Oxidation of Raw Starch Granules by Electrolysis in Alkaline Sodium Chloride Solution J
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F. F. FARLEY’ AND R. M. HIXON Iowa Agricultural Experiment Station, Ames, Iowa
dium hypochlorite. The process is used to study the c h b g e s of raw starch during oxidation and to determine the effect of various factors upon the course of oxidation, The changes during oxidation were followed b y determinations of hot viscosity, rigidity, gel strength, reducing power, turbidity, volume of swollen granules, quantitative birefringence, digestibility by @-amylase,and microscopic observation of swollen granules by photomicrographs. The course of oxidation was markedly affected by higher temperatures and increased alkalinity. A n explanation of the action of alkaline hypochlorite on raw starch granules is presented
Two phases of the oxidation of starch b y halogens are investigated. One is a study of the mechanism of the oxidation of starch paste b y bromine in neutral solution. Confirmatory evidence for the four expected types of oxidation ( 6 ) has been securednamely, for t h e oxidation of primary and secondary alcoholic groups, for the oxidative splitting of hexose units at a glycol grouping, and for the oxidation of aldehydic and ketonic groups to nonuronic acid units. Information regarding the chemical mechanism of the bromine oxidation allows a better interpretation of the second phase of the investigation. The second phase presents an electrolytic process for oxidizing raw starch granules by alkaline so-
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2. The oxidation of secondary alcoholic grou s to ketone groups, indicated previously by the reduction of Fehing solution (e), has been confirmed as far as the preparation of an oxime of the oxidized starch. The nitrogen content of the oxime was equivalent to one ketone group in 65 to 75 per cent of the CoHloO5 units when four equivalents of bromine per C ~ H I Ouuit O ~ were used to produce the oxidized starch. 3. Oxidative production of nonuronic carboxyl groups was indicated in earlier studies by the calcium content of the oxidized starch in excess of that calculated t o neutralize the uronic acids present (6). Upon hydrolysis and fractionation barium salts of nonuronic dibasic acids of less than six carbon atoms were obtained. 4. Indicative evidence for the splitting of hexose units at a glycol grouping was the oxidative decomposition of ketonic reducing groups in the later stages of oxidation (6). The instability of the oxidized starch is explicable by the resence of monoor diketo acid units. Such keto acid units coufd be further oxidized t o dibasic nonuronic acid units. The expected degradation of keto acid units to nonuronic acid units was likewise confirmed by the separation of barium salts of dibasic acids of less than six carbon atoms.
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UBLICATIONS regarding the oxidation of starch number well over two hundred. Nevertheless, the mechanism of the oxidation has been studied in a relatively small number of papers. Most oxidative treatments have been too mild or too extensive to throw light on the intermediate stages. The mechanism of one type of oxidation, periodic acid oxidation, has been successfully clarified due to its specificity (4, IO, 11). I n most of the other oxidations of starch the reactions are so involved and the oxidized products so difficult to isolate that little progress has been made in theoretical interpretations. A previous analytical study of the bromine oxidation of starch paste in neutral solution indicated four types of oxidation of the starch molecule ( 6 ) . Confirmation of these types was sought through hydrolysis and isolation of the oxidized glucose units. Extensive decomposition of the oxidized starch resulted when the usual hydrolytic methods and aqueous methylation procedures were used. I n order to stabilize the oxidized starch prior to hydrolysis, methylation was carried out by an anhydrous procedure using methyl iodide and silver oxide on the silver salt of the oxidized starch. Simultaneous methylation of the hydroxyl groups and esterification of the carboxyl groups resulted. The methylated product was hydrolyzed and fractionated. Analysis of the fractions contributed to the confirmatory evidence for the four expected types of oxidation: 1. The oxidation of primary alcoholic groups to uronic acid units was shown previously to produce as much as 50.7 per cent glucuronic acid anhydride equivalent when measured by evolution of carbon dioxide and somewhat less when measured by furfural yield (6). The presence of glucuronic acid units has been confirmed by hydrolysis of the oxidized starch and isolation of glucuronic acid as the cinchonine salt. This is the first isolation of glucuronic acid from a starch product. 1
When the chemical mechanism of the bromine oxidation of starch paste is considered in contrast to the small reducing power and uronic acid content of “chlorinated” starches, an explanation may be advanced for the somewhat localized attack of oxidizing agents on raw starch granules. Some of the physical and chemical aspects of the oxidation of raw starch granules have been investigated by means of electrolytic hypochlorite and the remainder of this paper is devoted to that investigation. In 1904 Leconte ( I d ) used electrolysis as one step in the preparation of a very white grade of rice starch. An alkaline suspension of the starch was electrolyzed between aluminum, zinc, or other metallic electrodes. Harvey (8) and White (14) patented a process in 1921-23 which was designed to convert starch to a glue. An aqueous suspension of the starch was made conducting by the addition of hydrochloric or sulfuric
Present address, University of Detroit, Detroit, Mioli.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
6x1 0
400
PER CENT CHLORINE 4 6 8
2 I
1
0
I 0
01 .
1
1
1
80
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I
HOT VISCOSITY (90°C>
I
I
I
0.2
0.3
0.4
0.5
EXTENT OF OXIDATION (EQUIVS. CHLORINE PER GLUCOSE UNIT)
FIGURE 1. CHANGE OF VISCOSITYAND VOLUMEOF S ~ O L LEN GRANULES DURING OXIDATION
acid, a salt, or an alkali. Iron or carbon electrodes were used with a direct current of 3.5 amperes a t 110 volts. The electrolysis was stopped before the starch reached the "soluble" stage. Fink and Summers (7) attempted to oxidize starch electrolytically between graphite electrodes in 2 per cent potassium bromide solution a t 40" C., kept neutral by means of calcium carbonate. They reported no reaction, stating that electrolytic oxidation in the bromide bath seemed specific for aldoses. The electrolytic oxidation process described in this paper was used t o oxidize raw starch granules in 2 per cent sodium chloride solution containing 0.4 per cent sodium hydroxide. The amount of sodium hypochlorite generated was varied from 0.025 t o 0.5 equivalent per C6H100~unit (0.5 to 11 per cent) of active chlorine.
Identification of Glucuronic Acid A sample of oxidized starch prepared from starch paste by
Vol. 34, No. 6
(5.02 per cent calcium) prepared by using four equivalents of bromine per CsHloOa unit (6) was dissolved in water, converted to the silver salt by the addition of the calculated amount of silver nitrate solution, and precipitated with two volumes of ethyl alcohol. After washing with 67 per cent alcohol and then with 95 per cent alcohol and drying in vacuo, 96 per cent of the carbohydrate was recovered. Silver found, 20.4 per cent; calculated from calcium content, 22.2 per cent. The dry silver salt was methylated and sirnultaneously esterified by using methyl iodide and silver oxide according to the customary methylation procedure with anhydrous methyl alcohol as diluent. The methylated material was removed from the silver residues with hot methyl alcohol. The same methylation procedure was applied four successive times to the silver residues from the first methylation. The final silver residue was extracted overnight with anhydrous methyl alcohol in a Soxhlet extractor. The products from this extract and from the five methylations were combined for further methylation. Two more methylations were carried out in the absence of methyl alcohol. HYDROLYSIS AND FRACTIONATION. A methylated product of 32.7 per cent methoxyl content was hydrolyzed in anhydrous methyl alcohol containing 2 per cent dry hydrogen chloride. The resulting carbohydrate acids were converted to the barium salts and fractionated by means of water, methyl alcohol, and chloroform. The analyses of the fractions were as follows: Fraction B (Two Preparations). Found: Ba 25.1 and 25.2 per cent, Me0 17.9 and 18.0 er cent; Ba 24.0 per cent Me0 22.6 and 22.3 per cent. Calcugted for barium dimethyl giucuronate. Ba 25.0 per cent, Me0 22.6 per cent. Calculated for barium dimethyl ketoglucuronate: Ba 23.9 per cent, Me0 21.5 per cent. The analyses indicate a mixture of methylated monobasic acids of six carbon atoms. Fraction E. Found: Ba 45.0 per cent, Me0 9.06 and 8.58per cent. Calculated for barium monomethyl glutarate: Ba 41.6 per cent, Me0 9.4 per cent. Calculated for barium monomethyl tartrate: Ba 45.8 per cent, Me0 10.36 per cent. The analyses of this fraction indicate a mixture of dibasic acids of fewer than six carbon atoms. Fraction G. Found: Ba 39.3 and 39.3 per cent, Me0 5.95 and 5.27 per cent. These analyses also indicate a mixture of dibasic acids.
the action of four equivalents of bromine per C6H100sunit (6) was hydrolyzed in 1N sulfuric acid, the sulfuric acid was reElectrolytic Oxidation Process for Raw Starch moved quantitatively with barium hydroxide, and the soluGranules tion of carbohydrate acids was concentrated in vacuo a t 45" C. Electrolyses were carried out in a crock or beaker of 4 liters to a small volume. A portion of the solution was heated for capacity, into which was lowered the electrode assembly and one hour on the water bath with excess cinchonine. The alkaline solution was cooled and filtered, the excess alkaloid removed by extraction with chloroform, and the aqueous layer evaporated O F VARIOUS FACTORS ON THE PROnUCTS O F ELECTROLYTIC TABLE I. EFFECT almost to dryness in vacuo. The cinchonine OXIDATION OF CORKSTARCH derivative crystallized and had a melting range Experiment No. 1 2 3s 40 50 6b 7C Sd of 172-175' C. After five or six recrystallizations from water, the cinchonine salt melted a t 0.25 0.25 0.1 0.1 0.1 0.1 0.1 0.1 5.5 2.2 5.5 2.2 2.2 2.2 2.2 2.2 199-200" C. sharply. A mixed melting point 1220 3000 3000 3000 3000 3000 3000 3000 15 25 0" 0" 9 9 5.5 On nith the cinchonine salt of glucuronic acid 42 53 450 55a 50 a 35 37 34.5 prepared from gum arabic was undepressed. 96.2 96.9 97.8 92.1 80.3 94 s 07.6 94.6 Relative' ' v h o s i t y , sec. (The reported melting point varies from 198' of flow 186 24 146 116 131 133 150 126 Rigidity, dynes/aq. om. 38 0 . .. . . .. . . , , . 145 145 145 to 204' C.) The cinchonine derivative of glu7.0 13.3 3.0 7.7 7.7 0.3 6.8 6.8 Reducing power, Rcu curonic acid from oxidized starch gave a positive Current d e n s i t y e , amp./ sq. om. 0.141 0.153 0.167 0.158 0.174 0.137 0 755c 0 074 naphthoresorcinol test for uronic acid. The 5.3 5.7 6.3 5.9 6.5 5.1 2.4 2.8 Current' amp. &Tin. t o (generate hypoextremely low yield of the glucuronic acid de163 186 172 72 76 69 88 190 chlorite rivative attested t o the suspected decomposition Hr. t o deplete hypochlorite after current was of the uronic acids during hydrolysis. Several 3
