Protein-stabilized emulsions

Some food O/W emulsions, including milk, cream, ice cream, and coffee whiteners and toppings, are stabilized by proteins such as casein that form a coating around the fat globules. These products also need to have some of the fat (oil) be partly crystalline to link the droplets in a network structure and to prevent complete coalescence of the oil droplets, especially under shear [78,825]. 

The complex structure of milk can be radically altered during processing. For example, changing the pH causes disintegration or rearrangement of the casein 

Part of the process to make cheese involves the flocculation of an electrostatically stabilized colloidal O/W emulsion of oil droplets coated with milk casein. The flocculation is caused by the addition of a salt, leading to the formation of networks which eventually gel. The other part of the process involves reaction with an enzyme (such as rennet), an acid (such as lactic acid), and possibly heat, pressure and microorganisms, to help with the ripening [811]. The final aggregates (curd) trap much of the fat and some of the water and lactose. The remaining liquid is the whey, much of which readily separates out from the curd. Adding heat to the curd (-38 °C) helps to further separate out the whey and convert the curd from a suspension to an elastic solid. There are about 20 different basic kinds of cheese, with nearly 1000 types and regional names. Potter provides some classification [811]. 

Having proteins present reduces the interfacial tension from about 27 mN/m for a pure oil/water system, to about 14 mN/m [827]. This may provide a sufficiently 

Casein or egg-yolk proteins are used as emulsifiers in a number of food products, such as O/W food emulsions (Table 13.1) [78,824]. A key difference here is that in caseinate-stabilized oil emulsions, the casein forms essentially monolayers and there are no casein micelles nor any calcium phosphate. Such emulsions are thought to be stabilized more by electrostatic repulsive forces and less by steric stabilization, in contrast to the situation in homogenized milk products [824]. 

Milk contains about 40 gl of fat and 32 gl of protein. The milk fat occurs in droplets stabilizedby layers of phospholipid and protein [16]. Ofthe protein, about 25gl is casein and 7 gl is serum proteins (mostly lactoglobulins). The caseins form casein micelles , which involve both casein and calcium phosphate, may contain 500-10000 casein protein molecules, and are of the order of 200-300 nm in diameter [16, 18, 34, 36]. The casein micelles are quite complex. There are four types of casein molecules of which most (a j -, a 2 P-caseins) complex with 

The complex structure of milk can be radically altered during processing. 

3) In cheese-making, the term whey refers to the dilute O/W emulsion that separates from the cot -ulated portion or curd. 

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S. Mohan and G. Narsimham Coalescence of Protein-Stabilized Emulsions in a High-Pressure Homogenizer. J. Colloid Interface Sci. 192, 1 (1997). 

E. Tomherg Punctional Characterization of Protein Stabilized Emulsions Emulsifying Behavior of Proteins in a Valve Homogenizer. J. Sci. Pood Agric. 29, 867 (1978). 

As reported in this chapter, the microscopic origin of both compressibility and elasticity of dense emulsions is rather well understood. Emulsions have elastic properties arising from either surface tension or surface elasticity and plasticity. Some protein-stabilized emulsions obey the same phenomenology as solid-stabilized emulsions they exhibit substantially higher osmotic resistances and higher shear moduli than surfactant-stabilized emulsions [38 0]. Moreover, they are strongly resistant to water evaporation. Proteins possess the ability to form... 

E. Dickinson and I. Chen Viscoelastic Properties of Protein-Stabilized Emulsions Effect of Protein-Surfactant Interactions. I. Agric. Food Chem. 46, 91 (1998). 

T. D. Dimitrova and F. Leal Calderon Bulk Elasticity of Concentrated Protein-Stabilized Emulsions. Langmuir 17, 3235 (2001). 

McClements, D.J. (2004). Protein-stabilized emulsions. Current Opinion in Colloid and Interface Science, 9, 305-13. 

Figure 3.4 Effect of polysaccharide on protein-stabilized emulsions. The diameter ratio, j43nuxtlire / J43protem is plotted against the molar ratio R (moles polysaccharide / moles protein). Here J43nuxtlire is average droplet diameter in fresh emulsion prepared with protein + polysaccharide, and d43pTOtQm is average diameter in emulsion stabilized by protein alone. Key , , legumin + dextmn (48 kDa) or legumin + dextran (500 kDa), respectively (0.5 w/v % protein, 10 vol% oil, pH = 8.0, /= 0.1 M) (Dickinson and Semenova, 1992) O, , asi-casein + pectinate and p-casein + pectinate at pH = 7.0, / = 0.01 M (2.0 w/v % protein, 40 vol% oil), respectively , p-casein + pectinate at pH = 5.5, / = 0.01 M (2.0 w/v % protein, 40 vol% oil) (Semenova et al, 1999). Reproduced from Semenova (2007) with permission.

Dickinson, E., Pawlowsky, K. (1997) Effect of i-carrageenan on flocculation, creaming, and rheology of a protein-stabilized emulsion. Journal of Agricultural and Food Chemistry, 45, 3799-3806. 

For a colloidal system containing a mixture of different biopolymers, in particular a protein-stabilized emulsion containing a hydrocolloid thickening agent, it is evident that the presence of thermodynamically unfavourable interactions (A u > 0) between the biopolymers, which increases their chemical potentials (thermodynamic activity) in the bulk aqueous phase, has important consequences also for colloidal structure and stability (Antipova and Semenova, 1997 Antipova et al., 1997 Dickinson and Semenova, 1992 Dickinson et al., 1998 Pavlovskaya et al., 1993 Tsap-kina et al., 1992 Semenova et al., 1999a Makri et al., 2005 Vega et al., 2005 Semenova, 2007). 

Figure 7.9 Effect of pectin (DE = 76%) on (a) creaming of protein-stabilized emulsions (11 vol% oil, 0.6 wt% protein, 0.28 wt% pectin, I = 0.01 M) containing (A) asi-casein (pH = 7), (A) p-casein (pH = 7), and ( ) o i-casein (pH = 5.5) and (b) steady-state shear viscometry of casein-stabilized emulsions (40 vol% oil, 2 vt% protein). Apparent shear viscosity at 22 °C is plotted against stress pH = 7.0, / = 0.01 M, (A) -casein, (A) p-casein, ( ) ocsi -casein + 0.5 wt% pectin, ( ) p-casein + 0.5 wt% pectin, ( ) p-casein + 1.0 wt% pectin, (O) as[-casein + 1.0 wt% pectin pH = 5.5,1 = 0.01 M, (x) ocsi -casein, (O) as[-casein + 0.5 wt% pectin, ( ) oc -casein + 1.0 wt% pectin. Reproduced from Semenova (2007) with permission.

In the study of Neirynck et al. (2007), the electrophoretic mobility data indicated that whey protein-stabilized emulsion droplets became gradually more negatively charged with pectin addition at pH = 5.5. This change was not only reflected in a smaller average droplet size, but also in a significant improvement in the creaming stability of the emulsions. 

Gancz, K., Alexander, M., Corredig, M. (2006). In situ study of flocculation of whey protein-stabilized emulsions caused by addition of high-methoxy 1 pectin. Food Hydro-colloids, 20, 293-298. 

Dickinson, E., Owusu, R.K., Williams, A. (1993b). Orthokinetic destabilization of a protein-stabilized emulsion by a water soluble surfactant. Journal of the Chemical Society, Faraday Transactions, 89, 865-866. 

Some products, like butter and margarine are stabilized by fat crystals. Salad dressings and beverage emulsions are stabilized by other emulsifiers. The stability of non-protein stabilized food emulsions, involving lower molar mass type molecules, tend to be better described by the DLVO theory than are protein-stabilized emulsions. An example of an O/W emulsifier whose emulsions are fairly well described by DLVO theory is sodium stearoyl lactylate [812],... 

Dalgleish, D.G. 1989. Protein-stabilized emulsions and their properties. In Water and Food Quality (T.M. Hardman, ed.), pp. 211-250, Elsevier Applied Science, London. 

Klemaszewski, J.L., Haque, Z., Kinsella, J.E. 1989. An electronic imaging system for determining droplet size and dynamic breakdown of protein-stabilized emulsions. J. Food Sci. 54, 440-445. 

The protein stabilized emulsions formed were made up of 40 % (w/w) soybean oil and 60 % (w/w) protein dispersion of 2.5 % (w/w) protein content. A quantity of 50 grams was emulsified... 

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