A Review on Food Values of Selected Tropical Fruits’ Seeds

Abstract

Fruit seeds are usually thrown out as waste during processing or after human consumption. Over the years, researchers have dedicated their effort to assess the food and nutritional values of many different fruit seeds. In this review, the research findings releated to the food values of ten different fruit seeds namely, guava, pumpkin, papaya, honeydew, mangosteen, rambutan, watermelon, mango, and durian were discussed. For several fruit seeds, the macro nutrient components such as oil, protein, and carbohydrate of are found to vary due to either varietal defferences or geographical variations. Among the seeds discussed, the range of oil content was from 1.8 to 49.0% while the range of protein content was from 6 to 40.0%. While the high oil bearing fruit seeds are potential new sources of oil, those with high protein content can be used for recovery of protein. As some of fruit seeds are edible and found to possess a host of phytonutrients, they can be harnessed for medicinal purposes. This review concludes that utilization of fruit seeds could not only bring health and wealth but also help to minimize the waste disposal problem of agro-based industries.

Keywords:

• Tropical fruit seeds
• Waste oil recovery
• Food ingredients
• Seed oil

INTRODUCTION

Asia is well-known for its rich fruit diversity with over 500 different species distributed in its diverse ecosystem.^[1] It has been reported that more than 70 different species of major and minor fruits are presently grown in the region, along with some other promising exotic tropical fruits. However, there are only about 20 species (banana, citrus, mango, pineapple, papaya, durian, rambutan, jackfruit, longan, tamarind, chempedak, langsat, guava, sour soup, salak, passion fruit), which are better known for large scale commercial cultivation.^[2] Normally, the tropical fruits such as guava, papaya, mango, pineapple, and durian are consumed in the fresh form. They are generally well-known for their nutritional values, which include vitamins and carbohydrates.^[3] Besides this, these fruits are also processed into end products such as jams, jellies, juices, pickles, and food flavors.^[1] It is commonly acknowledged that the fruit processing sector usually generates a large amount of seeds as wastes. In the past, investigations were undertaken to explore their potential uses in food applications due to their high nutritional value.^[4–7] For instance, papaya seeds have been used for decades as a vermi-fungal agent as well as a spicy flavoring substance. Owing to their abortive properties, they were also used as folk medicine to facilitate a good menstrual flow.^[8] Utilization of the seeds concurrently helps to deal with waste disposal problem, which causes environmental pollution.^[9] For this reason, many fruit seeds have been evaluated for specific industrial or food uses such as newer sources of oils.^[10,^11]

Oil is one of the important components of human diet since they are concentrated form of energy.^[12] Owing to the growing demand for vegetable oils, much interest is currently being focused worldwide on the possibilities of exploiting newer and underutilized plant resources for production of oils.^[13] With the new developments in the oilseed industry, the lipid quality and quantity of various fruit seeds can be assessed rapidly. Oil quantity varies depending on the types of seed, their size and shape^[14] while the quality characteristics of oils from different sources depend mainly on their fatty acid and triacylglycerol compositions. Apart from their nutritional role in human diet, oils are also utilized as food ingredients in delivering better texture, imparting flavor and mouthfeel in new formulations.^[15] There is no single oil that can satisfy all the requirements of the food industry.^[16] Since there has been an increasing nutritional interest for healthier oils with higher proportion of polyunsaturated fatty acids,^[17] a renewed focus is being given to the utilization of agricultural wastes as well as the utilization of byproducts from food processing.^[9,^18] The source of seeds such as guava,^[3,^19] pumpkin,^[20–23) okra,^[24,^25] and grape^[26–28] are reported to be highly stipulating due to their nutritional value.^[4] The objective of this article is to bring an update on the utilization of various fruit seeds as raw material for preparation of food ingredients.

Guava Seed

Guava, a member of the Myrtaceae family, is a crop widely grown in the tropical countries of the Asia and Africa. A quite a number of varieties of guava with differences in the color of the pulp, such as white, pink, red, etc., are grown in these countries.^[29] The most popular type of guava consumed by Malaysians is white guava while pink guava is grown for production of fruit juices. The content of vitamin C in guava has made it the richest source when compare to any other citrus fruit. Apart from the delicious taste when freshly consumed, guava pulp is also processed into products to be used in food and beauty care applications.^[29] The seeds of guava constitute about 6–12% by weight of the whole fruit.^[30] Being round in shape and pale yellowish brown in color, the seeds are reported to contain about 16% oil, 7.6% protein, and 61.4% crude fiber.^[19] Owing to this fact, guava seeds have the potential to become a source of oil that can be used in food products as well as supplement in dietary health. Zaini and co-workers^[5] made an attempt to utilize guava seed waste for formulation of fiber-enriched biscuits. The oil extracted from guava seeds are reported to possess phenolic compounds that display antioxidant properties such as radical scavenging activity.^[31] It has also been found to possess a higher iodine value (134.0 g I₂/100 g oil) with the refractive index of 1.4772 at 40°C. According to the fatty acid compositional analysis (Table 1), total unsaturated and saturated fatty acids in guava seeds oil were 87.3 and 11.8%, respectively. The major fatty acids of guava seed were linoleic acid (76.4%) followed by oleic (10.8%), palmitic (6.6%), and stearic acids (4.6%). According to Prasad and Azeemoddin,^[19] it is more or less similar to the fatty acid composition of safflower oil and can become a potential substitute for it. Until recently, there is hardly any attempt to utilize guava seed oil for preparation of food ingredients such as mono-acylglycerol (MAG) and di-acylglycerol (DAG).

TABLE 1 Fatty acid and other physico-chemical characteristics of fruit seed oils

Source of seed	Guava	Pumpkin	Papaya	Honeydew	Mangosteen	Rambutan	Water melon	Mango	Durian	Winter melon
Oil content (%)	12.6–16.0	27.83–45.4	29.16–30.7	25.0–32.3	21.8	38.0–38.9	22.1–35.65	8.05	0.50	10.9–53.76
Fatty acid:
Lauric acid (12:0)	–	–	–	0.014–0.2	–	–	–	–	–	–
Myristic acid (14:0)	0.1	0.1	0.2	0.073–0.2	–	0.13	–	–	0.12	–
Palmitic acid (16:0)	6.6–7.8	12.3–12.7	13.4–16.16	8.4–9.52	49.5	4.6	12.42–16.2	8.73	12.2	9.7–35.0
Palmitoleic acid (16:1)	–	0.1	0.2–0.27	0.083–0.3	Trace	0.72	Trace	–	1.15	0.21–0.3
Stearic acid (18:0)	4.6–5.8	5.4–6.99	4.6–4.73	4.6–6.089	1.33	7.88	10.2–13.8	37.7	1.42	3.0–14.0
Oleic acid (18:1)	8.2–10.8	36.2–37.8	71.3–76.9	16.8–31.5	34.2	43.09	14.2–23.0	44.75	8.42	7.0–26.2
Linoleic acid (18:2)	76.4–77.0	42.0–43.1	3.2–6.06	51.6–69.0	1.03	3.22	45.1–62.2	5.67	6.5	44.0–80.07
Linolenic acid (18:3)	0.1–0.3	0.3–0.36	0.2–0.22	0.19–0.2	Trace	0.74	1.02	0.59	11.3	0.1–0.13
Arachidic acid (20:0)	0.3–0.5	0.3	0.38–0.4	0.18–0.5	8.77	31.53	–	–	–	–
Eicosanoic acid (20:1)	–	–	0.3–0.32	0.11–0.164	0.1	5.89	–	2.48	–	–
Eicosadienoic acid (20:2)	–	–	–	–	0.11	–	–	–	–	–
Behenic acid (22:0)	0.1	–	0.23	Trace	Trace	2.10	–	–	1.21	–
Erucic acid (22:1)	–	–	–	0.247–0.4	–	0.10	–	–	–	–
Lignoceric acid (24:0)	0.1	–	–	0.12	Trace	–	–	–	–	–
Others	0.9	–	0.6	–	4.96	–	1.5–2.0	–	–	–
Iodine value	134.0	153.6	79.95	153.4	53.64	37.64–50.3	97.10–116.32	37.25	–	119.9–125.1
Saponification value	196.0	193.3–199.3	196.4	210.2	134.0	157.07–182.1	188.0–205.57	190.2	–	182.3–194.1
Unsaponifiable matter	0.49	0.67	1.35	0.9	–	0.5	1.02–4.30	–	–	–
Reference	[3,19]	[23,35,48]	[8,40]	[43]	[49]	[58]	[67,68]	[78]	[85]	[102]

Pumpkin Seed

Pumpkin, a member of the Cucurbitaceae family, is a diverse class of plants that consists of at least 119 genera and over 825 species.^[32] Pumpkin is popularly consumed as a vegetable since it is composed of several macro and micro nutrients useful for human health. The orange flesh of this fruit is usually covered by moderately hard rind, which even can be used for preparation of soup, purees, jams, etc. The central cavity of the pumpkin fruit consists of numerous seeds, which are flat, oval in shape, and light green in color^[22] with a testa that serves as a protectant around the seeds. The seeds are stated to contain about 27.83% crude oil, 39.25% crude protein, and 16.84% crude fiber.^[20] According to Soha and colleagues,^[33] they are natural sources of tocopherol and trace elements such as zinc. Pumpkin seeds are reported to have uses in culinary preparations mainly in the southern parts of Austria, Slovenia, and Hungary.^[22] For example, the kernels of pumpkin seeds have been utilized as additives in some food dishes.^[34] Being a food rich in oil and protein, pumpkin seeds are also consumed in significant amounts in these countries as a snack in the form of roasted seeds, rather than a source for oil extraction.

There are a number of reports on the chemical and nutritional characteristics of various pumpkin seed varieties as well as the edibility and stability of their oils. For instance, pumpkin seeds were reported to contain higher levels of potent phytochemicals, such as sterols, with a great promise for immunomodulation, reproductive health, as well as therapeutic effect over a range of disease conditions.^[33] According to some other studies, the intake of the whole extract of pumpkin seed is highly correlated with the reduced benign prostate hyperplasia-associated symptoms.^[35,^36] The capability of pumpkin seed as a stimulator of sexual desire has also been demonstrated in a number of studies.^[37] It is because of the fact that pumpkin seed oil has been found to possess a significant amount of vitamin E (tocopherol). Other than these, it has also been known to provide some other health benefits such as inhibiting the growth of the prostate. Despite the mentioned health benefits, pumpkin seed oil has been shown to exhibit anti-microbial activity. Hence, it is well-suited for improving nutritional benefits in food formulations.^[35]

Pumpkin seeds have been used in a number of countries either for oil or protein production. Yugoslavia, Austria, Hungary, and Nigeria are countries where pumpkin seed kernel has been used for extraction of edible oil.^[38] The production of oil from pumpkin seed provides the means of utilization of a renewable resource while adding value to the agricultural byproducts. Pumpkin seed oil varies from dark green to brown in color and the oil content of different varieties of pumpkin seeds were found to vary.^[35] Although the high iodine value (153.66 g I₂/100 g oil) of pumpkin seed oil indicated the high content of unsaturated fatty acids namely linoleic and oleic,^[38] these seed oils were still found to display good storage stability.^[22] Among the different fatty acids, the most abundant was oleic (44.11%), followed by linoleic (34.77%) and palmitic (15.97%).^[22] These results were in conformity with the findings of Stevenson et al.^[35] who also reported that the pumpkin seed oils contained a total of 73.1–80.5% unsaturated fatty acids, where the dominant were oleic and linoleic, followed by palmitic and stearic (Table 1). Although the high amount of linoleic acid makes pumpkin seed oil prone to oxidation, this fatty acid may have favorable nutritional implications and beneficial physiological effects in the prevention of cancer and coronary heart disease.^[39]

Papaya Seed

Carica papaya L. from the genus Carica has been distributed all over the tropical regions.^[8] The varieties cultivated extensively in Malaysia include Sekaki, Eksotika, Eksotika II, and Batu Arang. The flesh of papaya is not only consumed fresh, but it is also a raw material for jam, jelly, candies, and pickles. Although papaya is grown mostly for fresh consumption,^[8] it is also used for production of papain^[40] and cooking aids as well as medicinal products.^[41] Papaya fruit is a rich source of nutrients such as pro-vitamin A carotenoids, vitamins C, B, lycopene, minerals, and dietary fiber. Beta carotene, for instance, is already well-known to prevent damage caused by free radicals that may cause some forms of cancer. Papaya is laxative and a rich source of proteolytic enzymes that help in the digestion. Because of this reason, ripe fruit consumed regularly helps in habitual constipation.^[42]

Papaya usually possesses large amounts of seeds at the central cavity of the fruit, which is about 15% of the weight of the fruit. Papaya seeds are pungent and peppery making them to become unpalatable. However, the seed seems to have some potent medicinal values than the flesh. For instance, they have antibacterial properties and are against E. coli, Salmonella, and Staphylococcus infections. They are also believed to remove intestinal parasites, detoxify liver, and a remedy for toxin induced kidney failures.^[42] According to Malacrida et al.,^[8] papaya seeds were found to contain high concentrations of lipid (29.16%) and protein (25.63%). In a separate study, Samaram et al.^[14] reported that papaya has a potential to produce about 30–34% oils depending on the variety, seed size, and shape. Papaya seeds being rich in oil would make them economically attractive for industrial exploitation similar to other conventional oilseed crops such as corn (3.1–5.7%) and soybean (18–20%).⁽⁸⁾ As reported by the Puangsri et al.,^[40] the major fatty acids of the papaya seed oil were oleic (76.9%), followed by palmitic (13.4%), stearic (4.6%), and linoleic (3.2%) acids (Table 1). Apart from these, trace quantities of lauric, myristic, palmitoleic, margaric, linolenic, arachidic, and eicosenoic acids were also found. The lower content of linoleic acid in papaya seed oil indicated that it has a greater oxidative stability than any other seed oils.^[38] In a subsequent study, Malacrida et al.^[8] also confirmed that papaya seed oil exhibited a good oxidative stability without any added synthetic antioxidants. The consumption of high amounts of such oils might lead to a reduced risk of coronary heart disease. In addition, high-oleic oils have sufficient stability to be used in demanding applications such as frying. Another edible application is in the area of spray oils for snacks, crackers, cereals, and bakery products, where the oil is used to maintain product quality and improved palatability. The high proportion of unsaturated fatty acids would make the oil an acceptable substitute for other highly unsaturated oils. However, before being considered for food applications, toxicological studies are needed to be carried out to ascertain whether or not this oil is safe.^[40]

Honeydew Seed

Honeydew, a member of the Cucurbitaceae family, is one of the favorite fruits of Malaysia. Its fruit shape is round to oval while the pulp is sweet with a pleasant aroma.^[43] The greenish white color of its rind would become creamy yellow once it is ripened. In Malaysia, the fruit is commonly consumed fresh without processing into products. The hollow center of the fruits is found to contain large quantities of seeds, which are reported to have medicinal properties^[44] such as the potential anti-diabetic agent or beneficial remedy for treatment of chronic or acute eczema.^[42,^45] Medicinal properties of honeydew melon seed are due to the occurrence of phenolic glycosides in them.^[46] According to proximate compositional analysis, the seeds are found to contain about 25.0% oil, 25.0% protein, 23.3% crude fiber, and 19.8% carbohydrate.^[43]

In Malaysia, the seeds of honeydew melon are generally considered as waste to be discarded. If utilized for oil production, it could help to minimize waste accumulation in the edible fruit sector.^[43] The seed of honeydew melon is reported to have 49.4% total lipids (dry basis), which is comparably similar to those of other seeds such as sunflower, soybean, and peanut.^[4,^47] Hence, honeydew melon oil is an acceptable substitute for other highly unsaturated oils such as corn oil,^[48] and may also be used as salad oil. The extracted lipids were reported to have following physico-chemical characteristics: Appearance, light yellow at ambient temperature (25–30°C); odor, mild odor similar to the smell of ripen melon; refractive index 1.4662 at 40°C; iodine value (Wijs), 124.6 g I₂/100 g oil; free fatty acid content, 0.5 g oleic/100 g oil; unsaponifiable matter, 0.7%; and saponification number, 198.6. Honeydew seed has the potential to become a new source of edible oil due to high content of polyunsaturated fatty acids, which is an area of interest for researchers. The fatty acids content depends on the cultivar type and in majority of the cases the linoleic acid was consistently recorded as the predominant fatty acid (Table 1). According to Yanty et al.,^[43] the total unsaturated fatty acid content was roughly about 86.1% of which the predominant unsaturated fatty acids were linoleic acid (69.0%) followed by oleic acid (16.8%). Other than these, palmitic, palmitoleic, stearic, linolenic, and arachidic acids were present in lesser amounts (~5.6%).^[44]

Mangosteen Seed

Mangosteen (G. mangostana), a member of the Clusiaceae family, is one of the most widely recognized tropical fruits that has a universal appeal for its quality attributes such as color, shape, and flavor.^[49] It is rated as one of the most delectable of the tropics where the pulp is valued as one of the finest and most delicious of the fruits.^[50] Covered by a rounded berry and dark purple colored shell, the fruit has sweet, juicy flesh with a high content of sugars.^[51] Though slightly acidic, the pulp is generally sweet and delicious with an excellent flavor. The flesh of the fruit known as aril is reported to contain saccharose (10.8%), dextrose (1%), and kerrelose (1.2%).^[50] It is also reported to have medicinal properties such as anti-inflammatory, astringent, antibacterial, antitumor, and antioxidative.^[52]

Similar to other tropical fruits, mangosteens are generally consumed fresh and the seeds are discarded as waste. Although Mangosteen seed is a potential feedstock for biodiesel production,^[53] it is shown to be a promising source of carbohydrate and fat in food applications. The seed is stated to contain about 21.18% crude fat, 6.57% crude protein, 13.7% crude fiber, and 43.5% carbohydrates.^[49] It could also be utilized successfully as source of dietary fiber since it is reported to have relatively high crude fiber and carbohydrate contents, and can be used as roughage in feed formulation for livestock. Although low in protein content, it can be supplemented with other high protein sources such as groundnut cake, soybean cakes, etc. The seed could be a good source of edible oil as well as a potential food additive since it can be stored for longer period without spoilage.^[53] According to Quisumbing,^[54] the seeds of mangosteen were found to contain vitamin E, which might help reduce hydro-peroxidation of oil. According to oil compositional analysis (Table 1), the oil was reported to have 59.6 and 35.3% of saturated and unsaturated fatty acids, respectively. In the oil, palmitic (49.5%) was the most dominant fatty acid, followed by oleic (34.2%) and arachidic acids (8.77%).^[49] However, the seeds of Mangosteen from other sources were found to contain higher levels of stearate.^[55]

Rambutan Seed

Rambutan (Nephelium lappaceum L.), which is known as an exotic fruit is believed to have been originated from Malaysia. Rambutan, a member of the Sapindaceae family, is closely related to fruit species such as lychee (Litchi chinensis Sonn.), longan (Dimocarpus longan Lour.), and pulasan (Nephelium mutabile Blume).^[51,^56] Due to its Malay name rambut, which means hairy, it is sometimes called as “hairy litchi.” Rambutan fruit is generally ovoid or ellipsoid in shape and varies in color from pinkish to deep red or reddish orange to yellowish red.^[57] Beneath the skin, one could find a large seed, which is surrounded by a translucent, white juicy flesh or aril with a sweet and delicious flavor. Rambutan is usually eaten fresh, but it can also be canned in syrup or prepared into juice or jam. It is also believed to have medicinal properties such as astringent, anthelmintic, or antifebrile and can become curative for ailment such as severe dysentery and a warm carminative in dyspepsia.^[58]

The kernel of rambutan is edible after roasting but can have a slight bitter taste.^[59] As reported by Harahap et al.,^[60] the seed of rambutan is composed of 31.5% fat, 12.4% crude protein, relatively high content of carbohydrate (48.1%) and low amount of moisture 3.31%. The amino acid profile of the seed kernel showed that its protein was of a good quality.^[61] According to Harahap et al.,^[60] the rambutan seed can be better suited for food application when compared to watermelon seed and pumpkin seed owing to the finding that it is a good source of plant protein and carbohydrate. Some other studies also confirmed that the kernels of rambutan are rich in fat (ranging from 31.4 to 38.9%).^[61,^62] According to Table 1, the iodine and saponification values of rambutan seed oil are reported to be in the range of 41.8–49.6 g I₂/100 g and 157–190 mg KOH/g, respectively.^[62] Kheiri and Mohd. Som^[63] reported that the dominant triacylglycerols of fat from eight clones of rambutan kernel were found to be C56 (ranging from 34.6 to 39.1%), C58 (ranging from 28.8 to 47.2%), and C54 (ranging from 12.5 to 21.6%). The kernel fat was found to possess oleic and arachidic acids as the dominant fatty acids.^[61,^62] As fat of rambutan is solid white at room temperature, it could be possibly used in the formulation of high premium specialty fats like confectionary fats. According to Kalayasiri et al.,^[62] the fat might also be developed into specialty fat products, which are useful as a medium for cooking as well as base material for the manufacture of soaps and candles.

Watermelon Seed

Watermelon (Citrullus lanatus Thunb.), a member of the Cucurbitaceae family, is widespread across the tropical and subtropical regions of the world.^[64] It is primarily consumed as fresh fruit because of its sweet taste and pleasant flavor. Watermelon seeds have a high nutritional value because of the high protein (25.2–37%) and oil (37.8–45.4%) contents.^[65] They are used for human consumption in Arabian and Asian regions as snacks after salting and roasting. Watermelon seed oil has been studied in many parts of the world such as West Africa and the Middle East.^[65,^66] It is found to be stable at the room temperature conditions and has an amber color with a characteristic nutty flavor.^[67] The high oil content of the watermelon seed coupled with a fairly high concentration of unsaturated fatty acids make the seed suitable as food supplement. Taiwo et al.^[66] reported that watermelon seed oil is highly valued as a potential source of essential fatty acid (linoleic acid) with several health benefits (Table 1). This was in accordance with the previous findings of Mabaleha et al.^[11] who also reported the potential edible uses of watermelon seed. According to these two studies, the total unsaturated and saturated fatty acid levels of melon seed oil were 68.1–72.3 and 26.2–30.0%, respectively, and linoleic acid (45.1–51.2%) was the major component fatty acid followed by oleic acid (20.2–23.0%).^[67,^68] Although rich in linoleic acid, watermelon seed oil is preferred as frying and cooking medium in countries in the African and Middle East region due to the unique flavor.^[67] It may also enjoy applications as an industrial ingredient in the production of soap, cosmetics, and foam. The by-products which are seed and rind emanating from the processing could be useful to firing of industril boilers that are used in animal feed formulation.^[69,70]

Mango Seed

Mango (Mangifera indica L.) is one of the most important fruits worldwide and is cultivated in more than 100 countries of both tropical and subtropical regions. Being recognized for its attractive color, delicious taste, and exotic flavor, mango is a rich source of carotenoids and found to have high contents of ascorbic acid and phenolic compounds.^[71] As with many fruits, the edible flesh portion or pulp of mango fruit is relished to the extent of commercialization. A wide variety of processed products derived from mango pulp is known to include canned sliced mango pulp in brine or syrup, mango juice, nectar, jam, sauce, chutney, and pickle.^[16]

Mango has a single large seed covered with a shell that have the kernel inside, which represents around 20% of the whole fruit.^[72] Mango seed kernel has several medicinal uses in various parts of the World. In Fiji, fresh mango kernel is eaten for dysentery and asthma, while the juice is used in a nasal application for sinus trouble. In India, dried seed powder is applied to the head to remove dandruff and the kernel starch is eaten as a famine food. A hot water extract of the kernel is also recommended as an anthelmintic, aphrodisiac, laxative tonic.^[73] Depending on the variety, mango seed kernel (on a dry weight basis) contains average of 6.0% protein, 11% fat, 77% carbohydrate, 2.0% crude fiber, and 2.0% ash.^[74] Although mango seed kernel is reported to have a low content of protein, the quality of protein is said to be superior. According to Kittiphoom,^[75] mango seed kernel was high in potassium, magnesium, phosphorus, calcium, and sodium. It could be used as a potential source for preparation of functional food ingredients having natural antioxidants, antimicrobial compounds, etc. In addition, it could be further processed into therapeutic functional food products.^[75] After nutritional and toxicological evaluation of the mango seed kernel, Fahimdanesh and Bahrami^[76] indicated that mango seed kernel is a promising source of fat. Being nutritious and non-toxic, it could be a direct substitute for any solid fat without adverse effects on the quality. According to Rashwan,^[77] the lipids extracted from different mango varieties were free from toxic materials such as hydrocyanic acid. The lipid composition of various mango kernel varieties has drawn the attention of many researchers because of the potential application as a substitute for cocoa-butter in the confectionery industry.^[78] The neutral lipids of different mango seed kernel varied from 95.2 to 96.2%, phospholipids from 2.7 to 3.3%, and glycolipids from 1.1 to 1.4%.^[77] Triglycerides constituted of the major fraction of the neutral lipids and accounted for about 93.7 to 96.4%. The lipids of mango seed kernel consist of about 44–48% saturated fatty acids (majority stearic) and 52–56% unsaturated (majority oleic) (Table 1). Being a rich source of palmitic, stearic and oleic, mango kernel fat displayed some interesting physical and chemical characteristics.^[79]

Durian Seed

Durian (Durio zibethinus L.), a member of the Bombaceae family, is one of the well-known tropical fruits of Southeast Asia. Being native to Malaysia and Indonesia, durian is often regarded as the king of all fruits.^[80,^81] There are over 20 durian varieties known to exist, but only few of them have become popular for commercial production. The fruit of durian is either round or oval in shape while its rind is thick, semi-woody, and completely covered with strong sharp thorns.^[82] A medium size fruit may generally have three to five seeds, which are surrounded by edible custard-like pulp. With a unique strong sulfury odor, the pulp is found to be sweet in taste and yellowish to dark yellow in color.^[83]

The seed of durian is found to possess a white endosperm covered with a brown seed coat. It is usually discarded despite its usefulness as raw material for health care products. According to Amiza and Roslan^[84] durian seed flour could be incorporated into food products such as cake, cookies, soup, tempura, etc. since it is nutritious and has a high fiber content. Beside this, it can also be used as a thickening agent due to the presence of hydrocolloids as well as starch. Durian seed is not a rich source of lipids as the whole seed was found to have only a little amount of oil (1.8%). As reported by Berry,^[85] durian seed is found to contain palmitic acid (12.2%) as the major fatty acid followed by oleic (8.42%) and linoleic (6.50%) acids (Table 1). Nevertheless the utilization of this oil in food ingredients is yet to be discovered.

Winter Melon Seed

Winter melon, a member of the Cucurbitaceae family, is mostly distributed in many parts of the tropical regions. It also carries several other names that include ash pumpkin, tallow gourd, white pumpkin, gourd melon, Chinese watermelon, and so on.^[86] Winter melon is a good source of valuable nutrients that include natural sugars, amino acids, organic acids, vitamins, and mineral elements.^[87–89] In Malaysia, winter melon is mainly represented by two cultivars, which bear fruits of round and oval shapes. Locally known as Kundur fruit, winter melon is grown in many states of Malaysia mainly for fresh fruit consumption.^[89] In some countries of the Southeast Asian region, the mature fruits are also used in the preparation of soups. Beside this, they are also sliced and eaten as cooked alone or with meat as well as incorporated into other dishes.^[90,^91] A number of biological and medicinal properties such as anti-obesity,^[92] anti-inflammatory,^[93] anti-diarrheal,^[94] anti-pyretic,^[95] anti-compulsive,^[96] antioxidant,^[97] anti-ulcer and diuretic^[98,^99] have been ascribed to winter melon. A comprehensive review on the nutritional, medicinal and pharmacological properties of this multipurpose fruit could also be found elsewhere in the literature.^[89]

According to Jeffrey,^[100] most of the fruits from Cucurbitaceae species are valued for nutritional and medicinal properties and their seed are often discarded as agro waste. The seed, packed in the center of the fruit might change their color from white to yellowish brown as the fruit matures.^[101] They are also useful as food preservatives or as ingredient in animal feed formulations. The seeds, which are found to have considerable amount of oil, can be fried and used as a snack. As reported by Anwar et al.,^[102] the content of oil, protein, fiber, and ash ranged from 17.87 to 32.53%, 19.36 to 26.21%, 28.18 to 42.03%, and 5.02 to 11.81%, respectively. According to another study conducted by Sew et al.,^[103] the oil contained high proportion of linoleic acid (C18:2), accounting for more than 65% of the total fatty acids. Anwar et al.^[102] also found that oleic and palmitic were two other prominent fatty acids detected in the oil (Table 1). The seeds of the tested cultivars were found as rich source of oil, protein, and fiber and thus, could be used for dietary purposes. The extracted seed oils were found to have high amount of linoleic acid (63.10–70.64%), placing them in the category of high-linoleic vegetable oils. The presence of high amount of polyunsaturated fatty acid (linoleic acid), along with considerable amounts of tocopherol and phytosterol as beneficial components revealed its suitability and potential as functional ingredient in food applications.^[102]

CONCLUDING REMARKS

This article summarized the useful characteristics of several fruit seeds and the fatty acid composition of their oils. As majority of the fruit seeds discussed come from edible fruits, they have the potential to be used directly as raw material for food and pharmaceutical formulations. As some of them contain considerable amounts of oils, they can become source materials for functional lipids. The utilization of fruit seeds in future not only can minimize the accumulation of waste but it also can generate extra revenue for fruit processing sector. As many areas of fruit seeds are still needed to be investigated further, this article can be a useful reference for future research activities.