Introduction

Olive (Olea europaea L.), an ancient plant cultivated by humans, rich in bioactive compounds with medical and therapeutic benefits, grows in the Mediterranean basin and some parts of Asia, but olives are widely used for the production of table olives in many parts of the world. are grown [1, 2]. Olive has been produced as olive fruit and extra virgin olive oil (VOO) in this region for more than 5000 years [3]. Although olive cultivation has adapted to various regions of South America where suitable conditions exist for the production of most olives are produced in the Mediterranean basin [2, 4, 5]. It has been reported that the production of fermented table olives exceeded 2.6 million tons in the 2020–2021 harvest season and the main producing countries are Egypt, Tunisia, Morocco, Algeria, Spain and Greece [6]. Olive oil, which contains high amounts of unsaturated fatty acids (especially oleic acid), is also characterized by lower concentrations of saturated fatty acids such as stearic and palmitic fatty acids [2, 7]. Among the bioactive compounds found in olives, monounsaturated fatty acids are oleic acid, which accounts for approximately 65% of the total fatty acid profile of olives, followed by decreasing amounts of linoleic and linolenic acid [2, 8]. This fatty acid profile shows that olive oil provides a protective effect against cardiovascular diseases and various types of cancer [9,10,11]. In traditional table olive production, the bitterness removal of green olives is commonly done by keeping them in high brine concentration for 3 days and then fermenting them spontaneously in brine at certain concentrations [12, 13]. Various chemical and microbiological activities may occur during fermentation, and these olive fermentation methods can greatly affect the competitive activities of spontaneous fermentation’s microflora [14]. Microbiological and chemical changes may occur in the environment during the production of fermented table olives [15,16,17]. However, as a result of different debittering processes of green olives of “Ayvalık” and “Gemlik” varieties, the fatty acid map was not determined before and after fermentation. Fermented table olives are consumed with pleasure all over the world. Consumers’ perception of quality today necessitates the production of healthier products. The main purpose of table olive fermentation is to protect olives against spoilage and improve the sensory properties of the product, as well as to ensure consumer appreciation by contributing to the characteristic aroma resulting from the production of volatile compounds [18, 19]. Table olives, one of the most important fermented products of the modern world, have been produced since ancient times. Olive fruit differs from other single-seed fruits in terms of chemical composition as it contains lower sugar, higher fat content and contains a bitter substance called oleuropein. For this reason, it is not consumed when plucked from the olive branch. The bitter substance in its structure must be removed. Olive fruit can be ready for consumption by removing its bitterness using different methods. The most important thing to consider when choosing these methods is to choose the bitter removal method suitable to the olive processing technique (crushing, scratching, confit, etc.). Depending on the processing technique to be applied, the olives are fermented directly with brine or salt, after removing the bitterness with water or NaOH solution, or without pre-treatment for bitterness removal. Olives whose bitterness is removed with NaOH solution, popularly known as caustic, are processed olives. In natural olive production, olives are fermented directly with brine or salt without any pre-treatment [20]. No studies have been found on the oil contents of two commercially important types of olive fruits of Turkey after debittering them with different solvents and the fatty acid profiles of their oils as a result of fermentation. However, in addition to these features, the oil content and fatty acid profile distributions of the olives, which give taste and flavor to the olives, must also be observed before and after fermentation. Research work was carried out to investigate the oil results and fatty acid compositions of the oils obtained from “Ayvalık” and “Gemlik” olives debittered with different solutions and fermented.

Materials and methods

Material

Olives of “Ayvalık” and “Gemlik” varieties were provided from Mut district of Mersin province in October, 2022, and the olive fruits were pre-treated and had their bitterness removed using different solutions after being separated from foreign substances. The chemicals (NaOH and petroleum ether ) used in this study were obtained from Merck. The salt used in the brine was obtained from a salt seller in Konya as refined rock salt.

Methods

Debittering processing in olive fruits

Normal water, 10% salt water (salt:water; w/v) (brine), and 2% NaOH (NaOH:water; w/v) solution were used in the debittering process of olives. The olives, scratched, broken and whole, were subjected to the debitterness process in this solution.

Debittering with NaOH solution

The NaOH solution prepared at 2% concentration was kept at room temperature and cooled. To increase the penetration of the prepared NaOH solution into the olive fruit, 2% NaCl2 (NaCl2:water; w/v) was added to the solution. The solution was added to the olive fruits in the glass jar so that all of them remained in the NaOH solution. After the penetration rate of the NaOH solution into the olive fruit was checked using phenolphthalein, the NaOH solution was drained and the olives were washed with water until all of the NaOH was washed away.

Debittering by the lye

After boiled water at 50 °C was added to oak wood ash and olive fruit mixed in a 1:1 ratio, the olives were kept in the ash solution for 24 h, stirring at regular intervals. Then, the lye was filtered and removed from the olives, and the olives were thoroughly washed with tap water until there was no ash residue remaining.

Debittering by the brine

The debittering process with brine prepared at 10% concentration was carried out in a total of 5 days, changing the brine at 12-hour intervals.

Debittering by the water

The debittering process of olives with water was carried out for a total of 5 days by changing the water used to debitter the olives every 24 h. Then, the olives were subjected to fermentation.

After the bitter olives were placed in a 1 L glass jar, 10% brine (salt water) was added to the olives and the olives were fermented naturally for 6 days. The fermentation process continued for 60 days in total. Oil content and fatty acid composition analyzes of olives were performed on the 0th and 60th days of fermentation. Additionally, for control purposes, the olives were subjected to raw fruit analysis before processing (0th day).

Determination of oil contents

Once the fermented olives were finely chopped, they were dried at 70 °C to moisture content 14%, and ground into powder. After 10 g of the powdered olive sample were weighed into the Soxhlet cartridge, it was tightly closed with cotton and placed in the Soxhlet extractor. The extraction process was carried out with petroleum ether at 50 °C for 6 h. Then, the petroleum ether in the mixture was evaporated in vacuum rotary evaporator, and the remaining crude oil was calculated gravimetrically (%) [21].

Determination of fatty acid composition

Fatty acid methyl esters of olive fruit oil was esterified according to the method explained in ISO-5509 [22] were identified by gas chromatography (Shimadzu GC-2010) assembled with flame-ionization detector and capillary column (Tecnocroma TR-CN100, 60 m × 0.25 mm, film thickness: 0.20 μm). Nitrogen was used as carrier gas at a flow rate of 1.51 mL/min and a split rate of 1/50 µL/min. Using commercial standards (Fatty acid methyl esters (FAME)), different fatty acids were identified and their retention times were determined. Quantification of each compound was expressed as a percentage based on the area under the peak of the corresponding compound and the percentage of all detected compounds.

Statistical analysis

In interpreting the data analyses, the significance of the difference between the means was tested using ANOVA, using the Minitab 16 statistical package program (PSU-MSS). Duncan test was used to compare the groups that were significantly different in this test, and p < 0.01 and p < 0.05 values were considered to be statistically significant.

Results and discussion

Oil amounts of olive fruits fermented for 60 days after removing the bitterness

Oil contents of olive fruits fermented for 60 days after removing the bitterness of “Ayvalık” and “Gemlik” olives using different debittering solutions (drinkable water, brine (salt), lye and NaOH) in “scratched”, “crushed” and “whole” states are shown in Table 1. The oil amounts of the scratched “Ayvalık” and “Gemlik” olive varieties on the 0th day of fermentation were determined between 50.01% (lye) and 67.57% (NaOH) to 57.35% (salt) and 72.95% (NaOH), respectively. The oil results of scratched “Ayvalık” and “Gemlik” olives on the 60th day of fermentation were determined between 61.04 (salt) and 64.03% (water) to 55.89 (salt) and 61.95% (NaOH), respectively. The oil results of crushed “Ayvalık” olive fruits were recorded to be between 56.25 (salt) and 70.90% (NaOH) to 51.50 (NaOH) and 59.25% (salt) on the 0th and 60th days of fermentation, respectively. Additionally, the oil contents of the crushed “Gemlik” olive variety on the 0th and 60th days of fermentation were determined between 57.31 (lye) and 70.55% (NaOH) to 56.18 (lye) and 59.78% (salt), respectively. The oil results of whole “Ayvalık” and “Gemlik” olive varieties on the 0th day were determined between 48.98 (salt) and 68.93% (NaOH) to 45.61% (water) and 72.15% (NaOH), respectively. However, the amount of oil extracted from whole “Ayvalık” and “Gemlik” olive fruits on the 60th day were recorded between 54.80 (water) and 59.08% (NaOH) to 52.28 (water) and 58.99% (NaOH), respectively. While the oil contents of “Ayvalık” and “Gemlik” olive varieties that were scratched and crushed at the beginning of fermentation (day 0) were higher than the control, the oil contents of whole olives were lower than the control. However, it was generally observed that the amount of oil extracted from olives on the 60th day of fermentation was low compared to the oil amount of olives at the beginning of fermentation. In general, the greatest decrease in the oil result of both olive varieties was detected in olives that were subjected to crushing. It is thought that this decrease is probably due to the oil leaking out of the fruit due to the process applied during crushing. Depending on the debittering applied, in general, the highest oil content in olives at the end of fermentation was detected in NaOH-treated olives. The lower oil content of fermented olives compared to the beginning of fermentation may be due to the passage of soluble substances from the fruit into the brine due to tissue softening during fermentation. However, since the tissues of the olives treated with NaOH softened, these soluble substances passed into the brine more, and the oil content in these olives was higher than the others. As a result of debittering in fresh water, the oil content of olives was generally lower than those with other debittering processes. The highest oil amount was recorded in olives treated with NaOH, followed by lye, brine (salt) and potable water in decreasing order. In a previous study, while 69.7% oil was determined in Koroneiki olives, it was stated that this oil rate increased to 72.13% after 6 h of pectinase treatment [23]. Simoes et al. [24] reported that fresh fruits of “Ascolano” and “Grappolo” olive cultivars contained 15.38% and 20.15% oils, respectively. The findings regarding the oil content of olive fruits are consistent with the range of values reported in the literatüre. In addition, there are partial differences with the results of previous studies. The oil content of olive fruits varies depending on the olive variety, and it is thought that climatic conditions such as temperature and rainfall, agricultural practices such as fertilization and irrigation, and the olive oil production process also affect the oil content of olives.

Table 1 Oil contents of olive samples before and after fermentation (%)
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The fatty acid compositions of the oils extracted from fresh and fermented “Ayvalık” and “Gemlik” olive varieties debittered

The fatty acid profiles of the oils of “scratched, crushed, and whole” fermented Ayvalık and “Gemlik” olive varieties debittered in different debittering solutions (drinkable normal water, brine (salt), lye and NaOH) are offered in Table 2. Significant changes were monitored in the fatty acid compositions of olive oils basing on the olive variety and the bittering process (p < 0.01). In general, the bittering process did not have a significant impact on the fatty acid profile of olive oils. However, there were some small differences in the fatty acid content of olive oils depending on the bittering process used. The dominant fatty acids of the oils obtained from olives on the 0th and 60th days of fermentation were oleic, palmitic and linoleic acid. The fatty acid profile of the oils depending on the applied process and olive types differed at the beginning and end of fermentation (60 day). In general, the palmitic acid result of olive oils decreased slightly at the end of fermentation. The fatty acid amounts of olives partially increased compared to the control sample (0 day). Linoleic acid was the fatty acid that fluctuated the most depending on the olive variety, the applied debittering processes and the fermentation time. The linoleic acid content of olive oils was generally higher in “Ayvalık” olive oils than in “Gemlik” olive oils. The linoleic acid content of olive oils also varied depending on the bittering process used. For example, olive oils that were debittered with lye had a higher linoleic acid content than olive oils that were debittered with water or brine. Additionally, the linoleic acid content of olive oils generally decreased during fermentation. Palmitic acid results of scratched olives belonging to the “Ayvalık” olive variety on the 0th and 60th days were recorded between 15.01% (normal water) and 17.13% (NaOH) to 16.15% (normal water) and 22.05% (NaOH), respectively. Palmitic acid results of scratched olives belonging to the “Gemlik” olive variety were recorded between 15.41 (normal water) and 16.91% (NaOH) to 14.99 (normal water) and 17.40% (NaOH) on the 0th and 60th days of fermentation, respectively. The palmitic acid results of the oils obtained from crushed olives of “Ayvalık” and “Gemlik” varieties at the beginning (day 0) and at the end (day 60) of fermentation ranged from 14.86% (lye) and 16.18% (NaOH) to 15.77% (normal water) and 20.17% (normal water), and 15.82% (salt) and 16.34 (lye) to 16.52% (normal water) and 16.76% (lye), respectively. The palmitic acid amounts of the oils extracted from whole olives of “Ayvalık” and “Gemlik” olive varieties on the 0th and 60th days were determined between 15.29% (salt) and 16.29% (NaOH) to 14.79% (NaOH) and 16.02% (normal water), respectively. In general, although the palmitic acid content of olives decreased significantly compared to the control on the 0th day of fermentation, it was observed that it increased on the 60th day of fermentation. It can be said that on the 0th and 60th days of fermentation, the oleic acid contents of olives of both varieties generally increased compared to the control. The oleic acid results of the oils obtained from “ scratched “ olives of “Ayvalık” varieties at the beginning and end of fermentation varied between 70.10% (NaOH) and 71.66% (lye) to 64.91% (NaOH) and 71.51% (normal water), respectively. The oleic acid results of the oils extracted from the “crushed” olives of the “Ayvalık” olive variety during the same fermentation periods were identified to be between 69.63 (salt) and 71.28% (lye) to 67.28 (NaOH) and 70.31% (normal water), respectively. Also, the oleic acid amounts of all “Ayvalık” olive oils on the 0th and 60th days of fermentation were determined between 69.52 (NaOH) and 71.13% (salt) to 66.70 (NaOH) and 72.48% (normal water), respectively. However, the oleic acid contents of “scratched” olive oils of the olive variety on the 0th and 60th days of fermentation were between 70.42 (NaOH) and 72.36% (normal water) and 71.34 (NaOH) and 72.31% (normal water). Also, the oleic acid results of “crushed” Gemlik olive oils were determined between 71.55% (NaOH) and 72.71% (salt) and 70.36% (lye) and 73.22% (normal water) during the same fermentation periods (0th and 60th days), respectively. The oleic acid amounts of “Gemlik” olive oils in whole form on the 0th and 60th days of fermentation resulted between 71.51% (lye) and 72.75% (NaOH) to 71.84% (normal water) and 73.25% (lye), respectively. The oleic acid content of differently processed olives showed partial differences according to the applied parameters. The linoleic acid results of olive oils extracted from “Ayvalık” and “Gemlik” olives with different pretreatment and bittering treatments were recorded to be between 6.57% (gemlik, breaking, lye) and 10.70% (Ayvalık, breaking, salt) to 2.76% % (Ayvalık, scratching, NaOH) and 10.16% (Ayvalık, whole, lye) on the 0th and 60th days of fermentation, respectively. It can be seen that the linoleic acid content of “Ayvalık” olive oils was higher than that of “Gemlik” olive oils. The linoleic acid content of “Ayvalık” olive oils was generally higher than that of “Gemlik” olive oils, regardless of the debittering process used. For example, the linoleic acid content of “scratched” “Ayvalık” olive oils ranged from 7.74% (lye) to 10.47% (normal water) on the 0th day of fermentation, while the linoleic acid content of “scratched” “Gemlik” olive oils ranged from 7.09% (salt) to 8.01% (lye) on the 0th day of fermentation. However, it showed differences depending on the pre-treatments applied (drawing, breaking and whole). In general, on the 0th day of fermentation, the linoleic acid results of “Ayvalık” olive oils increased compared to the control, while the linoleic acid contents of “Gemlik” olive oils partially decreased. On the 0th day of fermentation, the linoleic acid contents of the oils extracted from the “scratching” olives of the “Ayvalık” and “Gemlik” olive varieties were detected to be between 7.74 (lye) and 10.47% (normal water) to 7.09 (salt) and 8.01% (lye), respectively. Linoleic fatty acid amounts of the same scratching-treated olive varieties were reported between 2.76 (NaOH) and 8.23% (salt) to 5.72% (NaOH) and 8.12% (salt) on the 60th day of fermentation. The highest amounts of linoleic acid in whole “Ayvalık” olive oils on 0th and 60th days were detected in olive oils that were debittered with normal water (10.35%) and lye (10.16%), respectively. The lowest and highest linoleic acid contents of “Gemlik” olive oil were detected in olive oil extracted from olives subjected to the “scratching” process, which were subjected to bitterness removal with NaOH (5.72%) and salt water (8.12%) on the 60th day of fermentation, respectively. In general, while the amount of saturated fatty acids in olives treated with NaOH increased, the amount of unsaturated fatty acids gradually decreased. This decrease may possibly be due to the reaction of the double bonds in unsaturated fatty acids with NaOH. On the 0th and 60th days of fermentation, the linoleic acid contents of olive oils belonging to the “Ayvalık” olive variety were higher than those of “Gemlik” olive oils. This may be due to plant genetics and variety. The stearic acid contents of the olive oils extracted from pre-treated olives of both olive varieties were found to be below 3.47% and although there was a statistical difference between the varieties and applications, the values were close to each other. It can be seen that debittering processes particularly affected the amounts of unsaturated fatty acids. In general, on the 60th day of fermentation, the oleic acid contents of “Ayvalık” and “Gemlik” olive fruits that were treated differently were partially reduced compared to the control (except Gemlik whole). This decrease may be due to the reaction of caustic and ash with fatty acids due to the fruits being exposed to the “Scratched” and “crushed” process. Ghanbari et al. [2] reported that fermented table olives have a wide range of functional properties as well as high oil and oleic acid content. It has been reported that oleic acid constitutes approximately 70% of the total major fatty acid in green “Nabali Baladi” fermented olives, while other fatty acids such as palmitic, linoleic and stearic acid constitute approximately 25% of the total fatty acids [5]. Dias-Martins et al. [25] determined 19.7% palmitic, 3.0% stearic, 2.4% palmitoleic, 21.4% linoleic, 0.02% linolenic, and 0.5% arachidic in the “Azapa” olive variety imported from Chile. In another study, as the ripening degree of Sarı Hasebi olive fruit progressed, the oleic acid content of Sarı Hasebi oil increased from 73.11 to 75.61%, in Gemlik olive oil from 71.84 to 74.57%, and in Halhalı olive oil it increased from 7.5 to 74.57%. It has been reported that it decreased from 73.20 to 67.28% [26]. El Riachy et al. [27] determined 11.89–17.76% palmitic, 0.37–1.16% palmitoleic, 2.01–4.42 stearic, 66.65–73.88 oleic, 7.58–14.69 linoleic, 0.54–1.01% linolenic 0.32–0.61% arachidic in the oils exracted from ten olive varieties. Ahmad et al. [5] reported that the oils obtained after the fermentation of “Nabali Baladi” olive fruits collected from Jordan in two harvest seasons (season 2017 and 2018) contain 14.3 and 14.3% palmitic, 1.5 and 1.6% palmitoleic, 2.6 and 2.6 stearic, 70.6 and 69.8% oleic, 8.2 and 8.2% linoleic, 0.73 and 0.74% linolenic and 0.4 and 0.4% arachidic, respectively. Lopez-Lopez et al. [28] reported that olive varieties were more effective on the fatty acid profile of olives and that olive processing and fermentation did not cause a significant change in fatty acids. Muhammad et al. [29] determined 15.4% palmitic, 1.5% palmitoleic, 3.5% stearic, 65.2% oleic, 16.3% linoleic, 0.7% linolenic, 0.3% arachidic and 0.2% gadoleic in “Kalamta” olive oil. The results obtained regarding the fatty acid profile obtained in this study were similar to the literature, but partial deviations were observed in their quantitative values. These changes may probably be influenced by factors such as different debittering processes applied to olives, fermentation, variety, genetic structure, harvest time, and growing conditions.

Table 2 Fatty acid compositions of oils from olive samples before and after fermentation (%)
Full size table

Conclusion

In general, the highest oil content in olives at the end of fermentation was detected in NaOH-treated olives. In addition, the fatty acid amounts of olives increased compared to the control sample (0 day). Linoleic acid was the most fluctuated fatty acid depending on the olive variety, the applied debittering processes and the fermentation time. In general, NaOH treatment increased the amount of saturated fatty acids in olives. Unsaturated fatty acids gradually decreased in amount. Olives processed for oil and for consumption should be subjected to a debittering process to break down the glycosides they contain. Generally, this process is carried out traditionally with either water or brine. In addition, it can be said that the bitterness removal process with lye used in this study had a positive effect on fatty acid composition.