17/ Seow Eng Keng
et al.
Total phenolics content (TPC)
TPC of coconut water was determined using Folin-Ciocalteu method as described by Tan
et al. (2014). Coconut
water (1 mL) was placed in a 100 mL volumetric flask followed by 70 mL of distilled water and 5 mL of Folin &
Ciocalteu's phenol reagent (10 times dilution). Mixture was incubated for 5 min
at room temperature before
adding 15 mL of 7.5% (w/v) sodium carbonate and topped up to 100 mL with distilled water. Mixture was
incubated for 2 h at room temperature. Measurement of absorbance was carried out in UV-1650
PC UV-Vis
spectrophotometer at wavelength of 765 nm. TPC was expressed as gallic acid equivalents (GAE) using units of
mg/L (mg GAE/L).
Stock solution of gallic acid at 500 mg/L was prepared in deionised water. Working standard solutions
of the mixture were prepared by dilution of stock solution to give final concentrations of 20, 40, 60, 80, and 100
mg/L in deionised water. All the standard solutions were treated as those of coconut water prior to absorbance
measurement at wavelength of 765 nm.
Sugars content
Glucose, fructose and sucrose contents in coconut water was determined using HPLC as described by Tan
et al.
(2014). Approximately 50 mL of coconut water was transferred into a 250 mL conical flask containing 2 g of
Amberlite MB-150 ion exchange resins. The mixture was allowed to rest (with occasionally swirling) for 15 min
to allow the ion exchange resins to bond with both the cations and anions present in the coconut water. After 15
min, the mixture was filtered with muslin cloth and a portion of the filtrate (5 mL) was filtered through a pre-
activated (with 5 mL of methanol followed by 5 mL of deionised water) Sep Pak C
18
(Waters
Corporation,
Milford, USA). The filtrate was then diluted 25 times with deionised water and filtered through membrane filter
0.45 µm (Whatman, 13 mm GD/X PTFE Filtration Media, Wycombe, UK) before being injected into the loop
(volume: 20 µL) of the system.
Stock solution of sugars (consists of fructose, glucose, and sucrose) at 1 mg/mL
were prepare in
deionised water. Standard solutions of the mixture were prepared by dilution of stock solution to give final
concentrations of 0.2, 0.4, 0.6, and 0.8 mg/mL in deionised water. All the standard solutions underwent cleaning
steps (Amberlite MB-150
ion exchange resins, Sep Pak C
18
, and membrane filter 0.45 µm)
similar to those of
coconut water prior to injection. Quantification of fructose, glucose and sucrose content in coconut water was
performed using HPLC system equipped with a pump system (Waters Corporation, Waters 515 HPLC Pump,
Milford, USA) and a refractive index detector (Waters Corporation, Waters 2414, Milford, USA). Separations of
these sugars were carried out using Waters Sugar-Pak 1 column 300 mm × 6.5 mm, (Waters Corporation,
Milford, USA) at 90°C. The flow rate was 0.5 mL/min and mobile phase used was 0.0001 M Ca-EDTA.
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