Carboxymethylcellulose (CMC; E466)
is a derivative of cellulose formed by
its reaction with alkali and chloroacetic acid.
The CMC structure is based on the β-(14)-D-glucopyranose
polymer of cellulose. Different preparations
may have different degrees of substitution, but it is generally
in the range 0.6 - 0.95 derivatives per monomer unit. [Back to Top ]
are somewhat shorter, on average, than native cellulose with
uneven derivatization giving areas of high and low substitution.
This substitution is mostly 2-O- and 6-O-linked, followed
in order of importance by 2,6-di-O- then 3-O-, 3,6-di-O-,
2,3-di-O- lastly 2,3,6-tri-O-.linked. It
appears that the substitution process is a slightly cooperative
(within residues) rather than random process giving slightly
higher than expected unsubstituted and trisubstituted areas.
CMC molecules are most extended (rod-like) at low
concentrations but at higher concentrations the molecules
overlap and coil up and then, at high concentrations, entangle
to become a thermoreversible gel. Increasing ionic strength
and reducing pH both decrease the viscosity as they cause
the polymer to become more coiled. [Back to Top ]
Most CMCs dissolve
rapidly in cold water and are mainly used for controlling viscosity
without gelling (CMC, at
typical concentrations, does not gel even in the presence
of calcium ions). As its viscosity drops during heating, it may
be used to improve the volume yield during baking by encouraging
gas bubble formation. Its control of viscosity allows use as
thickener, phase and emulsion stabilizer (for example, with milk
casein), and suspending agent. CMC can be also used for its water-holding capacity as this is high
even at low viscosity; particularly when used as the Ca2+ salt. Thus, it is used for retarding staling and reducing fat uptake
into fried foods.
The average chain length and degree of substitution are of great
importance; the more-hydrophobic lower substituted CMCs
are thixotropic but more-extended
higher substituted CMCs
are pseudoplastic. At low pH, CMC
may form cross-links through lactonization between carboxylic acid
and free hydroxyl groups.
The solution properties of a range of commercial CMC's have been investigated .
Interactive structures are available (Jmol). [Back to Top ]