Effects of Ph on Caramelization and Maillard Reaction Kinetics in Fructose-Lysine Model Systems E.H

Effects of pH on Caramelization and Maillard Reaction Kinetics in Fructose-Lysine Model Systems E.H. AJANDOUZ, L.S. TCHIAKPE, F. DALLE ORE, A. BENAJIBA, AND A. PUIGSERVER

ABSTRACT: The nonenzymatic browning reactions of fructose and fructose-lysine aqueous model systems were Food Chemistry and Toxicology investigated at 100 ЊC between pH 4.0 and pH 12.0 by measuring the loss of reactants and monitoring the pattern of UV-absorbance and brown color development. At all the pH values tested, the loss of fructose was lower in the presence than in the absence of lysine. And, in lysine-containing fructose solution, the sugar disappeared more rapidly than the amino acid. Lysine was moderately lost below pH 8.0. Caramelization of fructose, which accounted for more than 40% of total UV-absorbance and 10 to 36% of brown color development, may therefore lead to overestimating the Maillard reaction in foods. Keywords: Maillard reaction, caramelization, lysine, fructose

Introduction (Ellingson and others 1954; Bobbio and The aim of this study was, there- HE MAILLARD REACTION, WHICH others 1981; Baxter 1995). It has also fore, to describe the kinetics of the Tlinks the carbonyl group of reduc- been reported that the browning of nonenzymatic browning reaction in ing carbohydrates and the amino group fructose solutions was either more or fructose solutions heated either alone of free amino acids as well as of lysyl less extensive than that of glucose, de- or in the presence of lysine at 100 ЊC at residues in proteins, may have either pending on the heating conditions initial pH values ranging from 4.0 to beneficial or detrimental effects. At ear- (Kato and others 1969; Buera and oth- 12.0. The extent of both the caramel- ly stages of the reaction, an improve- ers 1987; Wijewickreme and others ization and the Maillard reactions was ment of the functional properties of 1997). On the other hand, the influence compared at the initial, intermediate, proteins was generally observed (Handa of pH on the Maillard reaction of amino and final stages. The kinetic behavior and Kuroda 1999), and afterwards anti- acid-containing glucose or fructose of the nonenzymatic browning reac- oxidant compounds are formed (Monti model systems has been studied at dif- tions of fructose was also compared to and others 1999) as well as highly ap- ferent pH ranges and conditions of that of glucose under the same experi- preciated browned flavors (Ho 1996). temperature and concentration of reac- mental conditions (Ajandouz and However, loss of lysine and decrease in tants, namely pH 4-6 (Buera and others Puigserver 1999). protein digestibility may also occur 1987), pH 5-7 (Petriella and others (Friedman 1996), together with some 1985), pH 5.5-7.5 (Baxter 1995) and pH Materials and Methods antinutritive (Oste and others 1987; 6-12 (Ashoor and Zent 1984). It is worth O’Brien and others 1994), toxic (O’Brien mentioning here that only the Maillard Materials and Morrissey 1989) or mutagenic browning intensity was measured in L-lysine, D-fructose, L-␣-amino-n- (Wang and others 1999) effects. The most of these studies. butyric acid and triethylamine (TEA) caramelization of sugars, which takes Very little attention has been paid so were purchased from Sigma Chemical place at the same time, also contributes far to the contribution of carameliza- Co. (St. Louis, Mo., U.S.A.). Phenyl- to nonenzymatic browning reactions. tion to the nonenzymatic browning re- isothiocyanate (PITC) and the standard In both the Maillard and caramelization actions of glucose or fructose, although mixture of amino acids were supplied reactions, highly UV-absorbing and col- studying the chemical reactions in- by Pierce Chemical Co. (St. Louis, Mo., orless compounds are formed at inter- volved in caramelization is a prerequi- U.S.A.). All the other chemicals used mediate stages, whereas the brown site for understanding the Maillard re- were of the purest commercially avail- polymers are formed at final stages action (Mauron 1981). It has, however, able grade. (Hodge 1953; Mauron 1981). been clearly established that the frag- The nonenzymatic browning reac- mentation of sugars occurs to a signifi- Heating procedure tion of fructose has not been as thor- cant extent at pH values below neutrali- An equimolar (0.05 M) mixture of oughly investigated as that of glucose, ty (O’Beirne 1986; Buera and others fructose and lysine (3 mL) was heated and it has usually been compared to the 1987) and increases considerably at in a 15-ml screw-sealed tubes for vari- latter. In several early studies (Maillard high pH values and temperatures, yield- ous periods of time in boiling water at 1912; Hodge 1953; Reynolds 1965), the ing colored N-free polymers (Myers various pH values. After 5, 15, 30, 60, 90, browning of fructose aqueous solutions and Howell 1992; Clarke and others and 120 min, the tubes were removed in the presence of amino acids in model 1997). The Maillard reaction in foods and immediately cooled in ice. Part of systems was found to take place more may then be overestimated and the em- the heated solution was used directly rapidly than that of glucose, although phasis placed on its detrimental rather for UV-absorbance, browning, and final the contrary was also reported to occur than its beneficial effects. pH measurements, while the rest was

926 JOURNAL OF FOOD SCIENCE—Vol. 66, No. 7, 2001 © 2001 Institute of Food Technologists Maillard and Caramelization Reaction Kinetics . . . stored at –20 ЊC for fructose and lysine intensity of the aqueous solutions con- well as in glucose solutions containing loss determinations. Fructose and taining fructose or fructose and lysine either lysine, or methionine or threo- lysine were separately heated under the were measured at room temperature at nine when heated to 100 ЊC at different same experimental conditions. The fol- 294 nm and 420 nm, respectively, using pH values (Ajandouz and Puigserver lowing buffers were used: 0.05 M sodi- a Beckman model DU 640 spectropho- 1999). In a number of studies in which um acetate adjusted to pH 4.0 with 1 M tometer (Beckman Instruments, Irwin, proteins were heated in the presence of acetic acid, 0.05 M sodium phosphate Calif., U.S.A.). When necessary, appro- reducing sugars at an intermediate adjusted to pH 6.0, pH 7.0 and pH 8.0 priate dilutions were made in order to moisture content and a wide range of using either monobasic or dibasic sodi- obtain an optical density of less than temperatures, a no-loss period was um phosphate, 0.05 M Tris-carbonate 1.5. found to occur when 50 to 75% of the adjusted to pH 9.0 with 1 M hydrochlo- All the experiments were carried out lysine was destroyed (Wolf and Thomp- ric acid, 0.05 M sodium carbonate-bi- in triplicate and the mean values (over- son 1977; Labuza and Saltmarch 1981). carbonate adjusted to pH 10.0 and pH all less than 10% standard deviation) The no-loss period might be due to 11.0 using either sodium carbonate or were used to draw up the kinetic plots. some limitation in the reactants or to sodium bicarbonate, and finally 0.05 M the release of amino groups at ad- sodium bicarbonate adjusted to pH Results and Discussion vanced stages of the Maillard reaction. 12.0 with 1 M sodium hydroxide. The decrease in pH throughout the Early stages in the browning heat treatment may also contribute to Fructose loss reactions the progressive lowering of the rates of The remaining nondegraded fruc- The initial stages in the nonenzymat- lysine loss. Under our experimental tose was monitored on high-perfor- ic browning reaction of the heated fruc- conditions, a pH drop of 0.25-3.8 units mance anion exchange-pulsed ampero- tose solutions, with or without lysine, and of 0.5 to 1.8 units occurred in fruc- metric detection equipment. Fructose was studied by measuring the extent of tose and fructose-lysine model systems Food Chemistry and Toxicology was eluted under isocratic conditions fructose and lysine degradation over after a 2-h heating period at starting pH from the CarboPac PA-100 (Dionex time. As expected, an increase in pH of values of 6.0 to 12.0, respectively. A Corp., Sunnyvale, Calif., U.S.A.) analyti- the heated solutions led to an increase more severe pH decrease has been re- cal anion exchange column (250 ϫ 4 in the initial rate of degradation of both ported in unbuffered solutions of gly- mm) equipped with an IonPac AG4A- fructose and lysine (Figure 1). Lysine cine and glucose (Nicoli and others SC (Dionex Corp., Sunnyvale, Calif., degradation was already reduced at pH 1993). More attention should, therefore, U.S.A.) guard column (25 ϫ 4 mm) con- 8.0, since it did not exceed 22% of the be paid to the pH decrease during non- nected to a gold working electrode cell. amino acid after 2 h of boiling, and only enzymatic browning reactions, espe- The eluent, a 5 mM sodium acetate so- 5% after 15 min as compared to higher cially at intermediate moisture content lution containing 0.1 M NaOH, was de- pH values (13 to 34% loss after 15 min and pH values below neutrality. livered at a rate of 1 mL.min–1 by a GP of heating at pH 9.0 to 12.0). Moreover, Studies on the Maillard reaction 40 Dionex gradient pump (Dionex lysine loss resulted almost completely, if placing emphasis on the degradation of Corp.,). The injection volume was deliv- not exclusively, from Maillard reaction, reducing sugars are not so numerous. ered by an AS 3500 Spectra System as no significant loss was observed Warmbier and others (1976) have re- autosampler from Thermo Separation when the amino acid was heated in the ported that lysine loss was higher than Products (Fremont, Calif., U.S.A.) and absence of fructose. Most of the loss that of glucose in a casein-glucose-glyc- Њ the detection was carried out with an was, therefore, observed during the be- erol model system at 45 C and a aw val- ED 40 electrochemical detector (Dionex ginning of heating; a no-loss period oc- ue ranging from 0.57 to 0.85, whereas Corp.). The area under the eluted peak curred at later heating stages. A no-loss Baisier and Labuza (1992) have shown was integrated with an Olivetti Pentium period has also been observed in a gly- that glycine and glucose losses oc- P 75i integrator (Olivetti, Paris, France) cine-containing glucose solution stored curred with the same rate constant in a using the Borwin chromatography Soft- at 37 ЊC (Baisier and Labuza 1992), as solution stored at 37 ЊC. More recently, ware program (JMBS, Grenoble, France), based on a 0- to 250-pmol fructose calibration chart.

Amino acid loss The unreacted lysine was monitored by performing reverse phase high-per- formance liquid chromatography after pre-column derivatization with phenyl- isothiocyanate as described by Bidling- meyer and others (1984). The eluted amino acid was detected at 254 nm using a model 486 variable wavelength detector (Waters Assoc., Milford, Mass., U.S.A.), and the resulting peak was integrated as described above for fructose.

UV-absorbance and brown color measurements Figure 1—Time course of fructose (A) and lysine (B) loss in lysine-containing The UV-absorbance and browning fructose solutions when heated at 100 ЊC and at varying pH values.

Vol. 66, No. 7, 2001—JOURNAL OF FOOD SCIENCE 927 might be due to the contribu-

The final stages in the nonenzymatic Intermediate stages in the nonenzy- Intermediate stages tion of some Maillard reaction products as well as to the to the UV-absorbance, accelerating effect of the amino acid on sugar caramelization reaction. As stated by Mauron (1981), most reactions that occur in pure sugars only at very high temperatures take also place at much lower temperature once they have re- acted with amino acids. Based on the curves of slopes of the UV-absorbance the solutions heated between pH 4.0 and pH 7.0, the caramelization reactions were found to account for as much as 40 to 62% of the UV-absorbing reaction products of fructose-lysine mixtures. matic browning reactions were detect- matic browning reactions at the UV-absorbance recording ed by by Lerici and oth- 294 nm as described shows that, in both ers (1990). Figure 3 of lysine in the absence and presence the higher the the fructose solution, the higher was the starting pH value, When fructose was heated absorbance. values ranging from alone at initial pH accumulation 4.0 to 7.0, a progressive degradation prod- of the intermediate ucts occurred as a function of time and no lag time was observed, whereas the hyperbolic curves which were obtained at pH 8.0 and pH 9.0 no doubt reflected the high level of fructose degradation occurring during the initial stages of the heating period (Figure 2). At higher pH quickly the UV-absorbance values, reached the maximum value and de- with in agreement thereafter, creased the almost complete degradation of fructose during the 1st stages in the in the UV- The decrease heating period. absorbance may result from the transformation of some intermediate products into brown polymers. In the presence of lysine, the amount of intermediate products accumulated was higher than in the absence of lysine and fitted a zero-order reaction model up to pH 9.0, as compared to pH 7.0 in the absence of the amino acid. Moreover, even at pH 12.0, no decrease in the absorbance was observed. The difference found in the kinetic behavior of fructose alone, and fructose in the presence of lysine, Final stages Intermediate stages Intermediate starting concentration of the sugar was the sugar of concentration starting to define It is not really easy the same. of the respective contribution precisely reaction to and Maillard caramelization nonenzymatic browning. the overall dation was actually lowered in the pres- in the lowered was actually dation to that of as compared ence of lysine effect alone. This protective fructose interac- to some reversible may be due the keto- takes place between tion which it was par- amino acid, since se and the values be- at pH ticularly conspicuous the 2 lysine amino tween 8.0-11.0 where A similar pro- groups are deprotonated. also been observed tective effect has heated in the pres- when glucose was or threonine ence of lysine, methionine What- 1999). (Ajandouz and Puigserver the active con- ever the pH conditions, in the presence of centration of fructose different from that lysine is significantly although the in the absence of lysine, C Њ —Vol. 66, No. 7, 2001 66, No. —Vol. C and increasing pH values. Њ fructose degra- ure 2, ure 2, C at increasing pH values. Њ JOURNAL OF FOOD SCIENCE As shown in Fig C for 8 min, both fructose and glucose both fructose and C for 8 min, 928 Figure 3—Time course of the appearance of UV-absorbance in aqueous solu- 3—Time course of the appearance of UV-absorbance Figure heat- tions of fructose (A) and in lysine-containing fructose solutions (B) after ing to 100 Figure 2—Percentage of fructose loss when it is heated for 5 min alone or in Figure 2—Percentage of fructose loss the presence of lysine at 100 at pH values ranging from 4.0 to 12.0 ranging from at pH values When 1999). and Puigserver (Ajandouz to 150 extracts were heated cocoa bean Њ whereas the loss were totally destroyed, did not exceed 50% of free amino acids A high temper- (Mohr and others 1971). some carameliza- ature may promote in a higher deg- tion reaction resulting sugar as com- radation of reducing and, consequent- pared to amino acids in changing the amino group-to-re- ly, ratio. ducing sugar molar Maillard and Caramelization Reaction Kinetics . . . . Kinetics Reaction Caramelization and Maillard more rapidly to be was found glucose or than lysine, or methionine degraded to 100 in solutions heated threonine

Food Chemistry and Toxicology Maillard and Caramelization Reaction Kinetics . . . browning reaction of fructose heated in beginning of the heating period, re- were 25% and 43% at pH 4.0 and pH the presence or absence of lysine were mained unchanged thereafter. The 6.0, respectively. It should be noted, studied by measuring the absorbance at promoting effect of pH on browning however, that a 4-fold molar excess of 420 nm. The time course of the devel- development was in agreement with fructose with regard to lysine was used opment of the brown color at increas- the literature data (Ashoor and Zent in their study and that in addition the ing pH values is shown in Figure 4. The 1984; Petriella and others 1985; Buera reactivity of lysine and glycine may not similarity between most of these curves and others 1987; Baxter 1995). be the same. and the UV-absorbance curves suggests The rating of fructose caramelization In all cases, the caramelization reac- that a large proportion of the interme- as a percentage of the overall brown tions contributed less to the browning diate products are precursors of brown color intensity of the lysine-containing intensity than to the overall UV-absor- polymers. No decrease in the absor- fructose solution between pH 4.0 and bance. The UV-absorbance values are bance at 420 nm of the fructose solu- pH 7.0 was found to be between 10 and more or less representative of interme- tion was observed at pH 10.0 and at 36%. These values were lower than diate compounds in the nonenzymatic pH 12.0, in sharp contrast with the UV- those obtained by Buera and others browning reactions (Hodge 1953; Lerici absorbance and the maximum brown (1987) in an aqueous glycine-fructose and others 1990), whereas the absor- color value, which was reached at the model system heated to 55 ЊC, which bance values at 420 nm may be related to the content in brown polymers, and the 294/420 nm absorbance ratio is in- dicative of the polymerization extent. Figure 5 illustrates the effect of pH on the transformation of intermediates into brown polymers in fructose and fructose-lysine model systems after a Food Chemistry and Toxicology 1-h heating period at 100 ЊC. As regards the caramelization reaction of fructose, a pH increase from 4.0 to 8.0 strongly enhanced the polymerization of the carbonyl compounds generated by the thermal degradation of fructose, whereas at a higher pH value, the intermediates were almost equally trans- formed into brown polymers. When fructose was heated in the presence of lysine at a pH value between 4.0 and 8.0, formation of the intermediate poly- Figure 4—Time course of brown color development in aqueous solution of merization products decreased, where- fructose (A) and in lysine-containing fructose solution (B) heated to 100 ЊC at as above pH 8.0 the rate of polymeriza- increasing pH values. The symbols are the same as in Figure 1. tion reactions of the intermediate carbonyl compounds seem not to be de- pendent on both the pH and the presence of the amino acid. Figure 5 strongly suggests that changes in the mecha- nism of the nonenzymatic browning reaction occurred around pH 8.0. Ac- cording to the general scheme of Hodge (1953), dealing with the chemistry of nonenzymatic browning reactions in model systems, the Amadori compounds are supposed to give rise preferentially to furfurals and related derivatives under acidic conditions, while reductones and highly reactive di- carbonyls are formed essentially under alkaline conditions. As pointed out in the introduction, there are some conflicting data about the reactivity of fructose as compared to glucose as far as browning is con- cerned. With respect to the intermediate stages of the Maillard reaction, volatile compounds such as furans, pyrans, and pyrroles have been obtained in Figure 5—Influence of pH on the transformation of UV-absorbing compounds into larger amounts from fructose than brown polymers in fructose and lysine-containing fructose solutions heated to from glucose when each sugar was 100 ЊC for 60 min. The plots at 30 min of heating are shown in the insert. heated in the presence of â-alanine

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YIELDING

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CONCLUSION??? ADDITION

N into the nonenzymatic browning re- into the nonenzymatic I especially at pH val- action of fructose, 7.0), the results of ues of foods (below some useful prac- our study may have at both the food tical implications levels as to technology and nutritional should be how low alkaline conditions managed, plant proteins such as those from soy- bean which may be exposed to high al- kalinity levels during their processing. Further studies are now required to be able to distinguish more clearly between the caramelization reaction and that initiated by the interaction between the sugar and protein amino groups in food. The increase in the ali- mentary use of free fructose during the last 2 decades, mainly in the form of high-fructose corn syrup (Park and the 1993), has brought to the fore Yetly question as to how it is degraded during heat treatment, especially at pH and what reac- neutrality, above values shown in Figure 6, the caramelization 6, the in Figure shown exponen- glucose increased reaction of while a pH 7.0 up to pH 12.0, tially from the cara- in the rate of linear increase oc- reaction of fructose melization No fur- the same pH range. curred in the was observed regarding ther change was added to glu- browning when lysine cose and fructose solutions. —Vol. 66, No. 7, 2001 66, No. —Vol. JOURNAL OF FOOD SCIENCE A possible explanation of this obser- A possible explanation 60 min. 930 Figure 6—Effect of pH on browning development in aqueous fructose, glu- Figure 6—Effect of pH on browning development in aqueous fructose, cose, fructose-lysine and glucose-lysine model systems heated to 100 Maillard and Caramelization Reaction Kinetics . . . . Kinetics Reaction Caramelization and Maillard Other 1993). Bernhard and (Nishibori from with the nitrogen atoms products (Amra- such as pyrazines amino acids, as well as and others 1995), ni-Hemaimi aspar- products from more specific were and Lawrence 1995) agine (Shu of the formed irrespective apparently Some sugar. of the reducing nature seem very likely mechanistic differences the nonenzymatic to exist between of fructose and that browning reaction for example, by of glucose as suggested, time in the brown- the existence of a lag and not in that ing of glucose solutions (Kato and others of fructose solutions 1987; Ajandouz 1969; Buera and others and Puigserver 1999). formation of some vation might be the rate-limiting intermediates during the nonenzymatic browning reaction of glucose, but not in that of fructose, as suggested by Kato and others (1969). In this connection, it seems very likely that a multiple Heyns products rather than single Amadori product are formed in the Maillard reaction (Suarez and others 1989). Another point which should certainly be taken into consideration when comparing the reactivity of glucose and fructose is the concomitant caramelization reactions. This should not be overlooked, as these reactions may also give rise to a variety of products from each sugar (Shaw and others 1996). As and Montgomery Yang 1968;

Food Chemistry and Toxicology Maillard and Caramelization Reaction Kinetics . . .

specification of caramel colours: an overview. Fd (6): 938-942. of glycerol on nonenzymatic browning in a solid Chem Tox 30 (4): 356-363. Park YK, Yetly EA 1993. Intakes and food sources of intermediate moisture model food system. J Food Nicoli MC, Anese M, Lerici CR. 1993. Effect of pH on fructose in the United States. Am J Clin Nutr 58 (Suppl) Sci 41 (3): 528-531. the kinetics of non-enzymatic browning in heat- : 737S-747S. Wijewickreme AN, Kitts DD, Durance TD. 1997. Reac- treated glucose-glycine model systems. Ital J Food Petriella C, Resnik SL, Lozano RD, Chirife J. 1985. Ki- tion conditions influence the elementary composi- Sci 5 (2): 139-144. netics of deteriorative reactions in model food sys- tion and metal chelating affinity of nondialyzable Nishibori S, Bernhard RA. 1993. Model systems for tems of high water activity: Color changes due to model Maillard reaction. J Agric Food Chem 45 (12): cookies: Volatile components formed from the reac- nonenzymatic browning. J Food Sci 50 (3): 622-625. 4577-4583. tion of sugar and ␤-alanine. J Agric Food Chem 41 Reynolds TM. 1965. Chemistry of nonenzymatic Wolf JC, Thompson DR. 1977. Kinetics of available (12): 2374-2377. browning. II. Adv Food Res 14: 167-283. lysine loss during thermal processing of a soy pro- O’Beirne D. 1986. Effect of pH on nonenzymatic Shaw PE, Tatum JH, Berry RE. 1968. Base-catalyzed tein isolate: Reversion of no-loss to rapid-loss browning during storage in apple juice concentrate fructose degradation and its relation to nonenzy- phase. J Food Process Preserv 1 (4): 271-278. prepared from bramley’s seedling apples. J Food Sci matic browning. J Agric Food Chem 16 (6): 979-982. Yang BY, Montgomery R. 1996. Alkaline degradation 51 (4): 1073-1074. Shu C-K, Lawrence BM. 1995. 3-Methyl-2(1H)-pyrazi- of glucose: effect of initial concentration of reac- O’Brien J, Morrissey PA. 1989. Nutritional and toxico- none, the asparagine-specific Maillard products tants. Carbohydr Res 280 (1): 27-45. logical aspects of the Maillard browning reaction formed from asparagine and monosaccharides. J MS 20000507 in foods. Crit Rev Food Sci Nutr 25 (3): 211-248. Agric Food Chem 43 (3): 779-781. O’Brien JM, Morrissey PA, Flynn A. 1994. Alteration Suarez G, Rajaram R, Oronsky AL, Garvinowicz MR. Ajandouz is grateful to the Institut National de la Recherche of mineral metabolism and secondary pathology in 1989. Nonenzymatic glycation of bovine serum al- Agronomique for a 2-year fellowship. We also thank Mr. C. Villard for technical assistance, and Mrs. J. Blanc for revising rats fed Maillard reaction products. In: Labuza TP, bumin by fructose (fructation). Comparison with the the English manuscript. Reineccius GA, Monnier V, O’Brien JM, Baynes J, Maillard reaction initiated by glucose. J Biol Chem editors. Maillard Reactions in Chemistry, Food and 264 (7): 3674-3679. Authors are with the UMR Université Aix- Health. Cambridge, U.K: The Royal Society of Chem- Wang MF, Jin Y, Li JG, Ho CT. 1999. Two novel beta- Marseille III/INRA, Marseille. Direct inquiries to istry. p. 397-401. carboline compounds from the author Puigserver at: Antoine.Puigserver@ Oste RE, Miller R, Sjostrom H, Noren O. 1987. Effects of Maillard reaction between xylose and tryptophane. Maillard reaction products on protein digestion. J Agric Food Chem 47 (1): 48-50. llbn.u-3mrs.fr Studies on pure compounds. J Agric Food Chem 35 Warmbier HC, Schnickels RA, Labuza TP. 1976. Effect Food Chemistry and Toxicology

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