Elsevier

Carbohydrate Polymers

Volume 49, Issue 2, 1 August 2002, Pages 207-216
Carbohydrate Polymers

Gelation and rheology of xanthan/enzyme-modified guar blends

https://doi.org/10.1016/S0144-8617(01)00328-9Get rights and content

Abstract

The rheological behavior and synergistic character of mixed polysaccharide systems are examined for blends of xanthan with enzymatically-modified guar. In particular, the enzyme α-galactosidase is used to selectively cleave off the galactose side chains of guar in order to obtain galactomannans with tailored molecular architecture: EMG1 with a relatively high galactose content of 33.6% and a mannose (M) to galactose (G) ratio of 1.85, and EMG2 with a lower galactose content (25.2%) and an M/G ratio of 2.86. Blends of xanthan with enzymatically-modified guar gum samples are examined in terms of their dynamic rheological properties and compared to those of xanthan — locust bean gum blends. The extent of synergism, illustrated by the gel elastic modulus G′ and yield stress τc, is found to increase with increasing extent of enzymatic modification. At constant ionic strength, the EMG2 and locust bean blends behave similarly, with increasing extent of synergy as the temperature of mixing is increased. Additionally, at a fixed mixing temperature, the blends made in water have a higher elastic modulus than those made in salt. In contrast, the EMG1 blends are weaker and the dynamic moduli are unaffected by changes in the mixing temperature or ionic strength. These results are consistent with those of other researchers and are directly related to both the level of disorder in the xanthan molecule as well as the galactose content and fine structure of the galactomannan.

Introduction

Natural biopolymers are used extensively in many applications, from hydraulic fracturing fluids in enhanced oil/gas production to coatings and food additives, because of their ability to modulate rheological properties. In particular, galactomannans such as guar and locust bean gums are used as rheology modifiers in food products ranging from dressings, soups and sauces to gelated desserts and baked goods to impart desirable textures, modulus, mouthfeel and shelf-stability (Fox, 1992, Maier et al., 1992). The water binding properties of these gums are used to advantage in preventing ice crystal growth and synerisis in dairy and frozen dessert products (Morris, 1990, Morris, 1995, Urlacher and Dalbe, 1992). Galactomannans can synergistically bind with the helices of other biopolymers such as xanthan and carrageenans, hence bringing about gelation of two components which are individually non-gelling, resulting in improved product quality and reduced production costs (Dea, Morris, Rees, Welsh, Barnes & Price, 1977). In addition to the above, low cost and abundance are major factors contributing to the popularity of galactomannans in the food industry.

At present, guar gum and locust bean gum are the only two galactomannans that are utilized on an industrial scale. Although the chemical structures of the two gums are the same, differences in the mannose to galactose ratios and the fine structures bring about significant differences in their properties. For example, guar undergoes rapid hydration in cold water to produce highly viscous solutions whereas locust bean gum can be dissolved only after heating to high temperatures (Fox, 1992, Maier et al., 1992). In addition, due to the greater proportion of galactose side chains, guar is unable to bind with the helices of xanthan or carrageenans to form synergistic gels (like locust bean gum) and can only produce an increase in viscosity (Dea & Morrison, 1975). Hence, the potential applications of guar gum are limited. This is unfortunate in light of the high costs and diminishing world supply of locust bean gum owing to factors such as long maturation period and competition from other cash crops (Bulpin, Gidley, Jeffocat & Underwood, 1990).

An alternative lies in tailoring the fine structure of guar gum using enzymes to produce a range of galactomannans with varying functionality. In particular, α-galactosidase from germinating legume seeds is effective in cleaving off the galactose side chains only, producing guar with a range of mannose to galactose ratios including that comparable to locust bean gum (Bulpin et al., 1990, McCleary et al., 1981, McCleary et al., 1984). Previous research in our laboratory has dealt with rheological changes, molecular weight distributions and diffusion issues arising due to enzymatic action on guar solutions and gels (Burke et al., 2000, Tayal and Khan, 2000, Tayal et al., 1998, Tayal et al., 1999a, Tayal et al., 1999b). The primary interests in these studies have been on the utilization of β-mannanase, a back-bone cleaving enzyme, and on investigating the properties of guar by itself. In this work, in contrast, we focus on understanding the rheological properties of guar, enzymatically modified using a side-chain cleaving α-galactosidase, in synergistic blends with xanthan.

d-galacto-d-mannans are reserve carbohydrates found in the endosperms of some legume seeds. They consist of a β-d-(1→4)-mannan backbone which is partially substituted by single (1→6)-α-d-galactopyranosyl groups (McCleary, 1979, McCleary et al., 1985). Well-known galactomannans, guar and locust bean gum, have galactose substitution degrees of 38 and 23%, respectively. Chemical procedures and hydrolysis techniques using highly purified enzymes indicate that the d-galactosyl groups in guar are arranged mainly in pairs and triplets. However, locust bean has a non-regular distribution of d-galactose units, with a high proportion of substituted couplets, lesser amount of triplets and regions of unsubstituted d-mannosyl residues (McCleary et al., 1983, Hoffman and Svensson, 1978, Hoffman et al., 1975, McCleary et al., 1985, Painter et al., 1979). Evidence from NMR spectroscopy is also consistent with a random arrangement of d-galactosyl groups in the two galactomannans (Grasdalen & Painter, 1980).

Xanthan gum is an extracellular polysaccharide obtained from the bacterium Xanthomonas campestris. The primary structure of xanthan consists of a (1→4)-linked β-d-glucopyranosyl backbone, alternate units of which are substituted at the O-3 position by a trisaccharide side chain containing a d-glucoronosyl unit between two d-mannosyl units (Jansson et al., 1975, Melton et al., 1976). The terminal β-d-mannopyranosyl unit is glycosidically linked to the O-4 position of the β-d-glucopyranosyluronic acid unit, which in turn is glycosidically linked to the O-2 position of an α-d-mannopyranosyl unit. Approximately half of the terminal d-mannosyl units contain a pyruvic acid residue as a 4,6-cyclic acetal. Finally, the nonterminal d-mannosyl unit is stoichiometrically substituted at O-6 with an acetyl group (Jansson et al., 1975, Melton et al., 1976). The secondary structure of xanthan can undergo a unique transition from a disordered random coil configuration where the side chains are oriented away from the backbone to a rigid, helical structure where the side chains are folded along the backbone. The transition is dependent on temperature and ionic strength of the solution with the ordered form being stabilized at low temperatures and high ionic strengths (Morris et al., 1977, Norton et al., 1984).

In this study, we use rheology to investigate synergistic effects in blends formed using xanthan and enzymatically-modified guar. In this regard, the structure of guar gum is tailored using highly specific α-galactosidase, a side-chain cleaving enzyme, to obtain two samples, one (EMG1) with a relatively high galactose content (33.6%) and an M/G ratio of 1.85 and the other (EMG2) with a lower galactose content (25.2%) and an M/G ratio of 2.86. We use dynamic rheology, a sensitive probe for material microstructure to probe gel formation and interactions of these enzymatically-modified guar with xanthan (Raghavan et al., 2000a, Raghavan et al., 2000b). Previous studies in this area have revealed that enzymatic modification of guar with α-galactosidase (McCleary et al., 1981) enhances its interaction with xanthan, agarose (McCleary et al., 1984) and κ-carrageenan (Bulpin et al., 1990). In particular, increases in compressive yield stress, compressive fracture strain and elastic modulus (measured at a single frequency) have been observed in these blends (McCleary et al., 1981, McCleary et al., 1984). However, no detailed rheological studies have been undertaken for any of these systems.

The present study expands the scope of previous research and addresses some unresolved issues that include the effect of temperature of preparation (Tp), xanthan to galactomannan ratio and ionic strength of xanthan solutions on the viscoelastic behavior of blends containing enzyme-modified guar and xanthan. In particular, our study reveals that enzymatic modification produces blends with significantly higher elastic modulus as compared to native guar blends, with gel strength increasing with decreasing galactose content. EMG1 blends containing galactomannan with a high galactose content (33.6%) are unaffected by changes in temperature of preparation or changes in solvent environment produced by addition of salt. In contrast, at a fixed ionic strength, EMG2 blends containing galactomannan with a low galactose content (25.2%) exhibit a higher elasticity (G′) with increasing temperature of mixing. However, at a fixed temperature, gel strength decreases when ionic strength of the solution is increased. These results are explained in terms of the change in xanthan conformation in response to temperature or ionic strength and/or the galactose content and fine structure of the galactomannan.

Section snippets

Materials

Samples of Uniguar 150 (Cyamopsis tetragonolobus) and locust bean gum (Ceratonia siliqua) were obtained from Rhodia Food Ingredients, Cranbury, New Jersey. These commercial samples were found to contain insoluble impurities like seed husks and other cellulosic material, which produced hazy solutions upon dissolution of the gums in water. Hence, an ethanol extraction procedure was used to purify the galactomannans to remove insoluble impurities (Mannion, Melia, Launay, Cuvelier, Hill, Harding &

Solution rheology

We begin by investigating the rheological behavior of each of the biopolymers — guar, locust bean and xanthan when dissolved in water. The concentration of each biopolymer was maintained at 0.8% w/w in every case. Fig. 1a depicts the frequency spectrum of 0.8% w/w guar solution alone. We observe that guar behaves like a typical macromolecular entangled biopolymer in solution, with the elastic modulus G′ dominating over the viscous modulus G″ in the high frequency range. A crossover point is

Summary

We use dynamic rheology to investigate synergistic effects in blends formed using xanthan and enzymatically-modified guar. In particular, the structure of guar gum is tailored using α-galactosidase, a side chain cleaving enzyme, to obtain two samples, one with a high (33.6%) galactose content (EMG1) and the other with a low (25.2%) galactose content (EMG2). We find that at high extents of enzymatic modification, guar (EMG2) can interact synergistically with xanthan to produce gels with elastic

Acknowledgements

The authors gratefully acknowledge the United States Department of Agriculture for supporting this work.

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