Dilute solution properties of guar and locust bean gum in sucrose solutions

Food Hydrocolloids 12 (1998) 339±348

Dilute solution properties of guar and locust bean gum in sucrose solutions Paul H. Richardson*, Juliette Willmer, Tim J. Foster Unilever Research Laboratory, Colworth House, Sharnbrook, Bedford MK44 1LQ, UK Received 19 May 1997; received in revised form and accepted 21 July 1997

Abstract The dilute solution properties of guar, native and puri®ed locust bean gum (LBG) in sucrose solutions (0±40% w/w) have been assessed. The intrinsic viscosity of LBG is arti®cially high due to a contribution from polymer/polymer associations. For both galactomannans, the addition of sucrose was shown to initially decrease the intrinsic viscosity, possibly due to either a reduction in solvent quality or a reduction in polymer/polymer association, and then increase towards a maximum value at 20% sucrose owing to an increase in solvent quality. At 40% w/w sucrose concentrations, the solvent quality decreased owing to competition for water, enhancing the extent of polymer contraction. The initial coil overlap concentration, c*, and observation of the onset of shear thinning, were detected at approximately 0.1% w/v for both galactomannans. This low concentration, along with the relatively low concentration dependence above c*, c1.8±2.6, indicated a second transition was present at even higher concentrations, c**, marking the onset of the concentrated regime. As opposed to other fully ¯exible polymer chains, it is proposed that these two transitions were more easily detected for these less compressible, semi-rigid polymers. At dilute concentrations, guar exhibited slightly greater speci®c viscosities at the same degree of space occupancy than LBG. This was attributed to a higher chain rigidity and possibly a consequence of the higher molecular weight resulting in poorer solubility. Furthermore, large Huggins coecients were determined for guar in sucrose solutions re¯ecting guar's poorer solubility than in water. # 1998 Published by Elsevier Science Ltd. All rights reserved.

1. Introduction Of all the plant seed galactomannans, guar and locust bean gum (LBG) are the most widely used as industrial thickeners. Their chemical structure is based on a 1,4linked b-d-mannan backbone with 1,6-linked a-dgalactose side groups (Dea & Morrison, 1975). Guar possesses a higher level of galactose substitution along the mannan backbone (approximately 40%) compared with 20±23% for LBG (Maier, Anderson, Karl, Magnuson, & Whistler, 1992). Native guar usually has a higher molecular weight (approximately 106 gmolÿ1) (Robinson, Ross-Murphy, & Morris, 1982) than LBG (3105 gmolÿ1), and as with all naturally occurring polysaccharides, these biopolymers are polydisperse in terms of molecular weight and galactose content (Doublier & Launay, 1981; Richardson, Clark, Russell, Aymard, & Norton, 1998) i.e. individual chains possess di€erent galactose contents. In terms of chemical structure/property * Corresponding author.

relationships, the average galactose content has been shown to strongly in¯uence the physical properties of these galactomannans. A lower galactose content yields stronger synergistic interactions with other biopolymers (Morris, 1990) and a greater individual gelling capacity based upon galactose uninhibited mannan interactions (Dea, Morris, Rees, Welsh, Barnes, & Price, 1977; McCleary, Clark, Dea, & Rees, 1985). The distribution of the galactose units along the main-chain (®ne structure) also in¯uences the physical properties of these biopolymers. Whereas it has been shown that the galactose monomers are distributed in a closely random fashion along the mannan backbone (McCleary et al.), galactomannans that possess longer galactose uninhibited mannan regions exhibit greater functionality (McCleary, 1979; Launay, Doublier, & Cuvelier, 1986). In addition, a third and less widely recognised dependence of the chemical structure of galactomannans upon its physical properties, is the in¯uence of the galactose polydispersity between chains. As recently highlighted (Richardson et al.), at the same average

0268-005X/98/$19.00 # 1998 Published by Elsevier Science Ltd. All rights reserved PII: S0268-005X(98)00025-3

340

P.H. Richardson et al./Food Hydrocolloids 12 (1998) 339±348

galactose content, galactomannans with a broader galactose distribution are more functional as they possess a greater proportion of chains with lower galactose content. The solution properties of guar and LBG (Launay et al., 1986) and other less widely used galactomannans in water have been characterised. The solution property parameters referred to throughout this paper are de®ned and their physical characteristics described in the general background section below. Robinson et al. (1982) and Sabaters de Sabate (1979) have provided intrinsic viscosity versus molecular weight Mark±Houwink relationships, [Z]=KMvw , for guar and alcohol fractionated LBG, respectively. Their results gave Mark±Houwink exponents, v, of 0.723 and 0.8 for guar and LBG, respectively, and K=3.810ÿ4 for guar giving a characteristic ratio, C1 of 12. This parameter is a measure of the rigidity of a polymer chain and as with most polysaccharides, this value represents a relatively sti€ chained polysaccharide (persistence length <10 nm) (Robinson et al., 1982). For LBG fractions, K was shown to be dependent upon the galactose content (Sabaters de Sabate). For LBG and guar, Morris, Cutler, RossMurphy, Rees, & Price (1981) have used the term hyperentanglement to describe the lower concentration deviation of the coil overlap parameter at c[Z]&2.5 from that for random coil polymer solutions, c[Z]&4. This deviation to higher viscosity at lower concentrations along with the greater concentration dependence has been attributed to speci®c chain/chain interaction between galactose free mannan segments (Morris et al., 1981; Morris, 1990), although alternative suggestions have been raised (Gidley et al., 1991; Launay et al., 1997). Possibly in support of this interaction, the intrinsic viscosity increased as the average galactose content was reduced (11.7 dl/g for 19% galactose and 9.9 dl/g for 25% galactose) (McCleary, Dea, Windust, & Cook, 1984) and the level of hyperentanglement more prevalent for galactomannans with a low galactose content (Kapoor, Milas, Taravel, & Rinaudo, 1996). More recently, Goycoolea, Morris, and Gidley, (1995) removed this speci®c chain/chain association by dissolving LBG and guar at pH=10 and thus converting these neutral polysaccharides to weak polyelectrolytes. This resulted in a signi®cant reduction in the intrinsic viscosity, [Z], for LBG, 12.1 to 5.2 dl/g and to a lesser extent for guar, 12.5 to 11.9 dl/g. These intrinsic viscosities were then in accordance with the relative actual molecular weights for these two galactomannans. Upon neutralisation, the intrinsic viscosities returned to their original values clearly showing that reversible aggregation does occur. Thus, not only does the speci®c association of galactomannan chains possibly contribute to higher viscosity dependencies with concentration, but it also increases the intrinsic viscosity of the macromolecule (a measure of the hydrodynamic volume), even though the latter is determined through extrapolation to in®nite dilution.

Although the proposition that mannan/mannan intermolecular associations are responsible for this hyperentanglement is widely reported (Morris et al., 1981), it is less widely appreciated that self-association tending towards precipitation also occurs in semi-dilute solutions. It has been reported that this aggregation occurs slowly over several days at ambient and the precipitation rate increases as the temperature is decreased (Morris, 1990). In support of this observation, recent work studying the gelation behaviour of LBG at relatively high concentrations in 60% w/w sucrose solutions has shown that the critical gelling concentration of LBG was approximately 1% w/w, that gelation was very slow with a maximum in gelation rate at ÿ5 C (Richardson & Norton, 1998). Sucrose is a widely used low molecular weight additive, however, limited work has studied the in¯uence of sucrose upon the solution properties of hydrocolloids. For the case of guar and LBG, Elfak, Pass, Phillips, and Morley (1977) found the intrinsic viscosity to decrease upon the addition of sucrose and other low molecular weight additives. Very recently, in a similar but more extensive study by Launay et al. (1997), no change in intrinsic viscosity was measured for 0, 10 and 40% sucrose concentrations. We report here a detailed analysis of the in¯uence of sucrose (5±40% w/w) upon the dilute solution properties of guar, native and puri®ed LBG. The aim of this paper was to assess the solvent quality of di€erent sucrose concentrations for these galactomannans through measures of the hydrodynamic volume extrapolated to in®nite dilution (through intrinsic viscosity measurements) and the concentration dependence of the zero shear speci®c viscosity. In addition, the dilute solution properties of sucrose/galactomannan solutions have been evaluated in light of more recent progress regarding LBG self-association. 2. General background The intrinsic viscosity, [Z], is not a very speci®c parameter and depends upon several factors (Bohdanecky & KovaÂrÆ , 1982). It is dependent upon the hydrodynamic volume occupied per unit mass of the macromolecule, which consists of the intrinsic volume occupied by the polymer chain and its excluded free volume. It is also in¯uenced by hydrodynamic properties which include a measure of the permeability of the polymer coil to solvent (if it is free draining, then [Z] is higher) and chain anisotropy. Deviations from spherical geometry add a frictional component to viscosity. One approach to determine the intrinsic viscosity is through extrapolation to in®nite dilution using the Huggins (1942) and Kraemer (1938) empirical expressions below: sp ˆ ‰Š ‡ l‰Š2 c …1† c

P.H. Richardson et al./Food Hydrocolloids 12 (1998) 339±348

ln…r † ˆ ‰Šl0 ‰Š2 c c

…2†

where the Huggins coecient l is a measure of polymer/ polymer interaction in dilute conditions and also depends upon the extent of coil expansion of the polymer coil. This analysis is normally restricted to a range of dilute concentrations due to less well de®ned contributions of chain/chain interactions at higher concentrations and the high viscosity dependency of impurities at lower concentrations. The excluded volume of a polymer coil is a result of chain units not being able to occupy the same volume element at the same time. It depends upon the attractive and repulsive forces within its vicinity and consequently depends upon the solvent environment as well as temperature. In good solvents, polymer coils tend to repel each other. The polymer chain units prefer to be solvated by the solvent and thus possess a high excluded volume. This expansion is de®ned accordingly: < r2 >ˆ 2r < r2 >0

…3†

where < r2 > and < r2 >0 are the actual polymer chain and unperturbed polymer chain root mean square endto-end distances and 2r is the expansion coecient (Flory, 1953). Good solvents are often de®ned from classical polymer solution theories (Flory) by a polymer± solvent interaction parameter, w>0.5, or a positive second virial coecient, A2, of the osmotic pressure in terms of concentration. Theta conditions are said to exist when the solvent and polymer chain units are mutually thermodynamically compatible. Under these conditions, w=0.5 and A2=0, the polymer is unperturbed, has zero excluded volume other than a minor contribution from steric hinderance and thus minimal expansion. A bad solvent is de®ned when A2<0 and w<0.5. Under these conditions, attractive forces between polymer chain units dominate and the polymer chains tend to aggregate and precipitate. Dense particles are formed with a decrease in the expansion coecient. Consequently, owing to the strong dependence of the intrinsic viscosity upon coil expansion, intrinsic viscosity values can be used to assess the quality of the solvent. The molar mass dependence of the intrinsic viscosity is frequently described by the Mark±Houwink±Kuhn± Sakurada equation: ‰Š ˆ KMv

…4†

where M is the molar mass and K and v are empirical constants. For ¯exible polymer chains, v varies between 0.5 (under theta conditions) and 0.8 for good solvents (Launay et al., 1986). Slightly higher values are possible for free draining coils but this is unlikely as strong hydrodynamic interactions are invariably always present for

341

macromolecules. For sti€-chain polymers, the exponent is higher and less dependent upon solvent quality, n1.8 for a rod polymer (Bohdanecky & KovaÂrÆ , 1982). K is directly related to the characteristic ratio, C1, which de®nes the ¯exibility of a polymer chain with respect to a freely jointed one: C1 ˆ< r2 >0 =nl2

…5†

where n is the number of repeat units in a polymer chain of ®xed length l (Flory, 1953). Thus, it is evident from Eq. (4) that if polymer aggregates exist at the practically de®ned low polymer dilutions used to determine the intrinsic viscosity, then the intrinsic viscosity will represent the aggregate chains and be higher than that for single chains owing to the higher excluded volume. The Huggins coecient in Eq. (1) usually varies between 0.3 (in very good solvents) and 1 (Launay et al., 1986; Bohdanecky & KovaÂrÆ , 1982), increasing as the solvent quality decreases, resulting in polymer coil contraction. Larger values possibly indicate a poorer solvent, and/or polymer aggregation (Bohdanecky & KovaÂrÆ ). Whereas no theoretical link has been established between the Huggins coecient and molar mass, higher values have been experimentally observed at higher molecular weights (Ho€man, 1957; Bohdanecky & KovaÂrÆ ). It has also been assumed that branching will lead to higher Huggins coecients as the polymer will be more compact than a linear polymer of comparable molecular weight (Bohdanecky & KovaÂrÆ ). 3. Materials and methods 3.1. Materials Locust bean gum and guar were supplied commercially from Meyhall. In addition, the LBG sample was puri®ed by solvation in water at 80±90 C for 30 minutes and then centrifugation at 27,000 g for 30 minutes at 25 C, followed by precipitation of the supernatant in isopropanol. The precipitate was washed with acetone and dried overnight in a vacuum oven at 30 C. Sucrose/galactomannan solutions were prepared by ®rst dry mixing the appropriate amounts of sucrose and galactomannan. Cold de-ionised water was then added and the solutions were heated at 90 C for 30 minutes whilst stirring with a magnetic ¯ea. It was necessary to heat the LBG solutions at such high temperatures to solvate the low galactose, high water temperature soluble fraction of LBG (Richardson et al., 1998). Molecular weight analyses (Russell, 1997) showed that the polymer did not degrade at these temperatures unless excessive mechanical stirring was used. Dilute galactomannan solutions were prepared (0.01% to 0.4% w/w) in a range of sucrose concentrations (5±40% w/w). The galactomannan

342

P.H. Richardson et al./Food Hydrocolloids 12 (1998) 339±348

concentrations were converted to weight by volume for the subsequent intrinsic viscosity analyses using density values for the appropriate sucrose concentration. 3.2. Characterisation of galactomannans The average galactose content of LBG was determined using a method by Taylor and Conrad (1972). LBG was hydrolysed using 100 microlitres per mg of 12 M sulphuric acid (cooled on ice), heated for 1 hour at 35 C and diluted with water, and then heated for 3 hours at 100 C. After neutralisation with concentrated ammonium hydroxide and dilution to 25 ml, the component monosaccharides were then analysed using a high performance anion exchange column (HPAEC). The molecular weight characteristics were determined using high performance size exclusion chromatography with multi-angle laser light scattering coupled with refractometric detection (HPSEC-MALLS) (dn/dc= 0.155 ml/g). For the molecular weight evaluation, LBG solutions were prepared by ®rst dispersing the polymer in a phosphate bu€er (50 mM KH2PO4:NaOH 1:3, pH=8), and then heating at 80±90 C for 30 minutes with continuous magnetic ¯ea stirring. The phosphate bu€er solution was used to remove any polymer aggregation. 3.3. Viscosity measurements Solution viscosities were determined for a range of shear rates using a Low Shear Contraves 30 rotational viscometer with coaxial cylinder at 25 C. The experimental viscosities, Zexp, were converted to speci®c viscosities, Zsp by accounting for the sucrose solution viscosity, Zsuc. solution, using: exp ÿ suc:solution …6† sp ˆ suc:solution The experimentally derived sucrose solution viscosities at 25 C‹0.1 C were 0.89 at 0%, 1.04 at 5% w/w, 1.19 at 10% w/w, 1.60 at 20% w/w and 4.98 mPa s at 40% w/w sucrose. 4. Results 4.1. Galactomannan characterisation Fig. 1 displays the size exclusion pro®le for LBG eluted in water and in phosphate bu€er respectively. The phosphate run did not display the high molecular weight aggregate peak at low elution times and also the main peak did not possess a higher molecular weight shoulder with respect to LBG in water. The high molecular weight aggregate peak in water was clearly detected by light scattering whereas a very small peak was observed from the refractometric signal. This is due to

Fig. 1. Size exclusion pro®les for native LBG in water and phosphate. Scattered light at 90 (Ð), refractive index (. . .).

the scattered light intensity being proportional to m2 where m is the aggregate mass, and the refractive index is independent of m and proportional to the polymer concentration. The resultant molecular weight characteristics for LBG in water and phosphate, and for guar and puri®ed LBG in phosphate as well as the average percentage galactose contents are given in Table 1. 4.2. Viscosity measurements A typical increase in speci®c viscosity with concentration was observed for all galactomannans. At di€erent sucrose concentrations, a small but distinct in¯uence upon the speci®c viscosities of each polymer was detected for all polymer concentrations and at all shear rates measured. The solutions studied were dilute and their viscosities typically shear rate independent. A slight shear thinning reduction in speci®c viscosity, however, was observed for guar and LBG at concentrations as low as 0.1% w/v. Di€erent solution regimes can be distinguished by plotting the speci®c viscosity versus concentration on a log±log scale (Morris et al., 1981), see Figs. 2 and 3 for the case of native LBG and guar, respectively, for all the sucrose concentrations studied. As seen in Figs. 2 and 3, a discontinuity in the concentration dependence of Zsp/c was detected at c*, de®ned as the concentration of initial onset of polymer coil overlap marking the transition from the dilute to the semi-dilute region. For each galactomannan, all sucrose concentrations were closely superimposed onto one master curve. However, slight di€erences for each sucrose concentration were detected and the concentration dependencies for both regimes as well as the concentration at the discontinuity for each sucrose concentration are listed in Table 2. The best ®t lines depicted in Figs. 2 and 3 are for LBG and guar in water, respectively. The intrinsic viscosities, [Z], were determined for each sucrose solvent concentration using the Huggins and Kraemer empirical expressions [Eqs. (1) and (2), respectively]. Fig. 4 displays the Huggins and Kraemer analyses

P.H. Richardson et al./Food Hydrocolloids 12 (1998) 339±348

343

Table 1 Physical characteristics of guar and locust bean gum Galactomannan solution

Number average molecular weight (Mn)

Weight average molecular weight (Mw)

Polydispersity (Mw/Mn)

% Galactose

Impurity (% w/w)

Native LBG in water

695 000a

812 000a

1.17

20.4

16

Native LBG in phosphate

288 000

370 000

1.29

20.4

16

389 000

460 000

1.18

23.0

±

1 059 000

1 220 000

1.15

37.0

±

Puri®ed LBG in phosphate Guar in phosphate a

The high molecular weight aggregate peak was not included in the molecular weight analysis.

for guar and native LBG in water and Table 3 lists the intrinsic viscosities and Huggins coecients for guar, native and puri®ed LBG at the di€erent sucrose concentrations studied. These analyses were closely linear and restricted to concentrations in the range 0.02±0.1% w/v. For guar, the Kraemer analysis gave a positive gradient (negative l0 ) which re¯ects the high Huggins coecients as theoretically, l+l0 ought to be 0.5. Huggins coecients were not determined for puri®ed LBG owing to too few data points in the concentration range, 0.02±0.1% w/v. Qualitatively, puri®ed LBG showed very similar dilute solution characteristics to that of native LBG. 5. Discussion As shown in Fig. 1 and Table 1, the size exclusion pro®les for LBG in water and phosphate are di€erent. In addition to the small peak at initial elution times representing a high molecular weight aggregate, the main peak elutes at earlier times and appears to consist of two peaks, suggesting the presence of polymer/polymer interactions for LBG in water. As with the removal of polymer associations at pH=10 by Goycoolea et al. (1995) for LBG and to a lesser extent for guar, in phosphate

Fig. 2. Speci®c viscosity versus concentration for native LBG at different sucrose concentrations. *=0% sucrose, *=5% sucrose, ~=10% sucrose, !=20% sucrose, ^=40% sucrose. c* is the critical coil overlap concentration.

bu€er the aggregate peak was no longer present and the main peak eluted at longer times indicating the removal of polymer chain association. Consequently, the resultant molecular weight characteristics in phosphate bu€er are a more accurate representation of LBG, and it is argued that the variation in molecular weight often reported for LBG (e.g. Lopez da Silva, Goncalves, & Rao, 1993) is partly a result of variations in the degree of association (as well as variations in botanical source). Clearly, the extent of solvation will also alter the extent of polymer association present in a LBG solution. With respect to the action of phosphate, Sharman, Richards, and Malcolm (1978) in phosphate bu€er, pH=6.9, using an indirect method to determine the molecular weight based upon sedimentation and di€usion coecients, found Mw=329,000 for LBG. The lower pH used by Sharman et al. (1978) suggests that the aggregate inhibition action of phosphate is not solely due to polarisation of the hydroxyl groups, if at all, but possibly due to speci®c association of phosphate onto galactose uninhibited mannan regions. The immediate return to the original intrinsic viscosity values for LBG and guar upon neutralisation (Goycoolea et al., 1995), and the fact that these polymer associations persist even at dilute concentrations as con®rmed by the high intrinsic viscosities in water, indicate that these associations are energetically very favourable. The same

Fig. 3. Speci®c viscosity versus concentration for guar at di€erent sucrose concentrations. &=0% sucrose, *=5% sucrose, ~=10% sucrose, !=20% sucrose, ^=40% sucrose.

344

P.H. Richardson et al./Food Hydrocolloids 12 (1998) 339±348

Table 2 The critical coil overlap concentration c*, and the speci®c viscosity concentration dependencies above, x and below, y, the critical coil overlap concentration for native LBG and guar in di€erent sucrose concentrations % Sucrose, w/w (native LBG) 0

x of Zsp=f(cx)

y of Zsp=f(cy)

c* (w/v)

% Sucrose, w/w (Guar)

x of Zsp=f(cx)

y of Zsp=f(cy)

c* (w/v)

1.16

2.28

0.10

0

1.30

2.44

0.10

5

1.21

2.13

0.09

5

1.42

2.54

0.14

10

1.12

2.23

0.09

10

1.37

2.47

0.15

20

1.25

1.82

0.11

20

0.98

1.97

0.09

40

1.07

2.58

0.12

40

1.25

±a

±a

a

Insucient data points at higher concentrations to detect a discontinuity in concentration dependence of the speci®c viscosity.

conclusion has been reached regarding mannan/mannan associations through gelation studies of high temperature water soluble fractions of LBG (Richardson et al., 1998). A large standard molar enthalpy of ÿ99 kJ/mol for cross-link formation was determined and these associations were found to dissociate at temperatures close to 100 C. The contribution from these chain/chain associations to the intrinsic viscosity indicates that the present Mark±Houwink constants for LBG [see Eq. (4)], are not solely representative of single chains. Often, the Mark±Houwink constants found for the lesser associated galactomannan, guar, by Robinson et al. (1982) are used to determine the molecular weight of LBG from intrinsic viscosity measurements. Owing to the contribution of polymer associations in water, use of these constants will produce an over estimated value for the molecular weight for LBG single chains. In addition, the slow polymer/polymer association process detected for LBG (Morris, 1990; Richardson & Norton, 1998) indicates the transient nature of LBG solutions and thus the resultant solution property characteristics are weakly time dependent. The concentration dependencies of the speci®c viscosities around the break point, c*, for guar and native LBG in di€erent sucrose concentrations as shown in Figs. 2 and 3, and Table 2, are qualitatively very similar.

Fig. 4. Huggins (solid symbol) and Kraemer (open symbol) analysis for native LBG and guar in water. &=LBG, *=guar.

Below c*, they resembled that expected for random coil polymers (Morris et al., 1981), c1.3, and found by other workers for these (Doublier & Launay, 1981; Morris et al., 1981) and other galactomannans (Ganter, Milas, Correa, Reicher, & Rinaudo, 1992; Kapoor, Milas, Taravel, & Rinaudo, 1994, 1996). A slightly greater concentration dependence, however, was found for guar in 5, 10 and 40% sucrose solutions. The concentration of initial coil overlap, c*, were again very similar, at approximately 0.1% w/v for both guar and LBG in all sucrose concentrations. One noticeable di€erence between guar and LBG was the break point for guar was noticeably less well de®ned than that for LBG. Figs. 5 and 6 display the degree of space ®lling, expressed by the coil overlap parameter, c[Z], for guar and native LBG at different sucrose concentrations, and the critical values for initial coil overlap, c*[Z] are shown in Table 3. Generally, these critical values at c*, were similar to those predicted for compressible spheres (polymer coils) as presented by Eq. (7) and found experimentally by Doublier & Launay (1981). The criteria for initial coil overlap for random coil polymers assumed to be hexagonally packed spheres can be expressed as (Simha & Zakin, 1960): …c ‰Š†cr ˆ

21=2 63=2   1:1 8NA

…7†

where f is the Flory viscosity function, taken as 2.551023 and NA is Avogadro's constant. These values were, however, slightly higher than those found by Launay, Cuvelier, and Martinez-Reyes (1997) which were comparable to an alternative overlap criteria, c*[Z]= 0.77 (see Graessley, 1980) and signi®cantly smaller than those reported by other workers for LBG and guar and well below that found for several other polymers (Morris et al., 1981) where, c*[Z]&4. Such a large discrepancy is a result of extrapolating viscosity versus concentration data from high and low concentrations to one critical point (in this study, only the Low Shear Contraves 30 rotational viscometer was used, which was unable to measure the more viscous, higher concentrated solutions). As discussed by Castelain, Doublier, and Lefebvre

P.H. Richardson et al./Food Hydrocolloids 12 (1998) 339±348

345

Fig. 5. Variation of the zero shear speci®c viscosity with the degree of space occupancy for native LBG in sucrose solutions. &=0% sucrose, *=5% sucrose, ~=10% sucrose, !=20% sucrose, ^=40% sucrose.

Fig. 6. Variation of the zero shear speci®c viscosity with the degree of space occupancy for guar in sucrose solutions. &=0% sucrose, *=5% sucrose, ~=10% sucrose, !=20% sucrose, ^=40% sucrose.

(1987) in their study on carboxymethylcellulose and hydroxyethylcellulose and recently by Launay et al. (1997) for guar, LBG and xanthan in sucrose solutions, closer inspection of viscosity versus concentration data often displays a transitional region between c* and c**. The existence of this second transition at c**, which marks the onset of the concentrated region, was predicted from scaling arguments by de Gennes (1979) (see also Tirrell, 1994). At concentrations greater than c*, the polymer coils compress as well as interpenetrate. This compression continues until the polymer chains reach their unperturbed (theta) state at c**, where Zspc3.3±5. The magnitude of these two transitions will therefore depend upon solvent quality as well as hydrodynamic volume, with reportedly (Tinland, Maret, & Rinaudo, 1990), more persistent chains increasing the breadth of the semi-dilute region. Consequently, such an extrapolation from concentrations above c** and concentrations below c*, would yield a critical point within the transitional, semi-dilute region, giving higher than actual values for c*[Z] (see also Launay et al., 1997). This also explains the lower concentration dependencies determined here after c*. At c**>c>c*, Zspc2.4 whereas as found by other authors (Morris et al., 1981; Launay et al., 1986), at c>c**, Zspc3.3±5.

The degree of space occupancies for guar and native LBG are compared in Fig. 7. It is clear that guar exhibits a greater dependence of speci®c viscosity with polymer chain space ®lling and slightly lower critical coil overlap values than LBG. The source of di€erence is not likely to be due to galactose uninhibited mannan/ mannan associations as these are less prevalent for guar. (It is noteworthy that owing to the polydispersity in the level of galactose substitution, guar is likely to possess some chains of lower than average galactose contents, which are able to aggregate. Evidence for this aggregation has been reported in the literature: reduction of [Z] in phosphate bu€er (Goycollea et al., 1995) and more directly by the observation of curved Zimm plots at low scattering angles (Doublier & Launay, 1981) and relatively high intensity SAXS scattering at low angles (Aymard, 1997)). Two possible factors respinsible for this behaviour are the more rigid nature of guar and the relative poorer solubility for guar in these solvents (especially in sucrose) than for LBG. The resultant more compact structure for guar is revealed by the larger Huggins coecients as shown in Table 3. Fig. 8 displays the intrinsic viscosities as a function of sucrose concentration for guar, native and puri®ed LBG. The maximum at approximately 20% w/w

Table 3 Dilute solution properties for standard LBG (NLBG), puri®ed LBG (PLBG) and guar in di€erent sucrose concentrations Sucrose (w/w) (%)

NLBG [Z] (dl/g)

NLBG l

NLBG c*[Z]

PLBG l (dl/g)

PLBG l

PLBG c*[Z]

Guar [Z] (dl/g)

Guar l

Guar c*[Z]

0

13.79

0.41

1.38

14.4

±a

±

9.25

0.80

0.93

5

12.83

0.54

1.15

12.5

±

±

8.11

1.87

1.14

10

12.95

0.46

1.17

13.3

±

±

8.55

1.53

1.28

20

15.36

0.29

1.69

17.9

±

±

9.59

1.46

0.86

40

11.47

0.49

1.38

17.0

±

±

7.56

1.95

±

a

No measurement.

346

P.H. Richardson et al./Food Hydrocolloids 12 (1998) 339±348

Fig. 7. Comparison of the degree of space occupancy for guar (open symbols) and native LBG (solid symbols) for all sucrose solutions. &=0% sucrose, *=5% sucrose, ~=10% sucrose, !=20% sucrose, ^=40% sucrose.

Fig. 8. Variation of the intrinsic viscosity for guar and LBG at di€erent sucrose concentrations. &=native LBG, *=guar, X=puri®ed LBG.

sucrose for both LBG and guar indicates that this is the best solvent for these polysaccharides and whereas this maximum does not necessarily imply that speci®c sucrose/galactomannan interactions are present, these results are contrary to those found by Elfak et al. (1977). In addition, they are also di€erent to the similar study performed by Launay et al. (1997), where no variation in intrinsic viscosity was found for guar and LBG in 0, 10 and 40% sucrose solutions. Perhaps not surprisingly, it is argued that the solvent quality is better at 20% w/w sucrose as the galactomannan is a polysaccharride. A similar concentration of sucrose was found to be the best solvent environment for other biopolymers (Antipova & Semenova, 1995). Presumably, this level has been reached through a balance of competition for water and sucrose/biopolymer mutual compatibility. At 40% w/w sucrose, the intrinsic viscosity has decreased due to competition for water resulting in a tendency for polymer chain collapse. This is analogous to the `salting out' of proteins. At even higher concentrations of sucrose, 60% w/w, the incompatability of galactomannan in sucrose is so high, owing to competition for water, that the polysaccharide will have a tendency to form included polymer rich regions in a continuous solvent. This can result in a loss of the galactomannan's contribution to the overall viscosity of this mixed system, as reported elsewhere for LBG (Richardson & Norton, 1998). Similar e€ects of concentrated sucrose solutions upon the rheology of other biopolymer systems have been reported, for example gelatin (Marrs, 1982). An explanation for the initial reduction from 0% sucrose (water) to 5% sucrose is less evident. Invoking solvent quality arguments, the reduction suggests that 5% sucrose is a poorer solvent than water. However, as described earlier, the intrinsic viscosity of LBG is arti®cially high (and guar to a lesser extent) due to polymer/polymer associations at very

dilute concentrations. Thus, the reduction in the intrinsic viscosity at 5% w/w sucrose may be attributed to a decrease in the size of the polymer species under investigation [see Eq. (4)], through a reduction in the level of polymer association. These two competing factors: a reduction in polymer/polymer association resulting in a decrease in the intrinsic viscosity (normally a consequence of a better solvent) and good solvents increasing the intrinsic viscosity through coil expansion were present throughout the sucrose concentration range studied. The fact that guar and LBG show similar trends of intrinsic viscosity with sucrose concentration infers that solvent quality e€ects dominate as guar's tendency to self-associate is signi®cantly less than that for LBG. The intrinsic viscosities of puri®ed LBG mimicked that of native LBG with respect to sucrose concentration. Their values were similar at low levels of sucrose suggesting a comparable contribution to the viscosity from the insoluble fraction (impurity) compared with LBG. At 20% w/w sucrose concentration, the higher intrinsic viscosities for puri®ed LBG indicate a signi®cant reduction in the viscosity of the insoluble component. There may also be an additional chain association contribution to the intrinsic viscosities for puri®ed LBG, supported by observations of gelation at lower concentrations for puri®ed LBG than native LBG (Willmer, 1997). Possibly, the removal of the insoluble low galactose chains and the proteinaceous component during puri®cation, no longer inhibits mannan/mannan association. Additional information with respect to the solvent quality and polymer/polymer interactions can be obtained from the Huggins coecients displayed in Fig. 9 for guar and native LBG. Relatively very large values were found for guar, especially in sucrose, referred to earlier through a higher concentration dependence

P.H. Richardson et al./Food Hydrocolloids 12 (1998) 339±348

347

values possess a signi®cant level of experimental uncertainty, the trends displayed in Fig. 10 for LBG and guar are noteworthy. Particularly at 20% sucrose concentration, the critical coil overlap values for LBG and guar were very di€erent. Such di€erences may be partly attributed to the approximation of using intrinsic viscosity values to calculate the initial coil overlap, rather than using less readily accessible hydrodynamic volumes. 6. Conclusions Fig. 9. Variation of the Huggins constant with sucrose concentration for guar and native LBG. &=native LBG, *=guar.

Fig. 10. Comparison of the intrinsic viscosity and the critical coil overlap value for guar (open symbols) and LBG (solid symbols) at di€erent sucrose concentrations.

below c*. These values were signi®cantly larger than those found by Launay et al. (1997), which may be a consequence of a variation in botanical source and the slightly di€erent solvation conditions used. These larger values in water can be attributed to higher degree of branching and possibly due to its higher molecular weight (lower solubility) than LBG, all reducing the ease of solvent drainage through the polymer (Bohdanecky & KovaÂrÆ , 1982). The even higher Huggins coecients for guar in sucrose suggest a relatively poorer solvent than for guar in water, and compared with LBG in sucrose. For LBG and somewhat for guar, over all sucrose concentrations, the Huggins coecients complimented the changes in intrinsic viscosities. That is, as the intrinsic viscosity increased, the Huggins coecient decreased and vice versa, displaying the changes in excluded volume (coil expansion). However, as shown in Fig. 10, at di€erent sucrose concentrations, the critical coil overlap values were not constant, as would be predicted based upon volume ®lling arguments of impermeable spheres. Although these coil overlap

Guar and LBG are highly viscous polysaccharides. They possess low initial coil overlap concentrations, c*=0.1% w/v, commensurate with the detection of shear thinning, and exhibit higher viscosity when compared to typical random coil polysaccharides. At concentrations within the semi-dilute region above c*, Zspc2.4. These c*[Z] values and Zsp versus c dependence above c* were apparently di€erent to those values reported by many other authors for LBG and guar, and other fully ¯exible polymers. This discrepancy was rationalised in terms of previous extrapolations to a single critical point, rather than treating the speci®c viscosity versus concentration data as two distinct transitions. Furthermore, at dilute and semi-dilute concentrations, guar exhibied slightly greater speci®c viscosities at the same degree of space occupancy than LBG. This was attributed to the more compact nature of guar polymer chains over LBG (a consequence of the higher level of galactose substitution) and possibly its higher molecular weight resulting in poorer solubility. LBG's intrinsic viscosities are arti®cially high due to a contribution from polymer/polymer associations. Addition of sucrose has been shown to initially decrease the intrinsic viscosity, possibly due to either a reduction in solvent quality or decrease in polymer/polymer association. A maximum value was found at 20% w/w sucrose for both galactomannans owing to an increase in solvent quality. At higher sucrose concentrations, the solvent quality decreased owing to competition for water, enhancing the extent of polymer contraction and thus reducing the intrinsic viscosity. Large Huggins coecients for guar indicate that guar is more compact in water and sucrose than LBG, again, possibly due to poorer solution solubility as well as the higher level of branching for guar. Acknowledgements The authors wish to thank Alison Russell and Dave Cooke for the physical characterisation of the galactomannan samples and Pierre Aymard, Allan Clark and Ian Norton for helpful discussions and advice.

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