Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
is chain cleavage at esterified uronic acid residues to form galacturonic acid oligomers
terminated by an arabinitol residue. Subsequent analysis of the galacturonic
acid containing fragments by high performance anion exchange chromatography
directly gives the distribution of unesterified uronic acid sequences. Coupling the
chromatography to electrospray-ionization mass spectroscopy provides confirmatory
evidence on structure of the oligomers produced. Obtaining this structural information
is a key step that should enable the refinement of structure–function relationships
which are sensitive to polymer charge and its distribution.
12.3 EXTRACTION OF PECTINS
Procedures for the extraction of pectic polysaccharides from cell wall materials have
developed in a semiempirical way, without necessarily a very detailed understanding
of the underlying physical and chemical mechanisms involved in the extraction
procedure. For example, suppose that it has been possible to isolate a cell wall
material with a minimum of physical and chemical modification. One common crosslink
involved in the pectin network of the plant cell wall involves calcium counterions.
If a powerful chelating was available, then this might potentially disrupt the
cross-link and enable the solubilization of pectic polysaccharides. One such powerful
Ca 2+ chelating agent is cyclohexane diamine tetraacetic acid (CDTA). Treatment of
parenchymatous tissues, such as tomato pericarp, with an aqueous solution of CDTA
at neutral pH causes cell separation, and it is possible to solubilize a pectin fraction
from a cell wall material at room temperature. 9 CDTA extracts only a fraction of
the pectin when used at neutral pH. It becomes a much more effective calciumchelating
agent as pH is increased, but it is surprising that it has rarely been used
as a pectin extractant at higher pHs. Although CDTA is popular as a pectin extractant,
it is worth remembering that its effect on the plant cell wall is unlikely to be limited
to the disruption of calcium-mediated ionic cross-linking. Gel-forming interactions
between pectin and basic peptides have been shown to be disrupted by CDTA. 28
Even at neutral pH and room temperature, CDTA might cause some degradation of
the borate cross-link of RG-II.
In cell wall studies, increasing concentrations of alkali are then used to extract
a series of pectin fractions and hemicellulosic polysaccharides. The key linkages or
associations disrupted in this alkali treatment are not known, but it has been suggested
that ester linkages are included.
For commercial extractions of pectins, acid conditions are typically used from
pH 1 to 3 and at elevated temperature of 50 to 90°C. 2 These acidic conditions
inevitably result in some hydrolysis of glycosyl linkages, including the hydrolysis
of the rhamnosyl linkage in the RG-I backbone and the more labile arabinofuransyl
linkages, galacturonosyl sequences being relatively resistant to hydrolysis. It has
been shown that the borate ester of RG-II is hydrolyzed under relatively mild acidic
conditions, being completely hydrolyzed within 30 min at room temperature at pH
1. 18 A further consequence of the acidic extraction conditions is the suppression of
charge on the uronic acid and the solubilization of the associated calcium counterions.
Therefore, the acidic conditions used disrupt a range of linkages and