Chemistry Reference
In-Depth Information
Chapter 17
Interactions during the Acidification of
Native and Heated Milks Studied by
Diffusing Wave Spectroscopy
Marcela Alexander, Laurence Donato and Douglas G.
Dalgleish
DEPARTMENT OF FOOD SCIENCE, UNIVERSITY OF GUELPH,
GUELPH, ONTARIO, CANADA, N1G 2W1
17.1 Introduction
The acid gelation of milk has often been thought of as a classic case of the
destabilization of a colloidal system to form a particulate gel. Skim milk is a
dispersion of particles (casein micelles) that are essentially composed of proteins
and protected by a surface 'hairy' layer of
k
-casein.
1
This surface layer carries
some charge (with a reasonably large negative z-potential of around -20 mV),
2
but the layer is believed to exert its stabilizing effect mainly because it protrudes
from the surface to provide steric stabilization of the particles.
3
Many studies
suggest that the hairy layer has a hydrodynamic thickness between 5 and 10 nm,
and that its removal by chymosin treatment at neutral pH destabilizes the casein
micelle.
4,5
The chymosin treatment diminishes both the surface charge and the
steric stabilization at the same time.
Conversely, during the acidification of milk, the hairy layer remains on
the micellar surface. But it is believed that, as the pH decreases, the surface
charges are titrated and the hairy layer collapses, so that steric and electro-
static stabilization are diminished, and at pH
5.0 the casein micelles
coagulate to form a gel.
6
This gel is weak,
7
presumably because the hairy
layer, although collapsed, prevents touching of the (putatively hydrophobic)
micellar 'cores', so that the forces holding the particles together are not
as strong as for the case when the hairy layer is removed. This is evident
when the strengths of acid gels made from partially renneted milks are com-
pared with those from unrenneted milk.
8
Thus far, therefore, it has been
possible to regard acid milk as simply a destabilizing hard-sphere colloid
E
257
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