Solution Properties of Polysaccharides - American Chemical Society

4 Xanthan Gum with Improved Dispersibility P. A. SANDFORD, J. BAIRD, and I. W. C O T T R E L L

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Kelco Division of Merck & Co., Inc., 8225 Aero Drive, San Diego, C A 92123

Xanthan gum (1) is the high molecular weight natural exopolysaccharide produced by the bacterium Xanthomonas campestris. Xanthan gum owes i t s commercial importance to i t s thickening, suspending and pseudoplastic properties in aqueous systems (2-9). Both industrial and food approved xanthan gum are available. Factors Affecting Gum Dispersibility Most high molecular weight hydrocolloids such as xanthan gum, guar gum, carboxymethylcellulose, etc., generally require a combination of vigorous and/or lengthy mixing times to produce uniform dispersion and complete hydration. The problem of "fish-eye" formation i s often encountered during dispersing of gums. When a gum particle begins to hydrate, often a gelatinous layer of p a r t i a l l y hydrated gum forms on the outside of the particle and prevents water from penetrating to complete hydration and dissolution of the p a r t i c l e . Ideally, what is desired is a lump-free solution/ dispersion that forms rapidly without vigorous agitation. However, in the industrial application of gums, often many d i f f i c u l t and varied hydration conditions are encountered (Figure 1). Often agitation is severely limiting thus making dispersion of gums d i f f i c u l t . Also, because of their many different uses, gums encounter a large variety of pH values, ionic environments, temperatures, e t c . , and gums are often mixed with surfactants, fats, o i l s , proteins, and various carbohydrates. Further, regulatory restrictions are placed on the use of gums in foods. Methods of Improving the Dispersibility of Gums The most common physical means of dispersing gums are l i s t e d in Figure 2. Some benefit can be obtained by generating bubbles (10) in situ (e.g., CO evolution from reacting NaHCO 2

0097-6156/81/0150-0031$05.00/0 © 1981 American Chemical Society In Solution Properties of Polysaccharides; Brant, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3


32

SOLUTION PROPERTIES O F POLYSACCHARIDES

and c i t r i c a c i d ) , but o f t e n the high shear of a mixer or vigorous a g i t a t i o n encountered i n the use of a dry powder eductor ( a s p i r a t o r ) i s more ef fective(JL). A technique (JL) o f t e n used i n d i s p e r s i n g gums i n t o food preparations i s the s e p a r a t i o n of gum p a r t i c l e s by b l e n d i n g with the d r y components of a formulation (e.g., sugar, s t a r c h ) . Another technique used t o d i s p e r s e gums i s to s l u r r y powder i n e i t h e r water m i s c i b l e non-aqueous l i q u i d s such as vegetable o i l p r i o r t o adding water. These non-aqueous s o l v e n t s allow the gum p a r t i c l e s to remain separated and hydrate slowly without forming lumps. Encapsulation of gums (11) a l s o accomplishes the same e f f e c t of keeping h y d r a t i n g gum p a r t i c l e s separated. Agglomeration (12) of gum p a r t i c l e s can a l s o r e s u l t i n a d i s p e r s i b l e gum p r e p a r a t i o n . Agglomeration b a s i c a l l y i n v o l v e s moistening gum p a r t i c l e s which then s t i c k together while the moisture i s removed, and the agglomerated p a r t i c l e s hydrate r a p i d l y without lumping. However, agglomeration i s quite costly. Sometimes by proper s e l e c t i o n of p a r t i c l e s i z e , lumping of some gums during d i s p e r s i o n i n water can be minimized. In a d d i t i o n t o the various p h y s i c a l means used to o b t a i n lump-free gum s o l u t i o n s / d i s p e r s i o n s , v a r i o u s chemical treatments of h y d r o c o l l o i d s have been a p p l i e d s u c c e s s f u l l y (Figure 3 ) . The most e f f e c t i v e chemical methods f o r e f f e c t i n g d i s p e r s i b i l i t y are the use o f c r o s s - l i n k i n g agents. The most common chemical method used i n rendering gums d i s p e r s i b l e i s t h e i r r e a c t i o n with aldehydes and, i n p a r t i c u l a r , dialdehydes. D i v a l e n t metal ions can c r o s s - l i n k xanthan gum, p a r t i c u l a r l y under a l k a l i n e c o n d i t i o n s . For example, xanthan gum p r e c i p i t a t e d with c a l c i u m ions a t pH 10-12 i s i n s o l u b l e a t n e u t r a l pH (13, 14). Such a xanthan gum/calcium complex can be d i s p e r s e d r e a d i l y ( s i n c e i t i s r e a l l y i n s o l u b l e ) and, by adding a c i d ( p a r t i c u l a r l y i f the a c i d complexes the calcium) such as c i t r a t e , the xanthan gum/calcium complex can be broken which allows the gum t o hydrate and develop lump-free s o l u t i o n s . Knowledge of the behavior of the v a r i o u s s a l t forms of xanthan gum allows the design of other ways f o r d i s p e r s i n g the gum. Borate a l s o forms complexes with xanthan gum under a l k a l i n e c o n d i t i o n s and allows the design of a system f o r i n c r e a s i n g the d i s p e r s i b i l i t y o f xanthan gum i n a l k a l i n e waters. More d e t a i l s on t h i s process are presented l a t e r . Other chemical d e r i v a t i v e s o f xanthan gum would be expected to have a l t e r e d d i s p e r s i b i l i t y and hydration behavior. G l y o x a l Treatment

of Polyhydroxyls (14-19)

G l y o x a l , because of i t s unique chemistry, i s probably the most commonly used chemical agent f o r treatment of gums f o r improving d i s p e r s i b i l i t y i n non-food a p p l i c a t i o n s . The most important r e a c t i o n s o f g l y o x a l are l i s t e d i n Figure 5• Near n e u t r a l pH v a l u e s , one g l y o x a l molecule tends to r e a c t

In Solution Properties of Polysaccharides; Brant, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

4.

SANDFORD E T A L . Xanthan

Agitation

—

Very low to vigorous

PH

-

2 to 12

I — ) -

Metal ions Salts » Temperature

—

> Gum particle size • Other components in hydration fluid


33

Gum

Figure 1.

i Soft water, hard water, brines I Na , K , C a , M g , A l , Cr+++ +

+

+ +

++

+ + +

0to120°C 20- to 325-mesh (Tyler standard screen) j Surfactants, oils/fats, proteins, I carbohydrates (sugar, starch, other gums)

Factors affecting gum dispersibility

Agitation • low shear—in situ bubble formation (CO2) • high-shear mixer • eductor/aspirator Separation/coating of gum particles • dry-mix dispersion (sugar, starch, clay) • liquid-mix dispersion -miscible non-aqueous liquids (glycols, alcohols) -non-miscible non-aqueous liquids (vegetable oil) • encapsulation Selection of particle size * mesh size • agglomeration

Figure 2. Physical means of improving the dispersibility of gums

CrossHnking/^solubilization • Aldehydes — Formaldehyde — Glyoxal — Gluteraldehyde • Metal ions — Divalent (Ca++, Mg++) — Trivalent(AI ) — Higher valences

it

Resolubilization Method Base-catalyzed hydrolysis

pH adjustment

+++

• Borates

Figure 3.

Metal-complexing agents pH adjustment

Chemical means of improving the dispersibility of gums



34

SOLUTION PROPERTIES OF

POLYSACCHARIDES

with the s i n g l e C-6 primary hydroxyl of the p o l y s a c c h a r i d e to form hemiacetal-I (Figure 4 ) . At lower pH v a l u e s , g l y o x a l tends to form a chemical bridge between two C-6 primary hydroxyls, hemiacetal-II (Figure 4 ) . I n c r e a s i n g the temperature during g l y o x a l treatment at low pH values can r e s u l t i n complete r e a c t i o n of the g l y o x a l with four primary hydroxyls. However, t h i s a c e t a l - I I I i s much more s t a b l e to a l k a l i n e h y d r o l y s i s r e l a t i v e to the hemiacetals I and I I . I t i s the r e l a t i v e ease of h y d r o l y s i s of hemiacetals I and I I from a polysaccharide t h a t i s so a t t r a c t i v e i n e f f e c t i n g d i s p e r s i b i l i t y . G l y o x a l - t r e a t e d xanthan gum and other gums are very d i s p e r s i b l e because of t h e i r r e l a t i v e i n s o l u b i l i t y but as the a c e t a l s are hydrolyzed, the gums begin to hydrate at a r a t e t h a t allows each gum p a r t i c l e to remain separated t o form lump-free s o l u t i o n s . By a l t e r i n g the g l y o x a l treatment, a spectrum of g l y o x a l t r e a t e d gums r e s u l t s , each of which has d i f f e r e n t hydration and d i s p e r s i b i l i t y properties. F i g u r e 6 l i s t s r e p r e s e n t a t i v e methods d i s c l o s e d i n the l i t e r a t u r e used to t r e a t various h y d r o c o l l o i d s such as xanthan gum with g l y o x a l . Gums have been t r e a t e d using g l y o x a l i n i t s l i q u i d (e.g., 40% i n water) form (14, 15, 16, 17, 19), s o l i d ( c r y s t a l l i n e dihydrate, mp 15°C) form (18), and vapor (bp-jj^ 51°C) form (15). The most commonly used g l y o x a l reagent i s a 40 percent s o l u t i o n i n water. Also to be noted i n Figure 5 i s t h a t g l y o x a l has been added not o n l y to gum s o l u t i o n s but a l s o to gum p r e c i p i t a t e s and dry gum powder. F a c t o r s A f f e c t i n g D i s p e r s i b i l i t y and Hydration of G l y o x a l Treated Xanthan Gum The data i n the next f i v e f i g u r e s (Figures 6 through 11) h i g h l i g h t some of the most important f a c t o r s a f f e c t i n g d i s p e r s i b i l i t y and h y d r a t i o n of g l y o x a l - t r e a t e d xanthan gum. These are shear r a t e during mixing, pH of the hydration f l u i d , s a l t content and l e v e l s i n h y d r a t i o n f l u i d , mesh s i z e , and temperature. The e f f e c t of high pH (approximately 10) and shear r a t e on hydration o f g l y o x a l - t r e a t e d xanthan gum i s shown i n the next two f i g u r e s (Figures 6 and 7 ) . In both f i g u r e s , the same hydration f l u i d s and samples were used, the o n l y d i f f e r e n c e being t h a t the shear r a t e used f o r a g i t a t i o n i n Figure 7 was lower (200 rpm) than t h a t used i n F i g u r e 8 (800 rpm). Under both shear c o n d i t i o n s , s i m i l a r r e s u l t s were obtained. With no pH adjustment, v i s c o s i t y developed r a p i d l y , and a c t u a l l y exceeded t h a t normally expected i n the 10-30 minute time i n t e r v a l . The reason f o r t h i s higher than-usual v i s c o s i t y i s not w e l l understood. I f more r a p i d h y d r a t i o n i s d e s i r e d , then NH4OH or NaOH can be added to the d i s p e r s i o n as noted i n Figure 7.


4. SANDFORD E T A L . Xanthan

glyoxal, H+ CH OH ^ 2

35

Gum

OH | CH 0 — CH

OH

OH OH glyoxal, H+ | | ^ CH 0 _ CH _ CH _

/ CH

2

I (Hemiacetal)

II (Hemiacetal)


Forward Reaction Favored < pH 7.0 < [^O] > T

CH 0

OCH

2

*

CH — CH

/

Reverse Reaction Favored > pH 7.0 > [H 0] > T

CH 0 2

OCH

III (Acetal) Glyoxal reactions with polyhydroxyls

Patent

U.S. Patent 3,297,583 U.S. Patent 3,489,719 U.S. Patent 3,997,508 U.S. Patent 4,041,234 U.S. Patent 4,041,234

(1967) (1970) (1976) (1977) (1977)

U.S. Patent 4,041,234 (1977) U.S. Patent 4,041,234 (1977) British Patent 1,547,030 (1979) British Patent 1,547,030 (1979)

Figure 5.

Glyoxal Treatment Glyoxal Reagent Acetone solution Glyoxal vapor Powder Aqueous formic acid Powder Aqueous acetic acid Powder Dry powder (heated) Powder Aqueous solution Isopropanol Aqueous isopropanol precipitate Aqueous solution Aqueous solution Aqueous solution Fermentation beer Aqueous solution Fermentation beer Aqueous solution Aqueous solution Gum Powder

U.S. Patent 2,879,268 (1959)

Glyoxal treatment of gums for improved dispersibility

>,° Tap water Lightnin' mixer, 200 rpm 1% gum

-J oM .2 C

.2 - 2000 1 w o & So m >

Figure 6.

J 0

10

20

I

30 40 50 60 TIME (minutes)

70

80

2

* \

2

Figure 4.

OCH

2

90

Hydration of glyoxal-treated xanthan gum (low shear)


2

2

36

SOLUTION PROPERTIES


10

Figure 7.

O F POLYSACCHARIDES

20 30 40 TIME (minutes)

Hydration of glyoxal-treated xanthan gum (high shear)

ZERO TIME

HYDRATION DELAY TIME

Figure 8.

Hydration delay test

Particle Size (mesh*) (urn)

Hydration Delay Time (min.)

Dispersibility

60 250

20.1

Excellent

100 150

20.8

Excellent

150 106

16.2

Excellent

200

75

19.5

Excellent

325

45

20.5

Excellent

* mesh size = Tyler standard screen Figure 9.

Effect of particle size on the hydration of glyoxal-treated xanthan gum



SANDFORD E T A L . Xanthan Gum

0 1

2

3

4

5

6

7

8

FINAL pH Figure 10.

UJ

Hydration of glyoxal-treated xanthan gum: effect of pH

80

0

1

2

3

4

5

6

7

NaCl CONCENTRATION (% in Dl water)

Figure 11.

Hydration of glyoxal-treated xanthan gum in NaCl solutions



38

SOLUTION PROPERTIES OF

POLYSACCHARIDES

Since, i n c e r t a i n a p p l i c a t i o n s of g l y o x a l - t r e a t e d xanthan gum, i t i s d e s i r a b l e to delay the development of v i s c o s i t y (hydration d e l a y ) , a simple t e s t was developed to monitor t h i s behavior (Figure 8 ) . In the hydration delay t e s t discussed, the gum i s added to the vortex (created by a magnetic s t i r r e r ) of the hydration f l u i d ( u s u a l l y s y n t h e t i c tap water) and the time t h a t i s r e q u i r e d f o r the vortex to disappear because of v i s c o s i t y development i s c a l l e d the hydration delay time. The p a r t i c l e s i z e of g l y o x a l - t r e a t e d xanthan gum as seen i n Figure 9 has l i t t l e i f any e f f e c t on the hydration delay time. The e f f e c t of pH of the hydration f l u i d on the hydration delay time (or v i s c o s i t y development) i s shown i n Figure 10. The lower the pH, the greater the hydration delay time becomes. This r e s u l t s because h y d r o l y s i s o f g l y o x a l from the gum i s slower at low pH values. Thus, lowering the pH of the hydration f l u i d can be used to obtain longer hydration delay times with g l y o x a l t r e a t e d xanthan gum without adversely a f f e c t i n g d i s p e r s i b i l i t y . Hydration of gums i n s a l t s o l u t i o n i s g e n e r a l l y more d i f f i c u l t than i n deionized water. The e f f e c t on g l y o x a l - t r e a t e d xanthan gum of adding NaCl to the hydration f l u i d i s shown i n Figure 11. Note t h a t when the pH i s unadjusted, h y d r a t i o n i s delayed with i n c r e a s i n g NaCl l e v e l s . I f the pH of the s a l t s o l u t i o n s i s adjusted to 10 with NH4OH p r i o r to adding the g l y o x a l - t r e a t e d xanthan gum, hydration i s r a p i d and independent of the NaCl l e v e l s . Hydration delay times of these adjusted samples are l e s s than f i v e minutes. Borate-Treated Xanthan

Gum

Although g l y o x a l - t r e a t e d gums have e x c e l l e n t d i s p e r s i b i l i t y i n a c i d or n e u t r a l systems, they are not d i s p e r s i b l e i n s t r o n g l y a l k a l i n e systems. The problem ( a t t r i b u t e d to a l k a l i n e h y d r o l y s i s of glyoxal) of d i s p e r s i n g g l y o x a l - t r e a t e d xanthan gum i n a l k a l i n e f l u i d s , can be overcome by using a b o r a t e - t r e a t e d xanthan gum (Figure 12). Borate treatment of xanthan gum at pH values above 8.0 r e s u l t s i n a c r o s s - l i n k e d xanthan gum t h a t i s e s s e n t i a l l y i n s o l u b l e at t h i s pH range (Figure 12). The presumed mechanism i s by borate r e a c t i n g with the v i c i n a l hydroxyls of mannose to form c r o s s l i n k s . T h i s phenomenon has been explored i n preparing d i s p e r s i b l e guar gum d e r i v a t i v e s (20_, 21). Although the i n t e r a c t i o n of borate with xanthan gum i s l e s s pronounced at high pH values, i t i s s u f f i c i e n t to produce a d i s p e r s i b l e product. Behaving almost opposite to t h a t of g l y o x a l - t r e a t e d xanthan gum, the a l k a l i n e borate/xanthan gum complex can be broken by the lowering of the pH. Borate-treated xanthan gum r e a d i l y disperses above pH 7.0, and hydrates at pH values l e s s than 7.0. T h i s can be accomplished by lowering the pH by a d d i t i o n of common a c i d s such as HC1, H2SO4, and H3PO4. Borate-treated



Gum

Xanthan gum + Borate + Xanthan gum pH > 8.0

pH < 7.0


\| O —CH

HC — O Xanthan gum

Xanthan gum

B HC—O

O—CH

Borate-crosslinked Xanthan gum Figure 12.

Borate-treated xanthan gum

>.9

10

Figure 13.

20 30 40 TIME (minutes)

50

60

Hydration of borate-treated xanthan gum blended with fumaric acid (2%) and Na CO (1%) 2

s


40

SOLUTION PROPERTIES O F POLYSACCHARIDES

xanthan gum can be dispersed r e a d i l y i n tap water a t pH l e s s than 8.0 (Figure 13) by blending b o r a t e - t r e a t e d xanthan gum with a base (e.g., Na2(X>3, 1%) and a slowly s o l u b l e a c i d (e.g., fumaric a c i d , 2%). The added Na C0 causes the pH t o be a l k a l i n e s u f f i c i e n t l y l o n g t o allow e x c e l l e n t d i s p e r s i b i l i t y . The added fumaric a c i d e v e n t u a l l y d i s s o l v e s causing the pH t o drop t o near n e u t r a l i t y ; t h i s breaks the borate complex thereby a l l o w i n g the xanthan gum t o hydrate. 2

3


Summary The d i s p e r s i b i l i t y o f xanthan gum can be improved most r e a d i l y by treatment with g l y o x a l o r a l k a l i n e borate. With g l y o x a l - t r e a t e d xanthan gum, i t has been shown t h a t the mesh s i z e , r a t e of shear o f mixing, pH, and s a l t l e v e l s a l l a f f e c t the h y d r a t i o n and d i s p e r s i b i l i t y . Borate-treated xanthan gum has e x c e l l e n t d i s p e r s i b i l i t y and i s e s p e c i a l l y s u i t e d t o a l k a l i n e s o l u t i o n s . Borate-treated xanthan gum blended with a base (e.g., Na C0 ) and a slowly s o l u b l e a c i d (e.g., fumaric acid) a r e d i s p e r s i b l e i n n e u t r a l s o l u t i o n s . By proper s e l e c t i o n of c o n t r o l l a b l e f a c t o r s , a d i s p e r s i b l e xanthan gum with d i f f e r i n g hydration r a t e s can be produced and t a i l o r e d t o s p e c i f i c i n d u s t r i a l needs. 2

Literature

3

Cited

1.

"Xanthan gum/KELTROL®/KELZAN®/A Natural Biopolysaccharide for Scientific Water Control", Second Edition, Kelco Div. Merck and Co., Inc., San Diego, California, 1975.

2.

McNeely, W.H. and Kang, K.S., "Xanthan and some other biosynthetic gums", In Industrial Gums, Whistler, R. L. and BeMiller, J. N., Eds., Second Edition, Academic Press, New York, 1973, pp. 473-497.

3.

Andrew, T. R., ACS Symposium Series No. 45, 1977, pp. 231241.

4.

Kovacs, P. and Kang, K. S., "Xanthan gum," In Food Colloids, Graham, H., Ed., Avi Publishing Co., Westport, Connecticut, 1977, pp. 500-521.

5.

Cottrell, I.W. and Kang, K.S., "Xanthan gum, a unique bacterial polysaccharide for food applications", In Development in Industrial Microbiology, 1978, pp. 19, 117-131.

6.

Racciato, J. S., Textile 46-50.

7.

Kang, K. S. and Cottrell, I. W., "Polysaccharides", In Microbial Technology, Vol. 1, Peppier, H. J., Ed., Second Edition, Academic Press, New York, 1979, pp. 417-481.

Chemist and Colorist

1979, pp. 11,



4.


Gum

41

8.

Pettitt, D. J., "Xanthan gum", In Polysaccharides in Food, Blanshard, J. M. V. and Mitchell, J. R., Eds., Buttersworth, Boston, 1979, pp. 263-281.

9.

Sandford, P. A., "Exocellular microbial polysaccharides", In Advances in Carbohydrate Chemistry and Biochemistry, Vol. 36, Tipson, R. S. and Horton, D., Eds., Academic Press, New York, 1979, pp. 265-313.

10.

U.S. Patent 3,236,657 (February 22, 1966), Raymond E. Cox/ General Foods Corp.

11.

Sliwka, W., Angew. Chem. (International 539-550.

12.

U.S. Patent, 3,551,133 (December 29, 1970), Billy A. Sprayberry and Richard L. Urbanowski/Diamond Shamrock Corp. C. L., Biotechnol,

Ed.), 1975, pp. 14,

13.

Mehltretter, 171-175.

Bioeng., 1965, pp. 7,

14.

U.S. Patent 4,095,991 (June 20, 1978), Pierre Falcoz, Pierre Celle, and Jean-Claud Campagne/Rhone-Poulenc Industries.

15.

U.S. Patent 2,879,268 (March 24, 1959), Elof Ingvar Jullander/ Mo Och Domsjo Aktg.

16.

U.S. Patent 3,297,583 (January 10, 1967), Wolfgang Dierichs and Werner Sammet/Henkel & Cie, GmbH.

17.

U.S. Patent 3,489,719 (January 13, 1970), Albert B. Savage and Ronald L. Glomski/Dow Chemical Co.

18.

U.S. Patent 3,997,508 (December 14, 1976), Horst Ziche/Henkel & Cie GmbH.

19.

U.S. Patent 4,041,234 (August 9, 1977), Fred J. Maske/ General Mills.

20.

British Patent 1,547,030 (June 6, 1979), Ian William Cottrell and Henry George Hartnek/Merck & Co., Inc.

21.

Goldstein, A. M., Alter, E. N., and Seaman, J. K., "Guar gum", In Industrial Gums, Whistler, R. L. and BeMiller, J. N., Eds., Second Edition, Academic Press, New York, 1973, pp. 303-321.

RECEIVED September 30, 1980.


Solution Properties of Polysaccharides - American Chemical Society

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