Pectins

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Pectins (also pectin substances ) (v. Greek : πηκτός pektós = solid, coagulated) are vegetable polysaccharides (multiple sugars), more precisely polyuronides , which essentially consist of α-1,4-glycosidically linked D - galacturonic acid units. From a nutritional point of view, pectins are dietary fibers for humans . Many microorganisms, on the other hand, are able to use pectins in their metabolism . In industry, pectins are mainly used as gelling agents .

Occurrence

Pectins are found in all higher land plants. Here you can find pectins in all solid components, for example the stems , flowers, leaves, etc. The pectins are contained in the central lamellae and primary cell walls , where they have a firming and water-regulating function. The pectin composition not only differs from plant to plant, it also depends on the type and age of the plant tissue. Plant parts with relatively tough / hard components are particularly rich in pectin, e.g. B. Citrus fruits or fruit bunches of sunflowers. On the other hand, soft fruits are low in pectin, e.g. B. Strawberries.

The peel of citrus fruits is a natural source of pectin.
Apple liquid pectin can be obtained from apples.

Content of pectins in fruits and vegetables (based on fresh weight):

When pectins are extracted from the plant tissues, they are chemically changed. This is why the native vegetable pectins are called protopectins to differentiate them from the modified pectins.

Extraction

Around 40,000 tons of pectin are produced worldwide every year. Pectin is obtained from vegetable raw materials with a high pectin content, for example apple , citrus or beet pomace . Different types of pectins with different properties result from the different starting materials and the extraction methods and modifications that vary depending on the manufacturer. Despite this wide range of variation, a basic scheme of extraction and modification can be formulated:

  1. Extraction of all water-soluble substances from the raw materials with hot water;
  2. Separation of the pectins from the other water-soluble substances by precipitation with ethanol , methanol or isopropanol;
  3. Multiple centrifugation / filtration and washing;
  4. Modification with hydrochloric acid to reduce the degree of esterification or with ammonia to form amidated pectins;
  5. Filtering, washing and drying again produces a white to greyish pectin powder;
  6. The pectin is adapted to its area of ​​application by means of additives (e.g. cane sugar for standardization, buffer substances for regulating the pH value and calcium availability).

Chemical structure and properties

The substance class of pectins occurs in a multitude of structures. What they all have in common is that they are polysaccharides, the main component of which (at least 65% by weight) is α- D- galacturonic acid ( pK a value 2.9) as a monomer . These galacturonic acid monomers are linked to one another via α-1,4-, mostly also to a small extent via β-1,4-glycosidic bonds and thus form the backbone of the pectin molecule.

Structural features of various pectins
Pectin1.svg
Section from the backbone of pectins:
poly-α- (1 → 4) -galacturonic acid.
Pectin2.svg
Partly esterified section of the spine
Pectin3.svg
Rhamnogalacturonan: backbone with "kink"
due to built-in rhamnose

This linear backbone is periodically interrupted by 1,2-bonds with α- L - rhamnose . Therefore the systematic name for pectin is rhamno-galacturonic acid . The incorporation of rhamnose units leads to disruptions in the formally straight polygalacturonic acid chain: the chains are "kinked". The rhamnose building blocks in natural pectins, in turn, have oligomeric side chains made from the sugars arabinose , galactose or xylose . These neutral sugar side chains can in turn be subdivided into arabinans, galactans and arabinogalactan-I and arabinogalactan-II, which is linked to proteins, but is often also classified as hemicelluloses. The side chains are usually between one and 50 sugar units. In the industrial production of pectins, most of these side chains, but especially the acid-labile arabinofuranose, are lost. The branches in the chain through L- rhamnose and its side chains do not occur regularly, but accumulate in the so-called hairy regions . In contrast, the linear parts of the chain are called smooth regions .

In addition to the branches of the main chain, there are other features of the pectin macromolecule. The hydroxyl groups on the C2 or C3 atom of the galacturonic acid units are acetylated to a small extent or substituted by other neutral sugars such as D- galactose, D- xylose, L -arabinose or L- rhamnose - here too, mainly in the hairy regions . The carboxy groups of polygalacturonic acid are often esterified with methanol . The degree of esterification and acetylation varies with the origin of the pectin, but has a decisive influence on the chemical properties. That is why pectins are classified according to their mean degree of esterification VE.

history

Pectin was first discovered in fruit juices in 1790 by the French chemist and pharmacist Louis-Nicolas Vauquelin . However, it did not get its name until 1824, when the French botanist and chemist Henri Braconnot continued his research and named the gelatinous substance pectic acid. One hundred years later, K. Smolenski was the first to suspect that pectin could consist of polymeric galacturonic acid. In 1930 Meyer and Mark recognized the chain form of the pectin molecule and in 1937 Schneider and Bock developed a formula for pectin. It was only at the beginning of the 20th century that people recognized the practical uses of pectins, which can be used for the production of food thanks to their good gelling properties.

Probably the best-known brand for pectin-containing gelling agents since 1928 was Opekta .

Classification of pectins

Highly methylated or highly esterified pectins

  • have a degree of esterification greater than 50% by definition,
  • gel with a sugar content of at least 55% by weight,
  • need a pH value of 1–3.5 for this,
  • can therefore only be used in acidic, high-sugar products, e.g. B. in jams and fruit fillings.

Low methylated, low methylester pectins

  • have a degree of esterification of 5–50% by definition,
  • can form jelly even without sugar in the presence of polyvalent cations ,
  • need a pH value of 1–7 for this,
  • are used for the production of pumpable fruit preparations , especially for dairy products.
  • are used to make low-calorie fruit spreads.

Pectic acids

  • have a degree of esterification of less than 5% by definition,
  • gel like low methylated pectins (see above),
  • and fall at high pH values and high levels of multivalent cations as pectates , salts of the unesterified pectic acids, from.

Amidopectins (amidated pectins, E 440ii)

  • fall into the group of low methylated pectins, whereby a certain proportion of the carboxylic acid groups is present as an amide due to the reaction with ammonia ,
  • gel like low methylated pectins with both sugar and polyvalent cations,
  • the properties of the jelly, however, are less influenced by the content of polyvalent cations, especially calcium. They are therefore more robust to use.

The main difference in application technology between the modifications lies in the tendency to form gels from aqueous solutions and in the properties of the gels formed. With the degree of esterification and the use of other additives, the speed of gel formation, gel strength, flavor release and spreadability can be controlled in a targeted manner.

Applications

Due to their ability to form gels , pectins are an indispensable component of many products in the food industry , the pharmaceutical industry or for cosmetics , in which gelling agents , thickeners and / or stabilizers are used for various reasons . In the food industry and sometimes also in the household, pectin is used to make jellies , jams and jams , confectionery , baked goods, to stabilize drinks and in milk products. Pectins, like agar-agar , carrageenan or alginic acid, can serve as purely vegetable substitutes for gelatine .

Low methylated pectins are used in yoghurt production and in the pasteurization of sour milk products, where the coagulation of casein is prevented by the presence of pectin.

They are approved in the EU as a food additive with the number E 440 without numerical maximum quantity restriction ( quantum satis ) for almost all foods. According to this, however, products that contain pectin and are obtained from dried residues of squeezed apples or from dried peel of citrus fruits or from a mixture thereof by treatment with dilute acid and subsequent partial neutralization with sodium or potassium salts are not considered food additives in this sense ("liquid Pectin").

Their properties as thickeners, protective colloids and stabilizers are also used in the pharmaceutical and cosmetics industries to increase the viscosity and stability of emulsions and suspensions and to produce various gels , creams and pastes .

Further medical applications of pectins result from the ability to act as a complexing agent in the detoxification of heavy metal poisoning and through their property of lowering the cholesterol level in the blood. They are also used in some medicines to treat diarrhea.

Gelling mechanisms

Pectin molecules are macromolecules and should be difficult to dissolve in water due to their high molar mass . In fact, however, relatively large amounts of pectin can be dissolved in just a little warm water. The cause of this phenomenon are the free carboxylic acid groups of the galacturonic acid building blocks. The acid groups dissociate in aqueous solution . This creates anionic acid residues that are more or less evenly distributed over the macromolecule. The negative charge ensures that the pectin molecules repel each other electrostatically . Furthermore, large hydration shells form around these charge carriers , which additionally prevent the molecules from approaching one another. In this way, pectins remain in suspension , which is why they are assigned to the group of hydrocolloids .

In order to gel , the barrier of electrostatic repulsion and hydration shells must be overcome. There are two mechanisms for this:

Gelation with polyvalent cations

A polyvalent cation is bound by two or more anionic carboxylate groups in a chelate complex , so that a gel is formed in which polyvalent cations hold the pectin chains together in a three-dimensional network. The gelation takes place according to the so-called "egg-box" model.

Gelation with sugar and acid

The acid converts many of the anionic acid residues into acid groups, which reduces the electrostatic repulsion between the pectin chains. Large amounts of sugar have a dehydrating effect, i. That is, they also bind water from the large hydrate shells of the pectins. This allows the pectin chains, e.g. Sometimes with the inclusion of sugar, they approach each other and form a three-dimensional network linked by hydrogen bonds .

Web links

Individual evidence

  1. Make pectin yourself. Home Remedies & Medicinal Plants Blog, February 9, 2014.
  2. Entry on pectins. In: Römpp Online . Georg Thieme Verlag, accessed on April 2, 2012.
  3. Max Wichtl (Ed.): Herbal Drugs and Phytopharmaceuticals. CRC Press, London, New York, and others 2004, p. 520 ( Rosae pseudofructus cum fructibus ).
  4. Rose hip in PharmaWiki, as of May 25, 2018.
  5. Entry on quince. In: Römpp Online . Georg Thieme Verlag, accessed on June 8, 2013.
  6. Report on pectins (PDF; 621 kB) by the manufacturer Herbstreith & Fox GmbH & Co. KG, Neuenbürg.
  7. pectins . In: Kompaktlexikon der Biologie, Wissenschaft Online; Retrieved July 19, 2010.
  8. Article 3 Paragraph 2 Sentence 2 iv) Regulation (EC) No. 1333/2008 of the European Parliament and of the Council of December 16, 2008 on food additives. In their Appendix II Part E for various food groups with restrictions on pectins