Abstract
The authenticity of olive oil has been a significant long-term challenge. Extra virgin olive oil (EVOO) is the most desirable of these products and commands a high price, thus unscrupulous individuals often alter its quality by adulteration with a lower grade oil. Most analytical methods employed for the detection of food adulteration require sample collection and transportation to a central laboratory for analysis. We explore the use of portable conventional Raman and spatially-offset Raman spectroscopy (SORS) technologies as non-destructive approaches to assess the adulteration status of EVOO quantitatively and for SORS directly through the original container, which means that after analysis the bottle is intact and the oil would still be fit for use. Three sample sets were generated, each with a different adulterant and varying levels of chemical similarity to EVOO. These included EVOO mixed with sunflower oil, pomace olive oil, or refined olive oil. Authentic EVOO samples were stretched/diluted from 0% to 100% with these adulterants and measured using two handheld Raman spectrometers (excitation at 785 or 1064 nm) and handheld SORS (830 nm). The PCA scores plots displayed clear trends which could be related to the level of adulteration for all three mixtures. Conventional Raman (at 785 or 1064 nm) and SORS (at 830 nm with a single spatial offset) conducted in sample vial mode resulted in prediction errors for the test set data ranging from 1.9–4.2% for sunflower oil, 6.5–10.7% for pomace olive oil and 8.0–12.8% for refined olive oil; with the limit of detection (LOD) typically being 3–12% of the adulterant. Container analysis using SORS produced very similar results: 1.4% for sunflower, 4.9% for pomace, and 10.1% for refined olive oil, with similar LODs ranging from 2–14%. It can be concluded that Raman spectroscopy, including through-container analysis using SORS, has significant potential as a rapid and accurate analytical method for the non-destructive detection of adulteration of extra virgin olive oil.
Highlights
Olive oil is obtained from the fruit of the olive tree (Olea europaea) and extra virgin olive oil (EVOO) has been prized throughout the ages for possessing unique qualities based on its medicinal, cosmetic, nutritional and taste, as well as even ceremonial value
We explored the potential of Raman spectroscopy combined with chemometrics approaches for exploratory analysis and multivariate statistical regression methods to identify adulterants and quantify their concentration in Extra virgin olive oil (EVOO)
In the present study we have employed three handheld Raman instruments to investigate whether it is possible to develop models that allow for the quantification of specific adulterants in EVOO
Summary
Olive oil is obtained from the fruit of the olive tree (Olea europaea) and extra virgin olive oil (EVOO) has been prized throughout the ages for possessing unique qualities based on its medicinal, cosmetic, nutritional and taste, as well as even ceremonial value. Adulteration of EVOO with low-cost or poor-quality edible oils for economic benefit is of considerable concern to the olive oil industry Trade organisations such as the International Olive Council (IOC) have laid out clear definitions for the different categories of olive oil and olive-pomace oil, as well as having clear trade standards and approved testing methods for their identification. These include fatty acid composition analysis by gas chromatography, stigmastadiene content by liquid chromatography [8], as well as organoleptic tests. In addition to the IOC approved approaches, several analytical techniques have been applied to detect the authenticity of EVOO, and these include mass spectrometry, gas chromatography, high-performance liquid chromatography and nuclear magnetic resonance (NMR) spectroscopy [5,10,11,12,13,14]
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