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Effect of maltodextrins on the surface activity of small-molecule surfactants
Colloids and Surfaces B: Biointerfaces 31 (2003) 47 /54 www.elsevier.com/locate/colsurfb
Effect of maltodextrins on the surface activity of smallmolecule surfactants Maria G. Semenova *, Larisa E. Belyakova, Anna S. Antipova, Yu. N. Polikarpov, Lida Klouda, Anna Markovic, Michael M. Il’in Institute of Biochemical Physics, Russian Academy of Sciences, Vavilov str. 28, Moscow 119991, Russia Received 19 August 2002; received in revised form 13 November 2002; accepted 22 January 2003
Abstract We report on the effect of commercially important polysaccharides (maltodextrins with variable dextrose equivalent (Paselli SA-2, MD-6 and MD-10) on the surface activity at the air /water interface of small-molecule surfactants (sms), possessing different hydrophobic /lipophilic balance ((SSL (Na ), the main component is a sodium salt of stearol / lactoyl lactic acid, and PGE (080), polyglycerol ester of C18 fatty acid), and widely used in food products. A marked change of the surface activity of sms was found in the presence of maltodextrins by tensiometry. The combined data of laser multiangle light scattering and mixing calorimetry have suggested that this result is governed by specific complex formation between maltodextrins and sms in aqueous medium. Measurements have been made of the molar mass, the second virial coefficient and the enthalpy of intermolecular interactions in aqueous solutions. The implication of a degree of polymerization of maltodextrins in this phenomenon was shown. The interrelation between the molecular parameters of the formed complexes and their surface activity at the air /water interface has been revealed and discussed. # 2003 Elsevier B.V. All rights reserved. Keywords: Maltodextrin; Small-molecule surfactants; Molecular parameters; Thermodynamics of interactions; Surface activity
1. Introduction It is well known that the major components and derivatives of starch, which are widely used in food manufacture because of their unique gel-forming characteristics, are able to form complexes or
* Corresponding author. Tel.: /7-95-135-9290; fax: /7-95135-5085. E-mail address: [email protected] (M.G. Semenova).
inclusion compounds with a variety of ligands such as lipids, emulsifiers and flavor substances [1 /11]. It was established that this interaction could influence starch gelatinization [12 /14] or gelation [8,14 /17]. Alternatively, one would expect that these polysaccharides could have an effect on properties of bound ligands and, in particular, on the surface behavior of small-molecule emulsifiers, but until recently this effect was not practically investigated and understood. There are only a few works [10,11,18 /20] that disclose the effects of
0927-7765/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0927-7765(03)00042-0
48
M.G. Semenova et al. / Colloids and Surfaces B: Biointerfaces 31 (2003) 47 /54
different in nature polysaccharides on the surface behavior or emulsifying ability of small-molecule surfactants (sms). In our recent previous works, devoted to this subject, we have revealed the dramatic distinction between the effects of the maltodextrins on the surface behavior of two sms, namely,model */ sodium caprate and industrially important */CITREM (the main component is a citric acid ester of monostearate) [10,11]. An unexpected marked increase in the surface activity of Na-caprate in mixture with maltodextrins was found, increasing with both concentration of Na-caprate and a decrease in a dextrose equivalent (DE) of maltodextrins, and vice versa a pronounced decrease in the surface activity of CITREM. Hence the study underlined need for further systematic investigations with use of other classes of sms. Thus, this work follows on from our earlier research [10,11] and investigates the effect of commercially important polysaccharides (maltodextrins with variable DE (Paselli SA-2, MD-6 and MD-10)) on the surface activity at the air / water interface of sms, possessing different hydrophilic/lipophilic balance ((SSL (Na). The main component is a sodium salt of stearol /lactoyl lactic acid), and PGE (080), polyglycerol ester of C18 fatty acid), widely used in food products [21]. We have also attempted to establish a likely molecular mechanism underlying the effects studied using a number of thermodynamic methods.
salt of stearol /lactoyl lactic acid) and PGE (080) (polyglycerol ester of C18 fatty acid) [21] were supplied by Danisco Cultor (Denmark). Under experimental conditions (phosphate buffer, pH 7.2, ionic strength 0.05 mol dm 3) the critical micelle concentrations (cmc) for SSL (Na) and PGE (080) are 3.5 and 1.3 mg dm 3, respectively. 2.2. Methods 2.2.1. Preparation of maltodextrin solutions Maltodextrins with variable DE were dissolved in an aqueous phosphate buffer (pH 7.2, ionic strength 0.05 mol dm 3, 0.01 wt.% sodium azide as an antimicrobial agent) by mechanical stirring at 85 8C for 1 h. Thereafter the solutions were allowed to air cool to room temperature. These solutions were used in all experiments. Concentration of the maltodextrins in solutions was checked using a Shimadzu (Japan) refractometer, on the known value of maltodextrin refractive index increment ground (0.150/103 m3 kg1).
2.1. Materials
2.2.2. Preparation of mixed solutions of smallmolecule surfactants with maltodextrins Stock solutions of the sms (0.01 wt.%) were prepared by ultrasound sonication 4.5 MHz over 1 h while the solutions were shaking at 65 8C (CPLAN water bath shaker, type 357, Poland). Thereafter the stock solutions, cooled to room temperature, were used to prepare mixed solutions with the maltodextrins of the required concentrations, whereupon the mixed solutions were shaken at 40 8C for 1 h (CPLAN water bath shaker, type 357, Poland) and then were allowed to be cooled to room temperature.
Three commercial samples of maltodextrins (Paselli SA-2, MD-6 and MD-10, Avebe) were used as supplied. They were enzymatic products of the hydrolysis of a potato starch having values of a DE equating with 2, 6 and 10, respectively. Phosphate buffered (pH 7.2, ionic strength 0.05 mol dm 3) solutions were prepared using analytical grade reagents (of 99.9% purity), and a double distilled water; 0.01 wt.% sodium azide was added to the buffer as an anti-microbial agent. Sms: SSL (Na) (the main component is a sodium
2.2.3. Determination of the surface tension, g, at the planar air /water interface Values of the surface tension, g , of the solutions of the sms, the maltodextrins and their mixtures were monitored with an accuracy of 1 mN m1 with the Wilhelmy plate technique [22] using a Kruss GmbH digital tensiometer K 10 (Hamburg, Germany). All measurements were made in the thermostatic cell at 259/0.5 8C. A platinum plate was used in measuring the surface tension. The surface tension was monitored continuously as a
2. Experimental
M.G. Semenova et al. / Colloids and Surfaces B: Biointerfaces 31 (2003) 47 /54
function of time. The values of g presented in this work are averaged data for at least two repetitions of each of the experiments. The value of the surface tension between a double-distilled water and air did not differ from the standard value within the range of the experimental error and was 72 mN m 1 at 25 8C independently of time. A slight decline in the surface tension for pure maltodextrin solutions (0.5% w/v) (Figs. 1 and 2) is most likely attributable to the contribution from impurities of both a protein and lipid nature, found in potato starch [23]. 2.2.4. Estimation of molecular parameters of maltodextrins and thermodynamic parameters of the maltodextrin /maltodextrin interaction in a bulk aqueous medium The weight-average molecular weight, Mw, and the second virial coefficient, A2 (the thermodynamic interaction parameter, A2, measures the strength and nature of the interaction between pairs of macromolecules in biopolymer solutions and thereby characterizes thermodynamic affinity of macromolecules for aqueous medium or for
49
each other) for maltodextrin molecules at concentration of the sms below the cmc were determined in dilute aqueous solutions of maltodextrins (0.1 / 0.5% w/v) from laser static light scattering. All measurements were made in the thermostatic cell at 259/0.5 8C. The Rayleigh ratio Ru was measured using vertically polarized light (633 nm) at angles in the range 40 5/u ]/1408 with a VA Instruments LS-01 apparatus (St Petersburg, Russia) calibrated with dust-free benzene (R90 / 11.84 /106 cm 1). Solutions were filtered through a membrane (a pore size of 0.45 mm (Millipore)) directly into the light scattering cell, and the biopolymer concentration was checked each time after filtration. The raw data were used to plot the angular and concentration dependencies of the ratio (HC /(DRU )) according to the Zimm method [24]. Here C is the biopolymer concentration g ml1, DRU is the excess light scattering over that of the pure solvent at angle u , and H is an instrumental optical constant equal to 4p2n2n2/NAl4 where NA is Avogadro’s number; l , the wavelength of an incident light in vacuo; n, the refractive index of the solvent and n is the refractive index increment of the biopolymer.
Fig. 1. Effect of maltodextrins (Cmaltodextrin /0.5 wt.%) with variable DE on the surface behavior of SSL (Na ) at the planar air / water interface (0.05 M phosphate buffer, pH 7.2, 25 8C). The interfacial tension, g , is plotted as a function of time for: maltodextrins without SSL (Na ) (×/ ×/ ×/j×/ ×/ ×/); mixtures of SSL (Na ) with maltodextrins: (-'-) SA-2; (-"-) MD-6; (-m-) MD-10; SSL (Na ) alone ( ×/ ×/ ×/k×/ ×/ ×/). (a) CSSL (Na) /2 mg dm 3 B/cmc/3.5 mg dm 3; (b) CSSL (Na) /6 mg dm 3 /cmc/3.5 mg dm 3.
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Fig. 2. Effect of maltodextrins (Cmaltodextrin /0.5 wt.%) with variable DE on the surface behavior of PGE (080) at the planar air /water interface (0.05 M phosphate buffer, pH 7.2, 25 8C). The interfacial tension, g , is plotted as a function of time for: maltodextrins without PGE (080) ( ×/ ×/ ×/j×/ ×/ ×/); mixtures of PGE (080) with maltodextrins: (-'-), SA-2; (-"-), MD-6; (-m-), MD-10; PGE (080) alone (×/ ×/ ×/k×/ ×/ ×/). (a) CPGE (080) /1 mg dm3 B/cmc/1.3 mg dm 3; (b) CPGE (080) /2 mg dm 3 /cmc/1.3 mg dm 3.
Values of the weight-average molecular weight, Mw, were estimated as average from the intercepts of both the concentration dependence of HC /DRU as u 0/0 and the angular dependence of HC /DRU as C 0/0. Values of the second virial coefficient A2 /Apol pol were estimated from the slope of the concentration dependence of HC /DRU as u 0/0. The errors in the determination of the value of weight-average molecular weight and the second virial coefficient are 10%. Refractive indexes increments, n, for maltodextrins in the absence or presence of the sms were determined at 436 nm using a Shimadzu differential refractometer (Japan). All measurements were made in the thermostatic cell at 259/0.5 8C. The values of n at l /633 nm were calculated from an equation for dispersion of the refractive index [25] with the correction coefficient taken as 0.9. Values of the refractive index increment for maltodextrins within the experimental error (9/ 10%) were the following for maltodextrins without sms and in the presence of SSL (Na/) and PGE (080): n/0.15 /103; 0.137 /10 3 and 0.136 / 103 m3 kg1, respectively.
2.2.5. Mixing calorimetry Calorimetric measurements were made using a LKB 2277 flow calorimeter set at 22 8C. A peristaltic pump pumped the reactants into the instrument. The pump was calibrated by measuring the time required to pump a known volume of solution into the calorimeter. The flow rate was equal to 9/106 l s 1. The ratio of the flow rates in the two channels was close to 1, i.e. the solutions of the maltodextrins or the sms in all cases were diluted by a factor of two under mixing. Both solutions were thermally equilibrated before entering the reaction vessel. A calibration of the calorimeter itself was done electrically at the temperature of a measurement. The sensitivity of the calorimetric measurement is no less than 3 / 106 J s 1. Thermal effects were observed during dilution of: (i) the maltodextrin solution by the pure buffer Qmaltodextrin buffer; (ii) the solution of sms by the pure buffer Qsmall-molecule surfactant buffer; (iii) the maltodextrin solution by the solution of sms QS. These thermal effects Q were measured in thermal power units (J s1).
M.G. Semenova et al. / Colloids and Surfaces B: Biointerfaces 31 (2003) 47 /54
The specific enthalpy of the interaction between the maltodextrins and the sms was obtained from the relationship: DHmaltodextrinsmallmolecule
surfactant
(QSQmaltodextrinbuffer Qsmallmolecule
surfactantbuffer =Dn
(3)
where Dn is the number of grams of maltodextrin mixed with sms per second (g s 1).
3. Results and discussion A marked decrease in the surface activity of both SSL (Na ) and PGE (080) (a rise in the surface tension, g ) was found in the presence of the maltodextrins (0.5% w/v) as shown in Figs. 1 and 2. As this takes place, the most pronounced increase in the surface tension (a decrease in the surface activity) of the mixed solutions was observed, on the one hand, for concentrations of sms above their cmc (Fig. 1b, Fig. 2b, Fig. 3), and
Fig. 3. Effect of maltodextrins (Cmaltodextrin /0.5 wt.%) with variable DE on the surface behavior of SSL (Na ) at the planar air /water interface (0.05 M phosphate buffer, pH 7.2, 25 8C). The difference between values of the interfacial tension, g , of mixture (maltodextrin/sms) and sms alone is plotted as a function of concentration of SSL (Na ) for: ', maltodextrin SA-2; ", maltodextrin MD-6.
51
on the other hand for maltodextrins with a lower degree of hydrolysis, that is DE. By way of illustration Fig. 3 shows a sharp decay of the surface activity of SSL (Na ) in the close vicinity and above its cmc in the presence of maltodextrins SA-2 and MD-6 (0.5% w/v). To gain greater insight into the effect found, we have attempted to elucidate the nature of the interaction between the maltodextrins and sms in aqueous medium by measurement of the enthalpy of their interactions from mixing calorimetry first. In the case of SSL (Na ), Fig. 4a shows the dominant endothermic character of the interaction between the maltodextrins and SSL (Na ), that is more pronounced at SSL (Na ) concentrations above cmc. As this takes place, maltodextrin SA-2 (with the largest extent of polymerization) shows the most pronounced endothermic character of the interaction. This result suggests that the interaction between the maltodextrins and SSL (Na) in aqueous medium is of a hydrophobic character [26], probably through a rather long hydrocarbon chain in the SSL (Na ) molecule (C17) and the hydrophobic parts of the glucose units in the polysaccharide molecules. The distinctive feature of the interaction between the maltodextrins and PGE (080) in aqueous medium is the pronounced exothermic character of it (Fig. 4b). As this takes place, the maltodextrin SA-2 (with the largest extent of polymerization) once again shows the greatest thermal effect of the interaction. In this case it is considered that the observed exothermic character is governed for the most part by hydrogen bonding between the maltodextrins and PGE (080), owing to the great number of hydroxyl groups in their molecules [3,21]. Besides, this effect could be attributable to formation of a helical conformation of the maltodextrins due to hydrogen bonding, especially for maltodextrin SA-2. The capacity of starch polysaccharides to undergo a conformational transformation upon interaction with lipophilic compounds giving inclusion complexes is well known in the literature [2,27,28]. Some contribution to the total exothermic character from the breakdown of micelles of PGE (080) could also be suggested.
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Fig. 4. Specific enthalpy, DH , of the interaction between the maltodextrins and sms in aqueous medium (0.05 M phosphate buffer, pH 7.2, 25 8C) vs. the sms concentration in the mixed solutions: (a) SSL (Na ) and (b) PGE (080), ', maltodextrin SA-2; ", maltodextrin MD-6; m, maltodextrin MD-10.
Overall, the mixing calorimetry data suggest complex formation between the studied sms and the maltodextrins through mainly the hydrophobic interaction in the case of SSL (Na ) and hydrogen bonding in the case of PGE (080), depending on their principal structural distinctions. Table 1 shows the molecular parameters: weight-average molecular weight, Mw, and second virial coefficient, Apol pol, of the formed complexes in a bulk aqueous medium, which have been obtained by laser static light scattering at the concentration of sms below the cmc. Data from laser static light scattering point up a marked increase in the thermodynamic affinity of the
maltodextrins for aqueous medium that is an increase in positive values of the second virial coefficients [29 /31], as a result of the interaction with both sms. This increase is more pronounced in the cases of the maltodextrins with higher DE, namely, MD-6 and MD-10. For maltodextrin SA2, the only slight change in the thermodynamic affinity of the formed complex for aqueous medium is most likely attributable to the formation of the inclusion complex with hiding of the most parts of the attached sms molecules in the interior of the complex. Moreover, an intensive association of the maltodextrin molecules follows the interaction be-
Table 1 Effect of sms on the molecular parameter of maltodextrins in aqueous medium (pH 7.2, ionic strength 0.05 mol dm3, 25 8C): the weight-average molecular weight, Mw, and the thermodynamics of pair interactions (the second virial coefficient Amd md) System
Maltodextrin SA-2 Maltodextrin MD-6 Maltodextrin MD-10
Without sms
With 2 mg dm 3 SSL (Na )
With 0.75 mg dm 3 PGE (080)
Mw (kDa)
Amd md 105 (m3 mol kg2)
Mw (kDa)
Amd md 105 (m3 mol kg 2)
Mw (kDa)
Amd md 105 (m3 mol kg 2)
260 100 45
20.5 8.1 2.2
515 192 183
25.6 39.9 59.2
540 175 92
34.0 50.1 129.5
M.G. Semenova et al. / Colloids and Surfaces B: Biointerfaces 31 (2003) 47 /54
tween the maltodextrins and sms, as if the sms molecules play the role of the component bridging molecules of the maltodextrins through hydrogen bonding or hydrophobic interactions. Based on molar concentrations of the components used in light scattering experiments (CSSL (Na) /4.67 / 106 mol dm 3; CPGE (080) /1.48 /10 6 mol dm 3; Cmaltodextrins from 4/106 to 1/104 mol dm 3 depending on the DE) the calculation of the molecular weight of hypothetical maltodextrin /surfactant complexes, which was rested on the assumption that all amount of added surfactants were bound equally by the maltodextrin molecules in solution, provides evidence for this suggestion by virtue of the fact that the calculated values are in several times smaller than those found by light scattering. On the strength of the combined data of mixing calorimetry and static light scattering, it may be safely suggested that the pronounced decrease in the surface activity of both SSL (Na ) and PGE (080) in the presence of the maltodextrins is governed principally by the formation of highly hydrophilic and consequently lower surface active complexes between them by the way when the hydrophobic patches of the interacting molecules are hidden in the interior of the complexes, whereas the hydrophilic ones are directed into aqueous medium.
4. Conclusions 1) The effect of maltodextrins on the surface activity of sms is mainly governed by formation of rather hydrophilic complexes between them in aqueous medium. In addition, a plausible formation of inclusion complexes in the case of maltodextrins with lower DE could also contributes to the marked decrease in the surface activity of sms found. 2) Both hydrophobic interactions and hydrogen bonding between sms and maltodextrins could underlie the complex formation between them, depending on the specific structural features of the sms structures. 3) The interaction of maltodextrins with micelles of sms promotes the complex formation
53
between them, supposedly by virtue of a marked increase in the number of their molecular contacts.
Acknowledgements The authors are most grateful to Danisco (Denmark) for the free supply of the SSL (Na) and PGE (080) samples.
References [1] B. Conde-Petit, F. Escher, J. Rheol. 39 (1995) 1497. [2] J. Nuessli, B. Sigg, B. Conde-Petit, F. Escher, Food Hydrocolloids 11 (1997) 27. [3] I.J. Harkema, Ingredients Handbook. Fat Substitutes, Leatherhead Food RA, Surrey, 1998, p. 103. [4] F. Osman-Ismail, J. Solms, Die Starke 24 (1972) 213. [5] F. Osman-Ismail, J. Solms, Lebensm.-Wiss. Technol. 6 (1973) 147. [6] M.A. Rutschmann, J. Solms, Lebensm.-Wiss. Technol. 23 (1990) 70. [7] M.A. Rutschmann, J. Solms, Lebensm.-Wiss. Technol. 23 (1990) 84. [8] J. Nuessli, B. Conde-Petit, U. Ruth Trommsdorff, F. Escher, Carohydr. Polym. 28 (1995) 167. [9] M.G. Semenova, A.S. Antipova, L.E. Belyakova, Y.N. Polikarpov, T.A. Misharina, M.B. Terenina, R.V. Golovnya, in: D. Roberts, A. Taylor (Eds.), Flavor Release: Linking Experiments, Theory and Reality, American Chemical Society, Washington, DC, 2000, p. 260. [10] M.S. Myasoedova, M.G. Semenova, L.E. Belyakova, A.S. Antipova, Colloids Surf. B 21 (2001) 179. [11] M.G. Semenova, M.S. Myasoedova, A.S. Antipova, in: E. Dickinson, R. Miller (Eds.), Food Colloids 2000: Fundamentals of Formulation, The Royal Society of Chemistry, 2001, p. 233. [12] K. Ohashi, G. Goshima, H. Kusuda, H. Tsuge, Starch/ Starke 32 (1980) 54. [13] S. Akuzawa, S. Sawayama, A. Kawabata, Biosci. Biotech. Biochem. 59 (1995) 1605. [14] E. Svensson, K. Autio, A.-C. Eliasson, Food Hydrocolloids 12 (1998) 151. [15] S.N. Raphaelides, Food Hydrocolloids 7 (1993) 479. [16] N. Krog, Starch/Starke 6 (1971) 206. [17] N. Krog, B. Nybo-Jensen, J. Food Technol. 5 (1970) 77. [18] E. Dickinson, M.I. Goller, D.J. Wedlock, Colloids Surf. A 75 (1993) 195. [19] E. Dickinson, M.I. Goller, D.J. Wedlock, J. Colloids Interf. Sci. 172 (1995) 192.
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[20] V.G. Babak, in: E. Dickinson, R. Miller (Eds.), Food Colloids 2000: Fundamentals of Formulation, The Royal Society of Chemistry, 2001, p. 91. [21] N.J. Krog, in: K. Larsson, S.E. Friberg (Eds.), Food Emulsions, Marcel Dekker, 1997, p. 141. [22] G.L. Gaines, Insoluble Monolayers at Liquid /Gas Interfaces, Interscience, New York, 1996. [23] H.D. Belitz, W. Grosch, Food Chemistry, Springer, Berlin, 1987, p. 549 (Chapter 17). [24] J.M. Evans, in: M.B. Huglin (Ed.), Light Scattering from Polymer Solutions, Academic Press, London, 1972, p. 89. [25] M.B. Huglin, in: M.B. Huglin (Ed.), Light Scattering from Polymer Solutions, Academic Press, London, 1972, p. 165.
[26] W.P. Jenks, Catalysis and Chemistry in Enzymology, McGraw-Hill, New York, 1969. [27] I. Appelqvist, M. Debet, Food Rev. Int. 13 (1997) 163. [28] M.C. Godet, H. Bizot, A. Buleon, Carboydr. Polym. 27 (1995) 47. [29] M.G. Semenova, in: N. Parris, A. Kato, L.K. Creamer, J. Pearce (Eds.), Macromolecular Interactions in Food Technology, American Chemical Society, Washington, DC, 1996 (Chapter 3). [30] C. Tanford, Physical Chemistry of Macromolecules, Wiley, New York and London, 1961. [31] M.G. Semenova, L.B. Savilova, Food Hydrocolloids 12 (1998) 65.
Effect of maltodextrins on the surface activity of smallmolecule surfactants Maria G. Semenova *, Larisa E. Belyakova, Anna S. Antipova, Yu. N. Polikarpov, Lida Klouda, Anna Markovic, Michael M. Il’in Institute of Biochemical Physics, Russian Academy of Sciences, Vavilov str. 28, Moscow 119991, Russia Received 19 August 2002; received in revised form 13 November 2002; accepted 22 January 2003
Abstract We report on the effect of commercially important polysaccharides (maltodextrins with variable dextrose equivalent (Paselli SA-2, MD-6 and MD-10) on the surface activity at the air /water interface of small-molecule surfactants (sms), possessing different hydrophobic /lipophilic balance ((SSL (Na ), the main component is a sodium salt of stearol / lactoyl lactic acid, and PGE (080), polyglycerol ester of C18 fatty acid), and widely used in food products. A marked change of the surface activity of sms was found in the presence of maltodextrins by tensiometry. The combined data of laser multiangle light scattering and mixing calorimetry have suggested that this result is governed by specific complex formation between maltodextrins and sms in aqueous medium. Measurements have been made of the molar mass, the second virial coefficient and the enthalpy of intermolecular interactions in aqueous solutions. The implication of a degree of polymerization of maltodextrins in this phenomenon was shown. The interrelation between the molecular parameters of the formed complexes and their surface activity at the air /water interface has been revealed and discussed. # 2003 Elsevier B.V. All rights reserved. Keywords: Maltodextrin; Small-molecule surfactants; Molecular parameters; Thermodynamics of interactions; Surface activity
1. Introduction It is well known that the major components and derivatives of starch, which are widely used in food manufacture because of their unique gel-forming characteristics, are able to form complexes or
* Corresponding author. Tel.: /7-95-135-9290; fax: /7-95135-5085. E-mail address: [email protected] (M.G. Semenova).
inclusion compounds with a variety of ligands such as lipids, emulsifiers and flavor substances [1 /11]. It was established that this interaction could influence starch gelatinization [12 /14] or gelation [8,14 /17]. Alternatively, one would expect that these polysaccharides could have an effect on properties of bound ligands and, in particular, on the surface behavior of small-molecule emulsifiers, but until recently this effect was not practically investigated and understood. There are only a few works [10,11,18 /20] that disclose the effects of
0927-7765/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0927-7765(03)00042-0
48
M.G. Semenova et al. / Colloids and Surfaces B: Biointerfaces 31 (2003) 47 /54
different in nature polysaccharides on the surface behavior or emulsifying ability of small-molecule surfactants (sms). In our recent previous works, devoted to this subject, we have revealed the dramatic distinction between the effects of the maltodextrins on the surface behavior of two sms, namely,model */ sodium caprate and industrially important */CITREM (the main component is a citric acid ester of monostearate) [10,11]. An unexpected marked increase in the surface activity of Na-caprate in mixture with maltodextrins was found, increasing with both concentration of Na-caprate and a decrease in a dextrose equivalent (DE) of maltodextrins, and vice versa a pronounced decrease in the surface activity of CITREM. Hence the study underlined need for further systematic investigations with use of other classes of sms. Thus, this work follows on from our earlier research [10,11] and investigates the effect of commercially important polysaccharides (maltodextrins with variable DE (Paselli SA-2, MD-6 and MD-10)) on the surface activity at the air / water interface of sms, possessing different hydrophilic/lipophilic balance ((SSL (Na). The main component is a sodium salt of stearol /lactoyl lactic acid), and PGE (080), polyglycerol ester of C18 fatty acid), widely used in food products [21]. We have also attempted to establish a likely molecular mechanism underlying the effects studied using a number of thermodynamic methods.
salt of stearol /lactoyl lactic acid) and PGE (080) (polyglycerol ester of C18 fatty acid) [21] were supplied by Danisco Cultor (Denmark). Under experimental conditions (phosphate buffer, pH 7.2, ionic strength 0.05 mol dm 3) the critical micelle concentrations (cmc) for SSL (Na) and PGE (080) are 3.5 and 1.3 mg dm 3, respectively. 2.2. Methods 2.2.1. Preparation of maltodextrin solutions Maltodextrins with variable DE were dissolved in an aqueous phosphate buffer (pH 7.2, ionic strength 0.05 mol dm 3, 0.01 wt.% sodium azide as an antimicrobial agent) by mechanical stirring at 85 8C for 1 h. Thereafter the solutions were allowed to air cool to room temperature. These solutions were used in all experiments. Concentration of the maltodextrins in solutions was checked using a Shimadzu (Japan) refractometer, on the known value of maltodextrin refractive index increment ground (0.150/103 m3 kg1).
2.1. Materials
2.2.2. Preparation of mixed solutions of smallmolecule surfactants with maltodextrins Stock solutions of the sms (0.01 wt.%) were prepared by ultrasound sonication 4.5 MHz over 1 h while the solutions were shaking at 65 8C (CPLAN water bath shaker, type 357, Poland). Thereafter the stock solutions, cooled to room temperature, were used to prepare mixed solutions with the maltodextrins of the required concentrations, whereupon the mixed solutions were shaken at 40 8C for 1 h (CPLAN water bath shaker, type 357, Poland) and then were allowed to be cooled to room temperature.
Three commercial samples of maltodextrins (Paselli SA-2, MD-6 and MD-10, Avebe) were used as supplied. They were enzymatic products of the hydrolysis of a potato starch having values of a DE equating with 2, 6 and 10, respectively. Phosphate buffered (pH 7.2, ionic strength 0.05 mol dm 3) solutions were prepared using analytical grade reagents (of 99.9% purity), and a double distilled water; 0.01 wt.% sodium azide was added to the buffer as an anti-microbial agent. Sms: SSL (Na) (the main component is a sodium
2.2.3. Determination of the surface tension, g, at the planar air /water interface Values of the surface tension, g , of the solutions of the sms, the maltodextrins and their mixtures were monitored with an accuracy of 1 mN m1 with the Wilhelmy plate technique [22] using a Kruss GmbH digital tensiometer K 10 (Hamburg, Germany). All measurements were made in the thermostatic cell at 259/0.5 8C. A platinum plate was used in measuring the surface tension. The surface tension was monitored continuously as a
2. Experimental
M.G. Semenova et al. / Colloids and Surfaces B: Biointerfaces 31 (2003) 47 /54
function of time. The values of g presented in this work are averaged data for at least two repetitions of each of the experiments. The value of the surface tension between a double-distilled water and air did not differ from the standard value within the range of the experimental error and was 72 mN m 1 at 25 8C independently of time. A slight decline in the surface tension for pure maltodextrin solutions (0.5% w/v) (Figs. 1 and 2) is most likely attributable to the contribution from impurities of both a protein and lipid nature, found in potato starch [23]. 2.2.4. Estimation of molecular parameters of maltodextrins and thermodynamic parameters of the maltodextrin /maltodextrin interaction in a bulk aqueous medium The weight-average molecular weight, Mw, and the second virial coefficient, A2 (the thermodynamic interaction parameter, A2, measures the strength and nature of the interaction between pairs of macromolecules in biopolymer solutions and thereby characterizes thermodynamic affinity of macromolecules for aqueous medium or for
49
each other) for maltodextrin molecules at concentration of the sms below the cmc were determined in dilute aqueous solutions of maltodextrins (0.1 / 0.5% w/v) from laser static light scattering. All measurements were made in the thermostatic cell at 259/0.5 8C. The Rayleigh ratio Ru was measured using vertically polarized light (633 nm) at angles in the range 40 5/u ]/1408 with a VA Instruments LS-01 apparatus (St Petersburg, Russia) calibrated with dust-free benzene (R90 / 11.84 /106 cm 1). Solutions were filtered through a membrane (a pore size of 0.45 mm (Millipore)) directly into the light scattering cell, and the biopolymer concentration was checked each time after filtration. The raw data were used to plot the angular and concentration dependencies of the ratio (HC /(DRU )) according to the Zimm method [24]. Here C is the biopolymer concentration g ml1, DRU is the excess light scattering over that of the pure solvent at angle u , and H is an instrumental optical constant equal to 4p2n2n2/NAl4 where NA is Avogadro’s number; l , the wavelength of an incident light in vacuo; n, the refractive index of the solvent and n is the refractive index increment of the biopolymer.
Fig. 1. Effect of maltodextrins (Cmaltodextrin /0.5 wt.%) with variable DE on the surface behavior of SSL (Na ) at the planar air / water interface (0.05 M phosphate buffer, pH 7.2, 25 8C). The interfacial tension, g , is plotted as a function of time for: maltodextrins without SSL (Na ) (×/ ×/ ×/j×/ ×/ ×/); mixtures of SSL (Na ) with maltodextrins: (-'-) SA-2; (-"-) MD-6; (-m-) MD-10; SSL (Na ) alone ( ×/ ×/ ×/k×/ ×/ ×/). (a) CSSL (Na) /2 mg dm 3 B/cmc/3.5 mg dm 3; (b) CSSL (Na) /6 mg dm 3 /cmc/3.5 mg dm 3.
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M.G. Semenova et al. / Colloids and Surfaces B: Biointerfaces 31 (2003) 47 /54
Fig. 2. Effect of maltodextrins (Cmaltodextrin /0.5 wt.%) with variable DE on the surface behavior of PGE (080) at the planar air /water interface (0.05 M phosphate buffer, pH 7.2, 25 8C). The interfacial tension, g , is plotted as a function of time for: maltodextrins without PGE (080) ( ×/ ×/ ×/j×/ ×/ ×/); mixtures of PGE (080) with maltodextrins: (-'-), SA-2; (-"-), MD-6; (-m-), MD-10; PGE (080) alone (×/ ×/ ×/k×/ ×/ ×/). (a) CPGE (080) /1 mg dm3 B/cmc/1.3 mg dm 3; (b) CPGE (080) /2 mg dm 3 /cmc/1.3 mg dm 3.
Values of the weight-average molecular weight, Mw, were estimated as average from the intercepts of both the concentration dependence of HC /DRU as u 0/0 and the angular dependence of HC /DRU as C 0/0. Values of the second virial coefficient A2 /Apol pol were estimated from the slope of the concentration dependence of HC /DRU as u 0/0. The errors in the determination of the value of weight-average molecular weight and the second virial coefficient are 10%. Refractive indexes increments, n, for maltodextrins in the absence or presence of the sms were determined at 436 nm using a Shimadzu differential refractometer (Japan). All measurements were made in the thermostatic cell at 259/0.5 8C. The values of n at l /633 nm were calculated from an equation for dispersion of the refractive index [25] with the correction coefficient taken as 0.9. Values of the refractive index increment for maltodextrins within the experimental error (9/ 10%) were the following for maltodextrins without sms and in the presence of SSL (Na/) and PGE (080): n/0.15 /103; 0.137 /10 3 and 0.136 / 103 m3 kg1, respectively.
2.2.5. Mixing calorimetry Calorimetric measurements were made using a LKB 2277 flow calorimeter set at 22 8C. A peristaltic pump pumped the reactants into the instrument. The pump was calibrated by measuring the time required to pump a known volume of solution into the calorimeter. The flow rate was equal to 9/106 l s 1. The ratio of the flow rates in the two channels was close to 1, i.e. the solutions of the maltodextrins or the sms in all cases were diluted by a factor of two under mixing. Both solutions were thermally equilibrated before entering the reaction vessel. A calibration of the calorimeter itself was done electrically at the temperature of a measurement. The sensitivity of the calorimetric measurement is no less than 3 / 106 J s 1. Thermal effects were observed during dilution of: (i) the maltodextrin solution by the pure buffer Qmaltodextrin buffer; (ii) the solution of sms by the pure buffer Qsmall-molecule surfactant buffer; (iii) the maltodextrin solution by the solution of sms QS. These thermal effects Q were measured in thermal power units (J s1).
M.G. Semenova et al. / Colloids and Surfaces B: Biointerfaces 31 (2003) 47 /54
The specific enthalpy of the interaction between the maltodextrins and the sms was obtained from the relationship: DHmaltodextrinsmallmolecule
surfactant
(QSQmaltodextrinbuffer Qsmallmolecule
surfactantbuffer =Dn
(3)
where Dn is the number of grams of maltodextrin mixed with sms per second (g s 1).
3. Results and discussion A marked decrease in the surface activity of both SSL (Na ) and PGE (080) (a rise in the surface tension, g ) was found in the presence of the maltodextrins (0.5% w/v) as shown in Figs. 1 and 2. As this takes place, the most pronounced increase in the surface tension (a decrease in the surface activity) of the mixed solutions was observed, on the one hand, for concentrations of sms above their cmc (Fig. 1b, Fig. 2b, Fig. 3), and
Fig. 3. Effect of maltodextrins (Cmaltodextrin /0.5 wt.%) with variable DE on the surface behavior of SSL (Na ) at the planar air /water interface (0.05 M phosphate buffer, pH 7.2, 25 8C). The difference between values of the interfacial tension, g , of mixture (maltodextrin/sms) and sms alone is plotted as a function of concentration of SSL (Na ) for: ', maltodextrin SA-2; ", maltodextrin MD-6.
51
on the other hand for maltodextrins with a lower degree of hydrolysis, that is DE. By way of illustration Fig. 3 shows a sharp decay of the surface activity of SSL (Na ) in the close vicinity and above its cmc in the presence of maltodextrins SA-2 and MD-6 (0.5% w/v). To gain greater insight into the effect found, we have attempted to elucidate the nature of the interaction between the maltodextrins and sms in aqueous medium by measurement of the enthalpy of their interactions from mixing calorimetry first. In the case of SSL (Na ), Fig. 4a shows the dominant endothermic character of the interaction between the maltodextrins and SSL (Na ), that is more pronounced at SSL (Na ) concentrations above cmc. As this takes place, maltodextrin SA-2 (with the largest extent of polymerization) shows the most pronounced endothermic character of the interaction. This result suggests that the interaction between the maltodextrins and SSL (Na) in aqueous medium is of a hydrophobic character [26], probably through a rather long hydrocarbon chain in the SSL (Na ) molecule (C17) and the hydrophobic parts of the glucose units in the polysaccharide molecules. The distinctive feature of the interaction between the maltodextrins and PGE (080) in aqueous medium is the pronounced exothermic character of it (Fig. 4b). As this takes place, the maltodextrin SA-2 (with the largest extent of polymerization) once again shows the greatest thermal effect of the interaction. In this case it is considered that the observed exothermic character is governed for the most part by hydrogen bonding between the maltodextrins and PGE (080), owing to the great number of hydroxyl groups in their molecules [3,21]. Besides, this effect could be attributable to formation of a helical conformation of the maltodextrins due to hydrogen bonding, especially for maltodextrin SA-2. The capacity of starch polysaccharides to undergo a conformational transformation upon interaction with lipophilic compounds giving inclusion complexes is well known in the literature [2,27,28]. Some contribution to the total exothermic character from the breakdown of micelles of PGE (080) could also be suggested.
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Fig. 4. Specific enthalpy, DH , of the interaction between the maltodextrins and sms in aqueous medium (0.05 M phosphate buffer, pH 7.2, 25 8C) vs. the sms concentration in the mixed solutions: (a) SSL (Na ) and (b) PGE (080), ', maltodextrin SA-2; ", maltodextrin MD-6; m, maltodextrin MD-10.
Overall, the mixing calorimetry data suggest complex formation between the studied sms and the maltodextrins through mainly the hydrophobic interaction in the case of SSL (Na ) and hydrogen bonding in the case of PGE (080), depending on their principal structural distinctions. Table 1 shows the molecular parameters: weight-average molecular weight, Mw, and second virial coefficient, Apol pol, of the formed complexes in a bulk aqueous medium, which have been obtained by laser static light scattering at the concentration of sms below the cmc. Data from laser static light scattering point up a marked increase in the thermodynamic affinity of the
maltodextrins for aqueous medium that is an increase in positive values of the second virial coefficients [29 /31], as a result of the interaction with both sms. This increase is more pronounced in the cases of the maltodextrins with higher DE, namely, MD-6 and MD-10. For maltodextrin SA2, the only slight change in the thermodynamic affinity of the formed complex for aqueous medium is most likely attributable to the formation of the inclusion complex with hiding of the most parts of the attached sms molecules in the interior of the complex. Moreover, an intensive association of the maltodextrin molecules follows the interaction be-
Table 1 Effect of sms on the molecular parameter of maltodextrins in aqueous medium (pH 7.2, ionic strength 0.05 mol dm3, 25 8C): the weight-average molecular weight, Mw, and the thermodynamics of pair interactions (the second virial coefficient Amd md) System
Maltodextrin SA-2 Maltodextrin MD-6 Maltodextrin MD-10
Without sms
With 2 mg dm 3 SSL (Na )
With 0.75 mg dm 3 PGE (080)
Mw (kDa)
Amd md 105 (m3 mol kg2)
Mw (kDa)
Amd md 105 (m3 mol kg 2)
Mw (kDa)
Amd md 105 (m3 mol kg 2)
260 100 45
20.5 8.1 2.2
515 192 183
25.6 39.9 59.2
540 175 92
34.0 50.1 129.5
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tween the maltodextrins and sms, as if the sms molecules play the role of the component bridging molecules of the maltodextrins through hydrogen bonding or hydrophobic interactions. Based on molar concentrations of the components used in light scattering experiments (CSSL (Na) /4.67 / 106 mol dm 3; CPGE (080) /1.48 /10 6 mol dm 3; Cmaltodextrins from 4/106 to 1/104 mol dm 3 depending on the DE) the calculation of the molecular weight of hypothetical maltodextrin /surfactant complexes, which was rested on the assumption that all amount of added surfactants were bound equally by the maltodextrin molecules in solution, provides evidence for this suggestion by virtue of the fact that the calculated values are in several times smaller than those found by light scattering. On the strength of the combined data of mixing calorimetry and static light scattering, it may be safely suggested that the pronounced decrease in the surface activity of both SSL (Na ) and PGE (080) in the presence of the maltodextrins is governed principally by the formation of highly hydrophilic and consequently lower surface active complexes between them by the way when the hydrophobic patches of the interacting molecules are hidden in the interior of the complexes, whereas the hydrophilic ones are directed into aqueous medium.
4. Conclusions 1) The effect of maltodextrins on the surface activity of sms is mainly governed by formation of rather hydrophilic complexes between them in aqueous medium. In addition, a plausible formation of inclusion complexes in the case of maltodextrins with lower DE could also contributes to the marked decrease in the surface activity of sms found. 2) Both hydrophobic interactions and hydrogen bonding between sms and maltodextrins could underlie the complex formation between them, depending on the specific structural features of the sms structures. 3) The interaction of maltodextrins with micelles of sms promotes the complex formation
53
between them, supposedly by virtue of a marked increase in the number of their molecular contacts.
Acknowledgements The authors are most grateful to Danisco (Denmark) for the free supply of the SSL (Na) and PGE (080) samples.
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