10 De Smet Plants are processing rice bran oil.
Origin
Originating from Eastern Asia and China, rice culture has expanded to India, Iran and Arabia.
Rice is the cereal which is used by the largest part of the world population : at least, half of the planet.
Rice is an annual gramineae; its structure is the one of most common cereals. One plant is made of 10 to 15 herbaceous stems of 0.7 to 1.8 m high supporting alternate leaves with a long lamina.
The inflorescence is a branching cluster, of 20 to 30 cm long and composed of 100 to 150 spikes. Every spike carries a grain surrounded by two rudi-mentary husks which are in their turn surrounded by two small leaves or bracts.
A very fine integument covers the grain; it is also called ‘ricebran’ or ‘silvery skin’ on which the husks remain adhesive even after the threshing.
The grain with its husks and bracts is called "paddy", the decorticated rice is named "cargo rice", and the rice cleaned from its integument is the white rice or polished rice.
We can distinguish :
- mountain rices, cultivated at an altitude up to 2.000 m in tropical countries and which require only 600 to 700 mm annual rain. They yield 2.500 kg of paddy per hectare.
- irrigated rices, which need a clayey ground able to retain the water above the ground. They yield up to 5.000 kg of paddy per hectare.
Both varieties have the same chemical characteristics and do not make really different botanical species.
The crop is done with sickles and the sheaves are dried and put in stacks. Threshing is mechanical or manual.
On top of the use of rice as food, it also serves in the manufacture of face powder (thanks to tiny pieces from the decorticating), in preparation of brandies and starch.
The rice bran from polishing of rice is a highly nutritive food : starch 35%, fatty matters 10-20% and proteins 10-12%.
Composition
Rice bran oil contains :
- 2% of gadoleic acid
- 30-35% of linoleic acid
- 40-45% of oleic acid
- 18-20% of palmitic acid
- 2-3% of stearic acid
- Iodine Value : 99-108
- saponification value : 180-194
- melting point : ....°C
- titer : .....°C
- unsaponifiables : 2-5%
It is a semi-siccative oil.
Process
Ricebran oil can easily be obtained in direct solvent extraction of the ricebran. But due to the powdery nature of the ricebran, it requires a preparation before extraction to ensure a good percolation of the solvent. To that respect, it will be heated and humidified before palletising (to compress through a die).
Due to the acidity of ricebran oil, special care should be taken to select the appropriate construction material. Moreover, as sand is often present in ricebran, heavy wear can be expected.
The crude oil generally has a very high acidity (up to 50% of free fatty acids)! It is due to the fact that the oil contained in the rice bran rapidly detoriates in humid atmosphere and under the action of a lypolitic enzyme (lipase); this enzyme favours the hydrolysis of the triglycerides and make free the fatty acids. It is currently observed that after 2-3 storage days, the acidity of the oil rises to 10%. During the first hours, the acidity may rise from 1% per hour !
It is thus necessary :
- either to process the ricebran immediately after the decorticating; this is quite rarely possible due to the fact that the solvent plants are usually far away from the fields where threshing is done.
- or sterilise the rice bran at the rice plants by heating it at 90-100°C and drying it to stop the action of the lipase and so to allow a storage before transport and extraction.
Ricebran oil, due to its high acidity, has often been used in soap factory, but of course, as it is produced in countries with high population, it has been used for human food. Much research has already been developed on this subject of refining ricebran oil.
The classical chemical neutralisation with centrifugal separator is not very appropriate because of the excessive losses consecutive to the high acidity and the important presence of waxes and stearines. Moreover the classical refining is limited in the dark colour reduction.
The neutralisation in miscella refining is probably one of the best alternatives as from one side losses are much more reduced and fatty acids are directly usable in soap factory and on the other side bleaching and deodorising are much easier.
The ricebran oil still contains 5 to 10% of waxes and stearines that can be removed easily in a miscella phase winterising.
End uses
The deoiled rice bran is quite poor in protein content (20%) and is used in mixture with others to feed animal and poultry, as well as fertiliser.
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