Castor seed commonly tests in the mid to high 40s percent oil, for illustration. There is no single universal standard. The figure that matters is the basis written in your contract, tested by an agreed method on an agreed moisture basis.
Oil content is the main thing you pay for when you buy castor seed. A few points of difference across a full container changes how much oil your crusher gets, so it is worth defining carefully.
What changes oil content
- Variety and hybrid: different seed types carry different oil levels.
- Growing season: drought, heat or unseasonal rain during seed fill can lower oil.
- Picking: castor is harvested in several pickings, and later pickings can differ from early ones.
- Maturity: immature and shrivelled seeds hold less oil.
- Admixture: husk, stalks and dust add weight with no oil, which dilutes the result.
- Moisture: the same seed shows a lower oil figure on an as-is basis when it is wetter.
Why the moisture basis matters
An oil result can be reported on an as-is basis, on a dry basis, or adjusted to a stated moisture. These give different numbers for the same seed. A supplier quoting on dry basis can look better than one quoting as-is, even if the seed is identical.
Write the basis into the contract. For example, state the oil content on a named moisture basis, or ask for both as-is and dry-basis results on every certificate. Then compare quotes on the same footing.
How oil content is tested
The classic method is solvent extraction, often called the Soxhlet method, where oil is extracted with a solvent such as hexane or petroleum ether and weighed. Some labs and crushers also use NMR, which is quick and non-destructive once calibrated.
Results from different methods and labs can differ slightly. Agree one method and one lab for settlement, and keep sealed counter-samples in case of dispute. Reading the result is covered in reading a lab certificate.
Quality of the oil, not just the quantity
If you crush the seed yourself, oil quality also matters. Damaged, mouldy or badly stored seed tends to give oil with higher free fatty acids and darker colour, which costs more to refine. Castor oil is valued for its high share of ricinoleic acid, which is what makes it useful in industry.
Ask for free fatty acid on oil extracted from the sample, as well as oil content, if your end use is sensitive. Store seed dry and off the floor, and avoid long storage in humid months.
A practical spec line
- Oil content: basis figure, with minimum accepted and price scale per point.
- Moisture basis for the oil result.
- Moisture maximum, for illustration in the high single digits, confirmed with your crusher.
- Admixture maximum, with its own price scale.
- Test method and settlement lab named.
Where we come in
We agree the oil basis, moisture basis and test method with the Gujarat supplier before booking. At loading we draw joint samples and send them to an NABL-accredited lab, so settlement rests on a result both sides accepted in advance. Start with a free sourcing review. For grades and terms, see what grades of castor seed are traded.