Small-scale oilseed processing
Practical Action
and then filtering the clear oil by gravity through fine cloth. A better but more expensive method
is pumping the crude oil through a filter press.
Clarification
Filtering will remove insoluble contaminants such as fibre but fresh extracted oil will also
contain moisture, resins and colorants from the seed. Clarification is a relatively simple method
of removing these unwanted elements and can be done by letting the oil stand undisturbed for a
few days and then separating the upper layer, or by using a clarifier in which the oil is held in a
tank with a heat source. This can be an old oil drum above a fire or commercially available
equipment. The oil is boiled to drive off water and to destroy naturally occurring enzymes and
contaminating bacteria. After heating the oil is allowed to stand and the contaminants separate
out. The oil is filtered through a cloth and is reheated to ensure that all the moisture has been
removed.
The typical system described above consists of the following equipment.
Decorticator with blower to remove shell, 150 kg/hr
Boiler, 50 kg steam/hr
Cooker
Expeller (motorised) 75 -100 kg/hr
Filter pump and press
Clarifier
Refining
In many local markets further refining is not required as the complex flavours of unrefined oils
are preferred. International markets tend to prefer lighter less intense oils for cooking which
means further processing of the oil.
There is a series of refining processes that can be carried out after the oil has been filtered.
De-odorising
Volatile compounds that produce bad odours can be eliminated through the process of sparging,
i.e. bubbling steam through the oil, under a vacuum.
Wintering
Allowing the oil to stand for a time at low temperatures so that glycerides, which naturally occur
in the oil, with higher melting points solidify and can then be removed from the oil by filtering.
Over time glycerides can degrade releasing fatty acids into the oil increasing the acidity levels
and reducing the quality.
Neutralisation
Fatty acids can be neutralised by adding a sodium hydroxide solution, also known as caustic
soda, or by stripping, which is a similar process to de-odorising.
Bleaching
Some oils have a very dark colour
to them that is unpopular with
consumers. The appearance of the
oil can be lightened by bleaching
De-gumming
Degumming is a way of treating
seed that have a high phosphotide
content. The phosphotide, which
makes a gummy residue, is
removed by mixing the oil with 2 or
3 % water. This hydrated
phosphotide can then be removed
by settling, filtering or centrifuged.
Packaging
Figure 6: The final product in Zimbabwe
Photo credit: Practical Action / Mike Battcock
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