Pectin (Ancient Greek: πηκτικός pēktikós: 'congealed' and 'curdled') is a heteropolysaccharide, a structural polymer contained in the cell walls and middle lamellae of terrestrial plants. The principal chemical component of pectin is galacturonic acid (a sugar acid derived from galactose). Commercially produced pectin is a white-to-light-brown powder, produced from citrus fruits for use as an edible gelling agent, especially in jams and jellies, dessert fillings, medications, and sweets.
Biology
Natural occurrence
Pectin was isolated and described by Henri Braconnot in 1825.He showed that it was present in the leaves, stems and fruits of many plants. Pears, apples, guavas, quince, plums, gooseberries, and oranges and other citrus fruits contain large amounts of pectin, while soft fruits, like cherries, grapes, and strawberries, contain smaller amounts. Typical levels of pectin in fresh fruits and vegetables are:
Apples, 1–1.5% Apricots, 1% Cherries, 0.4% Oranges, 0.5–3.5% Carrots 1.4% Citrus peels, 30% Rose hips, 15% Pectin is composed of complex polysaccharides that are present in the primary cell walls of a plant, and are abundant in the green parts of terrestrial plants. Pectin is the principal component of the middle lamella, where it binds cells. Pectin is deposited by exocytosis into the cell wall via vesicles produced in the Golgi apparatus. The amount, structure and chemical composition of pectin is different among plants, within a plant over time, and in various parts of a plant. Pectin is an important cell wall polysaccharide that allows primary cell wall extension and plant growth. During fruit ripening, pectin is broken down by the enzymes pectinase and pectinesterase, in which process the fruit becomes softer as the middle lamellae break down and cells become separated from each other. A similar process of cell separation caused by the breakdown of pectin occurs in the abscission zone of the petioles of deciduous plants at leaf fall.
Human nutrition Pectin is a natural part of the human diet and although not digested in the small intestine, it is fermented in the large intestine. It has a positive effect on decreasing overall LDL cholesterol levels as well as lowering post-prandial glucose levels. The daily intake of pectin from fruits and vegetables can be estimated to be around 5 g if approximately 500 g of fruits and vegetables are consumed per day. In human digestion, pectin binds to cholesterol in the gastrointestinal tract and slows glucose absorption by trapping carbohydrates. Pectin is thus a soluble dietary fiber. In non-obese diabetic (NOD) mice pectin has been shown to increase the incidence of autoimmune type 1 diabetes. A study found that, after consumption of fruit, the concentration of methanol in the human body increased by as much as an order of magnitude due to the degradation of natural pectin (which is esterified with methanol) in the colon. Consumption of pectin has been shown to slightly (3–7%) reduce blood LDL cholesterol levels. The effect depends upon the source of pectin; apple and citrus pectins were more effective than orange pulp fibre pectin. The mechanism appears to be an increase of viscosity in the intestinal tract, leading to a reduced absorption of cholesterol from bile or food. In the large intestine and colon, microorganisms degrade pectin and liberate short-chain fatty acids that have a positive prebiotic effect.
Other Pectin has been observed to have some function in repairing the DNA of some types of plant seeds, usually desert plants. Pectinaceous surface pellicles, which are rich in pectin, create a mucilage layer that holds in dew that helps the cell repair its DNA.
Chemistry
Definition and structure Pectin is a heteropolysaccharide with a high proportion of D-galacturonic acid (≈ 65%) in its repeat units. As the polymer's main chain contains α-L-rhamnose in addition to galacturonic acid, the systematic name for pectin is rhamno-galacturonic acid. The incorporation of rhamnose units disrupts the otherwise linear poly(galacturonic acid) chain, introducing bends (or "kinks"). Many rhamnose units in pectin carry oligomeric side chains of neutral sugars such as arabinose, galactose, or xylose. These branched sections are referred to as "hairy" regions, while the unbranched stretches composed mainly of galacturonic acid are termed "smooth" regions. In further detail, the hairy and smooth regions can be divided into distinct structural domains (that exist within the same pectin molecule): Smooth regions comprise homogalacturonan (HG), xylogalacturonan (XGA), and apiogalacturonan (APGA), while the hairy regions are made up of rhamnogalacturonan I (RG-I) and rhamnogalacturonan II (RG-II). The carboxyl groups of polygalacturonic acid are frequently esterified with methanol or acetic acid. The degree of esterification and acetylation varies depending on the source of the pectin and has a decisive impact on its chemical properties. Pectins are therefore classified according to their degree of methylation (DM) and degree of acetylation (DA), which represent the ratio of esterified galacturonic acids (methylated or acetylated) to total galacturonic acids. Functionally, three types of pectins are distinguished:
Pectic acids: degree of methylation less than 5% (DM<5) Weakly methylated (LM) pectins: degree of methylation less than 50% (DM<50) Highly methylated (HM) pectins: degree of methylation greater than 50% (DM>50) Amidated pectin shows enhanced tolerance to varying calcium concentrations. Thiolated pectin, capable of forming disulfide crosslinks, exhibits superior gelling properties beneficial for pharmaceutical and food applications.
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