member of Curcurbitaceae family. Like other members of Curcurbitaceae, its fruit has high water content of about 95%. Its rind has high potassium and magnesium contents which help to relax nerves, muscle and keeps blood circulating smoothly (Muhammad et al., 2014). It offers great support to digestion of foods in human system and aids emptying of the bowels.

Watermelon juice is highly appreciated but its seeds are discarded by some consumers, probably due to ignorance of its nutritional and health benefits (even therapeutically). Discarding of water melon seeds is economic and postharvest lose. It attracts flies and constitute environmental nuisance. Also pertinent comparative evaluation of melon, watermelon and cucumber seeds is not widely known to consumers. This paucity of information obviously does not support their wider use domestically and industrially.

The results of this study will reveal the nutritional values of seeds from melon, watermelon and cucumber.  The results would further encourage the consumption of the seeds and help prevent postharvest loses especially from watermelon seeds which are merely discarded by some people while consuming the pulp or juice. This research would also enlighten consumers on the importance of consuming such products as single strength or composite. Revelation of the respective nutrient potency of the seeds (melon, watermelon and cucumber) will enable consumers make their choices or consume each of them with specific interest. Furthermore, highlighting the phytochemical constituents of the seeds will depict their potential health benefits while ascertaining their functional properties will definitely unfold their potential industrial applications. The economic importance of the seeds will generally improve. Therefore the objective of this work is to comparatively evaluate the nutrient, phytochemical composition and functional properties of melon, watermelon and cucumber seeds. 

Materials and Methods

The melon (Curcubita citrullus L.), watermelon (Citrullus lanatus) and cucumber (Cucumis sativus) fruits were purchased from Ama-Ekwe Market, Umuisiama-Obokwu Avu in Owerri West Local Government Area, Imo State. The fruits were sorted, cleaned and washed to remove sand and other adherents. They were then dissected and their seeds extracted. The extracted seeds were respectively analyzed for proximate composition, vitamins, minerals and phytochemical concentrations following the methods of AOAC (2010). Data generated were subjected to analysis of variance (ANOVA) while Tukey test was used to separate the means and significant difference was determined at p < 0.05.

Results and Discussion

Proximate Composition of the Seeds

The results in Table 1 show that the moisture content of the seed samples was respectively 6.46%, 12.27% and 3.75% for watermelon, cucumber and melon. There were significant (p < 0.05) differences among all the seed samples. The cucumber seed had the highest (13.27%) moisture content followed by the watermelon seed with value 6.46% while the melon seed had the least value (3.75%). The value found for cucumber in this study is comparable with 12.30% reported by Oluwagbenle et al. (2019). There were significant (p < 0.05) difference among all the seed samples in terms of fat. Melon seed had the highest fat content (50.96%) followed by the watermelon (22.11%) while the cucumber seed had the lowest fat content (0.81%). The high value of fat in the melon seeds justifies its inclusion as oils seed.

The fat content of the cucumber was comparable to 0.08 - 0.98% reported by Waziri and Saleh (2015) while 50.96% discovered in melon seed was comparable to 49.05% observed by Jacob et al. (2010) but it is higher than 40.26-45.21% reported by Abiodun and Adeleke (2010). It was revealed that the least (2.48%) ash content of the seed samples was recorded in melon seed followed in ascending order by 3.71% and 9.48% found in watermelon and cucumber seed respectively. This implies that cucumber seed is a better source of ash compared to the watermelon and melon seed.  The range of the crude fibre content in this study is 2.37% to 9.44%. The result indicates that watermelon seed had the highest (9.44%) fibre content followed by cucumber seed (3.59%) while the least fiber content (2.37%) of the seeds was obtained from melon seed.  According to Jacob et al. (2010), fibre reduces the level of cholesterol in human blood and decreases the likelihood of different cancers. The implication is that watermelon seed may perform this function more effectively compared to cucumber and melon seeds.

Table 1: Proximate composition of the seeds (%)

Values are means of triplicate analysis and standard deviation. Means with different superscript along the columns are significantly (p < 0.05) different.

In terms of protein content, the result indicates that the cucumber seed had the least protein content (23.32%) followed by melon seed (25.67%) while watermelon seed had the highest (29.08%) protein content. The protein content is comparable to 28.63% reported for Colocynthis citrullus seeds by Bankole et al. (2005), 30.63% for Citrullus lanatus by Jacob et al. (2010) and 27.48% for Cucurbitapepo L seeds reported by Elinge et al. (2012). The findings in this study proves that water melon seeds, melon seeds and cucumber seeds would be good sources of protein for humans, especially vegetarians. The results also justifies the assertion by Jacob et al. (2010) that Citrullus lanatus (melon) seeds could provide the necessary protein requirement for the rural populace. The values of the carbohydrate content of the seed samples include 29.18%, 49.51% and 14.77% for watermelon, cucumber and melon seeds respectively. The cucumber seed had the highest value (49.51%) followed by the watermelon seed with value (29.18%). The results proved that melon and water melon seeds cannot be considered as good sources of carbohydrate especially when compared with other sources like cereals and tubers.

Mineral Content of the seed samples

Cucumber seeds had the least iron content (9.52mg/100g) followed by melon seed (42.16mg/100g) while the highest value (77.29mg/100g) was found in watermelon seed. The implication is that watermelon seed is a better source of iron compared to the cucumber and melon seeds. Iron helps in the formation of blood and in the transfer of oxygen and carbon dioxide from one tissue to another (Guthrie, 2000). Iron is an essential mineral micronutrient in life that participates in the critical metabolic progress in our body, including electron transport of the respiratory chain, oxygen transport, enzyme catalysis, and gene synthesis (Abbaspour et al., 2014; Vogt et al., 2021).  Iron deficiency is a severe nutrition deficiency problem worldwide, mainly causing iron deficiency anemia (IDA), neurodevelopment delay and even death, especially among pregnant women and children (Camaschella, 2019; Georgieff, 2020).

The zinc content of the seed sample were 46.59mg/100g, 05.16mg/100g and 19.14mg/100g for watermelon, cucumber and melon seeds respectively. The highest value (46.59mg/100g) was recorded in the watermelon seed while the least (05.16mg/100g) was found in the cucumber seed. The zinc content (5.46mg/100g) for cucumber seed found in the study by

Table 2: Mineral composition of the seed samples (mg/100g)

Values are means of triplicate analysis and standard deviation. Means with different superscript along the columns are significantly (p < 0.05) different.

Oluwagbenle et al. (2019) is comparable with 05.16mg/100g recorded in this study. Also the zinc content (21.05mg/100g) of melon seed recorded by Jacob et al. (2010) is slightly higher than 19.14mg/100g found in this present work. According to Abitogun et al. (2010), zinc is essential element in protein and nucleic acid synthesis. Zinc boosts the health of our hairs, plays a role in the proper functioning of some sense organs such as ability to taste and smell (Payne, 1990), helps in carbohydrate and protein metabolism and also assists in metabolism of vitamin A from its storage site in the livers and facilitates the synthesis of DNA and RNA necessary for cell production (Guthrie, 2000). Of all the minerals analyzed, potassium was most abundant. The watermelon, cucumber and melon seed samples contained 410.71mg/100g, 391.92mg/100g and 738.98mg/100g, respectively. Potassium is very important in maintaining body fluid volume and osmotic equilibrium, the pH of the body, regulation of muscles and nerve irritability, control of glucose absorption and enhancement of normal retention of protein during growth (Oluwagbenle et al., 2019).

Comparatively, water melon seed may not be regarded as a good source of calcium since it contains 8.76mg/100g whereas melon seed and cucumber seed had 34.69mg/100g and 166mg/100g each.  Calcium is a constituent of bones and helps the body to contract correctly, blood to clot and the nerves to convey messages. It is essential for disease prevention and control and may therefore contribute to the medicinal influences of the plant (Aliyu et al., 2008). The result also indicated that melon seed had the highest magnesium content (538.05mg/100g) while the lowest (68.36mg/100g) was found in the cucumber seed. According to Jacob et al. (2010) the daily requirement for magnesium is 400mg for adults and children aged 4 or older. This implies that melon seed is a richer and dependable source of magnesium compared to watermelon and cucumber seeds. Magnesium is beneficial to blood pressure and helps to prevent sudden heart attack, cardiac arrest and stroke. Like calcium, magnesium is an important component of bone and contributes to its structural development while calcium stimulates muscles, magnesium relaxes the muscles (Guthrie, 2000).

Vitamin Composition of the Seed Samples

The vitamin C content of the seed samples were 6.25mg/100g, 7.79mg/100g and 5.99mg/100g for watermelon, cucumber and melon seeds respectively (Table 3). The melon seed was found to contain the lowest (5.99mg/100g) vitamin C compared to watermelon and cucumber seeds. The cucumber seed had the highest value (7.79mg/100g) followed by the watermelon seed with value (6.25mg/100g). The vitamin C content of the seed samples were high compared to 4.74mg/100g and 3.78mg/100g reported by Olayinka and Atejere (2018) for cucumber pulp and rind. Vitamin C is an important water-soluble vitamin that has being implicated in many life processes apart from its antioxidant property (Abitogun et al., 2010).

The study also proved that watermelon, cucumber and melon seeds contain 0.04mg/100g, 0.03mg/100g and 0.05mg/100g of thiamin (vitamin) B1 respectively. It was observed that the vitamin B1 values were the least of all the vitamins analyzed.

Table 3: Vitamin composition of the seed samples

Values are means of triplicate analysis and standard deviation. Means with different superscript along the columns are significantly (p < 0.05) different.

Thiamine plays an important role in the utilization of carbohydrate for supply of energy, where it functions as the coenzyme thiamine pyrophosphate, or cocarboxylase, in the oxidation of glucose (Norman and Joseph, 2007). Absence of vitamin B1 results in a specific deficiency disease called beriberi.

Cucumber seed had the lowest riboflavin (vitamin B2) content (0.09mg/100g) followed by watermelon seed (0.17mg/100g) while the highest vitamin B2 content (0.28mg/100g) was found in melon seed. The United State recommended dietary allowance (USRDA) for vitamin B2 is 1.70 mg/100g (Usman, 2012), this implies that all the seed samples analyzed are poor sources of vitamin B2. Riboflavin helps in the oxidation processes of living cells and is essential for cellular growth and tissue maintenance (Norman and Joseph, 2007).

The vitamin B3 content of the seed samples as shown in Table (3) are 3.26mg/100g, 0.52mg/100g and 3.88mg/100g for watermelon, cucumber and melon seeds respectively. The low values obtained proved that cucumber seed is not a good source of vitamin B3 compared with water melon and melon seeds. Vitamin B3 is important as it helps the body to convert food into glucose, used to produce energy, maintain normal skin and mucous membranes as well as aid the function of the nervous system (Asdrubal, 2012).

Phytochemical Properties of the Seed Flour samples (mg/100g)

Water melon, cucumber and melon seeds were analyzed for their respective content of some phytochemicals (Table 5). Watermelon seed recorded the highest phenol content (0.81mg/100g) followed by melon seed with value 0.66mg/100g while the cucumber seed had the least value (0.52mg/100g). The relationship between phenol content and antioxidant activity has been studied in different food stuff (Jayaprakasha et al., 2008). Antioxidant activity of food stuff increases with the presence of total phenolic and flavonoid content (Okafor et al., 2015). Therefore the presence of phenol in seed samples indicates that the seeds especially the watermelon seed would have the potential for antioxidant activities. The analysis also revealed that watermelon seed had the highest phytate content (0.49mg/100g) followed by the cucumber seed (0.40mg/100g) while melon seed had the lowest phytate content (0.37mg/100g). Phytate at high concentration bind to iron and calcium and could prevent their absorption into the body causing deficiencies (Phillippy et al., 2012) but at low concentration (as obtained in this study), phytate (or phytic acid) is thought to have a positive impact as an antioxidant, effective in the prevention of carcinogenesis.

Table 5: Phytochemical properties of the seed samples

Values are means of triplicate analysis and standard deviation. Means with different superscript along the columns are significantly (P < 0.05) different.

The tannin content of the seed samples were 0.17mg/100g, 0.12mg/100g and 0.15mg/100g for watermelon, cucumber and melon seeds respectively. According to Jacob et al. (2010) high value of tannin in foods interferes with protein absorption and digestive enzymes. Tannins inhibit the activities of some enzymes like trypsin, amylase, lipase resulting from the formation of complexes with protein (Adewusi et al., 2008). The saponin content of the seed samples was within 0.35mg/100g to 0.89mg/100g. The values are 0.89mg/100g, 0.35mg/100g and 0.84mg/100g for watermelon, cucumber and melon seeds respectively. This indicates that the highest saponin (0.89mg/100g) was found in the watermelon seed followed by the melon seed with value 0.84mg/100g while the least value (0.35mg/100g) was obtained from cucumber seed. According to Okafor et al. (2015) saponin has been reported to show medicinal properties. It also serves as antibiotic, helping the body to fight infection and microbial invasion (Nijveldt et al., 2001).

It was observed that the concentration of flavonoid was the highest of all the phytochemicals analyzed. Watermelon and melon seed samples had no significant (p>0.05) difference but they differed significantly from cucumber seed flour in terms of flavonoid content. The result indicates that the watermelon seed had the highest flavonoid (1.37mg/100g) followed by the melon seed with value of 1.33mg/100g while the lowest value (1.09mg/100g) was found in cucumber seed. Studies revealed that the consumption of flavonoids can be used in the management of coronary heart disease (Kneki et al., 1996). This further encourages the consumption of the water melon, cucumber and melon seeds. From the result in Table (5), the alkaloid content of the seed flour samples are 0.32mg/100g, 0.36mg/100g and 0.31mg/100g. The statistical analysis revealed that there were no significant (p>0.05) difference between alkaloid content of watermelon and cucumber seeds as well as between watermelon and melon seeds. Therapeutically, alkaloid has been speculated widely to be ranked the most effective phytochemical for analgesics, antispasmodic and bactericidal effect (Okafor et al., 2015). The alkaloid value is low compared to 4.40% recorded by Okafor et al. (2015) on the quantitative analysis of watermelon peel extract.    

Conclusion and Recommendations

The findings made of this study have shown that watermelon, cucumber and melon seeds are good sources of vitamins and minerals. The analytical information has showed that watermelon and melon seeds are good sources of protein and fat. Also, they are highly rich in iron, potassium, magnesium, vitamins C and B3 needed for blood formation and transportation of oxygen and carbon dioxide between tissues. However, results indicate that the seeds are poor sources of vitamins B1 and B2 especially cucumber which is also low in zinc and iron. The phytochemical concentration of the seeds respectively justifies their safe consumption with or without heating and this in addition to the nutrient content suggest their potential application in food formulations. It is therefore recommended that the findings of this study be disseminated to encourage the consumption of the seeds and discourage littering of the environment especially with the cucumber seeds. It is also recommended that the seeds be used for fortification of foods to combat micronutrient deficiency.

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