SEPTEAIBER 15, 1940
ANAL'k-TICAL EDITION
(S) Holtzer, H., 2. anal. Chem., 95, 392 (1933). (9) Ishibashi, M., and Kishi, H., Bull. Chem. SOC.Japan, 10, 362 (1935). (IO) Ishibashi, M.,and Kishi, H., J . Chem. SOC.J a p a n , 55, 1060 (1934): Brit. Chem. A b s . , A720 (1935). (11) Meulen, H. ter, and Hesslinga. J., "New Methoden der organisch-chemischen Analyse", Leipzig. Akadeniische 1-erlagsgesellschaft. 1927.
*
(12) (13) (14) (15) (16)
531
Keuberger, .4.,2. anal. Chem., 116, 1 (1939). Pearson, Th. G., Ihid., 112, 179 (1038). Reif, K., .Vikrochemie, 9, 424 (1931). Riley, H. L., J . Chem. SOC.,1933, 895. Tougarinoff, M.,Ann. SOC. sci. Rrztselles, 54B, 314 (1934).
PRESENTED before t h e Division of Physic31 and Iiiorgiiiiic Ctiriniatry at
the
99th Meeting of the ;\iiirrican Ctieniical Society, Cincinnati. Ohio.
A Simplified Alkali-Lability Determination for Starch Products 1
THO3IAS JOHS SCHOCH A?D C. C. JENSEN, Corn Products Refining Company. Edgewater, N. J.
A
SIMPLIFIED alkalimetric method has been devised to estimate the relative hydrolytic degradation of starch products, analogous to the concept of alkali-lability (6, 7 ) . If the starch molecule is conceived as a glucopyranose chain terminating in a free aldehyde group, then any glucosidic hydrolysis should be reflected in increased aldehydic properties. Since the ordinary analytical procedures for reducing sugar are not applicable to starch, Richardson, Higginbotham, and Farrow (6) hare developed a special copper reduction technique to est'imate aldehyde content. The writers have found this method of questionable value, since the reducing value is largely influenced by the degree of dispersion of the starch in the alkaline copper medium. Raw starch slon-ly decomposes in hot aqueous alkali to give simple acidic substances, principally formic, acetic, and lactic acids, as well as pyruvic aldehyde. This immediately suggests that the reaction is initiated by enolization of free terminal aldehyde, as formulated by Evans ( 3 , 4 ) for the aldose sugars. T i t h acid-modified starches, alkaline decomposibion proceeds more rapidly, indicative of increased aldehyde content. Taylor (6, 7 ) has employed this concept of alkalilability as an index of starch hydrolysis, by determining the amount' of iodine-reducing substances produced during an arbitrary period of alkaline digestion. However, this iodometric technique is both complicated and tedious, and results cannot be duplicated by various operators. Also, starches which have been precipitated with acetone or et'hyl alcohol consume abnormal quantities of iodine, apparently because of retention of these solvents even after prolonged drying. Estimation of the acidic substances produced by decomposition of starches in hot alkali affords a much simpler and more precise measurement of alkali-lability. The procedure resembles that employed for saponification number of a fat :digestion of the starch in a measured volume of standard sodium hydroxide follon-ed by titration of the unconsumed alkali. The rate of decomposition of the starch, here termed the alkali number, is expressed as the cubic centimeters of 0.1 N sodium hydroxide consumed by 1 gram of st,arch during digestion in alkali for 1 hour a t 100" C.
3Iethod REQUIREDREAGESTS.Approximately 0.4 N sodium hydroxide, free from carhonate; 0.2 N sulfuric acid, accurately standardized; and 0.1 per cent alcoholic thymol blue solution. PROCEDCRE.The starch should be pulverized to pass a 60mesh sieve. Moisture is separately determined by drying 4 hours in VUCZLO a t 105' C., and the alkali number is calculated to the dry basis. Occasionally, a starch product will be encountered with added alkali or acid sufficient to affect the alkali number. In such cases, 1 gram of the starch is gelatinized in hot water and neutralized to t,hymol blue with standard acid or alkali, properly correcting the alkali number for this titer. The alkaline digestion may be conveniently run in an 8ounce narrowmouthed Pyrex nursing bottle. rldequate pro-
tection against carbon dioxide is afforded by the gum rubber caps marketed for bhis type of bottle, pierced with a hot needle to provide an exit for steam. Five hundred milligrams of the powdered starch are introduced into the bottle, 10 cc. of distilled water are added, and the contents are gently sn-irled to wet and suspend the sample. Then 25.00 cc. of 0.4 N sodium hydroxide are added by pipet, meanwhile agit,ating the sample to ensure uniform gelatinization in the alkali. If lumping occurs at this point, the determination should be repeated. Finally, 65 cc. of hot distilled water are added, and the bottle is immediately capped and placed in a vigorously boiling water bath. If a number of determinations are to be run, digestions may be commenced at intervals of 10 to 12 minutes. With starch products n-hich gelatinize in cold water, a slightly different technique is advised, to avoid the formation of insoluble lumps. The sample is introduced into a perfectly dry digestion bottle and \vetted with 1 to 2 cc. of benzene; 25.00 cc. of 0.4 N sodium hydroxide are then added with agitation, followed by 75 cc. of hot water. In this Tvay, complete dispersion of the starch is readily obtained, even with starches which have been precipitated from boiled pastes by alcohol. The bottle is heated for exactly 60 minutes, then placed in cold water, the cap is removed, and 50 t o 75 cc. of cold distilled water are quickly added. In this manner, the decomposition is halted almost immediately. One cubic centimeter of t,hymol blue indicator is added and the excess alkali titrated to a yellow end point with standard acid. Phenolphthalein has likewise been employed, but thymol blue seems somewhat more satisfactory. With highly converted starches, the end point is difficult because of the amber color which develops during digestion. In such instances, titration with the glass electrode t o a pH of 8 gives excellent results, though with practice visual titration to thymol blue can be applied t o all starch products. The titer value of t,he alkali is det'ermined by neutralizing 25.00 cc. of 0.4 A ' sodium hydroxide to thymol blue with standard acid. This procedure acts as a blank, balancing out indicator errors. Then the alkali number is calculated as: (cc. of acid to titrate blank - cc. of acid to titrate sample) normality of acid x 10 weight of sample on dry basis
x
Providing excess alkali is present, the concentration of sodium hydroxide employed for digestion does not materially affect the alkali number. However, variations in the amount of starch and T-olume of the digestion medium have an appreciable influence, and close adherence to the recommended procedure is advised. From 365 determinations on 164 different starch products, the average deviation calculated +0.17 unit in the alkali number. Each value here reported represents the average of two or more determinations.
-4pplications This method of analysis has been applied to a number of theoretical and pract'ical starch problems, of n-hich the following are typical inst,ances. RAW STARCHES.Commercial corn and wheat starches possess alkali numbers which are consistently higher than
INDUSTRIAL AND EKGISEERING CHEMISTRY
532
those of the common tuber starches. This is attributed to different molecular configuration, rather than to hydrolytic conditions which might prevail during manufacture. On a laboratory scale, starch has been isolated from cracked corn under nonaqueous steeping conditions which should preclude acidic or enzymatic action, and the alkali numbers of such products did not differ materially from that of commercial cornstarch. Starch Alkali S u m b e r Tapioca (3 different commercial s a n i p l ~ s ) 5 . 9 , 6 . 8 , 6 9 5 7, 6 . 6 , 6 . 9 Potato (3 different samples) 6 7 , 7 5 Rice 9.7,ll.j Wheat 9.8,10.6,11.0,11.2,11.9,12.1 Corn (commercial samples) 10.6. 1 1 . 4 Corn (nonaqueous steeping)
HETEROGENEITY OF STARCH. Starch is not necessarily comDosed of identical molecules. By leaching a t temperatures just below the gelatinization point, Baldwin ( 1 ) has extracted a small amount of soluble material from potato starch. Similar procedure applied to raw cornstarch yields a minor fraction of soluble carbohydrate possessing a high alkali number. Initial raw cornstarch Soluble fraction Insoluble residue
VOL. 12, NO. 9
Excepting the somewhat higher values in the last two instances, Where mineral acids were employed during dextrinization, the alkali numbers are substantially constant, bearing no relation to viscosity or solubility. An interesting sidelight on this subject is presented by the progressive drop in alkali-lability when a white dextrin is oven-dried a t 105" C. Initial product Dried 4 hours Dried '"2 hours
54.6 37.8 29.7
Some sort of anhydride formation may occur during dextrinization, rendering the product less susceptible to attack by alkali.
55t-
-4
o\
11.2
35.9 8.8
This soluble material probably represents smaller molecules occurring naturally in the starch granule. I n like manner, a typical acid-modified thin-boiling starch was found to be heterogeneous in character, since it could be fractionated by extraction with cold water. Initial thin-boiling starch Fraction soluble in cold water Insoluble residue
44 1 50 6 36 9
3
When raw cornstarch is digested in hot alkali, then neutralized and precipitated with alcohol, the resulting product possesses an alkali number lower than the original starch. However, even when such alkali predigestion is continued until 80 to 90 per cent of the starch is destroyed, the alkali number of the residual carbohydrate does not drop below 4.0. This would indicate the absence of any fraction completely stable toward alkali. ACID-MODIFIEDSTARCHES.With increasing degree of acid conversion, the alkali number rises progressively; Thin-boiling starch 20-fluiditp 40-fluidity 60-fluidity 75fliiidit~. gO-fluidirj, White dextrin, 25TG cold water-soluble White dextrin, 655% cold water-soluble Lintner's soluble starch a
14.5 15.0 15 7 20.7 41.5 56.3 62.6 66.4
Determined b y the method of Rue1 ( 8 : .
hence the so-called white dextrins should be considered as highly acid-modified starches. As another instance where this method has been employed to detect hydrolysis, 2 per cent raw cornstarch pastes were carefully buffered to various pH levels, and autoclaved for 4 hours a t 19 pounds steam pressure. The starch was then precipitated with alcohol, filtered, and dried. Subsequent alkali number determinations indicate maximum stability in the narrow pH range of 5.9 to 6.3 (Figure 1). Autoclaved starch solutions show a slight yellow discoloration above 6.3, indicative of alkaline decomposition. TORREFACTIOK DESTRISS. The Brit'ish gums and canary dextrins present a different picture, with alkali numbers only slightly higher than that of raw cornstarch. Gum, 5.5%. soluble, buffered conversion British gum, 17.57, soluble British gum. SO% soluble Gum, 857* soluble Dextrin, acetic acid conversion, 98% soluble Extra dark yellow dextrin, nitriq acid conversion, 85% soluble Dark yellow dextrin, hydrochloric acid conversion, 977a soluble
16.5 16.6 16 6 16.3 16.4 19.9 23.0
4
5
7
6
PH D U R I N G A U T O C L A V I N G
FIGURE 1
OXIDIZED STARCHES, Oxidation with alkaline hypochlorite is commonly employed for the manufacture of thinboiling starches. The alkali-lability of such commercial products is generally somewhat lower than that of the parent raw cornstarch. Low conversion Medium conversion High conversion
10.4 10.2 8.6
Interpretation of these result,s must await further investigation of the mechanism of starch oxidation. Alkali-lability, however determined, must not be construed as a quantitative evaluation of aldehyde content. The method simply affords a n empirical index of hydrolysis. Anhydrous glucose gives a n alkali number value of 85.2. This merely represent's the limiting maximum for substances of glucosidic configuration, and is not comparable Tyith starch values, since glucose is completely destroyed during the first few minutes of alkaline digestion. Similarly, maltose hydrate gives a value of 84.7. As might be anticipated, the following sugar derivatives are completely stable against alkali : gluconic acid, alpha-methylglucoside, sucrose, trehalose, raffinose, and melezitose.
Literature Cited (1) Baldwin, J . Am. Chem. Soc., 52, 2907 (1930). (2) Buel, 8th Intern. Congr. Pure Applied Chem., Orig. Com., 13, 63 (1912). (3) Evans and Benoy, J . Am. Chem. SOC..52, 294 (1930). (4) Evans, Edgar, and Hoff, Ibid., 48, 2665 (1926). ( 5 ) Richardson, Higginbotham, and Farrow, J . Teztile Inst., 27, 131 (1936). (6) Taylor, Fletcher, and Adams, ISD. ENG. CHEX.,Anal. Ed., 7, 321 (1935). (7) Taylor and Salzmann, J . Am. Chem. SOC.,55, 264 (1933). PRESENTED before the Division of Sugar Chemistry a t the g6th Meeting of the American Chemical Society, Milwaukee, R'is.
