Food Hydrocolloids 25 (2011) 361e367. Contents lists available at ScienceDirect. Food Hydrocolloids. journal homepage:

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1 Food Hydrocolloids 25 (211) 361e367 Contents lists available at ScienceDirect Food Hydrocolloids journal homepage: Micellar properties of OSA starch and interaction with xanthan gum in aqueous solution Veljko Krstonosic a, *, Ljubica Dokic b, Jadranka Milanovic b a Faculty of Medicine, University of Novi Sad, Hajduk Veljkova 3, 21 Novi Sad, Serbia b Faculty of Technology, University of Novi Sad, Bulevar Cara Lazara 1, 21 Novi Sad, Serbia article info abstract Article history: Received 22 February 21 Accepted 26 June 21 Keywords: OSA starch Critical micellar concentration Xanthan gum Interaction Octenyl succinate (OSA) modified starches are used, as emulsifiers and stabilizers, in many food, cosmetics and pharmaceutical products. The aim of this study was to determine critical micellar concentration () of two different octenyl succinate modified waxy corn starches at 25 C, and to examine possibility of their interactions with xanthan gum in aqueous solution. The was determined by viscometry, conductometry, surface tension and dye solubilization. The values for two OSA starches ( and OS2) varied from.5 to.88 g/1 cm 3 and from.41 to.81 g/1 cm 3 respectively, depending on applied technique. The same techniques were used for investigation of the interactions between OSA starch and xanthan gum. The addition of xanthan gum decreases the specific viscosity and increases surface tension and the values compared to the single OSA starch solutions. Ó 21 Elsevier Ltd. All rights reserved. 1. Introduction Starch is widely used in many industrial products due to its functional properties and nutritional value. It has been most often used as thickening agents, because of characteristic that after dispersing in hot water starch granules swell irreversible and form a viscose paste (Banks & Greenwood, 1975). However, the applications of native starches are limited due to their storage and process instabilities. That was the reason for developing the techniques for starch modifications. One of the techniques is chemical modification, which is applied in production of OSA starches by treating starches with octenyl succinic anhydride (Bhosale & Singhal, 26). Modification with hydrophobic octenyl succinic groups gives the starch molecule amphiphilic nature and thus surface active properties. Hydrophobic part of OSA starch molecule contains a carboxylic acid which can be negatively charged (Nilsson & Bergenstahl, 27). The OSA starches for food applications characterize low degree of substitution (DS value), between.1 and.3, and it could be considered as weekly charged polyelectrolyte (Shorgen, Viswanathan, Felker, & Gross, 2). They are used in many food, cosmetics and pharmaceutical products, as emulsifier and stabilizer (Ortega-Ojeda, Larsson, & Eliasson, 25; Varona, Martin, & Cocero, 29). * Corresponding author. Tel.: þ address: veljkokrst@yahoo.co.uk (V. Krstonosic). Amphiphilic molecules form the boundary layer, whereby the hydrophobic part of molecule is oriented toward the air or lipophilic phase and hydrophilic part stays in the water. At the certain concentration the surface is saturated and no more molecules can enter the boundary layer. Above this concentration the monomers form aggregates called micelles. The minimum concentration of surfactant above which micelles are formed is critical micellar concentration () (Varona et al., 29; Posa et al., 27). OSA starches are amphiphilic molecules and is one of the most important parameters designating their application as emulsifier. Xanthan gum is a microbial anionic heteropolysaccharide. Its main chain is based on a linear backbone consists of 1,4-linked b-dglucose, with a charged trisaccharide side chain at the C-3 position on every alternate glucose residue. The trisaccharide side chain contains D-glucuronic acid unit between two D-mannose units. Approximately one half of terminal D-mannose unit contains a piruvatic acid residue by a ketal linkage to the O-4 and O-6 positions. The D-mannose linked to the main chain contains an acetyl group at position O-6 (Garcia-Ochoa, Santos, Casson, & Gomez, 2; Rodd, Dunstan, & Boger, 2). It has been shown that its secondary structure consist of a five-fold helical structure. The high molecular weight of xanthan gum and formation of aggregates via hydrogen bonding are the reasons why its solutions exhibit high viscosity (Katzbauer, 1998; Viebke & Williams, 2). Since, widespread applications in industrial products and formulations of the polymer and surfactant combination, existence of interactions are of huge research interest (Mata, Patel, Jian, Ghosh, & Bahadur, 26). According to Goddard (22) there are 268-5X/$ e see front matter Ó 21 Elsevier Ltd. All rights reserved. doi:1.116/j.foodhyd

2 362 V. Krstonosic et al. / Food Hydrocolloids 25 (211) 361e367 three categories of interactions between polymer and surfactant: polyelectrolyte/oppositely charged surfactant, uncharged polymer/ charged surfactant and uncharged polymer/uncharged surfactant. However, several authors reported existence of interactions between hydrophobically modified polymers with surfactants of the same charge (Bromberg, Temchenko, & Colby, 2; Burke & Palepu, 21; Colby, Plucktaveesak, & Bromberg, 21; Deo et al., 23). Some of the authors reported that xanthan gum and waxy corn starch attracted to each other in solutions (Wang, Sun, & Wang, 21). Hydrophobically modified polymers such as OSA starches are well known as associative thickeners due to interactions with other polymers and surfactants in aqueous solutions (Ortega-Ojeda et al., 25). Ntawukulilyayo, De Smedt, Demeester, and Remon (1996) investigated stabilization of pharmaceutical paracetamol suspension with OSA starch and noticed the dramatic increase in elastic modulus of suspensions after increase in OSA starch from 4% to 6% in presence of constant concentration of xanthan gum. They assumed that association of xanthan gum and OSA starch caused these specific viscoelastic properties of the system. The aim of this study was to determine of two different OSA starch representatives, and to examine their possible interaction with xanthan gum in solution. In this paper we treated OSA starch as surfactant, due to its surface activity and we tried to find out whether they interact with polymer of the same charge. Knowledge of their behavior, when they are used together in formulations, is important to predict properties of the final product. In this paper viscometry, conductometry, dye solubilization and tensiometry were used in order to clarify the OSA starchexanthan gum behavior. 2. Materials and methods 2.1. Materials Octenyl succinate modified waxy corn starches and OS2 were obtained from National Starch and Chemicals GmbH, Germany. is recommended as good natural emulsifier and OS2 is good to use in encapsulation. The grades of both OSA starches are for food and pharmaceutical use. Commercial xanthan gum (Xanthural 18 CP) was purchased from KELKO e Hamburg. Double distilled water was used for solution preparation Methods Stock solutions were prepared by dissolving 1 g of OSA starch in 1 cm 3 of double distilled water at C and diluting to obtain certain concentration..2 g of xanthan gum was suspended in 1 cm 3 of double distilled water and left at room temperature for two days. Different blends of OSA starch and xanthan gum were prepared by mixing their solutions (1 g/1 cm 3 of OSA starch and.2 g/1 cm 3 of xanthan gum) together at certain ratios and storage at room temperature. values of OSA starches and their interactions with xanthan gum were studied by comparing the above mentioned techniques. In all experiments OSA starch concentration varied from.1 to.5 g/1 cm 3, while during the examinations of OSA starchexanthan gum interactions, xanthan gum concentration was kept constant. The xanthan gum concentrations were different and depended on the special requirements of applied methods. It was.2 and.4 g/1 cm 3 for viscometry,.5 g/1 cm 3 for dye solubilization,.4 g/1 cm 3 for tensiometry and.1,.5 and.1 g/1 cm 3 in conductometric investigations. All measurements were done 24 h after preparation of blends. Viscosity measurements of, OS2 and xanthan gum solutions and their mixtures were done using Ubbelohde capillary viscometer (SCHOTT) in thermostatic bath at 25.1 C. Each concentration was measured in triplicates and average values were calculated.all results were expressed as specific viscosity. Specific viscosity (h sp ): h sp ¼ h h 1 (1) where h is solution viscosity and h is pure solvent viscosity. Conductivity measurements were carried out at 25.1 Cby adding portions of 1 g/1 cm 3 or OS2 solution into the 5 cm 3 of double distilled water. For interaction determinations the adding portions consist of mixture of 1 g/1 cm 3 or OS2 and certain xanthan gum concentration, which were added to the 5 cm 3 of xanthan gum solution of the same concentration. The solution or blend was stirred using magnetic stirrer after addition of every portion of OSA starch, until the steady value of conductivity was achieved. Consort C83 multi parameter analyzer was used to measure a specific conductance of solutions and blends. Surface tension measurements were carried out on a Sigma 73D tensiometer (Finland) using a du Nouy ring method. All measurements were repeated three times. In all measurements temperature were kept constant at 25.1 C. The method of dye solubilization implied the extent of water insoluble dye Sudan III solubilization in OSA starch solutions. Different concentrations of OSA starch solutions were prepared by diluting.2 g/1 cm 3 solution. 1 mg of Sudan III was dissolved in 1 ml of certain OSA starch solution and left stirring for 5 h. The analyses were carried out using an Agilent 8453 UVeVisible spectrophotometer. Absorbance at 533 nm was monitored to determine the extent of dye solubility. 3. Results and discussion 3.1. Viscosity measurements The viscosity characteristics of the dilute solutions are good indicator of polymer conformation and their changes in polymere surfactant mixtures. The modified polymer, OSA starch, played a role of surfactant, and xanthan gum of the polymer. Viscosity experiments were conducted in order to understand the nature of those components and to find out the possible interactions. The dependence of the specific viscosity on the concentration of and OS2 in the aqueous solution is shown in Fig. 1. After certain concentration (marked by arrow at Fig. 1) the curves of specific viscosities decreased slope from.17 to.11 for OS2 and from.47 to.14 for. At this concentration some changes occurred in solution. Because OSA starch has characteristics of polymer and surfactant, this point might be a critical overlap concentration c *, which is defined as concentration at which individual polymer molecules begin to physically interact (Rodd et al., 2), or critical micellar concentration which arises due to aggregation of hydrophobic groups which tend to minimize their exposure to water (Egermayer, Karlberg, & Piculell, 24). The information about molecule conformation can be obtained by measuring the intrinsic viscosity of polymer, applying Huggins equation which expresses the reduced viscosity of polymer as function of concentration: h sp ¼½hŠþk½hŠ 2 c (2) c where [h] is intrinsic viscosity, h sp /c is reduced viscosity, k is Huggins parameter and c is polymer concentration.

3 V. Krstonosic et al. / Food Hydrocolloids 25 (211) 361e η s p OS Fig. 1. Specific viscosity vs. OSA starch concentration in aqueous solutions at 25 C. OSA starches exhibited behavior typical for polyelectrolytes aqueous solutions, having high values of reduced viscosity at low polymer concentrations (Yang, Chen, & Fang, 29). Such behavior originates from octenyl succinic groups. That was the reason for determination of intrinsic viscosity in.1% and 1% NaCl aqueous solutions at 25 C. The reduced viscosity vs. concentration of in NaCl solutions at 25 C are shown in Fig. 2. It is obvious that after certain concentration reduced viscosity decreases. Calculation of the specific viscosity values for measurements done in NaCl showed the same slope change tendencies as in pure water. The change of slopes for specific viscosities in NaCl happened after the concentrations at which reduced viscosities reached maximum (Fig. 2). These concentrations, for both OSA starches, were lower that the concentrations in pure aqueous solution due to presence of NaCl, and decreased with increase in NaCl concentration. The decrease in reduced viscosity occurred because of reduction in the hydrodynamic size of molecule (Rochefort & Middleman, 1987), which is influenced by presence of hydrophobic groups. Nilsson, Thuresson, Hansson, and Lindman (1998) reported that the hydrophobic tails of /g) 3 s /c (1c m η p % NaCl 1% NaCl /g) 3 η sp /c (1c m Fig. 2. Reduced viscosity vs. concentration in.1% and 1% NaCl at 25 C. hydrophobically modified polymers in dilute solutions associate intra-molecularly in order to minimize contact with water. For this reason, after certain concentration the polymer chains probably start to curl and shrink and this concentration is critical micellar concentration. The micelles of OSA starches are different than those formed by small surfactant molecules, and it is obvious that hydrodynamic size of molecules became smaller after. Intrinsic viscosity for OSA starches was estimated before the (Fig. 2). The Huggins parameter k gives information about solvent quality. Values between.3 and.7 have been suggested for perfect solutions, and for k 1 formation of aggregates is encouraged (Braga, Azevedo, Marques, Menossi, & Cunha, 26; Millard, Dintzis, Willett, & Klavons, 1997). Parameters k for and OS2 were and 28.7 in.1% NaCl and and in 1% NaCl respectively, which are much higher than 1, meaning that OSA starches have high tendencies to self-aggregation. The obtained results for intrinsic viscosities for and OS2 were cm 3 /g and.15 1 cm 3 /g in.1% NaCl and cm 3 /g and cm 3 /g in 1% NaCl respectively. The increase in NaCl concentration present in OSA starch solutions, led to increase in Huggins parameters and to decrease in intrinsic viscosities of determined OSA starches, due to salt effect on polyelectrolyte. According to Hormnirum, Sirivat, and Jamieson (2) the critical overlap concentration can be estimated as c * ¼ 1/[h]. Therefore the critical overlap concentrations for and OS2 would be 5.36 g/1 cm 3 and 9.95 g/1 cm 3 in.1% NaCl and 6.31 g/1 cm 3 and g/1 cm 3 in 1% NaCl respectively. The change of slope in plot of specific viscosities vs. OSA starch concentration in aqueous solutions (Fig. 1) for and OS2 occurred at.88 g/1 cm 3 and.81 g/1 cm 3 respectively. Those values are lower that the values for critical overlap concentrations calculated above, which is a confirmation for the conclusion that values obtained from Fig. 1 are points of the critical micellar concentrations. Intrinsic viscosity is related to the molecular weight, M, through the MarkeHouwink equation (Casas, Santos, & Garcia-Ochoa, 2; Chuah, Lin-Vien, & Soni, 21): ½hŠ ¼K$M a (3) where K and a are specific constants to the solvent and temperature used in measurements. Since the information for K and a for OSA starches are not available, it is not possible to calculate molecular weight. Due to the fact that both OSA starches, determined in this work, have the same origin and DS values (.1e.3 because they are recommended for food and pharmaceutical use), they have the same values for constants K and a. So it could be concluded that has higher molecular weight than OS2, because it has higher value of intrinsic viscosity. In the presence of constant xanthan gum concentration the specific viscosity of xanthan gumeosa starch blends decreased for low OSA starch concentration. At concentration, determined as of OSA starch (Fig. 1), the viscosity of the blends reached the minimum (Fig. 3). This is unexpected behavior, and could be indicator of some conformational changes in presence of xanthan gum molecules, considering that the specific viscosity of single OSA starch solutions increased with the concentration increase (Fig. 1.). Mya, Jamieson, and Sirivat (1999) reported the same results for Triton X-1 and polyacrilamide. They explained those results by following equation (3): h sp ¼ 2:5N Ac M V h (4)

4 364 V. Krstonosic et al. / Food Hydrocolloids 25 (211) 361e367 η s p ,4 g/1cm 3 xanthan gum + OS2,4 g/1cm 3 xanthan gum Fig. 3. Specific viscosity of OSA starchexanthan gum blends vs. OSA starch concentration for constant xanthan gum concentration (.4 g/1 cm 3 )at25 C. where N A is Avogadro s number, c is concentration of molecules in solution, M is molecular weight and V h is polymer hydrodynamic volume. They concluded that consequence of the interactions between Triton X-1 and polyacrilamide was decrease in number of particles in solution which reflected as decrease in specific viscosity. The same observation can be used to explain our results. The reason why minimum was reached at was formation of mixed micelles between OSA starch and xanthan gum molecules. After that concentration, increase in viscosity with increase in OSA starch concentration occurred. Upon further increase in OSA starch concentration, the viscosity continued to increase, suggesting formation complexes between formed mixed micelles. Examinations were done with two different xanthan concentrations (.2 g/1 cm 3 and.4 g/1 cm 3 ) and the same results were obtained for both concentrations. Deo et al. (23) reported similar results for hydrophobically modified polyelectrolyte and surfactant sodium dodecyl sulfate (SDS) of the same charge, and also concluded that they formed mixed micelles. According to Wang et al. (21) waxy corn starch and xanthan gum are attracted to each other in solution. Besides that, formation of mixed micelles is argument that xanthan gum and OSA starch may interact through formation of inclusion complexes between OSA tails and xanthan gum helix Conductivity measurements Conductometric measurements are widely used for determinations (Fuguet, Rafols, Roses, & Bosch, 25; Onesippe & Lagerge, 28; Sovilj & Petrovic, 26). The specific conductance increases linearly with increase in concentration of ionic surfactant, in this work OSA starch, up to the. After that concentration, specific conductance continues to increase, but with a lower slope than before the. The point after which curve changes the slope represents value (Fig. 4). The point for and OS2 determined by conductivity measurements were.73 g/1 cm 3 and.62 g/1 cm 3 respectively. In the presence of xanthan gum, which concentration was constant, the specific conductivity dependence on OSA starch () concentration is presented on Fig. 5. The conductivity of surfactant with added polymer usually shows three regions. The first break point, at the end of the first region, below, is related to beginning of surfactantepolymer association and it is called critical aggregation concentration (CAC). The second break point, after second region, over, is polymer saturation point (PSP) by the surfactant (Sovilj & Petrovic, 26). The break points CAC and PSP of surfactant and polymer of the same charge (Burke & Palepu, 21) are less significant than for the charged surfactant/uncharged polymer (Sovilj & Petrovic, 26), as well as for surfactant and oppositely charged polymer (Onesippe & Lagerge, 28). The results obtained by determination of specific conductance of OSA starchexanthan gum blends (Fig. 5) indicated existence of OSA starchexanthan gum interactions. Namely, it is noticeable that after CAC the curve changed slope, which is the result of interaction between those two components. For.1 g/1 cm 3 of xanthan gum concentration more significant changes occurred after CAC. The specific conductance showed non-linear dependence on OSA starch concentration until PSP was reached. In our recent work (Krstonosic, Dokic, Dokic, & Dapcevic, 29) we reported that critical overlap concentration for xanthan gum was.82 g/1 cm 3. The significant behavior of specific conductance at.1 g/1 cm 3 of xanthan gum may be because OSA starch molecules build into xanthan gum network, became slower, which reflected on specific conductance. 9 8 A 9 8 B 7 7 k ( μ S/cm ) k ( μ S/cm ) Fig. 4. Determination of the of OSA starches. Plot of specific conductance vs. A), B) OS2 concentrations in aqueous solutions at 25 C.

5 V. Krstonosic et al. / Food Hydrocolloids 25 (211) 361e PSP 72 OS2 14 CAC k ( μ S/cm ) xanthan concentration (%),1,5,1 γ (mn/m ) Fig. 5. Specific conductance of exanthan gum blends vs. concentration for constant xanthan gum concentration in aqueous solutions at 25 C. It is obvious that CAC increases with increase in xanthan gum concentration, and PSP values were independent on xanthan gum concentration (Table 1). The similar results for CAC dependence on polymer concentration were reported by Onesippe and Lagerge (28) for chitosan and SDS Surface tension measurements A surface tension measurement is another method to study surfactant micellarization and possible interactions in solution. Below the surface tension strongly decreases while surfactant concentration increases. After reaching the surface tension remains constant. Therefore the can be determined as concentration after which surface tension does not change. The was determined by measuring the surface tension of different concentration of and OS2 (Fig. 6), and obtained values were.5 g/1 cm 3 and.41 g/1 cm 3 respectively. Varona et al. (29) reported higher values for OSA starch derived from waxy maze, but there were no information about its molecular mass and DS values which may have the influence on. Minimum surface tension about 54 mn/m was reached for and mn/m for OS2 which is in agreement with data reported by Varona et al. (29). Shorgen and Biresaw (27) investigated surface properties of water soluble starches, and they reported that the surface tension of OSA starch (DS.2e.4) declined to 42e43 mn/m which is a slightly lower value than we reported above. They did not determine, but from figure they presented it is obvious that after 2.5e3 g/ml of OSA starch, surface tension remains constant which is in agreement with our results. Several authors reported reduction of surface tension after adding polyelectrolyte to the surfactant solution due to their Fig. 6. Determination of the values for OSA starches. Plot of surface tension vs. OSA starch concentration in aqueous solutions at 25 C. synergistic effect (Deo et al., 23; Mata et al., 26; Onesippe & Lagerge, 28), but Fig. 7 presents opposite effect. Namely, addition of xanthan gum increase the surface tension and value of OSA starch solution. Prud homme and Long (1983) reported that xanthan gum aqueous solutions do not show the surface activities for low concentrations (.1 g/1 cm 3 ), which was confirmed by Secouard, Malhiac, and Grisel (26). Prud homme and Long (1983) and Benichou, Aserin, Lutz, and Garti (27), suggested also, a dramatic decrease of surface tension at 1 g/1 cm 3 xanthan concentration. Our results of xanthan gum surface activity showed that xanthan gum did not significantly change water surface tension between.1 and.1 g/1 cm 3. That is the reason why addition of xanthan gum raises the surface tension and values of OSA starch water solutions. As xanthan molecules do not adsorb at airewater interface, OSA starch molecules were brought into bulk from airewater surface, through their interactions with xanthan gum. After certain OSA starch concentration (.6 g/1 cm 3 for from Fig. 7) the γ (mn/m ) xanthan gum Table 1 Comparison of CAC and PSP values obtained by conductometric titration for and OS2. Xanthan gum concentration (g/1 cm 3 ) CAC (g/1 cm 3 ) PSP (g/1 cm 3 ) OS2 OS Fig. 7. Surface tension vs. concentration for aqueous solutions and for exanthan gum mixtures while xanthan concentration maintained constant (.4%) at 25 C.

6 366 V. Krstonosic et al. / Food Hydrocolloids 25 (211) 361e367 surface tension values for OSA starchexanthan gum blends became constant and the values of surface tension for blends and for single OSA starch solutions overlapped Dye solubilization measurements Sudan III is water insoluble, organic, azo dye, which solubility is possible in surfactant aqueous solutions. It was expected that solubility of Sudan III rapidly increase after OSA starch micelle formation in aqueous solutions. Fig. 8 represents solubility of Sudan III as function of OSA starch concentration. It is obvious; from Fig. 8 that for both investigated OSA starches was around.8 g/1 cm 3. The solubilization process is influenced by several different factors. Some of them are: structure of the surfactant and organic compound, temperature, presence of polymer, etc (Burke & Palepu, 21). In order to determine OSA starchexanthan gum interaction, we investigated dependence of Sudan III solubilization as function of OSA starch concentration in presence of constant xanthan gum concentration (.5 g/1 cm 3 ). The absorbance for those systems did not show any significant differences from absorbance for pure OSA starch solutions. That was because the method does not have adequate sensitivity for those investigations. Summarizing the results, it is obvious for all applied techniques that the values for were higher than for OS2. It was because has higher molecular weight, thus its molecules contain more hydrophilic groups, and they have higher tendency to stay in the water than OS2 molecules. The results obtained for are different for all applied methods, because the is moderately method-dependent (Moulik, 1996). According to Moulik (1996) the most frequently used methods for determination are tensiometry, conductometry and fluorometry. Ghosh and Banerjee (22) studied interaction between SDS and globular protein trypsin using tensiometric, conductometric, calorimetric, fluorimetric, viscometric, and circular dichroism techniques. For determination of the values of SDS they used only tensiometric, conductometric and calorimetric method. Viscometry is effective method in examination of conformational and rheological changes (Ghosh & Banerjee, 22), which we used in determination of values. In view of previous information we concluded that the values of obtained, in this work, by tensiometric and conductometric methods are the most reliable. A OS Fig. 8. Absorbance of solubilized Sudan III dye vs. OSA starch concentration. 4. Conclusions In this paper four different techniques were used to determine critical micellar concentration of two variety of octenyl succinate modified waxy corn starches in aqueous solution. The applied techniques provide comparable results of. 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