Biomedical Engineering Reference
In-Depth Information
polysaccharides are important in mixed polysaccharide gel systems, and so are brie
y
introduced here. Some of these polysaccharides can be trapped into a gel-like state, either
by applying freeze
thaw cycles ( Chapter 8 ) or by alkali treatment, but under most normal
conditions they form only viscous solutions. These mixed systems have a number of
industrial applications, particularly in foods (dessert jellies and spreads) and pharma-
ceuticals (capsule additives).
-
10.6.1.1
Glucomannans
The commonest of these, konjac glucomannan (KGM), is a neutral polysaccharide that
consists of
4)-linked d-mannose and d-glucose with about 1 in 20 units being
acetylated. The ratio of mannose to glucose is reported as 3/2. On its own KGM can form
a gel when heated in the presence of alkali agents, and these alkali-induced gels are not
thermoreversible. Mixed gels of KGM with other polysaccharides have been used
extensively in industry (Nishinari et al., 2007 ).
β
(1
10.6.1.2
Xyloglucans
These are major structural polysaccharides which occur in the primary cell walls of
higher plants, and include those from detarium (Wang et al., 1996 ) and tamarind sources
(Nishinari et al., 2009 ). The seeds of the tamarind tree (Tamarindus indica) contain
xyloglucan as a storage polysaccharide, tamarind seed xyloglucan (TSX), which has
been widely used in the food industry. TSX has a
4)-linked d-glucan backbone that
is partially substituted at the O-6 position of its glucopyranosyl residues with
β
(1
α
-d-
xylopyranose. Some of the xylose residues are
β
-d-galactosylated at O-2, although the
fine structure depends on the source of the plant. Alone it can form a thermoreversible gel
by interacting with catechins or dyes, but it also forms gels in combination with other
polysaccharides (Nishinari et al., 2007 ).
10.6.2
Polysaccharide mixed-gel interactions
It has long been known that galactomannans interact with certain gelling polysaccharides
such as carrageenan and agar(ose) to improve gelling ability. Only a small amount of
added locust bean gum (LBG) makes the gels
, i.e. of higher modulus, less brittle
and more elastic (with higher failure to break). Since LBG and, particularly, guar gum are
much less expensive than carrageenan and agar, it is an advantage to use a mixture of a
galactomannan and less of the other component.
The interaction of galactomannans with xanthan has also attracted much attention. It was
found that at, say, 1% concentration, both locust-bean gum and xanthan form only viscous
solutions, but heating and cooling a mixture comprising 0.5% of each forms a
'
rmer
'
firm gel. The
experimental
finding that these non-gelling polysaccharides can increase the modulus of
agar and carrageenan gels, and cause gel-formation in the presence of xanthan, led
researchers to suggest that galactomannans interact
'
'
with agar, carrageenan
and xanthan, and in doing so form mixed junction zones (Dea and Morrison, 1975 ).
The term
synergistically
so that the total effect is greater
than the sum of the two individually. In quite a few reported cases of synergism, the effect
'
synergism
'
literally means
'
acting together
'
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