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Palm oil carotenoids

Choo Yuen May

Abstract

Crude palm oil is the richest natural plant source of carotenoids in terms of retinol (provitamin A) equivalent. This article reports on

» the carotenoids found in palm oil, its fractions, byproducts, and derivatives from the Elaeis guineensis and E. oleifera palms, including their hybrids and a back-cross, as well as the carotenoids of pressed palm fibres, second-pressed oil, palm leaves, and palm-derived alkyl esters;
» two novel procedures for preparing highly concentrated sources of carotenoids (>80,000 ppm), by recovery by palm alkyl esters, and by retention and concentration in deacidified and deodorized red palm oil;
» the carotenoid content and profiles of the above sources obtained by high-performance liquid chromatography; and
» nutritional effects of palm oil carotenoids and their potential applications for health promotion and disease prevention.

Introduction

Palm oil is derived from the mesocarp of the oil palm fruit. Its physical and chemical properties are quite distinct from those of oil obtained from the kernel inside the nut. The oil palm Elaeis guineensis originated in West Africa. The current planting material in Malaysia is a cross of the dura and pisifera varieties known as tenera, all belonging to the E. guineensis species.

The oil palm is the most prolific oil-producing plant; the national average oil yield in Malaysia is 3.8 tons per hectare per year, with 11% of that quantity being palm kernel oil, a co-product. The production of palm oil in the world for 1991 was 11.41 million tons, of which Malaysia (53.8%) and Indonesia (23.3%) were the two major producers [1]. Both countries are expected to increase production, with Malaysia estimated to reach 8.38 million tons and Indonesia more than 4.48 million tons by 1995 [2]. In 1984 the market share of palm oil in the world's oil supply was 10.1%, and it is expected that by the year 2000 palm oil will be as important as soy-bean oil, contributing 21.1% of the world's total oils and fats [3].

The composition of palm oil

Palm oil consists mainly of glycerides made up of a range of fatty acids. Triglycerides constitute the major component, with small proportions of diglycerides and monoglycerides. Palm oil also contains other minor constituents, such as free fatty acids and non-glyceride components. This composition determines the oil's chemical and physical characteristics.

The fatty acid composition of crude Malaysian palm oil is given in table 1 [4]. About 50% of the fatty acids are saturated, 40% mono-unsaturated, and 10% polyunsaturated. It contains adequate amounts of n-6, 18:2 essential fatty acid. In its content of monounsaturated 18:1 acid, palm oil is similar to olive oil, which is as effective as the more polyunsaturated oils in reducing blood cholesterol and the risk of coronary heart disease [5].

Crude palm oil contains approximately 1% of minor components: carotenoids, vitamin E (tocopherols and tocotrienols), sterols, phospholipids, glycolipids, terpenic and aliphatic hydrocarbons, and other trace impurities [6] (table 2). The most important are carotenoids and vitamin E, both of which possess important physiological properties.

The iodine value is between 50 and 56.

TABLE 1. Fatty acid composition of Malaysian palm oil

Source: Ref.4.

TABLE 2. Minor components of crude palm oil

a. Estimated

Carotenoids

The carotenoids, whose name is derived from the fact that they constitute the major pigment in the carrot root, Daucus carota, are undoubtedly among the most widespread and important pigments in living organisms. They are present in numerous vegetable Oils, including yellow maize (corn) oil, groundnut oil, soy-bean oil, rapeseed oil, linseed oil, olive oil, barley oil, sunflower-seed oil, and cotton-seed oil [16-22]. The concentration of carotenoids in these vegetable oils is generally low, less than 100 ppm.

Of the vegetable oils that are widely consumed, palm oil contains the highest known concentration of agriculturally derived carotenoids [23]. In fact, crude palm oil is the world's richest natural plant source of carotenes in terms of retinol (provitamin A) equivalent. It contains about 15 to 300 times as many retinol equivalents as carrots, leafy green vegetables, and tomatoes, which are considered to have significant quantities of provitamin A activity [24] (table 3). It is these carotenoids that impart an orangey-red colour to crude palm oil.

Currently, interest is focusing on the nutritional aspects of carotenoids. Since they are likely to grow in importance and value, their recovery from palm oil and its by-products is important.

TABLE 3. Retinol equivalents (RE) of red palm oil compared with other foods

Source: Ref. 24.

a. Per 100 B edible portion (µg).
b. Times greater or less than red palm oil (RPO).

Sources of palm carotenoids

Palm species and hybrids

Although palm forests are found in West Africa, oil palms are actually cultivated in East Africa, South America, Malaysia, and Java [25-27]. Palm oil from the Far East and from Zaire contains 500-800 ppm of carotenes, whereas that from Côte d'Ivoire and especially Benin contains 1,000-1,600 ppm, but the oil yield is less [25, 26, 28]. Oil from the tenera variety that is widely planted in Malaysia has a carotenoid content of about 500-700 ppm [6].

Other oil palm species, such as Elaeis oleifera (or E. melanococca), a South American palm, have been found to contain a higher concentration of carotenes (4,600 ppm) [29]. Hybrids of E oleifera x E. guineensis also produce oils with high concentrations of carotenoids. The following values have been found for the total carotenoid content of oil from different species, varieties, and hybrids: E. guineensis var. pisifera (P), 428 ppm; var. dura (D), 997 ppm; E. oleifera (O), 4,592 ppm; the hybrid O x P. 1,430 ppm; O x D, 2,324 ppm; and the back-cross OD x P, 896 ppm (table 4). E. oleifera oil has the highest carotenoid content and E. guineensis oil the lowest, with the hybrids and the back-cross having intermediate concentrations. It can be seen from the table that the species and hybrids are comparable with regard to the major components, a- and ß-carotene.

TABLE 4. Carotenoid composition (percentages) of palm oil from different species and sources

tr = trace.

a. With two cis isomers.
b. With one as isomer.
c. With three cis isomers.

Extraction methods and by-products

Carotenoids from commercial crude palm oil are concentrated during extraction and fractionation. A new system of extraction based on a double-pressing technique has been implemented recently by several mills in Malaysia [30]. The oil from the second pressing has a higher concentration of carotenoids. This can be attributed to the fact that the first pressing is carried out at lower pressure to avoid cracking the nuts, and relatively more oil is extracted compared with carotenoids. After removal of the nuts, the fibre is subjected to higher pressure, and more carotenoids are extracted from the mesocarp together with some residual oil left by the first pressing. The advantages of double pressing over the conventional single pressing are a lower loss of oil in fibre, a higher kernel extraction rate, less wear on the screw worm and cages, and reduction of contamination of crude palm oil by kernel oil.

Carotenoids may also be concentrated in an industrial process called fractionation [31, 32]. Palm oil is a semi-solid fat at ordinary room temperature due to the presence of solid, fully saturated triglycerides and the high-melting-point mono-oleoglycerides and monolinologlycerides dispersed throughout the liquid dioleoglycerides and other more unsaturated glycerides. Fractionation extends the uses of palm oil; the products obtained are liquid oil (olein, 70%-80%) and solid fat (stearin, 20%-30%). The liquid oil is designated for cooking, and the solid fraction can be used as a component of harder frying fats, for the production of margarine, vanaspati, and as a cocoa butter substitute. The carotenoid content in the palm olein (lower-melting-point) fraction is enriched 10%-20%, as shown in table 5.

TABLE 5. Carotenoid content of various palm oil fractions

Total carotenoids estimated at 446 nm.

A carotenoid-rich oil can be obtained from the pressed fibre of the oil palm fruit [33, 34] which normally is burnt as fuel in palm oil mills. The pressed fibre contains about 5%-6% of residual oil, and the extracted oil contains 4,000-6,000 ppm of carotenes, six times the concentration in crude palm oil. The carotenoid content of the residual oil in the pressed fibre from hybrid oil palms is even higher, 6,0007,000 ppm [33]

Oil palm leaves also contain carotenoids, at 1,900 ppm [35].

Carotenoid profiles

The total carotenoids in palm oil are usually determined by ultraviolet-visible spectroscopy at 446 nanometres as ppm of ß-carotene. However, because of their complex composition, various analytical methods have been employed to determine their profile. Earlier studies used column chromatography with different absorbents [36-38]. More recently, reverse-phase high-performance liquid chromatography (HPLC) has proved to have several advantages for separating the carotenoids in the oil [29, 39, 40]. By this method, the major carotenoids are a - and ß-carotenes, which constitute about 80%-90% of the total carotenoid content (table 4), The others are g-carotene, phytofluene, phytoene, lycopene, neurosporene, z-carotene, a-zeacarotene, ß-zeacarotene, d-carotene, and some xanthophylls such as zeaxanthin, a-carotene-5,8-epoxide, and ß-carotene-5,6-epoxide [29, 36, 40] (fig. 1).

FIG. 1. HPLC chromatograms of palm oil carotenoids run at different wavelengths. (1) Presumed xanthophylls; (2) lycopene-bans and three cis isomers (a denotes cis isomers); (3) and cis isomers; a -zeacarotene; (4) ß-zeacarotene; (5) neurosporene-trans (6) -carotene; (10) a d -carotene; (7) g -carotene-bans and as isomers; (8) z -carotene-bans and two cis isomers; (9) cis-a -carotene; (11) ß-carotene; (12) phytofluene; (13) cis-ß-carotene; (14) phytoene.

The carotenoid profile for the oil extracted from fibre has a slightly different chemical composition. The major carotenoids are still a- and b-carotene, but they constitute only about 50% of the total. Phytoene, lycopene, g-carotene, and d-carotene are present at higher concentrations [33]. The carotenoid profile of the second-pressed oil is similar to that of the fibre oil.

New methods for the recovery and concentration of carotenoids

In addition to the work on carotenoid-rich palm oils described above, studies are being carried out to obtain an oil with high concentrations of palm-based carotenoids for nutrition and health applications. Numerous extraction methods have been developed, including saponification [41, 42], urea processing [43], adsorption [44-47], selective solvent extraction [48], molecular distillation [49], and transesterification followed by distillation of esters [5053]. Most of the methods are difficult to perform, inefficient, or costly.

Recently, volatile methyl esters have been produced on a large scale from palm oil for oleochemical or diesel substitution [54-59]. This mild reaction converts the palm oil triglycerides to esters, leaving the valuable minor components unchanged [57, 60] and allowing for recovery of the carotenoids. The carotenoids have been concentrated or recovered from the volatile esters by various methods, such as adsorption [60], solvent-solvent extraction [52], and distillation [61].

One method involves the selective adsorption of carotenoids obtained from reverse-phase adsorption material [60], with the esters of higher polarity being first eluted out from the column. A recovery rate of greater than 90% can be achieved, with a carotenoid concentration of 8,000-9,000 ppm (table 6), both of which are higher than those obtained by other methods. The column can be regenerated and reused more than fifty times without any loss of activity.

TABLE 6. Results of various methods of carotenoid recovery and concentration

Source: Ref. 33.

a. Total carotenoids estimated at 446 nm.

Another method involves distillation of the volatile alkyl ester using normal vacuum or molecular distillation [61, 62]. Residual concentrates of 2.0% carotenoids can be achieved by normal vacuum distillation, with recovery of about 46%. These residual carotenoids can be further concentrated to 8.4% by normal-phase column chromatography, and at the same time other separated minor components are being concentrated [61]. Total tocopherol and tocotrienol content is increased to 37% and sterols to 32%, with recovery of 65% and 82% respectively, based on the crude methyl ester. An oil with a final carotenoid concentration of 80,000 ppm has been achieved through molecular distillation. A carotene with a concentration of 72% has been obtained through both molecular distillation and column adsorption [63].

In the current technology of physical refining, the carotenoids in crude palm oil undergo thermal decomposition during deodorization-deacidification (240°C-270°C). As a result, the processed product, normally known as refined, bleached, and deodorized (RBD) palm oil, contains no carotenoids at all. In view of this, a process to prepare carotene-enriched palm oil has been developed [64] that involves degumming of the oil with phosphoric acid, followed by treatment with bleaching earth. The treated oil is then subjected to deodorization and deacidification at a mild reaction temperature to remove odoriferous materials as well as free fatty acids. More than 80% of the carotenoids originally present in the crude palm oil are retained. Analysis by HPLC shows that the profile of the carotenoids is similar to that of the starting material, again indicating that carotenoids are not destroyed during the process.

The quality of this red palm oil is good. According to a sensory panel, it is suitable for food preparation. This process has been demonstrated successfully on a pilot-plant scale, with 80% of the carotenoids and vitamin E originally present in the crude palm oil being retained. It should be noted that in this process the triglycerides remain intact, unlike that described above in which all the triglycerides are converted to alkyl esters. However, the former process can yield a higher concentration of carotenoids after the alkyl esters are removed.

Palm oil carotenoids in nutrition

Carotenes as provitamin A

Carotenes, in particular ß-carotene, have long been known for their provitamin A activity, as they can be transformed into vitamin A in vivo. In addition, a-carotene, g -carotene, and ß-zeacarotene, which are present in crude palm oil, have similar activity. The vitamin A equivalents of a-, ß-, and g -carotenes and ß-zeacarotene are 0.9, 1.67, 0.75, and 0.42 respectively.

Crude palm oil has long been used by Africans as a source of vitamin A. Elsewhere in the world, however, it is not considered acceptable as a result of its content of free fatty acids. On the other hand, RBD palm oil, which is marketed for consumption worldwide, contains practically no carotenoids, as they have been destroyed during refining. As a result, the process mentioned above [64] was developed to produce a deodorized and deacidified red palm oil as a more widely acceptable source of vitamin A.

Nutritional and toxicological studies

Carotenoids as antioxidants

ß-carotene has long been known to be an efficient quencher of singlet oxygen and, as such, is an effective antioxidant. a -carotene and lycopene are also effective singlet oxygen quenchers, as demonstrated in a recent study on singlet oxidation of lipids, cholesterol, and low-density lipoprotein (author's unpublished data, 1992). In the presence of palm oil carotenoids, no oxidation products increased. This area of work is important, as research findings show that three micronutrients, ß-carotene and vitamins E and C, have protective properties against free radical damage that is believed to be responsible for numerous degenerative diseases such as atherosclerosis, arthritis, and carcinogenesis. In fact, a - and ß-carotenes, lycopene, and phytoene have anticancer properties, with a-carotene ten times as potent as ß-carotene as an anticancer agent [65]. ß-carotene has also been reported to have anti-atherosclerotic effects [66].

A study was performed to determine the tissue distribution of carotenoids in palm oil and to correlate their accumulation with protection against oxidative stress in rats [67]. After two weeks of feeding, ß-carotene in the liver increased from 7.3 to 30 ng per gram of wet tissue. After ten weeks, a -carotene and lycopene were 74 and 49 ng per gram of wet tissue respectively. The ß-carotene content in heart and hind-limb skeletal muscles increased after ten weeks to 17 and 6 ng per gram of wet tissue respectively. No carotenoids were detected in the brain, adipose tissue, and skin during the period of feeding. After in vitro induction of lipid peroxidation in liver homogenates by an azo-initiator of peroxyl radicals, an inverse correlation between tissue carotenoid level and accumulation of lipid peroxidation products was observed: a -carotene>lycopene > ß-carotene.

Toxicology study

The carotenoid concentrate prepared by molecular distillation [62] was subjected to a toxicological study (H. T. Khor, D. Tan, Y. M. Choo, 1992). Four groups of Sprague-Dawley rats (12 rats per group) were fed a semi-purified diet supplement with 0.2% palm oil-based carotenoid concentrate (20,000 ppm), methyl ester, ethyl ester, and a control diet for 16 weeks. Histopathological examinations showed the major organs such as the heart, lungs, adrenals, kidneys, liver, and spleen to be normal for all groups. No extensive or significant amount of fat was deposited in the heart and coronary vessels, and the aorta was normal in all groups. It was concluded that the carotenoid concentrate and other diets do not have toxicological effects on the major organs of male rats.

The carotenoid concentrate [62] has also undergone studies of oral cancer in animals (K. H. Ng, personal communication, 1992).

The Food and Agriculture Organization has recently accepted and included palm oil carotenoids as a permissible food colourant.

Forms for ingestion

Carotenoid concentrate obtained by molecular distillation has been prepared in three different physical

forms for potential nutrition and health applications [68]: capsules (both soft and hard), powder, and emulsion. The powder can be made into tablets or placed in hard capsules. Preliminary results of stability tests show that the powdered carotenoids were not as stable as the concentrate in soft capsules during storage for one year at ambient temperature (28°C-32°C). This could be due to greater exposure of the powder to light and air, leading to increased oxidation or degradation of the carotenoids. However, the carotenoid content of the powder declined only slightly (<4%) when it was kept in a freezer at -50°C for one year.

Conclusion

Crude palm oil is the richest natural plant source of carotenoids in terms of retinol equivalent. Analysis by HPLC shows that these carotenoids are rich in a -and ß-carotenes. Besides their provitamin A activity, carotenoids are good singlet oxygen quenchers, which leads to their role in minimizing or preventing photo-oxidation and thus diseases in which free radicals are implicated.

Two novel methods are effective in producing carotenoid-rich products such as carotenoid concentrate with concentrations greater than 80,000 ppm, and deacidified and deodorized red palm oil. This concentrate has been tested for toxicological effects and found to be safe.

In view of the importance of carotenoids for health and disease prevention, methods have been developed to present it in different forms for pharmaceutical applications.

Oil palm (Elaeis guineensis and E. oleifera) products are good and viable sources of carotenoids.

Acknowledgements

The author thanks the Director-General of PORIM for permission to publish this paper. Thanks are also extended to the Malaysian Palm Oil Promotion Council for its invitation to contribute to the present collection of studies.

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