Method 78b Determination of oil in rapeseed

The species we have dealt with is winter rape (Brassica napus L.). See comments in Chapter 4, 'Oils, fats and waxes', and the reference to Hughes (1969).

Oilseeds with hard shells must first be crushed to allow the solvent to reach the seed contents. The hard round rapeseed is easily fractured using a pestle and mortar, but the seed must not be crushed finely, which would spread the oil over the surfaces of the grinder. (The flatter, shiny and softer linseed does not split open cleanly in the same way, and another method is required. We homogenize 4 g seed for 2 min at 11,500-13,000 rpm in a shortened polystyrene sample tube, occasionally tapping the tube and adjusting its height to ensure all the seeds are broken; see also Chapter 2, 'Homogenization'.) The weight of the seed before and after extraction gives the weight of oil extracted by difference, and this procedure permits multiple extractions in one large Soxhlet extractor.

Procedure. Weigh accurately about 5 g of the split rapeseed into a previously dried and weighed 180-mm filter paper circle, which is then neatly and tightly folded and stapled to form a sachet. The weight of the staple/s must be added using the average weight from a number of staples. Tightly pack the sachets on end, and in layers, into a Quickfit EX5/75 600 ml flanged Soxhlet extractor above a 2-l flask positioned on a heating mantle. The extraction assembly must be firmly clamped to a support stand. Fill the reservoir with petroleum spirit (40-60°C) until it just siphons over, then half fill again. Attach a CX5/25 double-surface condenser and ensure a steady flow of coolant water emerges from the condenser. Turn on the heater control, which should be solid-state for safety reasons, and reflux for 16 h. After this period, turn off the heat just before the level in the reservoir has reached the siphon tube. After the solvent has siphoned over, unclamp the flange, remove all the sachets and allow them to drain off. Place them on a large stainless steel tray on a warm hotplate (40-50°C) in a fume cupboard, and allow to dry until no smell of solvent can be detected. Finally, dry in an oven for 1 h at 100°C, cool in a desiccator and weigh immediately. Note: filter paper rapidly absorbs atmospheric moisture.

Calculation. Divide the weight lost by the weight of sample and multiply by 100 to obtain the percentage oil in the seed sample. A separate sample for moisture content must be weighed at the same time as the sample for oil extraction if the percentage oil in seed DM is required.

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