Most castor oil bottles say "pure". Fewer say USP grade. The difference is a published standard, and whether the oil has been tested against it.
What USP grade means
USP stands for the United States Pharmacopeia, which publishes quality standards for medicines and their ingredients. Its castor oil standard, called a monograph, defines castor oil as the oil from the seed of Ricinus communis with nothing added. An oil labeled USP grade has to pass each test in that monograph.
What the USP standard tests
The castor oil monograph sets limits like these (figures from the USP 32 edition):
| Test | What it checks | USP limit |
|---|---|---|
| Specific gravity | That the oil has castor oil's density | 0.957 to 0.961 |
| Hydroxyl value | Castor oil's signature chemistry, which comes from ricinoleic acid | 160 to 168 |
| Iodine value | How unsaturated the oil is; out of range can signal blending | 83 to 88 |
| Saponification value | The overall fatty acid makeup | 176 to 182 |
| Free fatty acids | Freshness and breakdown | Low limit set by titration |
| Heavy metals | Contamination | 0.001% maximum |
| Distinction from other fixed oils | That it isn't mixed with other oils | Pass or fail |
What "cosmetic grade" means
"Cosmetic grade" has no single published definition in the US. It usually means the oil is meant for skin and hair, but which tests it passed is up to the seller. Some cosmetic-grade oils are excellent; the label alone just doesn't tell you.
Which one should you buy?
For oil you put on your skin, scalp, lashes and brows every day, a published standard is the easier way to know what you're getting. Look for "USP grade" on the label.
BIOCENTIC castor oil is USP grade, USDA organic, cold-pressed and hexane-free, and every batch is lab tested.
Sources
USP 32 Castor Oil monograph, via DrugFuture
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