A New Technique to Monitor Olive Ripening

After an extended final exam-induced hiatus, I am finally back! Fortunately, food research didn’t slow down while I was paralyzed by the onslaught of papers and studying, so I have articles about sake, caseins, and Pastis in the pipelines. But first let’s take a look at a strikingly simple method for tracking the maturation of olive fruits.

As olives grow and ripen throughout the summer and autumn months, the endocarp (pit) matures and hardens within six weeks of blooming, followed by the slow growth of the mesocarp (flesh) and exocarp (skin), primarily as a result of oil accumulation in the flesh. As it grows, the fruit morphs from a green color due to chlorophyll and carotenoid pigments to purple and then black as anthocyanins replace the other pigment molecules, demonstrating full ripeness by early winter. This visible transformation is mirrored by alterations in the molecular structure of the fruit, which have been measured by such analytical techniques as gas chromatography, which can separate and identify particular components, especially long-chained alcohols (triterpene alcohols) and sterols, and high performance liquid chromatography (HPLC), used to quantify the presence of ringed alcohol structures (phenolics), chlorophylls and carotenoids, and acidic compounds. However, both of these methods look only at particular groups of olive fruit components and are thus of limited practicality.

López-Sánchez, et al. report the effectiveness of vibrational spectroscopy for tracking the changing composition of olive fruits during maturation. Vibrational spectroscopy encompasses a set of techniques that analyze the unique interactions between different types of molecules and light. Through this type of analysis, chemists are able to determine the functional groups present in the sample. To study olive growth, López-Sánchez, et al. employ two of the most common examples of vibrational spectroscopy, infrared (IR) and Raman spectroscopy. These methods provide a broader analysis of fruit composition than the aforementioned chromatographic techniques. Moreover, vibrational techniques are quicker because the samples are much easier to prepare. Samples using intact olives can be taken of the skin, while only slicing and removal of the flesh are necessary to prepare samples of the flesh and pit. In the Spanish study, IR and Raman spectroscopy of ripe fruit confirmed the presence of water, lipids, and polysaccharides (especially cellulose and pectins) in the flesh, waxes and polysaccharides in the skin, and the woody compound lignin, along with water, in the pit. These data provide verification that these spectroscopic techniques are capable of discerning the compounds known to exist in each portion of the fruit.

An example of an IR spectrum - this is the spectrum of beta-carotene (Spectrum courtesy of SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, date of access)

Upon comparison of the spectra taken of olives at varying levels of maturity, the most prominent changes were observable in the mesocarp, and could be attributed to oil accumulation. As the olives mature, their spectra increasingly resemble those of pure olive oil, clearly indicating that the proportion of oil in the fruit is increasing. By measuring the change in the area of oil-specific peaks in the spectra over the ripening period (June to February), a three-phase trend was observed. Before hardening of the pit, there was no change in oil content. However, this period was followed by a rapid increase in oil between July and October, after which the oil level stabilized again. Due to an artifact of the IR spectroscopy technique, the IR data suggest that after a period of time the oil levels decrease, but this trend does not appear when Raman spectroscopy is used, or when oil levels are measured by the traditional method of oil extraction. The researchers posit that this apparent decrease is a result of the puncturing of cells as pressure is applied to ensure tight contact between the fruit and the ATR crystal (the effective light source for the IR spectrometer). Water is released as the cells burst, thus decreasing the apparent proportion of oil in the sample. This only occurs in ripe fruit because textural changes occur during the maturation process, rendering the ripe olive more delicate. Due to this artifact, the authors recommend using Raman spectroscopy, where samples can simply be placed in a holder for spectroscopic readings, rather than IR when investigating the structure of ripe olives.

By again comparing the areas of specific peaks over the ripening period, the researchers were able to track changes in phenolic and carotenioid compounds of olive flesh during growth. Phenolics were found to rapidly decrease before the pit hardened, and then gradually increase again until about September, at which point the compounds decreased again, finally stabilizing by early winter. This seemingly erratic trend can be explained by the dual origin of phenolic compounds in olives. Lignins and their precursors are phenolics, thus before the stone hardens, these compounds are abundant in the flesh, but as hardening, or lignification, occurs, they become sequestered in the pit, resulting in their observed decrease in the flesh. The subsequent trend is consistent with previously observed behavior of phenols in olive flesh, with a gradual increase followed by a sharp decrease as the fruit blackens.

Carotenoids follow a similar evolution as observed in the behavior of phenolics post-lignification. As the fruit grows, carotenoid content increases along with the increasing fruit mass. Once the olive is fully developed, carotenoid levels fall off, in correspondence to the color transformation from green to black, as carotenoids and chlorophylls are replaced by red-purple anthocyanin pigments. Notably, in analyzing the spectra taken from olive skins, the researchers noticed that as the skin thins upon maturation, the Raman spectra taken of whole, intact olives comes to more closely resemble that of the olive flesh, indicating that the lasers used in Raman spectroscopy are able to penetrate the fruit’s skin. This observation suggests that Raman spectroscopy of intact olives could be used to monitor changes in the flesh, such as oil accumulation. Raman spectroscopy thus appears to be a practically straightforward and non-invasive method for tracking the molecular changes associated with olive maturation and growth, and could be used to determine when to harvest the fruits to obtain the desired oil content.

López-Sánchez, M.; Ayora-Cañada; M.J.; Molina-Díaz, A. Olive Fruit Growth and Ripening as Seen by Vibrational Spectroscopy. J. Agric. Food Chem. Published online November 18, 2009.