Biopolymer Interactions in Emulsion Systems: Influences on Creaming

Chapter 16

Biopolymer Interactions in Emulsion Systems: Influences on Creaming, Flocculation, and Rheology Eric Dickinson

Downloaded by CORNELL UNIV on September 5, 2016 | http://pubs.acs.org Publication Date: November 19, 1996 | doi: 10.1021/bk-1996-0650.ch016

Procter Department of Food Science, University of Leeds, Leeds LS2 9JT, United Kingdom The stability and rheology of food oil-in-water emulsions are sensitive to the nature and strength of the biopolymer interactions at the surface of the droplets. Addition of polysaccharides to emulsions stabilized by food proteins or small-molecule emulsifiers may lead to a greater or poorer degree of stability with respect to creaming— depending on the type of polysaccharide and its concentration. Small-deformation shear rheological measurements can be very useful for predicting emulsion creaming behavior, for acting as a sensitive indicator of the character of droplet-polysaccharide interactions, and for distinguishing between the postulated bridging and depletion flocculation mechanisms. These general statements are illustrated in this article for model emulsions at neutral pH containing a number of different proteins (i.e., casein(ate), bovine serum albumin and βlactoglobulin) and polysaccharides (i.e., dextran, xanthan, rhamsan, guar gum and dextran sulphate). Biopolymer interactions have an important bearing on the shelf-life and texture of numerous food products. The primary macromolecular stabilizing agent in a typical food emulsion of the oil-in-water kind (homogenized milk, ice-cream, salad dressing, etc.) is a multicomponent mixture of adsorbed proteins. In addition, polysaccharides may be present as thickening or gelling agents, and there may be interactions between adsorbed proteins and polysaccharides which may have implications for the stability and rheology of the system (1-6). To understand the general behavior of biopolymers in emulsions, it is convenient to distinguish between competitive and co-operative phenomena (6). Competitive adsorption involves the partial (or perhaps complete) displacement from the surface of one biopolymer by another. The rate and extent to which this occurs depends on a number of factors such as the concentrations and molecular characteristics of the biopolymers, the aqueous solution conditions (pH, temperature, etc.), the age of the 0097-6156/96/0650-0197$15.00/0 © 1996 American Chemical Society

Parris et al.; Macromolecular Interactions in Food Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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MACROMOLECULAR INTERACTIONS IN FOOD TECHNOLOGY

adsorbed protein layer, and the presence of other competing and/or interacting species (surfactants, calcium ions, etc.). Co-operative adsorption involves transient or permanent association of two (or more) biopolymers in one (or more) discrete layers at the interface. Whereas competition implies a net repulsion (or at least no strong attraction) between coadsorbing species, co-operation implies direct complexation between the biopolymers—due to covalent, electrostatic, hydrophobic or hydrogen bonding (or any combination of these). In many systems there is evidence for both competitive and co-operative effects within the same adsorbed layer. The stability and rheological properties of a food colloid containing a mixture of dispersed droplets + biopolymer molecules are sensitive to the nature and strength of the droplet-droplet and droplet-biopolymer interactions (7,2). Whether these pair interactions are net attractive or net repulsive (weak or strong) is determined by the chemical nature of the biopolymer(s) present and the solution conditions (pH, temperature, etc.). The primary emulsifier layer around the emulsion droplets may consist of protein or low-molecular-weight surfactant. Where protein is the primary emulsifier, the overall structure and composition of the layer depends on the balance between the different kinds of biopolymer interactions, i.e. with the droplet surface, with the aqueous medium, or with other biopolymer molecules in the system. A binary aqueous solution of protein + polysaccharide may exhibit either complex coacervation or thermodynamic incompatibility, depending on whether the overall molecular pair-wise protein/polysaccharide interaction is, respectively, net attractive or net repulsive. In turn, the average strength and nature of the droplet-biopolymer interactions determines whether the emulsion is colloidally stable, or is flocculated, either by a bridging or a depletion mechanism (4,5). This paper describes recent experimental results obtained at Leeds relating to the effect of added polysaccharide on the stability and rheology of oil-in-water emulsion systems. Two types of systems are mainly considered: (i) emulsions of smallmolecule surfactant-coated droplets containing an added microbial polysaccharide, dextran or xanthan, and (ii) emulsions of milk protein-coated droplets with an added anionic polysaccharide dextran sulphate. In the former emulsions the dropletpolysaccharide interaction is non-associative or net repulsive, whereas in the latter systems it is net attractive. Repulsive Droplet-Polysaccharide Interactions Creaming and rheological behavior have been studied (7,8) for oil-in-water emulsions prepared with nonionic Tween 20 (i.e. polyoxyethylene sorbitan monolaurate) as emulsifier and with various concentrations of dextran (or some other polysaccharide) added after emulsification. Figure 1 shows the effect of the concentration of nonionic dextran (5 x 10 daltons) on the creaming stability at 25 °C of emulsions (30 wt % mineral oil, 1 wt % Tween 20, pH 7, ionic strength 0.05 M) having a fairly narrow droplet-size distribution and an average volume-surface diameter of d = 0.55 pm. Stability of each emulsion sample was visually assessed by observing the rate of appearance of a distinct serum layer at the bottom of an undisturbed flat-bottomed glass cylinder of height 60 mm. Whereas there was found to be no discernible serum separation over a storage period of 15 days for emulsion 5

32



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DICKINSON

Biopolymer Interactions in Emulsion Systems 199

Time / days

Figure 1. Influence of added dextran on creaming of oil-in-water emulsions (30 wt% oil, 1 wt% Tween 20, pH 7, ionic strength 0.05 M, d = 0.55 pm). Serum volume fraction (expressed as a percentage of total emulsion volume) is plotted against the storage time at 25 °C: (a), •, 1 wt%), although even at 10 wt% polymer the very high apparent viscosity of the emulsion is still not sufficient to prevent the rearrangement completely. It is interesting to compare the above results with those obtained (8,13) for similar emulsions where dextran is replaced by different microbial polysaccharides, xanthan or rhamsan. Whereas dextran in aqueous solution at a relatively low concentration (say, 0.5 wt %) is a Newtonian solution of viscosity not much greater than that for water, an equivalent solution of xanthan or rhamsan is extremely shear-thinning with a limiting zero-shear-rate viscosity of the order of 10 -10 times larger. This shear rheological behavior of the polysaccharide solution containing 0.5 wt % xanthan or rhamsan confers excellent creaming stability on emulsions prepared with nonionic surfactant, anionic surfactant, or protein as emulsifier (8,13-17). At much lower concentrations of xanthan or rhamsan (say, 0.05 wt %) when the shear rheological properties of the continuous phase are not so predominant, the emulsions exhibit evidence for the same extensive depletion flocculation and greatly enhanced serum separation as found with the dextran-containing systems. This behavior is illustrated by the data in Figure 3 for the effect of xanthan added after emulsification on the 2

3

4

6

6

7


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DICKINSON

Biopolymer Interactions in Emulsion Systems


100000 i

0.001 -I—

' " I

I

0.001

0.01

'

0.1

I

—

1

i

10

Shear stress / Pa

Figure 2. Influence of added dextran on the rheology of oil-in-water emulsions (30 wt % oil, 1 wt % Tween 20, pH 7, ionic strength 0.05 M, d = 0.55 pm). The apparent shear viscosity is plotted against shear stress at 25 °C: A, A, 0.5 wt %; • , 0, 1 wt%; • , • , 10 wt%. The solid symbols refer to the controlled stress experiments. The open symbols refer to the controlled strain-rate experiments. Dashed line refers to the system without dextran. Reproduced with permission from reference 7. 32

Percentage serum phase

80 --

0

01 .

02 . 03 .

xanthan concentration (wt%)

Figure 3. Influence of added xanthan on the creaming of oil-in-water emulsions (20 vol % oil, 2 wt % Tween 20, d = 0.54 pm). The serum volume fraction (expressed as a percentage of total emulsion volume) is plotted against the polysaccharide concentration in the aqueous phase following storage at 30 °C for 2 days (o) and 15 days (•). 32


201


202

creaming stability at 30 °C of sunflower oil-in-water emulsions (20 vol% oil, 2 wt% Tween 20, d = 0.54 pm) (18). The results show that, whereas serum separation is rather limited at very low xanthan concentrations (< 0.03 wt %), it is very rapid and extensive over the concentration range 0.05-0.2 wt %; however, at higher polymer contents (> 0.2 wt %) there is a slowing down again of the rate of serum separation, and creaming effectively stops due to rheological control at a xanthan content of ca. 0.3 wt %. Qualitatively similar behavior can be found when the small-molecule surfactant is replaced by a protein emulsifier such as sodium caseinate (15 J 6) or the microbial polysaccharide is replaced by some other type of non-adsorbing hydrocolloid. For instance, Figure 4 shows some data for the effect of guar gum added after emulsification on the creaming stability at 5 °C of a triglyceride oil-inwater emulsion (10 vol% oil, 1 wt% sodium caseinate, pH 6.6, d 0.54 pm) (19). We can see that there is an optimum hydrocolloid content (~ 0.1 wt%) giving the greatest extent of destabilization of the emulsion. Substantially below this optimum value, the creaming stability is greater because the flocculation is weaker, and substantially above the optimum the creaming stability is greater because the flocculation is strong and the mechanical structure of the emulsion resists the gravitational settling (as with the higher concentration dextran systems in Figure 1).


32

=

32

Weakly Attractive Droplet-Polysaccharide Interactions Small deformation rheological behavior has been investigated (20) for concentrated oil-in-water emulsions prepared with protein emulsifier (bovine serum albumin (BSA), P-lactoglobulin or sodium caseinate) and containing various concentrations of anionic dextran sulphate (DS) (5 x 10 daltons) added after emulsification. These particular mixed biopolymers systems were chosen for detailed rheological study following the observations (5,21,22) that the two globular proteins BSA and Plactoglobulin behave differently with respect to their stability properties in emulsion systems containing dextran sulphate at neutral pH. Together with evidence from complementary surface shear viscosity experiments (6,22,23), the earlier results can be interpreted in terms of interfacial electrostatic complexation between BSA and DS at neutral pH, but not between P-lactoglobulin and DS under similar solution conditions. Figure 5 shows the effect of DS concentration on the complex shear modulus G* measured at 30 °C and 1 Hz for oil-in-water emulsions (40 wt% w-tetradecane, 2.6 wt% protein, pH 7, ionic strength 0.005 M) having a mean volume-surface diameter of d « 0.6 pm. Here, for convenience, the added polysaccharide concentration is expressed in terms of a (reduced) DS surface coverage r s> where r s 1 corresponds to an added DS amount equal to that required to cover completely all the surface of the emulsion droplets to an arbitrary surface concentration of 2 mg m (assuming all the DS is adsorbed). The results in Figure 5 indicate that the sensitivity of the emulsion rheology to the polysaccharide is very much dependent on the nature of the protein emulsifier present. With the BSA emulsion, addition of DS was found to produce (a) a large increase in G* at very low r > (b) a maximum complex modulus of G* « 150 Pa at r « 0.1, and (c) an approximately constant modulus (G* = 50 ±30 Pa) for r > 0.3. With the P-lactoglobulin emulsion, there was found 5

32

=

D

D

- 2

DS

D S

D S


16. DICKINSON


thickness (cm)

20 ±


15

time (days)

Figure 4. Influence of added guar gum on the creaming of oil-in-water emulsions (10 vol% oil, 1 wt% sodium caseinate, pH 6.6, d - 0.54 pm). The serum layer thickness is plotted against the storage time at 5 °C: o, 0.005 wt%; •, 0.01 wt%; A, 0.1 wt%; A, 0.2 wt%. Dashed line denotes total sample height. 32

300 J_

0

0.2

0.4

0.6

0.8

1

Dextran sulphate coverage

Figure 5. Influence of added dextran sulphate on shear rheology of oil-in-water emulsions (40 vol % oil, 2.7 wt % protein, pH 7.0, ionic strength 0.005 M, d 0.6 pm). The complex shear modulus G* at 1 Hz is plotted against the (reduced) polysaccharide surface coverage r : • , BSA; • , P-lactoglobulin; A, caseinate. w

32

D S


204


to be no significant change in G* for r < 0.2, and just a gradual increase in modulus up to G* » 50 Pa at r = 1. And, with the sodium caseinate emulsion, there was found to be a steady increase in G* from r s = 0 to r s « 0.5 with an approximately constant value of G* » 250 Pa reached at the higher levels of polysaccharide addition. The large increase in shear modulus of the BSA emulsion at very low added polymer concentrations—well below that required for full surface coverage—is consistent with droplet flocculation by a bridging mechanism (24). It has been noted that the addition of DS at r = 0.1 to dilute BSA emulsions at pH 7 leads to a substantial increase in the measured d^ value, indicative of bridging flocculation of protein-coated droplets by the added biopolymer, whereas addition at r s 5 (i.e. well above saturation coverage) gives no change in d 2 and hence no flocculation. Restabilization at high DS concentrations can be explained in terms of a complete secondary stabilization layer of the highly charged polysaccharide adsorbed on top of the original protein layer. Additional evidence for an attractive interaction at pH 7 between DS and adsorbed BSA is available from separate measurements of surface viscosity (22,23), electrophoretic mobilities (5,6), emulsion stability (22), and foam stability (25). It has been suggested (6,22) that, even though both biopolymers carry a net negative charge at neutral pH, the origin of the attractive interaction is predominantly electrostatic between the highly charged anionic polysaccharide and small positive patches on the globular protein. This explanation is consistent with the observed reduction in the maximum value of G* on increasing the ionic strength from 5 mM to 70 mM by addition of sodium chloride (20). When experiments identical to those described above were repeated with (3lactoglobulin instead of BSA, no evidence was found (6,21,25) for any significant attractive interaction of DS with the adsorbed protein layer at pH 7. The absence of any bridging flocculation at low r in the concentrated emulsions containing Plactoglobulin + DS is confirmed by the small-deformation rheological data in Figure 5. There are various differences in molecular properties between BSA and Plactoglobulin which may account for the difference in interaction with the anionic polysaccharide. It is known (26) that P-lactoglobulin monomer (18.4 kDa) has just one strong binding site for surfactants, whereas the larger BSA monomer (66.2 kDa) has up to 12 such sites (27). In addition, at pH 7, there are a much greater number of positively charged residues (e.g. 59 lysines) on the BSA molecule as compared with the P-lactoglobulin molecule, which has 15 lysines, of which some are buried in the interior of the molecule (28). Clustering of this smaller number of positive groups into patches on the surface of the p-lactoglobulin may be insufficient to generate complexation with the negatively charged DS under these conditions. It is interesting to note that the effect of polysaccharide on the rheology of the sodium caseinate emulsion is different from that found with either of the globular proteins. Whilst the data in Figure 5 show no evidence for bridging flocculation of the casein emulsions at r « 0.1, the relatively high value of G* for r s ^ 0.5 is certainly indicative of some substantial protein-polysaccharide interaction. The 'lag phase' in the plot may be indicative of some sort of cooperativity in the interaction. The structure obtained here is rather insensitive to disturbance by high shear fields, in contrast to the bridging floes which are irreversibly disrupted by high shear (20). The D S

D S

D

D

D S


=

D

3

D S

D S

D


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300

Dextran sulphate coverage Figure 6. Effect of ionic strength on rheology of caseinate-stabilized emulsions (40 vol % oil, 2.7 wt % protein, pH 7.0, d = 0.58 pm) with added dextran sulphate. The complex shear modulus G* at 1 Hz is plotted against the (reduced) polysaccharide surface coverage T : •, 5 mM imidazole; • , 5 mM imidazole adjusted with NaCl to ionic strength 70 mM; A, 50 mM phosphate buffer. 32

DS

electrostatic origin of the casein-DS interaction at pH 7 is seemingly indicated by the large reduction in G* on increasing the ionic strength from 5 mM to 50 or 70 mM as shown in Figure 6. As there is no evidence in this case for a specific attractive interaction, it could be that the origin of the high G* in the DS + casein emulsion is due to structuring and possible thermodynamic incompatibility (29) in the aqueous phase under the influence of electrostatic repulsion between charged biopolymers present at high concentration. Such interactions would be expected to be reduced by increasing the ionic strength (Figure 6) or by lowering the pH towards the protein's isoelectric point (Figure 7). The preceding set of results for emulsion systems containing mixed biopolymers with weak electrostatic protein-polysaccharide interactions shows that such systems are extremely sensitive to the nature of the protein adsorbed layer and to the aqueous solution conditions. It is clear that small-deformation rheology is an extremely sensitive and powerful technique for monitoring the biopolymer interactions and the mechanisms of flocculation in concentrated emulsions containing mixtures of proteins and polysaccharides. The reason for this sensitivity is that changes in smalldeformation rheological behavior are highly responsive to changes in large-scale network formation in concentrated colloidal systems. Statistical mechanical theory shows (30) that the addition of a low concentration of weakly interacting polymer, modeled as small weakly adsorbing spheres that can fit into the gaps between larger spherical oil droplets, can produce a very substantial enhancement of the gel-like character of concentrated emulsions. Depending on whether the droplet-polymer pair


206



300 -

Dextran sulphate coverage Figure 7. Effect of pH on shear rheology of caseinate-stabilized emulsions (40 vol % oil, 2.7 wt % protein, 5 mM imidazole, d = 0.58 pm) with added dextran sulphate. The complex shear modulus G* at 1 Hz is plotted against the (reduced) polysaccharide surface coverage T s- •» pH 7.0; • , pH 6.7. 32

D

interaction is weakly attractive or weakly repulsive, reversible (weak) gel formation may occur by either bridging flocculation or depletion flocculation. This article has presented several examples of these two types of flocculation behavior as induced by the presence of polysaccharides. Acknowledgment Supportfromthe Biotechnology and Biological Sciences Research Council (U.K.) is gratefully acknowledged. Literature Cited 1.

2.

3. 4. 5.

Dickinson, E. In Gums and Stabilisers for the Food Industry; Phillips, G. O.; Wedlock, D. J.; Williams, P. A., Eds; IRL Press: Oxford, 1988; Vol. 4, pp. 249-263. Dickinson, G.; Stainsby, G. In Advances in Food Emulsions and Foams; Dickinson, E.; Stainsby, G., Eds; Elsevier Applied Science: London, 1988; pp. 1-44. Dickinson, E.; Euston, S. R. In Food Polymers, Gels and Colloids; Dickinson, E., Ed.; Royal Society of Chemistry: Cambridge, UK, 1991; pp. 132-146. Dickinson, E. J. Chem. Soc. Faraday Trans. 1992, 88, 2973-2983. Dickinson, E.; McClements, D. J. Advances in Food Colloids; Blackie A & P: Glasgow, 1995; pp. 81-101.


16. DICKINSON Biopolymer Interactions in Emulsion Systems 207 6. 7. 8. 9. 10.


11. 12. 13. 14. 15. 16. 17.

18. 19. 20.

21. 22.

23. 24. 25.

26. 27. 28. 29. 30.

Dickinson, E. In Biopolymer Mixtures; Mitchell, J. R., Ed.; Nottingham University Press, Nottingham, 1995; pp. 349-372. Dickinson, E.; Goller, M. I.; Wedlock, D. J. J. Colloid Interface Sci. 1995, 172, 192-202. Dickinson, E.; Ma, J.; Povey, M. J. W. Food Hydrocoll. 1994, 8, 481-497. Vrij, A. Pure Appl. Chem. 1976, 48, 471-183. Fleer, G. J.; Scheutjens, J. H. M. H.; Vincent, B. In Polymer Adsorption and Dispersion Stability; Goddard, E. D.; Vincent, B., Ed.; ACS Symposium Series No. 204; American Chemical Society: Washington, DC, 1984; p. 245. Vincent, B.; Edwards, J.; Emmett, S.; Croot, R. Colloids Surf. 1988, 31, 267-298. Lekkerkerker, H. N. W.; Stroobants, A. Physica A 1993, 195, 387-397. Dickinson, E.; Goller, M. I.; Wedlock, D. J. Colloids Surf. A 1993, 75, 195201. Gunning, P. A.; Hibberd, D. J.; Howe, A. M.; Robins, M. M. Food Hydrocoll. 1988, 2, 119-129. Cao, Y.; Dickinson, E.; Wedlock, D. J. Food Hydrocoll. 1990, 4, 185-195. Cao, Y.; Dickinson, E.; Wedlock, D. J. Food Hydrocoll. 1991, 5, 443-454. Luyten, H.; Jonkman, M.; Kloek, W.; van Vliet, T. In Food Colloids and Polymers: Stability and Mechanical Properties; Dickinson, E.; Walstra, P., Eds; Royal Society of Chemistry: Cambridge, UK, 1993; pp. 224-234. Dickinson, E.; Ma, J.; Povey, M. J. W. J. Chem. Soc. Faraday Trans. 1996, 92, in press. Heeney, L. M. Phil. Thesis, University of Leeds, 1994. Dickinson, E.; Pawlowsky, K. In Gums and Stabilisers for the Food Industry; Phillips, G. O.; Wedlock, D. J.; Williams, P. A., Eds; Oxford University Press: Oxford, 1996; Vol. 8, in press. Dickinson, E.; Galazka, V. B. Food Hydrocoll. 1991, 5, 281-296. Dickinson, E.; Galazka, V. B. In Gums and Stabilisers for the Food Industry; Phillips, G. O.; Wedlock, D. J.; Williams, P. A., Eds; IRL Press: Oxford, 1992; Vol. 6, pp. 351-362. Larichev, N. A.; Gurov, A. N.; Tolstoguzov, V. B. Colloids Surf 1983, 6, 27-34. Dickinson, E.; Eriksson, L. Adv. Colloid Interface Sci. 1991, 34, 1-29. Izgi, E.; Dickinson, E. In Food Macromolecules and Colloids; Dickinson, E.; Lorient, D., Eds; Royal Society of Chemistry, Cambridge, UK, 1995; pp. 312-315. Coke, M.; Wilde, P. J.; Russell, E. J.; Clark, D. C. J. Colloid Interface Sci. 1990, 138, 489-504. Morr, C. V.; Ha, E. Y. W. Crit. Rev. Food Sci. Nutr. 1993, 33, 431-476. Brown, E. M.; Pfeffer, P. E.; Kumosinski, T. F.; Greenberg, R. Biochemistry 1988, 27, 5601-5610. Dickinson, E. In Food Polysaccharides and their Applications; Stephen, A. M., Ed.; Marcel Dekker, New York, 1995; pp. 501-515. Dickinson, E. J. Chem. Soc. Faraday Trans. 1995, 91, 4413-1417.


Biopolymer Interactions in Emulsion Systems: Influences on Creaming

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