Clarifications of the Carbonyl and Water Absorptions in Fourier Transform Near Infrared Spectra from Extra Virgin Olive Oil

Abstract

AbstractThe Fourier transform near infrared (FT‐NIR) spectrum of extra virgin olive oils (EVOO) shows two minor carbonyl absorptions at 5280 and 5180 cm–1 that has been used to assess their authenticity. To establish components absorbing at 5280 cm–1, volatile aldehydes and ketones, triacylglycerol (TAG), diacylglycerols (DAG), free fatty acids (FFA), phenolics, and water are investigated and sometimes added to refined olive oil (ROO). Except TAG, the remaining carbonyls contribute to 5280 cm–1 by broadening peak. Water absorption is demonstrated by its removal using Na2SO4 or deuterium oxide addition; FT‐NIR spectral changes are reconstituted by water addition. Water absorption depends on being free or complexed with polar compounds in oil. The size of absorption is not related to abundance, but on unique absorption specificity of components; water shows the strongest absorption. Heat removes water and volatiles, leaving behind DAG, FFA, and phenolics, and makes it possible to differentiate absorption of water, volatile and non‐volatile carbonyls. Cloudy olive oils are analyzed using FT‐NIR methodology after warming for 3 min at 50 °C. FT‐NIR index values are replaced by a new calibration model based on correlating gravimetric mass loss of water plus volatiles with spectral changes. The FT‐NIR methodology is expanded to include EVOOs with 15.5% to 21% linoleic acid.Practical Applications: Testing for authenticity of EVOOs remains a challenge because adulterations continue to be a problem due to economic gains. Spectroscopy methods, specifically FT‐NIR, are much preferred to targeted chemical methods because they measure all constituents in products and are non‐destructive and fast. The current universal FT‐NIR methodology assesses 13 different parameters: five major FAs, and the DAG and FFA contents. The FT‐NIR index value measuring the content of moisture plus volatiles is now replaced by a gravimetric determination. The methodology identifies four major types of adulterants, high in oleic acid, linoleic acid, palm olein or ROO. The composition of olive oils makes it necessary to develop five oil‐specific groups, but cloudy samples still need to be clarified by slight warming before measuring. The value of this universal FT‐NIR methodology will increase after being adopted by commercial and in regulatory settings.