iCHAPTER 3: EXPERIMENTAL METHODS, OPTIMISATION AND KINETIC STUDIES
3.3.3 Methods of analysis for solvent selection
The global yield was determined by weighing the masses as shown in section 3.3.3.1. The total phenolic content and antioxidant activity of the extracts was analysed at the oxidative
stress unit (CPUT). Figure 3.2 shows some of the dried extract after adding 5 ml of solvent which will be taken for analysis.
Figure 3.2: Samples ready for analysis
3.3.3.1 Determination of global yield
The global yield was calculated using the following formula:
BSG of mass Initial
extract BSG of yield Mass
Global Equation 3.1
The initial mass of BSG was weighed in a weighing boat using the weighing balance. The mass of the BSG extract was determined from the difference between the mass of the round bottom flask used to evaporate the solution after extraction and filtration and the mass of the empty round bottom flask. After weighing, a fresh solvent was added to the dried extract and the sample was taken for analysis of the total phenolic content and the antioxidant activity.
3.3.3.2 Determination of total phenolic content
The TPC of the extracts was determined by the Folin-Ciocalteu method as modified from Moreira, 2012. The Folin’s reagent was diluted with water at 1:10 ratio. The samples were prepared in a plate that has 26 wells. Each well was filled with 25µL sample, 125µL Folin’s reagent and 100µL of NaCO3 solution (7.5% w/v) in the relative order using a pipette. The pipette tips were changed after a single use to avoid contamination of wells. One sample was filled in three consecutives well so as to take the mean reading. The first well was filled with a blank followed by 7 standards of gallic acid (GA). After 2h of incubation at room temperature in the dark, the absorbance was measured at 740nm using a multiskan®
spectrum (Germany). The TPC measures the oxidative strength of the extracts relative to the
gallic acid. The total phenolic concentration was calculated from the calibration curve, using GA as standard (5-250mgL-1). Data for the TPC were reported as mean ± SD for duplicates.
3.3.3.3 Measurement of antioxidant activity
The antioxidant activity is influenced by several factors to be considered when selecting the best antioxidant for a specific application. These factors include structural features of the antioxidants, concentration, temperature, reaction kinetics and location of the system as well as presence of pro-oxidants and synergists (Shahidi & Zhong, 2015). Moreira, (2012) reported several methods used to determine the total antioxidant activity of food and beverage such as 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay, Oxygen radical absorbance capacity (ORAC) assay, 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assay and ferric reducing ability of plasma (FRAP) assay. These assays differ in the chemical reactions that take place with the targeted compounds and hence produce different results from each other. Depending on the chemical reactions involved, these assays fall into two categories of hydrogen atom transfer (HAT) reaction-based assays and single electron transfer (ET) reaction-based assays (Shahidi & Zhong, 2015).
There is no standard or best assay that can be considered since each assay react different with antioxidant. Therefore at least two assays should be used for evaluation to authenticate the research. In this work two methods, DPPH and FRAP assay were used to determine the antioxidant activity of the BSG extracts.
3.3.3.4 DPPH radical scavenging activity assay
DPPH is a stable free radical and with a deep purple colour. The method is based on the theory that antioxidants are hydrogen donors. The DPPH measures the electron transfer ability of antioxidants to neutralize the DPPH radical and therefore is an ET-based method (Prior et al., 2005). The scavenging of the DPPH molecules is accompanied with a colour change of DPPH from purple to yellow and a decrease of UV absorption at 517nm. The extent of the decolourisation indicates the efficacy of the antioxidant. In this work, the radical scavenging activity of each extract was measured according to the procedure of Brand- Williams et al., (1995).
The DPPH solution in methanol (6.6 × 10-5 M) was prepared and 275 µL of this solution was mixed with 25 µL of the sample extracts using a pipette. Trolox also known as 6-hydro- 2,5,7,8-tetramethylchroan-2-2carboxylic acid (Aldrich) was used as standard. The filling of the wells was similar to that of the measurement of TPC. The absorbance decrease of DPPH was measured using the spectrophotometer at 517nm after 10min. The disappearance of the purple colour indicates higher free radical scavenging activity. The antiradical power (ARP,
%) which is defined as (1/EC50 x 100) was used to determine the activity of extracts. EC50 is defined as the amount of BSG extract that can halve the initial concentration of the DPPH solution.
3.3.3.5 Ferric reducing antioxidant power (FRAP) assay
The ferric reducing antioxidant power was measured according to the method described by (Benzie & Strain, 1996) with some modifications. The FRAP assay is a typical ET-based method that measures the ability of the BSG extract to reduce the ferric ion (Fe3+)–ligand complex to the deep blue ferrous (Fe2+) complex in acidic media (Shahidi & Zhong, 2015).
However, the assay does not directly measure the antioxidant capacity of a potential antioxidant. Its antioxidant activity is determined as increase of absorbance at 593 nm, and results are expressed as micromolar Fe2+ equivalents or relative to an antioxidant standard, trolox (5-250mg L-1) (Antolovich M, Prenzler PD, Patsalides E, McDonald S, 2002). The FRAP reagent was prepared from 300mM acetate buffer at pH 3.4, 10mM 2,4,6-Tripyndyl-s- triazine (TPTZ) in 40mM HCl and 20mM FeCl3 solution in proportions of 10:1:1 (v/v), respectively. 300µL of this solution was added to 10µL of sample or blank or standards using a pipette for 30min. The filling of the wells was similar to that of the measurement of TPC.
Readings of the Prussian blue, ferrous tripyridyltriazine complex were then taken at 593nm.
The measurements were carried out for 2 sets of experiments and results expressed as mg TE/g DW of sample.
3.3.3.6 HPLC-DAD analysis for individual components
The phenolic composition for the extracts with high TPC and antioxidant activity was analysed using an HPLC system (Agilent technologies, 1200 series) modified from the method described by (Rubilar et al., 2007). The HPLC system consisted of a low-pressure quaternary gradient unit with an inline degasser and a photodiode array detector.
Separation of polyphenols was achieved on a C18 column (150mm x 4.6mm x 4µm) with temperature being kept at 30 . The chromatography had a flow rate of 0.3mLmin-1 and the analytical standard and injection volume was 20µL. Water was used as a mobile phase A and methanol used as mobile phase B. The following gradient system was used in this analysis: 100% B in 0 min, from 100% to 0% B in 110 min, followed by 100% A for 20 min and back to 100% B in 10 min and a period of 10 min was set in between injections. The photodiode array detection was performed at 320 nm for phenolic acids (p-CA) and at 370 nm for quercetin, kaempferol and rutin. The chromatography data system obtained identified the analytes by comparing their peak integration area. Peak purity was also monitored to minimize interference of other peaks. Results in samples were expressed in mg/g BSG.