Correlation between refractive index of vegetable oils measured with surface plasmon resonance and acid values determined with the AOCS official method

- A new SPR method to measure acid value (AV) of vegetable oils was developed.
- RI of SPR correlated well with AV of the AOCS official method.
- RI of SPR was most suitable for refined oils.
- RI of pressed and non-refined oils was improved by applying a membrane filter.
- The determination coefficient between RIs and AVs was more than 0.9.

The acid value (AV) of edible vegetable oils provides important information about the quality and the degree of purity of oil during processing and storage. The present study was designed to correlate the refractive index (RI) obtained using SPR with AV obtained using the official AOCS method (Cd 3a-63). The SPR curve obtained from methanol layer of oil-methanol mixture was converted into RI based on pure water. The value of RI increased with oleic acid content added in vegetable oil. Oleic acid content of soybean oil had the determination coefficient of 0.988 with RI of SPR and 0.997 with AV of the official method at the significant level of 0.01. Samples of refined vegetable oils (4 kinds and 12 brands) and pressed and non-refined vegetable oils (3 kinds and 9 brands) were analyzed. RI of SPR correlated well with the AV of the AOCS official method, with the determination coefficient of 0.91-0.98. Every correlation equations of oil samples have the same slope of 0.005 and y-intercept from 1.3297 to 1.3301.