High-pressure effects on Maillard reaction between glucose and lysine

Glucose-lysine model systems prepared over a range of pH values (5-10) in unbuffered and buffered media were incubated at 60 degrees C either under atmospheric pressure or at 400 MPa. The results obtained showed that high pressure affected in different ways the different stages of the Maillard reaction and that such effects were strongly influenced by pressure-induced changes in the pH of the systems. In unbuffered media, at an initial pH < or =8.0, the formation of Amadori rearrangement products (ARP) was not considerably affected by pressure, whereas the intermediate and advanced stages of the Maillard reaction were suppressed, suggesting a retardation of the degradation of the ARP. In buffered media, at pH values < or =8.0, pressure slowed the Maillard reaction from the initial stages. These effects are attributed to the pH drop caused by the pressure-induced dissociation of the acid groups. In unbuffered and buffered media at initial pH = 10.2, high pressure accelerated the formation and subsequent degradation of ARP, leading to increased levels of intermediate and advanced reaction products.     

Impact of pH on the kinetics of acrylamide formation/elimination reactions in model systems

The effect of pH on acrylamide formation and elimination kinetics was studied in an equimolar (0.1 M) asparagine-glucose model system in phosphate or citrate buffer, heated at temperatures between 120 and 200 degrees C. To describe the experimental data, a simplified kinetic model was proposed and kinetic parameters were estimated by combined nonlinear regression and numerical integration on the data obtained under nonisothermal conditions. The model was subsequently validated in a more realistic potato-based matrix with varying pH. By increasing acidity, the reaction rate constants at T(ref) (160 degrees C) for both acrylamide formation and elimination can significantly be reduced, whereas the temperature dependence of both reaction rate constants increases. The introduction of a lyophilized potato matrix (20%) did not affect the acrylamide formation reaction rate constant at reference temperature (160 degrees C) as compared to the asparagine-glucose model system; the elimination rate constant at T(ref), on the contrary, was almost doubled.     

Toward a kinetic model for acrylamide formation in a glucose-asparagine reaction system

A kinetic model for the formation of acrylamide in a glucose-asparagine reaction system is pro-posed. Equimolar solutions (0.2 M) of glucose and asparagine were heated at different temperatures (120-200 degrees C) at pH 6.8. Besides the reactants, acrylamide, fructose, and melanoidins were quantified after predetermined heating times (0-45 min). Multiresponse modeling by use of nonlinear regression with the determinant criterion was used to estimate model parameters. The proposed model resulted in a reasonable estimation for the formation of acrylamide in an aqueous model system, although the behavior of glucose, fructose, and asparagine was slightly underestimated. The formation of acrylamide reached its maximum when the concentration of sugars was reduced to about 0. This supported previous research, showing that a carbonyl source is needed for the formation of acrylamide from asparagine. Furthermore, it is observed that acrylamide is an intermediate of the Maillard reaction rather than an end product, which implies that it is also subject to a degradation reaction.     

Effect of high-pressure treatment on lipoxygenase activity

Solutions of commercial soybean lipoxygenase (100 microgram/ML in 0.2 M citrate-phosphate and 0.2 M Tris buffer were subjected to pressures of 0.1, 200, 400, and 600 MPa for 20 mm. The enzyme was stable at atmospheric pressure (0.1 MPa) over a wide pH range (5-9). In citrate phosphate buffer, the enzyme had maximum stability over the pH range 58 in untreated samples and after treatment at 200 MPa, but with increasing pressure, the pH stability range become narrower and centered around pH 78. The enzyme was more sensitive to acid than alkali, and at pH 9, it lost virtually all activity after pressurization at 600 MPa for 20 mm in both buffers. The activity of the crude enzyme extracted from tomatoes treated at 200 and 300 MPa for 10 mm was not significantly different from that of the untreated tomatoes, while a pressure of 400 MPa for 10 mm caused a significant decrease in activity and treatment at 600 MPa led to complete and irreversible activity loss. Compared to unpressurized tomatoes, treatment at 600 MPa gave significantly reduced levels of hexanal, cis-3-hexenal, and trans-2-hexenal, which are important contributors to "fresh" tomato flavor, and this was attributed to the inactivation of lipoxygenase.     

Effect of high pressure on isomerization and degradation of lactose in alkaline media

The combined effects of temperature (60 degrees C) and high-pressure treatments (400 MPa for 3 h) on the isomerization-degradation of lactose (10%) in basic media were studied. The formation of isomeric disaccharides (lactulose and epilactose) and galactose decreased by the application of high pressure in aqueous sodium hydroxide 4 mM (pH 10.2) and 8 mM (pH 10.6), and sodium carbonate-bicarbonate buffer (pH 10.0). In addition, no substantial color development was observed in the sodium hydroxide systems for high and atmospheric pressure, whereas the application of high pressure led to a noticeable decrease of color development in the carbonate-bicarbonate system.     

Effect of ultrahigh hydrostatic pressure on the activity and structure of mushroom (Agaricus bisporus) polyphenoloxidase

High hydrostatic pressure (HHP, treatment pressure ≤700 MPa) is approved to be the most successful commercial nonthermal processing due to its minimal modifications in nutritional and sensory quality. However, for some pressure stable enzymes such as PPO, this unique technology can hardly inactivate them at treatment pressure below of 700 MPa. This study investigated the effects of ultrahigh hydrostatic pressure (UHHP, treatment pressure >700 MPa) on the activity of Agaricus bisporus mushroom polyphenoloxidase (PPO) both in the phosphate buffer and in the mushroom puree, and on the structure of the enzyme by means of circular dichroism (CD), fluorescence emission spectra, and sulphydryl group detection. The results showed that UHHP treatment at pressure from 800 to 1600 MPa caused significant inactivation on the PPO both in the phosphate buffer and in the mushroom puree. UHHP treatment at 1400 and 1600 MPa for 1 min reduced the enzyme activity by 90.4% and 99.2% in the buffer;, however, higher enzyme activity remained in the puree after UHHP treatment at the same condition. CD and fluorescence spectra analysis showed that the secondary and tertiary structures of UHHP treated mushroom PPO were changed. The sulphydryl group (SH) detection revealed that the SH content on the surface of UHHP treated mushroom PPO was increased. It has been suggested that the inactivation of mushroom PPO by UHHP treatment at pressure higher than 1000 MPa was due to the synergistic effect of the pressure and the heat arising from pressurization, in which heat plays a major role.     

超高压处理对南美白对虾在冷藏过程中贮藏特性的影响

该文以不同的超高压条件(200,400,600,700MPa)对南美白对虾进行处理,通过考察与比较不同压力处理组冷藏过程中感官、微生物以及各化学指标的变化情况,揭示超高压技术对南美白对虾冷藏特性的影响。结果表明:超高压处理可以有效地杀灭南美白对虾中绝大多数微生物,抑制贮藏过程中挥发性盐基氮的积累,延缓pH值的变化,从而延长南美白对虾的货架期,且处理压力越高延长效果越显著;超高压处理会给鲜虾带来不同程度上的煮熟虾的风味;400MPa和600MPa处理使虾在冷藏过程的黑变提前,而700MPa处理可以完全抑制南美白对虾黑变现象的发生;超高压能够改变腺苷三磷酸(ATP)及其代谢产物的代谢情况,但不影响腺苷酸(AMP)的代谢途径。因此超高压处理可以改善南美白对虾在冷藏过程中的品质,对南美白对虾冷藏具有潜在的应用价值。     

Rye Flour Extraction Rate Affects Maillard Reaction Development,Antioxidant Activity,and Acrylamide Formation in Bread Crisps

This report shows the effect of rye flour extraction rate on Maillard reaction, antioxidant activity, and acrylamide formation during toasting of rye bread crisps. Four rye flours with extraction rates of 70, 85, 95, and 100% were tested. Maillard reaction development was studied by measuring browning development, hydroxymethylfurfural (HMF), and glucosilisomaltol (GIM) formation, as well as antioxidant activity. Results showed that HMF and GIM concentrations in toasted bread crisps were higher as the flour extraction rate increases. Antioxidant activity increased during toasting as a consequence of antioxidant Maillard reaction product formation. Acrylamide concentration was clearly affected by free asparagine content of flour, while no effect of dietary fiber and natural antioxidant content of flours had an effect on acrylamide formation. Overall data suggest that the rate of Maillard reaction is higher in whole flours because of their higher free amino acid and protein content.     

Correlation of acrylamide generation in thermally processed model systems of asparagine and glucose with color formation, amounts of pyrazines formed, and antioxidative properties of extracts

The relations between the formation of acrylamide and color, pyrazines, or antioxidants in an asparagine/d-glucose browning model system under various conditions were investigated. The highest level of acrylamide was produced in the asparagine/glucose (1:3) system heated at 170 degrees C for 30 min (2629 microg/g asparagine). Color intensity increased with temperature and heating time. The formation of pyrazines increased steadily with an increase of temperature (140-170 degrees C) and heating time (15-60 min). Antioxidant formation varied among the samples heated under different conditions. A clear correlation between formation of acrylamide and browning color was obtained. The formation of acrylamide was linearly correlated with the formation of total pyrazines during the initial stages of the Maillard reaction. No obvious correlation between formation of acrylamide and antioxidants was observed. However, excess amounts of asparagine increased the formation of antioxidants, whereas excess amounts of glucose reduced its formation.     

Impact of harvest year on amino acids and sugars in potatoes and effect on acrylamide formation during frying

Acrylamide is formed via the Maillard reaction between reducing sugars and asparagine in a number of carbohydrate-rich foods during heat treatment. High acrylamide levels have been found in potato products processed at high temperatures. To examine the impact of harvest year, information on weather conditions during growth, that is, temperature, precipitation, and light, was collected, together with analytical data on the concentrations of free amino acids and sugars in five potato clones and acrylamide contents in potato chips (commonly known as crisps in Europe). The study was conducted for 3 years (2004-2006). The contents of acrylamide precursors differed between the clones and the three harvest years; the levels of glucose were up to 4.2 times higher in 2006 than in 2004 and 2005, and the levels of fructose were 5.6 times higher, whereas the levels of asparagine varied to different extents. The high levels of sugars in 2006 were probably due to the extreme weather conditions during the growing season, and this was also reflected in acrylamide content that was approximately twice as high as in preceding years. The results indicate that acrylamide formation is dependent not only on the content and relative amounts of sugars and amino acids but also on other factors, for example, the food matrix, which may influence the availability of the reactants to participate in the Maillard reaction.     

Conjugation of bovine serum albumin and glucose under combined high pressure and heat

The effect of combined heat and pressure on the Maillard reaction between bovine serum albumin (BSA) and glucose was investigated. The effects in the range of 60-132 °C and at 0.1-600 MPa on the lysine availability of BSA were investigated at isothermal/isobaric conditions. The kinetic results showed that the protein-sugar conjugation rate increased with increasing temperature, whereas it decreased with increasing pressure. The reaction followed 1.4th order kinetics at most conditions investigated. A mathematical model describing BSA-glucose conjugation kinetics as a function of pressure and temperature is proposed. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry were used to verify BSA-glucose conjugation and to identify the glucosylated sites. These indicated that the application of combined high pressure and high temperature resulted in significant differences in the progression of the Maillard reaction as compared to heat treatments at atmospheric pressure.     

Formation of 2,5-dimethyl-4-hydroxy-3(2H)-furanone through methylglyoxal: a Maillard reaction intermediate

The caramel-like aroma compound, 2,5-dimethyl-4-hydroxy-3(2H)-furanone (DMHF) was quantified and verified by HPLC and GC-MS in the Maillard reaction based on methylglyoxal (MG). The reaction was performed in the 0.5 M phosphate buffer by heating MG with or without either glycine or cysteine at 120 degrees C for 1 h. MG alone or MG with cysteine could produce increased level of DMHF with pH increased, whereas MG with glycine had contrary trend. Experiments using a 1:1 mixture of [(13)C6]glucose and [(12)C6]glucose indicate that in the presence of glycine or cysteine, glucose skeleton kept intact during DMHF formation since a 1:1 mixture of [(13)C6]DMHF and [(12)C6]DMHF was formed. Acetylformoin was detected in the glucose with amino acid reaction system as a precursor of DMHF, while in the MG reaction systems, acetylformoin could not be identified. It is suggested different pathways of DMHF formation via MG and glucose.     

Influence of thermally processed carbohydrate/amino acid mixtures on the fermentation by Saccharomyces cerevisiae

The production of alcoholic beverages such as Tequila, Mezcal, whiskey, or beer includes the fermentation of a mash containing Maillard reaction products. Because excessive heating of the mash can lead to complications during the following fermentation step, the impact of Maillard products on the metabolism of Saccharomyces cerevisiae was investigated. For this purpose, fermentation was carried out in a model system in the presence and absence of Maillard reaction products and formation of ethanol served as a marker for the progression of fermentation. We found that increasing amounts of Maillard products reduced the formation of ethanol up to 80%. This effect was dependent on the pH value during the Maillard reaction, reaction time, as well as the carbohydrate and amino acid component used for the generation of Maillard reaction products. Another important factor is the pH value during fermentation: The inhibitory effect of Maillard products was not detectable at a pH of 4 and increased with higher pH-values. These findings might be of relevance for the production of above-mentioned beverages.     

On the effect of tetraborate ions in the generation of colored products in thermally processed glycine-carbohydrate solutions

The effect of tetraborate ions on Maillard browning was investigated in a series of monosaccharide-glycine reactions in aqueous bis-tris buffer at pH 7.2. Addition of borax (sodium tetraborate) in catalytic amounts led to enhanced browning measured by absorbance at 420 nm in the order xylose > arabinose > galactose approximately = fructose > ribose > mannose > rhamnose, and the degree of browning with borax was uniformly greater than that produced by phosphate on an equimolar basis. A mechanism is proposed for borax catalysis in which monosaccharide-borate complexation shifts carbohydrate equilibria to favor open-chain (carbonyl) forms, thereby enhancing the rate of the Maillard reaction.     

In-depth mechanistic study on the formation of acrylamide and other vinylogous compounds by the maillard reaction

The formation of acrylamide was studied in low-moisture Maillard model systems (180 degrees C, 5 min) based on asparagine, reducing sugars, Maillard intermediates, and sugar degradation products. We show evidence that certain glycoconjugates play a major role in acrylamide formation. The N-glycosyl of asparagine generated about 2.4 mmol/mol acrylamide, compared to 0.1-0.2 mmol/mol obtained with alpha-dicarbonyls and the Amadori compound of asparagine. 3-Hydroxypropanamide, the Strecker alcohol of asparagine, generated only low amounts of acrylamide ( approximately 0.23 mmol/mol), while hydroxyacetone increased the acrylamide yields to more than 4 mmol/mol, indicating that alpha-hydroxy carbonyls are much more efficient than alpha-dicarbonyls in converting asparagine into acrylamide. The experimental results are consistent with the reaction mechanism based on (i) a Strecker type degradation of the Schiff base leading to azomethine ylides, followed by (ii) a beta-elimination reaction of the decarboxylated Amadori compound to afford acrylamide. The beta-position on both sides of the nitrogen atom is crucial. Rearrangement of the azomethine ylide to the decarboxylated Amadori compound is the key step, which is favored if the carbonyl moiety contains a hydroxyl group in beta-position to the nitrogen atom. The beta-elimination step in the amino acid moiety was demonstrated by reacting under low moisture conditions decarboxylated model Amadori compounds obtained by synthesis. The corresponding vinylogous compounds were only generated if a beta-proton was available, for example, styrene from the decarboxylated Amadori compound of phenylalanine. Therefore, it is suggested that this thermal pathway may be common to other amino acids, resulting under certain conditions in their respective vinylogous reaction products.     

Acrylamide formation from asparagine under low moisture Maillard reaction conditions. 2. Crystalline vs amorphous model systems

The formation of acrylamide was investigated in model systems based on asparagine and glucose under low moisture Maillard reaction conditions as a function of reaction temperature, time, physical state, water activity, and glass transition temperature. Equimolar amorphous glucose/asparagine systems with different water activities were prepared by freeze drying and were shown to quickly move to the rubbery state already at room temperature and a water activity of above 0.15. The acrylamide amounts were correlated with physical changes occurring during the reaction. Pyrolysis and kinetics of acrylamide release in amorphous and crystalline glucose/asparagine models indicated the importance of the physical state in acrylamide formation. In amorphous systems, acrylamide was generated in higher concentrations and at lower temperatures as compared to the crystalline samples. Time and temperature are covariant parameters in both systems affecting the acrylamide formation by thermal processes. On the other side, the water activity and glass transition temperature do not seem to be critical parameters for acrylamide formation in the systems studied.     

Changes in sulfhydryl content of egg white proteins due to heat and pressure treatment

The sulfhydryl (SH) content of egg white proteins (10% v/v or 9.64 mg of protein/mL) after heat (50-85 degrees C) and combined heat- and high-pressure treatments (100-700 MPa, 10-60 degrees C) was determined using 5',5-dithiobis (2-nitrobenzoic acid) (DTNB), both for the soluble fraction and the total protein fraction. Only irreversible changes were taken into account. Both physical treatments were performed at two pH levels: pH 7.6, corresponding to the pH of fresh egg white, and pH 8.8, corresponding to that of aged egg white. Both heat and combined heat- and high-pressure treatment resulted in an exposure of buried SH groups. These exposed SH groups were involved in the formation of disulfide bond stabilized protein aggregates, as shown by gel electrophoresis. Under severe processing conditions (above 70 degrees C at atmospheric pressure or above 500-600 MPa, depending on the temperature applied), a decrease in total SH content could be observed, probably due to the formation of disulfide bonds by oxidation, especially at alkaline pH when the thiolate anion was more reactive. The high degree of exposure of sulfhydryl groups, and subsequent oxidation and sulfhydryl-disulfide bond exchange reactions resulting in soluble aggregates, can explain why pressure-induced egg white gels are softer and more elastic than heat-induced ones. When pressure treatment was performed at low temperatures (e.g., 10 degrees C), a lower pressure was required to induce similar changes in the sulfhydryl content, as compared to higher temperatures (e.g., 25 degrees C), indicating an antagonistic effect between pressure and temperature in the domain studied (10-60 degrees C, 100-700 MPa). Treatment conditions resulting in extensive protein insolubilization were accompanied by a transfer of free sulfhydryl groups from the soluble to the insoluble protein fraction. These SH groups were mainly accessible to DTNB.     

Formation of sulfur aroma compounds in reaction mixtures containing cysteine and three different forms of ribose

The headspace volatiles produced from buffered and unbuffered cysteine model systems, containing inosine 5'-monophosphate, ribose 5-phosphate, or ribose, were examined by GC-MS. Sulfur compounds dominated the volatiles of all systems and included mercaptoketones, furanthiols, and disulfides. The inosine monophosphate systems produced much lower quantities of volatiles than ribose phosphate or ribose systems. In the systems buffered with phosphate or phthalate buffers, both ribose and ribose phosphate systems gave similar quantities of sulfur volatiles. However, in the absence of buffer, the ribose system was relatively unreactive, especially for volatiles formed via the 2,3-enolization route in the Maillard reaction, where 4-hydroxy-5-methyl-3(2H)-furanone is a key intermediate. A number of keto-enol tautomerisms, which are known to be acid-base-catalyzed, occur in the 2,3-enolization route. This may explain the catalysis of the ribose systems by the buffers. In the ribose phosphate systems, however, Maillard mechanisms probably played a less important role, because ribose 5-phosphate readily dephosphorylated to give 4-hydroxy-5-methyl-3(2H)-furanone on heating and thus provided an easier route to aroma compounds than the Maillard reaction.     

Model studies on the stability of folic acid and 5-methyltetrahydrofolic acid degradation during thermal treatment in combination with high hydrostatic pressure

Stability of folic acid and 5-methyltetrahydrofolic acid in phosphate buffer (0.2 M; pH 7) toward thermal (above 65 degrees C) and combined high pressure (up to 800 MPa)/thermal (20 up to 65 degrees C) treatments was studied on a kinetic basis. Residual folate concentration after thermal and high pressure/thermal treatments was measured using reverse phase liquid chromatography. The degradation of both folates followed first-order reaction kinetics. At ambient pressure, the estimated Arrhenius activation energy (E(a)) values of folic acid and 5-methyltetrahydrofolic acid thermal degradation were 51.66 and 79.98 kJ mol(-1), respectively. It was noticed that the stability of folic acid toward thermal and combined high pressure thermal treatments was much higher than 5-methyltetrahydrofolic acid. High-pressure treatments at room temperature or higher (up to 60 degrees C) had no or little effect on folic acid. In the whole P/T area studied, the rate constant of 5-methyltetrahydrofolic acid degradation was enhanced by increasing pressure, and a remarkable synergistic effect of pressure and temperature on 5-methyltetrahydrofolic acid degradation occurred at temperatures above 40 degrees C. A model to describe the combined pressure and temperature effect on the 5-methyltetrahydrofolic acid degradation rate constant is presented.     

Degradation of the Amadori compound N-(1-deoxy-D-fructos-1-yl)glycine in aqueous model systems

The fate of the Amadori compound N-(1-deoxy-D-fructos-1-yl)glycine (DFG) was studied in aqueous model systems as a function of time and pH. The samples were reacted at 90 degrees C for up to 7 h while maintaining the pH constant at 5, 6, 7, or 8. Special attention was paid to the effect of phosphate on the formation of glycine and the parent sugars glucose and mannose, as well as formic and acetic acid. These compounds and DFG were quantified by high-performance anion-exchange chromatography. The rate of DFG degradation increased with pH. Addition of phosphate accelerated this reaction, particularly at pH 5-7. The rate of glycine formation increased with pH in both the absence and presence of phosphate. High glycine concentrations (60-70 mol %) were obtained, preferably at pH 6-8 with phosphate. However, the yield of glycine formed from DFG decreased at the advanced reaction stage for all pH values studied, both in water and in phosphate buffer. The rate of parent sugar formation increased from pH 5 to pH 7 in the absence of phosphate, leading to glucose and mannose in a constant ratio of 7:3. Addition of phosphate accelerated this reaction, yielding up to 18% parent sugars, most likely formed by reverse Amadori rearrangement. The formation rate of acetic and formic acid increased with increasing pH. The sum of both acids attained 76 mol %. However, the acetic acid concentrations were much higher than those of formic acid.