Pectin

Pectins ( from Greek: πηκτός fixed pektós = congealed ) are plant polysaccharides, more polyuronides, which consist mainly of α -1 ,4 -glycosidically linked D -galacturonic acid units. Considered Nutritionally are pectins for humans fiber. Many microorganisms, however, are to walk pectins able.

• 7.1 gelation with polyvalent cations
• 7.2 gelation with sugar and acid

Occurrence

Pectins are present in all higher land plants. Here you will find pectins in all firm constituents, such as the stems, flowers, leaves, etc. The pectins are included in the middle lamella and primary cell walls and take over there a firming and water- regulating function. The pectin is different not only from plant to plant, but also depends on the type and age of the plant tissue. Especially pektinreich are plant parts with relatively tough / hard components, such as citrus fruits or fruit stands of sunflowers. Pektinarm other hand, are soft fruits, such as strawberries.

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

• Apple 1-1.5% Apple pomace 15 %

• Citrus peel ( from oranges and lemons ) 30%

In the extraction of pectins from plant tissues they are chemically changed. Therefore, the native vegetable pectins protopectins are called, to distinguish them from the modified pectins.

Extraction

Worldwide, about 40,000 tons of pectin are produced each year. The extraction of pectin takes place from vegetable raw materials with a high pectin content, such as apple, citrus or beet pulp. The various starting materials and the varying depending on the manufacturer recovery methods and modifications arise different types of pectin with different properties. Despite this wide variation can formulate a basic scheme of the extraction and modification:

Chemical structure and properties

The class of compounds of the pectin occurs in a variety of structures. Common to all is that this is polysaccharides, whose main component is (at least 65 % by weight), the α -D -galacturonic acid ( pKa 2.9 ) as a monomer. This galacturonic acid monomers are α -1, 4 -, usually joined to form a small proportion on β -1 ,4 -glycosidic bonds with each other and thus form the backbone of the pectin molecule.

This linear backbone is periodically interrupted by 1,2- bonds with α -L- rhamnose. Therefore, the systematic name for pectin rhamnose - galacturonic acid. The incorporation of rhamnose units means that it may cause problems in the formal linear polygalacturonic: the chains are " kinked ". The rhamnose - blocks in natural pectins in turn carry oligomeric side chains of the sugars arabinose, galactose, or xylose. This neutral sugar side chains can in turn, is, however, often counted arabinans, galactans and arabinogalactan and arabinogalactan -I -II, which is associated with proteins and hemicelluloses to be divided. The side chains are usually between one and 50 sugar units. In the industrial production of pectins these side chains in particular the acid-labile arabinofuranose go for the most part, lost. The branches in the chain by L- rhamnose, and their side chains do not occur regularly, they abound in the so-called hairy regions. In contrast, the names of the linear parts of the chain smooth regions.

In addition to the branching of the main chain, there are other features of the Pektinmakromoleküls. The hydroxy groups at the C2 or C3 atoms of the galacturonic acid units are acetylated to small parts or substituted by another neutral sugars such as D- galactose, D-xylose, L- arabinose or L-rhamnose - again mainly in the hairy regions. The carboxyl 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. Therefore, pectins are classified by their average degree of esterification VE.

History

Pectin was first discovered in 1790 by the French chemist Louis -Nicolas Vauquelin and pharmacists in fruit juices. However, it received its name until 1824, when the French botanist and chemist Henri Braconnot continued the research and called the jelly- forming substance pectic acid. One hundred years later suspected K. Smolenski be the first that pectin could consist of polymeric galacturonic acid. Then the chain form of the pectin molecule a formula for pectin 1930 by Meyer and Mark recognized and in 1937 by Schneider and Bock placed. Only at the beginning of the 20th century was recognized the practical uses of pectins, which can be used by their good gelling for the production of food. The most famous brand of pectin gelling agent since 1928 was Opekta.

Classification of pectins

High ester pectins or Hochmethylierte

• By definition, have a degree of esterification greater than 50 %,
• Gel at a sugar content of at least 55 % by weight,
• Need for a pH of 1-3.5,
• Can therefore only be used in acidic, sugary products, such as jams and fruit fillings.

Niedrigmethylierte, low methoxyl pectins

• By definition, have a degree of esterification of 50 % and 5 %,
• Can form even without sugar jelly in the presence of multivalent cations,
• Need for a pH of 1-7,
• Are used primarily for dairy products for the production of pumpable fruit preparations.
• Be used for the preparation of reduced calorie fruit spreads.

Pectic

• By definition, have a degree of esterification of less than 5 %,
• Gel as niedrigmethylierte pectins (see above),
• And fall at high pH values ​​and high concentrations of polyvalent cations as Pectates, salts of unesterified pectic from.

Amidopektine ( amidated pectins, E 440ii )

• Fall into the group of niedrigmethylierten pectins, with a percentage of the carboxylic acid groups present by reaction with ammonia as an amide,
• Gel as niedrigmethylierte pectins with both sugar and with multivalent cations,
• The jelly properties but are less influenced by the level of polyvalent cations, particularly calcium. They are therefore more robust in use.

The main technical difference between the modifications lies in the tendency to form gels from aqueous solutions, as well as in the properties of the gels formed. With the degree of esterification and the use of other additives can speed of gelation, gel strength, flavor release and string strength be selectively controlled.

Applications

Because of their ability to form gels, pectins are in the food industry, the pharmaceutical industry or for cosmetics an indispensable part of many products, which are used for various reasons gelling agent, thickening agent and / or a stabilizing agent. In the food industry, and partly also in the household pectin for making jellies, jams and marmalades ( see the article Opekta ), Confectionery, Bakery, Beverage stabilization and used in milk products. Pectins can just as agar - agar, carrageenan, alginic acid or serve as a purely herbal substitutes of gelatin.

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

It is in the EU as a food additive number E440 without maximum limit ( quantum satis ) generally approved for almost all foods.

Their properties as thickening agents, protective colloids, and stabilizers can also be used in the pharmaceutical and cosmetic industries, in order to increase the viscosity and stability of emulsions and suspensions, and to produce various gels, creams and pastes.

For other medical applications of pectins result from the ability to participate as a complexing agent in the detoxification of heavy metal poisoning and by their ability to reduce cholesterol levels in the blood. In addition, it is used in some medicines for diarrhea treatment.

Gelling

Pectin molecules are very large ( macromolecules ) and should be because of their weight and their large surface area is difficult to solve. In fact, one can solve relatively large amounts of pectin in little warm water. The cause of this phenomenon is the free carboxylic acid groups of the galacturonic acid units. In aqueous solution to dissociate the acid groups. This results in anionic acid residues more or less uniformly distributed over the macromolecule. The negative charge ensures that repel the pectin molecules electrostatically. Furthermore, form around these charge carriers large hydration shells, which further prevent the molecules approach each other. In this way remain pectins in suspension, which is why they are in the group of hydrocolloids.

For gelling the barrier of electrostatic repulsion and hydration shells must be overcome. There are two mechanisms:

Gelation with polyvalent cations

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

Gelation with sugar and acid

Said acid converted, many of the anionic acid groups to acid groups, so that the electrostatic repulsion between the pectin is lowered. Large amounts of sugar have a dehydrating effect, that is, they bind water from the large hydration shells of the pectins. This allows the pectin chains, sometimes also involving the sugar, approach each other and form a linked by hydrogen bonds, three-dimensional network.