ABSTRACT
T he capacity o f food processing and form ulation is rapidly increasing. As a result, the dem ands on protein functional properties in food systems has become acute, including the ability to form stable foams. Not all proteins foam, and those that do vary widely in their foam ing capacity (Graham and Phillips, 1976; MacRitchie, 1978; Hailing, 1981; Kinsella, 1981; Kinsella and Phillips, 1989). In foams, the dispersed phase volume is large in com parison to the continuous phase. High surface energy created by large air-water interfacial surface area and substantial density differences between phases ren d er these dispersions therm odynamically unstable (Table 6.1). For a protein to be a successful foam ing agent, it m ust be able to stabilize the new surface area continuously being created during foam ing (MacRitchie, 1978). T he m igration o f proteins to the interface is energetically favorable because some of the conform ational
Enhanced stability Reduced stability
Protein’s flexible dom ains increase Drainage (gravitational) of viscosity aqueous phase
Protein concentration and him thickness D isproportionation Film mechanical strength and surface Mechanical shock
viscoelasticity Gibbs-M arangoni effect Capillary pressure Film net surface charge Perm eable him H eterogeneous proteins with residual Surface-active lipids
tertiary structure
Solubility Amphipathicity
Segmental flexibility Interactive segments
Disposition o f charged groups
Disposition o f polar groups
Rapid diffusion to the interface Distribution o f charged, polar, and nonpolar
residues for enhanced interfacial interactions
Facilitate unfolding at the interface T he disposition o f d ifferen t functional
segments facilitaties secondary interactions in the air, aqueous, and interfacial phases
Charge repulsion between contiguous bubbles
Prevents close approach o f bubbles; hydration, osmotic, and steric effects
energy and some of the energy o f hydration o f the protein is lost at the interface (Phillips, 1981). T he adsorption o f proteins to an interface is not necessarily irreversible, however, and for many proteins the net energy is not sufficient to maintain the protein adsorbed. This has led to a reform ulation o f the concept o f protein surface adsorption to contain both a diffusion coefficient term and a probability o f adsorption (Damo daran, 1990). Thus, a protein effective in foam ing m ust rapidly diffuse to the interface, adsorb, and then reorien t to form a viscous him to m aintain discrete bubbles until stabilizing interactions develop (Prins,
1988; Dickinson et al., 1988; Kinsella and Phillips, 1989). This ability is dependen t on intrinsic factors including the structure and conform ation o f the protein, which in tu rn depends on environm ental factors such as pH , ionic strength, and protein interactions (Dam odaran, 1990; Hailing, 1981; Kinsella and Phillips, 1989). In most food foams, we m ust place an additional requirem ent on the film properties o f proteins in that stabilizing interactions m ust be m aintained through the gelation and m atrix form ation phase that ultimately stabilizes the solid food foam (Table 6 .2).
