Competitive adsorption of proteins with methylcellulose and hydroxypropyl methylcellulose

Food Hydrocolloids 19 (2005) 485–491 www.elsevier.com/locate/foodhyd

Competitive adsorption of proteins with methylcellulose and hydroxypropyl methylcellulose Juan-Carlos Arboleya1, Peter J. Wilde* Institute of Food Research, Norwich Research Park, Colney Lane, Norwich NR4 7UA, UK

Abstract Surface-active polysaccharides are attracting increasing interest for use in a variety of applications. Amongst these, methylcellulose (MC) and hydroxypropyl methylcellulose (HPMC) have been developed, in part, for their foam and emulsion stabilising properties, together with their water holding and viscosity enhancing properties. The aim of this research is to quantify the competitive adsorption between proteins and MC/HPMC, as they are often used together in many applications, and the results of potential effects of competition are unknown. Two proteins were compared; b-lactoglobulin (BLG) and b-casein (BCAS). BLG forms an elastic interface, whereas BCAS does not. Hence, BCAS is displaced by surfactants more easily than BLG. The surface rheology, surface tension and foam stability of the mixed protein:polysaccharide systems were determined to elucidate the mechanism and consequences of competition. In contrast to surfactants, both MC and HPMC formed highly elastic interfaces, more elastic even than BLG. Both HPMC and MC were more surface active than the proteins, therefore at higher MC and HPMC concentrations, the polysaccharides began to dominate the interfacial properties. Whereas surfactants reduce the elasticity of the protein adsorbed layer, the elastic properties of the polysaccharides enhanced the overall strength of the interface, which will potentially result in more stable foams. q 2005 Elsevier Ltd. All rights reserved. Keywords: Methylcellulose; Hydroxypropyl methylcellulose; b-Lactoglobulin; b-Casein; Interface

1. Introduction The formation and stability of foams and emulsions is a key quality parameter in a wide range of applications. Particularly in food since consumer perception of quality is strongly influenced by appearance. Foam and emulsion functionality is strongly influenced by the surface and interfacial properties of the surface-active components in the system (Dickinson, 2001). Therefore, the role that these ingredients play is vital for the formation and stability of food foams and emulsions. In addition, the competition between the different surface-active species can also be important for the functionality offoams and emulsions. Competitive adsorption phenomena between proteins and surfactants have been found to have a major impact on foam and emulsion stability (Coke, Wilde, Russell, & Clark, 1990; Dickinson, Owusu, & Williams, 1993). * Corresponding author. Tel.: C44 1603 255258; fax: C44 1603 507723. E-mail address: [email protected] (P.J. Wilde). 1 Current address: AZTI (Instituto Tecnolo´gico Pesquero y Alimentario) Txatxarramendi Ugartea, z/g, 48395 Sukarrieta/Bizkaia, Spain. 0268-005X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2004.10.013

Therefore, this competitive adsorption behaviour has been studied extensively over recent years (Bos & van Vliet, 2001; Cornec et al., 1996; Cornec et al., 1998; Euston et al., 1996). The importance is derived from the fact that proteins and surfactants stabilise interfaces by very different mechanisms (Wilde, Mackie, Husband, Gunning, & Morris, 2004). Proteins form a visco-elastic adsorbed layer, which creates a mechanical barrier against coalescence. In contrast, the fluid layer formed by surfactants allows them to rapidly spread in response to surface tension gradients. Both of these mechanisms rely on very different molecular properties and interactions, and are therefore mutually incompatible, resulting in instability. More recently, surface-active polysaccharides are attracting increasing interest. Amongst these, methylcellulose (MC) and hydroxypropyl methylcellulose (HPMC) have been developed, in part, for their foam and emulsion stabilising properties (Dickinson, 2003; Dickinson & Izgi, 1996), together with their water holding (Sarkar & Walker, 1995) and viscosity enhancing properties (Wollenweber, Makievski, Miller, & Daniels, 2000). Although their main

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application in food emulsions is as a stabiliser or thickener (Whistler, 1973), both MC and HPMC have demonstrated significant surface activity (Sarkar, 1984). The properties of the interfacial layers, especially the ratio of train/loop and tail segments, and the elasticity of the layer, determine the emulsion stability (Daniels & Barta, 1994), particularly the coalescence stability of the oil droplets (Cardenasvalera & Bailey, 1995). Hydrocolloids have been effective in increasing the stability of foams through enhanced viscosity that retards drainage (Stanley, Goff, & Smith 1996). The foam activity of MC has also been shown to improve the whippability of cake batters through partial replacement of the egg white. The foam stability was further improved by the gelation of the MC after heating (Grover, 1984). The emulsion stabilizing properties of these compounds could be associated with their structural features. Nahringbauer (1995) has suggested that adsorption of hydrocolloids includes two consecutive or simultaneous stages: a slow diffusion of the macromolecules from the bulk phase to the subsurface region followed by adsorption of polymer segments to the interface. Also, it was proposed that, similarly to proteins, adsorption was followed by changes in molecular conformation. Initially, in bulk solution, the macromolecules are coiled, but after adsorption the polymer backbone starts to unfold. A model was proposed involving unfolding or spreading of the adsorbed molecules followed by attachment of the polymer segments from the subsurface to the surface. This leads to an increase in the number of adsorbed polymer segments causing a reduction of the surface tension. Protein–polysaccharide interactions are very important for a wide variety of applications (Benichou, Aserin, & Garti, 2002; Kato, 2002) including the formation, structure and physical properties of mixed gel systems (Turgeon, Beaulieu, Schmitt, & Sanchez, 2003). However, the number of studies on the foam and emulsion stabilising properties of protein polysaccharide mixtures has been limited, despite the functionality of certain gums, which rely on protein polysaccharide interactions for their functionality (Benichou at al., 2002; Dickinson, 2003). Some studies have been performed studying adsorption at solid surfaces (Fujimoto, Reis, Petri, & Campana, 2002; Sierakowski, Freitas, Fujimoto, & Petri, 2002), and looking at controlled multilayer formation for stabilising emulsions (Moreau, Kim, Decker, & McClements, 2003). However, the competitive adsorption of independent surface-active proteins and polysaccharides has rarely been studied. Therefore, the aim of this research is to quantify the effects of competitive adsorption between proteins and MC/HPMC.

molecular weight 42 kDa methyl substitution between 27.0 and 30%; hydroxypropyl substitution between 3.0 and 5.5%) were obtained from The Dow Chemical Company (Midland, TX). b-Casein (BCAS; C-6905 minimum 90% by electrophoresis), and b-lactoglobulin (BLG; L-2506 approximately 80% purity), both derived from bovine milk, were purchased from Sigma chemicals (Gillingham, UK). Measurements on the mixed protein–polysaccharide solutions were performed at a fixed protein concentration of 10 mM (equivalent to 0.018 wt% BLG and 0.024 wt% BCAS) and variable MC and HPMC concentrations (0–0.75 wt%). The protein concentrations were chosen, as they were functionally relevant in a separate study. The polysaccharide concentration range was chosen as it was commercially relevant, and resulted in the whole range of interfacial behaviours from protein dominated, through to polysaccharide dominated. All measurements were performed in 10 mM phosphate buffer at pH 7.0. Polysaccharide solutions were gently dispersed using a bench top magnetic stirrer for 1 h then stored at 5 8C for different periods prior to measurement.

3. Methods 3.1. Surface tension Surface tensions at the air–water interface of solutions of protein and polysaccharides were measured using the pendant drop technique. In this technique, surface/interfacial tension was calculated from the size and shape of a drop hanging from the tip of a syringe. The mathematical treatment of pendant drop shape is based on the fundamental equation of capillarity, which relates the interfacial/surface tension to the pressure difference across a surface and to the two principal radii of curvature of the surface at that point (Ambwani & Fort, 1979). Droplets of the aqueous phase were held by the tip of a syringe (diameter, 0.7 mm). The image was digitised with a Pulnix TM500 monochrome camera (resolution, 512!512 pixels) using a MuTech MV200 frame grabber and a personal computer. The shape of the drop was analysed by computer image analysis (Hansen & Rodsrud, 1991). Surface tension was also determined by the Wilhelmy plate method. This apparatus consisted of a home-constructed tensiometer, which was made by a 10 g force transducer (HBM, GmbH, Darmstadt, Germany) and a 25 mm wide roughened glass plate. The measuring system was calibrated with a known weight before any contact between plate and sample surface. Accuracy was estimated at better than 0.1 mN/m. Surface tension was monitored at room temperature for between 10 and 20 min, according to the adsorption rate of each solution.

2. Materials 3.2. Surface rheology Methyl cellulose (Methocel Premium A15, mean molecular weight 14 kDa, methyl substitution between 27.5 and 31.5%) and HPMC (Methocel HPM 450, mean

Surface shear rheological measurements were carried out to study the mechanical and flow properties of adsorbed

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layers at fluid interfaces (Murray & Dickinson, 1996), and are sensitive to surface structure and composition (Ridout, Mackie, & Wilde, 2004). Experiments at the air–water interface were made using a Bohlin CS10 controlled stress rheometer using a 7 cm diameter bicone as measuring geometry. The surface rheological response was tested by oscillation mode at a frequency and stress of 0.5 Hz and 1.5!10K4 mN/m, respectively. Measurements were performed at room temperature.

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Fig. 1 shows the results of preliminary surface rheological experiments designed to check for adverse storage or temperature effects on the surface properties of 0.1% MC and HPMC solutions. Fig. 1a shows the surface shear elastic modulus (G 0 ) after 30 min adsorption time, following

storage of the MC/HPMC solutions for different times. In general, there was a dramatic effect of storage time, showing a large increase in G 0 over the first 3–5 days, followed by a period of stability. Finally after 10 days storage, G 0 began to decrease. MC initially formed a weaker interface, but during the stable period it was stronger (G 0 z210 mN/m) compared to HPMC (G 0 z130 mN/m). Subsequent experiments used MC and HPMC solutions which had been stored for periods which resulted in stable values of G 0 (3–10 days storage). The reasons for the dramatic changes with storage time are not clear, it is possible that changes in the state of hydration of the MC or HPMC may affect molecular structure, and it has been found that changes in hydration can occur for periods of up to 4 days (McCrystal, Ford, & RajabiSiahboomi, 1997). Fig. 1b shows that the surface rheology did show some changes as a function of temperature, although, around room temperature, the changes were within experimental error at approximately 210G20 mN/m. At 10 8C, samples showed weaker surfaces at 140G25 mN/m. Higher temperatures provoked a dramatic change in their rheological properties, particularly above 50 8C (Kobayashi, Huang, & Lodge, 1999). All future experiments were based on the stable intervals of storage stability and experiments were performed at 20 8C. The surface tension of the individual components as a function of concentration is shown in Fig. 2. All the components seem to reach an initial saturation point around 0.001 wt%. However, the surface tension of the proteins continued to decrease at higher concentrations. This is in agreement with literature values (Wustneck et al., 1996). The MC and HPMC are distinguishable from the proteins in that they appear to be more surface active than the proteins at all concentrations. The lowest values for the MC and HPMC were between 46 and 48 mN/m, compared to

Fig. 1. Effect of storage time and temperature on the surface shear elasticity of MC (B) and HPMC (:) after 30 min adsorption as a function of (a) storage time and (b) temperature.

Fig. 2. Effect of concentration on the surface tension as measured by the pendant drop technique for MC (B); HPMC (6); b-casein (%) and b-lactoglobulin (&).

4. Results and discussion 4.1. Individual components

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50–52 mN/m for the proteins. It is interesting to note, that for HPMC, above 0.02 wt%, the surface tension values begin to increase slightly. This has been observed for some surfactant systems where purity and molecular polydispersity can result in these types of effects around the cmc (Fang & Joos, 1992). Considering the surface behaviour of the individual components, the mixtures of protein and polysaccharides where then studied to evaluate the effects of competitive adsorption. 4.2. Protein and MC The surface properties of MC–protein mixtures were determined. Fig. 3 shows the results of the surface tension and surface rheological measurements of MC with BCAS and BLG. The surface tension results (Fig. 3a) show that as the MC concentration is increased, that the surface tension of the protein solutions decreases, demonstrating that the greater surface activity of the MC is affecting the surface tension of the mixture. The values for the mixed systems becomes very close to the MC alone around 0.1 wt% MC. In the case of BCAS, above 0.1 wt% MC, all the values were

similar to MC alone, suggesting that MC was dominating the surface properties above a concentration of 0.1 wt%. In the other hand, BLG–MC mixture showed a decrease of surface tension from 52 to 46.5 mN/m up to 0.2%. From this concentration, surface tension increased slightly up to 47 mN/m at 0.75% MC. This suggests that BLG is either resisting displacement by MC, which has been found previously (Mackie, Gunning, Wilde, & Morris, 1999). Or that a complex between BLG and MC is resulting in a surface with different characteristics to the pure components. To investigate this further, the surface rheological properties of these mixtures was investigated. The surface shear elastic modulus (G 0 ) of MC–protein mixtures is shown in Fig. 3b. MC alone showed a high surface elastic modulus value (O100 mN/m) at low concentration and remained fairly constant above 0.3% (w220 mN/m). BCAS alone forms a very weak adsorbed layer (Dickinson, Rolfe, & Dalgleish, 1990) and seemed to dominate the surface at low MC concentrations because of the low values of elastic modulus. At higher MC concentrations, BCAS was gradually replaced by MC resulting in higher values of the surface elasticity (antagonistic effect). At 0.75% of MC, mixture showed similar elastic behaviour to MC alone, which agrees with the surface tension results and suggests more or less complete displacement of BCAS molecules from the air–water interface. Slightly different behaviour was found in the presence of BLG (Fig. 3b). In this case, at low MC concentrations, the mixture showed higher elastic modulus values than BCAS mixtures and it reached MC values between 0.1 and 0.2%. This behaviour from the mixture can be expected due to the viscoelastic nature of the BLG adsorbed layer. Both components seemed to work synergistically to enhance the elasticity of the interface. This is an interesting result from the practical point of view for an improvement of the functionality of BLG by adding small amounts of MC. At higher MC concentrations, the mixture recorded even greater elastic values than isolated MC, which may suggest synergistic interactions between both components. 4.3. HPMC and protein

Fig. 3. Effect of MC concentration on (a) surface tension and (b) surface shear elasticity of solutions containing: no protein (!); or 10 mM b-casein (6) or b-lactoglobulin (&).

The surface tension and rheological properties of protein–HPMC mixtures a shown in Fig. 4. As suggested earlier, the surface tension of HPMC alone recorded higher surface tension values as the HPMC concentration was increased. This unusual behaviour was confirmed by using another technique. Wilhelmy plate results showed how surface tension increased from 47.5 to 48.5 mN/m at a range concentration from 0.01 to 0.1%. At 0.3%, surface tension reached equilibrium at 53 mN/m up to 0.75% (Fig. 4a). It has been reported (Fang & Joos, 1992) that experiments of SDS showed an minimum in the surface tension close to the cmc caused by impurities in the product. Therefore, polydispersity in the molecular weight or degree of

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Fig. 4. Effect of HPMC concentration on (a) surface tension and (b) surface shear elasticity of solutions containing: no protein (!, B); or 10 mM bcasein (6) or b-lactoglobulin (&). Open circles represent surface tensions measured by the Wilhelmy plate technique for comparison.

substitution may result in this effect. HPMC with mixtures of BCAS and BLG showed a decrease in surface tension at low concentrations of HPMC with similar values to isolated HPMC. However, from 0.01 to 0.05% for BLG and BCAS, respectively, a dramatic increase occurred and reached values around 54.4 and 53 mN/m for BLG and BCAS, respectively, between 0.1 and 0.3%. From 0.3%, surface tension of BLG mixture decreased to reach similar values to HPMC alone, whereas BCAS mixture remained a constant at 53 mN/m, similar to HPMC alone. This was most unusual behaviour, even for mixtures, but it was consistent and repeatable, and possible connected with the unusual surface tension behaviour of HPMC alone over this concentration range. Surface shear rheological measurements were carried out over the same HPMC concentration range, alone and in the presence of BCAS and BLG at a constant concentration (10 mM). Elastic modulus of HPMC alone showed lower values compared to MC alone (Fig. 3b). At 0.1%, HPMC already reached a maximum at around 130 mN/m and

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remained high, but slightly lower values up to 0.75%. For the BCAS–HPMC mixture, displacement of BCAS appeared to occur at a low HPMC concentration (0.05% HPMC) as G 0 reached a value similar to HPMC alone. However, the surface became weaker again between 0.1 and 0.2% HPMC. This behaviour was highly reproducible. Above that concentration HPMC seemed to control the surface space because of the similar values of the isolated compound and the mixture. Similar behaviour was observed in BLG–HPMC mixtures. G 0 values of the mixture exceeded isolated HPMC values at lower HPMC concentrations and reached a maximum at 190 mN/m with 0.1% HPMC. G 0 then decreased to 50 mN/m at 0.3% HPMC and from that concentration, HPMC started to dominate the surface. At 0.5% HPMC, elasticity showed similar behaviour to HPMC alone. In addition, the higher elastic modulus compared to BCAS–HPMC mixture supports the idea to have synergistic behaviour in the presence of BLG. When an adsorbed surfactant molecule orients itself at the interface, there is a decrease of free energy resulting in a lowering of surface tension (Dickinson & Stainsby, 1982). Unfortunately, this simple model becomes extremely complicated for polymers, where hydrophilic and hydrophobic regions are not well defined. They form threedimensional structures that make adsorption at the interface a much more complex process (Dickinson, 2003; Norde, MacRitchie, Nowicka, & Lyklema, 1986). In addition, the higher molecular weight of HPMC compared to MC studied here can make the diffusion to the interface slower and therefore result in a higher surface tension. This molecule can show a non-uniform configuration of hydrophilic and hydrophobic blocks where the adsorption can be more difficult (Sarkar, 1984). At different concentrations, the different molecular configurations will be at different stages in their adsorption isotherm and this may result in complex adsorption behaviour. The main feature of this competitive adsorption study is that the molecule competing with the protein has a greater surface elasticity than the protein. Conventional studies have involved small molecular weight surfactants, which are generally mobile at the surface, and therefore have negligible surface elasticity. This incompatibility in fact drives molecular segregation at the interface. Whereas, in this case, with two competing polymers, that both form immobile interfaces, the surface dynamics will be completely different. In the case of MC, there appears to be a fairly simple competition with the proteins for the interface. The MC is more surface active than the proteins, and eventually dominates the interfacial properties at higher concentrations. This process is completed at lower MC concentrations in the case of BCAS. This is probably because BCAS forms a very weak surface, and is known to be displaced at lower surface pressures (Mackie et al., 1999), and lower concentrations (Cornec et al., 1998) than BLG. In the case of BLG, the situation is a little more complicated because we have two competing polymers,

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both of which can form highly elastic surfaces. In addition, BLG is able to bind with hydrophobic ligands (Coke et al., 1990). The results suggest that instead of a simple competitive displacement of the protein by MC, some synergism occurs between the two adsorbed molecules, and at relatively low concentrations of MC, resulting in higher surface tensions and surface elasticities than either component. In fact G 0 for the mixed BLG:MC is higher than the additive values of the individual components above 0.2 wt% MC (Fig. 3b). It is unlikely that the packing density is greater with the two components, as the surface tension of the mixtures is actually higher. Therefore, it is more likely that synergistic interactions, probably electrostatically mediated, that lead to the synergistic increase in G 0 . This has been observed with other protein:polysaccharide systems (Sarker & Wilde, 1999; Sarker, Wilde, & Clark, 1998). In the case of HPMC, similar behaviour between the two proteins was observed. BCAS was affected by the increase in HPMC concentration earlier than BLG, for the same reasons as discussed above for MC. However, the situation was complicated by the complex surface behaviour of HPMC alone. Once the HPMC had reached its minimum surface tension, around 0.01 wt%, the surface tension increased slightly with increasing HPMC concentration. This effect has been observed with the complex micelle formation of SDS solutions when dodecanol is present as an impurity (Fang & Joos, 1992), it has also been observed in mixed surfactant systems (Wickham, Wilde, & Fillery-Travis, 2002). Therefore, it is possible that the polydisperse nature of the molecular weight and level of substitution of the HPMC used in this study may result in this surface tension effect. The unusual behaviour of the surface elasticity of HPMC in the presence of protein, was remarkably reproducible. The competition between competing, adsorbing species is initially based on surface activity of the individual components, and controls the surface competition (Ridout et al., 2004), and hence the surface rheological properties. Therefore, the changing surface activity of the HPMC as the concentration is increased, is likely to strongly influence the competitive adsorption with the added proteins. Hence, resulting in the unusual behaviour of the mixtures. More specifically, as the HPMC concentration increased above 0.1 wt%, the surface tension of HPMC increased to levels above that of BCAS alone, and then above BLG alone (Fig. 4a). This will allow the proteins to adsorb in preference to the HPMC, and hence the surface G 0 began to decrease towards values associated with the protein alone. As the HPMC concentration increased further, the surface tension increased only a little more, but the dynamics of adsorption would become much quicker, and would therefore dominate the surface behaviour before significant protein adsorption took place. The surface rheology of the HPMC is great enough that significant displacement of HPMC by the protein is unlikely to occur (Mackie et al., 1999).

Another consideration is the interactions between the proteins and polysaccharides in solution. This is another area of widespread study, as it is known that interactions or incompatibilities between proteins and polysaccharides allow structures to develop in solution (Turgeon et al., 2003). This may well influence the rate and extent of adsorption. Consequentially, the order in which the different components arrive at the interface will influence the final, equilibrium surface composition (Damodaran & Rammovsky, 2003; Ridout et al., 2004). As a result of these interactions in solution, the adsorption and hence the functional properties of the system may well be affected. The impact may be positive in terms of synergistic interactions giving rise to increased surface elasticity, or they may be negative as a result of hindered adsorption and higher interfacial tensions. In any case, a thorough investigation of the effects of these interactions is necessary to understand the functionality of these complex mixtures.

5. Conclusion The competitive adsorption of MC and hydroxypropylmethyl cellulose with two different proteins (BCAS and BLG) was investigated. BCAS appeared to be displaced through simple competitive adsorption, more easily than BLG. Some synergism appeared to take place between the adsorbed polysaccharides and BLG, resulting in greater values of surface elasticity in the mixtures. Complex adsorption behaviour of HPMC alone had a strong influence on the competition with the proteins, resulting in complex, non-linear concentration dependent behaviour. This underpinning knowledge of the complex surface behaviour of these mixtures should allow the design of protein:polysaccharide mixtures with enhanced functionality.

Acknowledgements The authors would like to thank Fraser Hogg, Dr Neil Carr, and Dr Alan Holmes for their funding and support, and to Dr Annette Fillery-Travis for her help and advice for this project. PJW also acknowledges BBSRC for support through the Core Strategic Grant to the Institute.

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