Ionic strength

Ionic strength may significantly influence the stability of emulsions in various ways primarily depending upon the valency, charge, concentration, and nature of ions available in the continuous phase (McClements, 2005). Presence of salts either screens the charge of the proteinstabilized emulsions or the added ions may bind to oppositely charged groups on the droplet surface, reducing the ζ- potential magnitude and thus significantly decreasing the electrostatic repulsion between emulsion droplets, finally resulting in saltinduced aggregation effects. This mechanism of saltinduced aggregation influencing the overall ζ-potential either by charge screening of the electrostatic repulsion between the proteinstabilized emulsiondroplets by Ca2+_{, Na}+ _{and K}+ _{salts or by ion binding}

to the droplet surfaces (Agboola & Dalgleish, 1995; Kulmyrzaev et al., 2000; Güzey et al., 2004) can be predicted by classical colloid science theory (Hunter, 1986), which basically correlates the zeta potential (ζ) with the surface charge density (σ) as follows:

0

sgn( ) [2

kT

n

{exp(

z e

/kT) 1}]



 



 









(2.6-1)

where, εO is the dielectric constant of the vacuum, εR is the relative dielectric

absolute temperature, ni0 and zi are the number per unit volume and valency of

ions “i” respectively, e is the magnitude of electrical charge for a single electron and sgn describes the sign function i.e. sgn(x) = 0, if x > 0, and 1 if x < 0. A number of studies have been performed to understand the effects of addition of different concentration and types of salts on emulsion characteristics, which are briefly summarized in the following subsections.

2.6.2.1 Monovalent cations

Addition of Na+_{(Demetriades et al., 1997; Srinivasan et al., 2000) and K}+_(Hunt

& Dalgleish, 1996; Kulmyrzaev & Schubert, 2004) ions on the physicochemical characteristics of whey proteinstabilized emulsions have been explored. Authors have reported that presence of ≥ 150 mM NaCl can result in creaming and droplet flocculation in WPIstabilized emulsions (28.0 wt% corn oil, 2.8 wt% WPI) by electrostatic charge screening effects (Djordjevic et al., 2004). In presence of K+ ions, whey proteinstabilized emulsions are stable to aggregation upto ~ 100 mM KCl, provided the pH is adjusted to < 4 or > 6 (Kulmyrzaev & Schubert, 2004).

In case of sodium caseinatestabilized emulsions, presence of K+ _{or Na}+ _{ions not}

only influences the emulsion stability but also affects the surface composition and relative surface coverage of the individual casein fraction (αs1-casein and/or

β-casein). It has been observed that emulsions made with αs1-casein are generally

more susceptible to saltinduced flocculation by NaCl, showing extensive flocculation above 100 mM NaCl than that stabilized by β-casein, latter showing no aggregation under the same conditions (Murray & Stainsby, 1987; Dickinson, 1997a). Hunt and Dalgleish (1996) reported that the addition of KCl > 25 mM concentration significantly influenced the adsorption behaviour of sodium caseinate to the droplet surface with relative increase in levels of adsorbed αs1-

casein at the expense of β-casein. Similar observations were also reported in the presence of NaCl (Srinivasan et al., 2000). However, referring to relative surface concentration analysis, it appears that β-casein concentration at the interface remains largely unaffected by NaCl addition, whereas the surface concentration

of αs1-casein increases substantially, indicating that the influence of ionic strength

on emulsion stability also depends on the concentration and kind of the protein saturated at the droplet surface.

2.6.2.2 Divalent cations

A significant number of publications on the effects of Ca2+ ions (Agboola & Dalgleish, 1995; Kulmyrzaev et al., 2000; Ye & Singh, 2000; Dickinson et al., 2003) and to a lesser extent on Cu2+ ions (Silvestre et al., 1999) on flocculation of milk proteinstabilized emulsions have revealed important insights about the interactions of divalent cations with protein coated droplet surface. It was reported that the binding of Ca2+ ions to whey proteins reduces electrostatic repulsions and induces hydrophobic interactions (Baumy & Brulé, 1988), which may unavoidably affect the adsorption behaviour of whey proteins to the droplet surface. The aggregation of -lgstabilized emulsion by added Ca2+ ions has also been reported due to the possible salting out of globular proteins (Agboola & Dalgleish, 1995).

The effects of divalent cations on emulsion stability of WPCstabilized emulsion droplets (30.0 wt% soy oil, 0.5 or 3.0 wt% WPC) have been studied(Ye & Singh, 2000). In emulsions made with 3.0 wt% WPC, the emulsion stability decreased slightly at low CaCl2 concentrations (< 15 mM), but markedly at higher Ca2+

concentrations (20 mM). This decrease in emulsion stability was explained by divalent cations inducing charge screening effects, resulting in flocculation and coalescence. Interestingly, in another study, the influence of both CaCl2 (0 to 10

mM) and KCl (0 to 600 mM) in WPIstabilized emulsions (7.0 wt% soy oil, 0.35 wt% WPI) at neutral pH was investigated (Keowmaneechai & McClements, 2002). These authors reported extensive droplet flocculation and creaming instability of emulsions above critical CaCl2 (3 mM) and KCl (200 mM)

concentrations and also suggested that divalent Ca2+ ions are more effective than K+ _{ions in causing droplet flocculation and creaming via charge screening}

In case of emulsions containing caseins as the adsorbed layer, calciuminduced flocculation has been demonstrated (Dickinson et al., 1987; Agboola & Dalgleish, 1995; Dalgleish, 1997). Emulsions containing κ-casein were generally observed to be resistant to Ca2+_{induced aggregation. However, emulsions}

stabilized by either αs1-casein or β-casein were generally flocculated by 15 mM

CaCl2 (Dickinson et al., 1987), which can be explained by the specific binding of

the Ca2+ ions to the negatively charged phosphoserine residues (Swaisgood, 1982) available on the αs1-casein or β-caseins (not available in case ofκ-casein),

thus altering the molecular charge distribution, selfassembly behaviour and finally conformation of the adsorbed casein molecule. This conformational change possibly reduced the thickness of the interfacial layer, thereby decreasing the steric repulsion (Horne, 1998). Hence, the calciuminduced flocculation in sodium caseinate emulsions is not only related to the screening of electrostatic droplet charges by divalent cations unlike whey protein, but also associated with the bound Ca2+ ions reducing the efficacy of steric stabilization and binding of Ca2+ ions between phosphoserine residues on caseincoated droplets causing bridging flocculation (Horne & Leaver, 1995). It is worth noting that addition of a moderate concentration of CaCl2 (58 mM Ca2+ ions) prior to homogenization

can significantly enhance emulsion stability particularly in concentrated caseinate emulsion systems by inhibiting depletion flocculation (Dickinson & Golding, 1998), which is an interesting observation as compared to well known destabilizing role of calcium salts.