1. INTRODUCTION
⌅Nowadays, the use of omega-3 fatty acids in human nutrition is one the most contemporary topics worldwide due to their role in improving human health. The omega-3 fatty acids include α-linolenic acid (ALA, C18:3), eicosapentaenoic acid (EPA, C20:5), docosapentaenoic acid (DPA, C22:5), and docosahexaenoic acid (DHA, C22:6). These are the essential fatty acids that help manage chronic diseases such as type 2 diabetes, cardiovascular diseases, hypertension, neurodegenerative diseases (Alzheimer's disease), dementia, depression, visual and neurological development, and certain types of cancers (Shahidi and Ambigaipalan, 2018ShahidiF, AmbigaipalanP. 2018. Omega-3 Polyunsaturated Fatty Acids and Their Health Benefits.. Annu. Rev. Food Sci. Technol.9 (1), 345–381. 10.1146/annurev-food-111317-095850).
Fish oil is a widely used omega-3 supplement (Tur et al., 2012TurJA, BibiloniMM, SuredaA, PonsA. 2012. Dietary sources of omega 3 fatty acids: public health risks and benefits. Br. J. Nutr.107 (2), 23–52. 10.1017/s0007114512001456). However, most vegetarians and non-fish eaters who neglect to intake fish oil consider plant sources of omega-3 fatty acids. Flaxseed (Linum usitatissimum L.) oil is the richest plant source of omega-3 fatty acids, ALA which is proven to have many health benefits (Joshi et al., 2022JoshiA, HegdeM, ZanwarA. 2022. Flaxseed oil and palm olein blend to improve omega-6: omega-3 ratioJ. Food Sci. Technol. 59, 498–509. 10.1007/s13197-021-05033-4). Flaxseed has been well known for reducing the risks associated with cardiovascular diseases and some types of cancers owing to lignans and omega-3 fatty acids. Extensive studies related to flaxseed and flaxseed oil reported that they have cardioprotective, antidiabetic, anticancer, and hepatoprotective properties (Dunford, 2015DunfordNT. 2015. Hemp and flaxseed oil: Properties and applications for use in food. Specialty Oils and Fats in Food and Nutrition: Properties, Processing and Applications. 10.1016/B978-1-78242-376-8.00002-8). Flaxseed oil consists of 9% saturated fatty acids (SFAs), 73% polyunsaturated fatty acids (PUFAs), and 18% monounsaturated fatty acids (MUFAs). Even though flaxseed oil contains antioxidants such as tocopherols and beta-carotene, it is highly susceptible to oxidation (Ebrahimi et al., 2021EbrahimiB, Nazmara, Z, HassanzadehN, YarahmadiA, GhaffariN, HassaniF, HassanzadehG. 2021. Biomedical features of flaxseed against different pathologic situations: A narrative review. Iran. J. Basic Med. Sci. 24 (5), 551. 10.22038%2Fijbms.2021.49821.11378) making them the biggest challenge in food applications, even though several studies are still underway.
Oxidation is one of the main problems encountered by food industries, as oxidation deteriorates sensory and nutritional qualities. The oxidation of lipids occurs during storage as well as processing such as frying. The simplest technique for stabilizing frying oils is altering the fatty acid composition of the oil by blending it with another oil with high oxidative stability characteristics (Farhoosh et al., 2009FarhooshR, EsmaeilzadehKR, PoorazrangH. 2009. Frying stability of canola oil blended with palm olein, olive, and corn oils. J. Am. Oil Chem. Soc. 86 (1), 71–76. 10.1007/s11746-008-1315-x).
In the present study, coconut (Cocos nucifera) oil has been blended with flaxseed oil. Coconut oil is highly stable against oxidation owing to its high content of SFAs (more than 90% saturated fatty acids) (Ramos et al., 2019RamosT, De SouzaEF, SantosMN, FiorucciAR, CardosoCAL, Da SilvaMS. 2019. Evaluation of antioxidant potential and chemical composition blends of sunflower oil (Helianthus annuus L.) with Coconut Oil (Cocos nucifera L.). Orbital. 11 (4), 246–252. 10.17807/orbital.v11i4.1226). In addition to its superior oxidative stability, it contains a high percentage of medium-chain fatty acids, namely lauric acid, which provides beneficial health properties. Approximately 60% of the fatty acids in coconut oil are short-chain fatty acids and medium-chain fatty acids which are effortlessly digestible (Jayasekara et al., 2007JayasekaraC, UpaliSAR, YalegamaLLWC, LAbeygunawardhanaMJ. 2007. Organoleptic and chemical properties of coconut and sesame oils and their blends. Cocus.18, 67–76.). The World Health Organization (WHO) recommended to reduce the intake of SFAs which has been linked to a significant reduction in coronary heart disease when replaced with PUFAs (WHO, 2018WHO. 2018. Draft guidelines on SFA and trans fatty acids intake for adults and children. http://www.who.int/mediacentre/factsheets/fs394/en). The blending of various vegetable oils with different degrees of saturation can create new oils with desirable characteristics such as nutritional status and oxidative stability. Therefore, blending coconut oil and flaxseed oil can produce an oil blend with desirable SFA, MUFA, and PUFA contents with oxidative stability.
The studies on preparing blended oil using coconut oil and flaxseed oil and coconut oil are much limited. Considering the nutritional importance of flaxseed oil, this study aimed to produce a blend of flaxseed oil and coconut oil with high oxidative stability, a balanced fatty acid profile (SFA: MUFA: PUFA), and rich in omega-3 fatty acids suitable for various food processing applications.
2. MATERIALS AND METHODS
⌅2.1. Materials
⌅Coconut oil (virgin), flaxseed oil (cold-pressed), and fresh potatoes were purchased from the local market in Sri Lanka. All chemicals and Gas Chromatography standards (FAME 37 Component Mix) were purchased from Sigma Aldrich, USA.
2.2. Methods
⌅2.2.1. Sample preparation
⌅Coconut oil and flaxseed oil blends were prepared in different ratios on weight basis (95:5, 90:10, 85:15, 80:20, and 75:25). Oil samples were weighed and thoroughly mixed using a magnetic stirrer (witeg MSH-30A) for 30 minutes, flushed with nitrogen and stored at -20 °C until analysis within a week.
2.2.2. Oven storage
⌅The storage study was performed according to the AOCS Recommended Practice Cg 5-97 (AOCS, 2009aAOCS. 2009a. Recommended Practice Cg 5-97. 2009: Oven storage test for accelerated aging of oils. In Official method and recommended practices of the AOCS. (6th ed.): Champaign, IL.). The same amount of sample (50 g) was taken into sample bottles (100 mL, clear glass bottle, O.D. 51.7 mm × H. 94.5 mm) loosely capped and stored at 65 °C for 14 days in an incubator (Memmert IN55plus) provided with ventilation. Samples were taken on the 1st, 3rd, 7th, and 14th days of storage and analyzed.
2.2.3. Frying test
⌅Fresh potatoes were cut into uniformly-sized pieces (3cm ×1cm ×1cm) using a mechanical slicer and fried in the oils and their blends continuously for four hours, and the oil samples were collected for analysis in every one-hour interval. Each hour is considered as one frying cycle. Samples were collected up to the 4th frying cycle. The temperature was maintained at 180 ± 5 °C throughout the frying and the ratio of oil: potato was kept constant.
2.3. Analysis of samples
⌅2.3.1. Determination of acid value
⌅The AOCS method was used to determine the acid value of the samples (AOCS, 2009bAOCS. 2009b. AOCS Official Method Cd 3d-63: Acid value of fats and oils. Official method and recommended practices of the AOCS. In (6th ed.): Champaign, IL.). About 4 g of the sample was weighed precisely into a clean dry Erlenmeyer flask and 25 mL of ethanol were added. The flask was then shaken to dissolve the oil. Then the contents were boiled in a water bath for 30 min (Witeg-digital control system). The hot mixture was titrated with a 0.01 N ethanolic KOH solution with phenolphthalein as an indicator until the pink color persisted for 30 seconds.
2.3.2. Determination of iodine value
⌅The iodine value was determined according to the AOAC official method 993.2 (AOAC, 2000aAOAC. 2000a. AOAC official method 993.20: Iodine value of fats and oils - wijs (cyclohexane–acetic acid solvent). In Official methods of analysis, 17th Edition, AOAC International, USA.). About 0.2 g sample were taken into a glass conical flask with a stopper and dissolved completely in chloroform. Then Wijs reagent (25 mL) was added and kept in the dark for 30 min followed by the addition of 10 mL of 15% potassium iodide solution. Immediately, the mixture was titrated with a 0.01N sodium thiosulfate solution with starch indicator while shaking vigorously until the color disappeared. A blank was also carried out in parallel to calculate the iodine value.
2.3.3. Determination of peroxide value
⌅The peroxide value was determined according to the AOCS SURPLUS Method Cd 8-53 (AOCS, 2017AOCS. 2017. AOCS SURPLUS Method Cd 8-53: Peroxide Value—Acetic Acid-Chloroform Method. In Official methods and recommended practices of the AOCS. (7th ed.): Champaign, 1L.). About 5 g of sample was taken into a 250-mL Erlenmeyer flask and it was closed immediately with a glass stopper. Then, 30 mL of a mixture of glacial acetic acid and chloroform (3:2, v/v) were added. After that, 0.5 mL of potassium iodide solution (saturated) was added and shaken well. The flask was then stored in the dark for 1 minute. 30 mL of distilled water were added and shaken well and titrated with 0.01 N sodium thiosulfate solution using a starch indicator. A blank test also was carried out in parallel.
2.3.4. Determination of saponification value
⌅The saponification value of the samples was determined according to the AOAC method (AOAC, 2000bAOAC. 2000b. AOAC official method 920.160: Saponification value, In Official methods of analysis17th Edition, AOAC International, USA.). About 3 g oil sample was dissolved in a 0.5 N ethanolic KOH solution and the mixture was refluxed for complete saponification for 2 h. Then, the solution was titrated with 1 N sulfuric acid solution using phenolphthalein as an indicator.
2.3.5. Analysis of fatty acid profile
⌅Fatty Acid Methyl Esters (FAMEs) of the oil samples were prepared by transesterification (WHO, 2020WHO. 2020. Global protocol for measuring fatty acid profiles of foods, with emphasis on monitoring trans-fatty acids originating from partially hydrogenated oils: WHO Laboratory Protocol. Retrieved fromhttps://apps.who.int/iris/bitstream/handle/10665/338049/9789240018044-eng.pdf?sequence=1&isAllowed=y). FAMEs were analyzed by Gas Chromatography (GC). About 50 mg oil was dissolved in 2 mL toluene in a screw-capped glass test tube and 1 mL of 7% BF3 in methanol was added. Then the tube was heated at 100 °C for 45 minutes in an oven with occasional shaking. After 45 minutes, the tube was removed from the oven and allowed to cool to room temperature. Then 5 mL of distilled water, 1 mL of hexane, and 1 g of sodium sulfate were added and shaken well and left for the hexane to separate. The separated hexane layer was carefully transferred into the small vial. Finally, the vial with FAMEs was sealed with parafilm and stored at -20 °C until analysis by GC.
FAMEs were analyzed by injecting 1µl sample into the GC (Agilent technologies 7890B GC system), equipped with a flame ionization detector (FID) and a fused silica capillary column (100 m length, 0.250 mm diameter, 0.20 µm film) (SP®-2560 Capillary GC Column) using nitrogen as carrier gas (flow rate - 20 mL/min). The split ratio was 30:1 and the injector and detector temperatures were 220 and 260 °C, respectively. The initial column oven temperature was maintained at 140 °C for 5 minutes and increased to 240 °C at the rate of 4 °C/min, then maintained at that temperature for 10 minutes. Fatty acids were identified using authentic standards (FAME 37 Component Mix) by comparing their retention times (Sivakanthan et al., 2019SivakanthanS, JayasooriyaLJPAP, MadhujithT. 2019. Optimization of the Production of Structured Lipid by Enzymatic Interesterification from Coconut (Cocos nucifera) and Sesame (Sesamum indicum) Oils using Response Surface Methodology. LWT.101, 723–730. 10.1016/j.lwt.2018.11.085).
2.4. Statistical analysis
⌅All experiments were done in triplicate and the values are expressed as mean values. Statistical analysis was carried out by Completely Randomized Design (CRD) using the SAS for Academics. The mean values were compared using Duncan’s Multiple Range Test (DMRT at α = 0.05).
3. RESULTS AND DISCUSSION
⌅3.1. Initial quality of oils
⌅According to the Codex Alimentarius standard (Codex Alimentarius, 2021Codex Alimentarius. 2021. Standard for named vegetable oils, CXS 210-1999: Adopted in 1999. Revised in 2001, 2003, 2009, 2017, 2019. Amended in 2005, 2011, 2013, 2015, 2019. http://www.fao.org/fao-who-codexalimentarius/sh-proxy/en/?lnk=1&url=https%253A%252F%252Fworkspace.fao.org%252Fsites%252Fcodex%252FStandards%252FCXS%2B210-1999%252FCXS_210e.pdf), the maximum level of acid value for cold-pressed and virgin oils should be 4.0 mg KOH/g oil, and peroxide value should be 15 meq O2/kg oil.
In this study, the acid values for coconut oil and flaxseed oil were 1.06 ± 0.00 and 0.99 ± 0.01 mg KOH/g of oil, respectively. The peroxide value for coconut oil was 0.60 ± 0.00 meq O2/kg of oil, whereas the peroxide value for flaxseed oil was 1.99 ± 0.01 meq O2 /kg of oil. Therefore, the initial values for both coconut and flaxseed oil complied with the Codex Alimentarius standard.
3.2. Oxidative stability of oils and their blends under accelerated oven storage
⌅The accelerated oven storage test has been widely used to study the stability of edible oils against oxidation. The storage of oil at 60 ± 5 °C accelerates the oxidation rate of the oil. In the accelerated oven storage method, one day of accelerated oven storage is considered equal to one month at ambient temperature (Evans et al., 1973EvansCD, ListGR, MoserHA., ConanIC. 1973. Long term storage of soybean and cottonseed salad oils. J. Am. Oil Chem. Soc. 50, 218–220. https://doi.org/ 10.1007/bf02640494).
3.2.1. Acid value
⌅The acid value is a measure of rancidity as free fatty acids are generated during lipid oxidation and hydrolysis. The higher the acid value, the higher the amount of free fatty acids, which is interpreted as decreased oil quality (Choudhary, 2013ChoudharyM. 2013. Effect of Deep-Fat Frying on Physicochemical Properties of Rice Bran Oil Blends. IOSR-JNHS. 1 (4), 01–10. 10.9790/1959-0140110). Changes in acid value during the storage period are presented in Figure 1a. The acid value for oils and their blends increased with storage. There was a significant difference between coconut oil and flaxseed oil (p < 0.05). However, there was no significant difference between blend 1 and 2, nor was blend 5 significantly different (p > 0.05). The results obtained were in accordance with Ngassapa et al. (2017NgassapaFN, MwaisakaTR, NyandoroSS. 2017. Stability of traditionally processed vegetable oils and their blends under different storage conditions. Int. J. Biol. Chem. Sci.10 (5), 2341. 10.4314/ijbcs.v10i5.31), who reported that there was an increment in the acid value for oils and their blends stored at air-free ambient temperature compared to oil samples stored under air-tight dark conditions for 60 days. The initial acid value for flaxseed oil was lower than coconut oil. However, throughout the storage period, the acid value for flaxseed oil was higher than coconut oil. The blends also presented increased acid values but were lower than flaxseed oil. This observation could be attributed to the higher degree of unsaturation in flaxseed oil compared to coconut oil. A study evaluated the stability of oil blends, and found that the blend of rice bran oil with sesame oil had lower free fatty acid content than pure sesame oil throughout the twelve-month storage period (Gulla and Waghray, 2011GullaS, WaghrayK. 2011. Effect of storage on physico-chemical characteristics and fatty acid composition of selected oil blends. J. Life Sci. (Wilmington, DE,U.S. 3 (1), 35–46. 10.1080/09751270.2011.11885167).
3.2.2. Iodine value
⌅The iodine value is the indicator of the degree of unsaturation. Unsaturated fatty acids are more susceptible to oxidation. Therefore, there will be a decrease in the degree of unsaturation with the progress of oxidation. The iodine value for flaxseed oil was higher than coconut oil because of the higher unsaturation of flaxseed oil than coconut oil. The iodine values for oils and their blends decreased with storage time, which was attributed to the oxidation of unsaturated fatty acids. The initial iodine values for coconut oil and flaxseed oil were, 7.08 ± 0.23 and 190.03 ± 0.45, respectively. The initial iodine value for oil blends increased with an increasing proportion of flaxseed oil because of the higher unsaturation of flaxseed oil. However, throughout the storage period, oil blends exhibited a downward trend (Figure 1b). The iodine values for coconut oil, blend 1, and blend 2 were not significantly different (p > 0.05) throughout storage, whereas other blends and flaxseed oil showed significantly different (p < 0.05) iodine values. Anwar et al. (2007AnwarF, HussainAI, IqbalS, BhangerMI. 2007. Enhancement of the oxidative stability of some vegetable oils by blending with Moringa oleifera oil. Food Chem.103 (4), 1181–1191. 10.1016/j.foodchem.2006.10.023) reported that a blend of 0-80% Moringaoleifera oil with sunflower oil and soybean oil resulted in decline in the iodine value as a function of storage time. The slow decline in the iodine value of oil blends could be attributed to the induction period where fat is oxidized, slowly demonstrating the initiation of the oxidation reaction. The rapid alterations in the iodine value for oil blends may be due to the propagation of the oxidation process where hydroperoxides are formed from free radicals in fatty acids generated in the initiation stage of the oxidation reaction (Gulla and Waghray, 2011GullaS, WaghrayK. 2011. Effect of storage on physico-chemical characteristics and fatty acid composition of selected oil blends. J. Life Sci. (Wilmington, DE,U.S. 3 (1), 35–46. 10.1080/09751270.2011.11885167).
3.2.3. Peroxide value
⌅Peroxide value is denoted as an important indicator of oil stability (Bhardwaj et al., 2015BhardwajK, VermaN, BhardwajS. 2015. A novel approach for improvement of oxidative stability of flaxseed oil by blending with palm oil. Int. J. Adv. Res.3 (11), 1399–1407.). Peroxides are the primary oxidation products generated in the early stages of the oxidation of lipids, and they contribute to the successive oxidation reactions to form a range of nonvolatile and volatile secondary oxidative compounds (Shahidi et al., 2017ShahidiF, WangJ, WanasundaraUN. 2017. Methods fo r measuring oxidative rancidity in fats and oils. Food Lipids: Chemistry, Nutrition, and Biotechnology, Fourth Edition. (1), 519–542. 10.1201/9781315151854). The peroxide values for both oils and their blends increased with an increasing storage period (Figure 1c). There was no significant difference between the peroxide values for blends 1, 2, and 3, whereas blend 4 and blend 5 differed significantly (p < 0.05). The results obtained were in accordance with Siddique et al. (2010SiddiqueBM, AhmadA, IbrahimMH, HenaS, RafatullahM. 2010. Physico-chemical properties of blends of palm olein with other vegetable oils. Grasas Aceites61 (4), 423–429. 10.3989/gya.010710), who reported that peroxide value increased with storage time. Further, after 14 days of accelerated storage, flaxseed oil showed a significantly (p < 0.05) higher peroxide value (3.79 ± 0.01 meq O2/kg) compared to coconut oil (2.19 ± 0.01 meq O2/kg), and blends obtained lower peroxide values than pure flaxseed oil. The higher peroxide values for flaxseed oil and the blends with a high proportion of flaxseed oil are due to the higher degree of unsaturation of flaxseed oil than coconut oil. A study by Bhardwaj et al. (2015BhardwajK, VermaN, BhardwajS. 2015. A novel approach for improvement of oxidative stability of flaxseed oil by blending with palm oil. Int. J. Adv. Res.3 (11), 1399–1407.) on enhancing the oxidative stability of flaxseed oil by blending it with palm oil revealed that blending flaxseed oil with 60% palm oil was significantly stable throughout the storage period compared to blending flaxseed oil with 40% palm oil because of the oxidative stability of palm oil. A study by Hamed and Abo-Elwafa (2012HamedSF, Abo-ElwafaGA. 2012. Enhancement of oxidation stability of flax seed oil by blending with stable vegetable oils. J. Appl. Sci. Res. 8 (10), 5039–5048.) reported that blending flaxseed oil with Nigella seed oil inhibited hydroperoxide formation by 31.71% (relative to flaxseed oil) compared to that of a blend with sesame oil which showed only 9.75% hydroperoxide formation inhibition. This means blending Nigella seed oil with flaxseed oil reduced the peroxide value of flaxseed oil compared to blending with sesame oil.
3.2.4. Saponification value
⌅The saponification value is a measure of the average molecular weight of all the fatty acids present. The higher the saponification value, the shorter the fatty acids on the glycerol backbone (Marina et al., 2009MarinaAM, CheMYB, NazimahSAH, AminI. 2009. Chemical properties of virgin coconut oil. J. Am. Oil Chem. Soc. 86 (4), 301–307. 10.1007/s11746-009-1351-1). The initial saponification value for coconut oil was 249.33 ± 0.58 mg KOH/g and flaxseed oil was 188.24 ± 0.01 mg KOH/g. This indicates that coconut oil has a higher amount of short chain fatty acids than flaxseed oil. The initial saponification value of oil blends decreased with an increased proportion of flaxseed oil. The saponification value for oils and their blends showed an increasing trend with storage time (Figure 1d). Blends and oil exhibited significant differences (p < 0.05), although no significant differences were found between blends 2 and 3 (p > 0.05). An increase in the saponification value of the samples with storage indicates a decrease in the chain length of fatty acids, possibly due to the breakdown of fatty acids during oxidation. The results obtained were in accordance with Siddique et al. (2010SiddiqueBM, AhmadA, IbrahimMH, HenaS, RafatullahM. 2010. Physico-chemical properties of blends of palm olein with other vegetable oils. Grasas Aceites61 (4), 423–429. 10.3989/gya.010710), who reported that the saponification value for oil samples increased with storage time. This trend explains that with the extended storage of these oils, free fatty acids are formed, which increases the saponification value.
3.3. Oxidative stability of oils and their blends during frying
⌅During the frying of pure flaxseed oil, higher foaming was observed. It was difficult to determine frying conditions. However, blend 3 exhibited the lowest foam compared to blends 4 and 5.
3.3.1. Acid value
⌅The acid values for oils and their blends were significantly increased throughout frying (Figure 2a). After the 4th frying cycle, except for blends 1 and 3, all other samples showed a significant increase in acid value (p < 0.05). During frying, both oxidation and hydrolysis could lead to the formation of free fatty acids. Che and Wan (1998CheMYB, WanHWR. 1998. Comparison of the frying performance of refined, bleached and deodorized palm olein and coconut oil. J. Food Lipids. 5 (3), 197–210. 10.1111/j.1745-4522.1998.tb00120.x) also found that the acid value increased throughout frying for palm oil and coconut oil blends.
3.3.2. Iodine value
⌅The iodine value for oils and their blends showed a downward trend (Figure 2b). Other than coconut oil and blend 1, the iodine values for all samples differed significantly (p < 0.05). This indicates the progress of oxidation and coconut oil and blend 1 showed higher stability than the other blends and flaxseed oil. This is evident from the non-significant changes in the fatty acid profile before and after frying for these two samples. Alireza et al. (2010AlirezaS, TanCP, HamedM, Che ManYB. 2010. Effect of frying process on fatty acid composition and iodine value of selected vegetable oils and their blends. Int. Food Res. J. 17 (2), 295–302.) found a similar decreasing trend in the iodine value for oils and their blends during deep frying and reported that the decrease in the iodine value is due to the decrease in double bonds as oil becomes oxidized. Che and Wan (1998CheMYB, WanHWR. 1998. Comparison of the frying performance of refined, bleached and deodorized palm olein and coconut oil. J. Food Lipids. 5 (3), 197–210. 10.1111/j.1745-4522.1998.tb00120.x) also reported the same downward trend in the iodine value for palm oil and coconut oil blends.
3.3.3. Peroxide value
⌅The peroxide value was increased with frying cycles (Figure 2c). The values for blends 1 and 4 were not significantly different (p > 0.05). However, other samples were significantly differed (p < 0.05). After the 4th cycle of frying, flaxseed oil exhibited the highest peroxide value (3.90 ± 0.15 meq O2 /kg), while coconut oil had a peroxide value of 1.99 ± 0.001 meq O2/kg. Blend 1 exhibited a higher peroxide value (2.80 ± 0.00 meq O2/kg) than that of blend 2 (2.22 ± 0.03 meq O2 /kg). However, blend 5 obtained the highest peroxide value (2.94 ± 0.07 meq O2/kg) among all the blends studied. Blends 3 and 4 manifested similar peroxide values after the 4th frying cycle. This observation also can be explained based on the degree of unsaturation. Flaxseed oil and blend 5, which consisted of a high amount of flaxseed oil, presented more extensive oxidation than the other samples. Farhoosh et al. (2009FarhooshR, EsmaeilzadehKR, PoorazrangH. 2009. Frying stability of canola oil blended with palm olein, olive, and corn oils. J. Am. Oil Chem. Soc. 86 (1), 71–76. 10.1007/s11746-008-1315-x) observed a similar increasing trend in peroxide value for canola, olive, palm olein, and corn oils, and the blending of these four oils showed a lower peroxide value compared to pure oils. Che and Wan (1998CheMYB, WanHWR. 1998. Comparison of the frying performance of refined, bleached and deodorized palm olein and coconut oil. J. Food Lipids. 5 (3), 197–210. 10.1111/j.1745-4522.1998.tb00120.x) also found the same scenario in the peroxide value pattern and stated the same fact.
3.3.4. Saponification value
⌅The saponification value also increased throughout frying (Figure 2d). However, the values for blends 2 and 3 were not significantly different (p > 0.05). Flaxseed oil showed a higher increment compared to coconut oil (from 188.71 ± 0.66 mg KOH/g to 200.22± 0.19 mg KOH/g) and showed a significant increase in saponification value (p < 0.05). An increase in the saponification value indicates the formation of short-chain fatty acids due to the oxidative degradation of fatty acids. Goburdhun and Jhurree. (1995GoburdhunD, JhurreeB. 1995. Effect of deep-fat frying on fat oxidation in soybean oil. Int. J. Food Sci. Nutr. 46 (4), 363–371. http://dx.doi.org / 10.3109/09637489509012568) observed an increasing trend in the saponification value of soybean oil blended with palm kernel olein.
3.4. Fatty acids composition of oils and their blends before and after frying
⌅The SFA: MUFA: PUFA (Saturated fatty acids: Monounsaturated fatty acid Polyunsaturated fatty acids) ratios of coconut oil and flaxseed oil were 20.3:1:0 and 1:1.6:5.6, respectively. The blends showed a modification in the SFA: MUFA: PUFA ratio (Joshi et al., 2023JoshiAA, HegdeMV, ZanwarAA. 2023. Modulation of essential fatty acid levels in coconut oil with flaxseed oil. Grasas Aceites74 (1), e503. 10.3989/gya.1018212). In particular, blend 5 showed 7.3:1:1.5, and blend 4 showed 7.8:1:1, respectively (Table 2). The WHO recommends that a reduction in the intake of SFA has been associated with a significant reduction in coronary heart disease when replaced with PUFA (WHO, 2018WHO. 2018. Draft guidelines on SFA and trans fatty acids intake for adults and children. http://www.who.int/mediacentre/factsheets/fs394/en).
Blend 1-95:5, Blend 2-90:10, Blend 3-85:15, Blend 4-80:20 and Blend 5-75:25, Coconut oil: Flaxseed oil, respectively. The values are calculated based on the values given in Table 1. SFA – Saturated Fatty Acids, MUFA – Monounsaturated Fatty Acids and PUFA – Polyunsaturated Fatty Acid
The blends showed an increase in alpha-linolenic acid contents (omega-3 fatty acid). The alpha-linolenic acid content in blends 1, 2, 3, 4, and 5 were 3.47 ± 0.02, 5.26 ± 0.03, 5.41 ± 0.01, 6.95 ± 0.01, and 11.12 ± 0.01%, respectively. The results obtained were in accordance with Faiza et al. (2016), where the blending of coconut oil and flaxseed oil blends of 70:30, respectively, increased the linolenic acid content. Among the blends, blend 5 exhibited a favorable omega-3 and omega-6 fatty acid composition, it showed 11.12 ± 0.01% omega-3 fatty acid and 5.01 ± 0.01% omega-6 fatty acid (Table 1).
Blend 1-95:5, Blend 2-90:10, Blend 3- 85:15, Blend 4- 80:20 and Blend 5-75:25, Coconut oil: Flaxseed oil, respectively. Values are the means ± standard deviation of duplicates. Mean significant differences were tested using DMRT at α level 0.05. The means with the same letters in the same row are not significantly different.
After 4 hours of continuous frying, a change in fatty acid composition was observed. Alpha-linolenic acid (C18:3) contents were significantly decreased in flaxseed oil, blends 1, 2, 3 and 5, (p < 0.05); whereas the Alpha-linolenic acid content in blend 4 was not changed (p > 0.05). Linoleic acid (C18:2) content also significantly decreased in the original flaxseed oil and all blends. These results were in accordance with Che and Wan (1998CheMYB, WanHWR. 1998. Comparison of the frying performance of refined, bleached and deodorized palm olein and coconut oil. J. Food Lipids. 5 (3), 197–210. 10.1111/j.1745-4522.1998.tb00120.x). Linoleic acid content was reduced by 1% in flaxseed oil and by 56, 41, 45, 24, and 19% in blends 1, 2, 3, 4 and 5, respectively. The alpha-linolenic acid content was reduced by 32, 67, 56, 36, and 31% in flaxseed oil in blends 1, 2, 3, and 5, respectively.
Alpha-linolenic acid (C18:3) is a polyunsaturated fatty acid and is more susceptible to oxidation and cis-trans isomerization (Szabo et al., 2022SzaboZ, MarosvölgyiT, SzaboE, KoczkaV, VerzarZ, FiglerM, DecsiT. 2022. Effects of Repeated Heating on Fatty Acid Composition of Plant-Based Cooking Oils. Foods. 11 (2), 192. 10.3390/foods11020192). The multiple double bonds make alpha-linolenic acid highly susceptible to oxidative and thermal degradation, leading to the formation of lipid peroxides, aldehydes, and ketones. High temperatures exacerbate this process by breaking the carbon-carbon double bonds and generating free radicals, which propagate further degradation. The degradation of alpha-linolenic acid during the thermal treatment of edible oils is reported in the literature (Mao et al.,2020MaoX, ChenW, HuyanZ, SheraziSTH, YuX. 2020. Impact of linolenic acid on oxidative stability of rapeseed oils. J Food Sci Technol. 57 (9), 3184–3192. 10.1007/s13197-020-04349-x; Szabo et al., 2022SzaboZ, MarosvölgyiT, SzaboE, KoczkaV, VerzarZ, FiglerM, DecsiT. 2022. Effects of Repeated Heating on Fatty Acid Composition of Plant-Based Cooking Oils. Foods. 11 (2), 192. 10.3390/foods11020192).
In the case of coconut oil, some new fatty acids were observed after frying, such as gondonic acid (C20:1), heneicoisylic acid (C21:0) and behenic acid (C22:0). There were no new fatty acids generated in the flaxseed oil or all the blends. The blending of flaxseed oil with coconut oil evidently increased the fatty acids profile of coconut oil and improved the oxidative stability of flaxseed oil. Blending improved the SFA: MUFA:PUFA ratio as well.
4. CONCLUSIONS
⌅All the blends showed acceptable oxidative stability under both storage and frying conditions. Blends 1, 2, and 3 exhibited higher oxidative stability than blends 4 and 5. The acid value of the prepared blends was lower than that of flaxseed oil, although higher than coconut oil. Moreover, the acid value and peroxide value of all blends fell within the acceptable range recommended by Codex Alimentarius. The saponification value increased with storage and after frying; whereas the iodine value decreased throughout storage and after frying. As all the blends exhibited acceptable oxidative stability (Codex Alimentarius, 2021Codex Alimentarius. 2021. Standard for named vegetable oils, CXS 210-1999: Adopted in 1999. Revised in 2001, 2003, 2009, 2017, 2019. Amended in 2005, 2011, 2013, 2015, 2019. http://www.fao.org/fao-who-codexalimentarius/sh-proxy/en/?lnk=1&url=https%253A%252F%252Fworkspace.fao.org%252Fsites%252Fcodex%252FStandards%252FCXS%2B210-1999%252FCXS_210e.pdf) under both storage and frying conditions, all the blends can be stored for up to 14 months and utilized as a frying medium. Blending can be used to improve the nutritional quality in terms of fatty acid profile. This study also found that combining coconut oil and flaxseed oil at a ratio of 80:20 (coconut oil: flaxseed oil) can boost the contents of omega-3 fatty acids and omega-6 fatty acids and that it can be kept for up to 14 months and utilized as a frying medium.