139
lower the DM, lower will be the amount of methyl-esterfied target groups and faster
will be the rate of the pectin hydrolysis. Owing to this, a number of the properties
are influenced including gel strength resulting in the formation of weaker gel. For
obtaining LMP without any main-chain damage, a controlled or reduced temperature is sufficient. And in case of β-elimination, a double bond is formed between C4
and C5 in the GalA units as a result of the cleavage of glycosidic bonds (BeMiller
1986). In case of β-elimination, which is temperature-dependent, unlike of acid
hydrolysis which is pH dependent, the elimination occurs only in presence of a
methyl group at C6 (Keijbets and Pilnik 1974). Again the rate of β -elimination
decreases as the DM decrease. A kinetic study was carried out by Kravtchenko et al.
(1992a, b) and the workers proposed that as the temperature increased, the rate of
β-elimination reaction also increased. They reported similar values of activation
energy for β-elimination at pH values ranging from 4.5 to 11.0. Moreover, the
β-elimination reaction takes place under alkaline conditions, or even at near neutral
pH (Lopes da Silva & Rao 2006; Taylor 1982). The reaction rate is very low at weak
acidic pH.
Another most common industrial methods to de-esterify pectin involves the use
of ammonia in alcohol at relatively low temperatures. This technique is called as
alkaline de-esterification (ALMP). Besides ammonia, hyrroxylamine and other
ammonia derivatives can also be used to carry out the aminolysis (Racape et al.
1989). The benefits of using ALMP include the production of relatively high pectin
molecular weights. In this process, the saponification of the methyl-esterified GalA
residues takes place at a rate which is slower than with sodium hydroxide in water at
the same pH. Therefore, with the increase in time of reaction, DAm increases, DM
decreases and the GalA content remains constant (Renard and Thibault 1996). It may
also be noted that there is no major change in the molecular weight pectin derived
from this de-esterification which signifies that the reaction does not lead to drastic
depolymerisation (Anger and Dongowski 1988). Also the composition of the neutral
sugar and the number of side chains remains constant (Reitsma et al. 1986). When
enzyme de-esterification is followed, a block distribution of unesterified GalA units
is obtained (Denès et al. 2000; Savary et al. 2003). In such case, the de- esterification
of the pectin is carried out using pectinmethyl-esterase (PME), the most commonly
used enzyme. The de-esterification of pectin by PME can be explained via two
mechanism: (1) PME follows a single chain-reaction by acting on a single chain of
pectin to produce block pattern of carboxylates and (2) It produces shorter blocks of
de-esterified pectins by having multiple attachments. Overall enzyme de-esterification is an efficient and environmentally safe method for obtaining LMP.
Structure and Solution Behaviour
The chemical and structural properties of pectins are still a subject of study since
its discovery owing to the homogeneity of this polymer family. With respect to its
basic structure, this heteropolysaccharide is made up of atleast 17 different
Pectin
lower the DM, lower will be the amount of methyl-esterfied target groups and faster
will be the rate of the pectin hydrolysis. Owing to this, a number of the properties
are influenced including gel strength resulting in the formation of weaker gel. For
obtaining LMP without any main-chain damage, a controlled or reduced temperature is sufficient. And in case of β-elimination, a double bond is formed between C4
and C5 in the GalA units as a result of the cleavage of glycosidic bonds (BeMiller
1986). In case of β-elimination, which is temperature-dependent, unlike of acid
hydrolysis which is pH dependent, the elimination occurs only in presence of a
methyl group at C6 (Keijbets and Pilnik 1974). Again the rate of β -elimination
decreases as the DM decrease. A kinetic study was carried out by Kravtchenko et al.
(1992a, b) and the workers proposed that as the temperature increased, the rate of
β-elimination reaction also increased. They reported similar values of activation
energy for β-elimination at pH values ranging from 4.5 to 11.0. Moreover, the
β-elimination reaction takes place under alkaline conditions, or even at near neutral
pH (Lopes da Silva & Rao 2006; Taylor 1982). The reaction rate is very low at weak
acidic pH.
Another most common industrial methods to de-esterify pectin involves the use
of ammonia in alcohol at relatively low temperatures. This technique is called as
alkaline de-esterification (ALMP). Besides ammonia, hyrroxylamine and other
ammonia derivatives can also be used to carry out the aminolysis (Racape et al.
1989). The benefits of using ALMP include the production of relatively high pectin
molecular weights. In this process, the saponification of the methyl-esterified GalA
residues takes place at a rate which is slower than with sodium hydroxide in water at
the same pH. Therefore, with the increase in time of reaction, DAm increases, DM
decreases and the GalA content remains constant (Renard and Thibault 1996). It may
also be noted that there is no major change in the molecular weight pectin derived
from this de-esterification which signifies that the reaction does not lead to drastic
depolymerisation (Anger and Dongowski 1988). Also the composition of the neutral
sugar and the number of side chains remains constant (Reitsma et al. 1986). When
enzyme de-esterification is followed, a block distribution of unesterified GalA units
is obtained (Denès et al. 2000; Savary et al. 2003). In such case, the de- esterification
of the pectin is carried out using pectinmethyl-esterase (PME), the most commonly
used enzyme. The de-esterification of pectin by PME can be explained via two
mechanism: (1) PME follows a single chain-reaction by acting on a single chain of
pectin to produce block pattern of carboxylates and (2) It produces shorter blocks of
de-esterified pectins by having multiple attachments. Overall enzyme de-esterification is an efficient and environmentally safe method for obtaining LMP.
Structure and Solution Behaviour
The chemical and structural properties of pectins are still a subject of study since
its discovery owing to the homogeneity of this polymer family. With respect to its
basic structure, this heteropolysaccharide is made up of atleast 17 different
Pectin