Textural and Rheological Properties of Oat Beta-Glucan Gels with

Mar 26, 2014 - ABSTRACT: The impact of oat β-glucan concentration and molecular weight (MW) on gel properties was investigated. Mixed. MW gels/viscou...
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Textural and Rheological Properties of Oat Beta-Glucan Gels with Varying Molecular Weight Composition Yolanda Brummer,† Cheryl Defelice,† Ying Wu,†,‡ Melissa Kwong,§,∥ Peter J. Wood,†,⊥ and Susan M. Tosh*,† †

Guelph Food Research Centre, Agriculture and Agri-Food Canada, 93 Stone Road West, Guelph, Ontario, Canada, N1G 5C9 Department of Nutritional Sciences, University of Toronto, Toronto, Ontario, Canada

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ABSTRACT: The impact of oat β-glucan concentration and molecular weight (MW) on gel properties was investigated. Mixed MW gels/viscous solutions at 3, 4, and 5% β-glucan with high molecular weight (HMW):low molecular weight (LMW) ratios of 0:100, 25:75, 50:50, 75:25, and 100:0 were evaluated. The 100:0 and 50:50 gels had the lowest tan δ values. The 50:50 gels had the highest storage moduli (G′), whereas 100:0 solutions did not gel. Peak melting temperature (TP) was highest for 0:100 gels and decreased with the addition of HMW β-glucan. Hardness, at 40% compression, increased with concentration, and 25:75 and 50:50 gels were hardest at each concentration. Ordered microstructure, apparent in 0:100 gels, diminished with HMW β-glucan addition. Glucose addition resulted in lower tan δ values and firmer, harder gels compared to gels without glucose. Thus, the textural properties and melting profiles of β-glucan gels can be manipulated by adjusting the ratios of molecular weight fractions or addition of sugar for different applications. KEYWORDS: β-glucan, gelation, oat, microscopy, molecular weight, hardness, firmness

1. INTRODUCTION The polysaccharide (1→3)(1→4)-β-D-glucan is found predominantly in the cell walls of oats and barley and in lesser amounts in rye and wheat. The basic structure is a linear chain of glucopyranosyl mononers connected by single β-(1→3) linkage or consecutive β-(1→4) linkages.1 The arrangement of these two linkages is such that the molecule is primarily composed of β-(1→3) cellotriosyl and cellotetrosyl units. These structural features make up about 90% of the molecule, the rest being contributed by longer runs of consecutive β-(1→4) monomers.2 The relative proportion of β-(1→3) cellotriosyl and cellotetrosyl units varies depending on β-glucan source3,4 and gives rise to disparate properties.5 In aqueous solution, βglucans exist as random coil polysaccharides and, as such, generate viscosity in a molecular weight and concentration dependent manner.6 β-Glucans from cereals grains have been widely studied because they are associated with several health benefits, including the attenuation of blood glucose levels7−9 and the reduction of serum cholesterol levels.10−12 In the gut, β-glucans increase the apparent viscosity of the food bolus, slowing the hydrolysis of starch13,14 and affecting the rate of carbohydrate absorption from the gut.15,16 Similarly, a role for viscosity in the reduction of serum cholesterol levels has been determined .12 Because of their ability to bind water and increase apparent viscosity, β-glucans are hydrocolloids and their functionality could be extended to include thickening, stabilizing, and texture modification of ice cream, gravies, salad dressings, and low fat preparations.17 Additionally, β-glucans can form gels, which increases their prospects with respect to food applications. Early work evaluating the rheological behavior of β-glucan speculated that hydrogen bonding between cellulosic regions in the polymer could form junction zones, trapping water and leading to the formation of a gel.18 If structural regularity is © 2014 American Chemical Society

required to form junction zones, this made sense intuitively because there are more consecutive β-(1→4) linkages than β(1→3) and it is well-known that hydrogen bonds can form between cellulosic material to adopt an ordered structure. However, based on a comparison of gels formed from structurally diverse β-glucans from oat, barley, and lichenan by Böhm and Kulicke,19 a mechanism whereby β-(1→3) linked cellotriosyl units form junction zones and are responsible for gelation is more likely. Support for this mechanism has since been established by other researchers who showed a strong correlation between the percent of β-(1→3) linked cellotriosyl units in a polymer and the elasticity (G′) of gels,20,21 and that β-glucan hydrolyzed to disrupt consecutive β-(1→4) linkages and preserve β-(1→3) linked cellotriosyl units produced more elastic gels than those with intact consecutive β-(1→4) linkages and disupted β-(1→3) linked cellotriosyl units.22 The proposed mechanism for β-glucan gelling is one where consecutive β(1→3) linked cellotriosyl units of separate molecules form junction zones via hydrogen bonding, trapping water in the evolving structure. These strands consisting of a few β-glucan molecules aggregate to form gel networks through cluster− cluster aggregation.23,24 Microscopic images21 and fluorescent particle tracking25 confirm the presence of heterogeneity in the gel structure, with regions of dense aggregation and others less densely aggregated or ordered. There is a lot of variability in the properties of β-glucan gels, with the chief determinants being molecular weight (MW), fine structure, and concentration. Lower molecular weight β-glucans (1) or solid properties (tan δ 0.05 for 3, 4, and 5% gels), and there was no significant difference between hardness values for gels with a 25:75 and 50:50 HMW:LMW ratio at the 3 and 5% concentration. However, at the 4% level, the hardness of 50:50 gels was significantly lower than that of 25:75 gels (P = 0.031). Essentially, when more than 50% of the β-glucan in the gel was HMW, the gel hardness declined. Similarly, Young’s modulus increased with increasing gel concentration and the highest values were obtained for gels with either 25 or 50% HMW β-glucan. Essentially, gels with no HMW β-glucan or more than 50% HMW β-glucan deformed more with less force than did gels with an equal amount of HMW and LMW βglucan. There was a significant difference (P < 0.05) in Young’s modulus between the first two levels of HMW proportion (0 and 25%) for the 5% gels, however there was no significant

Figure 4. Firmness (A, Young’s modulus) and hardness (B) for 3, 4, and 5% β-glucan gels with high molecular weight:low molecular weight (HMW:LMW) ratios of 0:100, 25:75, 50:50, and 75:25.

difference between the 50:50, 25:75, and 75:25 HMW:LMW ratios. The maximum firmness for the 4% gels occurred at 25% (5.13 kPa) and 50% (5.1 kPa) HMW β-glucan, with significantly lower values obtained at 0 and 75% HMW. As shown by small deformation measurements, the 100% LMW gels were the most elastic, indicating a structure that stored and released energy, but the gel network was weak and did not require a lot of force to compress. The addition of up to 50% HMW β-glucan increased the firmness and hardness of the gels, but this effect diminished as more HMW β-glucan was added. After 25 or 50% HMW β-glucan, the high viscosity of the mixtures hinders the movement of molecules into conformations conducive to hydrogen bonding and, therefore, junction zone formation. The LMW molecules appear to stabilize the gel structure at short distances whereas the HMW polymers contribute entanglements and longer range resistance to deformation. Hence, the gels are more viscoelastic and offer less resistance to flow. In 2003, Lazaridou et al.20 found an increase in Young’s modulus with increasing concentration for oat β-glucan gels (Mw 110,000, 6−12%), however, at constant concentration, the values initially increased with molecular weight, then decreased from Mw 65,000 to 250,000. Similarly, we found that firmness and hardness increased with the 3164

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addition of HMW β-glucan to the gels, but after 50% addition, they decreased. By contrast, Vaikousi et al.27 found that Young’s modulus for 8% barley β-glucan gels decreased as molecular weight increased from 40,000 to 180,000, after which it stabilized. There is substantial variation in published β-glucan gel firmness values, and the results obtained here are between the extremes of values reported thus far. Irakli et al.26 reported Young’s modulus values under 1 for 6% barley gels ranging from Mw 450,000 to 1.3 million, Lazaridou et al.20 obtained values of 9.37−30.45 kPa for 8% β-glucan gels with molecular weights from 35,000 to 250,000, and values of 39.71 and 36.62 kPa for 8% oat beta-glucan gels where the Mw was 105,000 and 203,000, respectively, have been published.20 Disparate results are expected given the dependency of gel properties on solubilization temperature, 31 concentration, molecular weight,20,22,32 distribution,19 and molecular structure .22,32,33 Microscopic examination of the gels reveals that the range in gel hardness and firmness results from differences in the gel network structure. The micrographs in Figure 5 show the

where runs of consecutive cellotriosyl units are aligned and form junction points. In the LMW solution, the polymers have more freedom to diffuse and the junction points form more frequently. In the solutions containing HMW β-glucan the entanglements retard the diffusion of the polymers, which hinders the formation network junction points. Each junction point that does form further impedes the mobility of the molecules, further slowing network formation. The entanglements and slow diffusion rates prevents gelling of the HMW solution. The greater mobility of the LMW polymers in solution allows them to act as linker molecules which create hydrogen bonded bridges between the slower, entangled HMW polymers. Previous investigations using atomic force microscopy have shown that HMW β-glucan tends to form larger clusters than LMW β-glucan.24 After the initial clusters of polymers have formed, they rearrange in aggregated structures which make up the network visible in the light microscope. The dissimilarity in the aggregated network structures visible in the light microscope indicated that the differences in the initial gelation changed aggregation patterns and the final microstructure observed. Thus, mixed HMW−LMW gels exhibit high gel stiffness by maximizing both the entanglements and the number of junction points in the material. 3.2. Model Food Gel. A model food gel prepared from oat β-glucan solutions in HMW:LMW ratios of 0:100, 50:50, and 75:25 was used to assess the influence of added glucose on the gel properties. Measurements of molecular weight and purity for the β-glucans used to make model gel systems are available in Table 1. Although the terms HMW and LMW are used, it should be noted that oat β-glucans used for the model food gels were different from β-glucans used for the mixed molecular weight studies and, hence, had different molecular weights (Mw), polydispersities (Pd), and molar ratios (MR). The HMW oat β-glucan had a weight average molecular weight (Mw) of 580,000 versus 1,190,000 for the mixed molecular weight gels. Similarly, the MW of the LMW β-glucan used in the model food gel was higher (Mw 145,000 versus 31,200) than the βglucan used for the mixed molecular weight gels. The LMW βglucan was slightly more polydisperse than the HMW gum. Gelled samples and controls (without added glucose) were subjected to small deformation oscillatory measurements to determine the relative contribution of viscous and elastic factors to the overall behavior of the gels. Figure 6 shows the frequency dependence of G′ for samples and controls with differing HMW:LMW ratios. Overall, G′ increased with increasing proportion of HMW β-glucan, however the 100% LMW gels were the least frequency dependent, regardless of the presence or absence of glucose. The 0 and 75% HMW gels without glucose had higher G′ values than gels with the same ratio but no glucose. The behavior was different for the 50% HMW samples, which gave higher G′ values for the sample with glucose at frequencies below 10 Hz, but lower G′ values for the sample with glucose at higher frequencies. An examination of G′ values shows that those without glucose were more frequency dependent at each HMW:LMW ratio. The tan δ values at 1 Hz (Figure 7) increased with increasing proportion of HMW such that the values for 0% HMW, 50% HMW, and 75% HMW gels were 0.09, 0.3, and 0.5 respectively for gels with glucose and 0.1, 0.6, and 0.8 respectively for gels without glucose. The gels composed entirely of LMW β-glucan display more typical gel behavior, with tan δ values near 0.1, indicating behavior dominated by elastic properties. Overall, tan

Figure 5. Micrographs obtained using phase contrast light microscopy of 4% β-glucan gels with HMW:LMW ratios of A, 0:100, B, 50:50, C, 75:25, and D, 100:0.

structure of gels with a total β-glucan concentration of 4% using phase contrast light microscopy. The aggregated β-glucan structures are clearly visible in image A, which contains only LMW β-glucan. In this sample, the β-glucan exists as hydrated fibers in a liquid solution. Low molecular weight β-glucan does not form a stable gel network that can bind sufficient water, and the network collapses easily, as indicated by the low firmness and hardness values from the large deformation tests. Gels with 50 or 75% LMW material gave the hardest, most elastic gels, as indicated respectively by the compressive force and Young’s modulus data. Figure 5B shows a 50:50 gel which has a structure that is well-defined and regular in nature. In gels with a HMW:LMW ratio of 75:25 (Figure 5C) the gel structure is not as well-defined. The gelation process is diffusion limited, and the highly viscous, entangled HMW polymers retard gelation. However, the LMW polymers are able to diffuse through the solution and set up bridges between reactive sites on the longer polymers. Micrographs of gels composed entirely of HMW material (Figure 5D) do not show a regular structure, but bright spots may indicate aggregates. During gelation, the number of hydrogen bonds between polymer molecules increases as they diffuse into conformations 3165

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Table 2. Young’s Modulus and Hardness Measurements for Model Food Gels with and without Glucose Young’s modulusa

hardnessa,b

c

HMW:LMW Ratio: Gels with Glucose 0.86 ± 0.14 d 0.53 ± 0.10 0.79 ± 0.13 d 0.55 ± 0.12 0.88 ± 0.12 d 0.47 ± 0.10 HMW:LMW Ratio Gels without Glucose (Control) 0:100 0.51 ± 0.08 e 0.38 ± 0.04 50:50 0.54 ± 0.11 e 0.37 ± 0.10 75:25 0.82 ± 0.26 d 0.44 ± 0.12

0:100 50:50 75:25

d d ed e e ed

Values are means ± SD. Values with different letters are significantly different (p < 0.05). bHardness in Newtons at 40% compression (N). c High molecular weight:low molecular weight. a

whereas for a gel prepared with the same composition but with the addition of citric acid and a small amount of flavoring, firmness and hardness values of 0.54 and 0.37 N were obtained. While the gels with citric acid gave much lower large deformation results than those without, the effects of the acid may have been mitigated by the addition of glucose as, overall, hardness and Young’s modulus values were lower for the model food gels without glucose compared to gels with glucose. 3.3. Conclusions. Gel properties, as measured by small and large deformation measurements, varied with proportion of HMW or LMW β-glucan. The mix of 50% HMW and 50% LMW produced the hardest, but also the most elastic, gels. By mixing high and low molecular weight β-glucans together, the relative contribution by each type of molecule to gel structure and texture is exploited. The differences observed in gel texture based on proportion of HMW and LMW β-glucan are diminished by the incorporation of other components in the gel such as citric acid and glucose. Hence, by controll of βglucan molecular weight and product formulations, gel properties can be controlled in order to achieve desired textural properties.

Figure 6. Frequency dependence of G′ for model food gels with high molecular weight:low molecular weight (HMW:LMW) ratios of 0:100, 75:25, and 50:50.

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Figure 7. Tan delta at 1 Hz for model gels with high molecular weight:low molecular weight (HMW:LMW) ratios of 0:100, 50:50, and 75:25 with glucose and control gels without glucose.

AUTHOR INFORMATION

Corresponding Author

*Tel: 00 + 1 226 217-8067. Fax: 00 + 1 226 217-8181. E-mail: [email protected].

δ values were slightly higher for gels without glucose, suggesting a contribution to elastic behavior by the glucose. The hardness (N) at 40% compression and Young’s modulus (E) were determined from the force displacement curves for the three model food gels and controls without glucose (Table 2). When no glucose was present, the firmness increased with increasing proportion of HMW β-glucan, and the hardness values were highest for the gel with 75% HMW. However, these trends were not observed when glucose was added to the gels, where no significant difference between hardness and firmness values was found, although the gel with 75% HMW gave the lowest hardness values. Added glucose competes for water, thereby slowing diffusion rates. Glucose may mitigate the effect of varying molecular weight by effectively increasing the concentration of β-glucan in solution and inhibiting the formation of junction zones. Additionally, these gels were less firm and required less force to compress than the similar gels prepared for the mixed molecular weight trial, perhaps reflecting the addition of a small amount of citric acid. For example, the firmness and hardness values for the 50:50 HMW:LMW gel at 4% were 5.1 and 2.7 N respectively,

Present Addresses ‡

Department of Agricultural and Environmental Sciences, College of Agriculture, Human and Natural Sciences, Tennessee State University, Nashville, TN. Tel: 00 + 1 615 963-5824. E-mail: [email protected]. ∥ Lecturer, Glycemic Index Research Unit, Temasek Polytechnic, 21 Tampines Avenue 1, Singapore 529757. Notes

The authors declare no competing financial interest. ⊥ Deceased.

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