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A practical approach for identifying antioxidants from volatile MRPs was developed based on the comparison of volatile MRPs before and after their reaction with DPPH radical by GC MS analysis. Using the approach, 2,5-dimethyl-4-hydroxy-3(2H)furanone (DMHF) and 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one (DDMP) were identified as the contributors to the antioxidant activity of volatile MRPs from Glc-His system while DMHF, 2-hydroxy-3-methyl-2-cyclopenten-1-one (CP) and 3-acetyl-1,6-dimethyl-2,4(1H,3H)pyridinedione (ADMP) as the contributors to the antioxidant activity of volatile MRPs from Glc-Tyr system. To test the effectiveness of the approach, seven compounds (DMHF, DDMP, CP, 2-furanmethanol, Furfural, 2-methylpyrazine, 2-acetylpyrrole) were investigated individually for their DPPH radical scavenging activity, ABTS radical scavenging activity and reducing power by UV-Vis analysis, which further confirmed the antioxidant activity of DMHF, DDMP and CP. Thus, the approach could be used to identify antioxidants from other volatile mixture. Besides, the interaction analysis denied the existence of the synergistic effect between each two compounds from above seven compounds. (C) 2013 Elsevier Ltd. All rights reserved.

The antioxidant activity of glucose-histidine MRPs (GH) and fractionated GH, obtained from the elution of GH on Bio-gel P-2 column, was evaluated for their DPPH radical scavenging activity, ABTS radical scavenging activity and reducing power, which illustrated that fraction H7 was of the greatest antioxidant activity. The further investigation identified 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one (DDMP) as the major compound in fraction H7 by employing GC-MS analysis and HPLC analysis, with the latter also confirming the antioxidant activity of DDMP by comparison of HPLC chromatograms of fraction H7 (or DDMP) before and after reaction with DPPH radical, ABTS radical or ferric ion. Finally, the comparison of the antioxidant activity of DDMP, fraction H7 and GH by UV-vis analysis showed their antioxidant activity was in the following order: DDMP> fraction H7> GH, suggesting that the major contribution of DDMP to the antioxidant activity of fraction H7, thus, DDMP could be identified as a strong antioxidant in GH considering that fraction H7 was of the greatest antioxidant activity among all fractions obtained from the separation of GH by gel filtration chromatography. (C) 2013 Elsevier Ltd. All rights reserved.

Volatile compounds prepared by heating glucose-tyrosine and glucose-histidine models in water-ethanol and water-glycerol media were studied under different water contents (0%, 25%, 50%, 75%, 100%, v/v). Aliphatic compounds, cyclopentenones, pyrazines, furans, furanones and pyrones were identified by GC–MS and analysed by factor analysis which showed that the presence of ethanol or glycerol in water could result in the increase of most volatile compounds in the Maillard reaction with the higher amount of volatile compounds formed in water-ethanol media, especially in the 50% ethanol medium. The measurement of antioxidant activity revealed that volatile MRPs prepared in water-ethanol media showed greater DPPH radical scavenging activity than those in water-glycerol media with the maximal DPPH radical scavenging activity observed in 100% ethanol medium for the glucose-tyrosine model and in 75% ethanol medium for the glucose-histidine model. Hence, water-ethanol medium could be used to promote the formation of volatile compounds, especially those with antioxidant activity.

In this study, the antioxidant activity (including ABTS radical scavenging activity and reducing power) and UV-Vis spectra of Maillard reaction products (MRPs) prepared from four sugars (glucose/fructose/ xylose/ribose) and twenty amino acids were studied. The effects of reactants on the antioxidant activity and UV-Vis absorbance of MRPs were investigated using correspondence analysis, which revealed that MRPs from fructose-amino acid models showed the similar UV-Vis absorbance and antioxidant activity with MRPs from glucose-amino acid models, MRPs from ribose-amino acid models showed the similar UV-Vis absorbance and antioxidant activity with MRPs from xylose-amino acid models, while there was significant difference between MRPs from hexose (glucose and fructose)-amino acid models and MRPs from pentose (ribose and xylose)-amino acid models, especially for methionine and valine. The correlation analysis demonstrated that ABTS radical scavenging activity showed the highest linearity (r = 0.831) with UV absorbance at 280 nm while reducing power showed the highest linearity (r = 0.689) with UV absorbance at 355 nm, indicating that different antioxidant properties of MRPs might derive from different compounds. Besides, as compared with cysteine, tyrosine and tryptophan showed a certain ABTS radical scavenging activity in spite of little reducing power. (C) 2011 Elsevier Ltd. All rights reserved.

In this study, the effect of pH on the volatile generation over reaction time as well as the DPPH radical scavenging activity of volatile MRPs was investigated in the Maillard reaction of glucose (Glc) with tyrosine (Tyr) and histidine (His). Factor analysis showed clearly that volatiles generated in 5-h open system were of similar composition to those in 0.5-h close system regardless of pH level and amino acid, implying higher pressure in close system could accelerate the volatile formation in Maillard reaction. Besides, all volatiles showed increasing tendency over reaction time under different pH levels with the exception of pyrones which decreased with the extension of reaction time at pH = 7 and pH = 9. Furans, phenols and 2-acetylpyrrole that were favored under acidic condition showed larger increase at pH = 5 over reaction time, while other volatiles except pyrones, furans, phenols and 2-acetylpyrrole, which were favored under neutral and alkaline conditions, showed greater increase at pH = 7 and pH = 9 over reaction time. The measurement of DPPH radical scavenging activity of dichloromethane extract obtained from Maillard reaction mixture revealed that volatile MRPs from Glc–Tyr system showed stronger antioxidant activity under alkaline condition, while volatile MRPs from Glc–His system showed greater antioxidant activity under acidic condition.