2021 年 70 巻 9 号 p. 1201-1210
This review is aimed to provide a comprehensive overview of the physicochemical properties and extraction processes of red palm oil, its nutritional properties and applications in food. Crude palm oil is firstly extracted from the fruit mesocarp and processed into red palm oil using pre-treatment of crude palm oil, with deacidification steps, and deodorization via short-path distillation. These processes help to retain β-carotene and vitamin E in red palm oil. Palmitic, stearic and myristic acids are the saturated fatty acids in red palm oil, while the unsaturated fatty acids are oleic, linoleic and linolenic acids. It is reported to overcome vitamin A deficiency, promote heart health and have anti-cancer properties.
Oil palm, Elaeis guineensis, has been cultivated in Malaysia since 1917 and is now the most important economic crop in Malaysia, with export revenue from the crop reaching more than RM 67.12 billion in 2018 1) . Other leading palm oil-producing countries are Indonesia and Thailand. The crop produces almost ten times higher oil per hectare than any other leading vegetable oil crops 2) . Palm oil and its derivatives are mainly used for cooking, frying oils, margarine, shortenings, vanaspati and specialty fats 3) . Palm oil or its blends with with palm kernel or with other vegetable oils were the different product available in market.
Crude palm oil is extracted from palm mesocarp and rich in minor components, including carotenoids (precursor of vitamin A), tocopherols and tocotrienols (vitamin E), sterols, phospholipids, squalene, aliphatic hydrocarbons and aliphatic alcohols 4) . More research has been focusing on carotenoids and vitamin E in palm oil in recent years with its antioxidant properties. Despite their nutritional values, palm carotenoids are removed during the refining process, this is to gain clear oil which is better for consumer acceptance and industry purposes 5) . The concentration of carotenoids is reduced to more than half with application of high temperature and steps such as deacidification and deodorization. Goh et al. stated that tocopherols’ level decreased tremendously during this refining process 4) .
Red palm oil is a refined palm oil, in which although it has similar properties as refined, bleached, deodorized (RBD) oil, it still retains its content of carotenoids and the vitamin E 6) . It did not go through a few processes, such as refining and bleaching with causes carotenoid, vitamin E and sterol reduction 7) . Red palm oil has a distinctive red colour because it contains high level of carotene and low free fatty acid level 8) . Nagendran et al. reported that this process was patented and used to commercialize various red palm oil fractions and products 9) .
From Table 1, the free fatty acids content of red palm oil, red palm olein and crude red palm oil are 0.1%, 0.1% and 11.4%, respectively 10) , 11) , 12) . The iodine values of red palm oil and red palm olein range from 51 to 56, while the slip melting points of these two oils range from 23-37°C 10) , 11) , 12) . Moisture and impurities of red palm oil and red palm olein are 0.04% and 0.50%, respectively 10) , 11) . The peroxide value of crude red palm olein (7.2 meq O2 per kg) was higher than red palm olein (1.5 meq O2 per kg) 10) , 11) .
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Table 2 shows the fatty acid composition and phytonutrient content of red palm oil. Red palm oil has a balanced percentage of fatty acids, which comprise 50% of saturated fatty acids, 40% monosaturated 10% polyunsaturated fatty acids 13) . This balanced composition of fatty acid allows red palm oil to stay at a semi-solid state and is more resistant to lipid oxidation than other vegetable oils that contain high monounsaturated acids. From 50% of saturated fatty acids, 45% are palmitic acid, and 5% are stearic acid 13) . Other studies reported that approximate 0.2 and 1.0% to 1.5% of lauric acids and myristic acid are also present in red palm oil as saturated acid, respectively 14) , 15) . The only monounsaturated acid within red palm oil is oleic acid, and the two polyunsaturated fatty acids are linoleic acid and linolenic acid 14) , 15) .
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Carotenoids are one of the key compounds highly retained after a special refining process and give red palm oil its orange-red color. Studies have shown that the amount of retained carotenoids after refining ranges from 500 to 786 parts per million depending on the condition of crude (red) palm oil before refining 16) . About 80% of the carotenoids retained were found to be consisted of 0.2% phytoene, 0.6% phytofluene, 41.3% α-carotene, 10.2% cis-α-carotene, 41.0% β-carotene, cis-β-carotene, 0.6% ς-carotene, 0.8% γ-carotene, 0.8% δ-carotene, 0.2% neurosporene, 0.5% α-zeacarotene, 1.3% β-zeacarotene and 1.0% lycopene 17) . However, α-carotene and α-carotene represents 90% of the refined carotenoids in red palm oil 15) . Among the 13 carotenoids retained, only α-carotene, β-carotene and γ-carotene can show provitamin A activity and is has 15, 44 and 300 times more retinol than carrots, leafy vegetables and tomatoes respectively 17) .
Other than carotenoids, tocopherols and tocotrienols also contribute to the oxidative stability of red palm oils. About 85% of tocopherols and tocotrienols are retained after the refining process, ranging from 600 to 1000 parts per million 15) . The tocopherols and tocotrienols present are in the form of 19% α-tocopherol, 29% α-tocotrienol, 41% γ-tocotrienol and 10% δ-tocotrienol with 70% are tocotrienols, and 30% are tocopherols 14) . As an antioxidant, it may prolong food products’ shelf-life, stabilize oils and fats, and lower free radical damage 18) .
Other than carotenoids and vitamin E in red palm oil, minor phytochemicals such as phytosterols, ubiquinones, coenzyme Q10, squalene and polyphenols were detected to be present in small quantity 14) . In red palm oil, the amount of phytosterols ranges from 325 to 365 parts per million, and squalene is 14 to 15 parts per million, coenzyme Q10 is 18 to 25 parts per million ubiquinone is 18-25 parts per million 17) .
The processing of oil palm fruit into edible oils involves many different and complex steps. Besides using traditional ways of processing, there are also application of small, medium and large scall mills 19) . The processing steps of oil palm fruit can be broken into a few steps: bunch reception, fruit sterilization, fruit digestion, pulp extraction, and oil (Fig. 1). Bunch reception involves grading the oil palm fruit and the threshing process, removing fruit from the bunches 19) , 20) . After the fruits have been graded, the sterilization process will take place in the sterilizer. Sterilization is a crucial step that inactivates and destroys the enzymes to prevent free fatty acids (FFA) by using high-temperature steam. This process also softens and loosens the fruit structure for easier fruit digestion and extraction of oil. The mesocarp (flesh) and the kernel (seed) are separated in the digester. The steam-heated vessels with attached rotating shafts, and a few stirring arms that help destroy the exocarp of the fruit and reduce the oil’s viscosity.
Simple processing flowchart of red palm oil and refined, bleached and deodorized palm oil.
Palm oil extraction has two common methods: the “dry” method and the “wet” method. “Dry” method uses mechanical presses such as hydraulic press and screw press to extract the oil from the digested material. The hydraulic presses are usually used in the batch system, while the screw press is used in a continuous system more often 19) . The “wet” method, on the other hand, uses water to draw out the oil from the fruit. The hot water introduced to the fruit will break down gums and resins that cause foaming of the oil during high-temperature frying. The gums and resins will soon remove through the oil clarification process. The mesocarp fiber will retain about 5-6% of oil after the pressing 19) , 21) .
Oil clarification is to separate the impurities from the oil. A mixture of oil, water and solids from the bunch fibers is transferred to the tank, and the separation of the oil is based on the density of the materials. Hot water is added to provide a barrier to the lighter oil droplets and the heavy solids. The oil droplets will stay at the top of the tank, and the solids will sink to the bottom. The crude palm oil (CPO) is decanted into a reception tank and the moisture content reduced to 0.15% to 0.25% to prevent FFA increase through the autocatalytic hydrolysis of the oil. CPO is subjected to centrifugation for purification, followed by drying step. The purified and dried oil is then transferred to the oil storage tank 19) , 21) , 22) .
Red palm oil can be obtained from the mild processing of crude palm oil while the refined, bleached, deodorized (RBD) palm oil is obtained by physical refining or chemical refining of the crude palm oil 9) . There are two stages of processing for the refining of red palm oil from crude palm oil. The first stage involves the CPO’s pre-treatment, which uses phosphoric acid for degumming of the oil and treatment with bleaching clay. The main purpose of the pre-treatment is to remove the impurities in the CPO while retaining the carotenes. The bleaching clay is removed by filtration. The next stage of the process is deacidification and deodorization. The pre-treated oil is passed through the short-path distillation unit at about 150°C to 170°C under vacuum to remove the free fatty acids (FFA) without destroying the carotenes 23) .
For the RBD palm oil, physical refining involves a few degumming steps, bleaching, and deodorization. In the degumming process, the phospholipids are reduced, and gums are removed from the oil 24) . The next step is the bleaching process that uses bleaching clay to remove the color pigments and residual soaps from the oil 25) . Physical refining is usually carried out at high temperatures and pressures in the deodorization step to remove the odor and impurities such as the FFA, volatile oxidation products and phospholipids. However, high temperatures may also destroy the carotenes and tocopherols; hence, a lower deodorization temperature is highly recommended to reduce carotenes’ losses. The oil becomes bland and light yellow 26) , 27) .
On the other hand, chemical refining is similar to physical refining but involves the alkali neutralization process. The alkaline neutralization process removes FFA and phospholipids from the crude palm oil and forms a byproduct named soapstock, a mixture of fatty acids, impurities and phospholipids 28) . Besides, chemical refining is carried out at a low temperature and uses a shorter time than the physical refining process. The losses of tocotrienols and tocopherol usually higher in the physical refining process, and physically refined oil have a lower shelf life compared to chemically refined oil 29) , 30) .
The processing steps refining red palm oil are shorter and consume less time compared to the refining process of RBD palm oil. The mild processing steps of refining red palm oil allow it to retain most of the carotenes, tocotrienols, and the oil color. However, all the refining steps for red palm oil could not remove the volatile compounds hence red palm oil will have a slightly distinctive taste and odour. The consumers may find the taste and smell of the red palm oil unique and different from other vegetable oils 58) . In contrast, RBD palm oil can only retain some carotenes and lost most of the tocotrienols contents during the refining, bleaching and deodorization process 9) , 23) .
Red palm oil is well known as an excellent source of carotenoids for pro-vitamin A (PVA). It is frequently supplemented in diets to help to relieve vitamin A deficiency (VAD) disease. In developing countries, VAD is the common health issues faced by the public. Insufficient dietary intake and poor absorption are factors that will lead to a low level of vitamin A stores in the body 31) . Red palm oil, other than supplying the essential carotenoids, is also an excellent source in providing fats to enhance the bioavailability of dietary PVA carotenoids 32) . Consequently, fat intake coupled with carotenoids is assumed to be essential as it reported to improve the bio-efficacy of dietary PVA carotenoids up to more than 80% that correlate to a retinol equivalent value of 0.4 33) , 34) .
There are few studies have reported the potential of RPO as the precursors of vitamin A and its effect in alleviating VAD is similar compared to vitamin A supplements 35) , 36) , 37) , 38) . The effects of mean serum retinol after consuming red palm oil are shown in Table 3. According to a meta-analysis studied by Dong et al., the results showed that supplementation with a low level of red palm oil (≤ 8 g RPO) to children and adults lead to an increase in serum retinol levels and β-carotene levels, but the serum α-carotene levels have no significant differences upon consumption of RPO 36) .
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Few studies were found incorporating red palm oil into the diet as a source of β-carotene. Research by Zeba et al. found that by adding 15 mL of RPO to school children’s diet three times a week for a year, the serum retinol had increased significantly 32) . Additionally, another study in Burkina Faso by Zagré et al. using childbearing age women and children as subjects for 24 months shows that individuals with a lower initial concentration of serum retinol showed an improvement in serum retinol response after the intake of RPO 38) . These results supported that a small amount of RPO can help combat VAD by using serum retinol as a reference for vitamin A status in the body. The fatty acids profile of RPO is similar to the palm oil, which contains almost 50% saturated fatty acids and the effect of these saturated fatty acids present in palm oil on the heart is controversial. There are arguments on the saturated fatty acid composition of palm oil that will increase blood cholesterol, particularly lower-density lipoprotein (LDL), which will increase the risk of cardiovascular disease. However, the high antioxidant contents in red palm oil, which comprise carotenoids, vitamin E, and other polyphenols, seem beneficial to our heart.
Scholtz et al. conducted a study on the effect of red palm olein on lipids and hemostatic factors in hyper-fibrinogenaemic subjects and compared it with refined palm olein intake 39) . They found that the total cholesterol and LDL cholesterol increased significantly in the subject that consumed palm olein compared to RPO and sunflower oil. This study found that RPO did not show any hypercholesterolaemic properties and RPO intake resulted in decreased tissue plasminogen activator antigen, which has a significant association with cardiovascular heart disease 40) . A high level of tissue plasminogen activator antigen is also found to precede type 2 diabetes development. The risk of diabetes increased linearly with the concentration of tissue plasminogen activator antigen; therefore, the tissue plasminogen activator antigen lowering effect of RPO is beneficial; however, more studies need to be carried out to determine this relationship 41) .
Additionally, the findings of Wilson et al. also support the results that total cholesterol and LDL cholesterol concentrations in a hamster fed with RPO containing diets are significantly lower compare to diets containing coconut oil 42) . Hamsters fed with RPO containing diets also show the highest high-density lipoprotein (HDL) concentrations compared to refined, bleached and deodorized palm oil (RBD-PO) coconut oil. Lower levels of aortic total, free and esterified cholesterol were observed in the hamsters fed with RPO (74%, 50% and 225%, respectively), RBD-PO (57%, 48% and 92%, respectively) and the reconstituted RBD-PO (111%, 94% and 94%, respectively) compare to the coconut oil-fed hamsters. Also, RPO containing diets accumulate lesser aortic cholesterol concentrations compared to hamsters fed coconut oil. RPO is found to be less atherogenic than the RBD-PO and coconut oil.
Two recent studies by Bacova et al. and Katengua-Thamahane et al. are found using spontaneously hypertensive rats (SHR) as subjects to study the effects of using RPO on the myocardial antioxidant enzyme, heart function and nitric oxide synthase 43) , 44) . Both results reported that with 0.2 mL of RPO supplementation per day for continuously 5 weeks had resulted in a significant reduction of elevated blood pressure and total NOS activity. RPO intake have positive effect on systolic and diastolic heart function, while suppress the heart rate elevation and reduces coronary flow, during the early-phase of post-ischemic reperfusion. Antioxidant-rich RPO is beneficial to SHR in these studies; however, the research for humans’ effect is still not clearly stated; therefore, more studies for the cardioprotective and blood pressure-lowering effect of RPO should be considered in the future.
According to Loganathan et al. and Oguntibeju et al., RPO, other than playing an important role as vitamin A precursor and lowering cholesterol, also shows potential activity reducing cancer risk 17) , 45) . The vitamin E tocotrienols and carotenoids of RPO both can act as antioxidants that play a significant role in maintaining the immunity of the human body 46) . Chemotherapy and radiation therapy are some of the treatments available for cancer; however, it will generate reactive oxygen series (ROS) that will induce DNA damage 47) . Antioxidant-rich RPO is said to have a beneficial effect on the consequences of chemotherapy. Loganathan et al. reported in his review paper that by a colonic tumor biomarker (aberrant crypt foci) on the rat subjects with azoxymethane-induced colon cancer, supplementation of RPO for 13 weeks could decrease the number of aberrant crypt foci and activity in the subjects 17) . In contrast, glutathione S-transferase activity was found to increase in the liver. Oguntibeju et al. had also supported RPO’s anticancer properties by reporting that RPO that is rich in tocotrienol can induce apoptosis in highly malignant mouse mammary epithelial cell at treatment doses 45) .
Palm oil is gaining worldwide acceptance due to its specific properties that promote its use in a wide range of end products. Numerous fractions and refined forms that are available on the market that renders these properties even versatile. Currently, Malaysia’s most significant type of product is refined palm olein, which is commonly used in households as cooking oil. Changing lifestyle trends and people generally being more health-conscious led to other areas of application for palm oil and its fractions to contribute to their overall well-being. The Malaysian Palm Oil Board research and development team made an effort to formulating red palm oil related products and introducing them into niche markets. At present, red palm oil is still gaining acceptance due to its red color as the primary hindrance for widespread market adoption 14) . Another reason RPO may not be commonly used is that it is more expensive than other fat sources for its high Pro-Vitamin A content 57) . The market acceptance of RPO is dependent on whether or not consumers view RPO as a value-added commodity. Hence, the authorities play an important role on substantiating health claims and on the flexibility for a wide range of food applications.
Nevertheless, RPO can reduce the prevalence of some diseases due to their desirable nutritional and functional properties. Red palm oil having properties like high smoke point and oxidation resistance had become a satisfactory alternative to refined palm olein to be used as cooking or frying oil, shortenings, and fat substitute and confectionary fats 48) .
Wu et al. fully substituted vegetable oil and animal fat with red palm oil (RPO) in commercial gravy oil sachets 49) . They analyzed these RPO gravy’s physicochemical properties and sensory characteristics of oil flavorings. Compared to other palm oils and commercial oils and fats, the content of RPO carotenoid, β-carotene, and vitamin E in the gravy was found to be in higher content.
Red palm oil has potential as a margarine replacement material due to its compositions of fatty acids similar colors and textures for the end product. A study by Harianti demonstrated the use of red palm oil to produce high antioxidant cake products to discover its potential to be a functional food 50) . Full substitution of RPO suggests the best organoleptic results, the highest β-carotene level with a value of 40.74 ppm/100 g and 470.44 mg/100 g antioxidant activity.
RPO also raises carotenoid, phytosterol, tocopherol, and tocotrienol amounts varying from 3-27 folds. On the other hand, the cholesterol level had reduced significantly by about 10-50 percent, which could be a healthier alternative to the spreads currently available on the market. Besides, the blends’ radical scavenging behavior improved with an improvement in the quantity of PRPOL, suggesting the action of potent antioxidant potential. Another research performed by Kumar et al. found that the inclusion of 20 percent retained palm olein (PRPOL) phytonutrients with butterfat in a chocolate spread has demonstrated optimum market acceptability on sensory attributes 51) .
Red palm oil is also often found in different types of processed foods, commonly like protein bars, baked goods and cereals 48) . In Wan’s study, palm tocotrienol-rich fraction (TRF) was incorporated into the formulation of cereal bars to enrich the vitamin E content 52) . After baking, the granola bar’s enrichment with 0.6% (w/w) liquid palm TRF or 3.57 mg/g vitamin E showed around 10% loss in vitamin E. Nevertheless, the final product enriched vitamin E still had a significant difference in proximate composition and caloric value than other commercial granola bars.
Research by Sidhu et al. discovered that in the processing of snacks, biscuits and bread, the addition of red palm olein increased its contents phytochemicals in these foods 53) . In comparison, the content of β-carotene was extremely low in snacks made with red palm shortening (RPS). This analysis shows that RPOL and RPS may be used to generate antioxidant enhanced functional confectionary foods.
Related research was performed by Isa and found that 5%, 8% and 10% of red palm olein mixed with non-milk fat solids in ice creams had higher scores in terms of color, taste, body texture and meltability 54) . Such aspects added benefit to the food produced from ice cream since the amount of β-carotene and pro-vitamin A allows the food to be more nutritional. Other applications of red palm oil in food products are shown in Table 4.
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Red palm oil has almost equal amounts of saturated and unsaturated fatty acids with palmitic acid as the major saturated fatty acid. This oil meets the quality specifications of refined oils that are extracted via mild processing. Therefore, its carotenoids can be retained, and the oil can be used to relieve vitamin A deficiency disease. Besides, the oil is rich in vitamin E (Tocopherols and tocotrienols) and contains phytonutrients. The data collected indicates the potential efficacy of RPO in promoting various health benefits, including antiatherogenic and anticancer. RPO can be used as a dietary supplement or incorporated in various food products such as salad dressing, chocolate spread and non-dairy products. It can be concluded that red palm oil can replace butterfat or hydrogenated oil, providing nutritional benefits. More attention in future research can be given to study red palm oil’s health effects in large-scale human clinical studies. Research on the bioavailability of nutrients in red palm oil upon conversion to processed food is also warranted.
