Conventional and Green Extraction Techniques of Sour Cherry

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This entry is adapted from the peer-reviewed paper 10.3390/su16010032

Prunus cerasus, commonly known as sour cherry, is a fruit widely consumed during the summer season. Processing of sour cherries results in the generation of substantial amounts of by-products. Following the extraction of juice, the residual pomace (comprising of skin and flesh) and pits remain as by-products.

sour cherry tart cherry polyphenols anthocyanins green extraction techniques

1. Conventional Extraction Techniques Used for P. cerasus Pomace

A study to optimize the acquisition of phenolic components from PC pomace by conventional extraction was carried out by Yilmaz et al. ^[1]. The extraction was carried out in various proportions of ethanol: water (EtOH:H₂O) and specifically 0:100, 20:80, 50:50, 80:20, and 100:0 with 0.01% hydrochloric acid (HCl), at temperatures of 8, 25, 50, 75, and 92 °C. Initially, a small amount of solvent was placed in each flask, and mixing was carried out for 7–8 min. Thereafter, the following concentrations of 4, 6, 9, 12, or 14 mL/g were added for extraction in each flask separately and in various combinations with the previous factors. The extract was centrifuged at 6000 rpm for 10 min to obtain the test sample. In accordance with the results, the most suitable solvent ratio for the highest polyphenol recovery was 50:50, with the other factors not playing such an important role. Sample 2 appeared the most suitable combination of extraction factors, and the polyphenol content of PC pomace under these conditions was 14.46 mg/g. Furthermore, sample 2 turned out to have the most suitable combination of extraction factors for obtaining maximum anthocyanin yield, which reached 0.41 mg/g. However, sample 10 gathered the most optimal extraction condition for ensuring the highest amounts of the phenolic elements cyanidin-3-glucosylrutinoside, neochlorogenic acid, and catechin. Notably, sample 10 had the second-best extraction combination for obtaining high amounts of polyphenols (3.59 mg/g) and anthocyanins (0.38 mg/g).

2. Green Extraction Techniques Used for P. cerasus Pomace

A distinctive method was carried out by Cilek et al. ^[2], where they prepared microcapsules containing PC pomace extract and evaluated their antioxidant capabilities. Afterwards, condensation was performed at 40 °C in a vacuum evaporator and the final sample had a volume 13–14 times decreased compared to its initial value. The concentrated extracts were lyophilized at −52 °C for 48 h under 0.1 mPa. The dried sample was then manually ground to a fine powder. The coating materials used were maltodextrin (MD) and gum arabic (GA). Three MD concentrations with distilled water were tested (10, 12, and 16% w/v). They were left overnight at 27 °C in a 70-rpm stirring water bath. GA solutions were prepared at the percentages of 4 and 8% (v/v), 2 h before the encapsulation procedure. For the preparation of the coating solutions, the solutions were mixed using a magnetic stirrer at 1250 rpm to obtain the total solid content of 10% (w/w) with MD/GA ratios of 10:0, 8:2, and 6:4 in weight. The final step for the preparation of the different microcapsules was to mix the pomace powder with the coating solutions with pomace to coating ratios of 1:10 and 1:20 (weight). The mixtures were homogenized at high speed for 5 min and subjected to ultrasonic treatment at 160 W, 20 kHz frequency, and 50% pulse and various time periods (15, 20, and 25 min). Therefore, 15 final samples were generated and tested for their antioxidant activity and phenolic content. Following the TEAC method to determine the antioxidant activity, the highest amount was found in the sample where the MD/GA ratio was 6:4, pomace to coating ratio was 1:10, and was subjected to ultrasound for 15 min, recording a value of 181.7 mmol TEAC/g. When this sample when analyzed by the DPPH method, it exhibited antioxidant activity of 2.85 ppm DPPH/g. However, after determining the antioxidant activity by DPPH, the best value of 2.90 ppm DPPH/g was recorded to the sample with an MD/GA ratio of 10:0, pomace to coating ratio of 1:10, and ultrasonication treatment of 15 min, i.e., increased antioxidant activity by 1.75%. According to the same study, the same conditions as before but with a longer ultrasound time (20 min) should be applied to obtain the higher content of polyphenols (14 mg/g).

A green extraction method was employed by Precup et al. ^[3] in order to create a food industry by-product ingredient, PC pomace, to be added to classic desserts. After extraction, the sample was centrifuged for 10 min, then the supernatant was filtered, and the filtrate was evaporated to dryness under vacuum. The polyphenol content of the PC pomace extract was 50.89 mg gallic acid equivalent (GAE)/100 g, an increase of two and a half fold from the study by Yilmaz et al. ^[1] using a conventional extraction method. The belief that extractions using green extraction methods lead to the export of the maximum amount of nutrients and antioxidants is therefore demonstrated ^[4].

The pomace consists of 24% skin and 76% kernel. Maurício et al. ^[5], determined the whole pomace and the skin part of pomace to study the bioactive compounds and antioxidant activity of the PC. They followed two green extraction techniques, maceration (extraction method 1), and decoction (extraction method 2) using the conditions. As far as the pomace extract derived by extraction method 1 and the one made by extraction method 2 are concerned, the phenolic content was in the range of 40 and 173 mg/g, respectively, while as far as the PC skin extract derived by extraction method 1 and the one made by extraction method 2 are concerned, the phenolic content was in the range of 31 and 289 mg/g, respectively. Consequently, the extraction by decoction in boiling water (100 °C) for 15 min is considered to be the most appropriate of all the extraction methods tested as it ensures the maximum value of phenolics. Furthermore, the amount of anthocyanins seemed to have been enhanced by extraction method 1 with no major differences between the two methods for the two samples. In particular, the skin extracted by method 1 had 1.8 mg/g anthocyanin content while the same sample extracted by method 2 had 1.6 mg/g anthocyanin content. Moreover, although the phenolic content was benefited by extraction method 2, the antioxidant activity was favored by extraction method 1 in all cases. Specifically, the antioxidant activity measured by DPPH^● assay for PC skin that received extraction method 1 was 880 μg/mL and the one receiving extraction 2 was 330 μg/mL, i.e., reduced by 166.67% compared to that receiving extraction method 1. In addition, the PC pomace’s antioxidant activity (DPPH^● assay) using extraction method 1 was 946 μg/mL and the pomace that underwent extraction method 2 was 407 μg/mL, having a difference of 132.43%.

3. Combination of Conventional and Green Extraction Techniques Used for P. cerasus Pomace

Ciccoritti et al. ^[6] examined the pomace from two PC genotypes (Bianchi d’Offagna, BO and Bianchi Montmorency, BM) in three different ways. After the removal of the pomace from both genotype samples, 100 g of pomace formed the control sample (CS) and was studied fresh. Furthermore, 400 g of the pomace were subjected to oven drying (OP) at 60 °C, air velocity: 0.6 ms⁻¹, relative humidity < 0.5%, system power: 1.4 kW/h for 24 h, and the remaining part, i.e., 400 g, was lyophilized (FDP) at −54 °C and 0.075 mbar for 72 h. Drying of the samples, regardless of the dehydration method, continued until the samples had a moisture content of 9%. A combination of two extraction methods, one conventional (simple stirring) and an ultrasonic method, was used to obtain the analyzed samples. To obtain all polyphenols and other antioxidants, the extraction procedure was repeated twice using 15 mL of fresh solvent each time and the three extracts were combined. According to the overall picture of the results, the BO genotype seemed to be richer in antioxidants (e.g., polyphenols and ascorbic acid). In addition to the genotype, the drying method has also shown to play an influential role since freeze-drying is more suitable than oven-drying, probably due to the fact that freeze-drying, apart from removing the moisture, contributes to the preservation of all the nutrients in the tested sample ^[7]. In order to determine the antioxidant activity, the samples were examined for their content in total polyphenols (TPC), total anthocyanins (TAC), total flavonoids (TFC), and ascorbic acid (AAC). In the BO genotype, CS displayed the highest content, reaching 45 mg/g, while the FDS sample had a content of 40 mg/g, i.e., 12.5% less. Also, regarding anthocyanin content, both CS and FDS seemed to have the same content, close to 4.5 mg/g. In contrast, the OP bale showed total flavonoid content (TFC) only 2 mg/g. Furthermore, regarding the content of the other two antioxidants in both samples, again CS seemed to be more enriched, ensuring values close to 24 mg/g of total flavones and 2.5 mg/g of ascorbic acid. Therefore, the sample that ensures the highest antioxidant content is the fresh BO, but in case of the need to use a dried sample, freeze-drying will ensure maximum results.

The evaluation of the effect of different extraction techniques on polyphenol content and antioxidant activity in PC pomace were investigated by Okur et al. ^[8]. TPC and DPPH^● radical scavenging activity were analyzed to determine its antioxidant activity. The TPC was found to be 108.36 mg GAE/100 g fresh weight (fw) whereas the antioxidant activity reached 70%. Three green extraction methods were tested, namely UAE, MAE, and HHP. Considering the green technologies, MAE at 90 s had the highest TPC, at 275.31 mg GAE/100 g fw, while between the other two methods, HHP and UAE, the highest TPC values recorded were at pressure 500 MPa and time 10 min for HHP and time 15 min for UAE, with the values being 227.51 and 239.84 mg GAE/100 g fw, respectively. In other words, the use of MAE as the extraction mode can benefit TPC compared with UAE and HHP methods by 21.01 and 12.88%, respectively. Similar results were also presented in the antioxidant capacity of the samples. Specifically, extraction with MAE at 90 s strongly benefitted the antioxidant activity where 89.90% was obtained. Meanwhile, in the extractions with HHP (at 500 MPa pressure and 10 min time) and UAE (at 15 min time), the antioxidant activity was 84.33 and 85.77%, respectively, i.e., reduced by 6.60 and 4.81%, respectively. In conclusion, by comparing the results of green extraction methods with the conventional extraction method, it is evident that irrespective of the green extraction method used, both the TPC and antioxidant activity of the PC pomace sample were enhanced. This fact was expected as many researchers have reported that green extraction methods promote the extraction of bioactive compounds compared to conventional extraction methods ^[9].

Sezer et al. ^[10] carried out one of the latest studies on the PC pomace in order to examine all extraction methods, conventional and green, in order to find the most suitable one for obtaining the maximum antioxidant amount (TPC and antioxidant activity). In addition to extraction by conventional or green extraction alone, a combination of conventional and green methods together was also performed. Further, an emerging green method that is also used is the use of enzymes, which allows the release of the analyzed compounds through the degradation of plant cell walls. More specifically, the destruction of the cell wall aims increase the isolation of the bioactive components of the plant and, therefore, enhance the efficiency of the extraction ^[11]. Prior to the application of any extraction method, PC pomace was dissolved in citric acid buffer (50 mM) at an acidic pH of 5.0 and left for 24 h in a refrigerator for hydration. After hydration, the samples were subjected to digestion either before or after each green extraction method (thermal, high-pressure, microwave heating). However, in the enzymatic hydrolysis procedure, the samples after hydration were subjected to digestion using 0.1% (w/w) of the enzyme cellulase, with continuous stirring at 50 °C for 1 h. Following digestion, boiling was carried out for 5 min in order to inactivate the enzyme. The control sample was prepared receiving only hydration as treatment. The conventional extraction method employed was thermal treatment of the hydrated sample at 60 °C for 1 h and then heating at 100 °C for 5 min. The result of TPC was 3.12 mg/g and the antioxidant activity was 5.48 mmol DPPH/100 g. Compared to the control sample, heat treatment is perceived to promote the antioxidant activity of PC pomace, since the control sample exhibited 2.72 mg/g TPC (14.71% reduced content) and 2.83 mmol DPPH/100 g antioxidant capacity (93.64% reduced antioxidant capacity). As far as the green extraction modes are concerned, four different methods were used such as enzymatic hydrolysis at 60 °C for 1 h followed by heating at 100 °C to deactivate the enzyme (sample 1), high pressure at 300 MPa (sample 2) and 600 MPa (sample 3) for 15 min both. A microwave technique at 850 W for 60 s (sample 4) was also implemented. Regarding the results of TPC of the samples, the following results were presented, 3.22, 3.92, 4.04, and 5.18 mg/g for samples 1, 2, 3, and 4, respectively. According to the results the most suitable extraction method was microwave extraction where a higher TPC was exhibited by, 60.87, 32.14, and 28.22% with respect to samples 1, 2, and 3, respectively. Moreover, concerning the control sample, its TPC was reduced by 90.44% compared to the microwave-extracted sample. Finally, the antioxidant activity recorded was 3.89, 4.51, 6.76, and 9.51% for samples 1, 2, 3, and 4, respectively. Over 10 different combinations of extraction methods were performed and analyzed. In particular, sample 5 was subjected to a thermal treatment at 60 °C for 1 h followed by heating for 5 min at 100 °C after hydration, which was followed by a high pressure of 300 MPa for 15 min. Thermal treatment was also applied to sample 6 where the same conditions were maintained followed by a high-pressure method at 600 MPa for 15 min. Enzymatic hydrolysis was also applied to samples 7 and 8 with the pressure of 300 MPa and 600 MPa for 15 min, respectively. In addition, enzymatic hydrolysis was applied at 60 °C followed by heating for 5 min at 100 °C and then subjected to high pressure at 300 MPa and 600 MPa for 15 min in samples 9 and 10, respectively.

4. Green Extraction Techniques Used for P. cerasus Pomace and Peel

Şahin et al. ^[12] examined the effect of a green deep eutectic solvent (DES) on the phenolic content of PC peel using a green extraction method (MAE). The definition of DES indicates liquids that belong close to the eutectic composition of the mixtures ^[13]; in this case, the solvent was prepared with a 1:4 molar ratio, citric acid/ethylene glycol, following a previous study ^[14]. Specifically, sample 1—35% water: 0.5 g mass, sample 2—50% water: 0.5 g mass, sample 3—20% water: 0.5 g mass, sample 4—35% water: 0.3 g mass, sample 5—50% water: 0.3 g mass, sample 6—20% water: 0.3 g mass, sample 7—50% water: 0.1 g mass, sample 8—35% water: 0.1 g mass, sample 9—20% water: 0.1 g mass. The highest TPC was found in the samples with the lowest mass (0.1 g). In detail, in sample 7 polyphenols reached 16.62 mg/g, in sample 8 polyphenols reached 17.70 mg/g, and in sample 9 polyphenols reached 18.14 mg/g (maximum value). The anthocyanin content of the samples reached up to 3.31 mg/g, amount recorded in sample 7, whereas the highest amount of cyanidin-3-glucoside (5.49 mg/g) was also recorded to the same sample. In conclusion, it might be said that for PC peel the ratio of 50% water/50% DES: 0.1 g mass for UAE system can ensure high amounts of antioxidants.

5. Conventional Extraction Techniques Used for P. cerasus Kernel

The study by Górnas et al. ^[15] investigated the lipophilic bioactive compounds (essential fatty acids, carotenoids, tocopherols, sterols, and squalene) present in the kernel oils derived from six different cultivars from Baltic countries and Russia (“Haritonovskaya”, “Bulatnikovskaya”, “Tamaris”, “Shokoladnica”, “Latvijas Zemais”, and “Zentenes”) of sour cherry. The oil was extracted with a sole conventional technique with hexane. The mixture was centrifuged, the supernatant was collected, and the procedure was repeated, with the final combined mixture having hexane evaporated through vacuum rotary evaporator. In terms of the overall content of monounsaturated fatty acids, the concentration exhibited a range of 26.0–46.1%, with the cultivar Latvijas Zemais having the highest proportion. The total polyunsaturated fatty acids also showed a similar trend, ranging from 44.0–62.3%, with the highest value achieved by the same cultivar. In comparison with tocopherols, the observed values ranged from 118.2–163.6 mg/100 g oil, with the Zentenes cultivar presenting the highest concentration. The Tamaris cultivar revealed a statistically significant difference (p < 0.05) in both total carotenoid and total sterol content. Total carotenoids ranged from 0.54–1.18 mg/100 g, with Tamaris cultivar achieving 1.75 mg/100 g of oil. A greater amount of 1041.3 mg/100 g of oil was measured in total sterol content, compared to the other cultivars which ranged from 313.6–416.24 mg/100 g of oil. Finally, Bulatnikovskaya cultivar had the highest content in squalene, which ranged from 65.8–102.8 mg/100 g of oil. Squalene, the primary component of skin surface polyunsaturated lipids, has moisturizing properties and protects skin cells from free radical oxidative damage ^[16]. The results indicate that the composition of bioactive compounds in sour cherry kernel oils is significantly influenced by the cultivar. In addition, it was observed that oils extracted from cultivars with a high oil content may contain lower concentrations of sterols and carotenoids.

The primary focus of the study by Korlesky et al. ^[17] was the comprehensive analysis of sour cherry kernel oil, containing its inherent properties and chemical composition. Fine powder of rinsed sour cherry kernel was extracted with hexane through the Soxhlet apparatus. The results showed that the sour cherry kernel oil yielded high concentrations of both monounsaturated (48.7%) and polyunsaturated fatty acids (39.3%). The authors of the study observed the presence of a specific fatty acid, α-eleostearic acid, which displayed noteworthy antioxidant properties. This fatty acid was found at a concentration of 5.72%. High total tocopherol content (TTC) was also measured at 525.2 ppm, with γ-tocopherols yielding the highest amount from other tocopherols (400 ppm). Consequently, cherry kernel oil has the potential to be beneficial in various dietary applications and incorporated into cosmetic products.

The production of sour-cherry jam and juice results in several by-products, one of which is the sour-cherry seed. Both the sour-cherry kernels and the seed shells are useful byproducts of sour-cherry production. To that end, both parts have been examined and evaluated in a study conducted by Farhadi et al. ^[18]. Powdered sour-cherry seeds were analyzed for their fatty acid content, TPC, and DPPH^• scavenging activity. Oven-dried ground kernels were extracted through Soxhlet extraction. The results showed that total polyphenols were present in high concentrations in sour-cherry seed (27.02 mg GAE/g). Chlorogenic acid (1887.50 μg/mg), 3,4-dihydroxybenzoic acid (262.30 μg/mg), quercetin (13.5 μg/mg), and rutin (58.45 μg/mg) were all found through chromatographic polyphenol analysis. Regarding sour cherry kernel oil, TPC was measured at 6.41 mg GAE/g, whereas the oil analysis revealed a high concentration of unsaturated fatty acids with oleic acid (45.03%), linoleic acid (40.61%), and linolenic acid (3.87%). In sum, sour-cherry seeds have promising physical properties that could make them useful in a variety of applications, such as functional beverages and dietary supplements. Initial research on the sour cherry seed properties has revealed promising potential for its use in nutritional supplements and healthful food products.

There is ongoing research into discovering new plant oils that may provide a natural source of nutraceutical compounds. Eight Polish sour cherry cultivars (‘Koral,’ ‘Naumburger’, ‘Lucyna,’ ‘Montmorency’, ‘Wanda’, ‘Wigor’, ‘Wolynska’, and ‘Wróble’ varieties) were analyzed for their crude fat, protein, and oil content in a study by Stryjecka et al. ^[19]. The chemical properties of the oils were evaluated after they were extracted through a Soxhlet extractor. Crude oil ranged from 24.6–35.4%. Fatty acid composition ranged significantly (p < 0.05) in oleic acid (25.4–41.0%), linoleic acid (39.1–46.2%), and α-linolenic acid (0.09–0.43%). Non-significant differences (p > 0.05) were recorded to the measurement of α-eleostearic acid in seven cultivars (~8.0–8.5%) but was recorded considerably higher in cv. ‘Naumburger’ (15.62%). The data presented above showed that the dominant fatty acid content in cherry oil varies depending on the cherry cultivar. In addition, α-eleostearic acid was discovered to be third, after oleic and linoleic acids, and its content was found to be cultivar-specific. Due to the large amount of beneficial unsaturated fatty acids and relatively low proportion of unhealthy saturated fatty acids, sour cherry kernel oil has many uses in human nutrition and medicine. Total sterol content (averaged 495 mg/100 g oil), total tocochromanols (averaged 134 mg/100 g oil), TPC (averaged 22.8 μg GAE/g), and total carotenoids (averaged 0.84 mg/100 g oil) indicated the strong presence of bioactive compounds in the sour cherry kernel oil. It could be also indicated by the vast antioxidant activity, as in DPPH^• assay averaged 60.4 mg Trolox/100 g oil and ABTS^•+ 40.4 mg Trolox/100 g oil. These compounds are antioxidants that help prevent cell death and tissue damage throughout the body and possess medicinal effects against cancer, inflammation, bacteria, fungi, viruses, and allergies ^[20].

6. Green Extraction Techniques Used for P. cerasus Kernel

In recent years, cold-pressed seed oils have become increasingly popular in the West as a healthy alternative to conventional cooking oils. The study conducted by Başyiğit et al. ^[21] highlighted that the natural bioactive compounds, especially unsaturated fatty acids present in sour cherry kernel oil, have the potential to enhance the nutritional value of food and pharmaceutical materials. A “green” technique including a laboratory-scale cold pressing machine with a capacity of 12 kg kernels/h was employed to extract oil from sour cherry kernels. The results showed that sour cherry kernel oil had high monounsaturated (39.74%) and polyunsaturated (43.34%) fatty acids composition, yielding a sum of 83.08% of unsaturated fatty acids. The authors also quantified specific individual carotenoids in the oil samples. Zeaxanthin (13.15 mg/kg oil), β-carotene (1.75 mg/kg oil), and cryptoxanthin (0.08 mg/kg oil) were the bioactive compounds of interest that were quantified. The antioxidant activity of the oil is also enhanced by known individual polyphenols determined chromatographically. For instance, compounds such as resveratrol (66.14 μg/kg), p-coumaric acid (101.61 μg/kg), ellagic acid (612.93 μg/kg), kaempferol (135.00 μg/kg), and naringenin (1678 μg/kg) were identified in the study samples. Compared to other oils, this kernel oil could be tested as an enrichment ingredient in a wide range of formulations.

Kazempour-Samak et al. ^[22] investigated, for the first time, the extraction of oil from the kernel of Iranian sour cherry (Cerasus vulgaris Miller) using “green” technique. Indeed, a cold-press extraction technique was employed to extract oil from sour cherry kernel samples with the temperature rising at least 35 °C. The authors evaluated the antioxidant properties of sour cherry kernel oil. The content of kernel oil in total polyphenols (33.44 mg GAE/g dm), TFC (46.37 mg quercetin/g dm), total tannins (1.21 mg GAE/g dm), total anthocyanin content (177.84 mg cyanidin 3-glucoside/mL), and TTC (832.5 mg/kg oil) indicated its considerable antioxidant activity. The same research team conducted a study ^[23] that involved the extraction of sour cherry (C. vulgaris Miller) kernel oil, again using a cold-press extraction technique, followed by an assessment of its chemical composition. The authors presented comprehensive results regarding the bioactive properties of the oil dry matter. The study explored the TPC (33.0 mg GAE/g), total tannin content (1.50 mg GAE/g), TFC (45.87 mg quercetin/g), total anthocyanin content (177.34 mg cyanidin 3-glucoside/mL), and TTC (562.5 mg/kg).

The purpose of the study from Atik et al. ^[24] was to identify the bioactive properties of oils extracted from sour cherry kernels using the cold press method. Oils were extracted using a cold press machine that could process 6 kg of kernels/h. To protect the unique qualities of oil, the process did not exceed 50 °C. It has been established that the fruit’s kernels, which are commonly discarded as waste in the fruit juice industry, can be refined into vegetable oil and used as an edible component. The authors identified linolenic acid as the major fatty acid in sour cherry kernel oil and was quantified at ~42%, summing an average of ~54% in polyunsaturated fatty acids and ~39% in monounsaturated fatty acids. The oil exhibited considerable antioxidant activity (1.86 mmol TE/g), which was attributed to the high TPC at 33.65 mg GAE/kg, with benzoic acid being the major polyphenol measured at 79.7 ppm. TTC was also measured at 224.43 ppm. The sterol composition analysis revealed that β-sitosterols (6018.27 ppm) showed high concentration in the kernel oil. In the same study, the authors also investigated wild plum (Prunus spinosa) kernel oil. It was evident that sour cherry kernel oil had higher values of TPC and sterols from wild plum kernel oil (28.32 mg GAE/g and 2509.93 ppm of β-sitosterols). The antioxidant activity also found reduced in wild plum kernel oil (1.44 mmol TE/g).

3.7. Combination of Conventional and Green Extraction Techniques Used for P. cerasus Kernel

The objective of the study conducted by Yilmaz et al. ^[25] was a comprehensive analysis of the chemical composition of kernels from sour cherry, with the intention of exploring their potential applications as a viable source of oil and antioxidants. Conventional extraction required successive extraction of the grounded kernel sample with hexane. The results showed the impact of ethanol presence in the extraction process on the fatty acid composition of kernel oil, as it was found to be statistically insignificant (p > 0.05), as the most abundant acid (C18:1, oleic acid) ranged from ~45–48%. However, total tocopherol concentration (TTC) was measured at 428.62 mg/L in the conventional extraction without 3% v/v ethanol. A similar pattern was seen in the quantification of β-carotene. Conventional extraction was found to have high efficiency and it seems that the presence of ethanol has positive effects on the extraction of this compound. It was measured 8.47 and 10.03 mg/L with the conventional technique, respectively, verifying the vast impact of ethanol. In the same study, the authors also investigated a “green” SC-CO₂ technique. This method yielded the highest TPC at 27.86 mg GAE/L, significantly higher than any other technique by ~29–76%. It is of high importance to highlight that the application of ethanol in both techniques (conventional and “green”) resulted in a considerable increase of the overall TPC and β-carotene of the kernel oil samples. However, the “green” technique was found to be the preferable technique as it yielded higher TPC and better polyphenol recovery, despite the higher tocopherol recovery provided by conventional extraction. The implementation of an environmentally and human-friendly technique to extract the oil renders it suitable for consumption in food products.

The objective of the conducted study by Dimic et al. ^[26] was to assess the efficacy of Soxhlet extraction to isolate oil from cherry seeds. The investigation focused on the analysis of oils with regard to their extraction yield, fatty acid composition, TTC, and antioxidant capacity. The raw material underwent fractionation, resulting in fractions with sizes less than 800 μm and greater than 800 μm. In the Soxhlet extraction procedure, milled cherry seeds were subjected to extraction using an appropriate non-polar solvent (n-hexane or methylene chloride) in a ratio of 1:4 solid-to-liquid. Oleic acid content did not differ significantly between the different solvents (~42 g/100 g of sour cherry seed oil). Linoleic acid averaged ~47 g/100 g seed oil) and α-linolenic acid was measured at ~0.32 g/100 g seed oil. Regarding TTC, it ranged from 37–46 mg/100 g cherry seed oil. In the same study, “green” techniques were also used for the isolation of kernel oil, i.e., supercritical carbon dioxide (SC-CO₂) and cold-pressing extraction. The laboratory-scale high-pressure extraction plant was utilized to conduct the SC-CO₂ process. The maximum content was achieved using SC-CO₂ at 70 °C, 0.4 kg/h at 200 bar. Oil yield ranged from 2.50–13.02%, with the highest yield obtained from SC-CO₂ method with 350 bar, 70 °C, 0.4 kg/h, in which the particle size (<800 μm) had a significant role. Linolenic acid content ranged with non-significant differences (p > 0.05) from 46.68–47.37 g/100 g of sour cherry seed oil. The highest content was achieved with a “green” technique, cold-press extraction. Finally, antioxidant activity was measured in order to evaluate the healing effect of sour cherry seed oil. DPPH^• assay (2.18–6.22 μM TE/g oil) highlighted the pressure effect in SC-CO₂ method with 70 °C, 0.4 kg/h at 275 bar. Subsequently, it was proved that bioactive compounds yield was increased further with “green” techniques. More specifically, SC-CO₂ extraction parameters like pressure, temperature, and flow rate were tuned. tocopherol yield in oil was increased when these factors were kept to a minimum during extraction. Particle size also had a crucial impact on SC-CO₂ because it can affect the amount of the sample extracted and the amount of the tocopherol produced. Extracts potentially rich in bioactive compounds may be obtained through the SC-CO₂ method without the need for further purification.

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Theodoros Chatzimitakos

Vassilis Athanasiadis

Dimitrios Kalompatsios

Konstantina Kotsou

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Eleni Bozinou

Stavros I. Lalas

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