Classification pectin physicochemical properties

According to the nomenclature adopted in 1944, the, distinguish the following pectin: protopectin (see above.), Pectin, pectin, acid and pectic acid.

Pectin is a general term for a water-soluble pectin, free from cellulose, consisting of methoxylated galacturonic acid residues. Pectin is a product obtained as a result of the first stage protopectin natural decay. In unaltered as it can form a jelly with sugar, acid and water. In mature fruits bulk of pectin is in the form of pectin. In this state, the fruit destined for confectionery processing, should come into production.

Methoxylated pectin polygalacturonic circuit can be simplified so designated15.1

During the gradual hydrolysis of pectin cleavage of methoxyl groups (demethoxylation). Fully demethoxylated pectin (with a pristine chain) is the name of the new nomenclature pectic acid.

The structure of the pectic acid can be represented by the following formula:15.2

Pectic acid is a colloid having a polymerized molecule. Its general formula (C5Н7О4SOON)n. However, it has only a weak ability studneobrazovaniya.

Between the pectin and pectic acid has a number of intermediate decay products of varying degrees of demethoxylation. They are present in the natural mixture of pectin. Polygalacturonic their chain consists of links, more or less of which is saturated with methoxy. These compounds given the name of pectin acids. Studneobrazuyuschie Their properties vary depending on the degree of methoxylation.

The general formula of pectic acid15.3

As a result of deep decay of pectic acids (and pectic acid), accompanied by not only demethoxylation but polygalacturonic chains break into separate units obtained molecular monogalakturonovaya soluble acid:15.4

which behaves like a typical monobasic acid.

Consider the most important physical and chemical properties of the pectin.

Pectin is a pure white substance, which swells in water to form a colloidal solution gradually - sol.

Pectin solutions have considerable viscosity.

The viscosity of the diluted sol pectin increases in direct proportion to the concentration. This relationship only exists for pectin solution with a concentration of up to 1%. In solutions with higher concentrations there are significant deviations from this dependence. In the region close to the pectin gelled sol, there is a manifestation of abnormal viscosity, accompanied by the formation of structure.

By adding acetone or alcohol (with a concentration above 50%), ether or benzene to aqueous solutions or solid occurs pectin sol gelation, or precipitation 111 new pectin gel depending on the concentration of added pectin and precipitant.

Proteins and tannins pectic substances from aqueous solutions are not precipitated.

The particles have a negative pectin complex high-density charge. Last mainly due to the free carboxyl groups of pectic acids. With this pectin (especially in the presence in them of pectin and pectic acids) under certain conditions precipitate from the aqueous solution by the action of multivalent metal salts as soluble in mineral acids coagulates these salts.

Pectin is an optically active substance with the right rotation, and its specific rotation is changed depending on the fruit from which it occurs and of its degree of purity.

Pectic acids or acidic medium to form salts of various metals.

For 0,5-1,0% aqueous solutions of pectin acid pH values ​​fluctuate within 3,2-3,4.

Pectic acid in dry form is a white powder having a small solubility in water. It gives with alkalis soluble salts (pectates), and alkaline earth and heavy metals, it forms an insoluble salt. Tannins it unlike pectin is precipitated.

Molecular weight (M) of pectin varies with the origin of pectin and its degree of depolymerization.

Pectin substances are usually multilayer mixture, ie. E. A mixture of molecules of different sizes. Therefore, we can talk about average values ​​only M.

The particular difficulty in establishing pectins M is in the fact that pectins are the heterogeneous mixture, which requires purification.

In addition, there are a variety of methods for determining M which give different values ​​for the same product.

Schneider and Bock (1937), M determined the pectin from apples osmotic method (Nitrate pectin-free "satellite" presence), a mean value of up to 30000

100 000; molecular weight 150 000, 220 000 to lemon and beet 20 000-25 000 pectin was found to oranges.

Svedberg and Graham (1938), using the method of ultracentrifugation, found that the value of M for the apple, pear and plum pectin range from up to 40 000 50 000.

Somewhat higher values ​​obtained when determining M pectin viokozimetricheskim way.

Along with the value M of great interest to characterize the physicochemical properties of the high polymers are of the form data and the amount of their molecules. Pectic acid molecules are cylindrical in shape; molecule thickness of about 10 A.

Described physicochemical properties determine the ability to form gels of pectin with sugar and acid.

The ability of pectins to studneobrazovaniyu

Pectin jellies form different composition, differing from each other in physical and mechanical properties.

The most typical for the production of marmalade-Pastila products is the formation of jelly with acid and water in the presence of relatively large amounts of sugar (60-80% by weight of jelly). Preparation of gels of this type is the basis of the production of fruit marmalades, pastes, jams, chocolates.

Without dwelling on the question of the mechanism studneobrazovaniya pectin in a confectionery production (coverage of this issue will be given a special place in the future), it is necessary here to consider studneobrazuyuschie properties of pectins and the factors by which these properties are determined.

Ability to studneobrazovaniyu shown individually in various pectins, depending on their origin, and therefore they are unequal in value for confectionery manufacture.

It is known, for example, that the greatest value in its ability to represent studneobrazuyuschey apple pectin, citrus fruits (from orange and lemon peel), black currant, gooseberry, baskets of sunflowers and beets. With the proper conduct of technological processes of production of these pectins they give jellies, has the required strength and other valuable properties.

Less than full-fledged in this regard are pectins rowan, quince, apricot, peach, plum, cranberry. Pectin of fruits, giving jellies, have lower strength, do not meet the requirements of the production of confectionery products. Even less valuable in terms of their ability to studneobrazovaniyu pectins are pears, cherries, summer fruit, vegetables and grapes.

Within the same species and variety of plant and fruit pectin ability to studneobrazovaniyu varies during plant development, fruit ripening, during storage and processing of the raw material.

The current state of knowledge does not allow even the possibility of reliably establish the causes that determine the natural ability to studneobrazovaniyu in pectins from different sources.

The quantitative content of pectin in fruits and plants studied varies widely from the 1,8 28% to the dry weight of the plant material.

The content of pectin in the feedstock does not allow him to judge the ability studneobrazuyuschey. This is due to the fact that the applicable methods of quantifying pectin (known calcium pektatny method, alcohol precipitation method, etc.) Are shown in the sum with its studneobrazuyuschimi factions and those factions who are deprived of this property.

At present, it is assumed that studneobrazuyuschie properties of pectin are predetermined mainly by the following factors:

  1. chain length molecules pectin;
  2. methoxylated galacturonic acid residues;
  3. neuronidnyh the presence of components (organic and mineral).

There is evidence that the ability of the pectin to studneobrazovaniyu depends primarily on the size of its molecules. The latter are determined degree of polymerization of the main chain valences and are characterized by the value of the molecular weight of the pectin.

Besides the natural features of the pectin molecule its polymerization degree depends on the conditions of the plant and the nature of exposure to a production process.

During the natural decomposition of pectin substances in plants, the first stage of protopectin hydrolysis has a positive value from this point of view. In fruits this stage coincides with the so-called technical maturity of them, it represents the optimal phase for the formation of the state of pectin substances. With further decomposition of pectin substances under natural conditions of ripening of fruits on a tree or during ripening and overripening in lezhke, weakly bound components of pectin (methoxyl groups and associated substances such as galactose, araban, etc.), and sometimes deeper hydrolytic ones Processes.

When biochemical spoilage of fresh fruits (fermentation, putrefaction) by the enzymes of microorganisms occurs forced the collapse of the pectin molecule. Some enzymes cleave protopectin other ( "pektazy" or pectinesterase) produces demethylation of pectin, and others ( "pectinase" or polygalacturonase) cause its depolymerization, t. E. Pectin chain cleavage of the molecule, its break more or less short lengths.

Pectin have a sensitivity to thermal and chemical influences. Therefore depolymerization of the pectin molecules often occurs in processing of raw pectin. The more pectin is subjected to various treatments (heat, acids or bases), the greater the risk of depolymerization of pectin, which is a decrease of its molecular weight.

These studies and practical experience show that all the impacts that cause depolymerization of the pectin molecules inevitably entail deterioration of its ability studneobrazuyuschey.

Take that of the components of pectin complex studneobrazuyuschey ability to have pectin fraction having a molecular weight of not less than 10 000. The remaining fractions are not involved in pectin studneobrazovanii they are bulking agents.

Degree of polymerisation hydrophilic properties are defined and colloidal compounds, their water binding capacity. These properties of pectin are important in the production of marmalade-Pastila products as protect them from drying out or from getting wet.

Recently, considerable evidence has accumulated showing that the demethylation of pectin, which is not accompanied by depolymerization of its molecules does not lead to loss of its properties studneobrazuyuschih. On the other hand there is often depolymerization of pectin to its demethylation, t. E. The splitting of the pectin chain to methoxylated acid residues noligalakturonovoy.

It was also found that pectins retain their ability studneobrazuyuschuyu often while reducing the content of CH30 5% and to below. The resulting acid with pectin exhibit its ability to studneobrazovaniyu in somewhat different forms than the pectin, rich methoxy.

In general, recent research data leads to a new interpretation of the role of CH30. The latter consists in the fact that the content of CH30 in pectin determines only the conditions studneobrazovaniya: sugar, acid required for the formation of jelly, and the speed studneobrazovaniya process.

On the basis of the established production technology nizkometilirovannyh pectins. A distinctive feature studneobrazovaniya nizkometilirovannyh pectins is that jellies form with small quantities of sugar (about 35% by weight sugar jelly), unlike conventional vysokometilirovannogo pectin, which is capable of only jellies sugar concentration not lower 65%.

Additionally, pectins, containing not more than 7,5% CH30, have the ability to form strong jellies with ions of polyvalent metals.

Nizkometilirovannye pectins obtained by the enzymatic, acid or alkali hydrolysis of pectin materials. The conditions of preparation of these gels with calcium salts, sugar and acid. Recently added only for the taste.

The most stable calcium pectin jellies obtained from acids containing metiloefirnyh between groups before 3,5 6,0%.

The role of calcium or other metal ions in the formation of these gels is pectic acid molecules that are linked together by metal ions via free carboxyl groups. Therefore, this type of pectin gels called "ionnosvyazannymi" gels because communication Studneva grid made therefrom by polyvalent metal ions (in this case -Sa ++) replacing hydrogens carboxyl groups scheme15.5

In conventional sugar-pectin jellies acid (sugar with 65%) this connection is via the free COOH group, which are connected with each other to form hydrogen bonds. Therefore, these jellies are referred to as "hydrogen-bonded" gels.

There pectin jellies intermediate type, which contain sugar and calcium. In practice received very widespread in recent years, such jellies with 35 ~% sugar and appropriate amount of calcium (called "nizkosaharnye" jellies).

These provisions form the basis of modern ideas about the role of metiloefirnyh groups and their practical value in the process studneobrazovaniya pectin.

As for other nonuronid components of pectin (arabinose, galactose), they do not play any significant role in the pelleting of pectin. Their low content in native pectin or the artificial reduction of their content by purifying pectin preparations leads to a corresponding increase in the amount of galacturonic substances in pectin and to an increase in its gel-forming ability. Therefore, the percentage content of galacturonic acid pectin (the amount of the uro-nid part) is also an indicator characterizing its gel-forming ability.

From all that has been said above, it follows that the decisive role for pectin-forming ability of pectin has natural processes. Hydrolysis it in a plant, extracting it from plant material and then processing it in production. In view of this, it is necessary to avoid as far as possible all those factors that cause the depolymerization of the pectin molecule. In particular, care should be taken to prevent overripe fruit before collection and post-harvest period. It is necessary to protect the fruit and berry raw materials from spoilage in storage, from prolonged heating (during cooking and drying) and from the strong impact of chemical agents (acids, etc.) in the processing processes.

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