Antioxidant and Antiglycation Activity of Selected Dietary Polyphenols

Jan 28, 2014 - Antioxidant and Antiglycation Activity of Selected Dietary. Polyphenols in a Cookie Model. Xinchen Zhang,. †. Feng Chen,. § and Ming...
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Antioxidant and Antiglycation Activity of Selected Dietary Polyphenols in a Cookie Model Xinchen Zhang,† Feng Chen,§ and Mingfu Wang*,† †

School of Biological Sciences, The University of Hong Kong, Pokfulam, Hong Kong Institute for Food and Bioresource Engineering, College of Engineering, Peking University, Beijing, China

§

S Supporting Information *

ABSTRACT: Dietary polyphenols have been proposed to be promising functional food additives for their potent antioxidant capacity and other health-beneficial bioactivities. The current study prepared cookies fortified with five selected dietary polyphenols (naringenin, quercetin, epicatechin, chlorogenic acid, and rosmarinic acid). Results indicated that the enhancement of the antioxidant capacity was not as obvious as expected because the phenolics’ antioxidant activity was seriously lowered by the baking process due to thermal degradation and transformation. Meanwhile, the tested polyphenols, especially quercetin, showed inhibition against formation of both reactive carbonyl species and total fluorescent advanced glycation endproducts (AGEs). Polyphenol fortification could also induce colorimetric changes and alterations in selected quality attributes. Overall, the findings support dietary polyphenols as functional food ingredients in the purpose of health benefits associated with a higher intake of antioxidants and a lower load of reactive carbonyls and AGEs. The polyphenols’ stability and reactivity during thermal processing should be an important consideration. KEYWORDS: dietary polyphenol, antioxidant capacity, reactive carbonyls, advanced glycation endproducts

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INTRODUCTION Dietary polyphenols are a large class of natural compounds present in various food sources such as vegetables, fruits, wine, tea, and cocoa products.1 They have been reported to offer diverse bioactivities, including anti-inflammatory, antiallergic, antiviral/antibacterial, antimutagenic/anticarcinogenic properties, and protective effects against diverse diseases.1 Many of these biologically significant functions are believed to be mediated by phenolics’ free radical scavenging capability, which combats the oxidative stress induced by reactive oxygen species. Although the antioxidant activity of dietary polyphenols has been well characterized in chemical model systems and in vitro, studies on their thermal stability and how thermal processing would influence the quantity of phenolic compounds and their associated antioxidant capacity in food are limited. As we previously reported, the antioxidant capacity of dietary polyphenols may be lost through thermal degradation or thermal reaction with sugar fragments in the caramelization model.2 In black beans, Xu and Chang3 showed that boiling and steaming caused significant decreases in phenolic content and antioxidant power. Peng et al.4 found that although fortification of bread with grape seed extract (GSE) enhanced the antioxidant capacity, baking led to a 30−40% decrease in GSE’s antioxidant activity. Increase in antioxidant activity with thermal processing has also been observed in the cases of pressure-steaming yellow beans, cooking tomato, and highpressure processing of tomato and carrot purees; this phenomenon was correlated with an increase of particular phenolic contents.3,5,6 Altogether thermal or pressure processing should be an important factor affecting both dietary polyphenols’ amount and their antioxidant activity in processed food products. © 2014 American Chemical Society

Besides antioxidant power, dietary polyphenols have been recently demonstrated to possess antiglycation activity. The proposed antiglycation mechanisms include free radical and reactive carbonyl species scavenging. Most investigations, however, were performed with plant extracts composed of more than one natural compound, therefore providing limited information on the structure−activity relationship. Besides, a mild temperature, typically 37 °C, was commonly used to mimic physiological condition, but the temperature is not related to conventional food-processing conditions. A few studies have tried to explore the addition of polyphenols as functional food additives to reduce the formation of advanced glycation endproducts (AGEs). Grape seed extract fortification was elucidated to inhibit carboxymethyllysine (CML) generation in bread.4 In a sponge cake model, ferulic acid was capable of reducing CML and carboxyethyllysine (CEL) formation, which effect was suggested to be attributed to its free radical scavenging activity.7 More experiments conducted in a food model with pure phenolic compounds are necessary to illustrate dietary polyphenols’ antiglycation potential under thermal conditions and understand the relationship between their chemical structure and antiglycation activity. The cookie has been selected as a food model in a variety of studies regarding the formation kinetics of heat-induced toxicants including hydroxymethylfurfural, acrylamide, αdicarbonyls, and AGEs.8−15 Despite variations in specific proportions, the recipes all included sugar, flour, and fat as major components. The baking temperature ranged from 160 Received: Revised: Accepted: Published: 1643

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to 300 °C, and baking time varied between 5 and 30 min.8−15 Under these baking kinetics, a variety of complex physical and chemical phenomena would happen, including the important nonenzymatic browning reaction resulting in brown color and bakery flavor development. During cookie baking, α-dicarbonyls may be produced via caramelization, Maillard reaction, and lipid peroxidation. Their high reactivity with protein poses health risks associated with pathological events and carcinogenicity.10 Cookies are also rich sources of dietary AGEs, which can be produced with α-dicarbonyls as precursors and are implicated in the development of oxidative stress and inflammation-associated diseases. On the basis of the above discussions, cookies would be suitable food models to examine the antioxidant and antiglycation activities of dietary polyphenols under thermal processing. The current study prepared cookies fortified with five representative dietary polyphenols (naringenin, quercetin, epicatechin, chlorogenic acid, and rosmarinic acid), respectively. Examination of the antioxidant capacity of fortified cookies would allow characterization of how thermal treatment affected phenolics’ antioxidant activity in the cookie system. The antiglycation potential of polyphenols in cookie was also assessed with reference to the formation of both total fluorescent AGEs and their important precursors, the reactive carbonyl intermediates. Finally, the practicality of polyphenol fortification was evaluated on the basis of the influence of phenolic addition on the color and other quality attributes of cookies.

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content of cookie was measured by a moisture analyzer (HB43-S, Mettler Toledo, USA). Around 3 g of ground cookie was heated at 120 °C until stable weight, and the moisture content was calculated according to the weight loss. For measurement of pH, 0.4 g of ground cookie was vortexed in 20 mL of water for 3 min and then held for 1 h at room temperature. The pH of the supernatant layer was taken by a pH meter (pHI510, Beckman Coulter, USA). Measurement of Antioxidant Capacity. The antioxidant capacity was quantified by Trolox equivalent antioxidant capacity (TEAC) assay.16 The stock radical solution was prepared by reacting 7 mM ABTS solution with 2.45 mM potassium peroxosulfate in the dark for 16 h at room temperature. Working radical solution was freshly prepared by diluting stock radical solution with Milli-Q water to have an absorbance of 0.7 ± 0.05 at 734 nm. Ground cookie was dispersed in water and 30, 50, and 70% ethanol with a final concentration of 100 mg/mL. The samples were extracted by sonication for 30 min and then centrifuged at 5100g for 20 min. Fifty microliters of supernatant or Trolox standard was mixed with 1.9 mL of working radical solution, and absorbance at 734 nm was read by a spectrophotometer (UV1206, Shimadzu, Japan) after 6 min of incubation. The results were expressed as nanomoles of Trolox per milligram of cookie. Investigation of Thermal Products of Quercetin after Cookie Baking Process. Ten grams of ground cookie was weighed in a 100 mL volumetric flask, and extraction was conducted by 70% methanol under sonication for 1 h. The extract was filtered, and the filtrate was analyzed by HPLC-DAD and LC-MS/MS. The Shimadzu HPLC system is composed of a separation module (LC-20AT), an autosampler (SIL-20A), a degasser (DGU-20A3), and a photodiode array detector (SPD-M20A). The separation of reaction products was achieved on a YMC-pack Pro column (5 μm, i.d. 2.1 × 150 mm). The mobile phases were water with 0.1% formic acid (A) and acetonitrile (B), and gradient flow was as follows: 0 min, 3% B; 5 min, 3% B; 45 min, 50% B; 55 min, 80% B; 60 min, 90% B; back to 3% B in 1 min and held for 14 min. The total run time was 75 min, and flow rate was 0.2 mL/min. The injection volume was 10 μL. UV detection wavelength for quercetin was 366 nm. The LC-MS/MS instrument was equipped with an electrospray ionization (ESI) source interfaced to a 3200 Qtrap mass spectrometer (AB Sciex, Canada). Liquid chromatography was performed on an Agilent 1290 UHPLC system with a binary pump (G4420A) and a thermostated autosampler (G4226A). The separation of the reaction products was performed in the same conditions as described above. The MS parameters were as follows: negative ion mode; spray voltage, −4300 V; scan range, 100−1000 Da; temperature, 450 °C; declustering potential (DP), −65 V; collision energy (CE), −30 eV. Determination of Glyoxal (GO) and Methylglyoxal (MGO). The two typical representatives of reactive carbonyls, GO and MGO, were quantified by a derivatization HPLC-DAD method.10 Two grams of ground cookie was extracted by 20 mL of 50% methanol under sonication for 1 h. After centrifugation, the supernatant was filtered, and 15 mL of filtrate was concentrated by rotary evaporation. The extract was redissolved in 2 mL of 50% methanol. Derivatization was conducted by mixing 600 μL of cookie extract with 200 μL of OPD (10 mM) and afterward incubating the solutions for 3 h at 70 °C in an oven. After cooling, 200 μL of acetic acid was added to the derivatized mixture. The samples were then filtered by a 0.45 μm membrane before HPLC-DAD analysis. After derivatization with OPD, GO and MGO could be measured as quinoxaline (Q) and 2-methylquinoxaline (2-MQ) correspondingly detectable by UV absorbance at 315 nm. The same Shimadzu HPLC system is used as described above. Separation was conducted on an ACE C18 column (5 μm, 250 × 4.6 mm, Advanced Chromatography Technologies, Aberdeen, UK). Elution was performed with a mixture of (A) 0.5% (v/v) acetic acid in water and (B) methanol, and gradient flow was as follows: 0 min, 16% B; 2 min, 16% B; 22 min, 32% B; 37 min, 80% B; 42 min, 80% B; 43 min, 16% B; 53 min, 16% B. Flow rate was 0.8 mL/min, and injection volume equaled 10 μL. Quantitative interpolation was facilitated by calibration curves constructed by Q and 2-MQ standard.

MATERIALS AND METHODS

Reagents and Chemicals. Epicatechin was purchased from Biopurify Phytochemicals Ltd. (Chengdu, China). Naringenin, quercetin, chlorogenic acid, rosmarinic acid, sodium chloride, sodium carbonate, sodium bicarbonate, trolox, ABTS [2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)], potassium peroxosulfate, OPD (ophenylenediamine), quinoxaline, 2-methylquinoxaline, Tween-20, SDS (sodium dodecyl sulfate), Tris, and β-mercaptoethanol were all purchased from Sigma-Aldrich (St. Louis, MO, USA). The ingredients for cookie preparation including salt, sugar, baking powder, canola oil, and white flour were purchased at a local supermarket. All solvents and acids used were of analytical or HPLC grade and obtained from BDH Laboratory Supplies (Poole, UK). Cookie Model. The cookie recipe consisted of double-deionized water (1.5 mL), salt (0.1 g), baking powder (0.12 g), sugar (3.5 g), canola oil (1.5 mL), and white flour (7.5 g). The ingredients were thoroughly mixed with 0.25% (w/w) of individual phenolic compound (naringenin, quercetin, epicatechin, chlorogenic acid, and rosmarinic acid). Dough was rolled and baked in an electronic baking oven (Kenwood, China) at 200 °C for 10 min. After baking, the cookie samples were cooled to room temperature and kept at −20 °C until further analysis. Measurement of Color, Texture, Moisture, and pH of Cookie Samples. The three chromatic coordinates L*, a*, and b* of ground cookie were measured by a colorimeter (CR-400, Konica Minolta, Japan). The coordinate L* indicates the lightness of color (L* = 0 yields black and L* = 100 means diffuse white); a* characterizes the position between red and green (negative values indicate green, and positive values indicate red); b* suggests the position between yellow and blue (negative values indicate blue, and positive values indicate yellow). Chroma value was calculated as (a*2 + b*2)1/2; E index was calculated as (L*2 + a*2 + b*2)1/2. The textural analysis was conducted by a texture analyzer (TA-XT2, stable micro system, Surrey, UK) equipped with a cylindrical probe of 20 mm in diameter. The mode was set as “measure force in compression”, and the compression distance was 1 mm. Hardness of the cookies was estimated by the maximum force (g). The moisture 1644

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Table 1. Impacts of Dietary Polyphenols on the Color of Cookiesa L* control naringenin quercetin epicatechin chlorogenic acid rosmarinic acid a

41.75 41.51 38.67 40.22 42.46 43.35

± ± ± ± ± ±

a* 1.91 0.34 1.69 1.65 0.42 0.88

3.55 3.87 2.03 4.09 2.78 2.75

± ± ± ± ± ±

b*

0.10 0.23 0.19b 0.11a 0.03b 0.08b

12.28 13.67 16.70 11.78 14.25 13.64

± ± ± ± ± ±

E index

chroma value 0.44 0.36a 1.13a 0.50 0.08a 0.43a

12.78 14.21 16.82 12.47 14.52 13.92

± ± ± ± ± ±

0.39 0.41a 1.10a 0.49 0.08a 0.43a

43.66 43.88 42.17 42.11 44.88 45.53

± ± ± ± ± ±

1.94 0.23 1.98 1.72 0.43 0.97

“a” marks values significantly higher than control value, whereas “b” marks values significantly lower than control value (P < 0.05).

Measurement of Fluorescent AGEs. To extract the protein from cookie samples, 250 mg of ground cookie was extracted by 4.75 mL of extraction buffer (0.05% Tween-20, 1% SDS, 5% β-mercaptoethanol, 50 mM Tris-HCl at pH 7.4).14 Extraction was performed overnight at room temperature under gentle agitation. The supernatant containing extracted protein was collected after centrifugation. One hundred microliters of extracted protein solution from cookies was pippeted to each well of a 96-well plate, and fluorescent AGEs were indicated by fluorescence with excitation wavelength of 355/40 nm and emission wavelength of 405/10 nm (Victor X4 Multilabel Plate Reader, PerkinElmer, USA). Statistical Analysis. Statistical analyses were carried out by Graphpad Prism 5 software package (GraphPad Software, Inc., La Jolla, CA, USA). Data are expressed as the mean ± standard error of triplicate determinations, and differences with P < 0.05 were considered to be statistically significant.

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Table 2. Impacts of Dietary Polyphenols on the Selected Quality Attributes of Cookiesa hardness (g) control naringenin quercetin epicatechin chlorogenic acid rosmarinic acid

11613 8270 6273 8745 9653 8596

± ± ± ± ± ±

3448.6 1407.3 1257.8 421 792.3 1763.6

9.62 9.49 9.02 9.11 8.88 9.27

pH

moisture (%)

± ± ± ± ± ±

2.20 2.25 2.64 1.85 2.10 2.07

0.038 0.015b 0.017b 0.017b 0.092b 0.031b

± ± ± ± ± ±

0.065 0.072 0.061a 0.023b 0.031 0.006b

a “a” marks values significantly higher than control value, whereas “b” marks values significantly lower than control value (P < 0.05).

Polyphenol Fortification Enhances Antioxidant Capacity, but Primary Compounds Are Thermally Converted to Other Products. As shown in Figure 1, the

RESULTS AND DISCUSSION

Polyphenol Addition Alters Selected Quality Attributes. One key consideration in the utilization of dietary polyphenols as functional food ingredients is whether the polyphenols will adversely influence the appearance, flavor, taste, and other sensory or quality attributes of fortified foods. Green tea and grape seed extract fortification have been illustrated to alter the color of baked bread on the basis of instrumental analysis.4,17 In this study, the potential of polyphenol addition to cause colorimetric changes of cookie was examined. As shown in Table 1, cookies prepared with 0.25% (w/w) quercetin, chlorogenic acid, and rosmarinic acid showed decreased redness and increased yellowness; epicatechin or naringenin could induce a positive shift in the redness or yellowness of cookies, respectively. For a comprehensive analysis combining lightness, redness, and yellowness values, the chroma value of cookies fortified with naringenin, quercetin, chlorogenic acid, and rosmarinic acid was significantly higher than control, whereas no obvious effects were observed on the E index. The above observed colorimetric changes might be attributed to colorants originated from thermal transformation of polyphenols or newly formed unknown colorants from reactions between polyphenol and the nutritional components in cookies. Table 2 summarizes the influence of dietary polyphenol addition on three commonly used quality descriptive indices. In textural analysis, phenolics were found to cause no significant change in cookie hardness at the specific addition level used in the study. A 0.1−0.7 decrease in pH value, however, was recorded as a result of polyphenol addition, which is probably a result of acidic compound formation. Tested polyphenols resulted in 0.1−0.4% fluctuation in cookies’ moisture content, and the decrease in the moisture content induced by epicatechin or rosmarinic acid fortification might favor the shelf life of cookies.18

Figure 1. Antioxidant capacity of cookie extracts with or without addition of polyphenols and polyphenol equivalents. Extract with antioxidant capacity significantly higher than control is marked with “a” (P < 0.05).

antioxidant capacity of cookie extract, which was prepared by extraction with solvents of different polarity, was significantly elevated with phenolics fortification. The increase in the antioxidant capacity, however, was much lower than that carried by the quantity of polyphenol originally added to the cookie recipe (denoted polyphenol equivalent in Figure 1), especially in the cases of quercetin and epicatechin fortification. This observation was consistent with early findings that quercetin and epicatechin were relatively sensitive to thermal processing and suffered from obvious thermal loss of their antioxidant activity.2 Naringenin and quercetin were selected for further quantitative analysis (70% methanol extraction followed by HPLC/DAD quantification) of remaining primary polyphenol compound after baking process. Surprisingly, results showed that even after a 10 min short baking time, a >30% decrease in naringenin amount was found, and in quercetin’s case, there was around an 88% loss of detectable UV absorption signal of 1645

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chromen-4-one as previously identified by Hvattum et al.20 In all, during the high-temperature baking process, quercetin was possibly degraded to very small molecules beyond detection of current LC/MS-MS methodology or transformed to unstable oxidized compounds, which readily react with methanol during extraction, forming compounds I and II. In addition to degradation or oxidation loss of primary polyphenol structure carrying antioxidant activity, thermal processing likely favors reaction of phenolics with other nutritional components in cookie recipes, such as sugar fragments originated from caramelization and Maillard reaction, and thereafter affects the antioxidants’ content.2,21 Therefore, it seems to be necessary to consider the phenolics’ thermal stability and reactivity in the food matrix when the food product is fortified with phenolics for the purpose of increasing the level of health-beneficial antioxidants. Polyphenols Inhibit the Formation of Glycation Intermediates and Endproducts. The α-dicarbonyl compounds are important intermediate products originated from either sugar/lipid oxidation or thermal degradation of Schiff base or Amadori rearrangement products. GO and MGO are two major α-dicarbonyls that can react with amino groups on the protein backbone and contribute to the formation of AGEs.10,22 The GO and MGO levels in model cookie control were 12.6 and 16.9 mg/kg, respectively, which are comparable to the values reported for commercial cookies.10 Selected polyphenols, including tea catechins and theaflavins, chalcones represented by phloretin and phloridzin, stilbenes, and genistein, have been reported to trap these two reactive carbonyls in physiological conditions.23 Our results suggested that under baking conditions, all five tested polyphenols were effective in reducing the amount of GO but not MGO in cookies (Figure 2). Furthermore, the inhibitory activity against

primary compound. Quercetin has been previously reported to be thermally unstable after long heating: heating quercetin at 100 °C in aqueous solution caused a 25% decrease after 300 min at pH 5, whereas an alkalinic pH of 8 resulted in much more severe thermal loss that left no quercetin after 240 min of heating.19 Our results indicate that in a low-moisture environment with heating temperature elevated to 200 °C, it took a very short time for quercetin to transform to other compounds. Further investigation was made into the thermal transformation of quercetin. On the basis of the total ion chromatogram comparison of cookie extract without or with quercetin, three major additional peaks were identified in quercetin-fortified cookie extract, the information of which is summarized in Table 3. Remaining primary quercetin (m/z Table 3. LC-MS/MS Data of Three Major Additional Compounds in Quercetin-Fortified Cookie Extract peak

retention time (min)

m/z [M − H]−

MS/MS

suggested structure

1

20.2

349, 331

299, 271, 179, 151

2

24.4

363, 331

299, 271, 179, 151

3

28.7

301

273, 179, 151, 107

2-(3,4-dihydroxyphenyl)3,3,5,7-tetrahydroxy-2methoxy-2,3dihydrochromen-4-one 2-(3,4-dihydroxyphenyl)3,5,7-trihydroxy-2,3dimethoxy-2,3dihydrochromen-4-one quercetin

301) was eluted at 28.7 min. Compound I (retention time = 20.2 min) showed UV λmax at 290 nm and two main ions at m/ z 349 and 331 during enhanced mass spectrometry (EMS) analysis, indicating easy loss of one molecule of water. Enhanced product ion (EPI) analysis of the ion at m/z 331 further elucidated a product ion at m/z 299, which may be the [M − H]− ion of oxidized quercetin, and the neutral loss was likely a molecule of methanol (32 Da).20 Neutral loss of 28 Da from the ion at m/z 299, probably corresponding to a CO, gave rise to a product ion at m/z 271, whereas the same loss of CO from quercetin produced an ion at m/z 273. The product ion spectrum of both quercetin and its deprotonated ion at m/z 299 showed two ions at m/z 179 and 151. On the basis of the agreements on the UV absorption and LC-MS/MS data, compound I is likely to be 2-(3,4-dihydroxyphenyl)-3,3,5,7tetrahydroxy-2-methoxy-2,3-dihydrochromen-4-one, which was identified by LC-MS/MS and NMR as the reaction product of quercetin and 2,2-diphenyl-1-picrylhydrazyl (DPPH) in methanol.20 The compound has also been reported by Buchner et al.19 to be present in the methanol solution of thermally processed quercetin at 100 °C. The maximum UV absorption wavelength of compound II (retention time = 24.4 min) is 293 nm, and the main ions found from the mass spectrum were at m/z 363, 331, and 299. The base ion at m/z 363 was of low abundance and may be unstable to readily lose a molecule of methanol (32 Da) to m/z 331. The neutral loss from m/z 331 to 299 could represent a second molecule of methanol. The following fragmentation pattern was analogous to that of compound I, giving rise to product ions at m/z 271, 179, and 151. Therefore, compound II might be the reaction product of oxidized quercetin incorporating two molecules of methanol, namely, 2-(3,4dihydroxyphenyl)-3,5,7-trihydroxy-2,3-dimethoxy-2,3-dihydro-

Figure 2. Inhibitory activity of polyphenols on the formation of glyoxal and fluorescent AGEs in a cookie model. Bars with an asterisk indicate significant difference from control (P < 0.05).

glyoxal formation was positively correlated with polyphenols’ antioxidant activity (R2 = 0.975). This phenomenon led to a hypothesis that the inhibitory activity of tested phenolics against glyoxal formation during cookie baking might be attributable to the phenolics’ free radical scavenging capability. Quercetin was most effective against fluorescent AGEs formation (>80% inhibition), followed by naringenin, rosmarinic acid, and epicatechin (Figure 2). Chlorogenic acid showed no significant inhibition. It has been suggested that the food ingredient profile has substantial effects on AGE formation and, compared with 1646

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Journal of Agricultural and Food Chemistry sucrose, the lipid composition might play a more important role in AGE production in sponge cake.7 The model cookie was rich in canola oil, which is a kind of edible oil containing a high proportion of unsaturated fatty acids including oleic acid, linoleic acid, and α-linolenic acid. Glyoxal is a prominent product of lipid peroxidation, and phenolics are able to inhibit lipid peroxidation via free radical scavenging capability. Therefore, it is likely that the tested dietary polyphenols reduced glyoxal generation by inhibiting lipid peroxidation. In agreement with earlier studies’ viewpoints, the antiglycation effect of dietary polyphenols possibly relied on their free radical scavenging activity in the oxidative stage of glycation, when reactive carbonyl species are produced from sugar/lipid/ Amadori product oxidation.4,7 In this paper, the antioxidant and antiglycation activities of five selected dietary polyphenols in a cookie model were discussed. Further to their potential to induce changes in cookies’ color, pH, and moisture content, polyphenols were capable of increasing the health-beneficial antioxidants content in fortified cookies. The antiglycation activity of dietary polyphenols during cookie baking was indicated by lower levels of glyoxal and total fluorescent AGEs, which might be attributed to phenolics’ free radical scavenging capacity. Compared with other tested polyphenols at the same massbased fortification level, quercetin is suggested to be the most promising functional cookie additive given its significant promotion of antioxidant capacity together with strong inhibition against glycation toxicants formation. The fate of dietary polyphenols during thermal food processing is of high research significance because the thermal conversion of chemical structure, which is associated with destruction of some primary compounds and introduction of many new compounds, would lead to alterations in foods’ organoleptic properties and, more importantly, biological activities. The suggested reaction products of oxidized quercetin, for example, might be potential antioxidative and chemopreventive agents or in contrast be harmful to human health, which need further investigations. The current work suggests that during future functional food development by means of dietary polyphenol fortification before food processing, cautious measures are required to efficiently retain the primary chemical structure and the structure-associated health-beneficial bioactivities of polyphenols during thermal treatment; on the other hand, research efforts are expected elucidating the complex array of chemical conversions taking place during heating and characterizing the structure and bioactivity of reaction products formed.

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ABBREVIATIONS USED

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REFERENCES

ABTS, 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid); AGEs, advanced glycation endproducts; CE, collision energy; CEL, carboxyethyllysine; CML, carboxymethyllysine; DP, declustering potential; DPPH, 2,2-diphenyl-1-picrylhydrazyl; EMS, enhanced mass spectrometry; EPI, enhanced product ion; ESI, electrospray ionization; GO, glyoxal; GSE, grape seed extract; MGO, methylglyoxal; 2-MQ, 2-methylquinoxaline; OPD, o-phenylenediamine; Q, quinoxaline; SDS, sodium dodecyl sulfate; TEAC, Trolox equivalent antioxidant capacity

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ASSOCIATED CONTENT

S Supporting Information *

LC-MS total ion chromatograms of control cookie extract and quercetin-fortified cookie extract (showing the first 30 min only). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(M.W.) Phone: +852-22990338. Fax: +852-22990348. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 1647

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Journal of Agricultural and Food Chemistry

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dx.doi.org/10.1021/jf4045827 | J. Agric. Food Chem. 2014, 62, 1643−1648