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Effect of salt content on the rheological properties of salad dressing-type emulsions stabilized by emulsifier blends
Journal of Food Engineering 80 (2007) 1272–1281 www.elsevier.com/locate/jfoodeng
Effect of salt content on the rheological properties of salad dressing-type emulsions stabilized by emulsifier blends Inmaculada Martı´nez *, Ma Angustias Riscardo, Jose Ma Franco Departamento de Ingenierı´a Quı´mica, Universidad de Huelva, Facultad de Ciencias Experimentales, Campus del Carmen, 21071 Huelva, Spain Received 26 June 2006; received in revised form 14 September 2006; accepted 23 September 2006 Available online 20 November 2006
Abstract The effects that salt content and composition of emulsifier blends exert on the rheological properties of salad dressing-type emulsions were studied. Binary blends of egg yolk and different types of amphiphilic molecules (Tween 20, sucrose laurate and pea protein), in several proportions, were used to stabilize emulsions. Salt concentration was ranged from 0 to 2.3% w/w. Steady-state flow tests and smallamplitude oscillatory shear measurements within the linear viscoelastic region were carried out. Rheological tests were complemented with droplet size distribution measurements. Rheological properties and physical stability of the emulsions studied were significantly influenced by salt content and the nature of binary emulsifier blends. In general, the values of rheological parameters studied increased with salt content. However, salt affects in much higher extent the properties of emulsions stabilized by high proportions of egg yolk or pea protein in the emulsifier blend, rather than those mainly stabilized by non-ionic low-molecular-weight surfactants, which are less sensitive to changes in the ionic strength. In this sense, the increase observed in the values of viscosity and linear viscoelastic functions of emulsions is more important when a protein is predominant in the emulsifier blend. This effect was explained on the basis of a more apparent increasing interdroplet interactions and viscosity of the continuous medium, both of them induced by salt addition, which lead to the consecution of an extensively flocculated state and improved creaming stability. On the contrary, different blends of pea protein and egg yolk showed a quite similar evolution of the rheological parameters with salt concentration. 2006 Elsevier Ltd. All rights reserved. Keywords: Creaming; Emulsifier; Emulsion; Egg yolk; NaCl; Protein; Rheology
1. Introduction The use of additives, which help to increase ageing stability of typical egg yolk-stabilized foodstuffs such as mayonnaises or salad dressings, is a common practice in the food industry. It is well known that emulsions are thermodynamically unstable systems, so that two kinds of additives are often used with the objective of avoiding phase separation: emulsifiers, with surface activity, and stabilizers, which act increasing the viscosity of the continuous medium. Proteins and low-molecular-weight (LMW) surfactants are key components of many foodstuffs (Dickin-
*
Corresponding author. Tel.: +34 959 21 99 99; fax: +34 959 21 99 83. E-mail address: [email protected] (I. Martı´nez).
0260-8774/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2006.09.022
son, 1992; Pugnaloni, Dickinson, Rammile, Mackie, & Wilde, 2004). Both types of molecules may be adsorbed at fluid interfaces, reducing the interfacial tension, thus facilitating the formation of emulsions by providing stability to freshly formed droplets. However, their molecular properties may be very different (Wilde, 2000). LMW surfactants are very mobile at the interface and they are particularly efficient reducing the interfacial tension. As a result, they rapidly coat the freshly created oil–water interface during emulsification. On the contrary, proteins are macromolecules species consisting of large chains of polar, non-polar and ionic amino acids. Consequently, proteins contain a mixture of hydrophilic and hydrophobic groups inducing intermolecular bonds more readily, conferring not only surface activity but also further steric stabilization.
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Different proteins and LMW surfactants suitable to replace egg yolk and milk derivatives have been tested in food emulsions as emulsifying ingredients (Franco, Partal, Ruiz-Marquez, Conde, & Gallegos, 2000; Partal, Guerrero, Berjano, & Gallegos, 1999; Pugnaloni et al., 2004; Raymundo, Franco, Partal, Sousa, & Gallegos, 1999; Sosulski & McCurdy, 1987). In this literature, special attention on the adsorption of binary mixtures of proteins, mixtures of LMW surfactants or protein-surfactant mixed systems have been paid. Previous work (Riscardo, Franco, & Gallegos, 2003, 2005) showed that the rheological properties, droplet size and physical stability of emulsions stabilized by different blends of emulsifiers depend on the weight ratio of emulsifiers, although the emulsifier total concentration remained constant, as well as the nature of the substances blended. These results were explained on the basis of the relationship among droplet size distribution, continuous phase characteristics and interactions among different emulsifier molecules. When mixtures of proteins and/or LMW surfactants are exposed to an interface, the different species compete to adsorb and lower the interfacial tension (Mackie, Gunning, Wilde, & Morris, 2000). During the equilibration of the interface, which may take from seconds in the case of pure LMW surfactants up to several hours for protein-stabilized systems, the molecules adsorb and desorb dynamically. If one of the components in the emulsifier blend reaches the interface first, then the second component will tend to replace the adsorbed molecules of the first type, partially or totally depending on the relative surface-activity of the species and their mutual cross interaction. Interestingly, in some cases, the mixture does not adsorb homogeneously over the surface. Thus, interfacial regions rich in one of species are present either during the equilibration process or in metastable states (Pugnaloni et al., 2004). Salt addition may induce two opposite effects on emulsion stability. On one hand, as previously reported (Srinivasan, Singh, & Munro, 2000), NaCl can destabilize protein-stabilized emulsions by two mechanisms: (i) the reduction of electrostatic repulsion among droplets and (ii) the modification of hydrophobic interactions between non-polar amino acids residues due to the alteration of the structural organization of water molecules at the interface. On the other hand, the destabilizing effect of NaCl is compensated by an increase of effective adsorbed protein concentration stabilizing the film (Palazolo, Mitidieri, & Wagner, 2003). In the case of egg yolk dispersions, the addition of salt also produces disruption of granules, liberating thus low-density lipoproteins which, in their molecular form, are more efficient as surface active agents (Guerrero, Carmona, Martinez, Cordobes, & Partal, 2004). The ability of electrolytes to influence the conformation of globular proteins has been reported to depend on the concentration of salt and/or ionic strength (Shenstone, 1968). At low ionic strength, the influence that salt exerts on protein structure is governed by electrostatic interactions. At higher ionic strength, depending on the system, the ability
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of salts to stabilize protein structure has been related to the preferential hydration of the protein molecule as a result of salt induced alteration of water in the vicinity of the protein (Harrison & Cunningham, 1986). Consequently, regarding physical properties and stability of emulsions, it is important to understand how the addition of salt alters the interfacial protein coverage and composition, especially in complex foodstuffs such as emulsions stabilized by emulsifying blends. In this work, the different alterations of the rheological behaviour of food emulsions induced by salt content are discussed, considering the nature of emulsifying blends, as previously discussed (Riscardo et al., 2003). Taking into account these considerations, the overall objective of this work is to study the effect of salt together with the composition of several binary emulsifier blends on the rheological properties of o/w emulsions with a formulation similar to those commercialized like salad dressings. 2. Experimental 2.1. Materials Pea protein isolate (purity >88%) was purchased from Cosucra, S.A. (Belgium). Tween 20 (sorbitan polyoxyethylene (20) monolaurate) and sucrose laurate of HLB 15, were used as received from Sigma–Aldrich (Germany) and Mitsubishi Food Corp. (Japan), respectively. Egg yolk was kindly supplied by Hijos de Ybarra, S.A. (Spain) after the application of a pasteurization treatment. Modified maize starch from Cerestar Ibe´rica S.A. (Spain) was used as a thickening agent. Other ingredients, also supplied by Hijos de Ybarra, S.A. (Spain), added to the emulsions were commercial sugar, wine vinegar (acidity: 10.3% weight/volume in acetic acid) and salt. 2.2. Emulsion preparation Food emulsions with a composition similar to commercial salad dressings were prepared. Sunflower oil concentration was set at 35% and total emulsifier content was fixed at 8% (all concentrations expressed in dry weight). Several emulsifier blends made up of egg yolk, in all cases, and a second emulsifier of different nature were used to stabilize the emulsions. Two egg yolk/emulsifier weight ratios, as different as possible attending to stability criteria (Riscardo et al., 2003), were selected for each blend (see Table 1). The concentration of the rest of ingredients used were: 4% weight/weight (w/w) commercial sugar, 4% (w/w) wine vinegar, 0–2.3% (w/w) salt, 1–2.5% (w/w) modified maize starch and the corresponding amount of distilled water to complete the formulation. The starch concentration was kept constant for a given emulsifier blend (see Table 1). This starch concentration is the minimum necessary to obtain, in the less favourable emulsifier weight ratio, a physical stability of at least seven days. With this formulation, the pH of the aqueous phase ranged between 3.6 and
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Table 1 Emulsifier blends and starch concentration used to stabilize the emulsions studied Emulsifier blends
Emulsifier weight ratio
Starch concentration (%)
Egg yolk/pea protein Egg yolk/pea protein Egg yolk/sucrose laurate Egg yolk/sucrose laurate Egg yolk/tween 20 Egg yolk/tween 20
1/7 7/1 6/2
1.0 1.0 2.0
8/0
2.0
6/2 2/6
2.5 2.5
3.8. As a preliminary step to emulsification, mixtures of starch, sugar and water were heated under gentle mechanical stirring until starch gelatinization temperature (80 C) was achieved. This procedure was followed in order to achieve the thickening of the continuous phase. The rest of ingredients were added to this mixture previous to oil addition. Pea protein was also dispersed in water, heating to 70 C in order to favour a certain degree of denaturation (Franco et al., 2000). Dispersions of sucrose laurate in water were kept 24 h at 60 C for a better solubilization, whereas Tween 20 was easily dispersed at room temperature. Emulsions were manufactured using a pilotplant colloidal mill, model ‘‘Delmix MZM/VK-7’’, from Fryma (Germany), using a rotor–stator gap of 1 mm. Rotational speed was set at 2830 rpm and the total residence time was 5 min in all cases. Emulsions prepared were stored in a refrigerator at 4 C. 2.3. Rheological tests Rheological measurements were carried out in a controlled-stress rheometer, RS100 from Haake (Germany). A plate–plate geometry (PPR35; 35 mm diameter), with rough surfaces, (roughness: 0.4 mm), was used in order to avoid wall-slip phenomena in steady-state flow measurements (Sa´nchez, Valencia, Franco, & Gallegos, 2001). Steady-state flow measurements were sometimes completed using a controlled-shear rate viscosimeter CV100 (Haake, Germany) using a plate–plate sensor system with rough surfaces as well (PPR20; 20 mm diameter). CV100 viscometer was used, in this case, in order to extend the shear rate range into higher values. Small-amplitude oscillatory shear tests were performed in a controlled-stress rheometer, RS100 from Haake (Germany) using a cone–plate sensor system (60 mm diameter, 4 angle). Rheological tests were made at 25 C. 2.4. Physical stability and droplet size distribution (DSD) measurements Ageing stability was determined by observing the bottom of the samples located in a 100 ml glass (30 mm diameter) cylinder. The appearance of a layer of water was
chosen as a criterion of instability. Thus, the physical stability was considered the period of time for which the emulsion does not visually show aqueous separation. DSD were determined in a range of droplet size between 0.2 and 150 lm by laser light scattering using a Malvern Mastersizer-X analyser (Malvern, UK) in the Fourier conformation (focus: 100 mm). Following the standard protocol, samples were diluted in distilled water (1 g of each sample in 500 mL of water). Values of the Sauter mean diameter, which is inversely proportional to the specific surface area of droplets, were obtained as follows: P 3 ni d d 3;2 ¼ P i2 ð1Þ ni d i where ni is the number of droplets with a diameter di. Polydisperisity was evaluated through the distribution uniformity: P V i jdðv; 0:5Þ d i j P U¼ ð2Þ dðv; 0:5Þ V i where Vi is the volume associated to particles of diameter di and d(v, 0.5) is the median of the distribution. All measurements were carried out 6 days after emulsions manufacture and replicated at least three times. A statistical analysis (ANOVA) was performed in order to establish the influence of the different variables studied. The significance level was set at 95%. 3. Results 3.1. Stability of emulsions In all cases, emulsions destabilize through a creaming phenomenon which is detected by aqueous phase separation at the bottom of the storage flask. As shown in Table 2, salt concentration decisively influences stability against creaming. In general, independently of the type of emulsifier blend used, an increase in salt concentration leads to
Table 2 Physical stability of the different emulsions studied as a function of salt concentration (days) Emulsifier blends
Salt concentration (%) 0.0
1% 7% 7% 1% 6% 2% 8% 0% 6% 2% 2% 6% a
egg yolk pea protein egg yolk pea protein egg yolk sucrose laurate egg yolk sucrose laurate egg yolk 20 tween 20 egg yolk tween 20
0.4
1.0
2.3
7
>120
>180a
>180a
21
30
18
90
>180a
>180a
100
>120
>180a
>180a
>180a
>180a
>180a
>120 7
8
60
13
80
>180a
Emulsions without change after 180 days were considered as stable.
I. Martı´nez et al. / Journal of Food Engineering 80 (2007) 1272–1281
detected. The Carreau model fits this flow behaviour fairly well (Fig. 1):
significantly more stable emulsions, especially in emulsions containing lower concentrations of egg yolk in which the creaming stability improved progressively with NaCl addition. On the contrary, as previously pointed out (Srinivasan et al., 2000), low stability was generally found in either salt-free emulsions or emulsions containing small amounts of salts, i.e. 0.4%, excepting in those systems stabilized by high contents of egg yolk.
g g1 1 is ¼h g0 g1 1 þ ðc_ =_cc Þ2
Fig. 1 shows the flow curves, plotted in the form of apparent viscosity vs. shear rate, for selected emulsions. In all cases, emulsions show a non-Newtonian behaviour, with two apparent regions: a Newtonian region at very low shear rates and a shear-thinning region, at intermediate shear rates, in which a power-law evolution can be
6
10
5
10
4
10
3
10
2
10
1
10
0
η (Pa·s)
A
10
ð3Þ
where g0 is the zero-shear-rate limiting viscosity, c_ c is a critical shear rate for the onset of the shear-thinning region, ‘‘s’’ is a parameter related to the slope of the shear-thinning region, and g1 is the high-shear-rate limiting viscosity, included as a fitting parameter to fit the slight tendency to reach a high-shear rate Newtonian region. Tables 3–5 show the values of the Carreau model parameters. As can be observed, in all cases, viscosity values significantly increase (p < 0.05) with salt content, especially at low shear rates. Moreover, this effect is more apparent when egg yolk is predominant in the emulsifier blend, as
3.2. Viscous flow
10
1275
B
0.0 % 0.4 % 1.0 % 2.3 % Carreau model
-1
10
-5
10
-4
10
-3
-2
10 . 10
-1
10
0
10
1
10
2
10
-5
10
-4
γ ( 1/ s)
10
-3
-2
10.
10
-1
10
0
10
1
10
2
10
3
γ ( 1/ s)
Fig. 1. Influence of salt content on viscous flow curves. (A) Emulsions stabilized by 1% egg yolk and 7% pea protein blends (starch 1%). (B) Emulsions stabilized by 7% egg yolk and 1% pea protein blends (starch 1%).
Table 3 Carreau model fitting data and values of the Sauter diameter (d32) and uniformity (U) for emulsions stabilized by egg yolk and pea protein blends (starch 1%) g1 (Pa s)
c_ c (s1)
s
d32 (lm)
U
Emulsions with 1% egg yolk and 7% pea protein 0.0 3.25 · 103 0.4 2.32 · 104 1.0 3.46 · 104 2.3 6.51 · 104
0.11 0.15 0.12 0.10
2.8 · 104 2.7 · 104 2.1 · 104 2.7 · 104
0.42 0.45 0.44 0.46
4.1 3.4 3.3 3.3
0.78 0.87 1.05 0.99
Emulsions with 7% egg yolk and 1% pea protein 0.0 2.81 · 101 0.4 4.23 · 103 1.0 3.62 · 104 2.3 8.27 · 104
0.13 0.18 0.06 0.06
9.8 · 103 9.6 · 104 3.2 · 104 2.3 · 104
0.32 0.43 0.45 0.45
2.4 2.7 2.6 3.0
0.75 0.65 0.65 0.70
Csalt(% w/w)
g0 (Pa s)
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Table 4 Carreau model fitting data and values of the Sauter diameter (d32) and uniformity (U) for emulsions stabilized by egg yolk and sucrose laurate blends (starch 2%) g1 (Pa s)
c_ c (s1)
s
d32 (lm)
U
Emulsions with 6% egg yolk and 2% sucrose laurate 0.0 1.24 · 104 0.4 6.94 · 104 1.0 7.72 · 104 2.3 1.14 · 105
0.15 0.18 0.18 0.20
1.0 · 104 1.2 · 104 1.1 · 104 2.0 · 104
0.41 0.45 0.45 0.47
2.9 3.9 3.8 3.3
0.72 0.51 0.54 0.53
Emulsions with 8% egg yolk and 0% sucrose laurate 0.0 6.87 · 103 0.4 8.49 · 104 1.0 1.35 · 105 2.3 2.78 · 105
0.46 0.60 0.52 0.56
1.2 · 103 2.8 · 104 3.1 · 104 2.6 · 104
0.43 0.46 0.46 0.46
2.1 2.1 2.4 3.0
0.49 0.46 0.73 0.79
Csalt (% w/w)
g0 (Pa s)
1% egg yolk, 7% pea protein
5
η0 (Pa·s)
1.0x10
3
4
2
η 0 = 8.06·10 + 2.53·10 Csalt (r = 0.96)
7% egg yolk, 1% pea protein 2 1 4 η 0 = 2. 81· 10 + 3. 77· 10 Csalt (r = 0.94)
4
8.0x10
4
6.0x10
4
4.0x10
4
2.0x10
A
0.0 6% egg yolk, 2% SL
5
η0 (Pa·s)
2.8x10
4
4
2
η 0= 3.31· 10 + 3.80· 10 Csalt ( r = 0.81)
8% egg yolk, 0% SL 4 5 2 η 0= 2.18· 10 + 1.13· 10 Csalt ( r = 0.98)
5
2.1x10
5
1.4x10
4
7.0x10
B
0.0 5
1.5x10
6% egg yolk, 2% Tween20 3
4
2
η0= 9.32· 10 + 4.34· 10 Csalt ( r = 0.91)
2% egg yolk, 6% Tween20 3 3 2 η0= 3.86· 10 + 4.16· 10 Csalt ( r = 0.94)
η0 (Pa·s)
5
1.0x10
4
5.0x10
0.0
C 0.0
0.5
1.0
1.5
2.0
2.5
Csalt ( % w/ w) Fig. 2. Influence of salt content on the zero-shear rate-limiting viscosity for emulsions stabilized by (A) egg yolk and pea protein blends (starch 1%); (B) egg yolk and sucrose laurate blends (starch 2%) and (C) egg yolk and Tween 20 blends (starch 2.5%).
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can be clearly observed in Fig. 2 where the values of the zero-shear rate-limiting viscosity, g0, are plotted vs. salt concentration for emulsions stabilized with different blends of emulsifier. Thus, g0 increases almost linearly in emulsions stabilized by egg yolk/pea protein blends, being the slope slightly higher for the emulsion with higher egg yolk content in the emulsifier blend (Fig. 2A). This effect is much more remarkable in the case of emulsions stabilized by egg yolk/LMW surfactant blends (Fig. 2B and C). An increment in salt content slightly affects emulsions containing lower egg yolk/Tween 20 weight ratios but, however, significantly increases g0 values in emulsions stabilized by higher proportions of egg yolk (Fig. 2C). On the other hand, parameter ‘‘s’’, related to the slope of the shear thinning region, shows, in general, significantly lower values for salt-free emulsions in comparison to those found in emulsions with added salt (Tables 3–5). Other parameters of the Carreau model are not statistically affected by salt concentration (p > 0.05).
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3.3. Small-amplitude oscillatory shear measurements Figs. 3 and 4 show the mechanical spectra inside the linear viscoelastic range for selected emulsions. In general, the typical evolution of the linear viscoelastic functions with frequency for highly flocculated emulsions, i.e. a clearly developed plateau region, has been found (Franco et al., 2000). However, this typical evolution was not found for salt-free emulsions. In these cases, a higher slope in the evolution of G 0 and G00 with frequency and values of the loss tangent closer to one were found. Furthermore, increasing salt content yields higher values of G 0 and G00 and progressively lower values of tan d, which indicates a relatively more elastic response, together with a more developed plateau region. In addition to this, the linear viscoelastic region expands with salt content (data not shown), related with a more shear-resistant microstructure, even more as egg yolk concentration increases. The characteristic parameter of the plateau region is the so called plateau modulus, GoN , defined more precisely else-
Table 5 Carreau model fitting data and values of the Sauter diameter (d32) and uniformity (U) for emulsions stabilized by egg yolk and Tween 20 blends (starch 2.5%) g1 (Pa s)
c_ c (s1)
s
d32 (lm)
U
Emulsions with 2% egg yolk and 6% tween 20 0.0 3.77 · 103 0.4 6.47 · 103 1.0 6.83 · 103 2.3 1.38 · 104
0.10 0.11 0.11 0.14
1.8 · 104 1.1 · 104 9.0 · 105 1.2 · 104
0.41 0.41 0.41 0.42
1.4 1.3 1.3 1.3
0.30 0.31 0.30 0.30
Emulsions with 6% egg yolk and 2% tween 20 0.0 9.32 · 103 0.4 1.33 · 104 1.0 3.58 · 104 2.3 1.30 · 105
0.30 0.30 0.35 0.39
1.8 · 104 1.9 · 104 1.0 · 104 2.1 · 104
0.40 0.41 0.42 0.46
0.32 0.34 0.35 0.35
0.32 0.34 0.35 0.35
Csalt (% w/w)
g0 (Pa s)
2
10
G', G'' (Pa)
1
10
0
G'
10
G'' 0.0 % 0.4 % 1.0 % 2.3 %
-1
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
ω (rad/s) Fig. 3. Influence of salt concentration on the mechanical spectra of emulsions stabilized by 1% egg yolk and 7% pea protein blends (starch 1%).
I. Martı´nez et al. / Journal of Food Engineering 80 (2007) 1272–1281
1278 3
10
2
G', G'' (Pa)
10
1
10
G'
G'' 0.0% 0.4% 1.0% 2.3%
0
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
ω (rad/s)
Fig. 4. Influence of salt concentration on the mechanical spectra of emulsions stabilized by 8% egg yolk and 0% sucrose laurate blends (starch 2%).
where (Ferry, 1980). Fig. 5 shows the evolution of GoN with salt concentration for the emulsions studied. As identically found with g0, the plateau modulus significantly increases (p < 0.05) with salt concentration, but more rapidly at higher proportions of egg yolk in emulsions stabilized by egg yolk/LMW-surfactant blends (Fig. 5B and C). On the contrary, the slopes of GoN vs. salt concentration plots are quite similar for emulsions stabilized by blends of egg yolk and pea protein (Fig. 5A), in spite of the high differences in blend composition. 3.4. Droplet size distribution (DSD) Salt concentration does not exert a significant influence (p > 0.05) on mean droplet size and uniformity for the emulsions studied, as can be deduced from the values shown in Tables 3–5. Although not statistically significant, a slight tendency to increase the Sauter diameter and polydispersity with salt content may be observed for emulsions stabilized with the higher concentrations of egg yolk in the emulsifier blend. Consequently, droplet size does not seem to play an important role in physical stability. Thus, for instance, emulsions containing high Tween 20/egg yolk weight ratios show small droplet sizes but, however, they were more unstable towards creaming compared to emulsions mainly stabilized by macromolecules. 4. Discussion The experimental results obtained in this work indicate that salt addition produces a progressive increase of viscous and viscoelastic functions, relatively more elastic characteristics and improved stability against creaming for emulsions stabilized by binary blends composed of egg yolk and other emulsifier of different nature in several pro-
portions. However, salt addition does not exert, in general, a significant influence on droplet size distribution. In all cases, ionic strength affects in much higher extent the properties of emulsions mainly stabilized by macromolecules, i.e. egg yolk or pea protein. On the contrary, since the LMW emulsifiers used in several blends were non-ionic surfactants, these emulsions are less sensitive to changes in the ionic strength (So¨derman & Johansson, 2000). Consequently, the effect of salt on emulsion properties seems to be determined by changes in protein configuration occurred in egg yolk or pea protein isolate. In addition to this, in the case of egg yolk, these results can be also explained taking into account that the addition of NaCl disrupts granules and liberates HDL and phosvitin, which are available for adsorption at the oil–water interface in their molecular state (Anton, Beaumal, & Gandemer, 2000). Interfacial films made with native granules usually present a lesser viscoelasticity than those made with constituents liberated from granules. Moreover, the effective reduction of interfacial tension during emulsification requires unfolding of adsorbed proteins at the interface to allow their non-polar segments to come into contact with the oil (Parker, 1987). Native granules are likely to have a poor efficiency to decrease interfacial tension because large particles evidently would not spread over the interface as individual proteins do. Similar considerations can be given for the emulsion mainly stabilized by the pea protein isolate. These findings are in agreement with those of Walstra (1996) which observed that molecular aggregates of several sizes, contained in industrial protein preparations, are less efficient emulsifiers than individual proteins. The increasing interdroplet interactions induced by salt addition may lead to a higher development of the flocculation process, which could result in coalescence favoured by
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1279
160 1% 0 GN 7% 0 GN
80
0
GN ( Pa)
120
egg yolk, 7% pea protein 2 = 13.93 + 40.36Csalt ( r = 0.94) egg yolk, 1% pea protein 2 = 0.93 + 41.50Csalt ( r = 0.98)
40
A
0 300
0
GN ( Pa)
250
6% egg yolk, 2% SL 2 0 G N = 35.49 + 35.78Csalt ( r = 0.86) 8% egg yolk, 0% SL 2 0 G N = 66.42 + 88.87C salt ( r = 0.94)
200 150 100 50
B
0
GN ( Pa)
0 200
6% egg yolk, 2% Tween20 2 0 G N = 16.80+ 71.90Csalt ( r = 0.94) 2% egg yolk, 6% Tween20 2 0 G N = 27.27+ 13.18Csalt ( r = 0.96)
160 120 80 40
C 0 0.0
0.5
1.0
1.5
2.0
2.5
Csalt ( % w/ w) Fig. 5. Influence of salt content on the plateau modulus for emulsions stabilized by (A) egg yolk and pea protein blends (starch 1%), (B) egg yolk and sucrose laurate blends (starch 2%) and (C) egg yolk and Tween 20 blends (starch 2.5%).
a reduction in the electrostatic repulsion among droplets. This fact can be seen in Tables 3–5 in which an increase in mean droplet size and polydispersity with salt content is observed for emulsions mainly stabilized by egg yolk. On the other hand, as previously reported (Franco et al., 2000), the consecution of an extensive flocculated state could be related to an improvement in creaming stability by immobilization of droplets in this network. Attending to the above discussion, this phenomenon seems to be reinforced by salt addition, as can be deduced from the change in the shape of the mechanical spectra (i.e. more developed plateau region) found by increasing salt content. In addition to this, the evolutions experimentally found in the zero-shear rate-limiting viscosity, parameter ‘‘s’’ or the plateau modulus with salt concentration could also be associated to a higher extension and strength of this flocculated
state. On the contrary, the low stability found in emulsions with high Tween 20/egg yolk weight ratios must be related to both a depletion flocculation mechanism, favoured by an excess of a LMW surfactant (Dickinson & McClements, 1995), and lower interfacial viscoelasticity (Wilde, 1996). Furthermore, the effect of salt on emulsifier blends, especially on egg yolk constituents, does not only have interfacial repercussions but also on the properties of the continuous medium. This fact is demonstrated by measuring the rheological behaviour of the continuous medium, previously to the addition of the oily phase. The importance that the continuous medium exert on the final emulsion is illustrated in Fig. 6, which shows viscous flow curves and mechanical spectra of two different continuous media (with 8% w/w egg yolk and 2% w/w starch) greatly differing in salt content. As can be observed, the presence of salt
I. Martı´nez et al. / Journal of Food Engineering 80 (2007) 1272–1281
1280
10
3
10
0
A
B 0.0 % 2.3 %
2
10
1
tan (δ )
G', G'' (Pa)
10
G'
G'' 0.0 % 2.3 %
10
10
-1
0
10
-2
10
-1
10
0
10
1
10
2
10
-2
10
-1
10
η (Pa·s)
6
10
5
10
4
10
3
10
2
10
1
10
0
10
10
1
10
2
ω (rad/s)
ω (rad/s) 10
0
C
0.0 % 2.3 %
-1
10
-5
10
-4
10
-3
10
-2
10 .
-1
10
0
10
1
10
2
10
3
-1
γ (s ) Fig. 6. Influence of salt concentration (values referred to the final emulsion) on (A) the storage and loss moduli, (B) the loss tangent and (C) the viscosity for continuous media of an emulsion stabilized by 8% egg yolk, differing in salt content (starch 2%).
induces a significant increment in viscosity and higher values of the linear viscoelastic functions associated to a higher relatively elastic response. In this sense, for relatively low weight oil fractions, as those considered in this work, the continuous medium greatly influences the rheological behaviour of the final emulsion, even more when salt content increases. Thus, for instance, as can be observed in Fig. 6 and Table 4, both continuous media (salt-free and containing 2.3% salt in the final system) show values of viscosity very similar to those found in the corresponding final emulsion (see Table 4). This effect may be mainly attributed to the salt-induced gelation of egg yolk lipoproteins (Guerrero et al., 2004). Once again, these results can be explained on the basis of a decrease in electrostatic repulsion forces favouring protein associations which increase viscous and viscoelastic parameter (Kim, Renkema, & van Vliet, 2001; McClements, Monahan, & Kinsella, 1993). Following the same mechanism, droplets
may also flocculate more easily due to the contraction of the electrical double layer around the charged droplets. 5. Conclusions From the experimental results it may be concluded that rheological properties and physical stability of the emulsions studied are significantly influenced by salt content and the nature of binary emulsifier blends. In general, salt addition produces a progressive increase of viscous and viscoelastic parameters and improved stability against creaming for emulsions stabilized by binary blends composed by egg yolk and other emulsifier of different nature in several proportions. However, salt addition does not exert, in general, a significant influence on droplet size distribution. Salt affects the rheological properties of emulsions stabilized by high proportions of macromolecules in much higher extent than those containing high relative weight ratios of non-
I. Martı´nez et al. / Journal of Food Engineering 80 (2007) 1272–1281
ionic low-molecular-weight surfactants, which are less sensitive to changes in the ionic strength. The values of viscosity and linear viscoelastic parameters analysed increase with salt concentration more markedly when a protein is predominant in the emulsifier blend, which must be attributed to increasing interdroplet interactions and viscosity of the continuous medium, both of them induced by salt addition. On the contrary, different blends of pea protein and egg yolk show a quite similar evolution of the rheological parameters with salt concentration. References Anton, M., Beaumal, V., & Gandemer, G. (2000). Adsorption at the oil– water interface and emulsifying properties of native granules from egg yolk: effect of aggregated state. Food Hydrocolloid, 14, 327–335. Dickinson, E. (1992). An introduction to food colloids. Oxford: Oxford University Press. Dickinson, E., & McClements, D. J. (1995). Advances in food colloids. Glasgow: Blackie Academic and Professional. Ferry, J. D. (1980). Viscoelastic properties of polymers. New York: John Wiley & Sons. Franco, J. M., Partal, P., Ruiz-Marquez, D., Conde, B., & Gallegos, C. (2000). Influence of pH and protein thermal treatment on the rheology of pea protein-stabilized oil-in-water emulsions. Journal of the American Oil Chemists Society, 77, 975–983. Guerrero, A., Carmona, J., Martinez, I., Cordobes, F., & Partal, P. (2004). Effect of pH and added electrolyte on the thermal-induced transitions of egg yolk. Rheologica Acta, 43, 539–549. Harrison, L. J., & Cunningham, F. E. (1986). Influence of frozen storage time on properties of salted yolk and its functionality in mayonnaise. Journal of Food Quality, 9, 167–174. Kim, K. H., Renkema, J. M. S., & van Vliet, T. (2001). Rheological properties of soybean protein isolate gels containing emulsion droplets. Food Hydrocolloids, 15, 295–302. Mackie, A. R., Gunning, A. P., Wilde, P. J., & Morris, V. (2000). Orogenic displacement of protein from the oil/water interface. Langmuir, 16, 2242–2247. McClements, D. J., Monahan, F. J., & Kinsella, J. E. (1993). Effect of emulsion droplets on the rheology of whey-protein isolate gels. Journal of Texture Studies, 24, 411–422. Palazolo, G. G., Mitidieri, F. E., & Wagner, J. R. (2003). Relationship between interfacial behaviour of native and denatured soybean isolates
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and microstructure and coalescence of oil in water emulsions-effect of salt and protein concentration. Food Science Technology International, 9, 409–411. Parker, N. S. (1987). Properties and functions of stabilizing agents in food emulsions. Critical Reviews in Food Science and Nutrition, 25, 285–315. Partal, P., Guerrero, A., Berjano, M., & Gallegos, C. (1999). Transient flow of o/w sucrate palmitote emulsions. Journal of Food Engineering, 41, 33–41. Pugnaloni, L. A., Dickinson, E., Rammile, E., Mackie, A. R., & Wilde, P. J. (2004). Competitive adsorption of proteins and low-molecularweight surfactants: computer simulation and microscopic imaging. Advances in Colloid and Interface Science, 107, 27–49. Raymundo, A., Franco, J. M., Partal, P., Sousa, I., & Gallegos, C. (1999). Effect of the lupin protein/surfactant ratio on linear viscoelastic properties of oil-in-water emulsions. Journal of Surfactants and Detergents, 2, 545–551. Riscardo, M. A., Franco, J. M., & Gallegos, C. (2003). Influence of composition of emulsfier blends on the rheological properties of salad dressing-type emulsions. Food Science Technology International, 9, 53–63. Riscardo, M. A., Franco, J. M., & Gallegos, C. (2005). Rheological characterisation of salad-dressing-type emulsions stabilised by egg yolk/sucrose distearate blends. European Food Research and Technology, 220, 380–388. Sa´nchez, M. C., Valencia, C., Franco, J. M., & Gallegos, C. (2001). Wall slip phenomena in oil-in-water emulsions: effect of some structural parameters. Journal of Colloid and Interface Science, 241, 226–232. Shenstone, F. S. (1968). In Carter, T. C., Oliver, Boyd (Eds.), Egg quality: A study of the Hen’s egg. Edinburgh. So¨derman, O., & Johansson, I. (2000). Polyhydroxyl-based surfactants and their physico-chemical properties and applications. Current Opinion in Colloid and Interface Science, 4, 391–401. Sosulski, F. W., & McCurdy, A. R. (1987). Functionality of flours, protein fractions and isolates from field pea and faba bean. Journal of Food Science, 52, 1010–1014. Srinivasan, M., Singh, H., & Munro, P. A. (2000). The effect of sodium chloride on the formation and stability of sodium caseinate emulsions. Food Hydrocolloids, 14, 497–507. Walstra, P. (1996). Emulsion stability. In P. Becher. Encyclopedia of emulsion technology (Vol. 4). New York: Marcel Dekker, Chapter 1. Wilde, P. J. (1996). Foam measurement by the microconductivity technique: An assessment of its sensitivity to interfacial and environmental factors. Journal of Colloid and Interface Science, 178, 733–739. Wilde, P. J. (2000). Interfaces: their role in foam and emulsion behaviour. Current Opinion in Colloid and Interface Science, 5, 176–181.
Effect of salt content on the rheological properties of salad dressing-type emulsions stabilized by emulsifier blends Inmaculada Martı´nez *, Ma Angustias Riscardo, Jose Ma Franco Departamento de Ingenierı´a Quı´mica, Universidad de Huelva, Facultad de Ciencias Experimentales, Campus del Carmen, 21071 Huelva, Spain Received 26 June 2006; received in revised form 14 September 2006; accepted 23 September 2006 Available online 20 November 2006
Abstract The effects that salt content and composition of emulsifier blends exert on the rheological properties of salad dressing-type emulsions were studied. Binary blends of egg yolk and different types of amphiphilic molecules (Tween 20, sucrose laurate and pea protein), in several proportions, were used to stabilize emulsions. Salt concentration was ranged from 0 to 2.3% w/w. Steady-state flow tests and smallamplitude oscillatory shear measurements within the linear viscoelastic region were carried out. Rheological tests were complemented with droplet size distribution measurements. Rheological properties and physical stability of the emulsions studied were significantly influenced by salt content and the nature of binary emulsifier blends. In general, the values of rheological parameters studied increased with salt content. However, salt affects in much higher extent the properties of emulsions stabilized by high proportions of egg yolk or pea protein in the emulsifier blend, rather than those mainly stabilized by non-ionic low-molecular-weight surfactants, which are less sensitive to changes in the ionic strength. In this sense, the increase observed in the values of viscosity and linear viscoelastic functions of emulsions is more important when a protein is predominant in the emulsifier blend. This effect was explained on the basis of a more apparent increasing interdroplet interactions and viscosity of the continuous medium, both of them induced by salt addition, which lead to the consecution of an extensively flocculated state and improved creaming stability. On the contrary, different blends of pea protein and egg yolk showed a quite similar evolution of the rheological parameters with salt concentration. 2006 Elsevier Ltd. All rights reserved. Keywords: Creaming; Emulsifier; Emulsion; Egg yolk; NaCl; Protein; Rheology
1. Introduction The use of additives, which help to increase ageing stability of typical egg yolk-stabilized foodstuffs such as mayonnaises or salad dressings, is a common practice in the food industry. It is well known that emulsions are thermodynamically unstable systems, so that two kinds of additives are often used with the objective of avoiding phase separation: emulsifiers, with surface activity, and stabilizers, which act increasing the viscosity of the continuous medium. Proteins and low-molecular-weight (LMW) surfactants are key components of many foodstuffs (Dickin-
*
Corresponding author. Tel.: +34 959 21 99 99; fax: +34 959 21 99 83. E-mail address: [email protected] (I. Martı´nez).
0260-8774/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2006.09.022
son, 1992; Pugnaloni, Dickinson, Rammile, Mackie, & Wilde, 2004). Both types of molecules may be adsorbed at fluid interfaces, reducing the interfacial tension, thus facilitating the formation of emulsions by providing stability to freshly formed droplets. However, their molecular properties may be very different (Wilde, 2000). LMW surfactants are very mobile at the interface and they are particularly efficient reducing the interfacial tension. As a result, they rapidly coat the freshly created oil–water interface during emulsification. On the contrary, proteins are macromolecules species consisting of large chains of polar, non-polar and ionic amino acids. Consequently, proteins contain a mixture of hydrophilic and hydrophobic groups inducing intermolecular bonds more readily, conferring not only surface activity but also further steric stabilization.
I. Martı´nez et al. / Journal of Food Engineering 80 (2007) 1272–1281
Different proteins and LMW surfactants suitable to replace egg yolk and milk derivatives have been tested in food emulsions as emulsifying ingredients (Franco, Partal, Ruiz-Marquez, Conde, & Gallegos, 2000; Partal, Guerrero, Berjano, & Gallegos, 1999; Pugnaloni et al., 2004; Raymundo, Franco, Partal, Sousa, & Gallegos, 1999; Sosulski & McCurdy, 1987). In this literature, special attention on the adsorption of binary mixtures of proteins, mixtures of LMW surfactants or protein-surfactant mixed systems have been paid. Previous work (Riscardo, Franco, & Gallegos, 2003, 2005) showed that the rheological properties, droplet size and physical stability of emulsions stabilized by different blends of emulsifiers depend on the weight ratio of emulsifiers, although the emulsifier total concentration remained constant, as well as the nature of the substances blended. These results were explained on the basis of the relationship among droplet size distribution, continuous phase characteristics and interactions among different emulsifier molecules. When mixtures of proteins and/or LMW surfactants are exposed to an interface, the different species compete to adsorb and lower the interfacial tension (Mackie, Gunning, Wilde, & Morris, 2000). During the equilibration of the interface, which may take from seconds in the case of pure LMW surfactants up to several hours for protein-stabilized systems, the molecules adsorb and desorb dynamically. If one of the components in the emulsifier blend reaches the interface first, then the second component will tend to replace the adsorbed molecules of the first type, partially or totally depending on the relative surface-activity of the species and their mutual cross interaction. Interestingly, in some cases, the mixture does not adsorb homogeneously over the surface. Thus, interfacial regions rich in one of species are present either during the equilibration process or in metastable states (Pugnaloni et al., 2004). Salt addition may induce two opposite effects on emulsion stability. On one hand, as previously reported (Srinivasan, Singh, & Munro, 2000), NaCl can destabilize protein-stabilized emulsions by two mechanisms: (i) the reduction of electrostatic repulsion among droplets and (ii) the modification of hydrophobic interactions between non-polar amino acids residues due to the alteration of the structural organization of water molecules at the interface. On the other hand, the destabilizing effect of NaCl is compensated by an increase of effective adsorbed protein concentration stabilizing the film (Palazolo, Mitidieri, & Wagner, 2003). In the case of egg yolk dispersions, the addition of salt also produces disruption of granules, liberating thus low-density lipoproteins which, in their molecular form, are more efficient as surface active agents (Guerrero, Carmona, Martinez, Cordobes, & Partal, 2004). The ability of electrolytes to influence the conformation of globular proteins has been reported to depend on the concentration of salt and/or ionic strength (Shenstone, 1968). At low ionic strength, the influence that salt exerts on protein structure is governed by electrostatic interactions. At higher ionic strength, depending on the system, the ability
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of salts to stabilize protein structure has been related to the preferential hydration of the protein molecule as a result of salt induced alteration of water in the vicinity of the protein (Harrison & Cunningham, 1986). Consequently, regarding physical properties and stability of emulsions, it is important to understand how the addition of salt alters the interfacial protein coverage and composition, especially in complex foodstuffs such as emulsions stabilized by emulsifying blends. In this work, the different alterations of the rheological behaviour of food emulsions induced by salt content are discussed, considering the nature of emulsifying blends, as previously discussed (Riscardo et al., 2003). Taking into account these considerations, the overall objective of this work is to study the effect of salt together with the composition of several binary emulsifier blends on the rheological properties of o/w emulsions with a formulation similar to those commercialized like salad dressings. 2. Experimental 2.1. Materials Pea protein isolate (purity >88%) was purchased from Cosucra, S.A. (Belgium). Tween 20 (sorbitan polyoxyethylene (20) monolaurate) and sucrose laurate of HLB 15, were used as received from Sigma–Aldrich (Germany) and Mitsubishi Food Corp. (Japan), respectively. Egg yolk was kindly supplied by Hijos de Ybarra, S.A. (Spain) after the application of a pasteurization treatment. Modified maize starch from Cerestar Ibe´rica S.A. (Spain) was used as a thickening agent. Other ingredients, also supplied by Hijos de Ybarra, S.A. (Spain), added to the emulsions were commercial sugar, wine vinegar (acidity: 10.3% weight/volume in acetic acid) and salt. 2.2. Emulsion preparation Food emulsions with a composition similar to commercial salad dressings were prepared. Sunflower oil concentration was set at 35% and total emulsifier content was fixed at 8% (all concentrations expressed in dry weight). Several emulsifier blends made up of egg yolk, in all cases, and a second emulsifier of different nature were used to stabilize the emulsions. Two egg yolk/emulsifier weight ratios, as different as possible attending to stability criteria (Riscardo et al., 2003), were selected for each blend (see Table 1). The concentration of the rest of ingredients used were: 4% weight/weight (w/w) commercial sugar, 4% (w/w) wine vinegar, 0–2.3% (w/w) salt, 1–2.5% (w/w) modified maize starch and the corresponding amount of distilled water to complete the formulation. The starch concentration was kept constant for a given emulsifier blend (see Table 1). This starch concentration is the minimum necessary to obtain, in the less favourable emulsifier weight ratio, a physical stability of at least seven days. With this formulation, the pH of the aqueous phase ranged between 3.6 and
I. Martı´nez et al. / Journal of Food Engineering 80 (2007) 1272–1281
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Table 1 Emulsifier blends and starch concentration used to stabilize the emulsions studied Emulsifier blends
Emulsifier weight ratio
Starch concentration (%)
Egg yolk/pea protein Egg yolk/pea protein Egg yolk/sucrose laurate Egg yolk/sucrose laurate Egg yolk/tween 20 Egg yolk/tween 20
1/7 7/1 6/2
1.0 1.0 2.0
8/0
2.0
6/2 2/6
2.5 2.5
3.8. As a preliminary step to emulsification, mixtures of starch, sugar and water were heated under gentle mechanical stirring until starch gelatinization temperature (80 C) was achieved. This procedure was followed in order to achieve the thickening of the continuous phase. The rest of ingredients were added to this mixture previous to oil addition. Pea protein was also dispersed in water, heating to 70 C in order to favour a certain degree of denaturation (Franco et al., 2000). Dispersions of sucrose laurate in water were kept 24 h at 60 C for a better solubilization, whereas Tween 20 was easily dispersed at room temperature. Emulsions were manufactured using a pilotplant colloidal mill, model ‘‘Delmix MZM/VK-7’’, from Fryma (Germany), using a rotor–stator gap of 1 mm. Rotational speed was set at 2830 rpm and the total residence time was 5 min in all cases. Emulsions prepared were stored in a refrigerator at 4 C. 2.3. Rheological tests Rheological measurements were carried out in a controlled-stress rheometer, RS100 from Haake (Germany). A plate–plate geometry (PPR35; 35 mm diameter), with rough surfaces, (roughness: 0.4 mm), was used in order to avoid wall-slip phenomena in steady-state flow measurements (Sa´nchez, Valencia, Franco, & Gallegos, 2001). Steady-state flow measurements were sometimes completed using a controlled-shear rate viscosimeter CV100 (Haake, Germany) using a plate–plate sensor system with rough surfaces as well (PPR20; 20 mm diameter). CV100 viscometer was used, in this case, in order to extend the shear rate range into higher values. Small-amplitude oscillatory shear tests were performed in a controlled-stress rheometer, RS100 from Haake (Germany) using a cone–plate sensor system (60 mm diameter, 4 angle). Rheological tests were made at 25 C. 2.4. Physical stability and droplet size distribution (DSD) measurements Ageing stability was determined by observing the bottom of the samples located in a 100 ml glass (30 mm diameter) cylinder. The appearance of a layer of water was
chosen as a criterion of instability. Thus, the physical stability was considered the period of time for which the emulsion does not visually show aqueous separation. DSD were determined in a range of droplet size between 0.2 and 150 lm by laser light scattering using a Malvern Mastersizer-X analyser (Malvern, UK) in the Fourier conformation (focus: 100 mm). Following the standard protocol, samples were diluted in distilled water (1 g of each sample in 500 mL of water). Values of the Sauter mean diameter, which is inversely proportional to the specific surface area of droplets, were obtained as follows: P 3 ni d d 3;2 ¼ P i2 ð1Þ ni d i where ni is the number of droplets with a diameter di. Polydisperisity was evaluated through the distribution uniformity: P V i jdðv; 0:5Þ d i j P U¼ ð2Þ dðv; 0:5Þ V i where Vi is the volume associated to particles of diameter di and d(v, 0.5) is the median of the distribution. All measurements were carried out 6 days after emulsions manufacture and replicated at least three times. A statistical analysis (ANOVA) was performed in order to establish the influence of the different variables studied. The significance level was set at 95%. 3. Results 3.1. Stability of emulsions In all cases, emulsions destabilize through a creaming phenomenon which is detected by aqueous phase separation at the bottom of the storage flask. As shown in Table 2, salt concentration decisively influences stability against creaming. In general, independently of the type of emulsifier blend used, an increase in salt concentration leads to
Table 2 Physical stability of the different emulsions studied as a function of salt concentration (days) Emulsifier blends
Salt concentration (%) 0.0
1% 7% 7% 1% 6% 2% 8% 0% 6% 2% 2% 6% a
egg yolk pea protein egg yolk pea protein egg yolk sucrose laurate egg yolk sucrose laurate egg yolk 20 tween 20 egg yolk tween 20
0.4
1.0
2.3
7
>120
>180a
>180a
21
30
18
90
>180a
>180a
100
>120
>180a
>180a
>180a
>180a
>180a
>120 7
8
60
13
80
>180a
Emulsions without change after 180 days were considered as stable.
I. Martı´nez et al. / Journal of Food Engineering 80 (2007) 1272–1281
detected. The Carreau model fits this flow behaviour fairly well (Fig. 1):
significantly more stable emulsions, especially in emulsions containing lower concentrations of egg yolk in which the creaming stability improved progressively with NaCl addition. On the contrary, as previously pointed out (Srinivasan et al., 2000), low stability was generally found in either salt-free emulsions or emulsions containing small amounts of salts, i.e. 0.4%, excepting in those systems stabilized by high contents of egg yolk.
g g1 1 is ¼h g0 g1 1 þ ðc_ =_cc Þ2
Fig. 1 shows the flow curves, plotted in the form of apparent viscosity vs. shear rate, for selected emulsions. In all cases, emulsions show a non-Newtonian behaviour, with two apparent regions: a Newtonian region at very low shear rates and a shear-thinning region, at intermediate shear rates, in which a power-law evolution can be
6
10
5
10
4
10
3
10
2
10
1
10
0
η (Pa·s)
A
10
ð3Þ
where g0 is the zero-shear-rate limiting viscosity, c_ c is a critical shear rate for the onset of the shear-thinning region, ‘‘s’’ is a parameter related to the slope of the shear-thinning region, and g1 is the high-shear-rate limiting viscosity, included as a fitting parameter to fit the slight tendency to reach a high-shear rate Newtonian region. Tables 3–5 show the values of the Carreau model parameters. As can be observed, in all cases, viscosity values significantly increase (p < 0.05) with salt content, especially at low shear rates. Moreover, this effect is more apparent when egg yolk is predominant in the emulsifier blend, as
3.2. Viscous flow
10
1275
B
0.0 % 0.4 % 1.0 % 2.3 % Carreau model
-1
10
-5
10
-4
10
-3
-2
10 . 10
-1
10
0
10
1
10
2
10
-5
10
-4
γ ( 1/ s)
10
-3
-2
10.
10
-1
10
0
10
1
10
2
10
3
γ ( 1/ s)
Fig. 1. Influence of salt content on viscous flow curves. (A) Emulsions stabilized by 1% egg yolk and 7% pea protein blends (starch 1%). (B) Emulsions stabilized by 7% egg yolk and 1% pea protein blends (starch 1%).
Table 3 Carreau model fitting data and values of the Sauter diameter (d32) and uniformity (U) for emulsions stabilized by egg yolk and pea protein blends (starch 1%) g1 (Pa s)
c_ c (s1)
s
d32 (lm)
U
Emulsions with 1% egg yolk and 7% pea protein 0.0 3.25 · 103 0.4 2.32 · 104 1.0 3.46 · 104 2.3 6.51 · 104
0.11 0.15 0.12 0.10
2.8 · 104 2.7 · 104 2.1 · 104 2.7 · 104
0.42 0.45 0.44 0.46
4.1 3.4 3.3 3.3
0.78 0.87 1.05 0.99
Emulsions with 7% egg yolk and 1% pea protein 0.0 2.81 · 101 0.4 4.23 · 103 1.0 3.62 · 104 2.3 8.27 · 104
0.13 0.18 0.06 0.06
9.8 · 103 9.6 · 104 3.2 · 104 2.3 · 104
0.32 0.43 0.45 0.45
2.4 2.7 2.6 3.0
0.75 0.65 0.65 0.70
Csalt(% w/w)
g0 (Pa s)
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Table 4 Carreau model fitting data and values of the Sauter diameter (d32) and uniformity (U) for emulsions stabilized by egg yolk and sucrose laurate blends (starch 2%) g1 (Pa s)
c_ c (s1)
s
d32 (lm)
U
Emulsions with 6% egg yolk and 2% sucrose laurate 0.0 1.24 · 104 0.4 6.94 · 104 1.0 7.72 · 104 2.3 1.14 · 105
0.15 0.18 0.18 0.20
1.0 · 104 1.2 · 104 1.1 · 104 2.0 · 104
0.41 0.45 0.45 0.47
2.9 3.9 3.8 3.3
0.72 0.51 0.54 0.53
Emulsions with 8% egg yolk and 0% sucrose laurate 0.0 6.87 · 103 0.4 8.49 · 104 1.0 1.35 · 105 2.3 2.78 · 105
0.46 0.60 0.52 0.56
1.2 · 103 2.8 · 104 3.1 · 104 2.6 · 104
0.43 0.46 0.46 0.46
2.1 2.1 2.4 3.0
0.49 0.46 0.73 0.79
Csalt (% w/w)
g0 (Pa s)
1% egg yolk, 7% pea protein
5
η0 (Pa·s)
1.0x10
3
4
2
η 0 = 8.06·10 + 2.53·10 Csalt (r = 0.96)
7% egg yolk, 1% pea protein 2 1 4 η 0 = 2. 81· 10 + 3. 77· 10 Csalt (r = 0.94)
4
8.0x10
4
6.0x10
4
4.0x10
4
2.0x10
A
0.0 6% egg yolk, 2% SL
5
η0 (Pa·s)
2.8x10
4
4
2
η 0= 3.31· 10 + 3.80· 10 Csalt ( r = 0.81)
8% egg yolk, 0% SL 4 5 2 η 0= 2.18· 10 + 1.13· 10 Csalt ( r = 0.98)
5
2.1x10
5
1.4x10
4
7.0x10
B
0.0 5
1.5x10
6% egg yolk, 2% Tween20 3
4
2
η0= 9.32· 10 + 4.34· 10 Csalt ( r = 0.91)
2% egg yolk, 6% Tween20 3 3 2 η0= 3.86· 10 + 4.16· 10 Csalt ( r = 0.94)
η0 (Pa·s)
5
1.0x10
4
5.0x10
0.0
C 0.0
0.5
1.0
1.5
2.0
2.5
Csalt ( % w/ w) Fig. 2. Influence of salt content on the zero-shear rate-limiting viscosity for emulsions stabilized by (A) egg yolk and pea protein blends (starch 1%); (B) egg yolk and sucrose laurate blends (starch 2%) and (C) egg yolk and Tween 20 blends (starch 2.5%).
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can be clearly observed in Fig. 2 where the values of the zero-shear rate-limiting viscosity, g0, are plotted vs. salt concentration for emulsions stabilized with different blends of emulsifier. Thus, g0 increases almost linearly in emulsions stabilized by egg yolk/pea protein blends, being the slope slightly higher for the emulsion with higher egg yolk content in the emulsifier blend (Fig. 2A). This effect is much more remarkable in the case of emulsions stabilized by egg yolk/LMW surfactant blends (Fig. 2B and C). An increment in salt content slightly affects emulsions containing lower egg yolk/Tween 20 weight ratios but, however, significantly increases g0 values in emulsions stabilized by higher proportions of egg yolk (Fig. 2C). On the other hand, parameter ‘‘s’’, related to the slope of the shear thinning region, shows, in general, significantly lower values for salt-free emulsions in comparison to those found in emulsions with added salt (Tables 3–5). Other parameters of the Carreau model are not statistically affected by salt concentration (p > 0.05).
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3.3. Small-amplitude oscillatory shear measurements Figs. 3 and 4 show the mechanical spectra inside the linear viscoelastic range for selected emulsions. In general, the typical evolution of the linear viscoelastic functions with frequency for highly flocculated emulsions, i.e. a clearly developed plateau region, has been found (Franco et al., 2000). However, this typical evolution was not found for salt-free emulsions. In these cases, a higher slope in the evolution of G 0 and G00 with frequency and values of the loss tangent closer to one were found. Furthermore, increasing salt content yields higher values of G 0 and G00 and progressively lower values of tan d, which indicates a relatively more elastic response, together with a more developed plateau region. In addition to this, the linear viscoelastic region expands with salt content (data not shown), related with a more shear-resistant microstructure, even more as egg yolk concentration increases. The characteristic parameter of the plateau region is the so called plateau modulus, GoN , defined more precisely else-
Table 5 Carreau model fitting data and values of the Sauter diameter (d32) and uniformity (U) for emulsions stabilized by egg yolk and Tween 20 blends (starch 2.5%) g1 (Pa s)
c_ c (s1)
s
d32 (lm)
U
Emulsions with 2% egg yolk and 6% tween 20 0.0 3.77 · 103 0.4 6.47 · 103 1.0 6.83 · 103 2.3 1.38 · 104
0.10 0.11 0.11 0.14
1.8 · 104 1.1 · 104 9.0 · 105 1.2 · 104
0.41 0.41 0.41 0.42
1.4 1.3 1.3 1.3
0.30 0.31 0.30 0.30
Emulsions with 6% egg yolk and 2% tween 20 0.0 9.32 · 103 0.4 1.33 · 104 1.0 3.58 · 104 2.3 1.30 · 105
0.30 0.30 0.35 0.39
1.8 · 104 1.9 · 104 1.0 · 104 2.1 · 104
0.40 0.41 0.42 0.46
0.32 0.34 0.35 0.35
0.32 0.34 0.35 0.35
Csalt (% w/w)
g0 (Pa s)
2
10
G', G'' (Pa)
1
10
0
G'
10
G'' 0.0 % 0.4 % 1.0 % 2.3 %
-1
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
ω (rad/s) Fig. 3. Influence of salt concentration on the mechanical spectra of emulsions stabilized by 1% egg yolk and 7% pea protein blends (starch 1%).
I. Martı´nez et al. / Journal of Food Engineering 80 (2007) 1272–1281
1278 3
10
2
G', G'' (Pa)
10
1
10
G'
G'' 0.0% 0.4% 1.0% 2.3%
0
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
ω (rad/s)
Fig. 4. Influence of salt concentration on the mechanical spectra of emulsions stabilized by 8% egg yolk and 0% sucrose laurate blends (starch 2%).
where (Ferry, 1980). Fig. 5 shows the evolution of GoN with salt concentration for the emulsions studied. As identically found with g0, the plateau modulus significantly increases (p < 0.05) with salt concentration, but more rapidly at higher proportions of egg yolk in emulsions stabilized by egg yolk/LMW-surfactant blends (Fig. 5B and C). On the contrary, the slopes of GoN vs. salt concentration plots are quite similar for emulsions stabilized by blends of egg yolk and pea protein (Fig. 5A), in spite of the high differences in blend composition. 3.4. Droplet size distribution (DSD) Salt concentration does not exert a significant influence (p > 0.05) on mean droplet size and uniformity for the emulsions studied, as can be deduced from the values shown in Tables 3–5. Although not statistically significant, a slight tendency to increase the Sauter diameter and polydispersity with salt content may be observed for emulsions stabilized with the higher concentrations of egg yolk in the emulsifier blend. Consequently, droplet size does not seem to play an important role in physical stability. Thus, for instance, emulsions containing high Tween 20/egg yolk weight ratios show small droplet sizes but, however, they were more unstable towards creaming compared to emulsions mainly stabilized by macromolecules. 4. Discussion The experimental results obtained in this work indicate that salt addition produces a progressive increase of viscous and viscoelastic functions, relatively more elastic characteristics and improved stability against creaming for emulsions stabilized by binary blends composed of egg yolk and other emulsifier of different nature in several pro-
portions. However, salt addition does not exert, in general, a significant influence on droplet size distribution. In all cases, ionic strength affects in much higher extent the properties of emulsions mainly stabilized by macromolecules, i.e. egg yolk or pea protein. On the contrary, since the LMW emulsifiers used in several blends were non-ionic surfactants, these emulsions are less sensitive to changes in the ionic strength (So¨derman & Johansson, 2000). Consequently, the effect of salt on emulsion properties seems to be determined by changes in protein configuration occurred in egg yolk or pea protein isolate. In addition to this, in the case of egg yolk, these results can be also explained taking into account that the addition of NaCl disrupts granules and liberates HDL and phosvitin, which are available for adsorption at the oil–water interface in their molecular state (Anton, Beaumal, & Gandemer, 2000). Interfacial films made with native granules usually present a lesser viscoelasticity than those made with constituents liberated from granules. Moreover, the effective reduction of interfacial tension during emulsification requires unfolding of adsorbed proteins at the interface to allow their non-polar segments to come into contact with the oil (Parker, 1987). Native granules are likely to have a poor efficiency to decrease interfacial tension because large particles evidently would not spread over the interface as individual proteins do. Similar considerations can be given for the emulsion mainly stabilized by the pea protein isolate. These findings are in agreement with those of Walstra (1996) which observed that molecular aggregates of several sizes, contained in industrial protein preparations, are less efficient emulsifiers than individual proteins. The increasing interdroplet interactions induced by salt addition may lead to a higher development of the flocculation process, which could result in coalescence favoured by
I. Martı´nez et al. / Journal of Food Engineering 80 (2007) 1272–1281
1279
160 1% 0 GN 7% 0 GN
80
0
GN ( Pa)
120
egg yolk, 7% pea protein 2 = 13.93 + 40.36Csalt ( r = 0.94) egg yolk, 1% pea protein 2 = 0.93 + 41.50Csalt ( r = 0.98)
40
A
0 300
0
GN ( Pa)
250
6% egg yolk, 2% SL 2 0 G N = 35.49 + 35.78Csalt ( r = 0.86) 8% egg yolk, 0% SL 2 0 G N = 66.42 + 88.87C salt ( r = 0.94)
200 150 100 50
B
0
GN ( Pa)
0 200
6% egg yolk, 2% Tween20 2 0 G N = 16.80+ 71.90Csalt ( r = 0.94) 2% egg yolk, 6% Tween20 2 0 G N = 27.27+ 13.18Csalt ( r = 0.96)
160 120 80 40
C 0 0.0
0.5
1.0
1.5
2.0
2.5
Csalt ( % w/ w) Fig. 5. Influence of salt content on the plateau modulus for emulsions stabilized by (A) egg yolk and pea protein blends (starch 1%), (B) egg yolk and sucrose laurate blends (starch 2%) and (C) egg yolk and Tween 20 blends (starch 2.5%).
a reduction in the electrostatic repulsion among droplets. This fact can be seen in Tables 3–5 in which an increase in mean droplet size and polydispersity with salt content is observed for emulsions mainly stabilized by egg yolk. On the other hand, as previously reported (Franco et al., 2000), the consecution of an extensive flocculated state could be related to an improvement in creaming stability by immobilization of droplets in this network. Attending to the above discussion, this phenomenon seems to be reinforced by salt addition, as can be deduced from the change in the shape of the mechanical spectra (i.e. more developed plateau region) found by increasing salt content. In addition to this, the evolutions experimentally found in the zero-shear rate-limiting viscosity, parameter ‘‘s’’ or the plateau modulus with salt concentration could also be associated to a higher extension and strength of this flocculated
state. On the contrary, the low stability found in emulsions with high Tween 20/egg yolk weight ratios must be related to both a depletion flocculation mechanism, favoured by an excess of a LMW surfactant (Dickinson & McClements, 1995), and lower interfacial viscoelasticity (Wilde, 1996). Furthermore, the effect of salt on emulsifier blends, especially on egg yolk constituents, does not only have interfacial repercussions but also on the properties of the continuous medium. This fact is demonstrated by measuring the rheological behaviour of the continuous medium, previously to the addition of the oily phase. The importance that the continuous medium exert on the final emulsion is illustrated in Fig. 6, which shows viscous flow curves and mechanical spectra of two different continuous media (with 8% w/w egg yolk and 2% w/w starch) greatly differing in salt content. As can be observed, the presence of salt
I. Martı´nez et al. / Journal of Food Engineering 80 (2007) 1272–1281
1280
10
3
10
0
A
B 0.0 % 2.3 %
2
10
1
tan (δ )
G', G'' (Pa)
10
G'
G'' 0.0 % 2.3 %
10
10
-1
0
10
-2
10
-1
10
0
10
1
10
2
10
-2
10
-1
10
η (Pa·s)
6
10
5
10
4
10
3
10
2
10
1
10
0
10
10
1
10
2
ω (rad/s)
ω (rad/s) 10
0
C
0.0 % 2.3 %
-1
10
-5
10
-4
10
-3
10
-2
10 .
-1
10
0
10
1
10
2
10
3
-1
γ (s ) Fig. 6. Influence of salt concentration (values referred to the final emulsion) on (A) the storage and loss moduli, (B) the loss tangent and (C) the viscosity for continuous media of an emulsion stabilized by 8% egg yolk, differing in salt content (starch 2%).
induces a significant increment in viscosity and higher values of the linear viscoelastic functions associated to a higher relatively elastic response. In this sense, for relatively low weight oil fractions, as those considered in this work, the continuous medium greatly influences the rheological behaviour of the final emulsion, even more when salt content increases. Thus, for instance, as can be observed in Fig. 6 and Table 4, both continuous media (salt-free and containing 2.3% salt in the final system) show values of viscosity very similar to those found in the corresponding final emulsion (see Table 4). This effect may be mainly attributed to the salt-induced gelation of egg yolk lipoproteins (Guerrero et al., 2004). Once again, these results can be explained on the basis of a decrease in electrostatic repulsion forces favouring protein associations which increase viscous and viscoelastic parameter (Kim, Renkema, & van Vliet, 2001; McClements, Monahan, & Kinsella, 1993). Following the same mechanism, droplets
may also flocculate more easily due to the contraction of the electrical double layer around the charged droplets. 5. Conclusions From the experimental results it may be concluded that rheological properties and physical stability of the emulsions studied are significantly influenced by salt content and the nature of binary emulsifier blends. In general, salt addition produces a progressive increase of viscous and viscoelastic parameters and improved stability against creaming for emulsions stabilized by binary blends composed by egg yolk and other emulsifier of different nature in several proportions. However, salt addition does not exert, in general, a significant influence on droplet size distribution. Salt affects the rheological properties of emulsions stabilized by high proportions of macromolecules in much higher extent than those containing high relative weight ratios of non-
I. Martı´nez et al. / Journal of Food Engineering 80 (2007) 1272–1281
ionic low-molecular-weight surfactants, which are less sensitive to changes in the ionic strength. The values of viscosity and linear viscoelastic parameters analysed increase with salt concentration more markedly when a protein is predominant in the emulsifier blend, which must be attributed to increasing interdroplet interactions and viscosity of the continuous medium, both of them induced by salt addition. On the contrary, different blends of pea protein and egg yolk show a quite similar evolution of the rheological parameters with salt concentration. References Anton, M., Beaumal, V., & Gandemer, G. (2000). Adsorption at the oil– water interface and emulsifying properties of native granules from egg yolk: effect of aggregated state. Food Hydrocolloid, 14, 327–335. Dickinson, E. (1992). An introduction to food colloids. Oxford: Oxford University Press. Dickinson, E., & McClements, D. J. (1995). Advances in food colloids. Glasgow: Blackie Academic and Professional. Ferry, J. D. (1980). Viscoelastic properties of polymers. New York: John Wiley & Sons. Franco, J. M., Partal, P., Ruiz-Marquez, D., Conde, B., & Gallegos, C. (2000). Influence of pH and protein thermal treatment on the rheology of pea protein-stabilized oil-in-water emulsions. Journal of the American Oil Chemists Society, 77, 975–983. Guerrero, A., Carmona, J., Martinez, I., Cordobes, F., & Partal, P. (2004). Effect of pH and added electrolyte on the thermal-induced transitions of egg yolk. Rheologica Acta, 43, 539–549. Harrison, L. J., & Cunningham, F. E. (1986). Influence of frozen storage time on properties of salted yolk and its functionality in mayonnaise. Journal of Food Quality, 9, 167–174. Kim, K. H., Renkema, J. M. S., & van Vliet, T. (2001). Rheological properties of soybean protein isolate gels containing emulsion droplets. Food Hydrocolloids, 15, 295–302. Mackie, A. R., Gunning, A. P., Wilde, P. J., & Morris, V. (2000). Orogenic displacement of protein from the oil/water interface. Langmuir, 16, 2242–2247. McClements, D. J., Monahan, F. J., & Kinsella, J. E. (1993). Effect of emulsion droplets on the rheology of whey-protein isolate gels. Journal of Texture Studies, 24, 411–422. Palazolo, G. G., Mitidieri, F. E., & Wagner, J. R. (2003). Relationship between interfacial behaviour of native and denatured soybean isolates
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