Pectin is classified as a soluble fiber. It is found in most plants, but is most concentrated in citrus fruits (oranges, lemons, grapefruits) and apples. Pectin is obtained by the aqueous extraction of citrus peels and apple pulp under mildly acidic conditions. Pectin obtained from citrus peels is referred to as citrus pectin.
Pectin is a carbohydrate in the cell wall of plants. It is a breakdown product from hemicelluloses (protopectins) which is generated during ripening of the fruit. Pectin itself is later broken down to pectinic acid and finally pectic acid. During this chemical breakdown process, the fruit gets softer as the cell walls degenerate. Pectin is a "gum" found naturally in fruits that causes jelly to gel. Tart apples, crab apples, sour plums, Concord grapes, quinces, gooseberries, red currants and cranberries are especially high in pectin. Apricots, blueberries, cherries, peaches, pineapple, rhubarb and strawberries are low in pectin. Underripe fruit has more pectin than fully ripe fruit. Jellies and jams made without added pectin should use 1/4 underripe fruit.
Pectin is widely used in the food industry as a gelling agent to impart a gelled texture to foods, mainly fruit-based foods such as jams and jellies. It also has pharmaceutical applications. Pectin is used in combination with the clay kaolin (hydrated aluminum silicate) for the management of diarrhea. It is used as a component in the adhesive part of ostomy rings. Pectin is also marketed as a nutritional supplement for the management of elevated cholesterol.
Chemically, pectin is a linear polysaccharide containing from about 300 to 1,000 monosaccharide units. D-Galacturonic acid is the principal monosaccharide unit of pectin. Some neutral sugars are also present in the substance. The D-galacturonic acid residues are linked together by alpha-1, 4 glycosidic linkages. The molecular weight of pectin ranges from 50,000 to 150,000 daltons. The galacturonic acid residues in pectin may be esterified with methyl groups. There are different types of pectin. Pectin in which more than 50% of the galacturonic acid residues are esterified is called high methoxyl or HM pectin. Pectin in which less than 50% of the galacturonic acid residues are esterified is called low methoxy or LM pectin. Pectin is a nondigestible polysaccharide. So-called modified citrus pectin is pectin that has been hydrolyzed and otherwise modified to make it more digestible and absorbable.
The mechanism of the possible hypocholesterolemic activity of pectin is not well understood. It appears that the viscosity of pectin is related to its possible hypocholesterolemic activity. Pectin preparations with high viscosity appear to be more effective in lowering cholesterol than are pectin preparations with lower viscosity. High-viscosity pectin is thought to lower cholesterol levels by raising the excretion of fecal bile acids and neutral sterols. High-viscosity pectin may interfere with the formation of micelles and/or lower the diffusion rate of bile acid and cholesterol-containing micelles through the bolus, consequently diminishing the uptake of cholesterol and bile acids.
Pectin has been found to alter the characteristics of the fibrin-network architecture, suggesting that it may have some antithrombotic activity. Fibrin networks, following pectin supplementation, were found to have lower tensile strength, and to be more lysable and permeable. Acetate supplementation was found to have similar activity on fibrin networks. It is thought that the acetate resulting from bacterial fermentation of pectin in the large intestine may account for pectin's possible effect on fibrin networks. The putative protective activity of pectin against colorectal cancer may be accounted for by its fermentation in the large intestine to butyric acid. Butyric acid may upregulate apoptosis.
Pectins are mainly used as gelling agents, but can also act as thickener, water binder and stabilizer. Low methoxyl pectins (< 50% esterified) form thermoreversible gels in the presence of calcium ions and at low pH (3 - 4.5) whereas high methoxyl pectins rapidly form thermally irreversible gels in the presence of sufficient (e.g. 65% by weight) sugars such as sucrose and at low pH (< 3.5); the lower the methoxyl content, the slower the set. The degree of esterification can be (incompletely) reduced using commercial pectin methylesterase, leading to a higher viscosity and firmer gelling in the presence of Ca2+ ions. Highly (2-O- and/or 3-O-galacturonic acid backbone) acetylated pectin from sugar beet is reported to gel poorly but have considerable emulsification ability due to its more hydrophobic nature, but this may be due to associated protein impurities.