A rapid method to authenticate vegetable oils through surface-enhanced Raman scattering

Abstract

Vegetable oils are essential in our daily diet. Among various vegetable oils, the major difference lies in the composition of fatty acids, including unsaturated fatty acids (USFA) and saturated fatty acids (SFA). USFA include oleic acid (OA), linoleic acid (LA), and α-linolenic acid (ALA), while SFA are mainly palmitic acid (PA). In this study, the most typical and abundant USFA present with PA in vegetable oils were quantified. More importantly, certain proportional relationships between the integrated intensities of peaks centered at 1656 cm(-1) (S1656) in the surface-enhanced Raman scattering spectra of different USFA were confirmed. Therefore, the LA or ALA content could be converted into an equivalent virtual OA content enabling the characterization of the USFA content in vegetable oils using the equivalent total OA content. In combination with the S1656 of pure OA and using peanut, sesame, and soybean oils as examples, the ranges of S1656 corresponding to the National Standards of China were established to allow the rapid authentication of vegetable oils. Gas chromatograph-mass spectrometer analyses verified the accuracy of the method, with relative errors of less than 5%. Moreover, this method can be extended to other detection fields, such as diseases.

Figure 1

(a) SERS spectra and molecular structures of PA, OA, LA, and ALA. SERS spectra of (b) OA/PA, (c) LA/PA, and (d) ALA/PA with different molar ratios.

Figure 2

(a) Response curve between S₁₆₅₆ value and the USFA content for the OA/PA, LA/PA, and ALA/PA samples. (b) Ratios of S₁₆₅₆ values for LA/PA and ALA/PA to OA/PA at the same USFA content; horizontal lines - mean values of the ratios; and represents the S₁₆₅₆ value of OA/PA. Error bars are marked.

Figure 3

(a) SERS spectra of OA/ALA with different molar radios. (b) Relationship between the normalized S₁₆₅₆ value and the normalized ETC of OA corresponding to (a). (c) Relationship between the normalized S₁₆₅₆ value and the normalized OA content corresponding to Fig. 1(b). Error bars are marked.

Figure 4

(a) SERS spectra of pure OA and three types of vegetable oil (soybean, sesame, and peanut). (b) Box plots of S₁₆₅₆ corresponding to the ETC of OA, with the measured S₁₆₅₆ values of three types of vegetable oils (Luhua brand). Error bars are marked.