The Maillard reaction. 18. Radical inhibition of browning by sulfur compounds

ESR spectroscopic investigations on the mechanism of Maillard-reactions using the model D-glucose/glycine show radical formation in some of the reaction steps. These are possibly due to competitive reactions against ionic mechanism. The formation of radicals depends on the concentration of reactants and reaches a maximum in thermically induced browning reactions. The addition of L-cysteine leads to a decrease in the radical concentration to about 60%. It can be indirectly proven that the radicals are involved in the formation of browning products. The hyperfine structures of ESR spectra are supposed to be similar to pyrazine cation radicals.     

The Maillard reaction between beta-lactoglobulin and lactose. 2. Effect of thiol and disulfide groups on the properties of the macromolecular reaction products]

If a protein takes part in the Maillard reaction as the amino component, other functional groups in the protein molecule are also expected to react in addition under appropriate conditions. The beta-lactoglobulin-lactose system, first used by Freimuth and Trübsbach, served to demonstrate that the properties of the products from the Maillard reaction are affected by thiol and disulphide groups in a low-water medium at high temperature. The amount, solubility, swelling capacity and composition are modified in dependence of the presence or absence of these reactive groups.     

Kinetic aspects of the Maillard reaction: a critical review.

The literature concerning the kinetics of the Maillard reaction was critically discussed according to the initial, intermediate and advanced stages, as this is the way the Maillard reaction is traditionally analysed. For each stage, a division is made between simple kinetics and complex kinetics. Simple kinetics means that the general rate law is used and results are reported as zero-, first- or second-order reactions (sometimes a fractional order). It is emphasized that this approach for a complex reaction as the Maillard reaction only results in a mathematical fit procedure, not in mechanistic insight. The rate constants and activation energies derived are in fact composed of several elementary rate constants. With complex kinetics, i.e. trying to establish the kinetics for individual reaction steps, more mechanistic information can be extracted. However, there are conflicting results in literature and the interpretation is not always correct, as is shown in several examples. A summary of activation energies reported for the various stages in the Maillard reaction reveals large discrepancies, probably reflecting strong effects of experimental conditions on results that are obtained. Careful control of experimental conditions and proper kinetic analysis of the various stages in the Maillard reaction should lead to more consistent results in the future.     

Deterioration of protein fraction by Maillard reaction in dietetic milks.

The development of the Maillard reaction in pasteurized, UHT and in-bottle sterilized dietetic milks was studied. In these products damage caused by heat treatments could increase as a result either of the addition of various ingredients or of manufacturing processes that alter their content of reducing carbohydrates. Protein damage was evaluated by measuring furosine by reversed-phase ion-paired HPLC. The levels of furosine detected made it possible to assess the amounts of biologically unavailable lysine. In all milks analysed blocked lysine values were < 340-350 mg/g total lysine, the level at which lysine becomes the limiting amino acid in milk. Pasteurized dietetic milks had levels of blocked lysine similar to that in ordinary pasteurized cows' milk. In some UHT and in-bottle sterilized dietetic milks their different composition resulted in an increase in the blocked lysine content. In some in-bottle sterilized milks, protein damage greatly reduces the beneficial effects of milk as a dietary supplement. Lactose-free milks, which are more susceptible to protein deterioration because of their higher content of reducing carbohydrates, were also analysed after storage at 20 degrees C and at < or = 4 degrees C. At the end of their recommended storage times, they contained limited amounts of blocked lysine only if they had been stored at < or = 4 degrees C.     

Kinetic study on the Maillard reaction. Consideration of sugar reactivity

The aim of the present study was to compare five reducing sugars (ribose, xylose, arabinose, glucose and fructose) with respect to their relative reactivity in the Maillard reaction (MR) (55 °C, pH 6.5) with a shrimp hydrolysate. For each system, the extent of the MR was assessed for a 24-h period by monitoring browning intensity, free amino group disappearance and sugar consumption. Results revealed the prevailing propensity of pentoses over hexoses to react in the MR, with a distinguishable behaviour for the ribose-hydrolysate system. Interestingly, pentoses were shown to mainly differ in terms of reaction rate. Complementary data on the chemical composition of the MR products (MRPs) were provided by size-exclusion chromatography, thus demonstrating the occurrence of some molecular rearrangements detected at 294 nm. The modification of protein substrates through the MR could represent a key step in the formation of new molecules and constitute a promising means to produce high value-added ingredients with biological and techno-functional properties.     

Kinetics of the reaction of sorbic acid with sulphite species.

Under aerobic conditions, the reaction of sorbic acid with sulphite species, S(IV), involves a pH-dependent oxidative mechanism with the loss of S(IV). Sorbic acid is essential to the process. In the absence of air, a much slower nucleophilic addition of sulphite ion to undissociated sorbic acid takes place.     

Application of the plastein reaction to mycoprotein: I. Plastein synthesis

The plastein reaction has been applied for the first time to mycoprotein following in situ digestion of crude material to solubilise protein and produce the required initial peptide mixture. Pepsin was found to be superior to the other proteinases tested (trypsin, chymotrypsin, papain, Streptomyces griseus proteinase) in the plastein synthesis step although in all cases yields of insoluble plastein were low (10–15%). Unusually, plastein yield was not much influenced by pH over the range 3.0–7.5, but percentage yield increased almost linearly over the peptide concentration range 11–43% (w/w) while the absolute yield of plastein increased exponentially over this range. Incubation at different temperatures showed that the rate of plastein formation was highest at 65°C, reaching a maximum in 4–5 h but, although not reaching maximum yield for 24 h, a temperature of 37°C gave approximately 25% greater yield. In general terms, the results suggested that mycoprotein peptides represented a rather poor substrate for plastein synthesis and only opaque, viscous solutions were obtained rather than the more common thixotropic gel structures.     

Application of the plastein reaction to mycoprotein: II. Plastein properties

Mycoprotein peptide hydrolysates have been used as starting materials for plastein synthesis. Compared to other proteins studied, mycoprotein hydrolysates were a relatively poor substrate for the plastein reaction and generally led only to thixotropic viscous solutions, rather than to gelled products, and only low yields of insoluble plastein material. Once formed, however, the insoluble fraction remained insoluble over the whole pH range of 2–11. In contrast to many other plasteins, the mycoprotein material was not solubilised by detergents such as sodium dodecyl sulphate although, like others, it was largely solubilised by 50% (v/v) organic acids or 1 M NaOH and partially solubilised by chaotropic agents such as 8 M urea, 6 M guanidinium chloride and 7 M potassium thiocyanate. A combination of 8 M urea and 50% (w/v) citric acid completely solubilised the plastein to a clear solution. Gel filtration failed to reveal any change in peptide molecular weight distribution on plastein formation while ion-exchange chromotography showed some quantitative differences but these were difficult to interpret as most of the material did not adhere to the anion-exchange column under the conditions used. Amino acid analysis revealed a marked preferential incorporation of hydrophobic peptides into the plastein fraction. Differential scanning calorimetry results showed only broad peaks which suggested heterogeneous reaction mixtures and products with no well-defined structural elements. These results are entirely consistent with plastein formation proceeding via a purely physical aggregation pathway.     

News on the Maillard reaction of oligomeric carbohydrates: a survey.

The reaction behaviour of monosaccharides in the Maillard reaction is well investigated. Amadori compounds, for instance, form deoxyhexosuloses which are responsible for the formation of volatile flavour substances and melanoidins. These intermediates can be quantified by a trapping reaction with o-phenylendiamine as stable quinoxalines. However the reaction behaviour of oligo and polymeric carbohydrates in non-enzymatic browning reactions are hardly known. Therefore maltooligosaccharides and in some cases starch were used together with glycine as model substances to investigate the Maillard reaction mechanism of oligo and polysaccharides. In quasi waterfree reaction systems oligosaccharides form alpha-dicarbonyl compounds via a "peeling off" mechanism. The Maillard reaction with the amino compound starts at the reducing end of an alpha-glucane residue and results in the formation of 1,4-dideoxyhexosulose. This is the main alpha-dicarbonyl compound found by trapping reaction with o-phenylendiamine during thermally induced degradation of maltooligosaccharides. The thermal reaction of maltooligosaccharides is accompanied by transglycosylations leading to formation of branched carbohydrate structures and by dehydrations forming anhydrosugars. The reaction mechanism which is responsible for degradation of oligomeric carbohydrates in aqueous model systems differs significantly from pathways mentioned above for water free reaction conditions. As the main alpha-dicarbonyl compound 3-deoxypentosulose is formed which can be proved also by trapping reactions. Under aqueous reaction conditions the formation of other alpha-dicarbonyls plays only a marginal role in the Maillard reaction of oligomeric carbohydrates. In an aqueous system transglycosylation and dehydration of maltooligosaccharides are not important.     

Detection of Salmonella spp. in tropical seafood by polymerase chain reaction

The incidence of Salmonella spp. in tropical seafood was studied using standard microbiological techniques and polymerase chain reaction (PCR). Six of 20 finfish (30%), 4 of 20 clams (20%) and 1 of 20 shrimp (5%) were positive by culture techniques and by PCR. In a comparative study of different selective enrichment broths and selective plating media, more than one enrichment broth and selective agar were found to be necessary for efficient detection of Salmonella from seafood. Selenite cystine broth (SCB) was found to be more efficient compared to tetrathionate broth (TTB) while both bismuth sulfite agar (BSA) and hektoen enteric agar (HEA) were equally effective as selective plating media for fish. In the case of clams, HEA was found to be more effective. The presence of Salmonella spp. could be detected by PCR amplification of DNA extracted directly from the enrichment broths. In two cases, enrichment broths that were positive by PCR did not yield Salmonella by conventional methods.     

Kinetics and mechanism of the reaction between 3-deoxyhexosulose and thiols.

The kinetics of the reaction of 3-deoxyhexosulose, DH, with mercaptoethanol, ME, and glutathione, GSH, resemble those of the DH-sulphite reaction, but the stoichiometry of the DH-thiol reaction is 1:2 unlike that of the DH-sulphite reaction which is 1:1. However, the rate determining step in all these reactions is the spontaneous conversion of DH to a reactive intermediate, followed by a rapid reaction of this intermediate with the nucleophile. This is also true of the reaction between DH and N-acetylcysteine, NAC, but this thiol is less reactive than ME or GSH and less than one mole of NAC reacts with each mole of DH. Evidence for instability of NAC at pH 5.5 is presented. Aminothiols (cysteine, homocysteine, cysteamine) undergo a fast reaction with DH followed by a slow release of thiol. The initial reaction is probably formation of a thiazolidine. In the case of cysteine and homocysteine it is suggested that the subsequent slow step is a Strecker degradation reaction. The kinetic behaviour of thiols in cabbage homogenates is reported.     

Photooxidation of nitrosylmyoglobin at low oxygen pressure. Quantum yields and reaction stoechiometries

Photooxidation of the nitrite-cured meat pigment, nitrosylmyoglobin, in aqueous solution saturated with a 20% CO₂/80% N₂ gas mixture with varying oxygen contents (0.1, 0.5 or 1.0%) was found to depend linearly on the oxygen content for both visible (436 nm) and UV-light (366 nm). Quantum yields were similar for the two wavelengths of excitation in agreement with previous findings at higher oxygen pressures. The reaction stoechiometry for photooxidation was different from that of thermal oxidation (investigated at the same oxygen pressures) with a unity MbFe(II)NO/O₂ ratio in the thermal reaction and a ratio larger than one for the photooxidation. For cured meat products packed in modified atmospheres light exposure may be even more harmful for the oxidative stability than expected from the concentration of residual oxygen.     

Evaluating acrosome reaction steps with brightfield and differential interference contrast microscopy techniques.

Acrosomal integrity of bovine spermatozoa was evaluated using naphthol yellow S-erythrosine B stain with bright-field microscopy, .2% glutaraldehyde fixation with Nomarski optics, and live wet smears with Nomarski optics to compare the evaluation techniques. Four ejaculates per two Holstein bulls were enriched through a Percoll gradient, capacitated with heparin, and acrosome-reacted with lysophosphatidylcholine to prepare spermatozoa for in vitro fertilization. Spermatozoa samples were taken after each preparatory step, and percentage of intact acrosomes was evaluated with the stain, .2% glutaraldehyde, and live wet smear. Acrosomal integrity decreased with each preparatory step across all techniques. The decrease in integrity from heparin to lysophosphatidylcholine indicated that all methods detected the acrosome reaction. Technique means across preparation steps showed no differences. Correlations between microscopy techniques were high. The live wet smear used no fixation methods. Both the naphthol yellow S-erythrosine B stain and the .2% glutaraldehyde techniques employed fixation steps that may have introduced artifacts that could influence the data. The live wet smear evaluated by Nomarski optics allowed a comparable evaluation of the acrosome-reacted bovine spermatozoa. Comparing brightfield with differential interference contrast microscopy for detecting the acrosome reaction in bovine spermatozoa showed that both methods were equally accurate.     

生化反应中一类多分子反应的非线性分析

研究了生化反应中一类多分子反应方程:x=1-αx-xy,y=β(xy-y)(α>0, β>0,q≥2).主要讨论了平衡点的稳定性态,系统无环的充分条件以及极限环的存在性.     

不同反应条件下的Maillard反应产物对卷烟燃吸品质的影响研究

制备了在不同反应条件下混合氨基酸和葡萄糖的Maillard反应产物,并把它们加入空白烟丝中进行详细的感观评价,在该反应体系中,反应温度为105℃左右,pH为10左右,反应时间5 h左右条件下的MaiⅡard反应产物对卷烟加香效果较好.     

反应温度和反应时间对抗坏血酸-半胱氨酸模式反应形成香味化合物的影响

以固相微萃取-气相色谱-质谱联用(SPME-GC-MS)技术对抗坏血酸与半胱氨酸(ASA-Cys)的模式反应产物进行鉴定,研究了酸性条件(pH5.00)下反应温度和反应时间对ASA-Cys模式反应形成香味化合物的影响。模式反应的早期阶段主要是ASA和Cys的热降解,此段对反应温度的依赖性很大。140℃的反应温度能使ASA和Cys快速热降解,从而加快Maillard反应的进行,形成风味较佳的肉香味。在此温度下,反应150min,模式反应形成的各类香味化合物的量达到最大,继续延长反应时间,各类香味化合物的量逐渐减少。结果表明140℃的反应温度以及该温度下150min的反应时间对模式反应形成香味化合物是合适的。     

Reaction of sorbic acid in wheat flour doughs: reaction with thiols.

The general characteristics of the reaction between sorbic acid and thiols are reviewed. Cysteine adds to the conjugated diene in position 5 to form the substituted 3-hexenoic acid. This is labile in acid solution, yielding a quantitative amount of sorbic acid. When wheat flour doughs are treated with sorbic acid and heated, a significant amount of the sorbic acid is not recovered on extraction with methanol. The use of acidified methanol leads to a quantitative recovery of the preservation and evidence is presented to suggest that sorbic acid-thiol adducts are formed. This is the first report of 'reversibly bound' sorbic acid in a food.