616
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 13, No. 9
TABLE 111. FRACITONATION OF METHYL ESTERS OF UNSATURATED was removed. Hydrogenation in acetic acid with palladiumFATTY ACIDS barium sulfate catalyst produced an acid with a melting point Boiling Range Saponification Iodine of 61” C.; that recorded for palmitic acid is 62.9” C. (4). Fraction Weight (5 Mm.) Equivalent NO. The neutral equivalent found was 258.3, while that calcuGrams c. 1 2.2 120-123 lated for palmitic acid is 256.3. 248.5 24.8 2 3 4 5
1.8 3.6 10.7 3.2 6 6.9 7 6.6 8 7.2 9 4.5 10 6.2 11 3.7 12 5.5 13 4.4 Residue” 9.3 Dark brown in color.
123-135 135-140 140-147 147-155 155-160 160-165 165-170 170-175 175-180 180-185 185-190 190-195
260.0 254.7 272.0 280.0 291.0 292.0 298.0 308.0 310.0 325.0 311.5 352.0
.....
...
83.6 103.7 123.1 125.4 127.3 153.1 166.5 183.2 212.2 226.2 208.1 232.7
...
To confirm the composition as calculated from the sauonification equivalents, the Saponification product from fraction 4 was acidified, extracted with ether, and dried, and the ether
Literature Cited (1) Armstrong, E. F., and Allan, J., J . SOC.Chem. I d . , 43, 207T (1924). (2) Brown, J. B., and B e d , G. D., J . Am. Chem. SOC., 45, 1289 (1923). (3) Chargoff, E., 2. physiot. Chem., 199,221 (1931). (4) Francis, F., and Piper, S. H., J . Am. Chem. SOC.,61,577 (1939). IF\ Richardson, A. S., Knuth, C. A., and Milligan, C. H., IND. ENG. CHEM.,17,80 (1925). Shriner, R. L., Fulton, J. M., and Burks, D. J., J . Am. Chem. SOC., 55,1494 (1933). IND.ENG.CHEM.,32, 1217 (1940). (7) Stingley, D. 18) . , Twitchell, E., Ibid., 9,581 (1917).
v.,
PRESENTED before t h e Division of Agricultural and Food Chemistry a t the i o i s t Meeting of t h e American Chemical Society,
st. Louis, MO.
Reducing Power of Starches and Dextrins F. F. FARLEY
AND
R. &I. HIXON, Iowa Agricultural Experiment Station, Ames, Iowa
A rapid ferricyanide method for determining the reducing power of starches and dextrins is presented in which the reduced iron is measured directly by a ceric sulfate titration. For starches hydrolyzed by hot or cold acid or oxidized by alkaline hypochlorite, and for raw starches and dextrins, the reducing power values by this method parallel those determined by the longer procedure of Richardson, Higginbotham, and Farrow. Many other modified starches such as the “chlorinated” and “thin-boiling” types have been measured.
T
H E copper number has been used extensively for reporting the reducing power of cellulosic materials and recently by Richardson, Higginbotham, and Farrow (7) for starch This paper describes a much simpler method for determining the reducing power or copper number of starches and dextrins, which shortens the time required to less than 25 minutes. While measuring the reducing power of a series of starches by the Gore and Steele (2) modification of the Hagedorn and Jensen (3) method, the authors noticed that the apparent maltose equivalent of the more soluble products when converted to milligrams of copper gave values equal to the copper numbers determined by the Richardson, Higginbotham, and Farrow method. For raw starch and very slightly solubilized starch, high values were obtained by the Gore and Steele method. These high values were attributed to the visible entrapment in the starch of iodine which was very difficultly released for measurement in the thiosulfate titration. I n the determination of the maltose equivalent of starchmaltose mixtures Martin and Newton (6) avoided the diffi-
culty due to iodine entrapment by using Hassid’s (4) ceric sulfate titration. It seemed probable that the same ceric sulfate titration could be used to advantage here to measure the reducing power: not of a starch-maltose mixture, but of starch or dextrin alone. This proved to be true. Consideration of the above statements reveals that any reducing value obtained for maltose in the presence of dextrins and modified starches is only an apparent maltose value. TABLE I. REDUCISC POWER OF STARCHES ASD DEXTRIXS R c u , JIg./Gram
Sample Pearl starch (control) Other commercial cornstarches Waxy maize starch Thin-boiling starch, 40-fluidity Thin-boiling starch, 90-fluidity Chlorinated starch, 2.5% chlorine Chlorinated starch, 5% chlorine Electrolytically oxidized starch Alkali dextrin, A Alkali dextrin, B Alkali dextrin, C Acid dextrin, A Acid dextrin, B Acid dextrin, C Gore ( 1 ) starch 5-hour conversion Gore (1) starch: 42-hour conversion Gore (1) starch, 96-hour conversion LMaltose Glucose
6 I
8-7.9 . 7 , 9 . 4 , 10.1, 11.2. 11.6
12.0 25.5 39.0 19.6 61.6 120.0 1900 2800
The potentiometric measurement used by Martin and Newton was eliminated because the color change of the solution from green to yellow just before the large voltage change was a satisfactory indication of the end point. However, starch products with very little reducing power produced only a slight green color and made accurate determination of the end point difficult. The addition of a measured amount of glucose solution to each starch sample obviated this dificulty. A correction for the reducing power of this added glucose was made by a blank determination. When, in the hydrolysis of starch by acid, the RcU values (milligrams of copper per gram of starch) were plotted against the time, there resulted a straight line for RcU values up to
ANALYTICAL EDITION
September 15, 1941
617
1000. This is shown in Figure 1 where the simplified RcU determination is compared with the longer method of Richardson, Higginbotham, and Farrow. It is evident t h a t the two methods give the same results in the starch and dextrin range but not in the range of the sugars. Figure 2 presents the change of rotatory power, of alkali number (8), and of iodine titration (6) as the same sample of starch was hydrolyzed. The alkali number became constant before the starch was one third converted to glucose. This shorter method for copper numbers uses iron salts instead of copper salts, and the results, therefore, might be reported as R F ~units. It seems better not to introduce another such unit, but to report the results as Rcu units to allow comparisons with copper numbers. When the maltose, glucose, or iron equivalent is desired the following conversion factors are used: Per cent maltose equivalent = RcU X 100 , 1900 Per cent glucose equivalent = Rcu
x
100
2800
55.84
Rr. (mg. of Fe per gram of starch) = Rcu X 63.57
The utility of the method in the relative characterization of starches and dextrins is shown by Table I. The reducing power increases with increasing conversion of starch products except for the electrolytically oxidized starches, which, because of the method of oxidation and washing, have values equal to or lower than those of raw starches. This simplified determination of reducing power has been used t o follow the acid and alkaline conversion of starch-e. g., on the "thinboiling'' starches, on the Gore starches ( I ) , and on the dextrins formed by acid or alkaline catalyst (Table I).
Reagents ALKALINEFERRICYANIDE : 32.9 grams of potassium ferricya-
nide and 50 grams of anhydrous sodium carbonate dissolved in
S 3000 U
L
2500
5w W
b!
3
a
1500
63.57
-
- Rcu
Literature Cited
L1
5
1000
2E 3
oi"
x
For starch products which have an Reu greater than 100 a smaller sample should be used.
0
3
Procedure
M1. of ceric sulfate X normality of ceric sulfate grams in sample
E zoo0
"
water and diluted to 1liter. The solution is approximately 0.1 N with respect to potassium ferricyanide. GLUCOSE SOLUTION:a 0.2 per cent glucose solution containing a small amount of phenol. SULFURIC ACID SOLUTION ( 5 N ) : 139 ml. of concentrated sulfuric acid made up to 1 liter of solution. CERICSULFATE, 0.05 N : 26.5 rams of reagent ceric sulfate (G. Frederick Smith Chemical Cos added to about 900 ml. of a solution containing 100 ml. of concentrated sulfuric acid. The mixture is digested until the solid has dissolved, cooled, and made up to 1 liter. The ceric sulfate solution is standardized against a standard ferrous ammonium sulfate solution potentiometrically or in the presence of o-phenanthroline indicator.
Five hundred milligrams of the starch sample are weighed into a 300-ml. Erlenmeyer flask, and 25 ml. of alkaline ferricyanide reagent and 5 ml. of 0.2 per cent glucose solution are added. The flask is placed in a boiling water bath for exactly 15 minutes and is rotated while the starch is gelatinizing in order to give a uniform mixture. At the end of 15 minutes the flask is cooled under the tap to room temperature, 25 ml. of sulfuric acid solution are added, and the resulting green solution is titrated dropwise with standard ceric sulfate. The color change is from green to yellow. A blank determination is run on 5 ml. of the lucose solution. The equivalents of ceric sulfate are converted 8irectly to milligrams of copper and reported as RcU units (mg. of copper per gram of starch).
Di
LL
FIGURE 2. CHANGE OF SPECIFIC ROTATION, ALKALINUMBER, AND IODINE TITRATION DURING HYDROLYSIS OF STARCH
500
0
0
50
100 150 MINUTES I N O ~ NH,SO,
200 250 300 AT 9 4 . ~ 0 5 ~ ~ .
FIGURE 1. COMPARATIVE DETERMINATIONS OF REDVCING POWER DURING HYDROLYSIS OF STARCH
(1) Gore, IND.ENG.CHEM.,20, 865 (1928). (2) Gore and Steele, IND.ENG.CHZIM.,ANAL.ED., 7, 324 (1935). (3) Hagedorn and Jensen, Biochem. Z.,135,46 (1923). (4) Hassid, IND. ENG.CHEM.,ANAL.ED., 8, 138 (1936) : 9, 228 (1937) ; 12, 142 (1940). (5) Kline and Acree, Ibid., 2, 413 (1930). (6) Martin and Newton, Cereal Chem., 15, 456 (1938). (7) Richardson, Higginbotham, and Farrow, J . Textile Inst., 27, T131 (1936). (8) Schoch and Jensen, IXD.ENG.CHEW,ANAL.ED., 12, 531 (1940). JOURNAL Paper J-843 of the Iowa Agricultural Experiment Station, Ames, Iowa. Project 516. Supported in part by a grant from the Corn I n d u s t r i a Research Foundation.
