Dynamic Surface Tension and Surface Dilatational Elasticity Properties of Mixed Surfactant/Protein Systems

Transcription

1 Journal of Oleo Science Copyright 2008 by Japan Oil Chemists Society Dynamic Surface Tension and Surface Dilatational Elasticity Properties of Mixed Surfactant/Protein Systems Lok Kumar Shrestha 1, Yohei Matsumoto 1, Keiichi Ihara 2 and Kenji Aramaki 1 1 Graduate School of Environment and Information Sciences, Yokohama National University (79-7 Tokiwadai, Hodogaya-ku, Yokohama , JAPAN) 2 Food Research and Development Laboratory, Morinaga Milk Industry Co., Ltd. (1-83, 5-chome, Higashihara, Zama-City , Kanagawa, JAPAN) Abstract: We present the study on dynamic surface tension and surface dilatational elasticity properties of dilute aqueous systems of pentaglycerol fatty acid esters (pentaglycerol monostearate, C 18 G 5, and pentaglycerol monooleate, C 18:1 G 5 ), whey protein, sodium caseinate, and mixed surfactant and protein at room temperature. The adsorption kinetics at the air-liquid interface has been studied by bubble pressure tensiometer and the oscillation bubble (rising drop) method. It has been shown that the dynamic surface tension curve basically presents two-regions; namely induction region and rapid fall region. During the induction time the adsorption is the diffusion-controlled process of amphiphilic surfactant or protein molecules from the bulk of the solution to the interface. Whey protein and sodium caseinate showed longer induction time ms compared to the surfactant systems, where induction time was estimated to be 1000 ms. However, in both the protein and surfactant systems, the induction time goes on decreasing with increasing the concentrations. The similar behavior was observed in the mixed system, and lower surface tension values were observed at higher concentrations. The fitting of the experimental data to the theoretical equation shows the presence of two relaxation mechanisms of widely different time scale for the adsorption of surfactant or protein molecules at the interface. The relaxation time strongly varies with the concentrations following the power law, and at fixed concentration it was the highest for whey protein and the lowest for C 18:1 G 5 system. The surface dilatational elasticity determined within the frequency range of 0.1 to 1 cycle/s supports the dynamic surface tension data. Key words: dynamic surface tension, surface dilatational elasticity, polyglycerol fatty acid esters, whey protein, sodium caseinate 1 INTRODUCTION Polyglycerol fatty acid esters, which can be prepared from the renewable resources such as plant or animal oils, are the mixtures of isomers with various polyglycerol chain lengths or various degrees of esterifications 1). Moreover, commercially available polyglycerol fatty acid esters contain large amounts of unreacted polyglycerol 2). The use of polyglycerol fatty acid esters in foods 3,4), cosmetics 5,6), detergents 7) and pharmaceutics 8,9) are increasing day-byday due to its low toxicity and ease of biodegradability. Besides, due to their capability of a-solid formation in water 10,11) or oils 12), they are mostly desired for the efficient foam and emulsion stabilization purposes 13-15). Many attempts have been made to clarify the functions of the polyglycerol fatty acid esters. However, few basic studies have addressed their physicochemical properties due to the difficulties associated with their structural complexity. Kunieda et al. 16) described the effects of polyglycerol chain length distribution on the phase behaviors of polyglycerol laurates. It was found that esters with broadly distributed polyglycerol chains pack more tightly at the interface and showed greater ability of lowering surface tension and giving stable emulsions compared to the esters with sharply distributed polyglycerol chains. Correspondence to: Kenji Aramaki, Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama , JAPAN aramakik@ynu.ac.jp Accepted May 15, 2008 (received for review March 25, 2008) Journal of Oleo Science ISSN print / ISSN online 485

2 L.K. Shrestha, Y. Matsumoto, K. Ihara and K. Aramaki Whey proteins are valuable by-products in the cheese industry and casein is a phosphoprotein found in milk. These proteins are widely used in a variety of foods primarily for their superior gelling and emulsification properties. b-lactoglobulin is a main component of whey proteins and principal gelling agent. It was found that besides novel food, whey proteins are also suitable for nonfood applications. For example, whey protein gels may be used as a phsensitive hydrogels for the controlled delivery of biologically active substances 17). Casein accounts for 76 to 86% of the total milk proteins and is heterogeneous mixture of four principal proteins: a s1 -, a s2 -, b, and k-casein (the approximate proportions are 38, 10, 36, and 13% 18) ). All the caseins are relatively small phosphoproteins and strongly hydrophobic in the order of b k a s1 - a s2 -caseins 19). Caseins have in particular amphiphilic nature due to a separation between hydrophobic clusters and negatively charged ions along the peptide chains. Due to its amphiphilic nature, caseins exhibit a significant effect on the interfacial dynamic properties if added into water. Polyglycerol fatty acid esters are usually added to milk products to improve their stability. The study of interfacial dynamic properties such as dynamic surface tension and surface dilatational elasticity of aqueous mixture of polyglycerol fatty acid esters and whey proteins or casein is of great importance mainly in the commercial process where the surfactants are used to modify the interfacial properties. Besides, the study of interfacial dynamics contributes to better understanding of the mechanism of foaming and emulsifications. Experiments have shown that the increase in foamability appears in parallel with the lowering of the dynamic surface tension but not the equilibrium surface tension, which is almost constant in the same concentration range 20,21). Besides, the surface dilatational elasticity is considered to have a significant contribution to the foaming properties 22-26). Therefore, the knowledge of interfacial dynamics is basic to an understanding of the properties and application of the system. In the present contribution, we report the study on the dynamic surface tension and the surface dilatational elasticity properties of aqueous systems of the pentaglycerol fatty acid ester type nonionic surfactants, whey protein, sodium caseinate, and mixtures of the surfactants and proteins. supplied by Morinaga Milk Co. Industry, Japan and the sodium caseinate from bovine milk was purchased from Sigma Co. Ltd. Deionized and Millipore filtered water was used in all the measurements Sample preparations All the samples were prepared in clean and dry glass bottles. The samples were mixed and then homogenized by using a magnetic stirrer for 24 h at Surface tension measurements Equilibrium surface tensions were measured by a Wilhelmy-type automatic surface balance Model K100 (Krüss Co., Germany) at 25. We have first measured the surface tension versus time to ensure the equilibrium surface tension. Using the rising drop method by a Tracker (I. T. Concept, France) the surface tension was crosschecked. Dynamic surface tension was measured by maximum bubble-pressure method BP2 (Krüss, Germany), which allows us to access to the surface ageing time 5 ms to 50 s Surface dilatational elasticity measurements Surface dilatational elasticity was measured at 25 by the oscillation bubble (rising drop) method using Tracker. The method involved an automatically controlled sinusoidal interfacial compression and expansion of air bubble at desired amplitude and frequency and determination of surface tension and surface area by drop-shape analysis of the digital images of the bubble. The surface dilatational elastic modulus (E*) and phase angle were derived from the change in surface tension resulting from a small change in surface area. The sinusoidal oscillation for surface dilatational measurements was started after 60 min to ensure the equilibrium interfacial tension. The dilatational elasticity, E*, of an air-liquid interface of area A, which is in a periodic dilatation, is given by the equation E * = dγ dln A where dg is the small change in surface tension due to small change in area dlna. (1) 2 EXPERIMENTAL 2 1 Technical grade pentaglycerol monostearate and pentaglycerol monooleate surfactants (abbreviated as C 18 G 5 and C 18:1 G 5 respectively) were supplied by Taiyo Chemicals Corp. and used as received. The surfactants are 75% pure, and the rest is unreacted polyglycerol. Whey protein was 3 RESULTS AND DISCUSSION Surfactant systems Pentaglycerol fatty acid esters (C 18 G 5 and C 18:1 G 5 ) do not form micelle in aqueous system, but show critical aggregation concentration (CAC) behavior 14). Dynamic surface tensions (DST) of aqueous C 18 G 5 and C 18:1 G 5 were measured well above CAC at different concentrations and the results 486

3 Dynamic Surface Tension and Surface Dilatational Elasticity Properties are shown in. When a new interface is created, the surface tension is comparable to that of pure water, but as the time passes by, the surfactant molecules are adsorbed at the air-liquid interface and the surface tension decreases. With increasing surfactant concentration, the decay of DST becomes faster because of higher number of surfactant molecules transported to the interface. The reduction of dynamic surface tension involves several mechanisms and a typical surface tension-time curve can be divided into four regions 27) ; an induction region (short times), a rapid fall region, a mesoequilibrium region, and an equilibrium region (at a long time). In the present system, however, we have observed only first two regions due to the limitation of the instrument or the system dependence. The induction time (the time until which surface tension is almost independent of the surface age) of 0.1 wt% C 18 G 5 solution is long ( 1000 ms). The longer induction time is a general characteristic of nonionic surfactant systems with a low critical micelle concentration (CMC) 28). However, with increasing surfactant concentration, the induction time shortens, i.e., the rate of surface tension decay increases with the surfactant concentration as a result of increased diffusion of the surfactant molecules. Qualitatively we can say that upon increasing surfactant concentration by a factor of ten, the induction time decrease nearly by a factor of ten (see ). The present results support the results obtained for diglycerol monomyristate aqueous system in which the induction time decreases monotonously with increasing surfactant concentration 15). On comparing DST behavior of C 18:1 G 5 and C 18 G 5 with diglycerol monomyristate aqueous system at fixed concentration (0.1 wt%), it is found that the induction time in the latter system is nearly 10 times shorter than the former systems. Besides, lower surface tension values could be observed with diglycerol monomyristate system. These results support the molecular size dependence of DST behavior. Namely, smaller size molecule shows faster adsorption to the interface compared to the bigger size molecules. At fixed surfactant concentration, there is no big difference in the induction time between C 18 G 5 to C 18:1 G 5. However, a minute observation of the DST curves reveals that C 18:1 G 5 is more effective in reducing the surface tension of the solution. In, we have plotted the difference Fig. 1 (a) Dynamic Surface Tension of Aqueous Solutions of C 18 G 5 and (b) C 18:1 G 5 Systems with Different Concentrations at 25. In panel c, we have plotted the difference between the surface tension value at 100 and 1000 ms. For the sake of clarity the induction region and rapid fall region is separated by downward arrows in panels a and b for 0.1 wt% solution. 487

4 L.K. Shrestha, Y. Matsumoto, K. Ihara and K. Aramaki between the surface tension values at 100 and 1000 ms (g g 1000 ) at different surfactant concentrations for C 18 G 5 and C 18:1 G 5 systems. The higher value of the g 100 -g 1000 at all studied concentrations for C 18:1 G 5 system indicates that C 18:1 G 1 adsorbs to the interface prior to the C 18 G 5. Note that the size of the hydrophilic moiety in both surfactants is same; the only difference is in the hydrophobic part. C 18 G 5 has saturated skeleton, while C 18:1 G 5 has a double bond in the hydrocarbon chain. One possible reason of different adsorption between C 18:1 G 5 and C 18 G 5 is the phase behavior. C 18:1 G 5 forms lamellar liquid crystal dispersion above CAC in aqueous system, whereas C 18 G 5 forms a-solid dispersion 14). Since solid has a rigid structure than a liquid crystal, monomer diffusion from liquid crystal dispersion seems to be easier. Next, possible reason is difference in particle size of the dispersed phases, because the diffusion from smaller particles is relatively faster compared to the bigger particles. Laser diffraction measurements have shown that the particle size of liquid crystal dispersion in C 18:1 G 5 is smaller than the solid dispersion in C 18 G 5 aqueous systems 14). The surfactant concentration has similar effect on the DST of C 18:1 G 5 system as was observed in C 18 G 5 system, i.e., faster decay of surface tension with a reduced induction time at higher concentration Protein systems We present the DST behavior of whey protein and sodium caseinate as a function of concentration. The DST data of the protein systems measured under identical conditions to the surfactant systems are presented in. Similarly to the surfactant systems, only two regions (induction and rapid fall regions) could be detected in the DST curves of the protein systems. However, the induction times are dramatically increased mainly in the dilute protein solutions. For example, the surface tension of 0.1 wt% whey protein is almost unchanged up to 10,000 ms and afterwards a small drop in the surface tension value is detected. Similar behavior is observed for 0.1 wt% sodium caseinate system. In 0.1 wt% surfactant systems the drop in the surface tension value was detected in the early stage below 1000 ms. The longer induction time of protein can be attributed to their structure and molecular size, because the diffusion coefficient is inversely proportional to the radius of particle and viscosity of medium. In the dilute regions, viscosity of proteins and surfactants solutions may be similar, then the diffusion of protein molecules, due to its bulky size, is relatively slower compared to the surfactant system, and hence, results longer induction period in the DST curves. Increasing protein concentration decreases the induction period and also the surface tension of the solutions. At higher concentrations, a saturation point is reached and further increasing concentration does not modify the DST curves. Above 3 wt%, identical DST behavior is observed in whey protein systems. Similarly, above 2 wt% the DST curves are almost identical in sodium caseinate system. Minute observation of the DST curves reveals that the effect of the concentration is more sensitive in the sodium caseinate system, i.e., a small change in concentration brings about a major change in the DST behavior. If we compare the DST properties of whey protein and sodium caseinate in the dilute region (0.5 wt%), it can be seen that the small fall in the surface tension value appears earlier in the sodium caseinate compared to whey protein system. The data emphasize on the fact that the whey protein molecules relax longer on its way to the interface. For example, the induction time of whey protein is decreased to 100 ms upon increasing concentration from 0.1 to 2 wt%, while the same extent of decrease in the induction period can be achieved by simply increasing concentration of the sodium caseinate from 0.1 to 0.3 wt% Mixed surfactant and protein systems We have studied the dynamic surface tension properties of the mixed surfactant and protein systems at 25, and the results for the whey/surfactant systems are shown in Fig. 2 Dynamic Surface Tension of Aqueous Solutions of (a) Whey Protein and (b) Sodium Caseinate at Different Concentrations at

5 Dynamic Surface Tension and Surface Dilatational Elasticity Properties. Addition of the pentaglycerol fatty acid ester surfactants in the aqueous system of whey protein causes a significant change in the dynamic surface tension behavior. Namely, the DST of the mixed systems decays faster and the rapid fall region of the DST curve appear earlier than the system without surfactants. This indicates that faster adsorption rate of surfactant molecules contributes to the shorter induction times of the mixed systems. With successive increase in the concentration of C 18 G 5 at fixed concentration of whey protein (0.5 wt%), the induction time decreases monotonously and surface tension goes on decreasing with the C 18 G 5 concentration within the studied concentrations. On the other hand, the DST curve is apparently unaffected by increasing surfactant concentration above 2 wt% in whey protein/c 18:1 G 5 system. Such behavior may be due to the formation of self-assembled structure or some structural evolution so that the number of monomer in the system is kept constant. However, we cannot exclude the possibility of saturation point (the concentration above which DST curve does not change with surfactant concentration) in the DST curve in the whey protein/c 18 G 5 system. This is because the phase behavior of the C 18:1 G 5 and C 18 G 5 in water shows the dispersion of lamellar liquid crystal and a-solid in the dilute regions at 25. deals with the study of dynamic surface tension properties of the mixed sodium caseinate/surfactant systems. As was clear in, at fixed concentration of 0.5wt%, the induction time in the DST curve shortens significantly when sodium caseinate replaces whey protein. This indicates the faster adsorption of sodium caseinate compared to whey protein at this composition. Addition of C 18:1 G 5 and C 18 G 5 to 0.5 wt% sodium caseinate solution shifts the whole DST curve towards downside, i.e., smaller surface tension values. Successive increase in the surfactant concentrations favors a monotonous decrease in the DST curve throughout the surface age indicating the faster adsorption at higher concentrations. 3 2 For the diffusion-controlled adsorption of surfactant molecules at the interface Ward-Tordai equation can be Fig. 3 Dynamic Surface Tension of Aqueous Systems of (a) the Mixture of Whey Protein and C 18 G 5 and (b) the Mixture of Whey Protein and C 18:1 G 5 at 25. The concentration of whey protein is fixed at 0.5 wt%. Fig. 4 Dynamic Surface Tension of Aqueous Systems of (a) the Mixture of Sodium Caseinate and C 18 G 5 and (b) the Mixture of Sodium Caseinate and C 18:1 G 5 at 25. The concentration of sodium caseinate is fixed at 0.5 wt%. 489

6 L.K. Shrestha, Y. Matsumoto, K. Ihara and K. Aramaki used to describe the behavior of dynamic surface tension 29). Use of the Ward-Tordai equation to the experimental data from short to long term region, however, is not an easy task, so dynamic surface tension is usually estimated by approximate relationship for very short times (t 0) and very long times (t ). The present systems are not the micellar solutions rather the dispersion of particles (either solids or lamellar liquid crystal and protein phase depending on the system). Therefore, the validity of the Ward-Tordai equation in such systems is doubtful. Hence, we prefer to use a simple approximation. We have first normalized the dynamic surface tension defined as; γ n γ t γ = γ γ 0 where g o, g t, g are the surface tension at zero time, at time t, and after very long time when surface tension reaches equilibrium value, respectively. The g is determined by the method described in section. Following a common trend in dynamic process, g n is expressed by a series of exponentials of time 30) ; (2) where a i is a constant and t i is a parameter (having a unit of time) related to the decay or relaxation of surface tension. The relaxation mechanism can be understood by fitting the experimental data to equation ( ). In the present cases, we have first tried to fit the data with first exponential through equation ( ), i.e., considering the single relaxation mechanism. However, the fitting was poor. Next, we have tried to fit the data taking higher degree of exponent. A satisfactory fitting to the experimental data could be achieved using above equation involving two t i terms, as it is evident from the representative fittings shown in for individual surfactant and protein systems. Two t i values (t i t 2 ) indicate the presence of two relaxation mechanisms with widely different time scale for the adsorption of surfactant or protein molecules at the interface. Details on such behavior have been described elsewhere 31,32). According to Equation ( ), we have calculated the t 1 and t 2 for the individual surfactant and protein systems with three different concentrations, and the results are presentγ n n = ae i= 1 ( t/ τ i ) i (3) Fig. 5 Normalized Dynamic Surface Tension for Aqueous Solutions of (a) C 18 G 5, (b) C 18:1 G 5, (c) Whey Protein, and (d) Sodium Caseinate with Different Concentrations at 25. The solid lines in the figures represent the best fit curves to the data points. 490

7 Dynamic Surface Tension and Surface Dilatational Elasticity Properties ed in. t 1 decreases sharply with increasing of the surfactant or protein concentration but at higher concentrations the effect of concentration on t 1 is minimum. The values of t 1 for the protein systems significantly differ from the surfactant systems at all concentrations studied. The observed higher values of t 1 of the protein systems indicate that the protein systems have higher energy barrier for the molecular diffusion. If we recall the DST curves, we can see comparatively longer induction time of the proteins compared to the surfactant systems (see and ), which indicates the slow diffusion of the proteins to the surface. A small difference in the values of t 1 is observed in the surfactant systems. The t 1 of C 18 G 5 is slightly higher than that of C 18:1 G 5 at and below 0.5 wt% but at 1 wt% the values are close to each other (see ). Note that the present surfactants are commercial products. Therefore, the impurities might have some contribution to the relaxation time. If we compare the t 1 of the protein systems, it can be seen that the value of t 1 is higher for whey protein throughout the concentration up to 5 wt%. Thus, the present data unambiguously shows that sodium caseinate molecules tend to diffuse faster compared to whey protein. The observed poor correlation of t 2 with the surfactant concentration (t s) may be due to some other nondiffusive mechanisms of the surface tension decay, and partially due to insufficient data points at long time, which is beyond the working range of the instrument. However, the t 2 decreases with increasing protein concentrations and after a certain point (above 2 wt%), the concentration effect on t 2 is minimum. Thus, a very long induction time obtained in the protein systems mainly at lower concentration suggests that adsorption of the protein molecules at the interface is very slow. 3 3 We have measured the interfacial surface dilatational elasticity of air-liquid interface for the surfactant, protein, and the mixed surfactant/protein systems at different concentrations as a function of oscillation frequency. The interfacial elasticity is regarded as a measure of the perturbed interface to regain its equilibrium position, which mainly depends on the kinetics of the exchange of surfactant molecules between the bulk and interface. This kinetics is highly dependent on the dilatational frequency and nature of the surfactant molecules. When the interfacial dilatation is fast, the surfactant molecules have no time for bulk-interface exchange and the value of g deviates from the equilibrium value, and consequently, the magnitude of E* increases. At very fast dilatational frequency, however, the elasticity becomes independent of the oscillation frequency 33,34). shows the variation of the modulus of surface dilatational elasticity as a function of the oscillation frequency for C 18 G 5 and C 18:1 G 5 systems with different surfactant concentrations at 25. In all the measurements, the interface was allowed to attain equilibrium tension before inducing sinusoidal deformation. Within the studied oscillation frequency range ( cycle/s), 0.1 wt% C 18 G 5 system has larger elasticity value compared to C 18:1 G 5 system and its value increases with the oscillation frequency, which may be attributed to the very slow transport of the surfactant molecules to the interface in comparison to the time scale of the dilatation. The higher elasticity value of C 18 G 5 is attributed to the slower diffusion of the surfactant molecules to the interface. The result extracted from the elasticity measurement is in agreement with the DST data. For example, it was observed in the DST that the diffusion of C 18 G 5 molecules is slower compared to C 18:1 G 5. As a result, less number of molecules is transported to the interface in comparison to the time scale of the dilatation, and hence, the elasticity increases. On the other hand, increasing surfactant concentrations decreases the elasticity and eventually the elasticity becomes almost independent of surfactant concentration. Only a small difference is noticed in the elastic- Fig. 6 Variation of (a) t 1 and (b) t 2 as a Function of Surfactant and Protein Concentration at

8 L.K. Shrestha, Y. Matsumoto, K. Ihara and K. Aramaki ity value on changing surfactant concentration from 0.5 to 1 wt% in both surfactant systems (see ). Increasing of the surfactant concentration increases the extent of adsorption of surfactant molecules at the interface, and surface tension will not change appreciably compared to the surface area, and hence, elasticity decreases. In we display the surface dilatational elasticity values as a function of oscillation frequency for sodium caseinate at different concentration. Besides, we also compare the elasticity of aqueous systems of the surfactants and the proteins at fixed concentration. As shown in the, the surface elasticity of sodium caseinate aqueous system goes on increasing with the oscillation frequency due to slow transport of the protein molecules to the interface compared to the time scale of dilatation frequency. Similarly to the surfactant systems, increasing concentration of sodium caseinate decreases the dilatational elasticity throughout the studied frequency, although at higher concentrations ( 0.5 wt% sodium caseinate) the elasticity difference becomes small. If we compare the elasticity values among all the systems at fixed concentration, it can be seen that whey protein exhibits the highest and C 18:1 G 5 lowest elasticity value throughout the studied oscillation frequency range, as can be seen from. Thus, the elasticity data indicates that the molecular transport from bulk to the interface is slower in the whey protein system and faster in the C 18:1 G 5 system, which is in agreement with the DST data. If the molecular transport to the interface is slow, the interfacial equilibrium is perturbed in greater extent, and hence, small change in the interfacial area causes a large change in the surface tension value, and finally results in higher surface elasticity. However, if the molecules are transported to the interface very fast, then the equilibrium perturbation is nullified so that there is less possibility of increase of surface tension from its equilibrium value even if we change the surface area. Therefore, in such case the elasticity value becomes low. Nevertheless, it should be Fig. 7 Variation of Surface Dilatational Elasticity E* as a Function of Oscillation Frequency for Aqueous Solution of (a) C 18 G 5 and (b) the C 18:1 G 5 with Different Concentrations at 25. Fig. 8 Variation of Surface Dilatational Elasticity E* as a Function of Oscillation Frequency for (a) Aqueous Sodium Caseinate System at Different Concentrations and (b) Aqueous Systems of the Surfactants and the Proteins at Fixed Concentration of 0.5 wt% at

9 Dynamic Surface Tension and Surface Dilatational Elasticity Properties noted that the surface dilatational measurements were done in low frequency range (0.1-1 Hz), and it is possible that the perturbation of this time scale may not be crucial for the stability of the interface. 4 CONCLUSION We have studied the dynamic surface tension and surface dilatational elasticity properties for the aqueous systems of C 18 G 5, C 18:1 G 5, whey protein, and sodium caseinate at 25. Compared to the surfactant systems, in the protein systems we have observed longer induction time in the dynamic surface tension curves. The induction time goes on decreasing with increasing the concentration of the surfactants and proteins in the system. The longer induction period in the protein systems could be attributed to the slower diffusion of the protein molecules. The fitting of the theoretical equation to the experimental data shows the presence of two relaxation mechanisms with widely different time scale for the adsorption of surfactant or protein molecules at the interface. The relaxation time decreased with concentration and is highest in the whey protein system and lowest in the C 18:1 G 5 system at fixed concentration. The surface dilatational elasticity determined within the frequency range of 0.1 to 1 cycle/s supports the dynamic surface tension data. Thus, the present study shows that the polyglycerol fatty acid esters or proteins (whey or sodium caseinate) significantly modify the surface properties of the aqueous system depending on their concentrations. ACKNOWLEDGEMENTS Authors are thankful to the Morinaga Milk Co., Japan for supplying whey protein and sodium caseinate. Shrestha L.K. is thankful to the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan for the Monbukagakusho Scholarship. 1. Kosswig, K. Nonionic Surfactants, Organic Chemistry in Surfactant Science Series (van Os, N.M. eds.). Vol. 72, Chapter 4, Dekker, New York (1998). 2. Lewis, J.J. Nonionic Surfactants, Organic Chemistry in Surfactant Science Series (van Os, N.M. eds.). Vol. 72, Chapter 8, Dekker, New York (1998). 3. Richardson, G.; Beergensthl, B.; Langton, M.; Stading, M.; Hermasson, A.M. The function of a-crystalline emulsifier on expanding foam surfaces. Food Hydrocolloids 18, (2004). 4. Michael, M.; Maria, S.A.; Anokhina, M.G. Calorimetric study of the interaction between small-molecule surfactants and sodium caseinate with reference to the foaming ability of their binary mixtures. Food Hydrocolloids 19, (2005). 5. Gallarate, M.; Carlotti, M.E.; Trotta, M.; Bovo, S. On the stability of ascorbic acid in emulsified systems for topical and cosmetic use. Int. J. Pharma 188, (1999). 6. Shima, M.; Kobayashi, Y.; Kimura, Y.; Adachi, S.; Matsuno, R. Effect of the hydrophilic surfactants on the preparation and encasulation efficiency in course and fine W/O/W type emulsion. Colloids Surf A. 238, (2004). 7. Brumbaugh, E.H. Liquid dishwashing detergent. UP (1999). 8. Takatori, T.; Shimono, N.; Higaki, K.; Kimura, T. Evaluation ofsustained release suppositories prepared with fatty base including solid fats with high melting points. Int. J. Pharm. 278, (2004). 9. Yamagata, Y.; Iga, K.; Ogawa, Y. Novel sustainedrelease dosage forms of proteins using polyglycerol esters of fatty acids. J. Controlled Release 63, (2000). 10. Izquierdo, P.; Acharya, D.P.; Hirayama, K.; Asaoka, H.; Ihara, K.; Tsunehiro, T.; Shimada, Y.; Asano, Y.; Kokubo, S.; Kunieda, H. Phase behavior of pentaglycerol monostearic and monooleic acid esters in water. J. Dispers. Sci. Technol. 27, (2006). 11. Shrestha, L.K.; Shrestha, R.G.; Iwanaga, T.; Aramaki, K. Aqueous phase behavior of diglycerol fatty acid esters. J. Dispers. Sci. Technol. 28, (2007). 12. Shrestha, L.K.; Kaneko, M.; Sato, T.; Acharya, D.P.; Iwanaga, T.; Kunieda, H. Phase behavior of diglycerol fatty acid esters-nonpolar oil systems. Langmuir 22, (2006). 13. Krog, N. Crystallization properties and lyotropic phase behavior of food emulsifiers. Crystallization Processes in Fats and Lipids Systems (Garti, N.; Sato, K. eds.). Marcel Dekker, Inc. New York, (2001). 14. Shrestha, L.K.; Acharya, D.P.; Sharma, S.C.; Aramaki, K.; Asaoka, H.; Ihara, K.; Tsunehiro, T.; Kunieda, H. Aqueous foam stabilized by dispersed surfactant solid and lamellar liquid crystalline phase. J. Colloid Interface Sci. 301, (2006). 15. Shrestha, L.K.; Saito, E.; Shrestha, R.G.; Kato, H.; Takase, Y.; Aramaki, K. Foam stabilized by dispersed surfactant solid and lamellar liquid crystal in aqueous systems of diglycerol fatty acid esters. Colloid Surf. A. 293, (2007). 16. Ishitobi, M.; Kunieda, H. Effects of chain length distribution on the phase behavior of polyglycerol fatty acid esters in water. Colloid Polym. Sci. 278, (2000). 17. Gunasekaran, S.; Xiao, L.; Ould Eleeya, M.M. Whey 493

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