Mixed gels - Physical Gels from Biological and Synthetic Polymers - page 304

all cases, turbidity was observed at suf ciently elevated temperatures. Cloud point curves

are well reproduced even after repeated heating and cooling cycles.

10.4.2 Depletion interaction

A different type of phase separation, associated with an attractive interaction between
polymers, commonly referred to as depletion interaction, was reported by Tuinier et al.
( 2000 ). The solution had a more complex composition, containing whey protein particles
( Chapter 9 ) of colloidal size, mixed with exocellular polysaccharides from lactic acid
bacterium.
The native whey proteins were subjected to a preliminary heating step and became
denatured. Denatured whey proteins are mutually attractive and the proteins are partially
aggregated. Phase separation kinetics was evidenced by small-angle light scattering
(SALS) in this system. It was found that the overall value of the scattered intensity
increases with time. All curves went through a maximum in the scattered intensity as a
function of q. The first recorded value of the characteristic length was about 9 μm.

The solutions eventually phase separated because the partial aggregation of the whey
proteins did not prevent sedimentation of the phases by gravity, as was the case for
mixtures containing gelatin.

10.4.3 Protein - protein mixed-gel systems

A number of these, particularly those formed from dairy products, are of great commer-
cial importance, although speci

c details are not discussed here since whole volumes
(Fox and McSweeney, 2003 ), and indeed whole journal runs, are dedicated to them.
Model systems of, for example, BSAwith the whey protein components β -lactoglobulin

(Lg) and -lactabumin (La) ( Chapter 9 ) have been studied by a number of groups,

following the pioneering work by Hermansson and co-workers (Langton and
Hermansson, 1992 ). They studied microscopy of these mixtures using a variety of
techniques, and in later work extended the mixtures to include those with added gelatin,
and performed small-deformation measurements on the same mixtures (Walkenstrom
and Hermansson, 1994 ). They concluded from their gel formation studies that the
components gel individually, suggesting phase separation. They detected a change in
overall properties above c.10% whey protein and 3% gelatin; gel formation of the whey
protein mixture was independent of the presence of gelatin, while that of gelatin
depended on the whey proteins. At concentrations below these values, the mixed gels
were thermoreversible, so the system was gelatin-continuous. At concentrations above
this, the microstructure of the mixed gels suggested that a phase separated, bicontinuous
system was formed.
When a mixture of globular proteins is heated above the denaturation temperatures of
the components it appears that mutual aggregation allows mixed networks to form. Many
years ago it was suggested that immuno-staining would allow this effect to be studied in
more detail. Work by Comfort and Howell ( 2002 ) has applied this method to samples,
albeit rather crude, of soy and whey protein mixtures.

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