Chemistry Reference
In-Depth Information
CH
2
OH
O
CH
2
OH
O
O
O
OH
n
OH
OH
O
O
CH
2
OCCH
3
O
OH
OH
COO-M+
O
O
OH
OH
+M-OOC
OCH
2
O
O
C
OH
OH
O
H
3
C
M+ = Na, K, ½Ca
Fig. 5.4
The primary structure of xanthan gum.
et al.
, 1981; Cheetham and Norma, 1989; Hassler and Doherty, 1990).
It has been suggested that there is no continuous relationship between
pyruvate content and viscosity, but rather, that there is a steep increase
when going from below 2% to above 3% pyruvate (Flores Candia and
Deckwer, 1999). It has also been demonstrated that the viscosity of low
pyruvate xanthan is less sensitive to the addition of salts (Cheetham and
Norma, 1989).
There is one published paper that contradicts this and claims that
pyruvate content has no significant effect on solution viscosity, and the
authors attributed the differences observed in other studies to possible
differences in molecular weight (Bradshaw
et al.
, 1983). However, in
this study, viscosity was measured at shear rates between 8.8 and 88.8
per second. These relatively high shear rates may account for the lack
of difference in measured viscosity. Generally, the viscosity differences
are far more marked at shear rates below 0.1 per second. Christensen
et al.
have shown that the terminal β-mannose is relatively susceptible
to acid hydrolysis. Thus, low pyruvate samples prepared in this way
may also have reduced molecular weight due to removal of this sugar
(Christensen
et al.
, 1993). The presence of acetate, on the other hand,
