Pectin


Pectin 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 which was isolated and described by Henri Braconnot in 1825. 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; as a food stabiliser in fruit juices and milk drinks; and as a source of dietary fiber.

Biology

Natural occurrence

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 small amounts of pectin.
Typical levels of pectin in fresh fruits and vegetables are:
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 LD L 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 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 in the colon.
Consumption of pectin has been shown to slightly 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 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 chain, introducing bends. 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 : Smooth regions comprise homogalacturonan, xylogalacturonan, and apiogalacturonan, while the hairy regions are made up of rhamnogalacturonan I and rhamnogalacturonan 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 and degree of acetylation, which represent the ratio of esterified galacturonic acids to total galacturonic acids. Functionally, three types of pectins are distinguished:
  • Pectic acids: degree of methylation less than 5%
  • Weakly methylated pectins: degree of methylation less than 50%
  • Highly methylated pectins: degree of methylation greater than 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.
Structural features of various pectins

Section of the pectin main chain:
Poly-α--galacturonic acid.

Partially esterified section of the pectin main chain

Rhamnogalacturonan: backbone with a "kink"
due to incorporated rhamnose

Structural domains

Pectin is often described as having alternating ‘smooth’ and ‘hairy’ regions, with the ‘hairy’ regions representing the branched rhamnogalacturonan I and rhamnogalacturonan II, and the ‘smooth’ regions corresponding to the linear homogalacturonan backbone. More specifically, pectin consists of different galacturonic acid–containing domains—mainly homogalacturonan, rhamnogalacturonan I, and rhamnogalacturonan II —which differ in their sugar composition and linkage patterns. Additionally, xylogalacturonan and apiogalacturonan are often considered to be pectin because they have the same backbone as homogalacturonan.
Homogalacturonan is a linear homopolymer of α--linked D-galacturonic acid residues that comprises ~65 % of pectin. Generally, homogalacturonan comprises D-galacturonic acid residues monomers in long stretches of at least 72 to 100 residues linked together.
Rhamnogalacturonan I is a repeating disaccharide of n. The galactosyl residues of the side chains can be substituted with α--linked L-arabinose residues. Type II arabinogalactan is mainly associated with proteins, so called arabinogalactan proteins, which are rich in proline/hydroxyproline, alanine, serine, and threonine. D-galacturonic acid residues residues in the backbone of rhamnogalacturonan I may be highly O-acylated on O-2 and/or O-3, but they are not usually methyl esterified. Ferulic acid groups in rhamnogalacturonan I may be ester-linked to O-2 of the arabinose residues and to O-6 of the galactose residues.
Another structural type of pectin is rhamnogalacturonan II, which is a less frequent, complex, highly branched polysaccharide. Rhamnogalacturonan II is classified by some authors within the group of substituted galacturonans since the rhamnogalacturonan II backbone is made exclusively of D-galacturonic acid units.

Molecular weight

The molecular weight of isolated pectine greatly varies by the source and the method of isolation. Values have been reported as low as 28 kDa for apple pomace up to 753 kDa for sweet potato peels.

Substitutions

In nature, around 80 percent of carboxyl groups of galacturonic acid are esterified with methanol. This proportion is decreased to a varying degree during pectin extraction. Pectins are classified as high- versus low-methoxy pectins, with more or less than half of all the galacturonic acid esterified. The ratio of esterified to non-esterified galacturonic acid determines the behaviour of pectin in food applications – HM-pectins can form a gel under acidic conditions in the presence of high sugar concentrations, while LM-pectins form gels by interaction with divalent cations, particularly Ca2+, according to the idealized 'egg box' model, in which ionic bridges are formed between calcium ions and the ionised carboxyl groups of the galacturonic acid.
The non-esterified galacturonic acid units can be either free acids or salts with sodium, potassium, or calcium. The salts of partially esterified pectins are called pectinates, if the degree of esterification is below 5 percent the salts are called pectates, the insoluble acid form, pectic acid.
Some plants, such as sugar beet, potatoes and pears, contain pectins with acetylated galacturonic acid in addition to methyl esters. Acetylation prevents gel-formation but increases the stabilising and emulsifying effects of pectin.
Amidated pectin is a modified form of pectin. Here, some of the galacturonic acid is converted with ammonia to carboxylic acid amide. These pectins are more tolerant of varying calcium concentrations that occur in use.
Thiolated pectin exhibits substantially improved gelling properties since this thiomer is able to crosslink via disulfide bond formation. These high gelling properties are advantageous for various pharmaceutical applications and applications in food industry.
Amidated pectins behave like low-ester pectins but need less calcium and are more tolerant of excess calcium. Also, gels from amidated pectin are thermoreversible; they can be heated and after cooling solidify again, whereas conventional pectin-gels will afterwards remain liquid.