Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
associations. Unfortunately, pectin solubility is reduced at acidic pH, which is why
presumably elevated temperatures are required to increase the effectiveness of water
as a solvent for the polysaccharide.
Commercial acid-extracted citrus pectins typically have a uronic acid content of
85–90%, a neutral sugar content of less than 10%, and a degree of esterification of
55–80%. 2 In comparison, careful purification of cell wall material and extraction
with CDTA yields pectins with 80% uronic acid, 15% neutral sugars, and a degree
of methyl esterification of 68%. 29 Acid extraction appears to cause some degradation
of the pectin, but the overall chemical composition is broadly similar to that of the
native material. In contrast, in a recent study of pectin extraction from sugar beet
pulp, 30 acid-extracted pectins had a uronic acid content of 30 to 52%, depending on
the conditions of pH and temperature used. Decreasing pH led to the extraction of
pectins richer in RG-I. The difference observed between acid-extracted citrus and
sugar beet pectins clearly reflects differences in pectin composition of the starting
material rather than variability in method of extraction.
12.4 PECTIN MODIFICATION
The ester content of pectin may be modified through the use of acid or alkali to
deesterify the pectin. If ammonia is used as the alkali, some of the uronic acid
residues become amides, and an amidated pectin is formed. 2 Chemical deesterification
occurs in a random way. Pectin may also be deesterified with pectin methyl
esterases. Fungal enzymes have a random attack pattern, whereas plant pectin
methyl esterases tend to result in a block distribution of deesterified uronic acid
residues. 4,31 This categorization may require some modification in the future, as in
higher plants there is a whole range of pectin methyl esterases whose mode of
action has not been fully characterized. The conditions of deesterification are also
important. For an apple pectin methyl esterase, 31 a blockwise distribution of uronic
acid residues was generated. At pH 7, only a fraction of the chains in the pectin
polysaccharide preparation were modified in this way. At pH 4.5, shorter blocks
were produced in the entire chain population. One of the reasons for the interest
in different methods of deesterification is that pectins produced should behave
differently in terms of their interactions with counterions. It is suggested that
blockiness should favor network formation, whereas more randomly distributed
charge might have a more pronounced effect on the affinity of the polysaccharide
for water. The combination of methodologies for the production of different patterns
of esterification and those for characterizing the pattern of charge distribution in
pectins should help provide structure–function relationships for patterns of charge
distribution.
The availability of different pectin methyl esterases and enzymes for selective
cleavage of neutral sugar pectin side chains makes it possible to produce a range of
pectins with different functional behavior. For example, 32 it was found that removal
of side chains decreased the viscosity of semidilute aqueous solutions of pectins in
the absence of calcium, whereas the use of pectin methyl esterase enhance gel
properties.