Rheological and kinetic study of the ultrasonic degradation of xanthan gum in aqueous solutions

Food Chemistry 172 (2015) 808–813

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Rheological and kinetic study of the ultrasonic degradation of xanthan gum in aqueous solutions Ruoshi Li, Donald L. Feke ⇑ Department of Chemical and Biomolecular Engineering, Case Western Reserve University, Cleveland, OH 44106, USA

a r t i c l e

i n f o

Article history: Received 19 February 2014 Received in revised form 5 September 2014 Accepted 23 September 2014 Available online 30 September 2014 Keywords: Ultrasonic degradation Xanthan gums Salt Rheology Viscosity

a b s t r a c t The effectiveness of ultrasound to degrade the molecular weight of xanthan gum in aqueous solutions was investigated for sonication times up to 60 min at 20 °C and for polymer concentrations up to 0.1 g/dl. The Huggins equation was found to be applicable to the intrinsic viscosity of xanthan gum prior to sonication, while a truncated form was found to be adequate for estimating the intrinsic viscosity of the degraded xanthan. To better understand the influence of salting-in and salting-out salts (classified on the basis of the Hofmeister series) on degradation, xanthan-gum solutions were pre-mixed with 0.1, 102, 103, or 104 M NaCl or Na2SO4, prior to ultrasonication. A kinetic model was developed and successfully applied to quantify and predict the degradation rates and efficiency. The various reaction rate constants and reaction orders were found to correlate with the different salt species and concentrations used, suggesting that salting-in and salting-out salts could increase or inhibit ultrasonic degradation by adjusting the molecular conformation of the xanthan. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Because of their practical significance, degradable natural polymers, such as polysaccharides and proteins, continue to attract the attention of researchers. Oligosaccharides, which are obtained from polysaccharide degradation, have been extensively used for many industrial and food applications and are considered to be products with high added-value. Among such polysaccharides is xanthan gum, which has been broadly used for food, pharmaceutical and cosmetic applications as a thickener (Lund, Lecourtier, & Muller, 1990), stabilizer, emulsifier, and foaming agent since it was first approved by the U.S. Food and Drug Administration in 1969. These applications stem from the ability of xanthan to induce high viscosity at low polymer concentration in aqueous solutions. Xanthan is produced by aerobic fermentation of a pure culture of the bacterium Xanthomonas campestris (Richardson & RossMurphy, 1987). Xanthan, which consists of a b (1–4)-linked glucose backbone with orderly distributed trisaccharide side chains, is completely soluble in cold and hot water (Holzawarth, 1981). Xanthan gums have been shown to exhibit two different conformational structures: an ordered conformation (Jansson, Kenne, & Lindberg, 1975), which has been described as double-stranded helical structure (Sato, Kojima, Norisuye, & Fujita, 1984; Sato, ⇑ Corresponding author. E-mail address: [email protected] (D.L. Feke). http://dx.doi.org/10.1016/j.foodchem.2014.09.133 0308-8146/Ó 2014 Elsevier Ltd. All rights reserved.

Norisuye, & Fujita, 1984; Sato, Norisuye, & Fujita, 1985), and a disordered conformation, which has been described as a singlestranded random coil (Milas & Rinaudo, 1986) or a wormlike chain with a moderate persistence length (Milas, Rinaudo, Duplessix, Borsali, & Linder, 1995). As a result, the xanthan gum may undergo a thermally induced conformational transition that depends on the salt concentration and temperature of the solvents. To date, much work has attempted to elucidate the relationship between the rheological behaviour of xanthan, conformational change, intermolecular interactions, and stability (Higiro, Herald, & Alavi, 2006; Higiro, Herald, Alavi, & Bean, 2007; Liu & Norisuye, 1988a, 1988b; Stokke, Smidsrød, & Elgsaeter, 1989; Wang, Sun, & Wang, 2001; Wang, Wang, & Sun, 2002). Degradation of these naturally occurring biopolymers has become a key goal for xanthan studies, because the molecular weight or particle size is often required to be reduced, or to be confined within a narrow range, in order to meet the requirements of the application. Recently, several techniques for degradation have been studied and applied to xanthan, including bio-degradation (Ahlgren, 1993; Cadmus, Jackson, Burton, Plattner, & Slodki, 1982; Muchová et al., 2009), thermal degradation (Lambert & Rinaudo, 1985; Soares, Lima, Oliveria, Pires, & Soldi, 2005; Srivastava, Mishra, Singh, Srivastava, & Kumar, 2012), chemical degradation (Christensen, Smidsroed, Elgsaeter, & Stokke, 1993), and ultrasonic degradation (Milas, Rinaudo, & Tinland, 1985). Compared to other degradation techniques, ultrasonic degradation has been considered to be one of the best ways to control

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molecular weight of degradation products. In addition, under ultrasonic processing, degraded polymers have the same basic chemical structure as their parent molecule, whereas other methods might alter the chemical structure and thereby the material behaviour (Basedow & Ebert, 1977). High intensity ultrasound generates various physicochemical effects, including radiation pressure, streaming, cavitation, and interface instabilities (Mulet, Cárcel, Benedito, Rosselló, & Simal, 2002). Cavitation is broadly accepted as the mechanism of ultrasonic degradation of polymers (Basedow & Ebert, 1977). Ultrasonic degradation of xanthan gum was recently reported by Tiwari, Muthukumarappan, O’Donnell, and Cullen (2008) and a limiting molecular weight was observed. As a result, a more thorough study of the influence of the environmental conditions within the solution on the degradation efficiency and limits is of high interest. In this work, we specifically study the influence of salt on the ultrasonic degradation of xanthan gum in aqueous solutions, which provides a potential way to achieve degradation beyond the limits seen in other degradation studies and with a faster degradation rate. Two categories of salts were used over a range of concentrations. Observation of changes in the intrinsic viscosity of the solutions was used to monitor the effectiveness of the ultrasonic degradation. A degradation kinetics model has also been developed and used to quantify and compare degradation rates under different environmental conditions. 2. Materials and methods 2.1. Materials and solution preparation Xanthan gum (of molecular weight, ranging from 13,000 to 50,000 kDa) was purchased from Sigma–Aldrich (St. Louis, MO). Sodium chloride (NaCl) and sodium sulphate (Na2SO4) were purchased from Fisher Scientific (Fair Lawn, NJ). Solutions were prepared by dispersing 100 mg of dried xanthan gum in 100 ml of deionized distilled water, followed by heating to 40 °C and stirring for 60 min. After cooling to room temperature, the solutions were transferred into two 50 ml centrifuge tubes, and spun at 2200 rpm for 15 min to remove air bubbles. After centrifuging, the solutions were ready for ultrasonic processing. To understand the influence of ionic strength on the ultrasonic degradation of xanthan gum, the solutions were pre-mixed with either 0.1, 102, 103, or 104 M NaCl or Na2SO4 before sonication. 2.2. Density measurement The amount of dissolved xanthan gum was measured by filtering small samples of the centrifuged xanthan gum solutions with a Tisch Scientific 0.22 lm Nylon syringe filter (North Bend, OH). Aliquots were added to small vials and heated overnight at 80 °C to evaporate the solvent, and the amount of dissolved material was determined gravimetrically. All the measurements were performed in triplicate.

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to make sure that all viscosity measurements were taken at the same ionic strength. Each sample was diluted by using the corresponding salt solution with the concentration of 0.1 M, to produce concentrations of xanthan gum in the range of 0.10–0.02 g/dl. The viscosity of the solutions was determined using Cannon Ubbelohde viscometers (State College, PA) at 25 °C. Origin 8.5.1 (Origin Lab, Northampton, MA) was used to plot viscosities against concentrations, as well as to obtain linear and nonlinear regression lines with the corresponding equations and correlation coefficients (R2) in order to assess the best model. 2.4. Intrinsic viscosity determination The goal of the rheological measurements is to determine the intrinsic viscosity [g] of the solutions as a function of ultrasonication in order to quantify the degradation process. The intrinsic viscosity measures the contribution of individual polymer molecules to the solution viscosity. As indicated in Eqs. (1)–(3), to obtain the value of intrinsic viscosity, solution-viscosity measurements are extrapolated to zero shear-rate (q ? 0) and infinite dilution (C ? 0) in order to eliminate the interaction effects between the polymer molecules. Here, gsp is the specific viscosity, grel is the relative viscosity, gs is the viscosity of the solvent, and g is the viscosity of the solution. The intrinsic viscosity is determined by measuring the specific viscosities of a solution at its original polymer concentration, diluting it several times with solvent and measuring and calculating the specific viscosity after each dilution, and extrapolating the course of specific viscosity results to zero concentration. The zero-concentration intercept value is the intrinsic viscosity of the polymer.

½g ¼ lim q!0 C!0

gsp

gsp ¼ grel  1 ¼ grel ¼

ð1Þ

C

g  gs gs

g gs

ð2Þ

ð3Þ

The relationship between dilute polymer solution viscosity and concentration can be described by many empirical forms, the most common of which is the Huggins equation (Huggins, 1942), as shown in Eq. (4), wherein k is the Huggins constant.

gsp C

¼ ½g þ k½g2 C

ð4Þ

For very dilute solutions, the Huggins equation can be simplified, removing the second-order term and, in this case, the intrinsic viscosity can be determined from the slope of a plot of polymer concentration against relative viscosity (Tanglertpaibul & Rao, 1987).

grel ¼ 1 þ ½gC

ð5Þ

2.3. Sonication treatment and viscosity measurement 2.5. Ultrasonic degradation kinetics The xanthan gum solutions (25 ml, 0.10 g/dl) were transferred into a cooling cell, and sonicated by using a Cole-Parmer ultrasonic processor Model CP750 (Vernon Hills, IL) fitted with a horn, which has a diameter of 0.5 in and a nominal power output of 750 W. The frequency of the amplifier was fixed at 20 kHz, and the amplitude was fixed at 35%, which corresponds to approximately 92 W of power. The water–ice bath was replenished as needed in order to control the temperature of the solutions in the cell. After sonication treatment, the salt concentrations of all solutions were adjusted to 0.1 M through addition of corresponding salts, in order

In general, the rate of degradation can be modelled as an nthorder reaction, based on total molar concentration of the polymer, as is shown in Eq. (6). Here [M]t is the total concentration of polymer molecules (and fragments) at time t, k0 is the degradation rate constant, and n is the order of reaction (Taghizadeh & Mehrdad, 2003).

d½Mt 0 ¼ k ½Mnt dt

ð6Þ

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Note that, since ultrasonic degradation is a mechanical process, the value of n is expected to be negative. By integrating and applying the initial condition that, at t = 0, [M]t = [M]0, where [M]0 is the initial total molar concentration of polymer and t is sonication time, Eq. (7) is obtained: 0

½M1n  ½M01n ¼ ð1  nÞk t t

ð7Þ

The total molar concentration of polymer, which increases as degradation occurs, is related to the number-average molecular weight and polymer concentration in solution, as shown in Eq. (8). The viscosity-average molecular weight, Mv is related to the intrinsic viscosity through the Mark-Houwink equation (White & Kim, 2008), Eq. (9), and to the number average molecular weight through Eq. (10).

½M ¼

C Mn

ð8Þ

½g ¼ KMav

ð9Þ

M v ¼ ½ð1 þ aÞ

Z

1

1

a

1

ex xa dx Mn B½ð1 þ aÞC½ð1 þ aÞa M n

ð10Þ

0

By combining Eqs. (7) through (10), the relationship between intrinsic viscosity and sonication time shown in Eq. (11) can be obtained. Here, [g]t is the intrinsic viscosity of the polymer solution at time t, [g]0 is the intrinsic viscosity of the original polymer solutions before ultrasonic treatment, and a and K are the Mark-Houwink constants, and C is the standard gamma function. n1

n1

00

½gt a  ½g0a ¼ ð1  nÞk t

Fig. 1. Plots of reduced viscosity vs. xanthan gum concentration: xanthan gum solutions (0.10 g/dl) were premixed with 0.1 M NaCl, and then sonicated for the given time. After sonication, the sonication-treated solutions were diluted with 0.1 M NaCl solutions to produce polymer concentrations ranging from 0.10 to 0.02 g/dl.

ð11Þ

where 0

00

k ¼

k

K½ð1 þ aÞCð1 þ aÞC a

1n a

3. Results and discussion 3.1. Intrinsic viscosity Four intrinsic viscosity values of xanthan gum solutions, which correspond to no-sonication, 2-min, 10-min, or 60-min sonication treatments are shown in Figs. 1 and 2. Eqs. (4) and (5) were applied and compared for estimation of the intrinsic viscosities of xanthan gum before and after sonication treatment. Higiro et al. (2006) and (2007) reported that the Huggins equation failed to fit the viscosity data, and thus did not generate the intrinsic viscosity for xanthan gum solutions with concentrations lower than 0.008 g/dl. However, the results of our study showed a good fit to the model (R2 = 0.9727) for the case of unsonicated solutions of xanthan concentration ranging down to 0.02 g/dl with 0.1 M NaCl (see Table 1). With sonication treatment, the intrinsic viscosities obtained by using the Huggins equation also show acceptable correlation coefficients (R2 ranging from 0.7090 to 0.9802) which suggests that the Huggins equation is applicable for estimation of intrinsic viscosities of xanthan gum solutions with polymer concentrations higher than 0.02 g/dl. Using the simplified Huggins equation (Eq. (5)) in which intermolecular interactions are not considered, a plot of relative viscosity against concentration is linear. It was found that a better fitting linear plot could be obtained by using this second model, as indicated by correlation coefficient values ranging from 0.9849 to 0.9999 for xanthan gum solutions, with or without sonication treatment. However, it was also found that the estimated intrinsic viscosity (38.30 dl/g) obtained from Eq. (5) is much higher than

Fig. 2. Plots of specific viscosity vs. xanthan gum concentration: xanthan gum solutions (0.10 g/dl) were premixed with 0.1 M NaCl, and then sonicated for the given time. After sonication, the sonication-treated solutions were diluted with 0.1 M NaCl solutions to produce polymer concentrations ranging from 0.10 to 0.02 g/dl.

that obtained from the Huggins equation (16.91 dl/g), when no sonication was applied to the xanthan gum solutions. Similar results were also found for the intrinsic viscosities of xanthan gum solutions with short-duration sonication treatment, where the intrinsic viscosity (20.35 dl/g) obtained from Eq. (5) is nearly twice that yielded by the Huggins equation (11.91 dl/g). After a relatively long-time sonication treatment, the difference between the estimated intrinsic viscosities obtained from the two models became smaller. These results suggest that molecular interaction is essential and should not be neglected when molecular chains are long and tend to form entangled structures. On the contrary, after sonication treatment, since the molecular chains became shorter and stiffer, the influence of molecular interaction on

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Fig. 3. Plots of intrinsic viscosity vs. sonication time for xanthan gum with different concentrations of: (a) NaCl; (b) Na2SO4. Intrinsic viscosities were obtained from the Huggins equation.

intrinsic viscosity becomes smaller and less important. As a result, to maintain the comparison properly, we use Eq. (4) as the best model for intrinsic viscosity determination because of its acceptable linear fit for data obtained from both sonicated and non-sonicated xanthan gum solutions. 3.2. Influence of salt on the ultrasonic degradation of xanthan gum Two categories of salts, salting-in and salting-out salts, which are classified according to the Hofmeister series (Kunz, Henle, & Ninham, 2004), were utilized in this work. Hofmeister theory is used to describe the ability of salts to precipitate certain proteins in aqueous solutions. In this theory, high charge-density ions are expected to polarise water molecules adjacent to the polymer to form hydration complexes and subsequently increase the surface tension of water. That will inhibit the dissolution of polysaccharides and increase the energy of cavity formation surrounding the molecular chains. This effect is named salting-out of the polymer. By contrast, the low charge-density ions may bind directly to the molecular chain and yield electron transfer, which may increase the dissolution of polysaccharides, and result in the salting-in of the polymer (Zhang, Furyk, Bergbreiter, & Cremer, 2005). The ability of different anions and cations to lead to salting-in and salting-out effects was studied and reported by Freire et al. (2009), where it was found that anions had a stronger influence than had cations. As a result, we focussed our study on the influence of anions on ultrasonic degradation. NaCl and Na2SO4 were selected as representative salting-in and salting-out behaviour salts. For the case of ultrasonic treatment, significant differences were found between salt species and concentrations in terms of the intrinsic viscosity for xanthan gum solutions. When xanthan gum

solutions were premixed with 0–0.1 M NaCl, a limited but significant decrease or increase of the intrinsic viscosity was noticed. After sonicating the xanthan gum solutions for 0.5 min to 10 min, the lowest intrinsic viscosity of sonication-treated xanthan solution was obtained in xanthan gum solution premixed with 104 M NaCl, whereas the intrinsic viscosities of xanthan solutions with 103–0.1 M NaCl were higher than that of salt-free solutions (see Fig. 3). This result suggests an influence of salt concentrations on sonication degradation efficiency. Previous studies have suggested that the xanthan gum backbone is disordered and highly extended due to electrostatic repulsions between charges on the chains in salt-free solutions (Stokke et al., 1989). In this state, the chains are relatively stiff, but retain some degree of flexibility. When NaCl ions are present in solution but with very low ionic strength (lower salt concentration), the xanthan gum backbone may become more disordered and extended, which makes the molecular chains less stable. In addition, as a salting-in salt, NaCl also increases the solubility of xanthan gum and lowers the energy of cavity formation surrounding the molecular chains. Hence, the

Table 1 Comparison of intrinsic viscosities and correlation coefficients obtained from the two models. Sonication time (min)

0 0.5 5 10 30

gsp C

grel ¼ 1 þ ½gC

0

¼ ½g þ k ½g2 C 2

[g] (dl/g)

R

16.91 11.91 5.65 4.03 3.77

0.9727 0.9802 0.8124 0.7090 0.9371

[g] (dl/g)

R2

38.30 20.35 5.97 4.40 2.95

0.9849 0.9950 0.9998 0.9997 0.9998

Table 2 Reaction rate constant and reaction order of ultrasonic degradation of xanthan gum with NaCl and Na2SO4 using different Mark-Houwink constants. Solutions were sonicated for specific times, and then the salt concentration was adjusted to 0.1 M prior to the viscosity measurement. The exponent b within the units of k00 is dependent on the values of reaction order (n) and the Mark-Houwink constant (a), and ranges from 3.0 to 3.2 for the case of NaCl, and 1.9 to 2.0 for the case of Na2SO4. NaCl

Na2SO4

a

k00 ((g/dl)b min1)

n

R2

k00 ((g/dl)b min1)

n

R2

0.9 1.0 1.1 1.2

0.0080 0.0072 0.0065 0.0060

1.9305 2.1562 2.3818 2.6074

0.9914 0.9922 0.9983 0.9685

0.0144 0.0129 0.0118 0.0108

0.7577 0.9528 1.1479 1.3430

0.9867 0.9674 0.9961 0.9589

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Table 3 Reaction rate constant k00 and reaction order n obtained from Eq. (11) for xanthan solutions premixed with NaCl or Na2SO4 in various concentrations. The Mark-Houwink constant was fixed at 1.0. Solutions were sonicated for specific times, and then the salt concentration was adjusted to 0.1 M prior to the viscosity measurement. The exponent b within the units of k00 is dependent on the values of reaction order (n) and ranges from 2.7 to 3.3 for the case of NaCl, and 2.0 to 2.4 for the case of Na2SO4. Concentration (M)

Salt-free 104 103 102 0.1

NaCl

Na2SO4

k00 ((g/dl)b min1)

n

R2

k00 ((g/dl)b min1)

n

R2

0.0072 0.0093 0.0068 0.0052 0.0041

2.1562 2.2562 1.9425 1.8361 1.6789

0.9922 0.9918 0.9961 0.9882 0.9889

0.0129 0.0089 0.0052 0.0028 0.0023

0.9528 1.0852 1.1488 1.0602 1.4433

0.9674 0.9785 0.9952 0.9962 0.9911

molecular chains would reach a further state of degradation at corresponding sonication times. On the contrary, when NaCl ions are present in solution with higher ionic strength (higher salt concentration), the ordered double-helical or even network conformation is stabilized. Xanthan gum chains in the helical conformation are much more rigid and stable than are the disordered chains. Charge screening causes the side chains to collapse down to the backbone, hence making the molecular chains difficult to degrade. When the sonication time is extended to 30 min or more, the viscosity of 103 M xanthan gum became even lower than that of salt-free xanthan gum solution. It was also noticed that the viscosity difference between each salt solution and the salt-free solution diminished with prolonged sonication time. In contrast, when NaCl was replaced by Na2SO4, regardless of the concentration, the intrinsic viscosity of sonication-treated xanthan gum salt solution was always higher than that of the sonicationtreated salt-free xanthan gum solution (Fig. 3). This is attributed to the nature of Na2SO4 since, as a typical salting-out salt, the anions can polarise adjacent water molecules that are, in turn, involved in hydrogen bonding with the hydroxyl or carboxyl groups on the chains. Then, these species would increase the surface tension of the cavity surrounding the backbone and the side chains, and subsequently lead to hydrophobic behaviour of the macromolecules. Finally, these effects would lead to the salting-out of xanthan gum, thereby lowering the efficiency of ultrasonic degradation. 3.3. Kinetics study of ultrasonic degradation of xanthan gums To study the degradation kinetics according to Eq. (11), the value of the Mark-Houwink constant a, which is dependent on the temperature, ionic strength, molecular weight, pH value of solvent, and even the ratio between pyruvate and acetate groups, needs to be considered. Based on previous reports, the value of the Mark-Houwink constant may range from 0.9 to 1.2 (Kulicke, Oertel, Otto, Kleinitz, & Littmann, 1990). Since the molecular weight of xanthan gums may vary by sonication time and sonication environmental condition, it is necessary to use an estimated Mark-Houwink constant in order to make degradation results comparable. Therefore, the sensitivity of reaction order and reaction rate constant to the value of a was tested on the basis of ultrasonic degradation of salt-free xanthan gum solutions, and the results are summarized in Table 2. By screening the changes of reaction order and reaction rate constant, it was found that the values of reaction order ranged from 1.93 to 2.60 for NaCl solutions, and ranged from 0.76 to 1.34 for Na2SO4 solutions. The reaction rate constants ranged from 0.0060 to 0.0080 (g/dl)3.0–3.3 min1, and from 0.0108 to 0.0144 (g/dl)1.9–2.0 min1 with Mark-Houwink constants ranging from 1.2 to 0.9 for the two kinds of salt solutions. As a result, to simplify subsequent comparisons, we fixed the Mark-Houwink constant as a = 1.0 for both salt solutions in our studies. From the data listed in Table 3, it could be noticed that the reaction rate constants obtained from this model correspond to the

experimental results for both categories of salts. When NaCl was premixed with xanthan gum solutions, the fastest degradation rate was achieved in 104 M NaCl solutions, with a reaction rate constant of 0.0093 (g/dl)3.3 min1, which is almost 30% faster than that of salt-free solutions. By contrast, when more NaCl was added, the degradation rate was significantly decreased (Table 3). Especially for 0.1 M NaCl solutions, the reaction rate was 0.0041 (g/dl)2.7 min1, which is 43% slower than that of salt-free solutions. On the other hand, when Na2SO4 was premixed with xanthan gum solutions, the reaction rate constants corresponding to salt solutions were all lower than those of salt-free solutions. With increasing concentrations of Na2SO4, the reaction rate decreased sharply. When 0.1 M Na2SO4 was used, the reaction rate constant decreased to 0.0023 (g/dl)2.4 min1,which is 82% lower than those of salt-free solutions. This result is caused by the high ionic strength and strong salting-out behaviour of Na2SO4 at higher salt concentration. Besides the reaction rate constants, it was found that the values of all reaction orders were negative, which indicates that the degradation rate would be inhibited by increasing the polymer concentration. This is consistent with the mechanical mechanism of ultrasonic degradation. For the NaCl group, the reaction order ranged from 2.3 to 1.7, whereas the reaction order ranged from 1.4 to 1.0 for the Na2SO4 group. This difference of reaction order indicates that the degradation rate is more sensitive to the polymer concentration when salting-in salts were used. Finally, the values of correlation coefficient of the two groups ranged from 0.9674 to 0.9961, indicating that Eq. (11) is an appropriate model to quantify the degradation rate. 4. Conclusions In this work, the ultrasonic degradation of xanthan gum in saltfree solutions and salt solutions was systematically studied. The results confirmed that salting-in and salting-out salts have significantly different influences on the ultrasonic degradation of xanthan gum in aqueous solutions by tuning the molecular conformation. In NaCl solutions, lower concentrations may enhance the degradation efficiency whereas higher concentrations could reduce the degradation efficiency. By contrast, Na2SO4 solutions of both of lower concentration and higher concentration may slow the degradation process. These results are attributed to different molecular conformations of xanthan gum in the two different salt solutions. Acknowledgements This research was supported by Nestlé R&D in Marysville, OH. References Ahlgren, J. A. (1993). Characterization of xanthan gum degrading enzymes from a heat-stable, salt-tolerant bacterial consortium. Developments in Petroleum Science, 39, 55–63. Basedow, A. M., & Ebert, K. H. (1977). Ultrasonic degradation of polymers in solution. Advances in Polymer Science, 22, 83–148.

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