Morphological and quality characteristics of genus of Canarium L.: A review

The genus Canarium L. comprises 75 species of aromatic trees which are found in the rainforests of tropical Asia, Africa and the Pacific. A literature review was conducted through published works to gather information on nutritional and ethnic values, colour, shape, fatty acid and also antioxidant activities of Canarium sp. The phytoconstituents which are present in most Canarium L. genus especially its fruit are mainly phenolic and fatty acid which are responsible for the actions. This novel compound serves as a contributor to the antioxidant capacity of Canarium fruit and thus, the genus can be recommended as a major source of natural antioxidants. However, this genus still can be categorised as underutilised fruit and little study is paid on its collection and comparison amongst species of Canarium L. species. Thus, the present review summarizes some such attributes of the Canarium L. genus.

The fruits are usually plum-shaped, drupe-like, almost always blue-black when ripe (white in South East Asian Canarium album and red in Canarium euryphyllum, rough, especially near base and apex, or glabrous; pericarp fleshy, sometimes fibrous pyrene stony, 3-celled, 1 or 2 cells sometimes slightly to almost entirely reduced (in the latter case endocarp quite rarely absent) (Figure 1). Cotyledons are palmatifid to 3, contortuplicate or folded [5].

Distribution
The Canarium L. genus (Burseraceae) is a very large genus of trees occurring in the tropical areas, one which is India [9]. It comprises approximately 100 species, which are distributed from India and Bhutan to Australia and New Guinea, with 75 species primarily distributed in central Asia and the Pacific, [5] but are mostly found in mainland Asia [1]. Roughly 55 species from the tropics of southeast Asia have a significant role to play in the rich use of their oils. The family is distributed across a range of tropical areas, free and low-level, including rainforests, deserts, dry deciduous forest kernels, fragrant resins and wood [10].
Canarium L. species are typically adapted to high rainfall areas, with well-distributed annual rainfall. Optimum rainfall for good growth and fruit production is around 2,000-3,000 mm [11], but  [12]. Canarium L. species naturally well grow on a broad variety of soil types [11,13], ideally moist, rich, deep, crumbly, organically sandy loam land, with a pH of 4.5-6.5, but will tolerate alkalinity up to pH 7.4. Canarium L., though, can be contained in poorly drained woodland conditions [12].
In the tropical rainforest of Sarawak, Malaysia Sibu Olive, Sarawak olive, 'dabai'

Nutritional values
Different fruits have different valuable compounds. However, besides due to lack of marketing, the public considers Canarium L. are inferior nutritionally. Kernels of C. indicum are very healthy [19] and are highly valued for their role in traditional food and health [5], usually are consumed fresh, dried or roast. Its nutritional value is high in oil, protein, vitamin and mineral content. A new flavone glycoside and six recognized cytoprotective compounds were extracted from the stem bark of C. bengalense. This may be due to traditional tumor and liver damage use of this plant [20]. C. odontophyllum is highly nutritious as it is rich in 339 kcal, 26.2 g fat, 3.8 g protein (100 g of dabai fruit), phosphorus fiber and minerals, potassium, calcium, magnesium and iron [21][9].

Ethnic values
There are many traditional ways of utilizing this genus being practiced by locals, ultimately contributing to a better quality of life. Normally, the kernel, the most useful component is processed into different food products. Kernel oil of C. indicum contains about 50% saturated fat (34% palmitic and 13% stearic), 38% monosaturated (oleic) and 14% polyunsaturated (linoleic). Therefore, this species could potentially be sold as cooking oil or blended with other oils. In Chinese folk medicine, C. album L. has long been applied in herbal treatment as a cure for dysentery, cough-hematemesis, snake venom, enteritis, and wellfish and alcohol toxicosis [22]. While in China, it is used to treat infections, inflammation, poisoning and detoxification [23]. C. bengalense extracts from its leaf and root showed anti-inflammatory, antiseptic and anti-asthmatic activity [20]. The species was reportedly useful in skin rashes and snake bites [24]. It is also investigated as Bangladesh's medicinal plants for influential biological activities [25] [26].
The pulp of C. ovatum, C. schweinfurthii and C. odontophyllum are commonly eaten raw after blanch. Another way is by dipping it with sugar, fish sauce, salt, pepper or other condiments. C. ovatum is also beneficial in the processing of flour, butter, baking goods, pasta and other items. The souvenir items usually are handcrafted using its shell whereas the nut is commonly used in pastries, candies and butter [27]. The pigment extracts of C. ovatum exocarps as a functional food colorant,  30 to 50% of the oil used industrially for the production of shampoos and waxes and pharmaceutically for the treatment of wounds and microbial infections [28]. Sometimes the fruit is used by traditional practitioners as a remedy for diabetes mellitus in southern Senegal [29] while in Congo and the Central African Republic, the plant is used in fever as stimulant, emollient, postpartum, constipation, malaria, diarrhea, sexual infections and rheumatism [30]. Meanwhile, high energy and fat content in dabai or C. odontophyllum potentially lead to local development of high-value products such as cold-storage pulp and fruit, mayonnaise, snacks, sauce and paste.

Quality Characteristics
This research provides a body of knowledge on the determination of different varieties or genotypes of Canarium sp. found that including C. album L. [31], C. ovatum Engl. [32], C. schweinfurthii Engl. [33][34] [35] C. odontophyllum Miq. [36,37] as presented in Table 3. Fruit skin color is an important indicator of external quality and maturity in common fruits as fruit appearance greatly influences consumer choice. Colour is an essential attribute to be considered in choosing certain food products, and can greatly affect customer acceptability. Table 3 summarizes the list of five Canarium L. with their colour attributes and variety name. C. album has a similar appearance to Mediterranean olives and turns yellowish-green [31]. C. ovatum or Pili pigment appears as red due to the presence of anthocyanin and its colour stability is maintained when at refrigerated temperature. In a study by Aril-dela et al., the Pili pigment extract imparted a uniform purplish color to the developed yogurt similar to a commercial blueberry yogurt. The successful application as the natural colorant of the extracted pigment from Pili exocarp produced a uniform purplish-pink color in yogurt [17]. To the naked eye, Pili nut kernel is light green-yellow in color. In a study conducted by Millena & Sagum, pulp colours are different amongst variety of pili nuts which lighter-yellowish, green-yellow, and darker -reddish or purplish pigment [32]. Different pH conditions can affect the colour changes. When the pulp becomes more acidic, it changes from light purple to bright red (intense red color), and from light purple to greenish when subjected to a more alkaline condition. C. schweinfurthii fruit appears green when immature, turning black when ripe. Meanwhile the immature C. odontophyllum fruit is white in color changes blue-black or dark purple when ripe.

Fruit shape and texture
Generally, the fruits are ovoid to elliptical. C. album L. possess elliptical, oblong, oval and obovate shape. The fruit shape index ratio ranged from 1.23 to 2.09 (Table 4), which was similar to previously reported results for C. odontophyllum fruit [38,39] due to its high-water content, weight, and large fruit size. As C. indicum have ovoid drupe, C. schweinfurthii Engl. appears spherical and oblong, hence they can rather roll than slide. The Chinese olive fruit, C. album L. has strong sensory characteristics. A first bite, it is bitter and astringent and then it becomes fragrant, sour, and sweet after being chewed. Most of the time C. album L. is consumed fresh and sometimes it is also processed into beverages, candy, and preserves [40] [41]. Conversely, C. odontophyllum has a fine, creamy texture.

Oil content and fatty acid
Fats and oils are composed of molecules known as triglycerides, which are esters composed of three fatty acid units linked to glycerol. Purple or half-purple C. indicum fruit produced kernels with an oil content of approximately 76%. Oil content for purple and half-purple kernels was not significantly different but both of them are significantly higher oil content than green kernels which only 72.8 % ( Table 5) [31].
Hosseini Bai et al. tested the roasting method, resulting in no changes in fatty acid composition or excessive kernel color development [42]. Thus, both roasted and raw kernels have beneficial health effects. Comparing amongst the fruit component and variety of C. odontophyllum respectively, the dabai kernel contains the highest percentage of protein and fat while the highest protein content was found in Kanowit purple dabai fruit as it is a good source of unsaturated fatty acids [43]. The high nutritional value combined with high fatty acid content suggests that dabai fruit is highly suitable for commercial use, especially as a source of healthy oil and functional foods [37]. Overall, the dominant fatty acid among the Canarium species are oleic and palmitic acids but for C. album, linoleic acids are the major fatty acids. Dabai pulp is rich with polyunsaturated fatty acids characterized by both linoleic and linolenic fatty acids [37]. In comparing amongst the variety of dabai, Bulat genotype was the most potential with an extremely high polyunsaturated fatty acid ratio. Meanwhile, pulp and kernel of C. ovatum may be considered as superior sources of mono-unsaturated fatty acids in Canarium L. species [32].

Antioxidant activities
Activities with antioxidants have been reported in C. ovatum, C. album, C. odontophyllum, and C. schweinfurthii. Pigment extract by spectrophotometric analysis C. ovatum show that high phenolic and flavonoid content were found, specifically anthocyanins, which also showed high antioxidant activity determined by DPPH and FRAP assays. The thick, semi-solid dark purple paste extracted from the exocarp has been discovered to have an antioxidant action attributable to phytonutrients, particularly anthocyanin [17] and the study increases the pulp's nutritional content, an important source of dietary fiber and essential fatty acids. C. album is considered a good source of polyphenols and major fruit antioxidant due to its phenolic content. This is due to the fruit's genetic background rather than climatic conditions and agronomic practice. Phenolic compounds are essential for sensory qualities, lead to fruit astringency [44] and are potentially vital for body health. The differences in the total phenolic content between fruits could be partly due to environmental factors during fruit growth and fruit development. Extraction of tannin from the leaves, twigs and stem barks of C. album L. showed powerful antioxidant activity was shown in the radical DPPH scavenging activity and ferric reduction of dried tannin [45]. Novel compounds from this species include brevifolin, hyperin and ellagic acid, showing free radical activity in the DPPH assay [46].
C. odontophyllum skin is a major antioxidant source because of its high phenolic compounds as well [47] while its kernels have the lowest phenolic content. Purple dabai fruits from Kapit has been found to contain the highest total phenolic, flavonoid and anthocyanin content and to show natural antioxidant potential compared to red dabai fruit with significantly lower total phenolic, flavonoid and anthocyanin content [36]. The distinction can be closely related to the geographical location which is less exposed to pollutants, thus having a positive influence on the phytochemical properties of dabai fruits. Total Phenolic Content and Total Flavanoid Content of C. album fruits have been reported to be higher than C. odontophyllum [48].
The phytochemical analysis on C. Schweinfurthii only shown carbohydrates, flavonoids and steroids in ethanol extract. In a separate study, Vitamin C and E exhibited remarkable but varying antioxidant activities in different extracts and the highest levels were obtained in methanol extract (Table 6) [49]. This species may act as an alternative medicine to cancer sufferers, diabetes, hypertension and other cardiovascular diseases. The essential oil of C. schweinfurthii Engl. was analised for the antioxidant activity with the DPPH assay and by ß-carotene bleaching, which resulted in significantly antioxidant activity at 150 μg / ml activity of both tests [50].

Conclusions
This review highlights the fact that only about 10% of the total Canarium L. species have been studied for botanical description, morphology and quality characteristics. Indeed, as compared to many other genera in this family, Canarium L. is still very much understudied. The collection diversity of certain quality attributes such as fruit colour, shape, texture, oil and fatty acid content as well as some phytoconstituents such as phenolic and flavonoid related to antioxidant activities reviewed in this work demonstrates that there is much to be discovered in this family. The comparison amongst species of Canarium L. especially its ethical values and nutritional values reveal the importance of these underutilized fruits and further investigation should be performed on food product value-added and comparison on isolated bioactive compounds amongst Canarium L. species to develop a certified novel herbal product that would benefit the local people.