Insect oils: Nutritional and industrial applications
By Abdalbasit Adam Mariod
In searching for new sources of oils, many researchers have investigated wild plants, but our research group took a different approach: We looked at insects as an oil source for both nutritional and industrial applications.
According to Sudanese indigenous knowledge, many insects have food and medicinal uses. We targeted two of these insects for our research: Aspongopus vidiuatus (melon bug) and Agonoscelis pubescens (sorghum bug).
The melon bug (Pentatomidae) is about 20 mm long. It is found in most African countries, where it causes damage to watermelon and other cucurbit shoots. The adult bugs can usually be found by lifting the young melon plants from the ground and inspecting the undersides of the leaves. The nymphs pierce the leaves, stems, and young fruits and suck the sap, resulting in wilting, fruit drop, and the death of the plant. Melon bugs are considered to be edible in Namibia, where the last nymph stage is called "nakapunda." In this soft stage, the bug is cooked and eaten. Melon bugs are widely distributed in Kordofan and Darfor states of Sudan (locally known as Um-buga), where field watermelons are one of the most important crops for the traditional rain-fed agriculture. There, tons of melon bug adults can be collected in infested fields. Elobied Agricultural Research Station (North Kordofan state of Sudan) designed a handpicking program for melon bug adults in plots of about 5,000 hectares in four different areas of the state, for two seasons. A total of 15 tons of melon bug adults were collected in the first season and 226 tons in the second one (Bashir et al., 2002).
The adult sorghum bug (Pentatomidae), commonly known in Sudan as Dura andat, is shield-shaped, about 11-13 mm long, and 6-7 mm wide. Both the upper- and undersides of its body are covered with a fine silvery pubescence after which it is named. Agonoscelis pubescens is found in a number of African countries south of the Sahara. In Sudan, the Dura andat has a wide distribution through-out the country. The adults infest sorghum during the plant's milky stage. In Western Sudan, adult sorghum bugs are collected, fried, and eaten. Additionally, in some areas of Sudan the collected bugs are pressed, and the expressed oil is used for cooking and some medicinal purposes. In the Botana area of central Sudan, nomads use the tar obtained from high-temperature rendering of the bugs to protect their camels against dermatological infections (Mariod et al., 2004).
What is the chemical composition of bug oil?
Oils extracted from these two Sudanese edible insects (Figs. 1,2) have interesting physicochemical properties and fatty acid compositions. For instance the amounts of saturated and unsaturated fatty acids they contain are comparable with those of oils commonly used in Sudan, such as sesame, groundnut, sunflower, and cottonseed (Table 1).
Acting as chain-breaking antioxidants, tocopherols, which are minor components of naturally occurring oils, react with lipid radicals to convert them into more stable products. Tocopherols protect food lipids against autoxidation and thereby increase their storage life and their value as wholesome foods (Kamal-Eldin and Appelqvist, 1996). The oils of melon and sorghum bugs contain only low amounts of tocopherols in comparison to other common Sudanese edible oils (Table 2).
In our experiments the amount of sterols in melon bug oil (MBO) was 17.5 mg/100 g. In sorghum bug oil (SBO), the amount of sterols was 449.9 mg/100 g (Table 3). The main sterol of the two oils is β-sitosterol. In comparison with other oils usually used in human nutrition, SBO had higher amounts of total sterols than either sunflower or groundnut oils.
There is increasing interest in isolating sterols for nutraceutical applications and as ingredients for functional foods (Holser et al., 2004). Sterol fraction analysis can be used to identify a fat or oil, to detect the adulteration of more expensive oils with cheaper oils, and to distinguish between different qualities of the same oil.
Insect oils are highly stable
The oxidative stability of MBO expressed as the induction period determined by the Rancimat method at 120°C is remarkably high (38.0 hr) in comparison to other edible oils. For instance, the oxidative stability of sesame oil is 1.6 hr and for sunflower oil, 5.4 hr. The stability of SBO (5.1 hr) is also in this range. The high oxidative stability of MBO may be due to the low amounts of polyunsaturated fatty acids (PUFA) such as linoleic and linolenic acid. On the other hand, in spite of the higher content of tocopherols in SBO, the high portion of PUFA in this oil seems to be mainly responsible for its low induction period. Therefore, in this case the fatty acid composition has a much higher influence on the stability of the oil than the antioxidants present in the oil.
Blending sunflower oil with MBO resulted in an increase of oleic and a decrease of linoleic acid and improved the oxidative stability of sunflower oil. This stability increased with an increase of the percentage of MBO in blends (Mariod et al., 2005). When MBO and SBO were stored at 30 ± 2°C in the dark for 24 months, their fatty acid compositions remained almost unaltered. On the other hand, the tocopherols of the two oils gradually decreased. The two oils showed slight changes in their oxidative stability as indicated by the peroxide value (PV), and when this stability was measured by Rancimat method as an induction period, MBO showed a slight decrease with loss of 10% of its induction period during two years of storage (Mariod et al., 2008). SBO showed a gradual increase in the PV and a gradual loss of stability as measured by induction period IP during storage.
In laboratory refining experiments of crude oils, phosphatide, peroxide, tocopherol, and sterol contents as well as oxidative stability fell during processing, while free fatty acids were almost totally removed. The amounts of total volatiles as well as the amounts of hexanal were decreased during the different processing steps. The color decreased throughout the processing steps up to bleaching, then in the deodorization step it darkened sharply in all samples. No change in the fatty acid composition was observed.
Biodiesel from insect oils
MBO and SBO were transesterified using methanol and ethanol in the presence of sulfuric acid. The resultant fatty acid esters were compared with the DIN 51606 specifications for biodiesel. Most of the insect oil biodiesel characteristics met the DIN specifications (water content, iodine number, phosphorus). However, the kinematic viscosity values of all samples were much higher than those for biodiesel standards. These can be reduced by blending with other low-viscosity biodiesels.
Abdalbasit Adam Mariod is an associate professor in the Department of Food Science & Technology, Sudan University of Food Science & Technology (Khartoum, Sudan). He can be contacted at firstname.lastname@example.org.
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