Physicochemical properties and bioactive compounds of selected seed oils

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Abstract

The physicochemical properties and chemical composition of oil extracted from five varieties of plant seeds (bittermelon, Kalahari melon, kenaf, pumpkin and roselle seeds) were examined by established methods. The thermal properties of extracted oils by differential scanning calorimetry were also evaluated. Sensorial profiles of these seed oils were defined through the CieLab (L*, a*, b*) colour. Most of the quality indices and fatty acid compositions showed significant (P < 0.05) variations among the extracted oils. Physicochemical properties of the oils extracted were iodine value, 86.0–125.0 g I2/100 g oil; saponification value, 171.0–190.7 mg of KOH/g of oil; acid value, 1.1–12.9 mg of KOH/g of oil, free fatty acid, 0.6–6.5 g/100 g of oil, and peroxide value 1.5–6.5 meq of O2/kg of oil. Palmitic, oleic and linoleic acids were the major fatty acids in all of the extracted seed oils except for bittermelon, where eleostearic acid was the major fatty acid. Gallic, protocatechuic, p-hydroxybenzoic, vanillic, caffeic, syringic, p-coumaric and ferulic acids were identified in the extracted plant oils. Among these, vanillic acid was predominant in all extracted oils. The oils were rich in tocopherols with γ-tocopherol as the major components in all oil samples. Among the phytosterols, sitosterol was the major phytosterol extracted from the five plant seed oils. The seeds of these plants contain a great number of valuable minor compounds, which have a potential high value as food and for production of non-food products.

Introduction

Recently, more attention has been focused on the utilization of food processing byproducts and wastes, as well as underutilized agricultural products. Such utilization will contribute to maximizing available resources and result in the production of various new foods. Consequently, a major contribution to avoiding waste disposal problems can be made.

The problems of industrial waste are becoming harder to solve, and much effort will be needed to develop nutritional and industrial potential of byproducts, waste, and underutilized agricultural products. Only a small portion of plant material is utilized directly for human consumption. The remaining portion of this material or part of it may be converted into nutrients for either food or feed or into fertilizer; thus an important contribution to food resources or industrial products could be made. In this respect, bittermelon, Kalahari melon, kenaf, pumpkin and roselle seeds can potentially be used.

Bittermelon (Momordica charantia L.), also known as bitter gourd, is a monoecious climbing vine. It is a tropical crop, grown throughout Asia for food and medicinals (Chakravarty, 1990). The seeds contain oil in which the major fatty acid is eleostearic acid (ESA).

Kalahari melon (Citrullus lanatus) is the important source of water in the Kalahari during dry months of the year when no surface water is available. In the Kalahari, the fresh Kalahari fruits are used as a stock feed in times of drought (Van Wyk & Gericke, 2000). Seeds are roasted and ground into meal, a nutritious food with a pleasant nutty taste. Leaves and young fruits are utilized as green vegetables (Van Wyk & Gericke, 2000). There is a bitter form of the Kalahari melon (C. lanatus) which is poisonous, but which can be used medicinally. Reported medicinal uses include treatment of worms, renal stones, dropsy, alcohol poisoning, diabetes and as a purgative and emetic (Van Wyk & Gericke, 2000). The fruit has been shown to contain lycopene, the same substance found in tomatoes, which has been proven to prevent heart attacks.

For the last few years, kenaf (Hibiscus cannabinus L.) plant has been cultivated in Malaysia. Kenaf has been reported as a suitable source for livestock and horse feed since it is high in protein (more than 20%) and has potential to replace alfafa (Daham, 2005). Kenaf seed contains 9.6% moisture, 6.4% ash, 20.4% oil, 21.4% nitrogenous matter and 12.9% crude fiber. The dry press cakes contain 33.0% protein, 6.0% oil, 17.4% crude fiber and 6.0% ash (Duke & duCellier, 1993). Palmitic, oleic, and linoleic acids were reported as major fatty acids, and palmitoleic, linolenic, and stearic acids were the minor fatty acids. Kenaf's relatively high oil content and its similarity to cottonseed oil suggest that the seed oil may be used as a source of edible oil (Mohamed, Bhardwaj, Hamama, & Webber, 1995).

Pumpkin (Cucurbita pepo L.) seed oil is a common salad oil in Austria and is produced in Slovenia, Hungary and the southern parts of Austria. The content of vitamin E in pumpkin seeds is very high (Murkovic, Hillebrand, Winkler, & Pfannhauser, 1996). Pumpkin seeds are also rich in plant sterols, which have recently become of great interest because of their serum cholesterol-lowering effect (Jones et al., 2000, Miettinen et al., 1995). They may also be beneficial against colon cancer (Awad et al., 1998, Rao and Janezic, 1992).

Roselle (Hibiscus sabdariffa, Linn.) seed is a valuable food resource (Al-Wandawi et al., 1984, Balami, 1998) on account of its protein and calorie content and also substantial amounts of fiber and valuable micronutrients (Omabuwajo, Sanni, & Balami, 2000). It is also an excellent source of culinary oil (Ahmed & Hudson, 1979). Roselle seeds that are produced contain approximately 15 wt% on a dry weight basis (dwb) of highly unsaturated triacylglycerols and small amounts of other lipid components. The seed oils are composed predominately of oleic and linoleic fatty acids and have been investigated for potential edible applications (Holser & Bost, 2001). The seed also contains desmethylsterol compounds. Such phytosterol compounds are known to reduce the absorption of dietary cholesterol when included in the human diet (Jones et al., 2000).

To the best of our knowledge, very little information has been reported on the phenolic, tocopherol and sterol profiles of these five varieties of plant seeds. Therefore, the main objective of this study was to evaluate the physicochemical properties of oils extracted from five different plant seeds. The phenolic, tocopherol and sterol profiles of these plant oils were investigated to determine the functional compounds in these plant seeds and improve the economic utility of these seeds as a source of edible/non-edible lipids.

Section snippets

Seed material

Bittermelon (M. charantia L.) seed and pumpkin (C. pepo L.) seed were obtained from local market. Kalahari melon (C. lanatus) seed was obtained from northern part of Namibia. Kenaf (H. cannabinus L.) seed and roselle (H. sabdariffa Linn) seed were obtained from Malaysian Agricultural Research and Development Institute (Selangor, Malaysia). Two samples (about 1 kg/sample) from each plant species were investigated. All seeds were ground into fine powder using a coffee grinder (National, Osaka,

Proximate analysis of plant seeds

Proximate compositions of bittermelon, Kalahari melon, kenaf, pumpkin and roselle seeds are presented in Table 1. Significant (P < 0.05) differences were observed among the seeds in their content of moisture, crude protein, fats/oils, crude fiber, ash and carbohydrate contents. Kalahari melon seed and pumpkin seed had high levels of protein (24.5–26.5 g/100 g) and oil (30.5–34.9 g/100 g). Many Cucurbitaceae seeds are rich in oil and protein (Al-Khalifa, 1996). The high percentages of oil make these

Conclusion

Improved knowledge on the composition, analysis and properties of bittermelon, Kalahari melon, kenaf, pumpkin, and roselle seeds would assist in efforts to achieve industrial application of these plants. Data about these seeds are very few, and there are no reports of the detailed bioactive composition of these seed oils. These seeds give a considerable yield of oil and the oil seems to be a good source of lipid-soluble bioactives. The high linoleic acid content also makes the oil nutritionally

Acknowledgement

Financial support of this work by Ministry of Science, Technology and Innovation of Malaysia through Science Fund (05-01-04-SF0398) is gratefully acknowledged.

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