Detection of adulteration of olive oil with canola and other seed oils by reversed-phase high performance liquid chromatography

Abstract

Reversed-phase high performance liquid chromatography has been shown to be very promising in the detection of olive oil adulteration with seed oils. This is especially true for seed oils with a high content in linoleic acid. The objective of this research was to evaluate the RP-HPLC method for detection of a seed oil with low linoleic acid content as the adulterant in olive oil. Twenty two authentic virgin olive oil samples were obtained and analyzed for triacyglycerol composition by RP-HPLC. Optimization of the method allowed fast separation of the triacylglycerols. Less than 30 min was needed for analysis of one sample, including its preparation. Based on this analysis two useful factors were established for the assessment of olive oil purity. First, authentic olive oil should not contain more than 1% of triacylglycerol species with equivalent carbon number (ECN) of 42 and second, the ratio of the area % of the peak that represents triacylglycerols with ECN of 46 to the area % of the peak that represents triacylglycerols with ECN of 44 should not be less than the value of 3.9. The twenty two virgin olive oil samples were then mixed with various levels (2.5-30% w /w) of canola oil, a high-oleic acid type of oil like olive oil, and analyzed by RP-HPLC. At the 7.5% level of adulteration the use of the area % of the peak for ECN of 42 could detect only 64% of the samples as adulterated. At the same level of adulteration the use of the peak ratio revealed 77% of the samples as adulterated. All samples were considered as adulterated by both factors when they were mixed with ≥10% canola oil. The detection of corn, sunflower and soybean oils was feasible by both factors even at a 2.5% (w /w) admixture with olive oil. In addition, qualitative results from the RP-HPLC method led to distinction between olive oil samples adulterated with canola oil from those adulterated with soybean oil. All samples were also analyzed for refractive index, absorption of UV light and fatty acid composition by GLC. These methods were inferior to RP-HPLC as adulteration with the above mentioned seed oils below 5% was not detected. More specifically, RI analysis could detect the seed oils only when they were present in >30% in olive oil. UV analysis could detect ≥5% sunflower and soybean oils, ≥10% corn oil and ≥12.5% canola oil in olive oil. The determination of the fatty acid composition was useful in the detection of >25% corn oil, >20% sunflower oil, >10% soybean oil and >12.5% canola oil in olive oil. Finally, a survey on commercial olive oil products was also conducted. From the RI, UV and fatty acid composition data it was concluded that all samples were properly labeled. However, based on RP-HPLC methodology as suggested in this thesis, 64% of the samples fell outside of characteristic limits for olive oils and thus, were considered as adulterated. This research work illustrated the advantages of the RP-HPLC technique over other fast and commonly utilized analytical procedures. The method can detect as low as 2.5% of seed oils with high linoleic acid content but olive oils adulterated with less than 10% canola oil might escape detection. The oil of new developed canola varieties as well as other inexpensive oils with similar composition to that of olive oil, should be the subject of future investigation.