This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Nomenclature of the pectic substances. The different names used in the literature for pectin and its related compounds have been numerous. To avoid confusion, the Committee on the Nomenclature of the Pectic Substances (Dore, Chairman) have proposed dividing these into three groups: (1) protopectin, (2) pectin, and (3) pectic acid. The chemical identity of the various pectic substances is not definitely settled, but a great deal of investigational work is being done at the present time.
Function of pectic substances in plant life. The pectic substances play an important role in plant life. Dore states that the primary function of the pectic substances is the cementing together of the individual cells that compose the plant.
Sources of pectin in plants. The pectic substances are found in the leaves, bark, roots, tubers, stalks, and fruits of plants. The pectic compounds occur in two places in the plant cell - in the middle lamella, where they serve as cementing material, and as thickened places on the cell wall. These two sources may yield slightly different substances. In fruits the pectin is usually found in the pulp and not in the juice, though there are some exceptions. For example, currant juice often contains pectin. The skins and cores of fruit like apples contain large proportions of pectin. Hence, apples and crab-apples are cut into small pieces, but not peeled or cored, for making jelly. Some of the root stocks, such as sugar beets, carrots, rutabagas, and turnips, contain appreciable amounts of pectin.
The amount of pectin not only varies in different fruits, but also in the same fruit at various stages of ripeness and in the same fruit in different years or seasons.
Protopectin. Protopectin has been called pectose and pectinogen. It is the water-insoluble pectic substance in plants. It may be rendered soluble by the enzyme protopectinase or by treatment with acid and other reagents. The soluble substances thus formed are designated as pectins.
Sucharipa has presented evidence that the protopectin is combined with cellulose. In the plant, the separation of the pectin from the cellulose is brought about by hydrolysis by the enzyme protopectinase.
The protopectin is the precursor of pectin. As the fruit ripens, pectin increases while the protopectin decreases. Protopectin is also converted to pectin by boiling with dilute acid. Since fruit juices contain acid, some protopectin is changed to pectin when the fruit is boiled. This is the reason why cooked fruit juices are better for jelly than uncooked ones.
Conrad', influenced by Ehrlich's work, found that as the pectin of the fruit increases, a pentose-bearing polysaccharide is liberated at the same time from the protopectin, its proportions paralleling that of the pectin to a great extent. This furfural-yielding polysaccharide is separated from the pectic substances by its solubility in 70 per cent alcohol.
Pectin. Among the various names that have been applied to pectin are parapectin, pectinogen, and protopectin. The term is applied to the methylated pectic substances that form a colloidal solution in water. It is the jelly-forming substances of the pectic compounds, although the pectins with fewer methyl groups have less jellying power than those with a greater number.
According to Sucharipa the pectins may be classified into (1) free pectins, which are removable from the plant by water solution, and (2) hydrolysis 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 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.
Protopectin and pectic acid do not form jellies. Protopectin and pectic acid do not form jelly when cooked with sugar and a fruit juice containing acid, although pectin forms a jelly under these conditions. Since over-ripe fruit contains a larger proportion of pectic acid and a smaller percentage of pectin, this furnishes the explanation for the fact that many jelly makers have long known, that juice from partially ripe fruit makes a better jelly than juice from over-ripe fruit.
Cooked extracted fruit juice is better for jelly. Fruit juice extracted raw or without cooking will seldom make jelly, or it yields a jelly of poor quality. Since the protopectin is not soluble in the juice of the fruit, if the juice is extracted raw it remains with the pulp of the fruit. If cooked, some of the protopectin is changed to pectin. Goldthwaite has reported experiments with apple and some other fruit juices extracted raw. Either no jelly was obtained or it had a poor texture.
Extraction of and preparation of juice for jelly. Hard fruits like apples and crab-apples are cut into small pieces or ground with a food chopper, barely covered with water, and cooked. Soft berries need little water added, 1/4 cup to a pound of fruit often being used. Currants that are not too ripe should have a larger proportion of water. Gooseberries, if green, because of their very high acid and pectin content and the character of the pectin, can be completely covered with water, and sometimes will need additional water. Ripe gooseberries require the same proportion of water used for other berries. After cooking, the juice is drained without squeezing from the pulp by placing the cooked material in a cloth bag. Fruits that are very rich in pectin may have two or three extractions made. These second and third extractions need to be boiled down more than the first extraction to concentrate the pectin. Second and third extractions also do not have as much of the fruit flavor as the first one.
Apple juice sometimes contains starch, particularly that from under-ripe fruit, which renders the juice cloudy. Askew states that starch formation begins near the periphery and progresses toward the core; but as the fruit matures the core area is cleared of starch first, the area near the skin last. Commercially, diastase can be added to convert the starch to sugar and thus clear the juice.
Grape juice contains tartaric acid, which often crystallizes in long needle-like crystals in the jelly. It can be removed partially from the juice by putting the juice in a refrigerator (the colder the better, but it is less soluble at lower temperatures), over night, or for a few days, and then straining the crystals from the juice.