Sesame is an annual crop grown in tropical, subtropical and warm temperate regions. It thrives in temperate regions during the summer and in tropical lowlands under semi-arid conditions. Varieties may be classified either as shattering or non-shattering, according to whether the seed capsules open on drying. Under optimal conditions, some varieties take only 3-4 months to reach maturity; in less favourable conditions, some slower types may take 8 months. When harvesting by hand, the crop is cut close to the ground with a sickle, tied in bundles, and stacked to dry. The crop is cut when the lowest capsules on the stem begin to open. The oil content of sesame seed and the fatty acid composition of the oil are given in Table 2. Natural sesame oil derived from good quality seed has a very pleasant flavour and can be consumed without further purification. The natural oil has excellent stability due to the presence of high levels of natural antioxidants (Lyon, 1972).
Hot water flotation
Hot water flotation is a traditional method in Uganda and Sudan for the extraction of sesame oil. The following is a description of a laboratory method, as details of the traditional method have not been located.
Sesame seed is ground to a paste and heated to 80-90°C for 15 min. Enough boiling water is then added to suspend the ground seed on stirring. The mixture is boiled with stirring for 15 min. After cooling the upper oil layer is separated off and dried by heating. The oil recovery from 0.5 kg seed is 108 ml, equivalent to an oil extraction efficiency of 41%.
Laboratory trials at NRI have demonstrated that sesame seed is suitable for processing in a bridge press. However, no results of its application in the field are available. In the laboratory trials, sesame seeds were ground to a paste using a powered mincer incorporating a plate with 2 mm holes. It is important to grind the seeds as finely as possible. Oil extraction was improved by the addition of water, and optimum oil recovery was achieved at a moisture content range of 11-13%. Yields of over 70% were recorded with sesame paste containing 12.7% moisture pre-heated to 50°C before pressing (see Figure 15).
Work in Tanzania has indicated that sesame is suited to processing in the ram press. Pre-grinding is not required but pre-heating the seed by warming in the sun, preferably on metal roofing sheets, is strongly recommended. Use of the CAPU press at 4 strokes/mint gave an oil extraction efficiency of 57.5% in terms of clarified oil. The oil production rate was a little over 2.2 l/in. The oil extraction efficiency of the smaller CAMARTEC press under similar operating conditions was 62%, and the oil production rate was 1.5 l/in.
The ghani process (Sudan)
The following traditional method was described by Kamel-Eldin et al. (1992). The addition of oil to aid the extraction process is of particular interest.
Sesame seeds (12 kg, oil content 53.1% MFB) were ground in a camel-powered ghani with 0.5 l of water. Oil release was observed after 30 min. when the temperature of the mass was 41°C. After 40 min., 2 l of oil previously extracted (temperature 46°C) were added to assist extraction. Extraction was complete in 55-60 min., giving approximately 5 l of oil (temperature 50°C).
The following operation was monitored at Illiassa in The Gambia (see Plate 7).
Run 1: 34 kg seed were moistened with about 1.5 l of water.
The yield of crude oil and cake was 15 kg and 21 kg respectively. The speed of the diesel engine could be varied and much use was made of this facility during the warm-up period. The wormshaft speed varied from 20 to 30 r.p.m.
About 0.5 l of warm water was added to the crude oil and the mixture heated with stirring until the water boiled. The boiling pattern changed from an initial vigorous 'boiling' to a gentle frothing, during which time (about 30 min.), the sediment conglomerated to a 'jelly-like' consistency. At this stage the oil was filtered through the cleaning sieve to give a clear product.
Run 2: 20 kg of seed was taken and water was added. The mixture was processed in a single pass with a constant wormshaft speed of 28 r.p.m. Expeller temperature during the run was constant at 61°C. The process time was 22 min., which is equivalent to a seed throughput of about 55 kg/in. Product yields were as follows:
The initial seed contained nearly 55% oil with 4% moisture. Oilseed cake from the process contained 19-22% residual oil. The extraction efficiency was 73-76%.
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