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Characteristics and Antioxidant Activity ofMaillard Reaction Products from Psicose-Lysineand Fructose-Lysine Model SystemsYan Zeng, Xiaoxi Zhang, Yuping Guan, and Yuanxia Sun

Abstract: d-Psicose, an epimer of d-fructose isomerized at C-3 position, is a rare ketohexose that is thought to bebeneficial for obese people and diabetic patients as a noncaloric sweetener. In the present study, model Maillard reactionproducts were obtained from d-psicose (or d-fructose) and l-lysine heating at 120 ◦C up to 8 h with the initial pH 9.0. Thechanges in pH, UV-vis absorbance, and free amino groups during the reaction were detected. Moreover, the antioxidantpotential of the Maillard reaction products at different intervals was investigated. Although there was almost no differencein the oxygen radical absorbance capacity, the Maillard reaction products from psicose performed better than that fromfructose in the radical-scavenging activity of 2, 2′-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt and1, 1,-diphenyl-2-picryl-hydrazyl. The reducing power of the Maillard reaction products from psicose was also strongerthan that from fructose. These results indicated that psicose played an effective role in the Maillard reaction and its Maillardreaction products could act as potential antioxidants in food industry.

Keywords: antioxidant activity, d-psicose, heating time, Maillard reaction, physicochemical property

IntroductionThe Maillard reaction, also named as nonenzymatic browning,

is a set of reactions resulting from the initial condensation betweenan available amino group of amino acids, amines, peptides or pro-teins, and an aldehydic or ketonic compound (Hodge 1953). Thisreaction produces a large number of so-called Maillard reactionproducts, which play an important role in the aroma, taste, color,and antioxidant potential of the stored or processed foods. TheMaillard reaction can be influenced by many factors, includingreactant concentration, temperature, heating time, initial pH, andthe characteristics of reactants (Renn and Sathe 1997; Naranjoand others 1998; Lertittikul and others 2007). In order to reducethe complexity, sugar-amino acid model systems have commonlybeen used to study the phenomena and the mechanism of theMaillard reaction (Wijewickreme and others 1999; Kim and Lee2008). For example, glucose-lysine is a usually selected model sys-tem (Narayan and Cross 1992; Jing and Kitts 2000; Moreno andothers 2003).d-Psicose, an epimer of d-fructose isomerized at C-3 position, is

a rare ketohexose that is thought to be beneficial for obese peopleand diabetic patients as a noncaloric sweetener, because it cansuppress hepatic lipogenic enzyme activity and provide zero energy(Matsuo and others 2001, 2002; Baek and others 2010). Recently,increasing attention has been paid to the Maillard reaction ofpsicose with proteins due to the excellent functional propertiesof its corresponding Maillard reaction products (Sun and others

MS 20101020 Submitted 9/10/2010, Accepted 1/4/2011. Authors Zeng, Zhang,Guan, and Sun are with Tianjin Inst. of Industrial Biotechnology, Chinese Academyof Sciences, Tianjin 300308, China. Author Zhang is also with College of FoodEngineering and Biotechnology, Tianjin Univ. of Science and Technology, Tianjin300457, China. Direct inquiries to author Sun (E-mail: sun_yx@tib.cas.cn).

2005, 2008). Puangmanee and others (2008) found that wheyprotein glycated by psicose showed an excellent emulsion propertyand could improve the quality of ice cream. In the report ofSun and others (2004a), the gelling functional property of henovalbumin could be enhanced by psicose through the Maillardreaction. Furthermore, the use of psicose in the Maillard reactioncould improve the foaming properties of egg white and egg whiteprotein more efficiently than glucose and fructose with increasingwhipping time (Sun and others 2004b, 2008). Besides the above-mentioned functions related to the textural characteristics of food,proteins glycated with psicose have exhibit significant antioxidantactivity in food products (Sun and others 2007). Although themodel Maillard reaction of psicose with glycine was set up (Baekand others 2008) and the reaction factors such as temperature,reaction concentration, and pH were examined, the research onlyfocused on the changes in browning and color of the Maillardreaction products.

At present, the demand for natural food with functional prop-erties has been increasing. Psicose can be produced in large scalefrom microbial or enzymatic reactions now (Gullapalli and oth-ers 2007) so that it is expected to be approved for commercialuse as a substitute of alimentary sugar in foodstuffs. The practicalapplication of psicose in the food industry requires more detailedinvestigation into the effect of psicose in the Maillard reaction,especially in the relevant antioxidant activity that can evaluate theuse of the Maillard reaction products as potential antioxidants infood processing strategies. The sugar-amino acid model systemis just an effective and simple approach to explore the differentrole of psicose from other hexoses such as fructose in the Maillardreaction and the relevant antioxidant activity.

To build a foundation for the Maillard reaction of psicose withpolypeptides or proteins in food chemistry, 2 Maillard reactionmodel systems of psicose with lysine (psicose-lysine) and fructosewith lysine (fructose-lysine) were established in this article and

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the effect of heating time on the characteristics and antioxidantactivity of the corresponding Maillard reaction products was ex-amined. Moreover, the characteristics and antioxidant activity ofthe Maillard reaction products from these 2 systems were com-pared in order to find the difference between psicose and fructosein the Maillard reaction.

Materials and Methods

Materials2,4,6-Trinitro-benzenesulfonic acid (TNBS), 2, 2′-azobis (2 -

amidinopropane) dihydrochloride (AAPH), 6-hydroxy-2,5,7,8-tetramethylchroma-2-carboxylic acid (Trolox), 2, 2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt(ABTS), 1, 1,-diphenyl-2-picryl-hydrazyl (DPPH), K2S2O4,and ferrozine were purchased from Sigma Chemical Co. (St.Louis, Mo., U.S.A.). d-Fructose and trichloroacetic acid werepurchased from Alfa Aesar Chemical Co. (Ward Hill, Mass.,U.S.A.). d-Psicose was purchased from Fushimi PharmaceuticalCo., Ltd. (Marugame, Kagawa, Japan). Other reagents used wereof analytical grade.

Preparation of the Maillard reaction productsReducing sugar (psicose or fructose) and lysine were dissolved

in double distilled water (ddH2O) as 2:1 molar ratio of sugar (0.2M) and amino acid (0.1 M). After adjusting the pH value to 9.0 by5 M NaOH, the solution was transferred to a flask and heated at120 ◦C up to 8 h under refluxing. The samples of the heated mix-ture were removed at different intervals and cooled immediatelyin an ice-water bath for analysis.

Measurement of UV-vis absorbanceThe UV-vis absorbance of the Maillard reaction products at

420 and 294 nm was examined by UV1800 spectrophotometer(Shimadzu, Japan). For the absorbance at 420 nm, the Maillardreaction products were taken in at 1-h intervals and diluted to20-fold. For the absorbance at 294 nm, the Maillard reactionproducts were taken in at 2-h intervals and diluted to 160-fold.

Measurement of pH valueThe pH of the Maillard reaction products taken in at 2-h inter-

vals was measured at 25 ◦C using FE20 pH Meter (Mettler Toledo,Zurich, Switzerland) calibrated with buffer solutions of pH 4.00,6.86, and 9.18, respectively.

Determination of free amino group contentFree amino group content was determined by TNBS according

to the method of Benjakul and Morrissery (1997). The Maillardreaction products were diluted to 16-fold in the test for use. Theblank and the control were prepared in the same way as the sample,using ddH2O and corresponding solvents instead of the sample andthe test reagents, respectively. The change in free amino groupcontent was expressed as the relative concentration (%) in com-parison with the original content.

Oxygen radical absorbance capacity (ORACFL) assayThe ORACFL assay was carried out in 75 mM phosphate buffer

(pH 7.4) according to the procedure reported by Rufian-Henaresand Morales (2007). The sample of the Maillard reaction products(20 μL, 250-fold dilution) and 120 μL of fluorescein (70 nM, finalconcentration) were placed in a 96-multiwell microplate. After theincubation at 37 ◦C for 10 min, 60 μL of AAPH solution was

added rapidly (12.8 mM, final concentration). The fluorescencewas recorded at λex = 485 nm and λem = 528 nm every minutefor 90 min by SpectraMax M2e plate reader (Molecular Devices,Calif., U.S.A.). A blank containing fluorescein and AAPH wasanalyzed in each run. Calibration curves of Trolox (1 to 8 μM,final concentration) were also carried out. The area under thecurve (AUC) was calculated according to the following equation:

AUC = 0.5+F1/F0+F2/F0+ · · · +Fn/F0

where F0 is the fluorescence intensity at time 0, Fn is the fluo-rescence at time n min. The net AUC of the sample was calcu-lated by subtracting the AUC of the blank to the AUC of thesample. ORACFL values were expressed as Trolox equivalents byusing the standard curve and the final ORACFL values of theMaillard reaction products were expressed in μmol Trolox permilliliter.

Trolox equivalent antioxidant capacity (TEAC) assayThe TEAC assay based on ABTS was done according to the

method described by Re and others (1999). The ABTS+· so-lution prepared from ABTS and K2S2O4 previously was dilutedto an absorbance of 0.7 ± 0.02 at 734 nm by sodium phos-phate buffer (10 mM, pH 7.4). The sample (50 μL, 50-fold di-lution) was added to 3-mL diluted ABTS+ solution and kept inthe dark for 6 min at room temperature after shaking. Then, theabsorbance of the mixture at 734 nm was recorded as Asample. Thepercentage of ABTS radical-scavenging activity was calculated asfollows:

ABTS radical − scavenging activity (%)= [Ablank − (Asample − Acontrol)]/Ablank × 100%

A standard curve of Trolox ranging from 0.12 to 1.2 mM wasprepared in the same manner. TEAC values were expressed asTrolox equivalents by using the standard curve and the final TEACvalues of the Maillard reaction products were expressed as μmolTrolox equivalents per millliter.

DPPH assayThe DPPH radical-scavenging activity of the Maillard reaction

products was determined according to the method of Yen andHsieh (1995). To 400-μL sample (100-fold dilution), 2 mL of0.25 mM DPPH in methanol was added. The mixture was shakenvigorously and allowed to stand in the dark at room temperaturefor 30 min. Then, the absorbance of the mixture at 517 nm wasrecorded as Asample immediately. The percentage of DPPH radical-scavenging activity was calculated as follows:

DPPH radical − scavenging activity (%)= [Ablank − (Asample − Acontrol)]/Ablank × 100%

Determination of ferrous reducing powerThe ferrous reducing power of the Maillard reaction products

was determined according to the method of Oyaizu (1986) withK3Fe(CN)6 and the absorbance was measured at 700 nm. Thesample used in the test was diluted to 100-fold. The ferrous re-ducing power (A) was calculated by the increase in the absorbanceat 700 nm as follows:

A = Asample − Ablank − Acontrol

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Detection of the chelating capacity for Fe2+The chelating capacity for Fe2+ of the Maillard reaction products

was estimated by the method of Dinis and others (1994). Threehundred microlitre Maillard reaction products and 50 μL of 2.0mM FeCl2 were added into 2.55 mL ddH2O. The mixture wasallowed to stand at room temperature for 30 s, followed by theaddition of 100-μL ferrozine (5 mM). The absorbance at 562nm was recorded after keeping the mixture at room temperaturefor 10 min. The percentage of chelating capacity for Fe2+ wascalculated as follows:

Chelating capacity for Fe2+ (%)= [Ablank − (Asample − Acontrol)]/Ablank × 100%

Statistical analysisAll analyses were run in triplicate and all results given in the

tables and figures were expressed as mean ± standard deviation.Differences between the 2 systems were tested for significanceby one-way analysis of variance using the statistical analysis sys-tem (SPSS 12.0 for Windows, SPSS Inc., Chicago, Ill., U.S.A.).Differences at P < 0.05 were considered to be significant.

Results and Discussion

UV-vis absorbanceThe browning of the 2 Maillard reaction model systems was

investigated based on the absorbance at 420 nm, which wasoften used as an indicator of the reaction extent (Morales andJimenzez-Perez 2001). As shown in Figure 1, the browning of the2 systems increased significantly with the reaction time. Althoughlittle distinction occurred in the first 4 h (P > 0.05), thereafterthe browning of psiocose-lysine increased more quickly than thatof fructose-lysine. As is well known, the browning products arerelated to the formation of melanoidins and the antioxidant activ-ity of the Maillard reaction products (Manzocco and others 2001).We supposed that psicose played a more effective role than fructosein browning and in the antioxidant activity of the Maillard reac-tion products especially entering the advanced Maillard reactionstage from heating for 4 h. This conclusion was confirmed by thefollowing experiments.

Figure 1–Browning of psicose and fructose during the thermal treatmentwith lysine.

Moreover, under the same reaction conditions, the distinctionof browning was more obvious when the ratio of sugar to lysinewas 2:1 compared to the ratio of 1:1 (data not shown). Lysine with2 −NH2 groups can react with 2 equivalent psicose (or fructose)to form Schiff-base. Under the ratio of 1:1, lysine was consumedinefficiently at the initial stage of Maillard reaction therefore theformation of intermediate compounds and the distinction in theeffect of psicose and fructose were restricted. The small distinc-tion of browning under the ratio of 1:1 may be also explainedaccording to Monti and others (2000), who found the blockage ofthe Nα group of lysine determined a faster development of colorand antioxidant ability than free lysine in the Maillard reaction.They hypothesized that this phenomenon was caused by the fasterformation of melanoidins in the system of Nα-subsitituted lysinebecause less material was dispersed in reaction pathways, especiallyin the Strecker degradation.

The absorbance at 294 nm, which indicated the formation ofthe intermediate compounds (Ajandouz and others 2001), wasalso employed to detect the procedure of the Maillard reaction(Ajandouz and others 2008; Cheriot and others 2009). The similarchange of the absorbance at 294 nm in these 2 systems (P > 0.05)during the whole heating process suggested that there were nomajor differences between psicose and fructose at the early stageof Maillard reaction (the data shown in Table 1).

Free amino group contentThe free amino group content in psicose-lysine and fructose-

lysine at different heating time was tested by TNBS. FromFigure 2, it was easy to discover that the free amino group contentof the 2 systems both decreased sharply at first then slightly in-creased at the end of the thermal treatment. There was no correla-tion between the rate of browning and the loss of free amino groups(psicose-lysine: r = −0.6550; fructose-lysine: r = −0.5329). Thesesimilar phenomena were observed by Baxter (1995). Consideringthe mechanism of Maillard reaction, the loss of free amino groupcontent was attributed to the formation of Schiff-base, while therelease of free amino groups may be ascribed to the decompositionof Heyns compounds during the later stage of the Maillard reac-tion (Baisier and Labuza 1992; Brands and van Boekel 2002). Atthe same time, the loss of lysine may be leveled off while the for-mation of melanoidins that the browning indicated kept increasing

Figure 2–Free amino group content in psicose-lysine and fructose-lysinesystems during the thermal treatment.

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due to the retardation time between the early and later stages. Theloss of free amino groups was more obvious in psicose-lysine thanthat in fructose-lysine, and the distinction between these 2 systemswas about 10% after 8 h thermal treatment (P < 0.05), which in-dicated that psicose may have higher reactivity than fructose at theadvanced stage of Maillard reaction.

Changes in pHAs listed in Table 1, the pH of psiocse-lysine and fructose-lysine

decreased with the increasing heating time. During the wholeheating process, there was no obvious difference in the change ofpH between the 2 model systems (P > 0.05). Moreover, the pHwas negatively correlated with the absorbance at 294 nm (psicose-lysine: r = −0.8946; fructose-lysine: r = −0.9141). Some acidiccompounds, including formic acid and acetic acid (Martins andothers 2003; Chen and others 2005), had been reported to presentin the intermediate Maillard reaction products. So, we suspectedthe change in pH was mainly due to the formation of organicacids together with the consumption of amino group at the earlystage of the Maillard reaction.

Radical-scavenging activityThe antioxidants play their roles mainly based on the mecha-

nisms of hydrogen atom transfer and single electron transfer. TheORACFL assay is classified as hydrogen atom transfer reactions,while TEAC and DPPH assays are mainly attributed to singleelectron transfer reactions (Prior and others 2005). These 3 meth-ods were used to detect the radical-scavenging activity of theMaillard reaction products from psicose-lysine and fructose-lysineat different reaction time. The experimental result of ORACFL

assay (expressed in μmol Trolox equivalents per millliter of theMaillard reaction products) was listed in Table 1. The ORACFL

values of the Maillard reaction products from psicose-lysine andfructose-lysine both increased obviously after heating for 2 h butsince then the growth rate was reduced. During the whole heatingprocess, there was almost no difference in the ORACFL values ofthe Maillard reaction products from psicose-lysine and fructose-lysine (P > 0.05), which may indicate that the Maillard reactionproducts from psicose-lysine and fructose-lysine have the similarability to deactivate radicals through the hydrogen atom transfermechanism.

The TEAC and DPPH assays were carried out to detectthe antioxidant ability of the Maillard reaction products inthe aqueous media and in the organic media, respectively. Asshown in Figure 3A, the 2 model systems almost had no ABTSradical-scavenging activity initially but later on their ABTSradical-scavenging activity increased. After 8 h, the TEACvalue of Maillard reaction products from psicose-lysine reached

52.53 μmol Trolox equivalents per milliliter, while its ORACFL

value after heating 8 h was only 22.82 μmol Trolox equivalentsper milliliter. During the whole reaction stage, the ABTSradical-scavenging activity of the Maillard reaction products frompsicose-lysine increased faster than that from fructose-lysine,especially in the last 4 h (P < 0.05). The distinction of theirABTS radical-scavenging activity was about 8% at 4 h and 19%at 8 h. A similar phenomenon in the DPPH assay was observed(displayed in Figure 3B) and there was a positive correlation

Figure 3–Free radical-scavenging activity of the Maillard reaction productsfrom psicose-lysine and fructose-lysine at different heating time. (A) TEACassay; (B) DPPH assay.

Table 1–Absorbance at 294 nm (A294), pH, and ORACFL value of the Maillard reaction products from psicose-lysine and fructose-lysine.∗

Absorbance at 294 nm∗∗ pH∗∗ ORACFL (μmol Trolox/mL)∗∗

Heating time psicose-lysine fructose-lysine psicose-lysine fructose-lysine psicose-lysine fructose-lysine

0 h 0.00 ± 0.01a 0.00 ± 0.01a 9.00 ± 0.01a 9.00 ± 0.01a 1.11 ± 0.22a 0 .46 ± 0.01a

2 h 0.38 ± 0.01b 0.39 ± 0.02b 7.05 ± 0.08b 7.21 ± 0.09b 13.93 ± 1.91b 17.23 ± 1.82b

4 h 0.59 ± 0.03c 0.58 ± 0.05c 5.19 ± 0.18c 5.39 ± 0.20c 17.54 ± 2.26c 17.47 ± 0.99b

6 h 0.96 ± 0.02d 0.83 ± 0.04d 4.66 ± 0.13d 4.74 ± 0.09d 19.93 ± 1.40d 20.24 ± 1.75c

8 h 1.59 ± 0.02e 1.34 ± 0.04e 4.19 ± 0.22e 4.43 ± 0.11d 22.82 ± 2.13e 23.11 ± 1.12d

∗ The Maillard reaction products were diluted to 160-fold for the measurement of the absorbance at 294 nm and were diluted to 250-fold for ORAC assay (λec = 485 nm; λem =528 nm).∗∗There was no significant difference between psicose-lysine and fructose-lysine (P > 0.05) in the measurement of the absorbance at 294 nm, pH, and ORACFL value at the sameheating time.Different small letter superscripts (a to e) denote the significant difference (P < 0.05) in the same system during the heating process.

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between the DPPH and TEAC assays (psicose-lysine: r = 0.9961;fructose-lysine: r = 0.9890). These results revealed that comparedto the Maillard reaction products from fructose-lysine, theMaillard reaction products from psicose-lysine were better freeradical inhibitors based on the single electron transfer reactions.

Reducing powerBesides radical-scavenging activity, reducing power is also asso-

ciated with antioxidant activity (Yen and Hsieh 1995). The ferrousreducing power of the Maillard reaction products from psicose-lysine and fructose-lysine at different reaction time was determinedby the absorbance at 700 nm. In accordance with TEAC andDPPH assays, the ferrous reducing powers of the Maillard reac-tion products derived from psicose-lysine and fructose-lysine wereboth enhanced with heating time, especially after 4 h (Figure 4).During the whole heating time, the ferrous reducing power of theMaillard reaction products from psicose-lysine was higher than thatfrom fructose-lysine and was 35% over that from fructose-lysine at8 h (P < 0.05), which further indicated the better potential effectof psicose than fructose in the antioxidant activity of the Maillardreaction products.

Furthermore, the significant distinction in the reducing powerand free radical-scavenging activity (based on single electron trans-fer) between psicose-lysine and fructose-lysine occurred after heat-ing for 4 h (P < 0.05). Linked to the changes of absorbance at294 and 420 nm, this result implied that the better antioxidantactivity of the Maillard reaction products from psicose was mainlygenerated by the melanoidins in the advanced stage. Moreover, theresults of ORAC, TEAC, and DPPH assays with the test of reduc-ing power indicated that the difference in the antioxidant activityof the Maillard reaction products from these 2 reducing sugars wasmainly related to the mechanism of single electron transfer.

Chelating capacity for Fe2+The chelating capacity for Fe2+ of the 2 model systems was

shown in Table 2. Unlike the results of browning, radical-scavenging ability, and ferrous reducing power, the chelatingcapacity of psicose-lysine for Fe2+ decreased firstly and thenincreased, while in fructose-lysine, the chelating capacity for Fe2+kept decreasing during the whole heating time. It was interest-ing that the chelating capacity for Fe2+ in the 2 systems both

Figure 4–Reducing power of the Maillard reaction products from psicose-lysine and fructose-lysine at different heating time.

Table 2–Iron chelating activity of the Maillard reaction productsfrom psicose-lysine and fructose-lysine.∗

Chelating activity of Fe2+ (%)

Heating time psicose-lysine fructose-lysine

0 h 97.56 ± 2.33a, A 99.21 ± 1.64a, A

2 h 22.78 ± 1.84b, A 20.45 ± 0.17b, A

4 h 1.77 ± 0.50c, A 11.87 ± 1.36c, B

6 h 2.71 ± 0.20d, A 7.82 ± 0.81d, B

8 h 5.17 ± 0.58e, A 4.76 ± 0.28 e, A

∗ The chelating activity was estimated from the initial amount of free iron. The samplein the test of chelating capacity for Fe2+ was constituted of 300 μL of the Maillardreaction products, 2.55 mL ddH2O, and 50 μL of 2.0 mM FeCl2 by the followingaddition of ferrozine (100 μL, 5 mM).Different capital superscripts (A to B) denote the significant difference (P < 0.05)between psicose-lysine and fructose-lysine in the chelating capacity for Fe2+ at thesame heating time.Different small superscripts (a to e) denote the significant difference (P < 0.05) of thesame system in the chelating capacity for Fe2+ during the heating process.

decreased sharply in the first 2 h and then the decreasing trenddiminished. Associated with the loss of free amino groups, weproposed that free amine groups may have a great impact on thechelating capacity for Fe2+ besides the Maillard reaction products,because lysine could chelate almost all Fe2+ while psicose andfructose could only chelate extremely small amounts of Fe2+ un-der the same experimental conditions (data were not shown). Inaddition, Ruzi-Roca and others (2008) also thought though newcompounds with iron-binding properties were produced duringthermal treatment the significant decrease in iron chelating activityof the Maillard reaction products could be partially due to the lossof free amine groups.

ConclusionIn this study, 2 aqueous Maillard reaction model systems

of psicose-lysine and fructose-lysine were established. TheUV-vis absorbance, radical-scavenging activity, reducing power,and chelating capacity for Fe2+ of the corresponding Maillard re-action products from the 2 systems at different heating time wereexamined. The results showed that the properties of the Maillardreaction products derived from the 2 systems were both closelylinked with heating time. More importantly, under the same con-dition, the Maillard reaction products from psicose-lysine per-formed better in browning, free radical-scavenging activity, andreducing power than that from fructose-lysine, especially afterheating for 4 h, indicating that the Maillard reaction products ofpsicose presented a higher antioxidant activity than that of fruc-tose. The significant distinction between psicose and fructose inthe Maillard reaction is thought to be related to the constitutionof the corresponding Maillard reaction products at the advancedreaction stage and it needs a further research.

AcknowledgmentsThis study was supported by Natl. Natural Science Foundation

of China (20972181).

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Vol. 76, Nr. 3, 2011 � Journal of Food Science C403