Steam Distillation 1. Packing and fitting the Charge Basket
Updated: Feb 13
For newcomers who have not steam-distilled crops to extract their essential oils before, this is the first part of a series giving some basic guidelines for getting started.
The essential oil yields should have been thoroughly checked beforehand to find the optimum 'harvesting window': we will assume that the time is now right to distil without further delay. (see blog - Laboratory QC Analysis).
Line the base of the basket with muslin, making sure you cover all the holes in the charge basket's mesh base. The reason for this is to stop crop particles falling through the holes and eventually turning the water into a 'soup' that can clog the heating elements and possibly cause them to fail. These particles can also cause off-odours in the essential oil if they are allowed to remain in the water for prolonged periods.
If the operator requires the charge basket to hold its absolute maximum weight, then provided a large enough piece of muslin is used as a liner, this material can assist in keeping the charge in place. The charge basket can then be slightly 'overfilled' so that the plant material fills it to the point so that it is domed: the muslin will then be folded over and then knotted at the top to ensure the charge remains securely in place throughout the distillation. The charge should not be compressed down since this may impede the passage of steam and may then cause a slight build-up in pressure in the boiling vessel.
The harvested crop will have been brought to the Still in e.g. a trailer, or in 1 tonne 'Builders Bags' or other suitable containers, depending on the size of the charge basket to be filled. Many oils are quite volatile, so it assists in maintaining peak yields if the crop containers have some form of sealing or closure system in place. This will minimise essential oil loss that can sometimes occur through evaporation if prolonged storage is likely.
For larger operations, it helps if there is a ramp or other system in place that will allow the material to be packed into the charge basket from above: repeatedly lifting 500 litres volume of plant material up from the floor and into the charge basket can prove to be a pretty challenging exercise!
Once the basket has been filled with the crop, the basket should be weighed to determine the weight of the charge to be distilled - the tare weight of the empty basket will have been already recorded so the nett weight of the crop being distilled can then be determined and should be recorded.
The drainage tap on the boiling vessel should be checked that it is in the closed position before filling the vessel with tap water to the point that the heating elements are well covered. It is best to aim at covering the highest point on the elements by about 50mm to ensure that there is no possibility of the water in the boiling vessel ever becoming low enough so that the elements ever become exposed. Remember that once the water has boiled it will be returned to the vessel through the return trap, so the level will remain roughly constant throughout the distillation.
Take care not to overfill the vessel either. The water level must be below the base of the charge basket suspended above it so that the steam can easily pass up through the mesh base and then continue through the charge of plant material in the basket. The lifting bar is now inserted into the 2 side lugs fitted at the top of the charge basket and moved so that the slots in the bar secure it in place: the lifting hook from the sliding block and tackle is then fitted into the central eye on the bar and the slack removed by pulling on the lifting section of the chain. With a helper steadying the basket, gently continue to raise the basket until its base is just above the top of the boiling vessel. Manoeuvre the basket into position by moving the framework that holds the block and tackle and carefully lower the filled basket fully into the boiling vessel until it is seated.
The distillation can now proceed - see the following blogs to see how this is done.