1042
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
Vol. 16, No. 10
Recent Progress in the Chemistry of Pectin and Its Industrial Applications’ By W. H. Dore VXIVERSITY OF CALIFORNIA AGRICULTURAL EXPERIMBNT STATION, BERKELEY, CALIF.
D’
:RECTIOKS for the domestic preparation of jellies from quince, apple, and currant juices were published as early as 1750,l,* but the jelly-making substance of fruits, pectin, was not discovered until 1825. The commercial manufacture of pectin, however, is of very recent date, the first patent for that purpose being granted in 1912. A considerable industry has sprung up since that time. Because of the widespread occurrence of pectin in nature, the raw“ materials for its manufacture are practically unlimited. Expansion of the industry may therefore be expected in proportion to the uses that may be found for the product. Increased uses, however, depend upon a more complete understanding of the chemistry of pectin, which, in turn, is only to be acquired by painstaking research.
is the product remaining after dissolving out the araban and
then splitting off the calcium and magnesium by treatment with dilute acids. The product still contains the methoxyl groups and is spoken of as an “ester acid,” but is regarded by Ehrlich as essentially a galactose-galacturonic acidi. e., an equimolecular union of galactose and galacturonic acid, analogous to the paired glucuronic acids that occur in the animal body. No serious conflict exists between the views of von Fellenberg and Ehrlich. The “ester acid” described by Ehrlich has the same behavior as von Fellenberg’s partially methoxylated ester of pectic acid. I n the light of recent researches, pectin substances may be defined as “derivatives of galacturonic acid.” This view is consistent with the time-honored recognition of pectin CHEMISTRY OF PECTIN bodies as carbohydrates possessing acid properties. MethThe discovery of pectin in plant juices by BraconnotZ oxy1 groups occur in ester combination with galacturonic acid; in 1833 was followed by the extensive researches of Fremy.3 other groups also enter into the pectin complex, but their method of linkage to the galacturonic groups is not yet known. Further contributions were made by Scheibler,4 Herzfeld, Since galacturonic acid is now recognized as the characterisTollens,6 and others.’ The net results of the work of these earlier investigators were (1) to establish the pectin bodies tic constituent of pectin, it is highly important that an acas carbohydrate derivatives possessing acid properties, curate analytical method be devised for the determination of (2) to show the presence of arabinose, carboxyl, and mucic this constituent. Data should then be obtained for the acid yielding groups in the pectin complex, and (3) to name and galacturonic acid and methoxyl contents of various pectins, describe a long list of pectin bodies. The individuality of and the relation between these two constituents should be ascertained. Work on this problem is now under way in the many of these may well be doubted. The work of two modern investigators has given us much Laboratory ol Plant Nutrition of the University of California. Of the numerous pectin bodies described by Fremy, we are additional information concerning the constituents of pectin. Von Fellenbergs has shown the presence of methyl groups in today compelled to recognize at least three: pectose, the ester combination with pectic acid. Ehrlich’sa important mother substance (now usually called “protopectin”) ; solucontribution was the identification of the substance to which ble pectin (generally designated simply as “pectin”); and pectin owes its acid properties. Thissubstanceisgalacturonic pectic acid. Progressive hydrolysis, either by enzymes or acid, a n isomer of glucuronic acid, and a half-way oxidation dilute acids, converts protopectin into soluble pectin and this product between galactose and mucic acid, represented by into pectic acid. The three forms occur in nature, protopectin Ehrlich in succulent root vegetables and unripe fruits, soluble pectin the formula CeH1007 or COH.(CHOH).&OOH. was the first investigator actually to isolate galactose from in ripe fruits, and pectic acid in unripe fruits and rotten vegepectin, although its presence had been previously inferred table tissues generally. The real existence of protopectin from the fact that pectin yields mucic acid on oxidation, has been recently questioned by Tutin,12who finds that all the Calcium and magnesium were also found by Ehrlich in the pectin of apples may be extracted by boiling with water, provided that the material is sufficiently finely divided to ,: pectin complex. Von Fellenberg regards pectin as essentially the methyl break up all protective cell walls. On the other hand, Suchester of pectic acid, since the methoxy groups are split off by aripall has shown that finely divided lemon “albedo,” after saponification with sodium hydroxide. Not all pectins are thorough extraction of the soluble pectin, still contains other fully methoxylated, however; part of the methoxyl groups pectin bodies which are not soluble until after hydrolysis. may be replaced by hydrogen or metals. Other groups may They appear to be combined with celluloseas “pectocelluloscs.” also be substituted for methoxyl. Tutinf0considers that there While Tutin’s experiments prove that the pectin of apples is is a partial replacement of the methoxyl groups by isopro- all in the soluble form, it appears from Sucharipa’s work that penyl groups in apple pectin, and Sucharipall has submitted protopectin, or the insoluble form, exists in some of the more evidence that in the “albedo,” or white inner rind, of permanent vegetable tissues. Some insight has been obtained by von Fellenberg into the lemons methoxyl groups are replaced by cellulose units. Ehrlich describes the naturally occurring pectin of plants probable mechanism of the formation of fruit jellies. When as “the calcium-magnesium salt of a complex anhydro- fruit juices containing pectin are boiled with sugar and orarabino-galactose-methoxyl-tetragalacturonicacid,” but re- ganic acids under proper conditions, a viscous solution is gards the form of combination as uncertain, except that the obtained which sets to a jelly on cooling. Prolonged boiling arabinose units in the complex are very loosely bound while in the presence of organic acids, however, destroys the jellythe union of the galactose is unusually strong. Pectic acid making power of the pectin, probably because of the formation of pectic acid. According to von Fellenberg, and to Such1 Received June 9, 1824. aripa,13 jelly does not form when fully methoxylated pectin is * Numbers refer t o bibliography at end of article.
v
October, 1924
INDUSTRIAL A N D ENGINEERING CHEMISTRY
boiled with sugar alone. If organic acids, such as malic or tartaric, or their calcium or magnesium salts, are added to the pectin-sugar solution, jelly formation usually occurs. On the other hand, it has been found impossible to prepare a jelly from a mixture of pectic acid and sugar, either with or without acids. These findings indicate that jelly formation is accomplished by neither fully methoxylated pectin nor pectic acid, but by partially methoxylated intermediates. To produce a jelly, the substance must be soluble in the sugar solution and give a solid solution of sufficient rigidity to satisfy the well-known physical requirements of a good jelly. The fully methoxylated pectins fail because their solutions are too fluid. Pectic acid fails because it is not soluble in the sugar solution and therefore yields a noncohering suspension of gelatinous particles, distributed unevenly through the sirup, inqtead of a uniform solid mass.
INDUSTRIAL MANUFACTURE OF PECTIN
It has long been the practice of housewives and commercial jelly-makers to reinforce fruit juices of poor jellying power with apple juice, whereby a good jelly invariably results, due to the increased pectin content. To secure the same effect, agricultural and domestic science institutions have recommended the home preparation of stock pectin solution from apple juice for use in jelly-making, and have given complete directions for the household production of such solutions. The large-scale manufacture of concentrated pectin solution and dried pectin by similar methods constitutes the present pectin industry. Since 1912 about a dozen patents have been granted for the manufacture of pectin. At least three large concerns and many smaller ones are producing pectin, mostly from apple products, but at least one company is using citrus by-products for this purpose. The process varies in different factories and naturally some of the details are not made public. I n general, it consists in extracting the pectin by boiling with water and evaporating the resdting pectin solution to either a thick sirup or a dry powder. The raw material is usually, but not always, given a preliminary extraction with cold water to remove soluble substances that would otherwise dissolve with the pectin. Starch i s sometimes removed from the dilute pectin solution by treating with malt extract. The purity of the product varies according to the raw materials and process used. It is difficult to get a pure pectin, free from the coloring and flavoring substances of the raw materials, since these go into solution a t the same time as the pectin snd are therefore contained in the final product. Impure pectins are unsuitable for the production of delicately flavored jellies, since the flavor and appearance are modified by the accompanying substances. It is therefore important to obtain it commercial pectin which shall be as pure as possible. Pectin may be manufactured from cull apples, cores, and peelings from canning factories or the pomace from cider mills. Pomace is the most suitable of these materials, because much of the soluble coloring and flavoring substances have already been removed with the pressed-out juice. If this material is digested in cold water before extracting the pectin, some of the remaining objectionable substances are removed and a better product is obtained, which still contains, however, some apple color and flavor. Some factories using cull fruits, peelings, etc., apparently extract the pectin by boiling with water directly without any preliminary treatment. The pectin solution which is obtained on evaporation then contains all the soluble substances that were originally present, in the apple, and is therefore a low-grade product. A purer pectin may be obtained by precipitation from pectin solutions by adding alcohol. This process, however, is not
1043
practicable on an industrial scale. Suggestions have been made to purify pectin by precipitation with various inorganic salts, but apparently these have not been used commercially. ECONOMIC ASPECTS The raw materials that so far have been found suitable for pectin manufacture are apple by-products and citrus byproducts. The quantity of these substances available is apparently many times the present needs of the industry. Walton and Bidwell14 estimatJe that 22,500,000 bushels of apples of 48 pounds each, or 540,000 tons, are pressed for cider each year in commercial plants in the United States. There is no way of estimating the additional large quantities that are worked up on the farms. Assuming a pectin content of 1 per cent, 5400 tons of anhydrous pectin could be produced from this single source. Apple pomace is being used to same extent as a cattle food, but Walton and Bidwell15 believe that its feeding value is not appreciably diminished by the removal of the pectin. As these authors point out, it is good economic policy to utilize the pomace as far as possible for the production of pectin for human food before feeding it to cattle. It has been reported by PooreI6 that 30,000 tons of waste lemons are produced annually in California as by-products of the citric acid industry and that these contain 1.95 per cent of pectin. A possible pectin production of about 585 tons may thus be obtained from lemon residues alone. Beet pulp has so far not come into use in this country for the preparation of pectin. Approximately 100,000 tons of dried pulp are produced annually, and this amount could yield about 25,000 tons of pectin, since the dried pulp contains about 25 per cent. Beet pulp, therefore, constitutes an immense potential raw material for the pectin industry. It is now very completely utilized for cattle feeding, but the same considerations that apply to apple pomace probably hold in the case of beet pulp, and the pectin could be extracted without greatly diminishing the value of the residue as a cattle food. The foregoing figures are to be regardedas only very approximate, but they indicate that enormous quantities of material are available for the manufacture of pectin. No figures are obtainable for the quantity of pectin now produced, but it is certain that only a very small fraction of these raw materials is being utilized a t present. A great pectin industry may therefore be developed if a sufficient demand for the product arises. The only use that has been made of pectin is in jellymaking, commercial and domestic. This outlet for the product is open to some extension, but within comparatively narrow limits. Any great expansion of the industry must accordingly depend upon the discovery of new uses for pectin.
POSSIBLE USESFOR PECTIN The possibility of using pectin as a human food was suggested by Ehrlich during the war. He proposed to utilize for that purpose the extracted pulp of sugar beets, which he estimated could furnish Germany with 176,000tons annually. Pectin has been regarded as having but little food value, since it apparently resists the hydrolytic action of the digestive enzymes, but Ehrlich believes that its food value may be far greater than has been previously supposed. The few experiments that have been carried out on the animal nutrition of pectin-like substances have not drawn a sharp distinction between pectin and the mixture of pentosans and hexosans that is loosely designated as “crude fiber” or “cellulose.” In any event, Ehrlich considers it feasible to develop a hydrolytic process whereby the galactose of pectin might be split off and its food value utilized. Pectin offers possibilities as a commercial source of mucic acid. Ehrlich reports a yield of 40 per cent of mucic acid from
1044
I N D USTRIAL A N D ENGINEERING CHEMISTRY
beet pectin, or 18 per cent on the dried beet pulp, the mucic acid being derived partly from galactose and partly from galacturonic acid. Mucic acid is now being made commercially from the wood of Lariz occidentalis for use in the manufacture of baking powder. This wood contains only 10 per cent of galactan and accordingly yields not over 7 or 8 per cent of mucic acid. From the standpoint of yield the pectincontaining substances have a decided advantage over the larch as a source of mucic acid. While many new uses will probably be suggested by studies on the constitution of pectin and its reactions as an organic substance, other valuable applications may be expected as a result of researches upon its colloidal properties. Apparently very little has been done in that direction.
PECTIN IN OTHER FIELDS The reactions of pectin enter into the preparation oi certain of the fibers used for textiles, notably flax, hemp, ramie, etc. The separation of the celIulose fibers from their incrusting substances depends upon breaking down the pectin substances which act as the cementing agent. The processes, thenwhether enzymic or bacterial, as in the natura! rsfii-g pmeess, or strictly chemical-are primarily concerned with the degradation of pectin. A knowledge of its decomposition reactions will accordingly find application here. The produc-
Vol. 16, No. 10
tion of wood pulp for paper-making is a different problem, since in this case lignin, not pectin, is the cementing agent. The pectin content of wood is apparently very small. Pectin plays an important role in the life processes of plants. For that reason the solution of some of the problems in plant physiology and the application of these to practical agriculture will be materially aided by a more complete understanding of pectin chemistry.
BIBLIOGRAPHY l-ElLis, “Country Housewife’s Family Companion,” James Hodges and B. Collins, London. 2-Ann. chim. phys., 28, 173 (1825); through Czapek, “Bichemie der Pflanzen,” 1905, p 546. 3-J. pharm. ch?m., 26, 368 (1840). 4-Ber., 1, 58 (1868); 6, 612 (1873). 5-Chem. Zentr., 62, 618 (1891). 6-Ann., 285, 278, 292 (1895). 7 - p ~ a review of early work on pectin, see von Fellenberg, Biochem. Z.,85, 118 (1918), or Czapek, loc. cit. 8-von Fellenberg, loc. cit. 9-Chem. Zlg , 41, 197 (1917). 10-Biochem. J., 15, 494 (1921); 17, 83 (1923). 11-J. Am. Chem. Soc., 46, 145 (1924). 12-Biochem. J . , 17, 510 (1923). 13-J. Assoc. Oficial Agr. Chem., 7 , 57 (1923). 14-U. S. Dept. A g r . , Dept. Bull. 1166, 5 (19231 15-Ibid., p. 34. 16-Chem. A g e (N.Y . ) , 80, 433 (1922).
Potash from Cement Dust’” Concentration by Elutriation By E. J. Fox and C. W. Whittaker BUREAUOF
SOILS, WASHINGTON, D.
C.
Fractions of cement dusts, the potash contents of which are a p contrary, it Was assumed E M E K T dust has proximately double those of the original materials, were obtained by that the two were PreciPibeen shown to be air separation. I n no instance was all the potash obtained in One tated independently, and Potentially a very might therefore be sepaimportant Source of Amerifraction of the material, regardless of the size of the fraction. r a t e d by Some Physical can p ~ t a s hbut , ~ production I n all fractions of a giuen sample the potash content decreased as means. Because of the difcosts Published to date inthe size of the particles increased. ficulties attending any wet dicate that the technic of The potash is apparently present on the surface of the dust particles method of separation, it is recovery of Potash thereand the disfribufionof the potash ouer all surfaces is apparently egual. believed that any successful from has not yet been deThe concentration of potash obtained was apparently due to the veloped to a satisfactory increase in the aggregate surface obtained in the finer fractions. method for the recovery of degree. Researches have potash from cement kilns been inaugurated, theremust be on some basis fore, to investigate this phase of the potash problem to de- other than water as a means of separation. The advantermine if more efficient methods of recovery can be devised. tages would be manifold. Both the dust and the potash The results and tentative conclusions reached from a study salts would be obtained in a dry state. KO elaborate equipof one phase of the problem are presented briefly in the fol- ment, for treating the dust with water would be required, which would eliminate all evaporating and crystallizing problowing paragraphs. The potash volatilized in cement kilns escapes from the lems incident thereto; and at the same time the one overcharge as a vapor, and as such is carried along with the gas whelming difficulty with calcium sulfate precipitating in water stream issuing from the kiln until the temperature of the gases pipes and spray nozzles would be avoided. To accomplish this, elutriation in media that would not drops to a point where the potash condenses. The potash or the dust was undertaken. then exists as minute particles of potash salts in a highly react with the . potash dispersed state. The dust, on the-other hand, consists of DESCRIPTION OF ELUTRIATOR relativelv large, solid Darticles which are blown from the kiln. ’Cinderzhe influence of the electric precipitator all the The elutriator employed consists of a lower reaction of 2l/4dust particles and a greater part of the potash fume are inch brass pipe, 16 inches long, surmounted by a 4-inch brought down together. I n the absence of evidence to the section, 30 inches long. The two sections are joined by a frustum Of a 60” cone, and are securely soldered 1 Received April 1, 1924. Presented under t h e title “Elutriation a s a Means of Concentrating Potash in Cement Dust” before t h e Division of together. Industrial and Engineering Chemistry a t t h e 67th Meeting of t h e American is attached to the bottom of the lower A 60-degree glass Chemical Society, Washington, D. C., April 21 t o 26, 1924. section by means of a brass collar or yoke screwed to the bot2 Published with t h e permission of t h e Secretary of Agriculture. tom of the section, making an airtight joint by means of rubber 3 U. S. Dept. A g r . , Bull 672.
C
.
