Faculty Members

Obtained B.Sc. (Hons) in Biochemistry major in Cell and Molecular Biology and developed keen interest in the food and beverages industry after obtaining working experiences in fast food outlet, palm oil refinery and nutrition outlet. Awarded M.Sc. (Distinction) from University of Leeds in Food Science and granted a research scholarship by the European Union to study for a Ph.D. in Food Science. 

Research interests of our group encompass a broad range of topics in food science and technology especially in relation to Asian functional foods, including the study of nutraceutical components such as isothiocyanates, flavonoids, etc.; the effect of storage and processing of fruits and vegetables with respect to how these components interact; and the kinetics and mechanisms of reactions taking place in food. The latter includes the Maillard reaction and the mechanism of action of antioxidants. The Maillard reaction is very complex, but is essentially the name given to a reaction scheme that has as its starting reagents amino acids and sugars that occur in food. Due to the complex nature of these reactions and the vast variety of chemical present within food itself, our group focuses on much simplified model systems. In these systems, e.g., glycine reacting with glyceraldehyde, we carefully look at the possible reaction pathways that may occur under different conditions, pH, temperature, concentration etc. This work involves a combination of experimental and theoretical techniques ranging from conventional UV/Vis kinetic measurements, NMR and chromatographic analysis of reaction products and intermediates. The theoretical work includes computational chemistry and chemical kinetic computer modeling.

The chemistry of antioxidants, which are present as an additive or naturally occurring in some foods, is also examined in my group. An antioxidant compound is a species capable of removing harmful radicals (mainly peroxy radicals), naturally or unnaturally, present in biological systems. Polyphenolic compounds are examples of antioxidants. There are currently three overall possible mechanisms for antioxidant action. (i) If the radical is in an excited triplet state, then the polyphenolic could act as a quenching agent. (ii) If the radical is a doublet, then direct hydrogen atom transfer can occur to the radical. Successive hydrogen atom transfer produces a bi-radical that can subsequently rearrange to a dicarbonyl. (iii) If the radical is a doublet, then charge transfer may occur to yield a closed-shell anion and a radical polyphenolic cation. Proton transfer from either the solvent or the polyphenolic cation can then occur to the anion. The overall result is the same as direct hydrogen atom transfer, i.e., (ii). In my laboratory we simulate the peroxy radical using well known compounds (DPPH or ABTS+ radicals) and investigate experimentally, using conventional UV/Vis spectrometers, or with a stopped-flow UV/Vis instrument for faster reactions; and theoretically, using computational chemistry and chemical kinetic computer modeling, their reaction with various polyphenolics.

Objectives Various Maillard model systems are studied to obtain the rate of reactions under different conditions. Such model systems are divided into rate determining steps in an attempt to model the parallel and consecutive multi-step reaction. The kinetic of antioxidant reactions especially polyphenolic antioxidants are studied to compare the effectiveness and capacity of the antioxidants. Antioxidants isolated from extracts of fruits and vegetables as well as those available commercially are studied. Computational study of both the reactions above is also studied to understand the basic mechanism of the reactions.

Deliverables and key findings Several model systems to predict the formation of colour of the Maillard reaction has been studied. The effect of pH on the formation of Schiff base during the first step has been studied in more detail using simple carbonyl and amino compound model systems. The Maillard reaction has also been found to be acting synergistically in the presence of more than one starting materials which adds to the complication of the model systems. A computational study was performed on the glyceraldehydes – glycine reaction taking into account the possible pathways at various pH. The scheme as suggested by Hodge have been used as a guideline. Various antioxidants in local fruits and vegetables have been identified including that in star fruit, lady’s finger, ciku king, salak and ulam raja. Methods have been developed to separate the antioxidant components in the extract of these plants and identify the antioxidant compounds simultaneously. The structures of some of these antioxidants have also been identified. The kinetics of simple polyphenols and DPPH radicals has been studied to observe the effect of different substituted groups on the effectiveness of these antioxidants. All these reactions are studied concurrently with computer modeling which is used to predict the rate of reaction based on the hydrogen transfer mechanism. Computational calculations were performed both in gas and in aqueous phase.

Significance, impact The Maillard reaction contributes to many aspects of food including quality, health and safety. In addition, the Maillard reaction has also been found to be particularly important in medicine especially associated with diabetics and the formation of cataracts. It is hoped that by understanding the kinetics of the reaction in model systems, one is able to control the reaction for the production of various products such as colours, flavours, antioxidants, carcinogens etc. in foods and medicine. One of the major concerns of the world population today is the aging population which is susceptible to various degenerative diseases. The field of medical and life sciences used to focus on the cure of a disease. However, it has now been recognized that prevention is the key to resolve many health problems in life. Antioxidants obtained from food is believed to be able to prevent the onset of many degenerative diseases caused by reactive radicals which is produced through normal metabolism. Understanding the way they operate may lead us to the solution of the most effective antioxidants. It may also allow one to understand the synergistic behaviour of these compounds and as such, it is possible to identify the best combination of antioxidants for different biological systems.

• Shui G, Leong LP. (2004) Characterisation of Antioxidants and Change of Antioxidant Levels during Storage of Manilkara zapota L. J. Agric. Food. Chem. Accepted.
• Shui G, Leong LP. (2004) Improved method for the analysis of major antioxidants of Hibiscus esculentus Linn. Journal of Chromatography A. Accepted.
• Shui G, Leong LP. (2004) Two commonly-used reactive nitrogen species (RNS) scavenging methods for total antioxidant capacity assay: A comparison. Free Radical Biology and Medicine 36: S69-S69 Suppl. 1
• Shui G, Wong SP, Leong LP. (2004) A complete antioxidant profile of Achras sapota Linn and changes of antioxidant levels during storage. Free Radical Biology and Medicine 36: S124-S124 Suppl. 1
• Shui G, Leong LP. (2004) Residue from star fruit as valuable source for functional food ingredients and antioxidant nutraceuticals. Free Radical Biology and Medicine 36: S132-S132 Suppl. 1
• G. Shui & L.P Leong. (2004). Analysis of Polyphenolic Antioxidants in Star Fruit Using High Performance Liquid Chromatography and Mass Spectrometry. Journal of Chromatography A. 1022 (1-2): 67-75 JAN 2.
• Shui GH, Leong LP. (2003) Rapid screening and identification of antioxidants of salak (salacca edulis reinw) using high performance liquid chromatograph coupled with mass spectrometry. Free Radical Biology and Medicine 35: 130 Suppl. 1
• G. Shui & L.P Leong. (2002) Separation and Determination of Organic Acids and Phenolic Compounds of Fruit Juices and Drinks by High-Performance Liquid Chromatography. Journal of Chromatography A. 977: 89-96.
• L.P. Leong & G. Shui. (2002) An Investigation of Antioxidant Capacity of Fruits in Singapore Markets. Food Chemistry. 76:69-75.
• Leong LP and Bettens RPA (2002) NUS Students and Biggs’ Learning Process Questionnaire. CDTL Brief. 5 (7) 3.
• Leong LP, Wedzicha BL (2000) A critical appraisal of the kinetic model for the Maillard browning of glucose with glycine. (Food Chem., Vol.68, No.1, pp.21-28)
• Keller C, Wedzicha BL, Leong LP, Berger J (1999) Effect of glyceraldehyde on the kinetics of Maillard browning. (Food Chem., Vol.66, No.4, pp.495-501)
• Wedzicha BL and Leong LP. (1998) Modeling of the Maillard reaction: rate constants for individual steps in the reaction. In The Maillard reaction in foods and medicine ; [proceedings of the 6th International Symposium on the Maillard Reaction, held at the Royal College of Physicians, London, UK, 27-30 July, 1997] ; edited by John O’Brien … [et al.] Cambridge : Royal Society of Chemistry)
• Wedzicha BL and Leong LP. (1998) Modelling and Control of the Maillard Reaction. (European Food and Drink Review, Spring 98).