Evaluation of antioxidant capacity of Aidia borneensis leaf infusion, an endemic plant in Brunei Darussalam

We investigated the total antioxidant capacity of Aidia borneensis leaf infusion, a Bornean endemic plant, which is traditionally consumed as a home-remedy beverage in Brunei Darussalam. The antioxidant capacity of the infusion of A. borneensis leaves was evaluated by 2,2-diphenyl-1-picryhydrazyl (DPPH) radical-scavenging ability. We found that the infusion shows a relatively high antioxidant capacity, and it was attributed to its high phenolic, flavonoid, and flavanol contents which were evaluated by Folin–Ciocalteu reagent, colorimetric assay, and aluminum chloride colorimetric method, respectively. By comparing its total antioxidant capacity, we estimated that the infusion of A. borneensis leaves is in the middle rank among twelve different commercially available Camellia sinensis teas. Our findings would have significant implications on A. borneensis products from Brunei Darussalam and on the feasibility of establishing this new beverage among the commercially available conventional C. sinensis and herbal teas.


Introduction
Antioxidants are compounds that inhibit the formations of free radicals and lipid peroxidations, which are responsible for many diseases and aging process in the human body (Ostrowska et al., 2001;Karimi et al., 2011;Gonbad et al., 2015). Among the antioxidants, polyphenol derivatives such as phenolic, flavonoid, and flavanol contents naturally found in plants are the most beneficial (Salah et al., 1995;Zandi and Gordon, 1995;Kim et al., 2003;Karimi et al., 2011). In addition to those polyphenols, the existences of phytochemical constituents, such as ascorbic acid, tocopherols, and carotenoids in consumable plants enhance their total antioxidant capacity (Chatterjee et al., 2013). Therefore, entire leaves either raw, cooked or processed into beverages are commonly consumed. Several research on herbal plants have suggested that their vitamins A, C, and E, carotenoids, polyphenolics and flavonoid contents also have antioxidant capacities (Karimi et al., 2011). Thus, they have medicinal values in the treatment of many human diseases, including cancer, cardiovascular diseases, and inflammatory diseases (Krishnaiah et al., 2011). For instance, the herbal tea infusions (also known as tisanes) made from fresh or dried flowers, leaves, seeds or roots are also widely consumed worldwide. These herbal tea infusions are believed to have medicinal effects, especially for their stimulant, relaxant, or sedative properties (Kara, 2009).
Driven by such high health benefits of natural antioxidants, exploration on new potential plants possessing high antioxidant capacity is of interest (Ariffin et al., 2011). Through such exploration, one could expect that there will be more varieties of healthy and good quality beverages in addition to the existing C. sinensis teas. In this regard, we have focused our research efforts on plants locally grown in Brunei Darussalam, where endemic plant species are remarkably plentiful. Those plants are uncultivated, but many of them have been utilized in traditional medicines to treat many diseases. Several of those important medicinal plants have been reported by Goh et al. (2017), but extensive researches on those plants are currently pursued. One of those plants is Aidia borneensis, a Bornean endemic plant and locally known as Sambah bagangan, belonging to Rubiaceae family (Goh et al., 2017). In general, this family includes perennial evergreen plants, which require humid and warm environmental geographic regions. A. borneensis grows up to about 2 m high, as depicted in Figure 1A. This plant thrives well on hilly and drained soils under the shadow of big trees, and it has the unique pinkish-red color of young leaves and green color of matured leaves, as shown in Figure 1B-C.
Traditionally, infusion of A. borneensis leaves are treated as beverages, where a decoction is prepared from the crude or dried leaves. It is consumed as a hot drink and it is believed to be able to relieve body aches, tiredness, and gastric pains (Goh et al., 2017). Intensive researches characterizing and quantifying the beneficial constituents as well as principle substances in this particular plant leaves are indispensable. In the present study, we have evaluated the antioxidant capacity of infusion A. borneensis leaves ( Figure 1D).
In order to assess relative potential of infusion of A. borneensis leaves, we have also compared its total antioxidant capacity to those found in twelve different commercially available C. sinensis black, Pu Er, and herbal teas. We should consider that this is a comparison without assessing numerous factors that may affect chemical constituents in the commercial teas according to cultivation type, growth conditions, horticultural practices, and different treatments by the tea manufacturer. The relationship between the total antioxidant capacity and total phenolic, flavonoid, flavanol, catechin, and caffeine contents in the infusion of A. borneensis leaves were also investigated. This is the first report on the antioxidant capacity and chemical constituents in A. borneensis leaf infusion. Our findings would have significant implications on A. borneensis tea and on its feasibility among the conventional commercially available C. sinensis and other herbal teas.

Chemicals and equipment
Dried leaves or tea of A. borneensis was obtained from the local factory (3MPK Herbal Tea, Kampong Kiudang, Brunei Darussalam). All chemicals in this work were obtained from different sources; namely 2,2diphenyl-1-picrylhydrazyl radical (DPPH), gallic acid monohydrate, quercetin, and catechin hydrate were purchased from Sigma-Aldrich (St. Louis, MO, USA), Folin-Ciocalteu reagent, sodium carbonate, sodium nitrite, aluminium chloride, sodium acetate, and methanol were purchased from Merck (Darmstadt-Germany), and caffeine were obtained from Fluka (Buchs, Switzerland). All these chemicals were mainly in analytical grade, except for catechin hydrate (in HPLC grade), and they were used as received without any further purifications. Distilled water was used throughout the sample preparations and measurements. All absorption measurements were performed on a singlebeam UV-visible spectrophotometer (Optizen model 1412V, Korea). In all measurements, the sample was contained in a 2 mL disposable cuvette with 1.2 cm optical path, and the absorbance was recorded at a specific wavelength. Infusion of A. borneensis leaves was prepared using an aqueous extraction according to a standard procedure, similarly to typical household tea preparation. Briefly, 2 g of granular dried leaves of A. borneensis ( Figure 1D) was soaked in 200 mL distilled water while stirring thoroughly with a glass rod, allowing their constituents to infuse into the solution. The infusion was then passed through a filter paper (Whatman Grade 1) with the pore size of 11 m, and the filtrate was kept at room temperature to cool down to 20-25°C. For chemical analyses, depending on each specific assay the infusion was diluted with distilled water to achieve concentration of the substances roughly within 10-4000 ppm.

The effects of brewing temperature and brewing time
To evaluate the effect of brewing temperature and time on the antioxidant capacity, we have prepared an infusion by soaking 2 g of dried leaves of A. borneensis directly into 200 mL distilled water. For the effect of brewing temperature, the distilled water was set to be at 10, 20, 40, 60, 80, and 100°C for a constant brewing time at 3 min. On the other hand, the effect of brewing time was inspected from the samples prepared in the distilled water at 80°C with various brewing time in the range of 1-120 min.

Determination of total antioxidant capacity by DPPH assay
The total antioxidant activity of the infusion of A. borneensis leaves was determined using 2,2-diphenyl-1picrylhydrazyl (DPPH) free radical scavenging assay according to the modified procedures reported by Delgado-Andrade et al. (2005). Here, a 200 µL of the infusion with various concentrations within 10-1000 ppm was added into freshly prepared 1 mL DPPH (50 ppm) in methanol. The mixture was allowed to stand in the dark at room temperature for 30 min. The monochromatic absorption of the mixture then was recorded at 517 nm against methanol, and a DPPH solution without infusion was used as a blank or control solution. The DPPH free radicals in the solution are scavenged by an antioxidant or a free radical species, resulting in a decrease in the intensity of its absorption band. Consequently, lower absorption intensity of the mixture indicated higher free radical scavenging activity (Yen et al., 1997), and DPPH free radical scavenging activities of the infusions were calculated according to where is the absorbance of the control solution and is the absorbance of the infusion. We have plotted a curve of % inhibition of DPPH activity against concentration of the infusions (data not shown), and from the plot we deduced IC 50 value which was attributed to the concentration of the infusions required for 50% inhibition. Radical scavenging activities of wellknown antioxidants, namely gallic acid monohydrate and quercetin, have also been measured for comparison or calibration. Thus, the total antioxidant activity of the infusions associated with that of gallic acid calibration curve is considered as gallic acid equivalents (GAE) and it is expressed as in ppm standard DPPH equivalent.

Determination of total phenolic content
Total phenolic content (TPC) in the infusions was determined according to Folin-Ciocalteu method based on the procedure used by Chan et al. (2007). For this TPC determination, 300 L of diluted infusions were introduced into sample vial followed by 1.2 mL sodium carbonate (7.5% w/v) and 1.5 mL Folin-Ciocalteu reagent (diluted by 10 fold) in distilled water. The color of the mixture turned from yellow to blue, and this mixture was allowed to stand for 30 min at room temperature. Absorption of the mixture then was recorded at a single wavelength at 765 nm. The calibration curve of gallic acid was plotted in the range of 25-200 ppm. The TPC of the infusion was calculated from linear regression of the calibration curve. Thus, TPC was expressed as gallic acid equivalents (GAE) in mg/100 g dry sample.

Determination of total flavonoid content
Total flavonoid content (TFC) in the infusion was measured using a slightly modified colorimetric assay according to the procedure reported by Damiani et al. (2014). Briefly, 100 μL of the infusions was dissolved in 1.35 mL distilled water in a sample vial. Into the solution, 50 μL sodium nitrite (5% w/v) were added, followed by 50 μL aluminium chloride (10% w/v). The mixture was mixed well using vortex (Model V-1 plus, Biosan, Latvia) and it was allowed to stand at room temperature in the dark for 10 min. Similarly, we have appropriately prepared standard ethanolic solutions of catechin hydrate in the same way. Absorption of the mixture and the standard solutions was recorded at a single wavelength at 415 nm. The TFC of the infusion was determined from linear regression of a calibration curve, which was obtained from catechin hydrate with different concentrations in the range of 25-1000 ppm. Therefore, the TFC of the infusion was expressed as catechin equivalent in mg/100 g dry sample.

Determination of total flavanol content
Total flavanol (TF) content in the infusions was determined using the aluminium chloride colorimetric method according to the procedure reported by Iqbal et al. (2015) with slight modifications. Briefly, the infusion (1.5 mL) was put in a test tube, followed by addition of 1 mL aluminium chloride (2% w/v) in distilled water. Then, 3 mL sodium acetate (5% w/v) was added, and the mixture was mixed well using vortex. The mixture then was centrifuged at 3000 rpm for 20 min. The absorbance of the supernatant was measured at 440 nm. In this measurement, ethanolic quercetin solutions which were prepared similarly to the samples of the infusions were used as a standard, and the calibration curve was obtained from the absorbance of the standard solutions with different concentrations in the range of 1-100 ppm. The TF of the infusion was determined from linear regression of the calibration curve, and it is expressed as quercetin equivalent concentration in mg/100 g dry sample.

Data analysis
In this work, appropriate treatments were carried out throughout all the experiments. All measurements on the infusions and control solutions have been performed at least in triplicate and all data have been analyzed. The data presented in this report represent the average values.

Total antioxidant capacity of A. borneensis leaf infusion
The total antioxidant capacity of the infusion of A. borneensis leaves has been analyzed by quantifying their DPPH free radical scavenging. The IC 50 of 2 g dried leaves of A. borneensis infused in 200 ml water was measured to be within 312 to 805 ppm, equivalent to 1440 to 3730 mg gallic acid per 100 g dry sample of A. borneensis leaves. This indicates that the infusion of A. borneensis leaves is a significant source of antioxidants. According to the DPPH free radical scavenging, the total antioxidant capacity depends on the brewing time and temperature of the infusions, as described below.

Effect of brewing time
By plotting IC 50 of the infusion against brewing time, as shown in Figure 2A, we demonstrate that its total antioxidant capacity increases gradually with the brewing time. The similar trend has also been observed in all C. sinensis teas (Ryan and Petit, 2010;Fernando and Soysa, 2015;Nikniaz et al., 2016). This may not be surprising because there are more phytochemicals and polyphenol derivatives could be permeated from the leaves into the solution at longer infusion time. However, we should note that at brewing temperature, 80°C, more than 50% of the total antioxidant capacity was achieved during the first 3 min infusion time. The total antioxidant capacity increases exponentially with infusion time and it is saturated at infusion times longer than 1 hour, indicating that the optimum extraction capacity is reached. This implies that water could effectively extract the antioxidants from A. borneensis leaves, and most of them should pass through the openings or interstices on the leaves in the early time of infusion. We may consider that this phenomenon should be similar to the cases of a liquid passing through a membrane due to different concentrations, where its concentration increases exponentially. Therefore, the concentrations of the antioxidants in the infusion may obey first-or secondorder kinetics (Peleg, 1988;Lafka et al., 2013;Fernando and Soysa, 2015). To clarify this issue, we have fitted our data with a single exponential function. As shown in Figure 2A, the single-exponential fitting gives an infusion rate constant of the antioxidants to be 0.040±0.005 min -1 . The validity of this consideration is unambiguously supported by the infusion-time dependence of TPC, TFC, and TF content of A. borneensis leaves, where their infusion rate constant is also within the range of 0.035-0.040 min -1 , as shown in  Figure 2B. These findings further indicate that the total antioxidant of A. borneensis leaves is clearly related to its TPC, TFC, and TF contents. We may also note that TPC, TFC, and TF contents, which are considered to be an important criterion of tea quality, were measured to be in the range of 739-2110, 440-1005, and 3.0-66.5 mg per 100 g dried leaves of A. borneensis, respectively. Figure 3A shows that the total antioxidant capacity of A. borneensis leaves increases abruptly with brewing temperature (in the range of 10-100°C). A similar tendency has also been observed for C. sinensis black and green teas, where infusion with greater antioxidant capacity was obtained at longer brewing time and higher temperatures (Sharpe et al., 2016). This finding implies that more phytochemicals and polyphenol derivatives from the A. borneensis leaves are infused into the water due most probably to the existences of more openings and interstices on the leaves, increasing the diffusion rate at higher brewing temperature. We found that more than 90% of the total antioxidant capacity was achieved at temperature higher than 60°C, indicating efficient releases of the phytochemicals and polyphenol derivatives from A. borneensis leaves into the brewing solution due most probably to larger numbers of openings, interstices, denaturation, or disruption of their cells at temperature higher than 60°C.

Effect of brewing temperature
Accordingly, in Figure 3B, we show that TPC, TFC, and TF contents in the infusion of A. borneensis leaves also increase abruptly with brewing temperature. Therefore, we could draw a conclusion that the total antioxidant capacity is related to the concentrations of phytochemicals and polyphenol derivatives in the A. borneensis leaf infusion, as discussed above. Considering that TF content in the infusions to scavenge DPPH is one or two orders lower when compared with those of TPC and TFC (see Figures 2B and 3B), thus polyphenols and flavonoids should be the major constituents responsible for the total antioxidant capacity of A. borneensis leaves.
It is noteworthy that although caffeine does not belong to polyphenols, it has a capability to inhibit lipid peroxidation (Brezova et al., 2009). Thus, one may consider that caffeine content could have a contribution in the antioxidant capacity of A. borneensis leaves. In this sense, we have measured its caffeine content, and we found that it is negligibly small (data not shown). Therefore, the contribution of caffeine in the total antioxidant capacity of A. borneensis leaves is not important. Such negligible contribution of caffeine in the total antioxidant capacity has also been discussed for C. sinensis black tea infusions (Fernando and Soysa, 2009).

Antioxidant capacity of several commercially available teas
We should note that most of the health benefits of tea are based on its total antioxidant activity. The total antioxidant activity of the infusion of A. borneensis leaves, therefore, was compared with those of twelve different C. sinensis black, herbal, and Pu Er teas commercially available. With the same brewing time and temperature, color of the infusions clearly varies depending on their brands, as shown in Figure 4. We found that the total antioxidant activity, either in IC 50 , gallic acid equivalent antioxidant capacity (GEAC), or quercetin equivalent antioxidant capacity (QEAC), is also significantly different in the various brands of teas. As listed in Table 1, the minimum IC 50 (143 ppm) was observed in the tea infusion of brand A, while the maximum value (4078 ppm) was found in the infusion of brand M. This indicates that the total antioxidant capacity of black tea brand A is approximately 30-fold higher than that of brand M. We should note that  infusion of A. borneensis leaves (brand I) has an IC 50 of 592 ppm, equivalent to approximately 4-fold lower than that of brand A and it is 7-fold higher than of brand M. This means that the health benefits of A. borneensis infusion are in the middle rank among the twelve C. sinensis and herbal teas evaluated in the present study.
We should note that, in addition to the brewing conditions (brewing time and temperature), the total antioxidant capacity of teas should vary greatly depending on a wide variety of factors such as the location of plantation, harvesting season, drying method, and manufacturing. Although there have been several literatures comparing the total antioxidant capacity and polyphenolic contents of commercial teas from different various brands, those factors are simply neglected (Chan et al., 2007;Taheri et al., 2011;Carloni et al., 2013). In particular, black teas are produced by fermentation with a number of parameters which can also modify their antioxidant capacity. For instance, polyphenol oxidases released from the disrupted cells can react with catechins, thus during black tea manufacturing, the most considerable changes occur in black teas is the concentration of catechins. Therefore, it is obvious that there is a significant difference in the total antioxidant capacity found in commercial black teas from various brands. However, we should mention that, regardless of that wide variety of factors determining tea quality, the infusion of A. borneensis leaves which was processed traditionally is among those teas with considerable total antioxidant capacity. In order to explore the effect of processing, a study on the antioxidant capacity of A. borneensis leaves with different pretreatments is currently being pursued.

Conclusion
In summary, we have investigated the total antioxidant capacity of Aidia borneensis leaf infusion, a Bornean endemic plant, by measuring its DPPH free radical scavenging activity. We found that the antioxidant capacity of the infusion increases with brewing time and temperature, and we have attributed the total antioxidant capacity of the infusion to their TPC, TFC, and TF contents. We have also assessed relative potential of the infusion of A. borneensis leaves by comparing its total antioxidant capacity with twelve different C. sinensis black, herbal, and Pu Er teas commercially available. We found that the infusion of A. borneensis leaves is in the middle rank among the twelve commercially available teas. Therefore, the present preliminary study has highlighted significant implications on A. borneensis leaves and on the feasibility of establishing this new potential plant among commercially available conventional C. sinensis tea, which has been known as a rich source of antioxidants.