314 4 Carbohydrates
4.4.4.13 Pectin
4.4.4.13.1 Occurrence,Isolation
Pectin is widely distributed in plants. It is pro-
duced commercially from peels of citrus fruits
and from apple pomace (crushed and pressed
residue). It is 20–40% of the dry matter content
in citrus fruit peel and 10–20% in apple pomace.
Extraction is achieved at pH 1.5–3 at 60–100◦C.
The process is carefully controlled to avoid
hydrolysis of glycosidic and ester linkages. The
extract is concentrated to a liquid pectin product
or is dried by spray- or drum-drying into a pow-
ered product. Purified preparations are obtained
by precipitation of pectin with ions which form
insoluble pectin salts (e. g. AI^3 +), followed by
washing with acidified alcohol to remove the
added ions, or by alcoholic precipitation using
isopropanol and ethanol.
4.4.4.13.2 Structure,Properties.....................................
Pectin is a polysaccharide mixture with a com-
plicated structure containing at least 65% of
galacturonic acid (GalA). Three structural
elements are involved in the make-up of
a pectin molecule: a homogalacturonan (cf.
Formula 4.147) consisting of (1→4) linked
α-D-GalA, a galacturonan with differently
arranged side chains (building blocks: apiose,
fucose, arabinose, xylose), and a rhamnogalac-
turonan with a backbone consisting of the
disaccharide units [→ 4 )-α-D-GalA-(1→2)-
α-L-Rha-(1→] and with its rhamnose residues
linked by arabinan and galactan chains. In
pectins, the GalA residues
(4.147)are esterified to a variable extent with methanol,
while the HO-groups in 2- and 3-positions may
be acetylated to a small extent. Pectin stability
is highest at pH 3–4. The glycosidic linkage
hydrolyzes in a stronger acidic medium. In
an alkaline medium, both linkages, ester and
glycosidic, are split to the same extent, the latter
by an elimination reaction (cf. Formula 4.148).
The elimination reaction occurs more readily
with galacturonic acid units having an esterified
carboxyl group, since the H-atom on C-5 is more
acidic than with residues having free carboxyl
groups.
At a pH of about 3, and in the presence of Ca^2 +
ions also at higher pH’s, pectin forms a thermally
reversible gel. The gel-forming ability, under
comparable conditions, is directly proportional to
the molecular weight and inversely proportional
to the esterification degree. For gel formation,
low-ester pectins require very low pH values
and/or calcium ions, but they gelatinize in the
presence of a relatively low sugar content.
High-ester pectins require an increasing amount
of sugar with rising esterification degree. The
gelsetting time for high ester pectins is longer
than that for pectin products of low esterification
degree (Table 4.22).
Apart from the degree of esterification, gel forma-
tion is also influenced by the distribution of the
ester groups in the pectin molecule.(4.148)