October, 1925
INDCSTRIAL A N D ENGINEERING CHEMISTRY Airplane Possibilities
The development of the airplane exhaust and curtain smokes since the war has opened up new possibilities in putting down much more effective screens a t points that were hitherto more or less inaccessible for good screens and a study of airplane smoke tactics constitutes a field in itself. Naval Possibilities
Especially is this so in naval warfare, where previously screens had to be maintained by the use of an excessive amount of smoke for a very long period to enable destroyers
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to attack successfully. By use of airplanes, effective and economical screens can be established for any desired sea area, and the screen can be originated comparatively close to the target or object, instead of five miles or more away, as was previously necessary. I n addition to screening torpedo attacks by destroyers, these screens can be used to block out part of the enemy fleet, making possible a concentration of fire on almost any desired number of enemy ships a t a cost of very little material, and the probability of loss of airplanes would be much less than the probability of loss of one or more destroyers engaged in setting up such a screen.
The Manufacture of Pectin’ By C. P. Wilson RESEARCH LABORATORY. CALIFORNIA FRUITGROWERS EXCHANGE, SAX DIMAS,CALIF.
Although it is customary to boil fruit juices to obtain a ECTIN was discovered in 1825 by Braconnot,2n*who made artificial jellies with pectin obtained from proper concentration to produce a jelly, it has long been recogcarrots. Since then many chemists have been inter- nized that jellies can be produced without heat and we have ested in pectin from various standpoints. A study of the household recipes for ‘‘sun jelly” or “cold jelly.” Currants splendid work of FellenbergI5 with its accompanying bib- are the outstanding possibility for making cold-process jelly, liography brings one about up to the date of that publication as they normally contain plenty of pectin and acid without in his knowledge of pectin, though Ehrlich14 attacks the ques- concentration of the juice, but other fruit juices can be used tion of the chemical constitution of pectin in a fundamental when the fruit is in just the right condition. It was but natural that after the role of pectin in jellyway and makes a distinct contribution to that subject. making was- recognized a-tMany others have worked tempts should be made to or are working on pectin, isolate this substance and a n d t h e literature is inA brief review of patent and other literature bearing by its use secure better concreasing in volume and in on the manufacture of pectin prefaces a discussion of trol of the jelly-makingprocvalue. the principles utilized in the recovery of this article. ess and eliminate the uncerRaw materials are apple citrus and beet refuse pulp or Present Status of Pectin tainty or f a i l u r e d u e t o Work carrots. variation in different kinds The production of pectin is followed through the sevThe lack of standard of fruits or in the condieral stages in its development, from the use of apple nomenclature is recognized tion of fruit of the same pectin stock to assist in jelling the juice of other fruits to by everyone who studies variety. the production of a dry pure pectin from citrus fruits pectin literature, and alJelly Stock by the application of principles of colloid chemistry. though there is now some Probably the first use of t e n d e n c y toward better added pectin was by the agreement in this matter there is urgent need for absolute uniformity. Other great housewife, when she learned that by mixing juice from cooked needs are for a knowledge of the actual chemical identity of apples with the juice of berries she could make a firm jelly which pectin and a rapid accurate method for its determination. could not be produced from the berry juice alone. From There is disagreement as to the composit’ion of the various this the practice grew into the production of “stock,” which pectin bodies, and TutinZ8J9J0denies the existence of proto- was a pectin solution obtained by boiling apples, apple pomace pectin as the precursor of pectin as proposed by T s c h i r s ~ h , ~or~ apple skins, cores, and trimmings, and straining off the liquid. This mas the simplest method of extracting pectin. S u ~ h a r i p a and , ~ ~ others. Boiling converted protopectin into pectin and the latter being Function of Pectin in Jelly soluble in the dilute solution of malic acid, there was obtained Regardless of these differences of opinion as to the exact a solution of pectin that could be used to enrich juices deficient nature of pectin, or indeed as to whether pectin is a chemical in this substance. When adopted by the jelly manufacturers this practice was individual or a family of substances, the name is generally accepted as meaning that thing occurring in fruit and other subject to gross abuse, and thousands of tons of jellies paraded parts of plants by virtue of which, in combinations with proper under attractive labels indicating the presence of fruits which amounts of sugar, acid, and water, fruit jellies acquire and were either entirely absent or present in such small proportions maintain their consistency as jelly. The term “fruit jellies” that they gave no distinctive character to the product. This is used in a general sense to cover also jams or any other fruit abuse was the initiator of its own correction. Thanks to the preparations that normally have the consistency of jelly efforts a t proper legislation and its enforcement, bad conditions are being corrected and the better element of the jellywhether they contain fruit pulp or only clear juice. making business is actively interested in the proper use of 1 Presented as a part of the Symposium on Pectin before the joint pectin and in legal restrictions that will eliminate the abuses meeting of the Divisions of Agricultural and Food Chemistry and of Bioassociated with it. logical Chemistry with the Division of Sugar chemistry at the 70th Meeting Attempts a t the recovery of pectin and studies of its use in of the American Chemical Society, Los Angeles, Calif., August 3 to 8, 1925. jelly-making have not been confined to commercial firms, but * Numbers refer t o bibliography at the end of article.
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the federa15,9J2s24and some of the state governments and universities have expended considerable time and money on this problem. 4 'I 8 16,11,26,26 I
Vol. 17, No. 10
be used with the fruit, the fiber being strained out with the fruit pulp.
8
Early Manufacture
For many years commercial jelly-makers have made jelly stock from apples or, more often perhaps, from apple pomace or from the skins, cores, and trimmings left from the production of dried apples and other products of the fresh fruit. I n this way a pectin solution of variable quality and doubtful keeping quality was obtained. Attempts were made to concentrate the pectin solution in open kettles with varying success and later vacuum concentration was developed. Finally, methods were developed for the separation of pectin from its solution by precipitation, drying, etc. The activity along this line has naturally resulted in the issue of a number of patents based on a variety of pectin preparations and a still greater variety of methods for obtaining them. The principles used in the various methods proposed will be outlined here. Raw Materials
The principal sources of pectin so far suggested are apple pomace or the refuse from cider and vinegar mills, citrus peels or the refuse from the production of citric acid and other major citrus products, beet pulp or the residue from beet sugar manufacture, and carrots which can be raised quite cheaply. So far as is known only the refuse from operations handling apples or citrus fruits have been used commercially as sources of pectin. The amounts of pectin in these raw materials are given in Table I. Table I-Percentage MATERIAL Apple pomace Lemon pulp Orange pulp Beet pulp14 Carrots31
of Pectin in R a w Materials
Pectin in fresh material Per cent 1.5 to 2.5 2.5 to4.0 3 . 5 to 5 . 5 1.0 0.62
Pectin on water-free basis Per cent 1 5 . 0 t o 18.0 30.0 t o 38.0 30.0 to 40.0 28.0 t o 3 0 . 0 7.14
The percentage of pectin found in prepared products such as fruit or beet pulp or pomace depends on the processes to which it has been subjected. If cold-water extraction has leached out sugars, acids, etc., and excess water is removed by pressing, the pectin content remains relatively high, while hot water treatments, especially in acid solution, reduce the residual pectin.
Purification of Extracts
Various methods of purifying pectin extracts are proposed. Some specifyremoval of sugar13or sugar and acid from the raw material by cold-water leaching, one removes starch12 by means of a diastatic enzyme, and another removes protein6 by means of a proteolytic enzyme. The object of the removal of starch and protein is to get a pectin that will make clear jellies. Methods for Precipitation of Pectin
Four principles have been used in the processes for precipitating-viz., alcoholic precipitation, salting out, electrolytic precipitation, and colloidal precipitation. Precipitation with alcohol is probably the oldest known method and has been used commercially. The big factor is the cost of alcohol, which must be carefully recovered or the venture is not profitable. Specially denatured alcohols-Formula 2 B containing 0.5 gallon of benzol per 100gallons of ethyl alcohol, and 35a containing 5 gallons ethyl acetate per 100 gallons ethyl alcohol-have been approved for the commercial production of pectin. I n using alcoholic precipitation it is found necessary to maintain the concentration of alcohol in the mixture at 50 per cent or more. The work of PooreZ2 is a noteworthy contribution to our knowledge of the factors entering into the recovery of pectin from citrus fruit waste, and especially of the possible economic use of alcohol as a precipitant. Pectin has been precipitated by the salting-out method,10~2D but it is very difficult to remove the salt which is adsorbed by the pectin precipitate. Attempts have been made to precipitate pectin by elect r o l y ~ i s ,but ~ successful results have not yet been reported. The method developed by the author and his c o - w o r k e r ~ ~ ~ is based on the colloidal nature of pectin and involves the precipitation of a negatively charged particle by the introduction of a positively charged particle. Process of Manufacture
Lemon refuse from citric acid manufacture contains about 2.5 to 4.0 per cent of pectin. Pectin in this case means alcohol precipitate minus ash determined on the total extract of the lemon pulp. The extraction is made with water containing Extraction Methods 0.5 per cent sulfurous acid and consists of ten successive Pectin is usually extracted from the raw material by means extractions of 1hour each on a steam bath in beakers covered of warm or hot acid solutions. Of the organic acids, citric, with watch glasses. The figure is arbitrary and based on a n tartaric, lactic, acetic, and malic have been specified by vari- empirical method which gives comparative results. The lemon refuse is cut or ground and heated to 95" to ous workers, and some have preferred a mixture of two or three of these acids. Acetic has generally proved unsatis- 98" C. for about 10 minutes to destroy pectinase, which confactory. Of the inorganic acids hydrochloric, phosphoric, verts pectin to pectic acid, thereby destroying its usefulness sulfurous, and sulfuric have been tried. I n the work of this for jelly-making purposes. The pulp is then washed in cold water on a revolving screen to remove fine particles and the laboratory sulfurous acid has been found satisfactory. probably albuminoid suspensions, which if not Some have conducted the extraction under p r e s ~ u r e , ~milky ~ ~ ~liquids, ~ but it is more usual to boil a short time at atmospheric pres- removed cause cloudiness in the jelly in which the pectin is sure with a dilute acid. Douglas,12 for instance, subjects used. This preliminary heating step may be eliminated and in the "fruit pulp to the action of hot or boiling water containing practice is dispensed with when the peel is heated promptly for a small proportion of any suitable acid." Huberls extracts with an alkaline medium containing 2 to extraction, since that operation properly conducted destroys 4 per cent sodium hydroxide for 15 to 30 minutes a t 40" to the pectinase. EXTRACTION-The ground, washed peel is run into wood 80" C. He does not get pure pectin, but sodium pectate, from which pectic acid is precipitated. A peptizing agent tanks having false bottoms covered with filter cloth; 500 to such as sodium or potassium citrate or tartrate is used to get 1000 kg. is the usual charge of pulp. A 1 per cent sulfurous the pectic acid in solution. This product has not been util- acid solution is added in sufficient quantity to suspend the p d p and the mixture is brought as rapidly as possible to about 90" ized commercially. Bey&' extracts impurities from citrus peel with alcohol C. The mixture is held a t this temperature for 2 hours and is and leaves a mixture of pectin and cellulose fiber which can drawn off through the false bottom, the pulp itself making
October, 1925
INDUSTRIAL AND ENGINEERING CHEMISTRY
the filter mat. A second or third extraction is made in the same manner. Lemon pulp that has been cooked is a very soft mass and must be handled with extreme care or one has a tank full of mush from which it is almost impossible to get a clear pectin liquor. Probably the mechanical handling of the cooked pulp is the greatest single problem in the recovery of pectin from lemons. The combined extracts or any one of them are filtered through paper pulp if necessary, and the pectin solution, which usually contains about 0.50 per cent pectin, is rapidly cooled and excess sulfur dioxide removed by passing it over a series of wooden baffle plates as in an ordinary water-cooling tower. The solution is run into tanks of about 7500 liters capacity, the usual charge of liquor being 5000 liters. PRECIPITATION OF PECTIN-^^ sample is tested in the laboratory and the amount of reagents required for precipitation of the pectin determined. The theory of the precipitation is this: Pectin is a colloid of the emulsoid or lyophyllic type carrying a negative charge. Ammonia and aluminium sulfate produce aluminium hydroxide, which carries a positive charge, and a mutual precipitation of aluminium hydroxide and pectin occurs. The precipitation is ascribed by some to an actual chemical combination of the aluminium hydroxide and pectin, but the writer believes the mutual colloidal precipitation theory t o be correct.
The reagents are carefully weighed and the aluminium sulfate is used in the form of a 25 per cent solution. While the pectin solution is in violent agitation the ammonia charge is dumped in practically instantaneously and is immediately followed by the aluminium sulfate, which is also added as rapidly as possible. Rest results are obtained when the quantities of ammonia and aluminium sulfate are such that after the mixture is complete the solution in which the precipitate is suspended has a minimum viscosity, which is usually a t p H 4.0 to 4.2 determined colorimetrically using bromophenol blue as indicator. SEPARATION OF PRECIPITATED PECTIN-The tank in which the precipitation is carried out is provided with a powerful wooden agitator and a t several points around the sides of the tank there are heavy wood baffles. This construction makes it possible to agitate the mixture violently during precipitation and this whips air into the precipitate causing it to rise to the top immediately in a thick spongy mass. The mother liquor is withdrawn through a screen and excess aluminium sulfate is washed from the precipitate by a short agitation in cold water. It is finally drawn off through a screen and the excess solution removed by means of a basket centrifugal or a hydraulic press. The pressed pectin precipitate is dried in a hot-air drier a t about 65" C. The shorter the drying period the better the quality of the pectin. The dried pectin is a dark green, horny mass, which is ground in a mill of the swing hammer type and put through a 100-mesh screen. The preparation a t this stage may be used for making lowgrade jellies, but it usually contains about 10 to 12 per cent of ash which is mostly aluminium hydroxide and contains some calcium and other mineral salts that carry through from the original extract. By very careful preparation of the extract with some sacrifice in yield it is possible to produce green pectin of good grade with but 6 or 7 per cent of ash. Aluminium hydroxide is insoluble in alcohol, whereas aluminium chloride is soluble. The aluminium hydroxide existing as an impurity in the dry crude pectin is converted into aluminium chloride by suspending the ground dry pectin in 85 per cent alcohol containing about 10 per cent by volume of concentrated hydrochloric acid. The exact quantity of
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hydrochloric is calculated from a determination of the percentage of ash in the pectin to be washed. An excess of hydrochloric acid must be used, and this is removed by washing with neutral alcohol in which both hydrochloric acid and aluminium chloride are soluble. The alcohol is drawn off through a cloth filter and recovered by distillation. The washed pectin is dried a t a moderate temperature in a vacuum oven from which the alcohol is recovered and is then put through a screen for uniform sizing and packed for market. The product resulting from this treatment is a grayish powder, which in jelly is neutral in color and flavor. It will carry from 140 to 220 times its own weight of sugar, depending on the condition of the fruit and the details of the various process steps. A standardized product is readily obtained by blending. Present Commercial Products Three types of pectin preparations are now on the market. Liquid concentrated pectin has become fairly familiar, as it is much used by the housewife as well as the jelly- and jammaker. It contains usually about 4 to 4.5 per cent of pectin. There are dry mixtures of pectin and sugar, put up both for the housewife and the jelly-maker, containing varying amounts of pectin. There is no uniformity as yet in the jelly strength of these mixtures, but a powder that will jell about forty times its weight of sugar is a very practical one. Pure pectin is marketed in large quantities to jelly- and jam-makers or to producers of the pectin-sugar mixtures, and eventually will be available for use by the housewife in its purest and most economical and practical form. Acknowledgment The writer wishes to express his grateful acknowledgment for the assistance of Eloise Jameson, F. N. Taylor, H. P. Zeh, and W. E. Baier, of the Exchange Research Laboratory, who have done the experimental work on the development of the pectin process, and to H. M. May, manager, H. H. House, chief chemist, and C. C. Taylor, pectin chemist of the Exchange Lemon Products Company which is engaged in the commercial development of citrus pectin by this process. Bibliography 1-Beylik, U. S. Patent 1,393,660 (October 11, 1921). 2-Braconnot, A n n . chim. phys., [ I I ] 28, 173 (1825). 3-Brown, A m . Vinegar Ind., 2, 14 (1923). 4-Caldwell, Wash. E x p t . Sta., Bull. 147 (1917). b c h e r n o f f , A m . Food J., 18, 200 (1923). 6-Crawford, U. S. Patent 1,507,338 (September 2, 1924). 7-Cruess and McNair, J. Ind. Eng. Chem., 8, 417 (1916). 8-Cruess and La1 Singh, Calif.Agr. Ezpl. Sta., Circ. 243 (1922). Q-Denotn, Johnstin, and Yeatman, U . 5'. Dcpt. Agr., Circ. 264 (1923). 10-Doell and Maes, U.S. Patent 1,385,525 (July 16, 1921). 11-Dore, Ind. Eng. Chem., 16, 1042 (1924). 12-Douglas, U. S. Patent 1,235,666 (August 7, 1917). 13-Douglas, U.S. Patent 1,082,682 (December 30, 1913). 14--Ehrlich, Chem. Z t g . , 41, 197 (1917). I+Fellenberg, Biochem. 2.. 86, Nos. 1-2, 118 (1918). 16--Goldthwaite, Untw. Ill. Bull. 11, No. 31, 5th ed. (1917). 17--Hardy, Biochem. J., 18, 283 (1924). 18-Huber, U. S. Patent 1,410,920 (March 28, 1922). 19-Jameson, Taylor, and Wilson, U. S. Patent 1,497,884 (June 17, 1924). 20-Magoon and Caldwell, Science N. S. 47, 592 (1918) 21--McKee, U. S. Patent 1,380,572 (June 7, 1921). U . S. Dept. Agr. Bull. 1323 (1925) 22-Poore, 23--Sucharipa, J. A m . Chem. Soc., 46, 145 (1924). 24-Sucharipa, J. Assoc. Oficial Agr. Chem., 7, 57 (1923). ZS-Tarr, Del. A g r . Expt. Sta., Bul/. 134 (1823). ZB-Tarr, Ibid., 136 (1924). 27-Tschirsch, Dissertation, Berne, 1908. 28-Tutin, Biochem. J., 16, 494 (1921). 29-Tutin, Ibid., 17, 83 (1923). 3&Tutin, Ibid., 17, 510 (1923). al-Dore, personal communication.
