Evaluation and characterisation of Citrullus colocynthis (L.) Schrad seed oil: Comparison with Helianthus annuus (sunflower) seed oil

Abstract

The physicochemical properties, fatty acid, tocopherol, thermal properties, ¹H NMR, FTIR and profiles of non-conventional oil extracted from Citrullus colocynthis (L.) Schrad seeds were evaluated and compared with conventional sunflower seed oil. In addition, the antioxidant properties of C. colocynthis seed oil were also evaluated. The oil content of the C. colocynthis seeds was 23.16%. The main fatty acids in the oil were linoleic acid (66.73%) followed by oleic acid (14.78%), palmitic acid (9.74%), and stearic acid (7.37%). The tocopherol content was 121.85 mg/100 g with γ-tocopherol as the major one (95.49%). The thermogravimetric analysis showed that the oil was thermally stable up to 286.57 °C, and then began to decompose in four stages namely at 377.4 °C, 408.4 °C, 434.9 °C and 559.2 °C. The present study showed that this non-conventional C. colocynthis seed oil can be used for food and non-food applications to supplement or replace some of the conventional oils.

Introduction

Vegetable oil is a valuable food commodity whose consumption has increased in the past decade, particularly in emerging countries such China and India. The increase is mainly due to, among other things, population growth, improving living standards and changing diets (Rosillo-Calle, Pelkmans, & Walter, 2009). Global vegetable oil consumption has grown by over 5% per year since the mid 1970s. Furthermore, the increase of the needs of their derivatives, especially biodiesel, has led to a growing demand for conventional vegetable oils, especially sunflower, soybean, rapeseed and palm oils. This trend combined with droughts, bad weather circumstances, speculation, and a low growth in total oil production, has led to a gradual decline in world supplies of vegetable oils.

Oil World indicated in its 2012 report (Mielke, 2012) that world production of 7 oilseeds (soybean, sunflower, rape, cotton, peanut, palm kernel, copra) will turn out to be smaller than expected, enforcing an unprecedented decline in oilseed stocks, as well as demand rationing. This may lead to a price spike of vegetable oils and to an insufficiency to satisfy global demand for food and oleo chemical industries as well as for the energy sector. All these facts highlighted there is a need to find other unconventional oils that can partially replace some of the conventional oils and thus lower their prices.

One of the possible alternative oils is the non-conventional oil from seeds of Citrullus colocynthis (L.) Schrad. It belongs to the species of the genus Citrullus of cucurbitaceae family, which usually consists of a large number of varieties that are generally known as melons (Mabaleha, Mitei, & Yeboah, 2007). The fruits of C. colocynthis, commonly named bitter apple, egusi melon, ground melon, Handhal, Dellaa El-Wad were gathered as deadly poison. The pulp is an effective hydragogue, cathartic and laxative because of its glucosides content such as colocynthin (Dane, Liu, & Zhang, 2007). The ripe fruit of C. colocynthis is nearly globular, yellow in colour and characterised by a thin but hard rind. The fruits have a soft, white spongy pulp which is filled with numerous ovate compressed seeds, which are edible and when ground provide a rough bread for the Bedouins living in desert (Dane et al., 2007). The seeds contain approximately 17–19% oil (Zohary & Hopf, 2000), and consist mainly of unsaturated fatty acids (80–85%) (Schafferman, Behazav, Shabelsky, & Yaniv, 1998). It is estimated that the oil yield is approximately 400 l/ha (Schafferman et al., 1998).

C. colocynthis native from tropical Asia and Africa (Dane et al., 2007), is now widely distributed in the Saharo-Arabian phylogeographic region of Africa, in the Mediterranean region, eastward through Iran to India and other parts of tropical Asia. This perennial herb is a drought tolerant species, which can survive arid environments by maintaining its water content without any wilting of the leaves or desiccation, even under severe stress conditions. This is accomplished by extending its root system into deep ground water (Dane et al., 2007). Therefore, C. colocynthis can be placed as a serious oilseed crop candidate for medicinal, food and non-food applications. The first objective of this study was to determine tocopherol, fatty acids and thermal profiles, physicochemical properties, ¹H NMR, FTIR and UV–Visible spectra of the non conventional C. colocynthis seed oil grown in Saudi Arabia. The second objective was to compare the properties of this non conventional seed oil with the common Helianthus annuus (sunflower) seed oil.