Hydrocolloid Gums – Their Role and Interactions in Foods - Practical Food Rheology - page 72

polymer chains of the co-synergists in the formation of heterotypic junc-
tions (Goycoolea et al ., 1995b). Replacing KM by LBG (both having
a β-1,4-linked sugar backbone) shows the same trends; however, the
sol-gel transition is wider, producing a gel with a modulus thrice that of
a KM containing gel. Both these facts indicate that the LBG molecules
tend to form a greater number of shorter interactions rather than the
longer ones found in heterotypic junction zones containing KM.
The polymeric composition of galactomannans (galactose/mannose
with a galactose side-chain distributed on the mannan backbone) was
deemed to be a critical factor influencing heterotypic junction zone
formation. Guar has a high G:M ratio and it did not seem to interact
with xanthan. However, Goycoolea et al . (2000, 2001) have shown
that the G:M ratio is less important in low salt-containing mixtures of
mesquite gum and DX leading to a description of different types of
heterotypic junction zones. Mannion et al . (1992) also indicated two
modes of interaction between xanthan and LBG depending on the G:M
ratio. Smaller values for G:M were observed to lead to an increase
in the temperature of solubility. As a result of this, G data measured
for the mixtures depended not only on the G:M ratio but also on the
temperature of mixing, indicating that the fine structure of both polymers
affects the final gel properties. Indeed, Cronin et al . (2002) have shown
that enzymic debranching of galactomannans (removal of Gs) in the
presence of xanthan promotes the formation of strong gels. The influence
of temperature of mixing has recently been re-investigated, and while
the gels created upon cooling mixtures from high temperature are seen
to be stronger and more cohesive than those mixed at a temperature of
20 ◦ C (which is below that of the xanthan disorder-order transition), the
enthalpy of melting the mixed gels appears to be independent of thermal
history (Fitzsimons et al ., 2008b). Therefore, the lack of cohesiveness of
the gel created at low temperature is attributed to a disruption of network
formation during mixing. Agoub et al . (2007) have furthered the debate,
regarding the requirement of the xanthan molecule to be either ordered
or disordered, by comparing xanthan with low pyruvate content to a
commercial xanthan, which was progressively depyruvylated by low pH
treatment. While the overall conclusion is unclear, the findings from the
study by Agoub et al . (2007) demonstrated an interesting functionality
of low pyruvate xanthan when mixed with KM, as the resulting systems
produced melt-in-the-mouth gels.
The work on xanthan has recently received new evidence pointing
towards an interaction with xyloglucan (Kim et al ., 2006a, 2006b).
The existence of such interactions might not be unexpected, since the
xyloglucan molecule has a β-1,4-glucan (cellulose) backbone; nonethe-
less, the high frequency of di- and tri-saccharide side chains could have
been taken as an indication for the absence of any interaction.