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FoR many years the word "pectin" has been used in a general sense to designate a group of substances which, in the presence of sugar and a proper degree of acidity, will form the common jellies or jams familiar to everyone, The flavor of jelly or jam, of course, depends upon the fruit or flavor used in its preparation. As to the origin of pectins, they are a constituent of plants, being found in the middle lamella or intercellular spaces of the plant tissues. It is thought that pectin acts as a cement, interlacing the cellulose of the cell walls. Chemically, pectins are complex carbohydrates which have been found to be comprised primarily of methylated polygalacturonic acids closely associated with arabinose and galactose. These complexes generally occur as a methyl ester but may exist as a free acid or metal salt. The size of the molecule and the arrangement of its constituents are variable, but the properties that pectins exhibit have been well studied. Pectin substances are generally grouped into three distinct entities termed "protopectin", "pectin" and "pectic acid". Protopectin is the precursor to pectin, being a water-insoluble parent substance which can be changed * Published as Miscellaneous Paper No. 32, with the approval of the Director, of the Delaware Agricultural Experiment Station, contribution of the Department of Chemistry. August 25, 1947. to pectin by restricted hydrolysis and made soluble by treatment with acid, alkali or enzymes. In nature this change is found to take place during growth or ripening of plant tissues. Pectin, the intermediate group, is a water-soluble polygalacturonic acid of varying methyl ester content. This group may be subdivided into pectinie acids, provided free carboxyl groups are present, and pectinates, when the carboxyl groups are in combination with metallic salts. By means of enzymic or chemical hydrolysis, water-soluble pectins can be converted to insoluble pectic acid which is essentially free from methyl ester groups but whose salts may be either normal or acid pectates. The enzymes capable of accomplishing these changes are protopectinase, which acts on protopeetin and converts it to pectin; pectase, which changes pectin to pectic acid; and pectinase, which has the ability to convert pectin and pectic acid to their simplest cleavage products, presumably galaeturonic acid, galactose and arabinose. In the early stages of ripening of fruits, with apple as an example, no soluble pectin is found, but as the fruit ripens the pectin gradually changes to the soluble state until finally, when the fruit is over-ripe, the pectin content decreases along with the solubility. These changes may be retarded by lowering the temperature; consequently, the reason for refrigeration of fruit is quite apparent. Sources Pectin is found to some extent in practically all fruits and vegetables. The principal commercial sources, however, are apple pomace and citrus peels. These waste products afford a cheap source of pectin. Apple pomace contains 1.5 to 2.5 per cent of pectin, lemon pulp 2.5 to 4.0 per cent, orange pulp 3.5 to 5.5 per cent, and grapefruit waste 3.0 to 4.5 per cent. Most of the original pectin still remains in the pomace, pulp or waste after the fruit juice has been removed by pressing. Ideally, fruits for jelly should contain sufficient pectin and acid to produce a good jelly without addition of these substances. Some fruits contain enough pectin and acid to accomplish this, while others are deficient in one or both of these substances. For example, crab apples, currants, lemons, limes, grapefruit, cranberries and sour varieties of oranges are fruits rich in pectin and acid. On the other hand, fruits low in both pectin and acid are ripe peaches, Bartlett pears and apricots. When a deficiency of these substances is known it is customary to supplement them by using commercial pectin and citrus juice in the home. The introduction of commercial pectin has eliminated the possibility of a poor jelly in the home and has made possible the standardization of jelly-making in industry. Preparation The preparation of commercial pectin from apple pomace and citrus peels involves several step-wise treatments. The particular treatment used depends upon the nature of the raw material and the use to be made of the product. In general, the raw material is subjected to a controlled extraction by means of mild acid hydrolysis in water solution in order to convert the insoluble pectin to watersoluble pectin. Much of the pectin in pomace and peel is in the form of an insoluble alkaline-earth salt of pectinic acid; therefore, polyphosphates render a valuable aid in its extraction. The temperature, time and degree of acidity during the extraction should be carefully controlled in order to obtain the best yield and quality of pectin. The next step is to clarify the pectin liquor, which may consist of treatment with starchhydrolyzing enzymes, decolorizing carbon and filtration with the aid of various filter-aids. Precipitation of the pectin follows the clarification. This is accom. plished either by the addition of alcohol, after partial concentration in vacuo, or precipitation of the pectin in dilute solution by colloidal aluminum hydroxide. The success of the alcohol precipitation depends on the completeness with which the alcohol can be recovered, while with the aluminum hydroxide precipitation, the material has to be further treated with acidified alcohol in order to remove the aluminum salts. Drying, grinding and standardization of the pectin follow the precipitation. In the case of commercial liquid-pectin solutions, the pectin extract after clarification is concentrated and standardized as to quality, quantity and acid content of the pectin.Some manufacturers dry the pectin solution in a thin film on a drum or spray dry. In any case, the final pectin product, whether liquid or powder, is usually sold according to grade which is the number of pounds of cane sugar that one pound of pectin can support in a jelly of 65 per cent soluble solids content at optimum acidity. During the late thirties the scientific literature on the subject of pectic substances began to mention more than one kind of pectin, namely low-methoxyl pectinates and high-methoxyl pectinates. This terminology was based upon the degree of de-esterification or methoxyl content of the pectin, which may range from 0-16.32 per cent, the upper 3 per cent being theoretically possible. The arbitrary upper limit for low-methoxyl pectinates is 7 per cent; all pectinates above this value are considered to be the high-methoxyl products. Until this time the pectin compounds of commercial concerns were the high-methoxyl pectinates. With the introduction of the low-methoxyl pectinates, gels with low soluble solids content, that is, jellied products with little or no sugar, could be made which were suitable for desserts and salads. Only sufficient sugar for flavoring purposes needs to be added. The preparation of these low-methoxyl pectin gels was found to be slightly more complex than the conventional 65 per cent soluble solids jellies. In addition to pectin, sugar and acid being present, as in regular jelly, it was found necessary to have a polyvalent chemical element present in order to produce gelation. In most cases a calcium salt has been found quite satisfactory for this purpose. The preparation of low-methoxyl pectinates may be accomplished by three quite different methods. One method results from a high-acid treatment of the pectin over a long period of time at a relatively low temperature. The second method has been to utilize enzymes in order to accomplish the necessary deesterification, and the third method, to employ an alkaline medium under controlled conditions. All methods have their advantages and disadvantages, but the pectin prepared in any one of the three ways exhibits primarily the same properties. Perhaps the most outstanding characteristic between the low-solids gels and regular high-solids jellies is the superior flavor of the former which is due to the lack of excess sweetness which possibly masks the real fruit flavor in the high-solids jellies. Uses By far the greatest use for pectin is in jellies, jams and marmalades, although there are many other uses, some not commonly known. The use of pectin in many food products, cosmetics, pharmaceuticals, medicines, textiles, adhesives, rubber latex, and in the hardening of steel has resulted from research over a period of several years. These are not all the uses, by any means, but perhaps the most important. In considering the food products that utilize pectin, the original gum-drop, which was prepared with gum arabic and was gummy, chewy and tough, has a competitor using pectin that is more tender, sweeter and brighter in color and which is more closely akin to a jelly than a gum. Pectin is effective as a stabilizer for ices, sherbets and ice cream. Its use as a thickening agent for soda-fountain syrups and crushed fruit has become quite common. In ices it aids to prevent the freezing of water which causes the granulation and crystallization inhibiting the material from freezing smoothly and evenly. In fruit juices it not only thickens but acts as a stabilizer to assist in preventing separation of the pulp. When added to tomato juice or tomato catsup it makes them more viscous and improves the general appearance. It assists in stabilizing and emulsifying salad dressing. In dairy products pectin assists in preventing a heavy curd in milk by acting as an emulsifier and a thickener, while in cheese it preserves the texture during heating to kill certain organisms that may cause deterioration. In bakery products pectin improves the texture, yield and moisture-holding capacity, thereby tending to slow down the staling process. Many uses have been suggested for pectin in cosmetics and pharmaceuticals. In most cases it serves as a jellifier, emulsifier or binder. It has been employed in dentifrices, salves, pastes, emulsions, cosmetic creams, lotions and as a binder in tablets or pills. Whenever used it must be under neutral or slightly acid conditions because it is decomposed by alkali. In connection with the medicinal use of pectin, it has been employed in the treatment of diarrhea and constipation. The particular action of pectin in the treatment of diarrhea is thought to be due to the galacturonic acid of the pectin molecule combining with the toxic substances to aid in their removal. In constipation pectin is useful probably because of its emulsifying and water-holding ability. Pectin is claimed to have antihemorrhagic properties, perhaps due to the galacturonic acid ester of the pectin molecule aiding blood coagulation. One of the most recently reported uses of pectin has been in the treatment of burns and soft tissue wounds. No positive explanation has been made to explain its action, but it does speed up healing when used in the form of pastes or salves. Bacteriologically pectin has been used with excellent results in culture media and found suitable for use in removing inactive proteins during the preparation of antitoxins. For finishing .textiles pectin has been found to be a competitor of starch. The pectin treatment is claimed to be simpler and does not require the auxiliary substances which are generally used along with starch. Pectin has been found to be a good adhesive which is effective on wood, glass or tin. A preservative is usually used in its preparation. A two to three per cent solution may be used for tipping the end of cigars; however, for a stronger adhesive, the amount in solution should be increased. The so-called "creaming" of crude rubber latex is covered by Patents that recommend the use of pectin. The addition of pectin causes the latex to separate into two layers, which results from the coating of the rubber particle with pectin followed by an equilibrium upset due to a specific gravity difference. Thus the rubber particle is separated from the non-rubber constituents. Since the heat conductivity of pectin solutions is approximately the same as that of oils used for quench hardening of steel, it has been claimed that varying degrees of hardness may be imparted to steel by simply varying the pectin conceatration. A low pectin concentration produces a brittle steel, while a high concentration of pectin produces a hard, tough product. A pectin in water solution of 0.5 to 4.0 per cent has been found a satisfactory range. With continued progress in science being made day by day through chemical research, it is only natural to expect that the list of uses for pectin will be expanded. Since commercial pectin is a by-product of the fruit industry, its cost of manufacture will be the limiting factor for competing with other products. Even at the present time apple pectin is not so plentiful as citrus pectin, due mainly to competition with the expanding citrus industry on the West Coast. Consequently, as time goes on, we may expect an increase in by-product waste materials, simpler methods of extraction and improved yields, so that the potential uses of pectin can be expanded. |
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