Chemistry Reference
In-Depth Information
tends to reduce xanthan gum viscosity (Hassler and Doherty, 1990). It
has been shown that an acetate-free xanthan has higher viscosity than
native xanthan.
Initial studies on the mechanism of xanthan gum synergy with galac-
tomannans proposed a model in which the unsubstituted, galactose-free
(smooth) regions of the galactomannans bind to xanthan in its ordered
state (Morris
et al.
, 1977). This model has been used to explain the dif-
ference in the degree of interaction between galactomannans of differing
degrees of substitution. Subsequent studies, which showed that interac-
tions were enhanced after heating the mixture to temperatures above the
coil-helix transition of the xanthan, were interpreted as evidence that
the binding occurred with the cellulosic backbone of the xanthan gum in
the ordered state (Cairns
et al.
, 1986, 1987). An alternative explanation
was that when the hydrocolloids are mixed at temperatures below the
setting point of the gel, they form an inferior, disrupted network with
melting and resetting giving a stronger, coherent gel (Morris, 1990).
More recent studies tend to support this interpretation and favour the
original model of Morris
et al.
(Morris and Foster, 1994; Cronin
et al.
,
2002; Fitzsimons
et al.
, 2008; Sworn and Kerdavid, 2009).
The strength of gels of xanthan and LBG or konjac mannan have
been shown to be very dependent upon the degree of acetyl substitution
and have generally concluded that the interactions increase with de-
creasing acetylation (Shatwell
et al.
, 1991a, 1991b; Wang
et al.
, 2002;
Morrison
et al.
, 2004). The result of which is stronger gels with LBG
and konjac mannan. It has also been suggested that low-acetate xanthan
has stronger interactions with guar gum compared with that of stan-
dard xanthan (Lopes
et al.
, 1992; Morris and Foster, 1994; Milas
et al.
,
1996; Morrison
et al.
, 2004). For example, evidence for this was demon-
strated in the form of increased viscosity at 1 per second in low-acetate
xanthan/guar mixtures compared with standard xanthan/guar mixtures
(Morrison
et al.
, 2004). There is very little published evidence to suggest
that the pyruvate content of xanthan has an influence on interactions with
galactomannans. Gel strength with LBG has been shown to be reduced
slightly with the reduction of pyruvate level, but results were not con-
clusive because the molecular weight of the low-pyruvate samples was
lower than standard xanthan (Shatwell
et al.
, 1991a). They also showed
that a xanthan sample from a mutant strain of
Xanthomonas campestris
,
believed to lack the terminal mannose residue from the trisaccharide side
chains, formed very weak gels, suggesting that the side chain may play
an important role in the interaction with LBG. However, recent work has
identified a xanthan gum with high pyruvate content that has exception-
ally high synergy with galactomannans (Sworn
et al.
, 2009). This work
also showed that the synergy was not increased by selective removal of
