Lai Peng Leong

An investigation of antioxidant capacity of fruits in Singapore markets

The antioxidant capacity of a group of fruits obtained in the Singapore markets was investigated. A total of 27 fruit pulps were tested for their general antioxidant capacity based on their ability to scavenge 2,2′-azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) free radical. The contribution of l-ascorbic acid (AA) to the total antioxidant activity of fruits was investigated by using RP-HPLC. The antioxidant capacity of the fruit pulp was measured by monitoring the change of absorbance of the free radical solution at 414 nm in the test reaction mixture following addition of the fruit extract, as compared with AA. The results were expressed as mg of AA equivalents per 100 g, i.e. the quantity of AA required to produce the same scavenging activity as the extract in 100 g of sample (l-ascorbic acid equivalent antioxidant capacity, AEAC). Total antioxidant capacities of AA acid, trolox, hydroquinone, pyrogallol and several fruits were also evaluated based on its ability to scavenge the 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical. Results obtained were compared with those of ABTS assay. Every mol of AA, trolox or hydroquinone, was found to reduce about 2 mol of ABTS+ or DPPH. However, 4 mol of DPPH or 7 mol of ABTS+ were scavenged by every mol of pyrogallol. A good correlation of AEAC was observed between the two methods. Both methods have been recommended to be useful tools to evaluate antioxidant capacities of fruits. According to the AEAC value of binary extract solution of fruits in the ABTS model, ciku shows the highest antioxidant capacity, followed by strawberry, plum, star fruit, guava, seedless grape, salak, mangosteen, avocado, orange, solo papaya, mango, kiwi fruit, cempedak, pomelo, lemon, pineapple, apple, foot long papaya, rambutan, rambutan king, banana, coconut pulp, tomato, rockmelon, honeydew, watermelon and coconut water. The AA contribution to AEAC of fruits varied greatly among species, from 0.06% in ciku to 70.2% in rambutan.

Separation and determination of organic acids and phenolic compounds in fruit juices and drinks by high-performance liquid chromatography

A high-performance liquid chromatographic (HPLC) separation method with photo-diode array detection has been developed for the simultaneous determination of organic acids and phenolic compounds in juices and drinks. The chromatographic analysis of organic acids and phenolic compounds was carried out after their elution with sulphuric acid solution (pH 2.5) and methanol from C18 stationary phase. The mobile phase employed was sulphuric acid solution working at a flow-rate of 0.35 ml min(-1) for the whole run, while methanol was linearly increased to 0.45 ml min(-1) from 15 to 75 min followed by a 5-min isocratic elution. Ten organic acid acids were eluted in 30 min and 21 phenolic compounds, which include phenolic acids and flavonoids, were eluted in the following 50 min. Target compounds were detected at 215 nm. The repeatability (n=3) and between day precision of peak area (n=3) were all within 5.0% RSD. The within-day repeatability (n=3) and between-day precision (n=10) of retention times were within 0.3 and 1.6% relative standard deviation (RSD), respectively. The accuracy of the method was confirmed with an average recovery ranging between 85 and 106%. The method was successfully used to measure a variety of organic acids and phenolic compounds in juices and beverages. This method could also be used to evaluate the authenticity, spoilage or micronutrient contents of juices.

Temperature and Solvent Effects on Radical Scavenging Ability of Phenols

In this work we have demonstrated the free radical scavenging ability of two-hydroxy (catechol, hydroquinone, resorcinol) and three-hydroxy (phloroglucinol, pyrogallol, 1,2,4-benzenetriol) phenols against the diphenylpicrylhydrazyl radical at various temperatures (15-40 degrees C) and in different solvent media. Kinetic measurements, made by the stopped-flow method, showed that the phenols with OH groups in the ortho positions have the largest rate coefficients compared to those with OH groups in the meta and para positions at all temperatures and in all solvent media. Among the ortho-structured phenols catechol, pyrogallol, and 1,2,4-benzenetriol, pyrogallol (three OH groups ortho to each other) had the greatest radical scavenging ability. This suggested that intramolecular hydrogen bonding in phenols controlled the rate of radical scavenging ability. The radical scavenging ability of phenols was fastest in methanol and slowest in THF, which emphasized the importance of the interactive behavior of the phenolic OH with the solvent. We concluded from our kinetic data together with our theoretically calculated OH bond dissociation enthalpies of phenols that the OH position played a crucial role in addition to the temperature and nature of the medium in determining the rate of the radical scavenging ability of polyphenols.

A critical appraisal of the kinetic model for the Maillard browning of glucose with glycine

A critical analysis of kinetic data for the loss of sulphite species, S(IV), in the glucose-glycine–S(IV) reaction, and the browning (A470) of glucose–glycine mixtures (pH 5.5, 0.2 mol l−1 acetate buffer, 55°C) confirm the accuracy of the 3-step model (proposed by Davies, C.G.A., Wedzicha, B.L., & Gillard, C. 1997). (Kinetic model of glucose–glycine reaction. Food Chemisty, 60, 323–329). Rate constants k1 and k2 are obtained by regression of [S(IV)]-time data to the integrated rate equation for the glucose–glycine–S(IV) reaction. It is confirmed that neither rate constant depends on [S(IV)]. The integrated rate equation for the overall browning reaction is derived, and the rate constant k3 for melanoidin formation as well as the effective extinction coefficient E are obtained by regression. The value of E is confirmed for high molecular weight (Mr>3500) melanoidins by a radiochemical technique based on 14C-labelled glucose. This paper thus presents a protocol to obtain values of the significant rate constants in the browning of aldoses with amino acids.

The Millimeter- and Submillimeter-Wave Spectrum of the trans-trans Conformer of Diethyl Ether (C2H5OC2H5)

Since dimethyl ether is found to have a high abundance in hot cores, it is reasonable to search among such sources for detectable abundances of the more complex analogs ethyl methyl ether (C2H5OCH3) and diethyl ether (C2H5OC2H5). Indeed, preliminary detections of both complex ethers have been reported. Definitive interstellar searches do require laboratory spectra, however, but for many years only low-frequency data have been available for both of these species. Following our recent study of the millimeter-wave and submillimeter-wave spectrum of ethyl methyl ether, we report here the study of the millimeter- and submillimeter-wave spectrum of the lowest energy conformer of diethyl ether. With four different spectrometers, over 1000 new spectral lines have been measured and analyzed at frequencies up to 350 GHz. Fitting the data to a set of spectroscopic parameters from the Watson A-reduced form of the asymmetric-top Hamiltonian has allowed us to predict the frequencies and intensities of many more transitions through 400 GHz. A more precise determination of the electric dipole moment of diethyl ether was also carried out, resulting in μ = μb = 1.0976(9) D.

Metabolite profiling of edible bird's nest using gas chromatography/mass spectrometry and liquid chromatography/mass spectrometry.

RATIONALE Edible birds nest (EBN) is a renowned food item in the Chinese community due to the therapeutic effects claimed to be brought about by its consumption. However, very little scientific information has been revealed to support these claims. Thus, metabolite profiling was performed to identify the metabolites and to relate them to the medicinal properties of EBN. The study also aims to further extend the results to determine any possible differences when the EBNs are classified according to their coloration, countries or production sites. METHODS Extraction of the metabolites was performed via sonication of EBN with methanol and chloroform. Two analytical platforms, namely gas chromatography/mass spectrometry (GC/MS) and liquid chromatography/mass spectrometry (LC/MS), were utilized to detect the metabolites. This multi-analytical platform would provide a comprehensive coverage of the metabolites in EBN. GC/MS analysis was conducted in scan mode from m/z 50-650. On the other hand, LC/MS analysis was operated in both positive and negative ion mode from m/z 150-1000. RESULTS Metabolites were identified and their relationships with the medicinal properties of EBN were deduced. Classification with chemometrics illustrates that EBNs could be differentiated according to their coloration, countries and production sites. This differentiation was due to the environment where the EBNs are produced. Furthermore, GC/MS was demonstrated to be more suitable for classification as the processing methods of the EBNs did not cause a significant variation in the metabolites detected by GC/MS. CONCLUSIONS The overall findings suggest that the novel approach of metabolite profiling offers new insights to understanding EBN and provided evidence to support the medicinal properties of EBN. In addition, the success of classification of EBNs with metabolite profiling combining with chemometrics represents a paradigm shift in the quality control of this food item.

Effect of glyceraldehyde on the kinetics of Maillard browning and inhibition by sulphite species

Abstract The kinetics of browning ( A 470 ) are reported for glyceraldehyde+glycine and glucose+glycine+glyceraldehyde mixtures ([glucose]=0–1 mol l −1 ; [glycine]=0–1 mol l −1 ; [glyceraldehyde]=0–10 mmol l −1 ; pH 5.5, [acetate buffer]=0.2 mol l −1 with respect to acetate ion; 55°C). In the absence of glucose the rate of reaction depends only on glyceraldehyde concentration, whereas glycine is required for colour development. The rate of browning when all 3 reactants are present is much greater than calculated as the sum of the individual glyceraldehyde+glycine and glucose+glycine reactions. This synergistic behaviour is accompanied by the removal of the induction phase normally seen for the Maillard reaction, and a browning reaction whose kinetics depend on the concentrations of all 3 reactants. The apparent dissociation constant of the hydroxysulphonate of glyceraldehyde is 6.76×10 −5 mol l −1 (pH 5.5, 55°C, ionic strength≈0.2 mol l −1 ). Theoretical calculations suggest that the conversion of glyceraldehyde to its hydroxysulphonate could contribute to the mechanism of the inhibition of Maillard browning by sulphite species.