Changes in volatile compounds of Ayvalik (Edremit) and Uslu olive oils depending on conditions and time of storage

'n.d. not detected.

Data showed mean of two independent tests (mean ±sd).

a-c Values in the same row with different superscript letters differ significantly (p < 0.05).

The storage of olives in inappropriate conditions had a negative impact on the aroma profiles of oils. Uslu olive oil was characterized with high concentrations of terpene volatile compound such as a-Farnesene (614±2) and aldehydes as 2-hexenal (146±3) and hexanal (140.5±4.6). In contrast, Edremit olive oil had lower concentrations of a-Farnesene and 2- exenal, which were determined as 150±20 and 14±1.3, respectively. The increase of concentration of 2-hexanal (E) during holding periods could be explained by the activity of the fungal enzymes in LOX pathway of olive fruits (Schnurer et al. 1999). However, the concentration of hexanal (251±11) in Edremit olive oil was immediately increased after extraction. In this study, from alcohol compounds 1-Hexanol and 3-Hexen-1-ol were found as the most potent volatiles of the Edremit oils, which concentration increased over holding time in both holding types. In Uslu olive oils, the concentration of 1 -hexanol also increased significantly (p<0. 05) during the holding time as observed for Edremit oil. The concentration of 3-Hexen-1-ol in Uslu oil during holding time did not increase in contrast to Edremit oil. Lower values of hexanal, hexanol, cis-3-hexenol and higher values of methanol, ethanol and 2-methyl butanol (fusty) in samples from plastic bags were also observed by other researchers (Koprivnjak & Conte 2002). The presence of hexanol was in virgin olive oils also reported (Haddada et al. 2007; Krichene et al. 2010; Manai et al. 2008). However, Kaftan (2007) did not identify hexanol in Memecik olive oils of 2005 and 2006 harvesting seasons. These differences between the findings can be related to the activity of alcohol dehydrogenase enzyme. As seen from Table 3 and Table 4, phenylethyl alcohol was also detected and its content increased during holding periods in both type of olive oils. Its occurrence was due to the degradation of phenylalanine (Issaoui et al. 2009).

Previous studies focused on relationship between irradiation or weather conditions and content of volatile compounds in olive oil before harvesting. However, no studies determined the effect of weather conditions on the content of volatile compounds in olive oil after harvesting. Servili et al. (2007) found that the presence of several volatile compounds, such as hexanal, (E)-2-hexenal, and other lipoxygenase derivative products positively correlated with the irrigation rate. In our study hexanal was not detected between 7-14 days when weather was stormy and heavy rain in both two variety of olive oils, while dimethylpalmitamine was not detected in stormy weather only in Edremit variety olive oils. The content of 2-hexen-1-ol significantly increased in Edremit olive oils holded in nylon sacks between 7-14 days. However, it was not detected in oils holded in plastic boxes. Similarly, the content of cycloheptanone 2-methylene significantly increased in Edremit olive oils holded in nylon sacks between 14-21 days (cloudy weather), however it was not detected in Uslu and Edremit oils holded in plastic boxes. While decrease in a-Farnesene in Edremit olive oils holded in nylon sacks between 0-7 days (sunny weather) was significant, this decrease was not so great in Uslu olive oils holded in nylon sacks during the days of 0-7.

Hexanal, which is responsible for fatty, fruity, green odor perceptions and contributed to pleasant sensory perceptions, was formed due to activity of aldehyde-lyase enzyme (Table 2). Therefore, the content of hexanal has been correlated with the highest overall grading of virgin olive oil (Morales & Aparicio 1999). In this study, the concentration of hexanal decreased more in oils obtained from Edremit variety holded at cases compared to oils extracted from Edremit variety holded at nylon sacks during 7 days. It decreased to 29.74±0.14 and to 9±2 in nylon sacks and case, respectively. While the ratio of hexanal to nonanal was lower than 2, the oil was evaluated as rancid which detectes the beginning of oxidation (Kesen et al. 2013). Therefore, according to the results of 14th and 21th days, it has seen that hexanal did not occur in the oil because of it is under the detectable limits. This indicated that hexanal/nonanal ratio was below 2. However, the decrease in hexanal content was not significantly different in oils obtained from Uslu variety olives which were holded for 7 days in sacks and cases (p>0.05). Acetic acid which is responsible for pungent, sour, vinegar-like odor can be formed by microorganisms growing on or in the olives during storage and are undesirable for olive oils (Davis 2007). Because of the production of acetic acid, Acetobacter could develop and cause vinegary defect. Acetic acid concentration in this sudy was not effected significantly by holding time and holding type in both olive oils during 21 days of storage (p>0.05). While 5-Hepten-2-one 6-methyl and 1-Butanol 3-methyl volatiles compounds were not detected in Edremit and Uslu olive oils at initial day, they formed during both sack and box holding due to the activity of the microorganisms in olives (Table 3 and Table 4). The occurence of 5-Hepten-2-one 6-methyl can be explained by the presence of bacteria from the genus Pseudomonas which are capable to degradate terpenic alcohols (Morales 2005).

PCA was used to highlight the data and to determine the relationships between the content of volatile compounds and olive oil variety during different holding times and types (Figure 1-3).

Figure 1. PCA plots of volatile compounds of Uslu variety olive oils.

Figure 2. PCA plots of volatile compounds of Edremit variety olive oils

PCA permitted a reduction of volatile compounds found in olive oil to three principal components (with eigenvalues>1). PCA results showed that 5 components were found (eigenvalues >1) for both olive varieties (Figure 4), but eigenvalues of two components were lower than the calculated percentile 95% they were dropped and only the first 3 components which eigenvalues were greater than percentile 95% were retained. Yang et al. (2017) used PCA and AHC chemometric analysis to determine the sensitive oxidation marker compounds, including (E, E)-2,4-heptadienal marker, by lipid oxidation in the n-3 PUFA rich oils. They found that oxidized volatiles did not rise linearly during oxidation and volatile compounds of lipid oxidation were not clearly identicable with PCA analysis (Yang et al. 2017).

Three PCs were extracted represent 81.27 % of the total variance of olive oil samples extracted from Uslu cultivar and 80.14 % of the total variance of olive oil samples extracted from Edremit cultivar. The first principal components PC1, PC2 and PC3 represented 45.15% and 41.31%, 21.90% and 21.39%, 14.21% and 17.43%, respectively for Uslu and Edremit varieties. First component was mainly defined by A11, A17 and A2 with positive correlation and second component mainly by A42 and A33 (with a high negative correlation) in olive oils obtained from Uslu varieties (Figure 1). On the other hand, first component was mainly defined by A22, A40 and A43 and second component mainly by A19, A1 and A7 with a high positive correlation in olive oils extracted from Edremit varieties (Figure 2).

Figure 4. Eigenvalues of volatile compounds of Uslu and Edremit olive oils obtained by PCA

Conclusions

Principal component analysis (PCA) was used to evaluate the effect of different types of olive packaging (plastic box and nylon socks) and different period of olive storage in these packaging on oil aroma profiles and to find the relationships between volatile compounds and olive oil varieties. Content of hexanal decreased significantly in oils obtained from Edremit variety during 7 days of holding in case. However, the holding type had no effect on the hexanal content in Uslu olive oils. PC1, PC2 and PC3 identified the 81.27 % of the total variance of olive oil samples extracted from Uslu cultivar and 80.14% of the total variance of olive oil samples extracted from Edremit cultivar. The content of acetic acid, which is an undesirable compound in virgin olive oils, was not changed significantly in both two oils depending on holding type or time.

To provide high quality of olive oils conditions for its storage should be controlled.The recommended storage conditions for olives is about 5 °C in air conditioning boxes for at least 30 days to reduce development of fungi and protect aroma profiles of obtained oils from changes.

It's needed more studies on the investigation of olive oils aroma profile for reveal the effects of harvesting time and method, olive varieties, climatic factors, storage conditions, and extraction techniques.

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