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Modular elements (accumulate during enzymatic degradation of
pectins)
•
RG-I (Rhamnogalacturonan I): Backbone: -Rha-GalA-Rha-GalA… with
Gal-rich branches
•
RG-II (Rhamnogalacturonan II): Backbone: - GalA-GalA-…, complex
branches. Highly conserved, associated with uptake and binding of
boron (borate)
•
Homogalacturonan: Unbranched regions of 1
→
4-linked
α
-D-GalA. May
contain O-acetyl groups (on C2 and C3). Binds Ca
++
and induces
gelation
•
Esterified homogalacturonan regions. NOTE! Esterification
⇒
No
charge. These regions associate in the presence of high
concentrations of sucrose. Basis for use in jams.
Backbone
Primarily a polymer of
α
-D-galacturonic acid (homopolymer of (1
→
4)-
α
-D-
galacturonic acid) with varying degree of carboxyl groups methyl esterified,
and rhamnogalacturonan I (heteropolymer of repeating
α
-L-rhamnosyl (1
→
2)
linked to
α
-D-galactosyluronic acid disaccharide units).
Commercial pectins
High methoxyl pectins (high degree of esterification) are used as gelling
agents in foods, jams etc. Gelling requires:
a)
High degree of esterification => low content of COO
-
(since –COOCH
3
has no charge)
b)
Low pH (remaining -COO
-
→
-COOH). Citric acid often added
c)
High sucrose content => influences water activity