According to Sucharipa the pectins may be classified into ( 1 ) free pectins,
which are removable from the plant by water solution, and (2) hydrol-
ysis pectins, which result from treating protopectin with hydrolyzing
agents.
Pectic acid. In fruit, protopectin is changed to pectin by enzymes of
the fruit. With still further ripening, pectic acid is formed from pectin
by the enzyme pectase. These changes are gradual, but the larger portion
of the pectic acid is formed when the fruit is ripe and when decay begins.
In the presence of pectase and calcium, barium, or strontium salts, pectic
acid may form a gel.
Dore states that, when the protopectin is in combination with cellulose.
THE PECTIC SUBSTANCES 155
the under-ripe fruit is firm. As the fruit ripens it becomes softer as the
protopectin is changed to the pectin. Rotten fruit is entirely disintegrated
because the pectin has been changed to pectic acid.
In fruits the rate at which softening takes place depends largely upon
the temperature. Haller has shown this for a number of varieties of apples
in storage. The rate "at 40°F. was found to be slightly more than double
that at 32°, whereas that at 50° was slightly less than double that at
40°, and that at 60° nearly double the rate at 50°."
Pectinase is a term applied to the enzyme which hydrolyzes pectin and
pectic acid into their simplest cleavage products.
Constitution of the pectic substances. The constitution of the
pectic substances is not definitely known. The purest pectins, when treated
with a dilute alkali, yield 10 to 12 per cent of methyl alcohol, showing
that methoxy groups (CH3O) are found in the molecule. As the methoxy
groups are split off, different series of pectin compounds are formed, these
compounds having different jellying powers. The greater the number of
methoxy groups split off, the less the jellying power and the more sirupy
the jelly. Pectic acid is the demethoxylated pectin. Since pectic acid is the
simplest and the most easily purified of the pectic compounds, it has been
prepared in the purest state. Schryver and Haynes have suggested that
pectic acid is composed of one molecule of galactose, one of arabinose, and
four of galacturonic acid arranged in a ring, the last having the four car-
boxyls of the acid groups free for methyl ester formation.
Sucharipa states that the chemical difference between free pectins and
hydrolysis pectins is that arabinose is in the former, whereas it is split off
from the latter by hydrolysis.
Other investigators question the presence of arabinose as a constituent
of pectic acid, since it is impossible for it to be formed from galacturonic
acid. Acetic acid has been reported among the decomposition products of
pectic acid obtained from pectin of flax and sugar beets.
Olsen states that most of the recent work on the chemistry of the pectin
molecule fails to consider that pectins may vary depending upon their source.
He adds that it has long been known commercially that citrus and apple
pectins, irrespective of grade, form jellies of definitely different types,
though this has not been recognized in the scientific literature.
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According to Sucharipa the pectins may be classified into ( 1 ) free pectins,
which are removable from the plant by water solution, and (2) hydrol-
ysis pectins, which result from treating protopectin with hydrolyzing
agents.
Pectic acid. In fruit, protopectin is changed to pectin by enzymes of
the fruit. With still further ripening, pectic acid is formed from pectin
by the enzyme pectase. These changes are gradual, but the larger portion
of the pectic acid is formed when the fruit is ripe and when decay begins.
In the presence of pectase and calcium, barium, or strontium salts, pectic
acid may form a gel.
Dore states that, when the protopectin is in combination with cellulose.
THE PECTIC SUBSTANCES 155
the under-ripe fruit is firm. As the fruit ripens it becomes softer as the
protopectin is changed to the pectin. Rotten fruit is entirely disintegrated
because the pectin has been changed to pectic acid.
In fruits the rate at which softening takes place depends largely upon
the temperature. Haller has shown this for a number of varieties of apples
in storage. The rate "at 40°F. was found to be slightly more than double
that at 32°, whereas that at 50° was slightly less than double that at
40°, and that at 60° nearly double the rate at 50°."
Pectinase is a term applied to the enzyme which hydrolyzes pectin and
pectic acid into their simplest cleavage products.
Constitution of the pectic substances. The constitution of the
pectic substances is not definitely known. The purest pectins, when treated
with a dilute alkali, yield 10 to 12 per cent of methyl alcohol, showing
that methoxy groups (CH3O) are found in the molecule. As the methoxy
groups are split off, different series of pectin compounds are formed, these
compounds having different jellying powers. The greater the number of
methoxy groups split off, the less the jellying power and the more sirupy
the jelly. Pectic acid is the demethoxylated pectin. Since pectic acid is the
simplest and the most easily purified of the pectic compounds, it has been
prepared in the purest state. Schryver and Haynes have suggested that
pectic acid is composed of one molecule of galactose, one of arabinose, and
four of galacturonic acid arranged in a ring, the last having the four car-
boxyls of the acid groups free for methyl ester formation.
Sucharipa states that the chemical difference between free pectins and
hydrolysis pectins is that arabinose is in the former, whereas it is split off
from the latter by hydrolysis.
Other investigators question the presence of arabinose as a constituent
of pectic acid, since it is impossible for it to be formed from galacturonic
acid. Acetic acid has been reported among the decomposition products of
pectic acid obtained from pectin of flax and sugar beets.
Olsen states that most of the recent work on the chemistry of the pectin
molecule fails to consider that pectins may vary depending upon their source.
He adds that it has long been known commercially that citrus and apple
pectins, irrespective of grade, form jellies of definitely different types,
though this has not been recognized in the scientific literature.