Lead Ursolate in Relation to Fruit Cleaning R. F. COHEEAKD J. L. ST.JOHN Division of Chemistry, Agricultural Experiment Station, State College of Washington, Pullman, Wash.
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282.7" C. in c o m p a r i s o n with ANY washing solutions Lead ursolate was prepared f r o m ursolic acid 284' to 285" found by Sando. have b e e n i n v e s t i isolated f r o m apple wax. Analysis of this salt The lead salt of this compound gated to d e t e r m i n e indicated tetracalent lead. Data on the solubility was prepared by d i s s o l v i n g 6 their e f f e c t i v e n e s s in the reof this compound are presented. i7sing lead g r a m s of the purified ursolic moval of lead and arsenic spray ursolate or similar lead compounds, a n additional acid in 450 cc. of 95 per cent residues from fruit. The search alcohol and adding 30 cc. of 20 for more e f f e c t i v e c l e a n i n g method of approach to fruit cleaning problems per cent lead acetate. The salt agents, particularly in laborais available. Based on the existence of the lead was precipitated by adding 500 tory work, has frequently been salt of ursolic acid, it is suggested ihat the intercc. of warm water. The product based on a study of the relative ference of wax in fruit cleaning may be due in was filtered a n d washed solubility of lead arsenate in the part at leust to the formation of such a compound thoroughly with water to remove reagents investigated. It has excess lead a c e t a t e . I t was been generally accepted that the on fruit sprayed with lead arsenate. Ecidence further purified by dissolving in development of wax on apples is of the presence of an organic lead compound on alcohol, diluting with water, fila factor which may seriously insprayed fruit is presented. On this basis a tering, washing, and drying a t t e r f e r e w i t h t h e r e m o v a l of modiJication of f r u i t cleaning methods is postu105' C. The melting point of arsenic. The theory is that the lated. the resultant c o m p o u n d was wax forms a coating around the 199" to 200" c. lead arsenate particles which preThe percentage of lead in this compound was determined by vents the action of the washing solvents. The same theory has also been advanced to explain the interference of fish and wet-ashing with sulfuric and nitric acids, and finally weighing mineral oils, more particularly where hydrochloric acid is used the lead as the sulfate. Vsing triplicate one-gram samples as a wabhing solution. Since the recent establishment of a lead of the lead salt, the amount of lead found mas 0.1054, 0.1047, tolerance for food, the investigational work on fruit cleaning and 0.1052 gram, giving an average of 10.51 per cent of lead has pointed toward even greater interference of the wax in the in the compound. Based on Sando's formula this would indiremoval of lead than in the removal of arsenic. Under the cate a lead salt of ursolic acid which contains 4 moles of the cultural, spray, and storage conditions existing, there is possi- ursolic acid to 1 atom of lead, indicating tetravalent lead if bility of the formation of definite combinations or compounds the ursolic acid is monobasic. The theoretical percentage of between lead arsenate and the constituents of the wax. If such lead in such a lead salt is 10.21. To obtain evidence of the possible existence of lead ursocompounds are formed, a proof of their existence and a study of their properties should be of value in the further develop- late in apple wax from sprayed fruit, a n ether extract of apple pomace was filtered, the ether evaporated, and the residue ment of methods of removing lead residue from fruit. Sando (8,9)and Markley and Sando (2-5) have investi- wet-ashed, and a qualitative test showed the presence of gated the composition and properties of apple wax, and the lead. Since lead ursolate is soluble in ether, this result sugchanges in the various wax constituents that take place during gests that lead ursolate or some other ether-soluble lead salt fruit development and storage. From their work (4) it ap- is present on the fruit. pears that the outer, protective coating of the apple is comSOLCBILITY posed of about 56 per cent of cutin which is insoluble in ethyl ether and 44 per cent of an ether-soluble fraction. In maUnder the conditions outlined by Kamm ( I ) , ursolic acid ture fruit this latter fraction contains approximately 60 per is insoluble in water and petroleum ether, partially soluble in cent of ursolic acid and 40 per cent of an oily material con- dilute acetic, hydrochloric, and nitric acid, and soluble in taining n-nonacosane and IO-nonacosanol ( 2 ) . The ursolic concentrated acetic and sulfuric acids. It is also found to acid would, therefore, comprise approximately 26 per cent be slightly soluble in 10 per cent sodium silicate of the grade of the total protective coating of the apple. The formula used in the commercial fruit cleaning in this state. Measureof ursolic acid proposed by Sando (9) is that of a monobasic ments made in this laboratory show that 0.2 gram of ursolic hydroxy acid, CaH,OHCOOH. Since this is one of the acid is soluble in 100 cc. of ether a t 25" C. while 0.62 gram is principal constituents of the protective coating of the apple, soluble in 100 cc. of 70 per cent alcohol. It is also soluble an attempt was made to determine whether or not it would in a 2 per cent alcoholic solution of sodium hydroxide, and in form a lead salt. toluene, benzene, xylene, and chloroform. A mixture of equal amounts of ursolic acid and n-nonacosane mas soluble in a PREPARATIOX OF LEADURSOLATE mixture of acetic acid and gasoline. In preparing the ursolic acid, the method of Sando (8, 9) The solubility of apple wax was further studied by extractwas used. The source of the wax and acid was dried apple ing 50-gram samples of dried apple pomace for 2 minutes with pomace. The pomace was extracted with petroleum ether a number of organic solvents a t 25" C. These were filtered, to remove nonacosane and nonacosanol, and then with ethyl the solvent was evaporated, and the residue weighed. Chloether in order to remove the crude ursolic acid. After re- roform and acetone extracted over 0.5 gram per 100 cc. of the moval of the ether by evaporation, this crude product was solvent, ether 0.44, petroleum ether 0.19, ether following pecarefully extracted in a Soxhlet apparatus in order to remove troleum ether 0.24, and carbon tetrachloride 0.29. The wax impurities. It was further purified, according to Sando, is also soluble in benzene, toluene, and xylene. by preparation of the sodium salt and subsequent regeneration While ursolic acid is slightly soluble in 5 per cent hydroof the ursolic acid. The melting point of this product was chloric acid, the lead ursolate is insoluble in the 1.5 per cent 781
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INDUSTRIAL AND ENGINEERING CHEMISTRY
concentration of hydrochloric ordinarily used in commercial fruit cleaning. This was demonstrated by shaking the lead ursolate with the solvent, filtering, and adding ammonium sulfide to the filtrate. However, on adding 10 per cent of ether to the hydrochloric solution, lead is brought into solution, but the same result is accomplished with a mixture of 90 per cent water and 10 per cent ether. On the other hand, the lead ursolate is not soluble in water to which one per cent of ether, toluene, or benzene has been added. A series of experiments similar to the preceding was made with 1.5 per cent hydrochloric acid and also with a 10 per cent sodium silicate solution. These are the concentrations which have been frequently used in commercial fruit cleaning (6). About 0.01 gram of the lead ursolate was shaken for 1 minute with one of the above solutions to which had been added 1 per cent of the organic solvent investigated. After shaking and filtering, the presence or absence of lead in the filtrate was demonstrated by the addition of ammonium or sodium sulfide. Shaking the lead salt with 1.5 per cent hydrochloric acid, no lead was found in the filtrate after adding 1 per cent of kerosene, mineral oil, carbon tetrachloride, toluene, xylene, monochloroacetic and trichloroacetic acid, or alcohol up to 25 per cent. The use of 35 to 45 per cent alcohol materially increased the solubility of the lead under these conditions. The addition of 5 per cent of acetone, benzene, and butyric acid increased the lead solubility slightly. The same amount of chloroform and ethyl acetate markedly increased the solubility. Ten per cent sodium silicate containing also 10 per cent of alcohol was a good lead solvent, but with 5 per cent alcohol added, no lead was dissolved. Only a small amount of lead was dissolved by the silicate solution alone, but the addition of a small amount of soap apparently somewhat increased the solubility of the lead. The addition of kerosene to the silicate and soap solution had little effect. The addition of 1 per cent of benzene, toluene, or xylene to either 10 per cent sodium silicate solution or to this solution containing also a small amount of added soap greatly increased the solubility of lead from lead ursolate. This comparatively small amount of the organic solvent appeared to be a t least as effective as larger amounts. At 38" C. the relationship between the lead solubility in sodium silicate solution and in this solution containing added soap and also kerosene appeared nearly the same as a t room temperature. However, a t this higher temperature the addition of organic solvents such as benzene and toluene both with and without added soap produced a larger increase in lead solubility. Chloroform also seemed to be very effective. Robinson (7) found that alcohol and other organic solvents when added to dilute hydrochloric solutions were not satisfactory in practice for the removal of the arsenic residue. Kerosene in this connection did not promote a consistent reduction of the arsenic below the tolerance.
RELATION TO CLEANIKG The results presented above suggest that the theories which have been developed to explain the interference of apple wax in fruit cleaning processes are only a partial explanation. The lead salt of ursolic acid is insoluble in hydrochloric acid of the concentration used in fruit cleaning but is slightly soluble in 10 per cent sodium silicate solution. Added soap and kerosene very slightly increase this solubility. If lead ursolate or other combinations of lead with wax constituents exist on the fruit, as suggested above, difficulty in removing the last traces of lead by the fruit cleaning methods now in use might be expected. Therefore, the addition of small amounts of certain organic solvents to these solutions could be of value in removing the last traces of lead residue from the fruit if the addition of these solvents increased the soh-
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bility of wax constituents, lead ursolate, or other lead compounds. Results in this laboratory have shown that the solubility of lead ursolate in 10 per cent sodium silicate is increased by the addition of 1 per cent of chloroform, benzene, toluene, or xylene. Other solvents may produce similar results. The effectiveness of such combinations in commercial fruit cleaning should be determined. While chloroform may be the most efficient in increasing lead solubility under these conditions and also offers little fire hazard, i t is more volatile and thus subject to more rapid loss under practical conditions than the others which showed promise. Of the other three mentioned, xylene probably offers the least fire hazard. I n selecting an organic solvent for practical fruit cleaning, other factors such as objectionable fumes must also be considered. Apple wax itself is also soluble in these organic reagents so that they may help to remove any lead arsenate which may be mechanically coated with wax. Care may be necessary, however, in the use of such solvents in order to avoid the removal of enough of the waxy coating to allow more than normal wilting during storage after washing. Fish oil and mineral oil which are used in orchard sprays and may in some cases interfere with residue removal are also soluble in these solvents. Based on the hypothesis of Markley and Sando (6) the presence of mineral oil on the fruit may afford a better opportunity for the formation of lead salts of wax constituents. The results reported may help to explain the lack of relation noted by different investigators between the solubility of lead arsenate in different washing solutions and their relative effectiveness in actual fruit cleaning. The fact that in many instances the ratio of lead to arsenic on washed fruit is well above this ratio in lead arsenate may also be explained by postulating the formation of definite lead compounds with constituents of the wax. Also the information presented helps to explain the fact that hydrochloric acid solutions are less effective in cleaning waxy fruit than are sodium silicate solutions. Further light is also thrown on the reason for the increased effectiveness of sodium silicate to which small amounts of soap have been added. Such solutions cause increased solubility of lead ursolate, and in cleaning operations both this and an increased wetting power may be factors in the increased efficiency. The use of certain organic solvents with sodium silicate may also permit the use of lower temperatures or smaller concentrations of sodium silicate during fruit washing with a resultant decrease in fruit injury.
LITERATURE CITED (1) Kamm, "Qualitative Organic Analysis," John Wiley & Sons, N. Y.,1932. (2) Markley, Hendricks, and Sando, J. Biol. Chern., 98,103 (1932). (3) Markley and Sando, J. Agr. Research, 42, 705 (1931). (4) Ibid., 46, 403 (1933). (5) Markley and Sando, Plant Physiol., 8,475 (1933). (6) Overlev. St. John, Overholser, and Groves, State Coll. Wash. Agr.-Expt. Sta., Bull. 286 (1933). (7) Robinson, J. Econ. Entomol., 24, 119 (1931). (8) Sando, J . Biol. Chem., 56,457 (1923). (9) Ibid., 90,477 (1931).
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R E C ~ I V EApril D 2, 1934. Published as Scientific Paper 290, College of Agriculture and Experiment Station, State College of Washington.
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