Cold-pressed milk thistle seed oil: physico-chemical properties, composition and sensory analysis

2.1. Materials

Milk thistle seeds (Silybum marianum L. Gaert.) were purchased from Aktar Diyarı Co. (İzmir, Turkey). It was acknowledged that the seeds were harvested from cultivation plants located in the İzmir province of Turkey in the 2019 harvest season, and then cleaned and packed before marketing. The seeds were stored in deep-freeze (-18 ºC) until cold pressing in our laboratory for two months. All chemicals used in the analyses were purchased from Sigma (St. Louis, MO, USA) or Merck Co. (Darmstadt, Germany).

2.2. Analyses of the milk thistle seeds

The length, width and thickness values of the seeds were determined with a digital caliper (Leo, Nikko Ltd., China). The 1000-seed weight was determined gravimetrically by weighing (Sartorius ED224S, Sartorius, Göttingen, Germany) 25 randomly selected seeds several times, and then multiplying the weight by 40. Color values of the seeds were determined with a Minolta colorimeter CR 400 (Minolta Camera Co., Osaka, Japan). The moisture content of the seeds was measured with a OHAUS MB45 moisture analyzer (Ohaus, Pine Brook, NJ, USA). The total oil, protein, and ash contents were analyzed according to AOAC 920.39 (AOAC, 2002AOAC. 2002. Association of Official Analytical Chemists (17th ed.). AOAC International.), AOCS Aa 5-38, and AOCS Ba 5a-49 (AOCS, 1998AOCS. 1998. Official Methods and Recommended Practice of The American Oil Chemist’s Society (5th ed.). American Oil Chemist’s Society. AOCS Press, Champaigne, IL, USA.), respectively.

2.3. Cold pressing of the milk thistle seeds

As a result of the pre-experiments, the optimum seed moisture level, which provides maximum oil yield and ease of processing, was determined as 12%. The moisture content in the seeds was adjusted through incubation with added tap water in a closed vessel for 24 h. The amount of added water was calculated by total seed weight and initial seed moisture content. A laboratory scale cold press machine (Koçmaksan, ESM 3710, lzmir, Turkey), with a single head, 1.5 kW power and capacity for 12 kg seed/hour was used for cold-pressed oil production. The cold pressing process was carried out with a screw rotation speed of 18 rpm and a 12 mm exit die. The oil´s exit temperature did not exceed 40 ºC. After pressing, the oil was centrifuged (Sigma 2-16K, Postfach, Germany) (6797 xg, 10 min) and then filtered through Whatman no. 1 filter paper to remove suspended solid seed materials and moisture. Finally, the oil was placed in amber-colored glasses, flushed with nitrogen gas, and stored at 4 ºC during the analyses.

2.4. Physico-chemical analyses of the oil

An oil pycnometer was used to measure the specific gravity of the oil and the analysis was applied according to the AOCS Cc 10c-95 method (AOCS, 1984AOCS. 1984. Official Methods and Recommended Practice of The American Oil Chemist’s Society (3th ed.). American Oil Chemist’s Society. AOCS Press, Champaigne, IL, USA.). Specific extinction coefficient values were measured with a spectrophotometer (Shimadzu UV-1800, Shimadzu Co., Kyoto, Japan) according to the AOCS Ch 5-91 method (AOCS, 2017AOCS. 2017. Official Methods and Recommended Practice of The American Oil Chemist‘s Society (7th ed.). American Oil Chemist’s Society. AOCS Press, Champaigne, IL, USA.). An Abbe refractometer (Bellingham and Stanley, Tunbridge Wells, UK) was used to measure refractive index values of the oil. Color values were assessed with a Minolta colorimeter CR-400 (Minolta Camera Co., Osaka, Japan). Apparent viscosity was measured with a Brookfield DV II + Pro Viscometer (Brookfield Eng. Lab., Inc., Middleborough, MA, USA). This analysis was carried out at room temperature with a LV-SC4-18 spindle and rotation speed of 50 rpm.

Free fatty acidity, peroxide value, p-anisidine value, iodine number, saponification number, and unsaponifiable matter in the oil samples were determined according to AOCS Ca 5a-40, AOCS Cd 8-53, AOCS Cd 18-90, AOCS Cd 1-25 (AOCS, 1998AOCS. 1998. Official Methods and Recommended Practice of The American Oil Chemist’s Society (5th ed.). American Oil Chemist’s Society. AOCS Press, Champaigne, IL, USA.), AOCS Cd 3-25 (AOCS, 2017AOCS. 2017. Official Methods and Recommended Practice of The American Oil Chemist‘s Society (7th ed.). American Oil Chemist’s Society. AOCS Press, Champaigne, IL, USA.), and TSE 894 (TSE, 1970TSE. 1970. Yemeklik bitkisel yağlar-muayene metodlari. TSE 894. Resmi Gazete. Ankara, Turkey.) methods, respectively. The total phenolic content in the oil was determined according to the Folin-Ciocalteu technique as described by Yilmaz et al. (2015)Yılmaz E, Aydeniz B, Güneşer O, Arsunar ES. 2015. Sensory and physico-chemical properties of cold press-produced tomato (Lycopersicon esculentum L.) seed oils. J. Am. Oil Chem. Soc. 92, 833-842. https://doi.org/10.1007/s11746-015-2648-x..

2.5. Thermal analyses of the oil

Melting and crystallization parameters were determined with a Differential Scanning Calorimeter (Perkin-Elmer DSC 4000, USA) according to Dassanayake et al. (2009)Dassanayake LSK, Kodali DR, Ueno S, Sato K. 2009. Physical properties of rice bran wax in bulk and organogels. J. Am. Oil Chem. Soc. 86, 1163-1173. https://doi.org/10.1007/s11746-009-1464-6.. About 10 mg oil were placed into an aluminum pan and then sealed hermetically. The analysis was carried out against an empty aluminum pan. The thermal program cycle was as follows: 20 ºC to 110 (10 ºC/min), 110 ºC to −70 ºC (10 ºC/min), held at −70 ºC for 3 min and −70 ºC to 50 ºC (5 ºC/min). Curves were calculated with Pyris 1 Manager Software.

2.6. Determination of the fatty acid, sterol, and tocopherol compositions of the oil

Fatty acid composition was determined according to AOCS Ce 2-66 (AOCS, 1998AOCS. 1998. Official Methods and Recommended Practice of The American Oil Chemist’s Society (5th ed.). American Oil Chemist’s Society. AOCS Press, Champaigne, IL, USA.). First, 100 mg oil were weighed into a test tube and then 10 mL hexane were added and the oil was dissolved. Then, 100 μL 2 N methanolic KOH were added and mixed for 30 s. Finally, this mixture was centrifuged (Sigma 2-16K, Sartorius, Germany) (6461 xg, 10 min) and the clear phase was taken into a vial. Fatty acid composition was analyzed by GC-MS (GCMS-QP2010, Shimadzu Corporation, Nishinokyo, Japan) equipped with a Rxi-5MS column (30m x 0.25mm ID x 0.25µm film thickness, Restek Co.). The working conditions of GC were as follows: 2 μL injection volume, splitless, 0.83 mL/min flow rate, helium as carrier gas, 250 ºC injection temperature. The oven temperature program was as follows: held at 100 °C for 1 min, 100 °C to 160 °C (20 °C/min), held at 160 °C for 1 min, 160 °C to 180 °C (4 °C/min), 180 °C to 330 °C (30 °C/min), held at 330 °C for 4.70 min. The working conditions of the MS detector were as follows: 200 °C ion source temperature, 280 °C interface temperature, 5 min solvent-cutoff time. The library installed in the device was used for the evaluation of the peaks.

The sterol composition of the oil was determined according to the ISO 12228 method (ISO, 1999ISO. 1999. International standards official methods 12228:1999, animal and vegetable fats and oils-determination of individual and total sterols contents-Gas chromatographic method. International Organization for Standardization, Geneve, Switzerland.). First, unsaponifiable matters were extracted. Then, sterol fractions were obtained using Thin Layer Chromatography (TLC). Sterol composition was determined by using a Gas Chromatograph-FID (Agilent Technologies 7890B) equipped with a DB5 capillary column (30 m × 0.25 mm ID × 0.1 μm film thickness, J&W Scientific Co). 1 μL injection volume, 1:100 split ratio, 0.7 mL/min flow rate, 290 ºC inlet temperature, and 300 ºC detector temperature were used during the analysis. The carrier gas was hydrogen and detector gases were hydrogen (30 mL/min) and dry air (400 mL/min). The thermal program was as follows: held at 60 ºC for 2 min, heating to 220 ºC (40 ºC/min) and held at 220 ºC for 1 min, heating to 310 ºC (5 ºC/min) and held at that temperature for 30 min. Commercial standards were used for the identification of sterols. The amount of sterols was determined from the peak area of α-cholestanol as internal standard.

The tocopherol composition of the oil was analyzed according to Grilo et al. (2014)Grilo CE, Costa P N, Gurgel CSS, Beserra AFM, Almeida FNS, Dimenstein R. 2014. Alpha-tocopherol and gamma-tocopherol concentration in vegetable oils. Food Sci. Technol. 34, 379-385. https://doi.org/10.1590/S0101-20612014005000031. with minor modifications. 200 μL oil were dissolved in 4.8 mL dichloromethane. The mixture was mixed and placed into a vial. Tocopherol composition was determined by using a HPLC (Shimadzu Corporation, Japan) equipped with Inertsil ODS-3 column (250 mm× 4.6mm× 5 μm, GL Sciences Inc., Japan) and a RF-20A fluorescent detector. The analysis was carried out with 20 μL injection volume, isocratic elution with a flow rate of 1.6 mL/min and 30 ºC oven temperature. Methanol: water (97:3, v/v) mixture was used as the mobile phase. Detector wavelengths were set to 290 and 330 nm for excitation and emission, respectively. Commercial standards were used for identification and quantification.

2.7. Sensory descriptive analysis of the oil

Sensorial properties were determined through Quantitative Descriptive Analysis (QDA) (Meilgaard et al., 1991Meilgaard M, Civille GV, Carr BT. 1991. Sensory Evaluation Techniques. CRC Press.; Altug and Elmacı, 2005Altug T, Elmacı Y. 2005. Gıdalarda Duyusal Değerlendirme (Sensory Evaluations of Food). Meta Printing Services, İzmir, Turkey.). Five female and five male volunteer panelists, between 21 and 47 years of age, were trained for at least 15 hours. Six descriptive terms were developed under the leadership of the panel leader. The descriptive terms, their definitions and references used in the analysis are presented in Table 1. The sample was served in a glass covered with a metal lid to the panelists together with water, unsalted crackers and expectoration cups. The analyses were carried out in daylight at room temperature on different days and sessions. A 10-cm line scale (1 = minimum intensity, 10 = maximum intensity) was used. The analyses were replicated in a randomized order.

2.8. Consumer test

Appearance, smell/aroma, taste/flavor and general acceptance of the oil were evaluated by 50 volunteer consumers. A 5-point hedonic scale was used (1 = Dislike extremely, 5 = Like extremely). A sample was served in a glass covered with metal lid to consumers together with water, unsalted cracker and expectoration cups.

2.9. Statistical analysis

The same batches of seeds harvested in the 2019 season were divided into two parts for two replications of cold-press oil production. After each production, the oils obtained were designed as the replicates. For each replicate oil sample, analyses were performed in triplicate. The data presented were the mean ± SEM values.

3.1. Physico-chemical properties of the seeds

The physico-chemical properties of the milk thistle seeds are shown in Table 2. In one study (El-Haak et al., 2015El-Haak MA, Atta BM, Abd Rabo FF. 2015. Seed yield and important seed constituents for naturally and cultivated milk thistle (Silybum marianum) plants. Egypt. J. Exp. Biol. (Bot), 11 (2), 141-146. ), length, thickness and width of wild and cultivated milk thistle seeds were found to be about 7 mm, 2 mm and 3 mm, respectively. Our results concur with the literature. 1000-seed weight value is an important parameter to determine seed content. In the study of El-Haak et al. (2015)El-Haak MA, Atta BM, Abd Rabo FF. 2015. Seed yield and important seed constituents for naturally and cultivated milk thistle (Silybum marianum) plants. Egypt. J. Exp. Biol. (Bot), 11 (2), 141-146. , the 1000-seed weights of wild and cultivated milk thistle seeds were found as 25.23 and 27.40 g, respectively. In another study (Arampatzis et al., 2018Arampatzis DA, Karkanis AC, Tsiropoulos NG. 2018. Silymarin content and antioxidant activity of seeds of wild Silybum marianum populations growing in Greece. Annl. Appl. Biol. 174, 61-73. https://doi.org/10.1111/aab.12470.), 1000-seed weight values for the seeds in different milk thistle populations were found between 14.91 and 25.90 g. Our results are quite different from the literature, probably because of the differences in milk thistle species and cultivation conditions. L*, a* and b* values of the seeds were measured as 43.87, 4.70 and 16.73, respectively. In the study of Arampatzis et al. (2018)Arampatzis DA, Karkanis AC, Tsiropoulos NG. 2018. Silymarin content and antioxidant activity of seeds of wild Silybum marianum populations growing in Greece. Annl. Appl. Biol. 174, 61-73. https://doi.org/10.1111/aab.12470., L*, C*, h values were measured as 28.12-42.79, 5.50-14.09 and 72.04-80.71, respectively. Also, in the same study, the seed color was identified as brown. Although the color spaces used in the studies were different, we can say that the colors of the seeds are similar. Moisture, oil, protein and ash contents of the seeds were found as 6.78, 14.98, 17.31 and 4.14%, respectively. In one study (El-Haak et al., 2015El-Haak MA, Atta BM, Abd Rabo FF. 2015. Seed yield and important seed constituents for naturally and cultivated milk thistle (Silybum marianum) plants. Egypt. J. Exp. Biol. (Bot), 11 (2), 141-146. ), milk thistle seeds were found to contain 28.53-29.68% oil, 22.50-27.54% protein and 3.25-4.50% ash. In another study (Zhang et al., 2020Zhang Z-S, Wang S, Liu H, Li B-Z, Che L. 2020. Constituents and thermal properties of milk thistle seed oils extracted with three methods. LWT-Food Sci. Technol. 126, 1-8. https://doi.org/10.1016/j.lwt.2020.109282.), the proximate composition of milk thistle seeds was determined as 7.64% moisture, 21.09% oil, 15.46% protein and 4.72% ash. There are some minor differences, but the results are generally in line with the literature.

3.2. Physico-chemical properties of the oil

The Physio-chemical properties of the oil are shown in Table 3. Specific gravity and refractive index values mainly depend on the fatty acid composition and therefore these parameters vary according to the type of oil. In one study (Meddeb et al., 2017Meddeb W, Rezig L, Abderrabba M, Lizard G, Mejri M. 2017. Tunisian milk thistle: An investigation of the chemical composition and the characterization of its cold-pressed seed oils. Int. J. Mol. Sci. 18, 1-13. https://doi.org/10.3390/ijms18122582.), the specific gravity and refractive index values of cold-pressed milk thistle seed oils originating from different geographical areas in Tunisia were found as 0.91 and 1.46-1.47, respectively. In the study of Bahl et al. (2015)Bahl JR, Bansal RP, Goel R, Kumar S. 2015. Properties of the seed oil of a dwarf cultivar of the pharmaceutical silymarin producing plant Silybum marianum (L.) Gaertn. developed in India. Ind. J. Nat. Prod. Res. 6, 127-133. , specific gravity of solvent-extracted milk thistle seed oil was determined as 0.885. In the study of Fathi-Achachlouei et al. (2019)Fathi-Achachlouei B, Azadmard-Damirchi S, Zahedi Y, Shaddel R. 2019. Microwave pretreatment as a promising strategy for increment of nutraceutical content and extraction yield of oil from milk thistle seed. Indust. Crops Prod. 128, 527-533. https://doi.org/10.1016/j.indcrop.2018.11.034., the refractive index values of milk thistle seed oils were measured as 1.345-1.351. Our results are generally in line with the literature.

Specific extinction values are the indicators of oxidation products. In the codex, it was stated that E232 and E270 values should be 2.50 and 0.22 at the most for extra virgin olive oil, respectively (Codex, 2017Codex. 2017. Türk Gıda Kodeksi Zeytinyağı ve Pirina Yağı Tebliği (Turkish Olive Oil and Olive Pomace Oil Codex), Tebliğ no: 2017/26. Resmi Gazete, Ankara, Turkey). Oil types are different, but production techniques are similar (cold pressing). Therefore, these limit values could be used for comparison. As seen in Table 3, specific extinction values in the oil were lower than the limit values given in the codex. Consequently, it can be said that the oil was in accordance with the codex in terms of oxidation parameters.

Color is an important appearance property for cold-pressed oils in terms of consumer preference. In the study of Meddeb et al. (2017)Meddeb W, Rezig L, Abderrabba M, Lizard G, Mejri M. 2017. Tunisian milk thistle: An investigation of the chemical composition and the characterization of its cold-pressed seed oils. Int. J. Mol. Sci. 18, 1-13. https://doi.org/10.3390/ijms18122582., the L*, a* and b* values of milk thistle seed oils originating from different regions were determined as 41.94-66.07, -0.85-1.53 and 5.78-14.95, respectively. The results do not match exactly with this study, but they are similar.

According to codex (Codex, 2017Codex. 2017. Türk Gıda Kodeksi Zeytinyağı ve Pirina Yağı Tebliği (Turkish Olive Oil and Olive Pomace Oil Codex), Tebliğ no: 2017/26. Resmi Gazete, Ankara, Turkey), free fatty acidity and peroxide value should be max. 0.8 % and 20 meqO₂/kg oil for extra virgin olive oil produced by a similar production technique (cold pressing). As seen in Table 3, the peroxide value of the oil was low and within acceptable limits according to codex, but the free fatty acidity value exceeded the limit. The results indicate that during storage of the seeds and the oil, oil hydrolysis might have occurred. Therefore, it is recommended that seeds should be processed without long storage times, and seeds and oil should be stored at low temperature and low humidity. The p-Anisidine value is an indicator of secondary oxidation products. In one study (Rokosik et al., 2020Rokosik E, Dwiecki K, Siger A. 2020. Nutritional quality and phytochemical contents of cold pressed oil obtained from chia, milk thistle, nigella, and white and black poppy seeds. Grasas Aceites 71, 1-9. https://doi.org/10.3989/gya.0679191.), the p-anisidine value of cold-pressed milk thistle seed oil was determined as 0.091. In another study (Grajzer et al., 2020Grajzer M, Szmalcel K, Kuzminski L, Witkowski M, Kulma A, Prescha A. 2020. Characteristics and antioxidant potential of cold-pressed oils-possible strategies to improve oil stability. Foods 9, 1-18. https://doi.org/10.3390/foods9111630.), p-anisidine values for cold-pressed milk thistle seed oils were found between 0.13 and 2.19. The p-Anisidine value in our study is much higher than these values. Therefore, the storage conditions of the seeds and the oil should be improved.

In our study, the iodine number and the saponification value were found to be 85.86 g I₂/100 g oil and 198.30 mg KOH/g oil, respectively. In one study (Meddeb et al., 2017Meddeb W, Rezig L, Abderrabba M, Lizard G, Mejri M. 2017. Tunisian milk thistle: An investigation of the chemical composition and the characterization of its cold-pressed seed oils. Int. J. Mol. Sci. 18, 1-13. https://doi.org/10.3390/ijms18122582.), the iodine number and saponification value for cold-pressed milk thistle seed oils originating from different geographical areas in Tunisia were found between 112.41-118.32 g I₂/100 g oil and 128.08-205.16 mg KOH/g oil, respectively. In the study of Bahl et al. (2015)Bahl JR, Bansal RP, Goel R, Kumar S. 2015. Properties of the seed oil of a dwarf cultivar of the pharmaceutical silymarin producing plant Silybum marianum (L.) Gaertn. developed in India. Ind. J. Nat. Prod. Res. 6, 127-133. , the iodine number and saponification value for solvent-extracted milk thistle seed oil were reported as 97 g I₂/100 g oil and 199 mg KOH/g oil, respectively. There are some minor differences, but the results are generally in line with the literature.

The unsaponifiable matter content was measured as 1.90%. In the study of Meddeb et al. (2017)Meddeb W, Rezig L, Abderrabba M, Lizard G, Mejri M. 2017. Tunisian milk thistle: An investigation of the chemical composition and the characterization of its cold-pressed seed oils. Int. J. Mol. Sci. 18, 1-13. https://doi.org/10.3390/ijms18122582., the unsaponifiable matter content of cold-pressed milk thistle seed oils were found between 1.57 and 5.84%. In another study (Harrabi et al., 2016Harrabi S, Curtis S, Hayet F, Mayer PM. 2016. Changes in the sterol compositions of milk thistle oil (Silybium marianum L.) during seed maturation. Grasas Aceites 67, 1-6. http://dx.doi.org/10.3989/gya.0495151. ), milk thistle seeds were collected at different maturity stages and oils were obtained from the seeds following the solvent extraction technique. The unsaponifiable matter contents in these oils were reported as 1.9-3.8%. Our results are in agreement with the literature.

It is known that phenolic compounds in edible oils have various health benefits (Gioxari et al., 2016Gioxari A, Kogiannou DAA, Kalogeropoulos N, Kaliora AC. 2016. Phenolic compounds: Bioavailability and health effects. in Caballero B, Finglas PM, Toldra F (Eds.), Encyclopedia of Food And Health. Academic Press, 339-345.). In this study, the total phenol content was measured as 11.01 mg GAE/100 g oil. In the study of Grajzer et al. (2020)Grajzer M, Szmalcel K, Kuzminski L, Witkowski M, Kulma A, Prescha A. 2020. Characteristics and antioxidant potential of cold-pressed oils-possible strategies to improve oil stability. Foods 9, 1-18. https://doi.org/10.3390/foods9111630., the phenolic contents of cold-pressed milk thistle seed oils were found between 71.7 mg CAE/kg oil and 124.7 mg CAE/kg oil (CAE: Caffeic acid equivalent). In another study (Meddeb et al., 2017Meddeb W, Rezig L, Abderrabba M, Lizard G, Mejri M. 2017. Tunisian milk thistle: An investigation of the chemical composition and the characterization of its cold-pressed seed oils. Int. J. Mol. Sci. 18, 1-13. https://doi.org/10.3390/ijms18122582.), the phenolic contents in cold-pressed milk thistle seed oils were determined as 3.59-8.12 mg GAE/g. It is obvious that there is a great variation between milk thistle seed oils in terms of total phenolic content, probably because of the differences in milk thistle species and cultivation conditions.

3.3. Thermal properties of the oil

The melting and crystallization parameters are presented in Table 4. Just a single peak was observed during melting and crystallization. Melting and crystallization temperatures of the oil were determined as -20.18 °C and -3.71 °C, respectively. It is stated that higher amounts of saturated fatty acids cause higher melting temperatures (Mayfield et al., 2015Mayfield S, Van de Walle D, Delbaere C, Shinn SE, Proctor A, Dewettinck K, Patel A. 2015. CLA-rich chocolate bar and chocolate paste production and characterization. J. Am. Oil Chem. Soc. 92, 1633-1642. http://dx.doi.org/10.1007/s11746-015-2740-2.). The melting point of the oil was lower, as expected, because unsaturated fatty acids were dominant in milk thistle seed oil. In one study (Zhang et al., 2020Zhang Z-S, Wang S, Liu H, Li B-Z, Che L. 2020. Constituents and thermal properties of milk thistle seed oils extracted with three methods. LWT-Food Sci. Technol. 126, 1-8. https://doi.org/10.1016/j.lwt.2020.109282.), it was reported that milk thistle seed oil melts between -44 °C and 10 °C and two major transitions were at -24 °C and -15 °C. In the same study, two transitions, a small peak at -2 °C and a sharp narrow peak at -55 °C, were observed during cooling. In another study (Meddeb et al., 2017Meddeb W, Rezig L, Abderrabba M, Lizard G, Mejri M. 2017. Tunisian milk thistle: An investigation of the chemical composition and the characterization of its cold-pressed seed oils. Int. J. Mol. Sci. 18, 1-13. https://doi.org/10.3390/ijms18122582.), a major peak was observed at around -30 °C during the melting of the oils. Beyond this major peak, they also observed several small peaks from -22.92 °C to 11.62 °C. Although there are some differences, we can say that the results in our study are similar.

3.4. Fatty acid, sterol, and tocopherol compositions of the oil

The physical properties, stabilities, and usage areas of the edible oils mostly depend on their fatty acid compositions. Hence, the determination of fatty acid compositions of oils is very important. As seen in Table 5, milk thistle seed oil contained higher amounts of unsaturated fatty acids. Linoleic acid and oleic acid were the major fatty acids with 51.97 and 27.06%, respectively. The total unsaturated fatty acid content was determined as 79.48%. In one study (Zhang et al., 2020Zhang Z-S, Wang S, Liu H, Li B-Z, Che L. 2020. Constituents and thermal properties of milk thistle seed oils extracted with three methods. LWT-Food Sci. Technol. 126, 1-8. https://doi.org/10.1016/j.lwt.2020.109282.), linoleic acid (46.19%) and oleic acid (30.59%) were determined as major fatty acids, and total unsaturated fatty acid content was determined as 77.94% for cold-pressed milk thistle seed oil. In another study (Meddeb et al., 2017Meddeb W, Rezig L, Abderrabba M, Lizard G, Mejri M. 2017. Tunisian milk thistle: An investigation of the chemical composition and the characterization of its cold-pressed seed oils. Int. J. Mol. Sci. 18, 1-13. https://doi.org/10.3390/ijms18122582.), the major fatty acids of milk thistle seed oils were found as linoleic acid and oleic acid with 57.00-60.30% and 15.50-22.40%, respectively. In the same study, the total unsaturated fatty acid content was determined as 82.05-83.64%. Generally, the results in this study concur with the literature.

It is known that sterols have important health benefits (Berger et al., 2004Berger A, Jones PJH, Abumweis SS. 2004. Plant sterols: factors affecting their efficacy and safety as functional food ingredients. Lipids Health Dis. 3, 1-19. http://dx.doi.org/10.1186/1476-511X-3-5.). The sterol composition is shown in Table 5. Total sterol content was measured as 108.57 mg/100 g oil. β-Sitosterol was determined as the major sterol with 67.56 mg/100 g oil and it was followed by ∆7-stigmastenol and stigmasterol. In the study of Zhang et al. (2020)Zhang Z-S, Wang S, Liu H, Li B-Z, Che L. 2020. Constituents and thermal properties of milk thistle seed oils extracted with three methods. LWT-Food Sci. Technol. 126, 1-8. https://doi.org/10.1016/j.lwt.2020.109282., the total sterol content of cold-pressed milk thistle seed oil was found as 291.43 mg/100 g oil and Δ7-stigmastenol was determined as major sterol, followed by sitosterol. In another study, β-sitosterol was determined as the major sterol for cold-pressed milk thistle seed oil with a total sterol content of 1815.18 mg/kg oil (Rokosik et al., 2020Rokosik E, Dwiecki K, Siger A. 2020. Nutritional quality and phytochemical contents of cold pressed oil obtained from chia, milk thistle, nigella, and white and black poppy seeds. Grasas Aceites 71, 1-9. https://doi.org/10.3989/gya.0679191.). There are some differences in terms of total sterol content and sterol composition between different studies probably due to differences in milk thistle species and cultivation conditions.

Tocopherols are oil-soluble vitamins which have antioxidant activity. As seen in Table 5, the total tocopherol content was measured as 117.38 mg/kg oil and γ-tocopherol was determined as the major tocopherol with 53.60 mg/kg oil. In a study of Zhang et al. (2020)Zhang Z-S, Wang S, Liu H, Li B-Z, Che L. 2020. Constituents and thermal properties of milk thistle seed oils extracted with three methods. LWT-Food Sci. Technol. 126, 1-8. https://doi.org/10.1016/j.lwt.2020.109282., the total tocopherol content was 645.47 mg/kg oil and γ-tocopherol was determined as the major tocopherol for cold-pressed milk thistle seed oil. In another study (Meddeb et al., 2017Meddeb W, Rezig L, Abderrabba M, Lizard G, Mejri M. 2017. Tunisian milk thistle: An investigation of the chemical composition and the characterization of its cold-pressed seed oils. Int. J. Mol. Sci. 18, 1-13. https://doi.org/10.3390/ijms18122582.), α-tocopherol was determined as the major tocopherol for cold-pressed milk thistle seed oils with a total tocopherol contents of 49.57-318.29 mg/kg oil. The results in our study do not exactly match with these previous studies probably due to differences in milk thistle species and cultivation conditions.

3.5. Sensory properties and consumer preferences of the oil

The results from the sensory descriptive analysis are shown in Table 6. Six descriptors were selected to describe the cold-pressed milk thistle seed oil, which were sweet, spicy, raw vegetable, straw, roasted and throat-catching. It was observed that spicy flavor was quite dominant compared to other terms and it was followed by throat-catching. To the best of our knowledge, there is no data in the literature about the sensory descriptive terms of milk thistle seed oil. Therefore, this study presents important information to the literature for the first time. In one study (Dhouibi et al., 2020Dhouibi I, Jridi M, Flamini G, Jabeur H, Masmoudi M, Bouaziz M. 2020. Volatile and phenolic contents and antioxidant and antibacterial properties of Tunisian milk thistle and mastic oils. Euro-Mediter. J. Env. Integ. 5, 1-12. https://doi.org/10.1007/s41207-020-00200-z. ), volatile compounds in cold-pressed milk thistle seed oil were determined. 1,8-Cineole (minty, herbal flavor), methylpyrazine (roasted flavor), 2,5-dimethylpyrazine (roasted flavor) and hexanal (vegetative, herbal flavor) were found as the major volatile compounds in milk thistle seed oil. p-Cymene (spicy flavor), isocaryophyllene (woody, spicy flavor) and β-caryophyllene (woody, spicy flavor) were also detected in the oil. The descriptive terms determined for milk thistle seed oil in our study generally match the volatile compound descriptions detected in the study of Dhouibi et al. (2020)Dhouibi I, Jridi M, Flamini G, Jabeur H, Masmoudi M, Bouaziz M. 2020. Volatile and phenolic contents and antioxidant and antibacterial properties of Tunisian milk thistle and mastic oils. Euro-Mediter. J. Env. Integ. 5, 1-12. https://doi.org/10.1007/s41207-020-00200-z. .

The results from the consumer test are presented in Figure 1. Hedonic scores are generally higher than 3.00 points, which is the neutrality point. Therefore, it could be said that consumer satisfaction was moderate. To the best of our knowledge, there is no data in the literature regarding the consumer preferences of milk thistle seed oil. Therefore, this study presents important information to the literature for the first time.