Kinetics of the sulphite-inhibited browning of fructose.

Sulphite species, S(IV), inhibit the non-enzymic browning of fructose-amino acid mixtures. Inhibition of browning is accompanied by a loss of S(IV). The kinetics of the reaction of S(IV) in the system: fructose-glycine-S(IV) are described in detail; two distinct mechanisms have been identified. One involves only fructose in the rate determining step. The other requires fructose and both the glycine and S(IV). Some amines (e.g. taurine and ethanolamine) can markedly increase the rate of the S(IV)-independent step. Arginine and lysine are particularly effective for increasing the rate of reaction of S(IV) in the S(IV)-dependent reaction.     

Reaction of melanoidins with sulphur dioxide: stoichiometry of the reaction

Sulphur dioxide, in the form of sulphur(IV) oxoanions(S(IV)), e.g. HSO^-₃, SO^2-₃, reacts irreversibly with non-enzymic browning model melanoidins (M_r > 12000, prepared by combination of 1.25 mol glucose with 1 mol glycine (pH 5.5, 90°C, 22 hr)). When the reaction with S(IV) is carried out for > 39 days at 40°C and pH 5.5 with initial concentrations of melanoidin of 5.71 g/l and S(IV) of 0.0371 M (equivalent to c . 2400 ppm SO₂), up to one sulphur atom is incorporated for every two molecules of glucose used to form the polymer. This stoichiometry is discussed in relation to the degree of unsaturation of the melanoidin.     

Formation of the Heyns rearrangement products in dihydroxyacetone and glycine Maillard reaction: A computational study

Density functional theory computations at the standard state on the proposed mechanisms of dihydroxyacetone and glycine Maillard reaction under different pH conditions have revealed that dihydroxyacetone + deprotonated glycine and dihydroxyacetone + unprotonated glycine reactions are more favorable than dihydroxyacetone + protonated glycine and dihydroxyacetone + glycine zwitterion reactions for the formation of the Heyns rearrangement products (HRPs). The gaseous phase reaction is assumed more feasible than the aqueous phase reaction for the production of HRPs. Due to the possibility of the production of both of the enol and keto forms of HRPs, the rate of browning in the dihydroxyacetone + deprotonated glycine reaction is assumed higher than that of the others. Dihydroxyacetone + protonated glycine and dihydroxyacetone + glycine zwitterion reactions are not favorable for the formation of HRPs and, therefore, the reaction is assumed hindered under these conditions and the rate of browning is supposed to be lower than that of the others. Possibilities for the conversion of dihydroxyacetone to glyceraldehyde and formation of hydroxyacetaldehyde from dihydroxyacetone as a C2-fragmentation product have also been evaluated.     

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

The kinetics of browning (A₄₇₀) 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.     

不同模式体系美拉德反应程度及抗氧化活性比较

以葡萄糖、果糖、蛋氨酸、甘氨酸、精氨酸和大豆分离蛋白为美拉德反应原料,研究加热时间和混合比例对褐变程度的影响,并对褐变程度最强的产物进行抗氧化活性检测。结果表明：葡萄糖、果糖分别与甘氨酸按物质的量之比2:1混合(pH9.0),并于100℃恒温水浴锅中加热180min时褐变程度最强。葡萄糖-甘氨酸与果糖-甘氨酸模式体系的美拉德反应物都具有一定的抗氧化能力。在相同体积分数条件下,果糖-甘氨酸模式体系的美拉德反应物的还原能力和抗氧化值均要强于葡萄糖-甘氨酸模式体系的美拉德反应物。葡萄糖-甘氨酸模式体系的美拉德反应物对 ·OH和O2 ·的抑制率最高分别达50.65%和45.42%,而果糖-甘氨酸模式体系的美拉德反应物对 ·OH和O2 ·的抑制率最高则分别达57.55%和28.54%。     

Kinetics of the reaction between 3-deoxyhexosulose and sulphur(IV) oxospecies in the presence of glycine

The kinetics of the reaction between 3-deoxyhexosulose (DH) and sulphur (IV) oxospecies (S(IV)) in the presence of glycine (0·5m) at pH 5·5 and 55°C are reported. The initial reaction is of first order with respect to DH (k = 0·0164 h⁻¹) and of zero order with respect to S(IV). The presence of glycine increases the rate of reaction whilst S(IV) acts as an inhibitor through formation of the hydroxysulphonate of DH. A kinetically derived value for the dissociation constant of the hydroxysulphonate in the presence of glycine is 0·004m. Possible mechanisms and the application of the results to the S(IV)-inhibited Maillard reaction are considered.     

Rate of maillard browning in sweet whey powder

The objective of this study was to evaluate the rate of Maillard browning in 3 commercial sweet whey powders (WC1, WC2, and MW1), under accelerated shelf-life testing (ASLT) and under normal storage conditions (21 degrees C and 35% RH). Rate of brown pigment formation (k) obtained from short-term ASLT of whey powder was compared with actual findings obtained from the long-term shelf-life testing under normal conditions. Deterioration by Maillard browning, measured by spectrophotometer, was compared with changes in color (Hunter Laboratory), free moisture, titratable acidity, and sensory attributes. Results suggest that estimated k (from ASLT) was comparable with the observed rate (obtained at ambient temperature) for 2 producers (WC1, MW1). The actual k values observed for samples WC1, WC2, and MW1, stored under normal conditions, were 0.0031, 0.0080, and 0.0148 color units/g of solid per mo, respectively. The estimated values of k for samples WC1, WC2, and MW1 were 1.12, 4.90, and 1.35 times more than the observed values, respectively. The Q10 values (increase in reaction rate for a 10 degrees C temperature increase) ranged from 1.77 to 4.14, and the activation energies ranged from 15.9 to 28.4 kcal/mol. Hunter Laboratory values L* and a* appeared most sensitive to changes during storage. Free moisture content, and acidity increased significantly with storage. However, no significant changes were detected by the sensory panel in the attributes considered.     

Effect of pH on non-enzymatic browning reaction during γ-irradiation processing using sugar and sugar–glycine solutions

Effects of pH on the non-enzymatic browning reaction caused by γ-irradiation processing were investigated. The brown colour development of irradiated sugar–glycine solutions was greater in a buffer system than in deionized distilled water (DDW) with higher pH. Although browning of irradiated sugar solution without glycine was highly increased was in alkaline buffer, no browning was developed in DDW. The maximum browning of the solutions of sugar and sugar–glycine were observed at pH 10 and pH 8, respectively. The browning intensity was in the following order: sucrose ? fructose > glucose. Non-constant pH was observed in DDW (dropped pH between 1.58 and 2.03 units); however, the use of buffers was partially effective in keeping pH constant (pH dropped between 0.03 and 0.56 units). When the irradiated solutions of sugar with and without glycine were analyzed using reverse phase high performance liquid chromatography with diode-array detection (HPLC-DAD), three peaks were separated at 3.37 ± 0.04, 4.60 ± 0.02, and 2.53 ± 0.26 min, and the λ_max values of these peaks were the range 259–288 nm. The sum of the areas of these peaks at 260 nm increased with increasing pH. The results of this study indicated that conditions of the system, such as pH and media, can influence the non-enzymatic browning reaction during γ-irradiation.     

Free radical scavenging capacity of Maillard reaction products as related to colour and fluorescence

The free radical scavenging activity of Maillard reaction products (MRPs) produced by heating glucose or lactose with lysine, alanine or glycine was estimated directly by means of a 2,2-diphenyl-1-picrylhydrazyl radical (DPPH·) method. Model systems of sugar (lactose and glucose) and amino acid (alanine, glycine, and lysine) were heated without pH control at 100°C up to 24 h. Fluorescence (347 and 415 nm), browning (absorbance at 420 nm), colour parameters in the CIELab scale, and free radical scavenging activity were determined. Browning is not directly related to the free radical scavenging properties of MRPs formed at prolonged heating conditions. This study shows that fluorescence measurement of heated sugar/amino acid systems is more effective than browning to follow the formation of MRPs with free radical scavenging activities. This approach could be applied to optimise the formation of natural antioxidants during food processing where browning is not desirable for consumers.     

The dissociation constant of the hydroxysulphonate of 3,4-dieoxy-4-sulphohexosulose in the sulphite-inhibited maillard reaction of glucose and glycine

The dissociation constant of the hydroxysulphonate of 3,4-dideoxy-4-sulphohexosulose (DSH) was measured by iodimetric determination of free and reversibly bound sulphur (IV) oxospecies (S(IV) in mixtures of DSH and S(IV). The reaction medium was chosen to simulate the conditions used in the model Maillard system of McWeeny et al. (1969) and contained DSH ($\text{4·3 × 10}{}^{\mathrm{?3}}\text{,}\text{m}$), glycine (0·5 m) and S(IV) (0·0035 – 0·035 m) at pH 5·5 and 55°C. Under these conditions, DSH forms a monohydroxysulphonate with a dissociation constant of $\text{4·4 × 10}{}^{\mathrm{?3}}\text{,}\text{m}$, adding credence to the suggestion that DSH is a major contributor to the increase, with time, in reversible binding of S(IV) in the system glucose-glycine-S(IV).     

Formation of glyoxal in hydroxyacetaldehyde and glycine nonenzymatic browning Maillard reaction: A computational study

Density functional theory (DFT) computations at the standard state on the proposed mechanisms have revealed that glyoxal (Gox) is one of the most possible intermediates in the hydroxyacetaldehyde (Hald) and glycine nonenzymatic browning Maillard reaction under different pH conditions. By following the total mass balance, the gaseous state reaction has been found as more feasible for the formation of Gox than that of the aqueous solution. Hald + deprotonated glycine reaction under basic condition and Hald + unprotonated glycine reaction under neutral condition have been supposed to be more favorable for the production of Gox than Hald + protonated glycine reaction under acidic condition and Hald + glycine zwitterion reaction at the isoelectric point of glycine. Oxidation of Hald to Gox has been found as more plausible in the gaseous state than the aqueous solution. Oxygen has been found as necessary for the production of Gox from Hald. At the standard state, DFT calculations on the proposed mechanisms have evaluated that unprotonated and deprotonated glycine and glycine zwitterion are feasible for liberating NH₃, whereas protonated glycine has been found as unfeasible. DFT computations at the standard state on the proposed mechanisms have also revealed that the gaseous state Hald + NH₃ reaction is more feasible for the formation of Fald than that of the aqueous solution. Water, which is a by-product, has found as necessary for the initiation of both of Hald + glycine and Hald + NH₃ reactions.     

Isolation and characterisation of a wheat albumin

A major albumin from Cappelle-Desprez wheat flour has been isolated by carboxymethyl-cellulose chromatography in a sufficiently pure state for characterisation. The water-soluble protein is free from phenylalanine and histidine, and this enables the purity to be estimated as > 95 %. It appears to consist of a single chain with a mol. wt. of 26,000. The absorbance coefficient at 280 nm is 1.9 × 10³, the high value being due to 8 tryptophan and 8 tyrosine residues per molecule. There are 10 S. S bonds but no sulphydryl groups or sugar residues in the molecule. Sedimentation studies indicate that the molecules tend to aggregate in 0.2 M-NaCl. No protease or amylase activity was detectable.     

Nonenzymatic Browning In Liquid Model Systems of High Water Activity: Kinetics of Color Changes Due to Reaction Between Glucose and Glycine Peptides

The rate of browning of solutions (water activity adjusted to 0.90 by adding sodium chloride) of glucose-glycine peptides (diglycine and triglycine) was well described by a “mixed-order” kinetic model having two rate coefficients, k_o and k_l. Activation energies for these parameters were calculated and compared with values for the same reaction of glucose with the amino acid (glycine). At pH below 6 the rate of browning of peptide solutions was higher than that of the amino acid (glycine) at the same molar concentration; at higher pH the reverse was observed.     

Role of amino acids in non-enzymic browning

Loss of nitrogen in the form of freeamino groups during browning of various aqueous sugar-glycine mixtures has been studied, and the results are reported. It has been found that such loss depends on the type of sugar (pentose, aldohexose, or ketose), and is initially accelerated in the presence of buffering agents. the results also indicate that all the free primary amino nitrogen present during browning of sugarglycine mixtures can be accounted for as glycine. Browning of various saturated and unsaturated aldehydes with glycine has been shown to lead to pigments containing no nitrogen, and the significance of this is discussed in relation to the browning reaction. It is suggested that monoketoseamines are a major intermediate in browning reactions involving loss of amino nitrogen, and an extended reaction-scheme for the browning of aldose-amino systems is presented. the significance of the low loss of amino nitrogen in ketose-glycine systems is discussed in relation to the mechanisms of browning of these systems. Consideration is given to methods used in colour measurement in non-enzymic browning and to the limitations of colour measurement as an index of the extent of reaction.     

美拉德反应产物的褐变、荧光吸收及抗氧化性的研究

采用以还原糖-氨基酸(木糖/葡萄糖分别与甘氨酸、赖氨酸、谷氨酰胺、半胱氨酸)模式美拉德反应(Maillard Reaction,MR)制备得到美拉德反应产物(MRPs),研究反应条件对MRPs的褐变程度、荧光吸收强度以及其抗氧化活性的影响,并分析MRPs的光学特征与抗氧化活性之间的联系。结果表明,还原糖和氨基酸的种类对MR的褐变强度有很大影响,其影响程度由强到弱分别为:Xly>Glc,Lys>Gly>Glu>Cys。随着反应时间的延长,各体系MRPs的褐变程度逐渐变强;而荧光吸收强度随着加热时间的延长而增强,然后在出现最大值后趋于平缓或者表现出下降的趋势。MRPs的抗氧化能力也随反应时间的延长而增强,抗氧化活性与褐变强度呈正相关,与荧光吸收特性没有表现出明显的关联。     

Formation of pyrazines in hydroxyacetaldehyde and glycine nonenzymatic browning Maillard reaction: A computational study

By considering the formation of pyrazines (C₄N₂H₆ and C₄N₂H₄) as one of the possible final Maillard flavour compounds, Density Functional Theory computations at the standard state have been performed on the proposed mechanisms of glyoxal and glycine in the advanced stage of hydroxyacetaldehyde and glycine nonenzymatic browning reaction under different pH conditions. The results reveal that the basic condition is the most favourable for the production of pyrazines (Pzs), and the aqueous solution is more favourable than that of the gaseous state. The reactions at the isoelectric point of glycine and under neutral conditions are the second and third most favourable for the production of Pzs, respectively. The reaction under acidic conditions is the least feasible for the production of Pzs. Amino acetaldehyde is the most likely precursor of the pyrazine ring in the reaction. Presence of air or oxygen is necessary for the production of 2,3,5,6-tetrahydropyrazine (C₄N₂H₄) from 3,6-dihydropyrazine (C₄N₂H₆). Water is necessary with glyoxal and glycine species for the formation of Pzs and water is produced as a by-product during the formation of Pzs.     

不同贮藏温度下鲜切紫甘薯褐变相关因素研究

为研究鲜切紫甘薯的褐变条件与机理,本文对不同贮藏温度下(4和12℃)的鲜切紫甘薯的褐变程度、褐变相关酶活力以及褐变底物酚类物质进行分析。结果表明,12℃贮藏组的鲜切紫甘薯褐变度始终显著高于4℃贮藏组(P<0.05)。贮藏期间,褐变重的鲜切紫甘薯多酚氧化酶(Polyphenol oxidase,PPO)和过氧化物酶(Peroxidase,POD)活性更高,丙二醛(Malondialdehyde,MDA)含量更多。2~8 d,不同贮藏温度下鲜切紫甘薯绿原酸和阿魏酸含量显著不同(P<0.05),因此推断褐变相关酶活性、膜脂过氧化程度以及酚类物质含量可能是引起鲜切紫甘薯褐变的关键因素。通过相关性分析,多酚氧化酶(PPO)和过氧化物酶(POD)是引起鲜切紫甘薯褐变的关键酶。绿原酸和阿魏酸是鲜切紫甘薯褐变的主要底物。     

Kinetics of the Maillard Reaction Between Aspartame and Glucose in Solution at High Temperatures

Comparison was made of the extent of browning during accelerated storage tests of glucose/aspartame and glucose/glycine model systems under steady state conditions of 70, 80, 90, and 100°C and an a_w of 0.80. Browning of aspartame followed zero order kinetics with a time to 0.1 absorbance units at 420 nm of 11.40, 5.3, 2.15 and 1.0 hr for each respective temperature. The activation energy (E_A) was 22.0 Kcal/mole for the glucose/aspartame system and 15.5 Kcal/mole for the glucose/glycine system. The temperature sensitivities (Q₁₀) for the model systems were 2.4 and 1.9, respectively. The predicted shelf life to reach 0.1 absorbance units in an aqueous system at 45°C is 62 days compared to the actual value of 60 days.     

Technical note: Effect of low pH preservation on the colour and consistency of tomato juice

The changes in colour and consistency are reported for tomato juice produced by low pH preservation and stored under ambient conditions. The juice preserved under acid conditions showed higher consistency, probably due to the higher concentration of pectin and the presence of low methoxyl pectin. The furfural and hydroxymethyl-furfural which were detected in the juice indicated that darkening of the juice colour was due to non-enzymic browning involving both ascorbic acid and sugars (hexoses). Although the retention of ascorbic acid was higher under low pH conditions, it led to a significant darkening of the juice colour due to non-enzymic browning involving mainly ascorbic acid.