Maillard reaction products derived from thiol compounds as inhibitors of enzymatic browning of fruits and vegetables: the structure-activity relationship.

Some thiol-derived Maillard reaction products (MRPs) may exert antioxidant activity, depending on the reaction conditions as well as on the sugar and the sulphydryl compound. Recently, we reported that MRPs derived from glucose or fructose with cysteine (CSH) or glutathione (GSH) mixtures greatly inhibited polyphenoloxidases (PPOs), oxidoreductases responsible for discoloration of fresh or minimally processed fruits and vegetables. Glucose and GSH were shown to be the most active in producing inhibitory MRPs. Therefore, we examined the way in which the nature of the reactants affected their synthesis, in order to establish a structure-activity relationship for the inhibitory products. Various aqueous (0.083 M, 0.125 M, or 0.25 M) mixtures of a sugar (hexose, pentose, or diholoside) with either a CSH-related compound (CSH, GSH, N-acetyl-cysteine, cysteamine, cysteic acid, methyl-cysteine, cysteine methyl ester), an amino acid (gamma-glutamic acid, glycine, methionine), or other sulfur compound (thiourea, 1,4-dithiothreitol, 2-mercaptoethanol) were heated at 103 degrees C for 14 h. Soluble MRPs were compared for their ability to inhibit apple PPO activity. In the presence of CSH, the rated sugars (same molar concentration) ranked as to inhibitory effect were pentoses > sucrose > hexoses > or = maltose. In the presence of glucose, the simultaneous presence of an amino group, a carboxyl group, and a free thiol group on the same molecule seemed essential for the production of highly inhibitory compounds.

Antioxidant activity of malt rootlet extracts.

To improve the malt rootlet value, the antioxidant potentialities of this byproduct of the malting industry have been analyzed. Three extracts have been considered from the points of view of dry matter yield, level of antioxidant compounds, and efficiency and cost of the extraction protocols. They respectively contain rootlet oil, free phenolic compounds, and bound phenolic compounds. The rootlet oil contains only a low quantity of tocopherols (respectively, 20.6 and 4.2 microgram of alpha-tocopherol and gamma-tocopherol per gram of dry rootlets), and a weak antioxidant activity, evaluated in a stripped corn oil by following spectrophotometrically the conjugated dienes, has been pointed out. The bound compound extract presents a good antioxidant power mainly due to the presence of trans-ferulic and trans-p-coumaric acids, but the dry matter yield is low (2%). The free compound extract has a good antioxidant power, and the valuable dry matter, mainly composed of proteins (52%), sugars (33%), and reducing compounds (5.5%), has a yield of 12%. The mixing of bound and free compound extracts presents an antagonistic effect on the antioxidant power, but a synergistic effect has been pointed out for the mixing of alpha-tocopherol and free compound extract.

Determination of the antioxidant activity of phenolic compounds by coulometric detection.

The combination of reversed-phase high-performance liquid chromatography with coulometric detection allows the detection of phenolic antioxidants in complex matrices like plant extracts with a high degree of sensitivity and selectivity. According to their voltammetric behaviour, phenolic acids and tocopherols show maximal detector response at low potentials (100-450 mV) while flavonoids show optimal response at two different potential values (one at 0-300 mV and one at 600-900 mV). The potential corresponding to maximal detector response (MDRP) of phenolic acids was shown to be inversely proportional to their antioxidant efficiency as determined in a lipidic model system under strong oxidizing conditions (110 degrees C, intensive oxygenation) or by the DPPH() test. However, such a relationship was not observed for flavonoids.

Atmospheric pressure chemical ionization (APcI) liquid chromatography-mass spectrometry: characterization of natural antioxidants.

The combination of liquid chromatography with mass spectrometry (LC-MS) allows non-volatile and thermally sensitive compounds to be handled. The key feature, regarding LC flowrate and MS vacuum requirements, is the LC-MS interface system. Atmospheric pressure chemical ionization (APcI) is a sensitive and widely applicable method which gives primarily molecular weight information with the ability to provide structural information if required. Applications concerning the characterization of natural antioxidants from rosemary, sage and barley are reported: the main functional groups of diterpenes (rosmanol and derivatives), the aglycone part of glycosylated flavones and the different units of polymers of flavan-3-ols (procyanidins, prodelphinidins) are shown.