Food Emulsifiers and Stabilizers - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 239

proportions 4:4:1:1) linked by calcium ions and colloidal calcium phosphate. The
caseins

have fairly similar molecular masses and isoionic points (pH = 5.2),
but rather different molecular charges (-20e, -12e, and -4e) at neutral pH. 160 The
commercial emulsifier sodium (or potassium) caseinate lacks calcium phosphate and
is consequently less aggregated than the casein in milk, but has roughly the same
protein composition. A comparison of the properties of emulsions stabilized by
individual caseins,

α s1 , β , κ

with those stabilized by sodium caseinate will
provide insight into the competitive and/or cooperative aspects of protein adsorption
and colloid stabilization in casein-containing systems.
Adsorption experiments at fluid interfaces by Michel et al. 160 and Dickinson
et al. 161 showed that

α s1 , β , and κ

β -casein was more surface-active than α s1 -casein but direct

evidence for preferential adsorption of -casein in emulsions is rather limited. 163,164

Replacing a real complex emulsion system with a model system composed of mon-
odispersed polystyrene latex particles enables adsorbed proteins to be studied in a
systematic manner. Different monomeric caseins adsorbed separately on negatively
charged latex particles. 164,165 Consistent with it having a higher net charge in solution
than

β

α s1 -casein adsorbed on latex gave coated particles a higher
electrophoretic mobility compared to the other caseins. However, in the presence of
calcium ions, at concentrations greater than 1 mM, latex particles coated with

β - or κ -casein,

κ -casein

carry a higher effective negative charge than particles coated with either α s1 - or

β -casein. This is interpreted as being due to the strong binding of calcium ions to

α s1 - and

-caseins thereby reducing the net negative charge on the adsorbed protein
layer. The adsorption characteristics of sodium caseinate on lattices are intermediate
between those for

β

α s1 - and β -caseins, suggesting that for solid surfaces, β -casein does

not displace α s1 -casein over the short experimental time-scale. 164

Many semi-empirical studies on the emulsifying and foaming behavior of other
food grade proteins have been reported. Such semi-empirical information is essential
to food technologists but gives little insight into the key physico-chemical factors
involved. Many proteins have been reported to stabilize emulsions including
lysozyme, bovine serum albumin, myosin, soy protein, -lactoglobulin, gelatin, etc.

Each of these proteins has some advantages in the key factors affecting stabilization
such as adsorption (kinetics of diffusion, induction periods, ability to lower surface
tension, number of adsorbing sites, thickness of the film, extent of coverage, etc.),
desorption (reversibility of adsorption, competitive adsorption, conditions for inter-
facial replacement), and film properties (thickness of adsorbed protein film, dena-
turation on the surface, coagulation formation of mixed film with other proteins or
with low molecular weight surfactants, surface rheology).
A basic knowledge of the physico-chemical properties of food proteins and an
understanding of the many factors affecting the quality of foods is required to
fabricate acceptable food products. While soy proteins have largely been considered
as economical substitutes for more expensive protein ingredients, they should be
viewed as vital functional components for use by food technologists to fabricate
new foods.
The trend towards quick-service convenience foods requiring fabrication, dehy-
dration, rehydration, etc., brought the versatility of textured proteins to the forefront
in the mid and late 1960s. The economic performance of soy protein in chopped

β