Chemical and Physical Properties of Sweet Potato Starch F. H. THURBER, Bureau of Chemistry and Soils, Washington, D. C. and of cassava starch (57 granMETHOD for the proThe ratio of a- to @-amylose is approximately ules), 13 p . To measure t h e duction of a high-grade tlw same in sweet potnto and white potato starches. starch granules, a filar micromesweet potato starch has The maxinium viscosities of the two starches ter ocular ( X 12.5) was used been r e p o r t e d by Balch and differ greatly. The rdatizdy uniform visro.sity on the microscope together with Paine (3). In order to appraise a 4-nun. ( X 40) objective. The of sweet potdo sturch over long periods of healing as fully as possible all of the relative size and shape of the potential uses of this starch, an is a lialuable property since, under these condigranules are shown in Figure 1. investigation of its chemical and tions, a un$orni sizing bath can be easily mairiphysical properties has been unlained. Sizing tests show that sweet potato a- AND @-AMYLOSE CONTENT dertaken. In this paper the restarch possesses satisfactory coating and peneAs pointed o u t b y Baldwin sults of a comparison of certain trating qualities for cotlon yarn. (4, many i n v e s t i g a t o r s conproperties of sweet potato starch with those of white potato, corn, It is believed that in cull sweet potatoes there sider some s t a r c h granules to be made u p of t w o d i s t i n c t and C&SS&VR s t a r c h e s a r e reis a potential source of starch which is quite components, a n outer someported. High-grade commercial suitable for siziny cotton textiles and which may what insoluble p o r t i o n 4 e., starohes were used except in the he used to displace white potato starch that is a-amylose or amylopectin case of sweet p o t a t o s t a r c h , ii,ow k i n g imported for this purpose. If made and an inner more soluble which was prepared in the Deeomponent--i. e., p-amylose or partment of Agriculture expericommercially avuilable, it will in all probability granulose. a-Amylose yields mental plant. al.qo find uses in other industries. a violet color (10) with iodine The swect p o t a t o s t a r c h and eelatinizes when heated granules are relatively small. Slxty-three granules Rere measured and found to range in with water; @-amylose gives a blue eo101 with iodine and size from 4 to 27 i.r; the average diameter nas 12 p. The does not form a paste when heated with water. Starches average diameter of the white potato bhrch granules (153 which have approximately the same relative contents of the granules) was 26 p ; tliat of corn starch (52 granules), 11 p ; amyloses might be expected to manifest similarities in other properties, such as adhesiveness of pastes and type of dextrin formed on partial hydrolysis. For this reason a study of the determination of these components of sweet potato s t a r c h was
A
Y
Corn atarnh
sweet w t a t o etarob
Wbite potato atsrch
Froun~1, S T ~ L GRANULES II
35;
A number of methods for bringing about the separation of these components have been described in the literature (4,7,9, 10, fd,13). A f r e q u e n t l y used procedure (9, 12, IS) consists essentially in d i s i n t e g r a t i o n of the g r a n u l e followed by its subjection to the inftuence of an electric p o t e n t i a l difference in an electrophoresis cell. The insoluble n-amylose bearing the negatively charged acid radical migrates towards the positive pole. If this electrode is placed at the lower end of the cell, the separation is aided by gravity as well as by electrical migration (19). The arrangement of such a cell is shown in Figure 2. A 220-volt potential difference was used The shape of the cell illustrated by Taylor and Iddlrs (IS) was modified to some extent to aid in securing a more rapid settling of the a-amylose. To dibintegrate the starch granule, it might be thought that it would be necessary only to heat the starch suspension in water to a polnt somewhat above its gelatinization temperature for a sufficient length of time to make sure that the outer envelopes are ruptured. Sweet potato starch suspensions were prepared in this manner, and the s e p a r a t i o n was made in an electrophoresis cell. After each extraction the suspension was reheated and then fmzen with carbon
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dioxide snow to makc sure that the granules had burst. The temperature a t which the starch was gelatinized was found to influence greatly the ratio of the fractions obtained. Thus, when the starch was gelatinized a t TO" C., 54 per cent was recovered in the insoluble fraction; a t 75" C., 42 per cent, and a t 85'C., 34 per cent, was rccovered.
approximately 16 per cent of a-amylose when tested in this manner ( I S ) . The availability of the main componentthat is, B-amylose-in white potato and sweet potato starches may he quite different, as will be pointed out in a later section of this paper.
VIBCOSITX A property of starch which is widely used to determine its uniformity and also its suitability for a given purpose is its viscosity.' Comparison of the viscosity of sweet potato, white potato, corn, and cassava starches yielded some interesting results. Three per cent starch pastes were prepared by suspending the required amount of starch in 10 eo. of cold water, after which the remainder of the water was added from a pipet, allowance being made for the moisture content of the starch. In determining gelatinization temperatures, the water before being drawn into the pipet was held at a temperature of 50" C. For all other measurements this water was mintained a t a temperature of 98" C. To insure uniformity of the pastes, they were agitated only by the stream of water from the
F I G U ~2.E E ~ ~ c ~ n o ~ e o a CELI esrs
Microscopic examination of this fraction reveals that the starch granules are broken into fragments of various sizes, and that the starch envelopes have not been entirely separated from the interior of the granule as it was hoped they would be. From this investigation it appears that a-amylose makes up a t least a part of the interior of the granule and that the interior must he broken down before a true measure of the a-amylose can he obtained. As suggested by Taylor ( 1 1 ) , fragments of granules containing &amylose had migrated to the positive pole together with the a-amylose. It was thus necessary to resort to other means to bring ahout disintegration. Prolonged grinding of dry starch in a mortar or in a hall mill produces a modified product characterized by a very low viscosity and lack of characteristic granular structure. This phenomenon was ohserved by Alsherg (I) and later made use of by Taylor and Beckmann (19 in their studies on corn starch. Sweet potato starch samples were ground in a hall mill for periods of 24, 36, and 50 days, and were then separated in the electrophoresis cell, The results after 36 and 50 days of grinding were practically identical. tinder these conditions only 2.8 per cent of the starch remained in the a-amylose fraction. This is very close to the value which has been reported for white potato starch-that is, 1.8 per cent (18)and sets these root starches off from corn starch which yields
pipet (8). Measurements were made with a Stormer viscometer. I n this instrument a cylinder is suspended in the starch paste, and a weight of known value ex& its force to bring ahout the rotation of the cylinder. The time, expressed in fifth-secoods, required to produce one hundred revolutions of the cylinder is used as a measure of the viscosity of the paste. A series of measurements was niade to determine the temperature a t which the starch begins to swell or gelatinize, as is shorn by the increase in viscosity with increaw in temperature (2). Heating was then continued to determine the conditions required for securing the maximum viscosity. I visCoattymeaauremsoia were made by F W. Coleman, formerly of the Carbohydrate Divieion.
TEMPEEATURES AS SHOWN BY VISCOSITYMEABUREMENT~ TABLEI. GELATINIZATION 8TanCH (3% PAWrrl 5S0 C. 60' C. 37 37 sweet p"tat,o While potato CBB 8hY B
.6Ie C. 37
vzs~oai~i I N Firm-Snconoel 6 2 O C.
37
88"
C.
67' C.
37 106
%ls
63 72 Corn 37 37 37 37 37 Figurea i n paieirtliesea desig-nsto initial points of ohaoga ill viecuiity.
75'
b.
14; 171
37
82' C. 75 1700 607 37
83' C . 82 2180 (535)
(38)
84" C. 89 2800 502 39
85'
c.
88 3080 389 42
95'
c.
tal
5428 208 70
98' c.' 185 5868
177 72
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May, 1933
This may also serve as a measure of the point a t which complete gelatinization occurs. Readings were taken a t 2" C. intervals. The results of this test are given in Tables I and 11. First-grade starches were used in making these tests, but it should be remembered that different samples of the same variety of starch may not yield identical results in viscosity determinations. TABLE11. EFFECTO F
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viscosities of different starches, or even different samples of the same starch, under any one set of conditions may be seen from the rapid change in viscosity occurring in these starch pastes with variation of temperature and time.
ON C H A S G E I X
C O N T I N U E D HE.4TINQ
VISCOSITY
(In fifth-seconds) STARCH (3% PASTE) VISCOSITYAT 90' C. Min. 15 20 30 60 120 240 360 Sweet potato 170 180 188 188 149 108 91 White potato 9670 12,960 11,076 6324 3308 961 585 Caa s a v a 177 .. .. .. ,. 50 Corn 74 75 76 72 69 67 66
..
Cassava starch began to gelatinize a t 61' C., white potato starch a t 61', sweet potato starch a t 67", and corn starch a t 83". Cassava starch paste reached a maximum viscosity a t 83 ', whereas the remaining samples exhibited a gradual increase in viscosity up to the maximum temperature used, which was 99" C. On continued heating a t this temperature, white potato starch reached its maximum viscosity of nearly 13,000 units in 20 minutes, sweet potato starch 188 units in 30 minutes, and corn starch 76 units in 30 minutes. The comparison is more easily seen by reference to the curves shown in Figures 3-6. In Figure 3 the temperatures at which the curves leave the base line mark the beginning of the gelatinization period. The maximum viscosity for cassava starch paste is also shown. Only a small fraction of the white potato starch curve is visible, since the viscosity increases very rapidly with increase in temperature. I n Figure 4 the maximum viscosities of sweet potato and corn starch pastes, which occurred after 30 minutes of heating, are shown. The viscosity of white potato starch is too great to appear on this scale. Figure 5 is a combination of Figures 3 and 4,showing the complete curves for cassava, sweet potato, and corn starches over a 6-hour heating period. Figure 6 is on a much smaller scale and shows the comparatively extremely high viscosity of white potato starch paste. The necessity of using extreme care in comparing the I
I
1
I
I
I
In the sizing of cotton yarn in the textile industry, it is often necessary to maintain the size bath at or near the boiling point of water for several hours. Uniform viscosity of the starch paste is desirable in order to obtain consistently the desired degree of sizing. Thus, the behavior of a starch under these conditions is of importance in determining its value in this industry. A second series of tests was made in which the starch pastes were stirred slowly and continuously, much as might be done in a textile mill. These results are given in Table I11 and are shown graphically by the curves in Figure 7 . Since the viscosity of the cassava starch decreased so rapidly on long continued heating, as is shown in Figure 5, it was omitted from this test and from those which follow. TABLE111. VISCOSITY OF CONSTAXTLY AQITATED STARCH PASTES STARCH Sweet potatoo White potatob Cornn a 3% paste.
blin. 5
(In fifth-seconds)
VISCOSITY 10 15 20 30 60 120 240 127 148 153 147 131 115 104 95 897 1021 1997 1484 1204 562 134 100 81 94 89 76 73 72 71 83 b 2% paste.
The viscosity of white potato starch paste decreases rapidly when the paste is agitated. After 2 hours of stirring
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may be somewhat different, as is indicated by the viscosity curves. The extremely rapid reduction in the viscosity of white potato starch paste on continued heating indicates that the granules are being burst, thus setting free a large proportion of the &amylose. This conclusion is borne out by microscopic examination. I n the case of sweet potato stnrch the grains are swollen, but relatively large numbers of them still remain in granular form even after long continued heating. PENETIUTION AND COATINQ POWER OF &ARCHES
The relative penetration and coating power of sweet potato, white potato, and win starch pastes with respect to cotton yarn were compared in the follorring manner: Four per cent starch pastes were prepared hy heating starch suspensions in water for 30 minutes at 98" C. The mixture was stirred throughout the heating period. Numher 30 cotton threads were immersed in this size bath for 3 minutes. They wcre then removed, and, after the excess starch had been brushed off, they were dried and embedded in pyroxylin (6, 6, 14). Cross section8 were then cut, stained with iodine, and examined under the microscope. A typical set of cram sections is shown in Figure 8. The dark out.er portion marks the extent to which the starch hs,s penetrated the thread. These results represent only one set of sizing conditions. It is, however, quite evident from them under the conditions used, the viscosity becoIrles f&ly that sweet potato starch possesses satisfactory coating and constantand is not much greaterthan that of sweetpotato and corn starch pastes. The viscosity of pastes of these two penetrating qualities and from this standpoint should he entirely satisfactory for the purpose of sizing cotton textiles. starches is not greatly affected by moderate agitation. In so far as this test is concerned, sweet potato starch LITEKATUEE CITED should be well adaded for use in textile sizine. Its viscositv lies between that-of corn starch and white potato (1) Aleberg and Perry. Pmc. Soe. Ezpll. Bid. Md., 22, 60 (1924). which has been heated and stirred for some time. Moreover, ( 2 ) AIsberg and Rask. c%d chcm., I , 107-15 (1924).
White potato staroh ~
Sweet potato stsroh
Coin starch ~
U
8. H
ECnoss
SECTIONS OP COTTONT x n e a ~ s( X 200)
it shows relatively little change in viswsity on continued heating for long periods of time.
SIZINGOF COTTONYARN I n a number of textile mills, white potato starch is considered to he especially desirable for sizing purposes, presumahly because of its superior coating and penetrating qualities. It will be recalled that the @-amylose content of white potato starch, as determined hy the method described (13)),is very h i g h ~ v e r98 per cent of the whole granule. The pamylose content of sweetpotatostarch is high, the two might he expected to exhibit similar properand ties. The availability of the @-amylosein the two starches
(3) I3alch and Paiiie. IXD.Elio. CHX~M., 23. 1205-.13 (1931). (4) Raidwin. J . Am. Chem. Soc., 52, 2907 (1930). (5) Jeffrey, Yotan. Gm.,86,456 (1928). (6) Kisser and Andemon, Am. J . Bolnnw. 15, 4 3 7 4 1 (1928). (7) Ling and Nanji, J . C/tem. Soc., 123, 2866 (1923). ( 8 ) Ilipperton, Hawaii Agr. Enpt. Si.%., Bull. 63 (1931). (0) Sarneo and Mayer, Kolloidchcm.. Beil~efte,13, 272 (1921). (10) Tanret, RILL Sot. C h i w 141 17, 8:j (1915). ('I) ' pri>=te communication. (12) Taylor and Reckmann, J . A n . Chon&.Soc.. 51,294 (1929). (13) T ~and I~ ~ I I~ I I ~~~E ~~~,~ cnrhr.. . ~ . 18, 713 (1926). (14) Willows and Alonitnder. J . Trriile Inst., 12,99 (1921).
*.'
Racslvnn October 27. 1832. Presented before the Diviaion of Sugar Chcmietry st the 84th Meeting 01 the Arnerioan Chemical Sooiety. Denver. Colo.. Auguat 22 to 26. 1932, This pamr is Contribution 126 from the Carbohydrate ~ i ~ i ~B~~~~~ i ~ " , chemistry soils.
