According to the nomenclature adopted in 1944, the following pectic substances are distinguished: protopectin (see above), pectin, pectic acids and pectic acid.
Pectin is a generic term for a water-soluble cellulose-free pectin substance consisting of methoxylated galacturonic acid residues. Pectin is a product obtained as a result of the first stage of the natural breakdown of protopectin. In unchanged form, it is able to form a jelly with sugar, acid and water. In mature fruits, the bulk of pectic substances are already in the form of pectin. In this state, the fruit intended for confectionery processing should go into production.
Methoxylated pectin polygalacturonic circuit can be simplified so designated
In the process of hydrolysis of pectin, gradual cleavage of the methoxy groups (demetoxylation) occurs. Fully demethoxylated pectin (with an intact chain) is called pectic acid by the new nomenclature.
The structure of the pectic acid can be represented by the following formula:
Pectic acid is a colloid having a polymerized molecule. Its general formula (C5Н7О4SOON)n. However, it has only a weak ability of gelation.
Between pectin and pectic acid there are a number of intermediate decay products of varying degrees of demethoxylation. They are present in a natural mixture of pectic substances. Their polygalacturon chain consists of units, most or less of which are saturated with methoxyls. These compounds are named pectic acids. Their gelatinous properties vary depending on the degree of their methoxylation.
The general formula of pectic acid
As a result of the deep breakdown of pectic acids (and pectic acid), accompanied not only by demetoxylation, but also by breaking the polygalacturonic chain into separate links, we obtain molecularly soluble monogalacturonic acid:
which behaves like a typical monobasic acid.
Consider the most important physical and chemical properties of the pectin.
Pure pectin is a white substance that swells in water with the gradual formation of a colloidal solution - a sol.
Pectin solutions have considerable viscosity.
The viscosity of the dilute pectin sols increases in direct proportion to the concentration. Such a relationship exists only for pectin solutions with concentrations up to 1%. In solutions with a higher concentration, significant deviations from this dependence are observed. In the area close to the gelation of the pectin sol, anomalous viscosity is observed, accompanied by the formation of a 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 of the pectin complex have a negative charge of high density. The latter is mainly due to the free carboxyl groups of pectic acids. Due to this, pectic substances (especially in the presence of pectic and pectic acids in them) are precipitated under certain conditions from an aqueous solution under the action of salts of polyvalent metals in the form of coagulates of these salts soluble in mineral acids.
Pectin is an optically active substance with right-hand rotation, and its specific rotation varies depending on the fruits from which it originates and on the degree of its purity.
Pectic acids form medium or acidic salts of various metals.
For 0,5 — 1,0% solutions of pectic acids, pH ranges from 3,2 — 3,4.
Pectic acid in dry form is a white powder with low solubility in water. With alkalis, it gives soluble salts (pectates), and with alkaline earth and heavy metals, it forms insoluble salts. With tannins, it is precipitated, unlike pectin.
The molecular weight (M) of pectic substances varies depending on the origin of pectin and the degree of its depolymerization.
Pectic substances are, as a rule, a polymolecular mixture, i.e. a mixture of molecules of various sizes. Therefore, we can talk only about the average values of M.
The specific difficulty of establishing M pectins lies in the fact that pectins are a heterogeneous mixture that requires prior purification.
In addition, there is a wide variety of methods for determining M, which give different values for the same product.
Schneider and Bock (1937), which determined M of apple pectin by the osmotic method (for pectin nitrates free from the presence of "satellites"), obtained average values from 30000 to
100 000; for pectin oranges, the molecular weight of 150 000 was found, for lemon 220 000 and beet 20 000 — 25 000.
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 are obtained when determining M of pectin by the pericosimetric method.
Along with the value of M, data on the shape and size of their molecules are of great interest for characterizing the physicochemical properties of high polymers. Pectic acid molecules are cylindrical; molecule thickness about xnumx 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 is the formation of jelly with acid and water in the presence of relatively large amounts of sugar (60 — 80% by weight of jelly). Getting jellies of this type underlies the production of fruit and berry jams, marshmallows, jams, sweets.
Without dwelling on the question of the mechanism of jelly formation of pectin in the conditions of confectionery production (this issue will be given a special place in the future), it is necessary here to consider the gelatinous properties of pectins and the factors by which these properties are determined.
The ability to gelatinization is manifested individually in different pectins, depending on their origin, and therefore they are of unequal value for confectionery production.
It is known, for example, that pectins of apples, citrus fruits (made from a crust of oranges and lemons), black currants, gooseberries, sunflower baskets and beets are the most valuable in terms of their jelly-forming ability. With proper management of technological processes of production of these pectins, they give the jellies with the necessary strength and other valuable properties.
Less valuable in this respect are the pectins of mountain ash, quince, apricot, peach, plum, cranberry. Pectins of these fruits, giving the jellies with lower strength, do not meet the requirements of the production of confectionery. Even less valuable in terms of their ability to gel formation are pectins of pears, cherries, summer berries, grapes and vegetables.
Within the same species and variety of fruits and plants, the ability of pectin to gelatinization changes during the development of the plant, fruit ripening, during storage and processing of this raw material.
The current state of knowledge does not yet give an opportunity to reliably establish the causes that determine the natural capacity for gelatinization in pectins from various 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 this raw material does not make it possible to judge its gelatinous ability. This is due to the fact that the methods used for the quantitative determination of pectin (the well-known calcium-pectate method, the method of precipitation with alcohol, etc.) show in total with its gel-forming fractions and those fractions that are deprived of this property.
It is currently accepted that the jelly-forming properties of pectic substances are predetermined mainly by the following factors:
- chain length molecules pectin;
- methoxylated galacturonic acid residues;
- neuronidnyh the presence of components (organic and mineral).
It can be considered proven that the ability of pectin to jaundice depends primarily on the size of its molecule. The latter are determined by the degree of polymerization of the chains of principal valences and are characterized by the value of the molecular weight of pectin.
In addition to the natural features of this pectin, the degree of polymerization of its molecule depends on the conditions of plant development and the nature of the impact on it in production processes.
In the course of natural decay of pectic substances in plants, the first stage of the hydrolysis of protopectin 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 phase of the state of pectic substances that is optimal for gelling. With further decomposition of pectic substances in natural conditions of fruit ripening on a tree or during ripening and overriding in maturation, weakly bound components of pectin (methoxyl groups and related substances, such as galactose, araban, etc.) are split off, and sometimes deeper hydrolytic processes.
When biochemical spoilage of fresh fruits (fermentation, rotting) under the influence of enzymes of microorganisms, a forced disintegration of the pectin molecule occurs. Some enzymes break down protopectin, others (“pectases” or pectinesterases) demethylate pectin, and some (“pectinases” or polygalacturonases) cause its depolymerization, i.e. cleavage of the chain of pectin molecules, breaking it into shorter or shorter segments.
Pectins are sensitive to heat and chemicals. Therefore, the depolymerization of the pectin molecule often occurs in the processing of pectin-containing raw materials. The more pectin is subjected to various treatments (heating, the action of acids or alkalis), the greater the danger of the depolymerization of pectin, which is manifested in a decrease in 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.
It is assumed that of the constituent parts of the pectin complex pectin fractions with a molecular weight of at least 10 000 have jelly-forming ability. The remaining fractions of pectin are not involved in gelation, they are ballast substances.
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, a significant amount of data has been accumulated, showing that pectin demethylation, which is not accompanied by depolymerization of its molecule, does not lead to a loss of its gelling properties. On the other hand, depolymerization of pectin before its demethylation is often observed, that is, the cleavage of the pectin chain into methoxylated residues of polylacticuronic acid.
It was also established that pectins retain their gel-forming ability, often with decreasing CH content.30 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 predetermines only the conditions of gelation: the amount of sugar, acids required for the formation of jelly, and the speed of the process of gelation.
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 polyvalent metal ions.
Low-methylated pectins are obtained by enzymatic, acidic or alkaline hydrolysis of pectin-containing raw materials. Developed conditions for the production of these jellies with calcium salts without sugar and acid. The latter are added only for taste.
The most durable calcium jelly is obtained from pectic acids with a methyl ester content ranging from 3,5 to 6,0%.
The role of calcium or ions of other metals in the formation of these jellies is that the molecules of pectic acids are bound to each other by metal ions through free carboxyl groups. Therefore, this type of pectin jelly is called “ion-bound” gels, since the jelly mesh is bonded in them using polyvalent metal ions (in this case —Ca ++), which replace the hydroxides of carboxyl groups according to
In conventional pectin-sugar-acid jelly (with 65% sugar), this bond is carried out through free COOH groups, which are connected to each other to form hydrogen bridges. Therefore, these jellies are referred to as "hydrogen-bonded" gels.
There are intermediate type pectin jelly, which contain both sugar and calcium. In practice, such jellies with 35 ~% sugar and the corresponding amount of calcium (the so-called “low sugar” jellies) have become very widespread in recent years.
These provisions formed the basis of modern ideas about the role of methyl ester groups and their practical significance in the process of gelatinization of pectin.
As for the other neuronidic components of pectin (arabinose, galactose), they do not play any significant role in the gelation of pectin. Their low content in native pectin or artificial reduction of their content by purification of 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 of galacturonic acid in the pectin (the amount of the uronic part) is also an indicator characterizing its gel-forming ability.
From all the above, it follows that the processes of the natural are of decisive importance for the jelly-forming ability of pectin. hydrolyzing it in a plant, extracting it from plant materials and subsequent processing in production. In view of this, it is necessary to avoid as far as possible all the factors that cause the depolymerization of the pectin molecule. In particular, care must be taken to prevent over-ripening of fruits before harvesting and during the post-harvest period. It is necessary to protect fruit and berry raw materials from spoilage in storage, from prolonged heating (during cooking and drying) and from the strong influence of chemical agents (acids, etc.) in the processing processes.