Natural phenolic substances are ubiquitously-distributed secondary metabolites in berries, fruits, vegetables, coffee, herbs and edible oils. Main phenolic substances include phenolic acids, phenolic alcohols and flavonoids. Plant phenolic compounds have drawn the attention of researchers due to their antioxidant properties. The antioxidant mechanisms of the phenolic compounds in plants have been well established (Michalak, 2006; Zeb, 2020). The antioxidant properties of the phenolic compounds in plants are connected to beneficial health effects such as conferring protection against the development of cancer, diabetes, cardiovascular diseases and neurodegenerative diseases (Pandey and Rizvi, 2009). The phenolic compounds in plants can also act as food preservatives that can inhibit the lipid oxidation in edible oils and fish oils owing to their high polyunsaturated fat contents (Maqsood et al., 2014). The color and flavor properties of natural foods as well as processed foods and beverages are also controlled by plants’ phenolic compounds (Cheynier, 2012).

During the extraction of edible oils, the phenolic compounds of kernel materials are incorporated into the oils. Edible oils have saponifiable and unsaponifiable compounds. The saponifiable fraction may account for 90-98% of the total mass of oil while the unsaponifiable content varies from 2-10% (Narasinga Rao, 2001). The saponifiable fraction includes triglycerides, diglycerides, monoglycerides, fatty acids and other saponifiable lipids. The minor unsaponifiable fraction includes phenolic compounds, flavonoids, vitamin E, sterols, hydrocarbons, etc. (Moura Fe et al., 1975). In coconut oil, the saponifiable fraction may amount to up to 99.5% of the weight of the oil (Gutfinger and Letan, 1974). In edible oils with no sedimentations or insoluble materials, the saponifiable fraction can be observed as a clear solution and unsaponifiable compounds are dissolved in the saponifiable fraction.

Phenolic compounds in several edible oils have been reported and most of these studies are limited to assessing phenolic compounds in a particular oil (Mannino et al., 1999; Tripoli et al., 2005; Siger et al., 2008; Janu et al., 2013). Among them, a vast majority of research has been focused on the phenolic fraction of olive oil (Kiritsakis, 1998; Ryan and Robards, 1998; Boskou et al., 2005; Galvano et al., 2007; Servili et al., 2009). In addition to the studies on the phenolic compounds in specific edible oils, a recent report has comprehensively reviewed the different classes of phenolic compounds in several edible oils (Zeb, 2021). As phenolic compounds are polar molecules, their solubility in the oils is low. However, these minor components improve the quality of edible oils by enhancing health benefits and sensory properties (Visioli and Galli, 1998). The interest in the phenolic substances in coconut oil is also due to their antioxidant activity and related health benefits (Seneviratne et al., 2011; Lima et al., 2015; Narayanankutty et al., 2018; Rohman et al., 2021; Senanayake et al., 2021). In addition, the phenolic compounds present in coconut oil and coconut oil meal improve the shelf-life of coconut oil as well as other edible oils and baked food (Senanayake et al., 2019). Even though there is a wealth of literature regarding research conducted on the phenolic substances of olive oil and other common edible oils, the first report on the phenolic substances in coconut oil was published in 2008 (Seneviratne and Dissanayake, 2008). Since then, there has been a growing number of studies on the phenolic fraction of coconut oil. Emulsions of virgin coconut oil (VCO) containing ferulic acid and p-coumaric acid have been prepared with various sweeteners to improve the palatability of VCO (Wiyani et al., 2020). The addition of VCO to dark chocolate formulations has shown that the nutritional properties of dark chocolate are improved by the phenolic compounds in VCO (Rashid et al., 2017). New studies indicate that coconut oil improves the absorption of phenolic compounds in rats and humans, suggesting that the naturally-present phenolic compounds in coconut oil are more bioavailable than the phenolic compounds in aqueous foods (Prasadani et al., 2017; Weerakoon et al., 2021). Despite several health and nutritional advantages and applications as food preservatives, the quality and quantity of the phenolic compounds in coconut oil have not been reviewed. Therefore, the present review aimed to comprehensively review the up-to-date knowledge on the effect of extraction conditions, variety, country of origin and nature of kernel material on the phenolic composition of coconut oil.

The most important factor affecting the phenolic content in coconut oil is the method of extraction (Seneviratne and Jayathilaka, 2015; Seneviratne and Jayathilaka, 2016). The coconut kernel is different from all other seed kernels due to its bulkiness and high percentage (59%) of coconut oil in the dried kernel (Gutfinger and Letan, 1974). Therefore, the extraction of coconut oil is less cumbersome compared to the extraction of other edible oils. Different methods of coconut oil extraction are summarized in Figure 1. Coconut oil prepared by the wet extraction methods under cold conditions is called VCO. In addition, VCO is also produced by dry extraction by pressing high quality dried coconut kernels under cold conditions.

Most studies on the phenolic composition of coconut varieties are limited to those done on a particular variety and there are limited studies that compare the differences in composition among the different varieties. The TPC of the VCO extracted by freezing coconut milk from West African Tall variety is 160.2 mg/kg. Methanol (60%) was used to extract phenolic compounds in this study (Ahmad et al., 2015). Coconut kernels of Dwarf x Tall variety from India indicate the presence of coumaric acid, caffeic acid and chlorogenic acid. (Dhanyakrishnan et al., 2018). The TPC of coconut water from the Malayan green coconut variety is 95.15 mg/L; while that of the Malayan yellow coconut variety is 46.85 mg/L. (Adubofuor et al., 2016). Even though there is a limited number of reports on the phenolic composition of coconut oil from different coconut cultivars, most of such data cannot be compared because of the different conditions used for the extraction of coconut oil or different extraction conditions used for the extraction of the phenolic substances from the coconut oil. A systematic comparison of the TPC in coconut oil extracted from three coconut varieties and three hybrids of these varieties grown in Thailand has been reported (Arlee et al., 2013). According to this study, the TPC of VCO extracted by natural fermentation may vary significantly among cultivars and hybrids. However, the range of TPC in coconut oil extracted by the fermentation method for three cultivars and three hybrids is 481-554 mg/kg. The TPC in VCO prepared by cold pressed dry method may also vary significantly (p ≤ 0.05) among varieties and cultivars. Nevertheless, the range of TPC is 486-579 mg/kg. The TPC in VCO from two cultivars grown in Mexico also shows that there is a significant difference in TPC with cultivar. The TPC in this study varies in the range 600-780 mg/kg (Elodio-Policarpo et al., 2019).

VCO has become popular as a functional oil due to its health benefits. The major contributor to the numerous health benefits is phenolic compounds (Srivastava et al., 2018). Thailand, Indonesia, India, Malaysia, Sri Lanka and Vietnam are the main producers of VCO in the world. Even though phenolic compounds play a major role in providing health benefits to consumers, phenolic quality and quantity are not included in coconut oil standards. Detailed studies on the influence of geographical origin on the TPC in coconut oil have not been carried out. However, VCO samples obtained from the markets of Malaysia and Indonesia have been compared (Marina et al., 2009b). The results indicated that the TPCs in the Malaysian VCO samples were 118-292 mg/kg; while those of the Indonesian VCO samples were 78-251 mg/kg.

There are two components in the coconut kernel: white coconut meat and coconut testa. The interior part of the coconut kernel is the thick, fleshy, white coconut kernel, which is known as white coconut meat. The thin brown outer skin of the white coconut kernel between the white coconut meat and the coconut shell is known as coconut testa. When water evaporates from the coconut meat during the drying of coconut halves, white coconut meat and testa separates from the shell (Figure 2). Both testa and coconut meat are taken for dry or wet extraction of coconut oil unless coconut testa is purposely removed. The TPCs in coconut meat from three coconut types from Indonesia, Vietnam and Thailand have been compared (Ngampeerapong and Chavasit, 2019). Shredded coconut meat was extracted with 80% methanol. This study reported that the TPCs of coconut meat were 72,122 mg/kg (Thailand variety), 63,910 mg/kg (Indonesian variety) and 103,421 mg/kg (Vietnam variety). However, the same paper indicated that the percentages of moisture + protein + fat + carbohydrates of coconut meat added up to 99.09% (Thailand variety), 99.01% (Indonesian variety) and 99.06% (Vietnam variety). Therefore, the phenolic contents of ~7% (Thailand variety), ~6% (Indonesian variety) and 10% (Vietnam variety) may not be realistic. The moisture content in the coconut meat was about 50% according to the analysis of this research. Coconut kernels contain free amino acids (Baptist, 1963). These amino acids interfere with the determination of TPC by the Folin-Ciocalteu method (Everette et al., 2010; Bastola et al., 2017). Sugars also interfere with the Folin-Ciocalteu method (Sanchez-Rangel et al., 2013; Muñoz-Bernal et al., 2017). Proteins in coconut meat can be precipitated prior to the determination of TPC in coconut milk to avoid interference (Kwon et al., 1996; Nadeeshani et al., 2015). Interference by amino acids, proteins and sugars in the coconut meat may contribute to the high readings of TPCs in some studies. However, the reported phenolic contents of 98-100 mg/kg for coconut meat may be more reasonable (Mahayothee et al., 2016).

Coconut testa is a very minor component of coconut meat. It is difficult to give an exact estimate of the phenolic contents in pure coconut testa since part of white coconut meat also combines with coconut testa during the paring of coconut meat to separate testa. Coconut testa is the richer source of phenolic compounds compared to white coconut meat (Seneviratne et al., 2009; Seneviratne et al., 2016). A study indicates that the TPC in the coconut oil extracted from the whole copra with both white kernel and brown testa is 1.4 ± 0.19 mg/100 g (14 ± 2 mg/kg); while coconut oils extracted from white coconut kernel alone and coconut testa alone contain 1.1 ± 0.11 mg/100 g (11 ± 1 mg/kg) and 1.9 ± 0.12 mg/100 g (19 ± 1 mg/kg) of phenolic substances, respectively (Appaiah et al., 2014). Reports indicate that the extraction efficiency of phenolic compounds from coconut testa significantly (P ≤ 0.05) varies with the solvent system, while the acidification of the solvent increases the extraction efficiency. For example, the extraction of phenolic compounds from dry and defatted testa by 80% ethanol gives TPC of 72.7 ± 1.9 mg/g (72,700 ± 1900 mg/kg); while the extraction of dry and defatted testa with acidified 80% ethanol gives TPC of 93.9 ± 5.3 mg/g (93,900 ± 5,300 mg/kg) (Arivalagan et al., 2018). Phenolic compounds were extracted from powdered coconut testa with methanol using Soxhlet extraction. The reported phenolic quantities were unusually high with over 80% (822 mg/g) of the weight of the plant extract being phenolic compounds (Ojha et al., 2019). If these phenolic quantities were expressed for coconut testa powder instead of plant extract, a better comparison could have been made with other reported values. The phenolic composition of coconut testa also varied with the coconut cultivar. The TPCs in coconut testa powder produced from four different coconut cultivars grown in Sri Lanka, namely San Raman, Gon Thembili, Ranthembili and Tall x Tall varied in the range of 27,530-62,580 mg/kg due to cultivar (Marasinghe et al., 2019).

In both the wet and dry processing of coconut kernels for oil extraction, both coconut meat and testa can be used. Even though coconut testa is the main source of phenolic substances in coconut kernels, due to its brown color, coconut testa adds a light-yellow color to the coconut oil. Colorless coconut oil is commercially preferred. Therefore, coconut testa is removed during the preparation of desiccated coconut as well as in the large-scale preparation of VCO by both wet and dry methods.

The TPCs in coconut oil extracted from only white coconut kernels and white coconut kernels with coconut testa under wet hot extraction conditions were 78 ± 2 and 250 ± 4 mg/kg, respectively (Seneviratne et al., 2009). Higher phenolic contents in the coconut oil extracted by using white kernels with coconut testa was due to the richer phenolic contents in testa compared to white coconut kernels.

The phenolic contents in coconut kernel components (white coconut meat and testa) are not fully incorporated into coconut oil during wet or dry extraction methods. Therefore, the residue after the expulsion of coconut oil from coconut kernel components is also rich in phenolic antioxidants (Illam et al., 2017; Karunasiri et al., 2020b).

Table 1 shows that the TPC clearly depends on the method of extraction of coconut oil. According to Table 1, there is a wide range of TPC reported by different authors for copra oil. The range of TPC in copra oil is 91-644 mg/kg, based on the reported studies (Nevin and Rajamohan, 2004; Seneviratne and Dissanayake, 2005; Seneviratne and Dissanayake, 2008; Arunima and Rajamohan, 2013; de Azevedo et al., 2020). However, the TPC values reported by Ghani et al. (2018) for dry extracted coconut oil, the TPC values reported by Prapun et al. (2016) and by Ghani et al. (2018) for VCO extracted by fermentation methods and the TPC value reported for VCO prepared by the enzymatic extraction by Prapun et al. (2016) seem unusually higher than the values reported by other researchers. Even though the phenolic contents depend on many factors other than the extraction method, these extremely high phenolic contents compared to the other reported values may not be reasonably explained. The TPC in coconut oil prepared by oven-dried grated coconut meat and sun-dried grated coconut meat were reported in one study as 1.56 ± 0.24 mg/g (1560 ± 240 mg/kg) and 8.57 ± 0.36 mg/g (8570 ± 360 mg/kg), respectively (Ghani et al., 2018). The values are higher than any other reported values for coconut oil prepared by dry methods (Table 1). The difference in phenolic contents between coconut oils prepared by oven-dried and sun-dried coconut meat was attributed to the possibility of destroying phenolic substances during the drying process. However, the temperature used in the drying process was 40 ºC in this study. Even higher temperatures up to 60-70 ºC were used in the production of VCO and thermally unstable compounds were not affected by 60-70 ºC temperatures. The TPC in coconut oil varied in the order: fermentation > chilling > RBD, suggesting that the steps involved in the RBD process removed some phenolic substances (Marina et al., 2009a).

If unusually high values for TPC were omitted and if extraction method was considered the only variable, the maximum additional phenolic content that could be incorporated into coconut oil by changing the extraction method compared to the extraction method that gave the lowest phenolic content is about 527 mg/kg (Seneviratne and Dissanayake, 2008). Therefore, up to 527 mg/kg phenolic compounds can be incorporated into coconut oil by changing the extraction method. Not many studies have been conducted to evaluate how phenolic compounds in coconut oil vary with the variety of coconut. The limited data available for the variation of TPC with variety is up to 180 mg/kg. The influence of geographical origin on the TPC cannot be computed due to a lack of sufficient relevant data. TPC may change by up to 172 mg/kg due to the influence of the nature of coconut kernel components based on the reported values (Seneviratne et al., 2009).

The individual phenolic antioxidants in coconut oil have been identified and quantified. Caffeic acid, p-coumaric acid, ferulic acid, catechin, dihydrokaempferol, rosmarinic acid and quercetin were identified by mass spectroscopy coupled with liquid chromatography (LC-MS) (Seneviratne and Dissanayake, 2008; Illam et al., 2017). The structures of commonly reported phenolic compounds present in coconut oil are given in Figure 3. The quantities of the individual phenolic compounds were higher in coconut oil prepared by boiling coconut milk (hot extracted coconut oil) compared to copra oil prepared by pressing coconut copra (Seneviratne et al., 2009; Srivastava et al., 2016). The TPC was also higher in hot extracted coconut oil compared to VCO prepared by fermentation (Table 1) (Seneviratne et al., 2009; Srivastava et al., 2016). In the hot extraction method, coconut milk emulsion is heated for a long time till water in the emulsion evaporates. During this evaporation, water and oil layers separate and with the evaporation of water, hydrophilic phenolic compounds are concentrated in the aqueous phase. This allows higher partitioning of phenolic antioxidants in the oil layer. The high temperature used in the hot extraction method favored the dissolving of phenolic compounds in the oil layer (Seneviratne et al., 2009). Consistent with the higher TPC of hot extracted coconut oil, the number and the quantity of the phenolic compounds were also higher in hot extracted coconut oil (Table 2) (Seneviratne et al., 2009; Srivastava et al., 2016). In addition to the results in Table 2, another author also reported 4.04 x10^-2 ppm gallic acid in the phenolic extracts of VCO with no information on the extraction method (Librado and Von Luigi, 2013). Other reports have also indicated that the quantities of individual phenolic acids in coconut oil vary with extraction method (Marina et al., 2009a). For example, Table 2 shows that the quantities (mg/kg) of phenolic compounds in coconut oils prepared by fermentation method and by chilling/centrifugation method contain different amounts of individual phenolic compounds. Vanillic acid and caffeic acid were not detected in VCO prepared by chilling and centrifugation; while those two phenolic compounds were detected in coconut oil prepared by fermentation. Catechin and epigallocatechin are the common flavonoids present in coconut oil (Table 2). In addition, quercetin (1.62 ± 0.09 mg/kg) has been reported in hot extracted coconut oil (Srivastava et al., 2016). Myricetin-3-o-glucoside has also been reported to be present in VCO with no clear origin (Illam et al., 2017).

RBD coconut oil is prepared by procedures involving chemical and physical steps. In physical refining, degumming is done in the first step by treating the oil with phosphoric acid to remove phospholipids. Then the oil is heated to 80-90 °C and bleached by passing through a mixture of bleaching earth and activated carbon. Finally, the deodorizing and removal of free fatty acids are done by exposing the oil to a vacuum at 220-240 °C. The main difference in chemical refining is that free fatty acids are removed by treating the oil with a pre-determined amount of NaOH to neutralize fatty acids instead of removing free fatty acids under vacuum distillation. The resultant sodium salts in fatty acids (soap) are washed away with water. Refining, bleaching and deodorization remove most of the phenolic substances, tocopherols and sterols from coconut oil (Liu et al., 2019a; Deen et al., 2021). One study indicated that the TPC in crude copra oil was 618 mg/kg. However, the remaining TPC of this oil after the RBD process was 20 mg/kg (Pavan Kumar et al., 2018). The TPCs in commercial RBD coconut oil purchased from the Indian market were 21 ± 2 mg/kg; while those in crude oil samples were in the range of 27-191 mg/kg. The maximum total phenol content reported for RBD oil is 120 mg/kg, according to Table 1. Some studies also reported almost zero (0.1 ± 0.0 mg/kg) TPC for RBD coconut oil (Liu et al., 2019b). Individual phenolic compounds are also different in RBD coconut oil compared to other coconut oils. For example, the vanillic and syringic acids present in coconut oil extracted by fermentation are absent in RBD coconut oil. The ferulic acid content present in coconut oil extracted by the fermentation method is 5.09 ± 2.3 mg/kg; while RBD coconut oil contained only 1.39 ± 0.2 mg/kg (Marina et al., 2009a). Therefore, the nutritional properties expected from the phenolic antioxidants present in crude or virgin oils may not be fully expected from RBD oils. Due to the removal of phenolic compounds, RBD coconut oil is less stable against oxidation compared to VCO (Koh and Long, 2012). RBD also has different sensory properties compared to crude coconut oil or VCO due to the removal of phenolic compounds and different volatile organic matter (Chang et al., 2020).

The TPCs in olive oil have been reported in numerous publications. Commonly reported ranges include 45-532 mg/kg (Mannino et al., 1999; Bayram et al., 2012). The TPC in the olive oil of three Italian varieties show significant (p ≤ 0.05) differences and changes in the range of 367-530 mg/kg (Sicari, 2017); while the TPC of some traditionally prepared Turkish olive oils from different olive varieties ranges from 22.5 to 97.1 mg/kg (Tanilgana et al., 2007). The TPCs in the olive oils from six Italian olive varieties prepared by a novel cooling treatment of olive paste varied in the range of 410-1005 mg/kg (Veneziani et al., 2017; Veneziani et al., 2018). Olive oil from two Croatian olive varieties, Bianchera and Busa, contained 312 and 248 mg/kg, respectively (Skevin et al., 2003). Mean TPC and maximum TPC in olive oil from olive varieties in Greece were 483 mg/kg and 4003 mg/kg, respectively (Diamantakos et al., 2021). Due to the differences in phenolic profiles, the fingerprinting of phenolic composition with suitable chemometric methods can be used to differentiate the varietal origin of olive oil (Bajoub et al., 2017).

Geographical conditions also play an important role in the TPC of olive oil. The TPC in the same olive variety drastically changed from 513-1084 with the geographical area (Mansour et al., 2017). European olive varieties grown in Tunisia produced different phenolic profiles compared to the traditional growing areas and the most important factor affecting the phenolic composition was suggested to be altitude (Dabbou et al., 2009). In addition to the olive variety and geographical origin, the ripening stage of the olives and the crushing method also affect the phenolic composition according to studies done in Italy and Spain (Giovacchino et al., 2002; Navajas-Porras et al., 2020).

Several phenolic compounds present in coconut oil such as caffeic acid, ferulic acid, gallic acid, p-hydroxybenzoic acid, syringic acid, vanillic acid, p-coumaric and cinnamic acid have been reported to be present in olive oil as well (Ryan and Robards, 1998; Boskou et al., 2005). In addition to free phenolics and flavonoids, several bound phenolic compounds are also present in olive oil (Servili and Montedoro, 2002; Jimenez-Lopez et al., 2020). However, only three bound phenolic compounds have been reported so far for coconut oil (Seneviratne et al., 2009; Illam et al., 2017). The phenolic compounds in both olive oil and coconut oil are responsible for beneficial health effects (Covas et al., 2006; Seneviratne and Jayathilaka, 2015; Narayanankutty et al., 2018; Deen et al., 2021). The aroma and flavor of olive oil correlate with the phenol content (Kiritsakis, 1998; Genovese et al., 2018; Pedan et al., 2019). Even though the sensory properties of virgin coconut oil have been studied (Villarino, Dy and Lizada, 2007, Lukic et al., 2017; Fiorini et al., 2018), the correlation between phenolic compounds and sensory properties in coconut oil needs further research.

The common range of TPC for coconut oil extracted with any variable such as extraction method, variety, coconut kernel material, etc. is 72-650 mg/kg according to reported values. However, unusually high values for TPC have also been reported for coconut oil and coconut meat. Even though the extraction method, variety, hybrids, refining, etc. affect the TPC in coconut oil, these factors may not account for some unusually high TPC values reported. Interference by other compounds such as amino acids and sugars during the determination of phenolic content using the Folin-Ciocalteu assay and possible calculation errors during converting colorimetric readings to given quantities in (mg/g, mg/100 g or mg/kg) units may be possible causes for the large deviation of TPC values in some reports. The RBD process removes phenolic compounds from coconut oil. Depending on the efficiency of the RBD process, the TPC in RBD coconut oil varies between 0-120 mg/kg. The antioxidant activity and health benefits of the phenolic compounds in coconut oil have been sufficiently reported. Even though there has been substantial work done on the extraction method and phenolic composition, sufficient studies have not been conducted to assess the effect of the wide range of coconut varieties available in different geographical origins on the phenolic composition. Therefore, information on the variation in phenolic composition with coconut variety and geographical origin presented in the present review was based on the limited literature available. Careful review of the literature also indicates that there are very limited studies on the identification of phenolic compounds in coconut oil by HPLC methods. Most of the reported phenolic compounds in coconut oils are free phenolic acids and flavonoids. Several of these free phenolic compounds present in coconut oil are also present in olive oil. However, the presence of glycosides, aglycones, secoiridoid derivates or any other bound forms of phenolic compounds in coconut oil have not been reported. Therefore, further studies are necessary to elucidate the complete phenolic profile of coconut oil. In addition, further studies are necessary to investigate how the phenolic compounds in coconut oil are related to the sensory properties of coconut oil.