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Flaxseed gums and their adsorption on whey protein-stabilized oil-in-water emulsions
Food Hydrocolloids 23 (2009) 611–618
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Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd
Flaxseed gums and their adsorption on whey protein-stabilized oil-in-water emulsions Seddik Khalloufi, Milena Corredig, H. Douglas Goff, Marcela Alexander* Department of Food Science, University of Guelph, Guelph, Ontario, N1G 2W1 Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2008 Accepted 28 April 2008
The effect of the addition of flaxseed gum on the physicochemical properties of whey protein-stabilized (WPI) oil-in-water emulsions at pH 3.5 was investigated. Two different varieties (Emerson and McDuff) were tested at concentrations ranging from 0% to 0.33% (w/v), by measuring droplet size, z-potential, phase separation behavior, microstructure and apparent viscosity. With addition of flaxseed gum the zpotential of the droplets decreased from around þ30 mV to a negative value (10 mV) at concentrations >0.2%. These results indicated that the negatively charged polysaccharide fraction from flaxseed interacted with the protein adsorbed at the interface. An increase in apparent particle size was also noted with increasing flaxseed concentration, with destabilization becoming visually evident at concentrations higher than 0.1% (w/v). Microscopy, rheological data and size distribution analysis demonstrated for the first time that flaxseed gum interacts with protein-stabilized oil droplets at low pH, causing bridging flocculation. No significant differences were noted between flaxseed gums extracted from the Emerson and McDuff varieties. This research demonstrated that the electrostatic interactions between flaxseed gums and protein-stabilized emulsions need to be controlled when designing novel acidic beverages containing these polysaccharides. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Flaxseed gum Zeta potential Food emulsion Particle size Flocculation Creaming
1. Introduction Food emulsions of the oil-in-water type are often stabilized by proteins. Temperature, pH, ionic strength, protein concentration and protein/oil ratio, and oil volume fraction are among the major parameters that affect the physical properties of the emulsion (Guzey & McClements, 2006; McClements, 2005; Walstra, 2003). Polysaccharides are often added in food emulsions to further control the bulk properties of the final product. While non-interacting polysaccharides are added mostly to increase the viscosity of the solution, charged polysaccharides, under certain conditions, can interact with the charged proteins on the surface of the emulsion droplets. These interactions between polysaccharides and proteins at the interface can be controlled to increase the thickness of the surface layer surrounding the droplets and to create multilayer surfaces. Indeed, the electrostatic attraction between the oppositely charged biopolymers is the driving force for multilayer buildup in emulsion systems (Decher, 1997). Changes in environmental conditions, for example pH or ionic strength, can strongly affect the interactions occurring between polysaccharides and proteins at the interface (Bonnet, Corredig, &
* Corresponding author. Tel.: þ1 519 824 4120x56101; fax: þ1 519 824 6631. E-mail address: [email protected] (M. Alexander). 0268-005X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2008.04.004
Alexander, 2005; Gancz, Alexander, & Corredig, 2006; Roudsari, Nakamura, Smith, & Corredig, 2006). At a pH below their isoelectric point, the proteins adsorbed on the oil droplet will be positively charged, and negatively charged polysaccharides will form complexes at the interface. Although the effect of charged polysaccharides such as pectin (Bonnet et al., 2005; Gancz et al., 2006; Roudsari et al., 2006), carrageenan (Alexander & Dalgleish, 2007), alginate, carrageenan and gum arabic (Harnsilawat, Pongsawatmanit, & McClements, 2006), and xanthan gum (Sun, Gunasekaran, & Richards, 2007) on the particle size and bulk properties of proteinstabilized emulsions has been described, the effect of the addition of flaxseed gum is not known. Whey protein isolate (WPI) is one of the protein preparations most used in food emulsions. The major whey proteins of milk present in WPI have very good emulsifying properties, as they readily adsorb onto the oil droplet surfaces protecting them against coalescence. The physicochemical properties of WPI and its interaction with different polyelectrolytes have been widely studied (Gancz, Alexander, & Corredig, 2005; Gancz et al., 2006; ten Grotenhuis, Paques, & van Aken, 2000; Kim, Decker, & McClements, 2006, Sun et al., 2007; Weinbreck, Rollema, Tromp, & de Kruif, 2004). Flaxseed gums are commonly employed in the cosmetic industry as texturing agents; however, in the food industry, their application has not yet been extensively examined. Polysaccharides
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extracted from flaxseed have shown promise as a novel food ingredient, however, very little is understood on its effect when added to food emulsions. Flaxseeds are composed of about 40% fat, 28% dietary fiber and 20% protein (Flax Council of Canada). Epidemiological studies, animal experiments and human clinical trials suggest a role for flax in the prevention and treatment of chronic diseases/health problems such as heart disease (Zhao et al., 2007), diabetes (Hilpert et al., 2007), cancers (Lord, Bongiovanni, & Bralley, 2002) and osteoporosis (Griel et al., 2007). Flaxseed gum can be easily extracted with water either from the fiber portion or from the whole seed (Cui, Mazza, Oomah, & Biliaderis, 1994b; Warrand et al., 2003). The extraction yield, the level of protein, and the physicochemical properties of gum are a function of temperature, pH, ratio of water to seeds, duration of the extraction and variety of the raw material (Cui & Mazza, 1996; Cui et al., 1994b; Fedeniuk & Biliaderis, 1994; Oomah, Kenaschuk, Cui, & Mazza, 1995). The major monosaccharides in flaxseed gum are L-galactose, D-xylose, L-rhamnose, and D-galacturonic acid (Oomah et al., 1995). Other constituents such as L-arabinose, D-glucose, and L-fucose are also present in the gum but in smaller amounts (Oomah et al., 1995). Flaxseed gum contains a neutral and an anionic fraction (Cui, Mazza, & Biliaderis, 1994a; Fedeniuk & Biliaderis, 1994; Oomah et al., 1995; Warrand et al., 2003, 2005a, 2005b). According to the percentage of these constituents, the gum can be classified as more acidic or more neutral. Unpublished results from our laboratory show that the gum extracted from McDuff has a higher acidic/neutral ratio than that extracted from the Emerson variety. The neutral fraction is represented by the B-D-(1 / 4)-xylan backbone of the arabinoxylan component, with L-arabinose and D-galactose attached in position 2/ 3 of the side chains (Cui et al., 1994a). The acidic fraction consists of pectin-like polysaccharides containing L-rhamnose, D-galactose and D-galacturonic acid (Cui et al., 1994a). So far, very limited data are available on the behavior of flaxseed gum when used in proteinstabilized emulsions. Therefore, a better understanding of the interactions between this polysaccharide and other ingredients is required before considering the use of flaxseed gum in food systems. The objective of this study was to assess the behavior of flaxseed gums when mixed with a whey protein-stabilized oil-in-water emulsion at acidic pH. A pH of 3.5 was chosen partly to be in the same pH range as some commercial drinks and partly for direct comparison to already published data. The choice of the two varieties of flaxseed gums used in this work, Emerson and McDuff, was based on their differences in terms of acidic and neutral fractions. A model oilin-water emulsion stabilized by WPI at pH 3.5 was employed for this study. The results of this study will give practical information on how to incorporate this novel polysaccharide with a positive nutritional image in food beverages already fortified with proteins. 2. Materials and methods
7.0, followed by gentle stirring overnight. The solution was filtered through a 0.8 mm filter (Millex-HV Millipore Co., Billerica, MA). To inhibit microbial growth, sodium azide (0.02 wt %) was added to the buffer. A 12% (v/v) stock emulsion was prepared by mixing soybean oil with the WPI solution using a high-speed-blender (PowerGen 125 Fisher Scientific, Co., Nepean, ON) for 1 min, followed by homogenization (Emulsiflex C5, Avestin, Ottawa, ON) with five passes at 40 MPa. After homogenization, the pH of the emulsion was adjusted to 3.5 by addition of HCl and the emulsion was stored overnight at 4 C. Flaxseed gum extracted from Emerson and McDuff varieties was dissolved (1% w/v) in 20 mM imidazole buffer at pH 7.0 and stirred overnight. The pH of the gum solution was adjusted to 3.5 with HCl, and then added to the emulsion. Since GalA can be neutralized at pH of 2.9–3.2 and since it is the predominant uronic acid in the gum, the flax acidic fraction would also be neutralized at pH 2.9– 3.2%. Therefore, at this working pH, the flaxseed gums will still have a negative charge. Mixtures were prepared in a range of concentration from 0% to 0.33% (w/v) flaxseed gum, maintaining the oil and protein concentrations constant (9% (v/v) oil, 0.6% (w/v) WPI). 2.3. Emulsion droplet size measurements Measurements of size (Z-average) and z-potential were carried out using dynamic light scattering (Zetasizer Nano, Malvern Instruments, Worcestershire, UK). Each measurement was obtained from the average of three readings of two sub-samples. The z-potential provides an estimate of the net charge on droplets measured at the shear plane (Surh, Decker, & McClements, 2006). This zpotential is directly related to the charge of the droplets, emulsifier and biopolymer as well as the charge associated with any ions that move along with the droplet in the electric field (Surh et al., 2006). The apparent diameter of the oil droplets was determined using dynamic light scattering as Z-average diameter (D), which gives information about the hydrodynamic size of particles in suspension. The dilution ratio of emulsion to imidazole buffer at pH 3.5 was 1:2000 to avoid multiple scattering and the sample was placed in the spectrometer immediately after dilution. The particle size distribution of the emulsions was measured using integrated light scattering (Mastersizer X, Malvern Southborough, MA). Emulsion droplets were diluted in the same imidazole buffer as described earlier with pH 3.5, and a refractive index of 1.46 and 1.33 were used for the oil droplet and the solvent, respectively. The size distribution of the particles as a function of concentration of flaxseed gum added was taken as an indication of stability of the system. A monomodal distribution of size with a small average diameter commonly indicates a stable system. Each measurement was obtained from the average of three readings of two separated sub-samples.
2.1. Materials 2.4. Apparent viscosity Whey Protein Isolate (WPI, Alacen, 895) was donated by Fonterra (Mississauga, Ontario, Canada). The Emerson and McDuff flaxseed gums were supplied by Modern Research Station, Cereal Research Center, Agriculture and Agri-Food Canada (Manitoba, Canada); 15% seed/water ratio, at room temperature soaked overnight, was used for the extraction as described by Cui et al. (1994b). Soybean oil was purchased from Sigma (Louis, MO). Imidazole, Sodium azide, HCl and NaOH were analytical grade and were obtained from Fisher Scientific Co. (Atlanta, GA). MilliQWater was used to prepare all solutions.
Rheological measurements were carried out to determine the apparent viscosity of the system as a function of gum concentration. The measurements were made using a TA Instruments AR2000 (New Castle, Delaware USA) equipped with cone and plate (40 mm angle 2 ). The apparent viscosity was measured at 25 C and the samples were placed in the rheometer immediately after the addition of the gum. The apparent viscosity was measured at a shear stress of 0.05 Pa.
2.2. Sample preparation
2.5. Light microscopy observations
A stock solution of WPI (1.1%) was prepared by dissolving the required quantity of protein in the 20 mM imidazole buffer at pH
The microstructure of the emulsions as a function of different flaxseed gum concentration was assessed using phase contrast
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2.6. Kinetics of phase separation Immediately after addition of the gum, emulsion mixtures (10 ml) were poured in 15 ml-graduated plastic tubes. The tubes were sealed and stored quiescently at 4 C. The position of the creaming boundary was monitored daily for 28 days. The kinetics of phase separation was expressed as a function of the variation of serum volume, which is represented by the aqueous phase at the bottom of the tubes.
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microscopy (Olympus optical BX60F5, Japan). Before observation, the sample was gently mixed; aliquots (5 ml) of the emulsion were placed on a microscope slide and covered with a cover slide. The images of the sample were captured using a digital camera directly connected to the computer equipped with image processing software (Image Pro Plus, version 6. Media Cybernetics, Inc. Bethesda, USA).
3. Results and discussion The isoelectric point of WPI is around pH 5 (McClements, 2005), hence at pH 3.5 the oil droplets will be positively charged. As stated earlier, the flaxseed gum has a negative charge at this pH. Fig. 1 shows the z-potential of the emulsion droplets at pH 3.5 and as a function of polysaccharide concentration for the two gum varieties. Without any addition of gum, WPI-coated oil droplets show a positive charge of about þ33 mV. A small amount of polysaccharide decreases significantly the z-potential. Moreover, 0.005% of the McDuff gum decreased the particle charge by 86% while the same amount of Emerson gum decreased the z-potential by only 20% (Fig. 1). With the addition of the polysaccharide, the zpotential progressively moves from positive to negative values for both gum varieties. The charge remains positive for concentration <0.15% of polysaccharide. At higher concentrations of polysaccharides the emulsion droplets become negatively charged with values around 8 to 10 mV. This charge reversal on both varieties is the result of the interaction between the negatively charged gum onto the surface layer of the positively charged proteins (Decher, 1997; Roudsari et al., 2006). This finding is in agreement with the results reported by Gancz et al. (2005, 2006), Gu, Decker, & McClements (2007) and Harnsilawat et al. (2006) who studied the behavior of some polyelectrolytes on food emulsions at low pH containing negatively charged polysaccharides. In our study, charge neutralization is reached at a higher concentration of polysaccharide with the Emerson gum than with the McDuff gum. In addition, the change of z-potential is smooth and progressive in the case of Emerson (Fig. 1A), whereas in the case of McDuff (Fig. 1B) the initial slope is steep. Fig. 2 depicts the effect of gum concentrations on the apparent average size of the oil droplet measured by DLS. The insets show a detailed view of the droplet sizes up to 0.10% gum. In both Emerson and McDuff gum varieties, the particle size increases progressively with increasing polysaccharide concentration.
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2.7. Experimental design and statistical analysis
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The samples were kept at 4 C until use. All measurements were carried out at room temperature and all samples were gently mixed just prior to the experiment. Each experiment was replicated three times (in the case of Emerson) and two times (in the case of McDuff) using freshly prepared samples. The values reported are the means and standard deviation of these repetitions. An analysis of variance was performed using the General Linear Model procedure of the SAS statistical analysis software package (version 8.01 SAS Institute, Cary, NC 27513). Differences were considered significant at p < 0.05.
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Although in emulsions containing Emerson variety a six-fold growth in the average size is shown already at very low gum concentrations (Fig. 2A), statistical analysis reveals that the diameter is significantly different only at 0.1%. In contrast, the McDuff variety (Fig. 2B) shows a much slower growth in size, and with 0.10% polysaccharide, the average size is only half of that of the emulsions containing the Emerson gum variety. The difference in the standard error at low concentrations between the two varieties indicates a rather polydisperse sample in the case of the emulsions containing Emerson flaxseed gum while still quite monodisperse for the McDuff. At high concentrations, the high measurement errors are also a clear indication that the system now has a wide size distribution due to flocculation of the oil droplets. While the Emerson variety shows a continuous growth as a function of polysaccharide concentration, the McDuff variety shows an abrupt nine-fold increase in size between 0.10% and 0.15% gum, reaching a plateau at this concentration. This increase in size is a clear indication of the interaction of the polysaccharide with the WPI-covered oil droplets and corresponds directly to the differences in the drop of z-potential described in Fig. 1. Emerson gum shows a gentle but progressive decrease in the potential of the oil droplets accounting for the slower growth in size. McDuff instead, shows an abrupt decrease in potential thereby causing the droplets to flocculate rapidly. Furthermore, the significant change in average diameter occurs at about 0.15% gum, the concentration needed to
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achieve a charge near neutral (see z-potential data on Fig. 1). It is also possible to hypothesize that while the McDuff flaxseed gum is interacting strongly with the proteins at the surface of the droplet (attaching themselves onto the surface), thereby leading to a progressive but small increase of the particle size, in the case of Emerson gum the low net charge of the gum probably results in a more expanded conformation and more interactions of the flaxseed gum with proteins on more than one droplet surface (hence the higher standard error in the measurement of apparent diameter). It becomes clear that while at low concentration flaxseed gum interacts with the WPI on the surface of the oil droplets, destabilization occurs only when the charges have been neutralized. At higher amounts of flaxseed gum (>0.1%), bridging flocculation aids in the destabilization of the emulsion. At high concentrations (gum 0.15%), and for both polysaccharides, the particle size reached a plateau. Fig. 3 shows the apparent particle size distribution for both gums obtained by Mastersizer measurements. Due to the pumping action of this technique, the apparent floc sizes obtained will not correspond to the ‘‘actual’’ floc sizes that may happen in the sample or those measured by DLS (see below). However, this work is interested in the concentration-dependent differences between the two gums. Since the experimental protocol was similar with both gums, the results obtained are directly comparable. Overall, there are three ‘‘regions’’, depending on gum concentration (i) low concentrations, up to 0.017%, the
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Diameter (µm) Fig. 3. Particle size distribution of emulsions (9% soybean oil, 0.6% WPI, pH 3.5) as a function of added gum concentrations measured using integrated light scattering. Emerson (A) and McDuff (B).
distribution of the emulsion particles is monomodal with low particle size (z0.16 mm). This indicates that a critical amount of flaxseed gum needs to be added to cause emulsion destabilization. (ii) At intermediate concentrations (0.040.10%, the average particle size is much larger than in the original emulsion (>7.5 mm) and the emulsions show a monomodal distribution of droplet sizes. The bimodal distribution and the shift from small to high particle size is a clear indication of aggregation or bridging flocculation. Although the general trend and average sizes from Figs. 2 and 3 are quite comparable, they do not match exactly. The reason for this slight discrepancy is the nature of the measurements themselves. The Mastersizer is a static light scattering measurement while DLS is dynamic. This simply means that DLS measures the diffusion of the
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scattering particles in solution and calculates the apparent size associated to this diffusion via the Stokes-Einstein relation. The Mastersizer, instead, measures scattering as a function of angle and fits the intensity data to well-known theoretical models. In this case, the size will be weighted towards parts of the particle with the highest refractive index contrast. Therefore, a particle with dangling ‘‘hairs’’, for example, will have a dynamic apparent size much larger than its ‘‘static’’ Mastersize-measured size. Furthermore, DLS is optimal for the measurement of sizes up to a few microns, while Mastersizer works best on larger sizes (to 10s of microns). Nevertheless, the correspondence between our static and dynamic data is good. For the lower flaxseed concentration, the layer of protein on the surface of the oil droplets affects the diffusion of the particles. This results in a dynamic size slightly larger than that arising from the static measurements. At high gum concentration, the aggregates get too large to be properly measured by DLS. Furthermore, the presence of unaggregated particles will be picked up by DLS and not by the Mastersizer, skewing the results to lower values. Fig. 4 illustrates the degree of visual phase separation, reported as amount of serum present, with increasing gum concentration. At low concentrations (gum 0.1%), there was no visual creaming for emulsions containing either the Emerson or the McDuff gum throughout the 28 days of observation. These results confirmed the
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data on z-average diameter and z-potential, as a small amount of gum does not cause emulsion destabilization. At higher concentrations (gum 0.15%), a clear serum layer dividing top (opaque cream) from bottom (clear serum) becomes apparent after a few days of storage. This phase separation process is a clear indication of the emulsion’s instability. The kinetic stability of emulsions is controlled by the structural organization of the oil droplets and by the three parameters mainly involved in the Stokes’ law equation of terminal velocity, V (McClements, 2005):
2gr 2 ðrd rc Þ V ¼
mc
(3)
where r is the radius of the particle, g is the acceleration due to gravity, r is the density, m is the viscosity and the subscripts c and d refer to the continuous and dispersed phases, respectively (McClements, 2005). These results confirm what has already been shown in Figs. 1–3. At high concentrations (gum 0.15%) destabilization occurs mostly due to bridging flocculation. The highest value of creaming, about 40%, occurs at a concentration of 0.15%. At this concentration, flocs form and although some polysaccharide is present in the serum phase, the increase in viscosity due to the addition of gum is not yet high enough to significantly retard creaming. The extent of creaming after 28 days of storage is concentration-dependent: addition of more gum seems to decrease the final creaming value (Fig. 4). At the high concentration, more polysaccharide – most likely the neutral fraction – will be present in solution and increase the viscosity, causing a decrease in the creaming rate. This creaming retardation due to excess polysaccharide in solution has been reported before (ten Grotenhuis et al., 2000). Creaming occurred at a faster rate in emulsions containing flaxseed gum of the McDuff variety compared to the Emerson variety. More evidence on the extent of the instability of the emulsions and the difference between flaxseed extracted from different varieties can be derived from microstructural observations. Fig. 5 illustrates the results of optical microscopy observations. Although optical microscopy is not best-suited to follow size development of such small droplets, it nevertheless is a good and fast back-up technique to check the bulk appearance of this system. Therefore, not too much emphasis must be put on the individual appearance of the droplets (as it is quite common to obtain light artifacts) but rather look at the behavior of the overall system. The top four micrographs correspond to different concentrations of Emerson gum and the bottom four to different concentrations of McDuff. In the absence of gum, the images cannot clearly pick up individual droplets. This is due to the microscope’s resolution and points to the absence of large aggregates. Therefore, we can conclude that there is stability, as expected, in the emulsion with no added gum. Addition of flaxseed gum, either from Emerson or McDuff variety, causes microstructural changes. At 0.04%, the system containing McDuff gum seems to be slightly more uniform than that containing Emerson. Again, emphasizing that light microscopy must not be over-interpreted in these small-size regimes, these results seem to agree with both DLS and Mastersizer data which show a larger and bimodal distribution, respectively, for the Emerson gum at this concentration. At a concentration of 0.20% of either gum, the WPIstabilized emulsions show aggregation, although with no visual phase separation yet. This observation suggests that the system was no longer homogenous and that the emulsion had experienced some reorganization that led to flocculation/aggregationwithin the system. These results also correspond to those obtained with light scattering which show a markedly increase in size after the addition of both types of gum (Figs. 2 and 3). At high concentrations (gum ¼ 0.33%) the micrographs show large, distinct domains in the images. This corresponds to visual phase separation with droplet-
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Fig. 5. Phase contrast micrographs of emulsions at pH 3.5 containing different concentration of flaxseed gum for Emerson (top) and McDuff (bottom) varieties. Scale bar ¼ 50 mm.
rich regions within which oil emulsion droplets were entrapped and somewhat restricted in motion. Again here, these results correspond well with those of Figs. 2 and 3 in which the samples showed a large extent of aggregation. The impact of gum concentration on the apparent viscosity of the gum solutions (A) and the emulsion (B) is shown in Fig. 6. At very low concentrations of polysaccharide (<0.05%), the viscosity of the Emerson gum solution seemed not to be different from that of the McDuff solution. In contrast, at concentrations >0.10%, the viscosity of the Emerson gum solution is higher than that of the McDuff. This could explain why, in the mixed samples, the excess gum not attached to WPI-coated droplets decreased the rate of
creaming by increasing the background viscosity of the emulsion and decreasing the mobility of the droplets. Similarly, the emulsions containing Emerson gum exhibit a higher apparent viscosity than the emulsions containing McDuff. The cause of this difference is most likely caused by the higher ratio of neutral fraction in the Emerson flaxseed gum compared to that of the McDuff gum. However, the differences in the apparent viscosity of the emulsion with added Emerson or McDuff gum could also be attributed to the nature of the forces involved in the internal structure of the flocs. Visual observations (Fig. 4) demonstrated that phase separation is faster for emulsions containing McDuff gum than Emerson gum. When looking at the apparent viscosity at low stress for the whole
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Acknowledgments
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The authors wish to thank the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) for its financial support. The authors are also grateful to Dr. François Capel for the gum extraction.
References
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Gum concentration (%) Fig. 6. Effect of gum concentration for Emerson (C) and McDuff (-) varieties on the apparent viscosity. Flaxseed gum dispersed in buffer (A) or added to WPI-stabilized emulsions (B) (9% soybean oil, 0.6% WPI, pH 3.5). Viscosity measured at 0.05 Pa. Results are the average of at least two independent measurements.
emulsions, it is clear that up to 0.1% polysaccharide, the systems show low viscosity, while at high concentrations (gum 0.1%), both varieties (Emerson and McDuff) showed a significant increase in viscosity caused by bridging of droplets in large flocs.
4. Conclusions This study aimed at investigating the behavior of flaxseed gums when mixed with WPI-stabilized emulsion droplets at acidic pH. The results demonstrated the interaction and attachment of gums onto the surface of proteins due to electrostatic interaction between the oppositely charged molecules. Up to 0.1% concentration of gum, no visual phase separation was observed, demonstrating the kinetic stability of the emulsions. At higher concentrations of polysaccharide (gum 1.5%), the microstructure, particle size, particle size distribution and low-shear apparent viscosity of the emulsion begin to change and become more affected by the presence of the gum. These changes result in aggregation and phase separation. Despite the fact that Emerson and McDuff gums are different in terms of amount of acidic and neutral fractions, this study suggests that when these gums are added into emulsions, their overall effects are relatively similar. Our results show there is still much work to be done to fully understand and optimize the incorporation of these natural gums in food products.
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Warrand, J., Michaud, P., Picton, L., Muller, G., Courtois, B., Ralainirina, R., et al. (2003). Large-scale purification of water-soluble polysaccharides from flaxseed mucilage, and isolation of a new anionic polymer. Chromatographia, 58(5–6), 331–335. Warrand, J., Michaud, P., Picton, L., Muller, G., Courtois, B., Ralainirina, R., et al. (2005a). Flax (Linum usitatissimum) seed cake: a potential source of high molecular weight arabinoxylans? Journal of Agricultural and Food Chemistry, 53(5), 1449–1452. Warrand, J., Michaud, P., Picton, L., Muller, G., Courtois, B., Ralainirina, R., et al. (2005b). Structural investigations of the neutral polysaccharide of Linum
usitatissimum L. seeds mucilage. International Journal of Biological Macromolecules, 35(3–4), 121–125. Weinbreck, F., Rollema, H. S., Tromp, R. H., & de Kruif, C. G. (2004). Diffusivity of whey protein and gum arabic in their coacervates. Langmuir, 20(15), 6389–6395. Zhao, G., Etherton, T. D., Martin, K. R., Gillies, P. J., West, S. G., & Kris-Etherton, P. M. (2007). Dietary alpha-linolenic acid inhibits proinflammatory cytokine production by peripheral blood mononuclear cells in hypercholesterolemic subjects. The American Journal of Clinical Nutrition, 85(2), 385–391.
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Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd
Flaxseed gums and their adsorption on whey protein-stabilized oil-in-water emulsions Seddik Khalloufi, Milena Corredig, H. Douglas Goff, Marcela Alexander* Department of Food Science, University of Guelph, Guelph, Ontario, N1G 2W1 Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2008 Accepted 28 April 2008
The effect of the addition of flaxseed gum on the physicochemical properties of whey protein-stabilized (WPI) oil-in-water emulsions at pH 3.5 was investigated. Two different varieties (Emerson and McDuff) were tested at concentrations ranging from 0% to 0.33% (w/v), by measuring droplet size, z-potential, phase separation behavior, microstructure and apparent viscosity. With addition of flaxseed gum the zpotential of the droplets decreased from around þ30 mV to a negative value (10 mV) at concentrations >0.2%. These results indicated that the negatively charged polysaccharide fraction from flaxseed interacted with the protein adsorbed at the interface. An increase in apparent particle size was also noted with increasing flaxseed concentration, with destabilization becoming visually evident at concentrations higher than 0.1% (w/v). Microscopy, rheological data and size distribution analysis demonstrated for the first time that flaxseed gum interacts with protein-stabilized oil droplets at low pH, causing bridging flocculation. No significant differences were noted between flaxseed gums extracted from the Emerson and McDuff varieties. This research demonstrated that the electrostatic interactions between flaxseed gums and protein-stabilized emulsions need to be controlled when designing novel acidic beverages containing these polysaccharides. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Flaxseed gum Zeta potential Food emulsion Particle size Flocculation Creaming
1. Introduction Food emulsions of the oil-in-water type are often stabilized by proteins. Temperature, pH, ionic strength, protein concentration and protein/oil ratio, and oil volume fraction are among the major parameters that affect the physical properties of the emulsion (Guzey & McClements, 2006; McClements, 2005; Walstra, 2003). Polysaccharides are often added in food emulsions to further control the bulk properties of the final product. While non-interacting polysaccharides are added mostly to increase the viscosity of the solution, charged polysaccharides, under certain conditions, can interact with the charged proteins on the surface of the emulsion droplets. These interactions between polysaccharides and proteins at the interface can be controlled to increase the thickness of the surface layer surrounding the droplets and to create multilayer surfaces. Indeed, the electrostatic attraction between the oppositely charged biopolymers is the driving force for multilayer buildup in emulsion systems (Decher, 1997). Changes in environmental conditions, for example pH or ionic strength, can strongly affect the interactions occurring between polysaccharides and proteins at the interface (Bonnet, Corredig, &
* Corresponding author. Tel.: þ1 519 824 4120x56101; fax: þ1 519 824 6631. E-mail address: [email protected] (M. Alexander). 0268-005X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2008.04.004
Alexander, 2005; Gancz, Alexander, & Corredig, 2006; Roudsari, Nakamura, Smith, & Corredig, 2006). At a pH below their isoelectric point, the proteins adsorbed on the oil droplet will be positively charged, and negatively charged polysaccharides will form complexes at the interface. Although the effect of charged polysaccharides such as pectin (Bonnet et al., 2005; Gancz et al., 2006; Roudsari et al., 2006), carrageenan (Alexander & Dalgleish, 2007), alginate, carrageenan and gum arabic (Harnsilawat, Pongsawatmanit, & McClements, 2006), and xanthan gum (Sun, Gunasekaran, & Richards, 2007) on the particle size and bulk properties of proteinstabilized emulsions has been described, the effect of the addition of flaxseed gum is not known. Whey protein isolate (WPI) is one of the protein preparations most used in food emulsions. The major whey proteins of milk present in WPI have very good emulsifying properties, as they readily adsorb onto the oil droplet surfaces protecting them against coalescence. The physicochemical properties of WPI and its interaction with different polyelectrolytes have been widely studied (Gancz, Alexander, & Corredig, 2005; Gancz et al., 2006; ten Grotenhuis, Paques, & van Aken, 2000; Kim, Decker, & McClements, 2006, Sun et al., 2007; Weinbreck, Rollema, Tromp, & de Kruif, 2004). Flaxseed gums are commonly employed in the cosmetic industry as texturing agents; however, in the food industry, their application has not yet been extensively examined. Polysaccharides
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extracted from flaxseed have shown promise as a novel food ingredient, however, very little is understood on its effect when added to food emulsions. Flaxseeds are composed of about 40% fat, 28% dietary fiber and 20% protein (Flax Council of Canada). Epidemiological studies, animal experiments and human clinical trials suggest a role for flax in the prevention and treatment of chronic diseases/health problems such as heart disease (Zhao et al., 2007), diabetes (Hilpert et al., 2007), cancers (Lord, Bongiovanni, & Bralley, 2002) and osteoporosis (Griel et al., 2007). Flaxseed gum can be easily extracted with water either from the fiber portion or from the whole seed (Cui, Mazza, Oomah, & Biliaderis, 1994b; Warrand et al., 2003). The extraction yield, the level of protein, and the physicochemical properties of gum are a function of temperature, pH, ratio of water to seeds, duration of the extraction and variety of the raw material (Cui & Mazza, 1996; Cui et al., 1994b; Fedeniuk & Biliaderis, 1994; Oomah, Kenaschuk, Cui, & Mazza, 1995). The major monosaccharides in flaxseed gum are L-galactose, D-xylose, L-rhamnose, and D-galacturonic acid (Oomah et al., 1995). Other constituents such as L-arabinose, D-glucose, and L-fucose are also present in the gum but in smaller amounts (Oomah et al., 1995). Flaxseed gum contains a neutral and an anionic fraction (Cui, Mazza, & Biliaderis, 1994a; Fedeniuk & Biliaderis, 1994; Oomah et al., 1995; Warrand et al., 2003, 2005a, 2005b). According to the percentage of these constituents, the gum can be classified as more acidic or more neutral. Unpublished results from our laboratory show that the gum extracted from McDuff has a higher acidic/neutral ratio than that extracted from the Emerson variety. The neutral fraction is represented by the B-D-(1 / 4)-xylan backbone of the arabinoxylan component, with L-arabinose and D-galactose attached in position 2/ 3 of the side chains (Cui et al., 1994a). The acidic fraction consists of pectin-like polysaccharides containing L-rhamnose, D-galactose and D-galacturonic acid (Cui et al., 1994a). So far, very limited data are available on the behavior of flaxseed gum when used in proteinstabilized emulsions. Therefore, a better understanding of the interactions between this polysaccharide and other ingredients is required before considering the use of flaxseed gum in food systems. The objective of this study was to assess the behavior of flaxseed gums when mixed with a whey protein-stabilized oil-in-water emulsion at acidic pH. A pH of 3.5 was chosen partly to be in the same pH range as some commercial drinks and partly for direct comparison to already published data. The choice of the two varieties of flaxseed gums used in this work, Emerson and McDuff, was based on their differences in terms of acidic and neutral fractions. A model oilin-water emulsion stabilized by WPI at pH 3.5 was employed for this study. The results of this study will give practical information on how to incorporate this novel polysaccharide with a positive nutritional image in food beverages already fortified with proteins. 2. Materials and methods
7.0, followed by gentle stirring overnight. The solution was filtered through a 0.8 mm filter (Millex-HV Millipore Co., Billerica, MA). To inhibit microbial growth, sodium azide (0.02 wt %) was added to the buffer. A 12% (v/v) stock emulsion was prepared by mixing soybean oil with the WPI solution using a high-speed-blender (PowerGen 125 Fisher Scientific, Co., Nepean, ON) for 1 min, followed by homogenization (Emulsiflex C5, Avestin, Ottawa, ON) with five passes at 40 MPa. After homogenization, the pH of the emulsion was adjusted to 3.5 by addition of HCl and the emulsion was stored overnight at 4 C. Flaxseed gum extracted from Emerson and McDuff varieties was dissolved (1% w/v) in 20 mM imidazole buffer at pH 7.0 and stirred overnight. The pH of the gum solution was adjusted to 3.5 with HCl, and then added to the emulsion. Since GalA can be neutralized at pH of 2.9–3.2 and since it is the predominant uronic acid in the gum, the flax acidic fraction would also be neutralized at pH 2.9– 3.2%. Therefore, at this working pH, the flaxseed gums will still have a negative charge. Mixtures were prepared in a range of concentration from 0% to 0.33% (w/v) flaxseed gum, maintaining the oil and protein concentrations constant (9% (v/v) oil, 0.6% (w/v) WPI). 2.3. Emulsion droplet size measurements Measurements of size (Z-average) and z-potential were carried out using dynamic light scattering (Zetasizer Nano, Malvern Instruments, Worcestershire, UK). Each measurement was obtained from the average of three readings of two sub-samples. The z-potential provides an estimate of the net charge on droplets measured at the shear plane (Surh, Decker, & McClements, 2006). This zpotential is directly related to the charge of the droplets, emulsifier and biopolymer as well as the charge associated with any ions that move along with the droplet in the electric field (Surh et al., 2006). The apparent diameter of the oil droplets was determined using dynamic light scattering as Z-average diameter (D), which gives information about the hydrodynamic size of particles in suspension. The dilution ratio of emulsion to imidazole buffer at pH 3.5 was 1:2000 to avoid multiple scattering and the sample was placed in the spectrometer immediately after dilution. The particle size distribution of the emulsions was measured using integrated light scattering (Mastersizer X, Malvern Southborough, MA). Emulsion droplets were diluted in the same imidazole buffer as described earlier with pH 3.5, and a refractive index of 1.46 and 1.33 were used for the oil droplet and the solvent, respectively. The size distribution of the particles as a function of concentration of flaxseed gum added was taken as an indication of stability of the system. A monomodal distribution of size with a small average diameter commonly indicates a stable system. Each measurement was obtained from the average of three readings of two separated sub-samples.
2.1. Materials 2.4. Apparent viscosity Whey Protein Isolate (WPI, Alacen, 895) was donated by Fonterra (Mississauga, Ontario, Canada). The Emerson and McDuff flaxseed gums were supplied by Modern Research Station, Cereal Research Center, Agriculture and Agri-Food Canada (Manitoba, Canada); 15% seed/water ratio, at room temperature soaked overnight, was used for the extraction as described by Cui et al. (1994b). Soybean oil was purchased from Sigma (Louis, MO). Imidazole, Sodium azide, HCl and NaOH were analytical grade and were obtained from Fisher Scientific Co. (Atlanta, GA). MilliQWater was used to prepare all solutions.
Rheological measurements were carried out to determine the apparent viscosity of the system as a function of gum concentration. The measurements were made using a TA Instruments AR2000 (New Castle, Delaware USA) equipped with cone and plate (40 mm angle 2 ). The apparent viscosity was measured at 25 C and the samples were placed in the rheometer immediately after the addition of the gum. The apparent viscosity was measured at a shear stress of 0.05 Pa.
2.2. Sample preparation
2.5. Light microscopy observations
A stock solution of WPI (1.1%) was prepared by dissolving the required quantity of protein in the 20 mM imidazole buffer at pH
The microstructure of the emulsions as a function of different flaxseed gum concentration was assessed using phase contrast
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2.6. Kinetics of phase separation Immediately after addition of the gum, emulsion mixtures (10 ml) were poured in 15 ml-graduated plastic tubes. The tubes were sealed and stored quiescently at 4 C. The position of the creaming boundary was monitored daily for 28 days. The kinetics of phase separation was expressed as a function of the variation of serum volume, which is represented by the aqueous phase at the bottom of the tubes.
40
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microscopy (Olympus optical BX60F5, Japan). Before observation, the sample was gently mixed; aliquots (5 ml) of the emulsion were placed on a microscope slide and covered with a cover slide. The images of the sample were captured using a digital camera directly connected to the computer equipped with image processing software (Image Pro Plus, version 6. Media Cybernetics, Inc. Bethesda, USA).
3. Results and discussion The isoelectric point of WPI is around pH 5 (McClements, 2005), hence at pH 3.5 the oil droplets will be positively charged. As stated earlier, the flaxseed gum has a negative charge at this pH. Fig. 1 shows the z-potential of the emulsion droplets at pH 3.5 and as a function of polysaccharide concentration for the two gum varieties. Without any addition of gum, WPI-coated oil droplets show a positive charge of about þ33 mV. A small amount of polysaccharide decreases significantly the z-potential. Moreover, 0.005% of the McDuff gum decreased the particle charge by 86% while the same amount of Emerson gum decreased the z-potential by only 20% (Fig. 1). With the addition of the polysaccharide, the zpotential progressively moves from positive to negative values for both gum varieties. The charge remains positive for concentration <0.15% of polysaccharide. At higher concentrations of polysaccharides the emulsion droplets become negatively charged with values around 8 to 10 mV. This charge reversal on both varieties is the result of the interaction between the negatively charged gum onto the surface layer of the positively charged proteins (Decher, 1997; Roudsari et al., 2006). This finding is in agreement with the results reported by Gancz et al. (2005, 2006), Gu, Decker, & McClements (2007) and Harnsilawat et al. (2006) who studied the behavior of some polyelectrolytes on food emulsions at low pH containing negatively charged polysaccharides. In our study, charge neutralization is reached at a higher concentration of polysaccharide with the Emerson gum than with the McDuff gum. In addition, the change of z-potential is smooth and progressive in the case of Emerson (Fig. 1A), whereas in the case of McDuff (Fig. 1B) the initial slope is steep. Fig. 2 depicts the effect of gum concentrations on the apparent average size of the oil droplet measured by DLS. The insets show a detailed view of the droplet sizes up to 0.10% gum. In both Emerson and McDuff gum varieties, the particle size increases progressively with increasing polysaccharide concentration.
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2.7. Experimental design and statistical analysis
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The samples were kept at 4 C until use. All measurements were carried out at room temperature and all samples were gently mixed just prior to the experiment. Each experiment was replicated three times (in the case of Emerson) and two times (in the case of McDuff) using freshly prepared samples. The values reported are the means and standard deviation of these repetitions. An analysis of variance was performed using the General Linear Model procedure of the SAS statistical analysis software package (version 8.01 SAS Institute, Cary, NC 27513). Differences were considered significant at p < 0.05.
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Although in emulsions containing Emerson variety a six-fold growth in the average size is shown already at very low gum concentrations (Fig. 2A), statistical analysis reveals that the diameter is significantly different only at 0.1%. In contrast, the McDuff variety (Fig. 2B) shows a much slower growth in size, and with 0.10% polysaccharide, the average size is only half of that of the emulsions containing the Emerson gum variety. The difference in the standard error at low concentrations between the two varieties indicates a rather polydisperse sample in the case of the emulsions containing Emerson flaxseed gum while still quite monodisperse for the McDuff. At high concentrations, the high measurement errors are also a clear indication that the system now has a wide size distribution due to flocculation of the oil droplets. While the Emerson variety shows a continuous growth as a function of polysaccharide concentration, the McDuff variety shows an abrupt nine-fold increase in size between 0.10% and 0.15% gum, reaching a plateau at this concentration. This increase in size is a clear indication of the interaction of the polysaccharide with the WPI-covered oil droplets and corresponds directly to the differences in the drop of z-potential described in Fig. 1. Emerson gum shows a gentle but progressive decrease in the potential of the oil droplets accounting for the slower growth in size. McDuff instead, shows an abrupt decrease in potential thereby causing the droplets to flocculate rapidly. Furthermore, the significant change in average diameter occurs at about 0.15% gum, the concentration needed to
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achieve a charge near neutral (see z-potential data on Fig. 1). It is also possible to hypothesize that while the McDuff flaxseed gum is interacting strongly with the proteins at the surface of the droplet (attaching themselves onto the surface), thereby leading to a progressive but small increase of the particle size, in the case of Emerson gum the low net charge of the gum probably results in a more expanded conformation and more interactions of the flaxseed gum with proteins on more than one droplet surface (hence the higher standard error in the measurement of apparent diameter). It becomes clear that while at low concentration flaxseed gum interacts with the WPI on the surface of the oil droplets, destabilization occurs only when the charges have been neutralized. At higher amounts of flaxseed gum (>0.1%), bridging flocculation aids in the destabilization of the emulsion. At high concentrations (gum 0.15%), and for both polysaccharides, the particle size reached a plateau. Fig. 3 shows the apparent particle size distribution for both gums obtained by Mastersizer measurements. Due to the pumping action of this technique, the apparent floc sizes obtained will not correspond to the ‘‘actual’’ floc sizes that may happen in the sample or those measured by DLS (see below). However, this work is interested in the concentration-dependent differences between the two gums. Since the experimental protocol was similar with both gums, the results obtained are directly comparable. Overall, there are three ‘‘regions’’, depending on gum concentration (i) low concentrations, up to 0.017%, the
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Diameter (µm) Fig. 3. Particle size distribution of emulsions (9% soybean oil, 0.6% WPI, pH 3.5) as a function of added gum concentrations measured using integrated light scattering. Emerson (A) and McDuff (B).
distribution of the emulsion particles is monomodal with low particle size (z0.16 mm). This indicates that a critical amount of flaxseed gum needs to be added to cause emulsion destabilization. (ii) At intermediate concentrations (0.04
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scattering particles in solution and calculates the apparent size associated to this diffusion via the Stokes-Einstein relation. The Mastersizer, instead, measures scattering as a function of angle and fits the intensity data to well-known theoretical models. In this case, the size will be weighted towards parts of the particle with the highest refractive index contrast. Therefore, a particle with dangling ‘‘hairs’’, for example, will have a dynamic apparent size much larger than its ‘‘static’’ Mastersize-measured size. Furthermore, DLS is optimal for the measurement of sizes up to a few microns, while Mastersizer works best on larger sizes (to 10s of microns). Nevertheless, the correspondence between our static and dynamic data is good. For the lower flaxseed concentration, the layer of protein on the surface of the oil droplets affects the diffusion of the particles. This results in a dynamic size slightly larger than that arising from the static measurements. At high gum concentration, the aggregates get too large to be properly measured by DLS. Furthermore, the presence of unaggregated particles will be picked up by DLS and not by the Mastersizer, skewing the results to lower values. Fig. 4 illustrates the degree of visual phase separation, reported as amount of serum present, with increasing gum concentration. At low concentrations (gum 0.1%), there was no visual creaming for emulsions containing either the Emerson or the McDuff gum throughout the 28 days of observation. These results confirmed the
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data on z-average diameter and z-potential, as a small amount of gum does not cause emulsion destabilization. At higher concentrations (gum 0.15%), a clear serum layer dividing top (opaque cream) from bottom (clear serum) becomes apparent after a few days of storage. This phase separation process is a clear indication of the emulsion’s instability. The kinetic stability of emulsions is controlled by the structural organization of the oil droplets and by the three parameters mainly involved in the Stokes’ law equation of terminal velocity, V (McClements, 2005):
2gr 2 ðrd rc Þ V ¼
mc
(3)
where r is the radius of the particle, g is the acceleration due to gravity, r is the density, m is the viscosity and the subscripts c and d refer to the continuous and dispersed phases, respectively (McClements, 2005). These results confirm what has already been shown in Figs. 1–3. At high concentrations (gum 0.15%) destabilization occurs mostly due to bridging flocculation. The highest value of creaming, about 40%, occurs at a concentration of 0.15%. At this concentration, flocs form and although some polysaccharide is present in the serum phase, the increase in viscosity due to the addition of gum is not yet high enough to significantly retard creaming. The extent of creaming after 28 days of storage is concentration-dependent: addition of more gum seems to decrease the final creaming value (Fig. 4). At the high concentration, more polysaccharide – most likely the neutral fraction – will be present in solution and increase the viscosity, causing a decrease in the creaming rate. This creaming retardation due to excess polysaccharide in solution has been reported before (ten Grotenhuis et al., 2000). Creaming occurred at a faster rate in emulsions containing flaxseed gum of the McDuff variety compared to the Emerson variety. More evidence on the extent of the instability of the emulsions and the difference between flaxseed extracted from different varieties can be derived from microstructural observations. Fig. 5 illustrates the results of optical microscopy observations. Although optical microscopy is not best-suited to follow size development of such small droplets, it nevertheless is a good and fast back-up technique to check the bulk appearance of this system. Therefore, not too much emphasis must be put on the individual appearance of the droplets (as it is quite common to obtain light artifacts) but rather look at the behavior of the overall system. The top four micrographs correspond to different concentrations of Emerson gum and the bottom four to different concentrations of McDuff. In the absence of gum, the images cannot clearly pick up individual droplets. This is due to the microscope’s resolution and points to the absence of large aggregates. Therefore, we can conclude that there is stability, as expected, in the emulsion with no added gum. Addition of flaxseed gum, either from Emerson or McDuff variety, causes microstructural changes. At 0.04%, the system containing McDuff gum seems to be slightly more uniform than that containing Emerson. Again, emphasizing that light microscopy must not be over-interpreted in these small-size regimes, these results seem to agree with both DLS and Mastersizer data which show a larger and bimodal distribution, respectively, for the Emerson gum at this concentration. At a concentration of 0.20% of either gum, the WPIstabilized emulsions show aggregation, although with no visual phase separation yet. This observation suggests that the system was no longer homogenous and that the emulsion had experienced some reorganization that led to flocculation/aggregationwithin the system. These results also correspond to those obtained with light scattering which show a markedly increase in size after the addition of both types of gum (Figs. 2 and 3). At high concentrations (gum ¼ 0.33%) the micrographs show large, distinct domains in the images. This corresponds to visual phase separation with droplet-
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Fig. 5. Phase contrast micrographs of emulsions at pH 3.5 containing different concentration of flaxseed gum for Emerson (top) and McDuff (bottom) varieties. Scale bar ¼ 50 mm.
rich regions within which oil emulsion droplets were entrapped and somewhat restricted in motion. Again here, these results correspond well with those of Figs. 2 and 3 in which the samples showed a large extent of aggregation. The impact of gum concentration on the apparent viscosity of the gum solutions (A) and the emulsion (B) is shown in Fig. 6. At very low concentrations of polysaccharide (<0.05%), the viscosity of the Emerson gum solution seemed not to be different from that of the McDuff solution. In contrast, at concentrations >0.10%, the viscosity of the Emerson gum solution is higher than that of the McDuff. This could explain why, in the mixed samples, the excess gum not attached to WPI-coated droplets decreased the rate of
creaming by increasing the background viscosity of the emulsion and decreasing the mobility of the droplets. Similarly, the emulsions containing Emerson gum exhibit a higher apparent viscosity than the emulsions containing McDuff. The cause of this difference is most likely caused by the higher ratio of neutral fraction in the Emerson flaxseed gum compared to that of the McDuff gum. However, the differences in the apparent viscosity of the emulsion with added Emerson or McDuff gum could also be attributed to the nature of the forces involved in the internal structure of the flocs. Visual observations (Fig. 4) demonstrated that phase separation is faster for emulsions containing McDuff gum than Emerson gum. When looking at the apparent viscosity at low stress for the whole
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Acknowledgments
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The authors wish to thank the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) for its financial support. The authors are also grateful to Dr. François Capel for the gum extraction.
References
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Gum concentration (%) Fig. 6. Effect of gum concentration for Emerson (C) and McDuff (-) varieties on the apparent viscosity. Flaxseed gum dispersed in buffer (A) or added to WPI-stabilized emulsions (B) (9% soybean oil, 0.6% WPI, pH 3.5). Viscosity measured at 0.05 Pa. Results are the average of at least two independent measurements.
emulsions, it is clear that up to 0.1% polysaccharide, the systems show low viscosity, while at high concentrations (gum 0.1%), both varieties (Emerson and McDuff) showed a significant increase in viscosity caused by bridging of droplets in large flocs.
4. Conclusions This study aimed at investigating the behavior of flaxseed gums when mixed with WPI-stabilized emulsion droplets at acidic pH. The results demonstrated the interaction and attachment of gums onto the surface of proteins due to electrostatic interaction between the oppositely charged molecules. Up to 0.1% concentration of gum, no visual phase separation was observed, demonstrating the kinetic stability of the emulsions. At higher concentrations of polysaccharide (gum 1.5%), the microstructure, particle size, particle size distribution and low-shear apparent viscosity of the emulsion begin to change and become more affected by the presence of the gum. These changes result in aggregation and phase separation. Despite the fact that Emerson and McDuff gums are different in terms of amount of acidic and neutral fractions, this study suggests that when these gums are added into emulsions, their overall effects are relatively similar. Our results show there is still much work to be done to fully understand and optimize the incorporation of these natural gums in food products.
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