Rapid Detection of the Oxidation of Almonds Based on Surface Enhanced Raman Spectroscopy

The oxidation of almonds during the storage and processing would seriously affect their nutrition and quality. The purpose of this study was to establish a sensitive and reliable method for rapid detection of the oxidation of almonds. A rapid and simple phase-transfer of gold nanoparticles (AuNPs) from aqueous solution to toluene was achieved via a strategy of surface ligand exchange. The results of UV-Vis and transmission electron microscopy showed that the morphology of AuNPs after phase-transfer didn’t change obviously. The AuNPs in toluene could be successfully used as an efficient SERS substrate for detecting the oxidation of almond oil. It was showed that the characteristic Raman signal of the cis-double bond at 1655 cm⁻¹ of almonds oil gradually decreased during oxidation. Raman signal at 1747 cm⁻¹ of the ester bond was selected as the reference, and the relative intensity of I₁₆₅₅/I₁₇₄₇ showed a good linear relationship with the time of accelerated ageing of almonds (R²=0.98). By virtue of principal component analysis, the developed method is able to quickly and accurately distinguish the almonds with different oxidation levels. This study also provided a new strategy for the preparation and application of SERS substrates for fatty foods.