Oliiiive!

Nutritionally useful carotenoids and phenolic compounds increase as olives grow but then decrease as they ripen. Now, researchers have demonstrated that monitoring two Raman bands can help growers keep an eye on these changes and so optimise their product for olive oil production.

Macarena López-Sánchez, María José Ayora-Cañada, and Antonio Molina-Díaz of the Department of Physical and Analytical Chemistry, Faculty of Experimental Sciences, University of Jaen, in Spain, hoped to demonstrate the potential of ATR-FTIR (Attenuated total reflectance-Fourier transform infrared spectroscopy) and Raman spectroscopy in evaluating changes that occur during the development and maturation of olive fruit.

Olives are the fruit of the Olive tree (Olea europaea) and represent a major component of the agriculture and cuisine of the Mediterranean region of southern Europe, North Africa, and the Middle East, not least because of the highly prized oil extracted from the fruit. Olives are also now grown in Chile and Australia, but today Spain is the world's largest producer, with 36 per cent of the total international output. Italy produces a quarter and Greece about 18 per cent of the world production 2.5 million tons (presumably those figures include stones).

With the quality of product, both fruit and oil depending keenly on the ripening process, researchers in the field are constantly looking for methods of determining the optimal point to harvest and press olives. Now, Ayora-Canada and colleagues have analysed the spectra of different parts of the olive (skin, flesh and stone) at different stages of development.

"The determination of several components at different maturity stages has been approached using different techniques like gas chromatography for triterpene alcohols and sterols and HPLC for phenolics, chlorophyll and carotenoids and organic acids," the researchers say. Even magnetic resonance imaging has been used to obtain information about changes in oil and water within olives, they add.

The team now reports, in the current online issue of the Journal of Agricultural & Food Chemistry, the evolution of the different spectral bands and how they relate to the concentrations of major olive constituents such as triglycerides, water, antioxidant carotenoids and phenolic compounds.

The team found that oil accumulation can be followed using both FTIR and Raman spectroscopy and that increases in bands at 1746 cm^-1 (ATR-FTIR) and 1440 cm^-1 (Raman) correlate well with the oil content in the fruit determined using the standard Soxhlet extraction method.

They point out that the ATR-FTIR spectra of overripe olives does not provide a representative picture of the olive flesh because the accumulation of water on the surface of the ATR crystal hinders the application of this technique. However, Raman spectroscopy can accurately reveal the increase in carotenoids and phenolic content during olive growing and show how they decrease during the ripening phase by monitoring the 1525 and 1605 cm^-1 Raman bands, respectively.

"Following the maturation progress with vibrational spectroscopic techniques, oil accumulation in the fruit could be easily monitored providing a rapid tool to classify olives depending on their maturation stage," the researchers say, "This could be useful, for example, to diagnose if a fruit is mature enough to be collected, with a spectroscopic criteria, more objectively than by visual inspection of colour changes."

"The evolution of the content of phenolic compounds and carotenoids derived from the Raman spectra can be an important indicator of the optimum harvest time," the team concludes.