Sirup from Sweet Potato Starch A Pilot-Plant Investigation LAWRENCE E. STOUT AYD CARL G. RYBERG, JR.
A sweet sirup has been prepared from sweet potato starch. The effects of pressure and of acid concentration on the rate of hydrolysis were studied and compared with their effects on the rate of hydrolysis of cornstarch. The isoelectric point of the undesirable humus, and the amount of activated carbon and the length of time necessary to decolorize the sirup were determined. The clarity, color, flavor, and aftertaste of the resulting sirup were observed.
Washington University, St. Louis, Mo.
7W E E T potatoes are rated as the second most important
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vegetable crop grown in the United States, yet only about 70 per cent of it is marketable. The remainder consists of potatoes which have the same composition and quality as the marketable ones, but are larger than the produce markets will accept. Very little or no use is made of these oversized potatoes, and they represent a considerable loss to the growers. The development of a sweet potato byproducts industry, which could convert these potatoes into salable products, would be of benefit to the growers. Many products derived from sweet potatoes have been prepared, and some have been exploited commercially (3, 9 ) . However, none of these commercial ventures have been successful. Starch and sirup seem to have the best commercial possibility today. Sweet potato sirup was produced as far back as 1870 by Delamarre (4). Other workers have devised methods of producing sirup from sweet potatoes (6-9). I n general, these workers cooked the sweet potatoes to a pulp, converted the carbohydrates present with enzymes, then filtered and concentrated the resulting liquor. This type of sirup production directly from sweet potatoes offered theoretical advantages. Some of the sugars normally present in the sweet potato would be recovered in the sirup, and thus a higher yield mould be obtained than from starch alone. Moreover, the refining of starch is an expensive operation and this cost mould be avoided if a satisfactory sirup could be produced directly from the potatoes. However those substances which impart color, odor, and taste to the sweet potato were also present in the sirup, and although it was fair in quality, i t had no distinctive advantages which enabled it t o compete with sirups already on the market-e. g., corn sirup. The method of making sirup followed by the corn sirup manufacturers seems preferable to the one mentioned above, since it is rapid and easy to control. They extract the starch from the corn grains, rid i t of undesirable matter, and then convert i t with dilute mineral acid under pressure. The literature reveals no reports of attempts to apply this procedure to sweet potato starch. The reason may be the difficulty of purification of sweet potato starch. McDonnell (11) in 1908, and others ( I , $ , 10, 14-19) studied the extraction of sweet potato starch and the removal of the color, odor, and taste but until recently have not exploited their results. Sweet potato starch has recently become available in the United States in commercial quantities (13). The starch produced is white, odorless, tasteless, and of high quality.
It can compete satisfactorily on a quality basis with the best grades of imported white potato and cassava starches ( I d ) . The following average analysis of the starch was reported by Thurber ( 1 7 ) : Ash, Moisture Protein (N X 6 , 2 5 )
0.096% 12.6 0.053
Sulfur dioxide is used in the purification of the starch. The substances responsible for the color, odor, and taste of the sweet potato may have been reduced into compounds which will be readily oxidized by atmospheric oxygen into the original substances. If they have been removed, products derived from this starch should be free from the obnoxious properties. This paper attempts to adapt the procedure used to make corn sirup to the production of sirup from sweet potato starch on a pilot-plant scale. The standard procedure for cornstarch in this district is as follows ( 4 A ) : Forty pounds of concentrated hydrochloric acid are mixed with 40 gallons of water and poured into a hot converter. Twenty-two hundred gallons of 26" BB. starch slurry are then run into the converter a t such a rate that pasting occurs. The converter is closed, heat is applied, and the starch is converted under pressure until a sample gives a wine-red color with iodine. The sirup is blown from the converter into a storage tank where it is neutralized with sodium carbonate until the sirup has a definite pH. This p H value varies with the type of starch and the individual conditions in each plant but is usually within the range 3.0 t o 5.5. After neutralization, the sirup is filtered, partially decolorized with carbon, concentrated to about 26" BB., decolorized, and then concentrated to the desired consistency. The adaption of this procedure to sweet potato starch called for a study of two variables: (a)the effects of pressure and acid concentration on the rate of hydrolysis of sweet potato starch, and (b) the conditions necessary for the .prothe length of hyduction of a high-grade sirup-namely, 1451
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drolysis, the degree of neutralization, and the amount of activated carbon and time of contact needed t o produce a water-white sirup.
Experimental Procedure APPARATUS.The conversions were made in a 20-gallon acidproof bronze (90 per cent copper-10 per cent tin) autoclave (Figure 1). The open-type steam coil and the mechanical stirrer were also of acid-proof bronze. Steam at 100 pounds gage pressure was available for the coil. The starch slurry was made and stored in a large earthenware crock, equipped with a stirrer. A gear
FIQURE 1
pump pumped the slurry from the crock into the autoclave. The pump and the mechanical stirrer were driven by l/a-horsepower motors. The crock served also as the neutralizing vessel. Filtration was effected by .a 6-inch Shriver plate-and-frame press, A 10-gallon steam-Jacketed vacuum evaporator was available for evaporating the sirup to the desired consistency. RAWMATERIALS.Sweet potato starch was purchased from the starch plant at Laurel, Miss. Cornstarch was obtained from
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the Corn Products Refining Company. The water used in the starch slurry was the ordinary tap water supplied by the St. Louis County Water Works. The acid used in the tests was concentrated c. P. hydrochloric acid. The sirup was neutralized with sodium carbonate. Filtration was facilitated by the use of Johns-Manville Hy-flo Supercel filter aid. Fehling solution was used in the analysis of the sirup for the reducing sugar content. The Fehling solution was prepared from c. P. potassiumsodium tartrate, sodium hydroxide, and CuS04.5H10. It was standardized against c. P. anhydrous dextrose. Decolorizing tests were made with both Norite and with Mallinckrodt activated carbon. PROCEDURE. The autoclave was preheated until it reached the temperature of the steam. This was done by closing the autoclave and introducing steam a t the desired pressure. The steam pressure fluctuated widely until the autoclave reached the temperature of the steam, at which point the pressure assumed a constant value. Next the autoclave was emptied of condensate and charged with the prepared slurry. This slurry had been pre ared by mixing 10 pounds of starch with 15 pounds of water in t i e crock and stirring until a smooth slurry was obtained. The calculated weight of hydrochloric acid was then added, and the mixture pumped into the autoclave. The stirrer was kept in operation until the end of the conversion. When the charging was completed, the autoclave was closed and the pressure raised to the desired value. The conversion was continued for the desired length of time and was stopped by blowing the sirup into the crock. The sirup was neutralized to a pH of 5.1. While the conversion was in progress, samples were withdrawn from the autoclave through the sampling valve. After the removal of each sample, the sampling line was cleaned with steam (Figure 1). These samples were analyzed by the method of Eynon and Lane (6) which consists briefly of titrating a known amount of standard Fehling solution with the unknown solution to the methylene blue end point. The pH of neutralization is known as the break point because at this pH the humus formed in the reaction recipitates. A solution of 2 N sodium carbonate was made. Bortions of this solution were added to each of a series of 100-cc. samples of unneutralized sirup. The first sample received 1 cc. of solution; the second, 2 cc.; the third, 3 cc.; etc. I n one of the samples a flocculent precipitate formed, leaving a clear liquid. Adjacent samples remained turbid. The pH of this sample was determined colorimetrically. On a second series of samples the pH of the break point was determined within 0.1 pH unit. After neutralization about 2-3 ounces of filter aid were added, and the sirup was pumped through the filter press. The filter cloths had been precoated with filter aid. The filtered sirup was colored slightly but was quite clear. This sirup was treated with activated carbon. The amount of carbon and the length of time needed for completely decolorizing the sirup were determined on 100-cc. samples of the sirup. The sample was treated with a known amount of carbon and allowed to stand for a measured length of time. After filtration the decolorized sirup was compared with fresh water for color and clarity. After decoloration the sirup was evaporated to about 30' B6. This sirup was tested for clarity, color, and flavor.
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TIME IKMINUTES
FIGURE 2
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I E 10 I5 TIME INMINUTES
FIGURE 3
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TABLEI. EFFECT OF VARIATION IN PRESSURE ON HYDROLYSIS RATEOF SWEET POTATO STARCH
-
Time, Min. 2 4 6 8 10 12 15 18 20 21 a
Mg. Reducing Sugar/Cc. Sirup 0.0044 G. HC1/ 100 0.starch 40 50 35 141 ... 223 61 37 261 95 45 265 120 71 275 147 91 275 171 110 275 196 144 212 169 280 2c6 275 ...
d . 0 2 0 G. HC1/100 G. starch30" 35 40 45 40 78 118 144 94 148 212 208 131 208 261 273 148 240 278 286 187 265 280 286 200 273 280 286 228 275 280 286
...
241
275
...
...
...
...
-
...
,..
ziil
,,,
Pressure, pounds per square inch.
TABLE11. EFFECT OF VARIATION IN ACIDCONCENTRATION ON HYDROLYSIS RATU OF SWEET POTATO STARCH AT 40 POUNDS PER SQVARE INCHPRESSURE Time, Min. 2 4 6 8
10 12 15 18 21 a
-Mg.
0.02Oa 118 212 261 278 280 280 280
Reduoing Sugar/Cc. Sirup0.0044 0.0066 0.0088
... 61
95 120 147 17' 196 212 230
...
...
... 65
100 138 163 193 203 220 233
... 88
124 173
lg5 245 255 268 280
5
... 87
TABLE 111. COMPARISON OF HYDROLYSIS RATESOF SWEET POTATO STARCH AND CORNSTARCH Time, Min. 2 4 6 8 10 12 15
Effect of Acid Concentration and Pressure om Hydrolysis Rate
8, 5
I 1 10 16 TIME I N MINUTES
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FIGURE 4 is increased, the rate of hydrolysis increases. The rate of increase is not directly proportional to the increase of pressure. T h e rates obtained a t pressures of 40, 45, and 50 pounds per square inch do not differ greatly. Figure 4 represents the effect of variation of acid concentration on the rate of hydrolysis. It shows that the rate increases with the acid concentration. The highest acid concentration studied is greater than is used in the com-
Reducing Sugar/Cc. SirupCornatarch Sweet Potato Starch 0.020a 0.0066 0.020 0.0068 35b 40 35 40 121 78 150 142 64 65 214 208 96 100 250 143 240 138 265 273 168 163 273 280 195 193 280 215 275 203 223 220 is0 ... 275 233 233
-Mg.
Acid conoentration, gram per 100 grams starch.
I n these tests the starch was converted for about 20 minutes. A preliminary conversion indicated that the starch was converted past the sirup stage within this length of time. The data are given in Tables I and I1 and plotted in Figures 2, 3, and 4. The abscissas are the elapsed time in minutes, and the ordinates are the reducing sugar calculated as mg. of dextrose per cc. of sirup. Figures 2 and 3 represent the effect of variation of pressure on the rate of hydrolysis. They show t h a t as the pressure
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FIGURE 5
0.0110 137 183 213 240 255 280 280
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10 15 TIME IN MINUTES
18
20 21 a
b
...
...
...
... ...
...
Aoid concentration, gram per 100 grams staroh. Pressure, pounds per square inch.
mercial production of corn sirup. The commercial concentration is in the vicinity of the lowest concentration utilized in this work. The higher acid gives a higher rate of hydrolysis but also seems to produce more humus, since the liquors were of a darker color than liquors hydrolyzed a t the lower acid concentrations. Another objection to the use of high acid concentration is that the acid is neutralized t o sodium chloride. Too much salt may give a n off-flavored sirup. Tests made on the samples withdrawn in the rate studies with iodine solution revealed that when the reducing sugar content had reached a value of about 125 mg. per cc. of sirup, the blue starch-iodine color was no longer produced; instead, a deep wine-red color resulted. When the reducing sugar content had reached a value of about 250 mg. per cc. of sirup, no color was produced with iodine. These concentrations represented conversions of about 35 and 70 per cent, respectively. As Figure 4 shows, this concentration is reached in a very short time. With the higher acid concentration, the time is so short that the hydrolysis would be difficult to control. With the lower acid concentrations, the time of hydrolysis is longer and the reaction is easier t o control. I n the corn sirup industry the elapsed time from the start of the filling of the converter t o the end of the discharging of the autoclave is about 30 minutes. Figure 4 shows t h a t an acid concentration lower than 0.0044 gram per 100 grams of starch can be used and hydrolysis still occurs within 30 minutes. Figure 5 and Table 111 show a comparison of the rates of hydrolysis of sweet potato starch and cornstarch under the same conditions. These curves show that cornstarch is only slightly easier to hydrolyze than sweet potato starch.
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Production of Water-White Sirup The starch was hydrolyzed to the wine-red color with iodine solution. After neutralization and filtration, the sirup had a slight yellow color. Two activated carbons were used in the tests on the ease of decoloration. Carbon 1 was satisfactory in its action whereas carbon 2 was unsatisfactory. A treatment for 15 minutes with 0.5 gram of carbon 1 per 100 cc. of sirup gave a water-white sirup. After decolorization the sirup was concentrated to about 30" B6. Sirups prepared from sweet potato starch and cornstarch were both water-white and clear. They were both sweet and had very little aftertaste. Addition of about 0.5 cc. of vanilla flavoring per 100 cc. of sirup masked the aftertaste.
Conclusion The results obtained in this work indicate that sweet potato starch does not differ greatly from cornstarch in its behavior to dilute mineral acid under pressure. They also indicate that a sweet sirup can be prepared from sweet potato starch that is comparable in appearance and flavor to cornstarch prepared under similar conditions. The con-
ditions required for the production of sweet potato sirup do not differ greatly from those required for the industrial conversion of cornstarch.
Literature Cited (1) Balch and Paine, I N D . ESG. C H E W ,23, 1205 (1931). ( 2 ) Caeser and Moore, Ibid.,27, 1447 (1935). (3) Carver, Tuskegee Inst. Ala., Bull. 38 (1923). (4) Delamarre, U. S. Patent 109,991 (Dec. 6 , 1870). (4A) Dieta, private communication, Dee. 1937; Ebling, Ibid. (5) Eynon and Lane, J . Chern. I n d . , 42, 32 (1923). (6) Gore, Chem. A g e (N. Y.), 29, 151 (1921). (7) Gore. J. B i d . Chem., 44, 19 (1920). (8) Gore, 5. S. Patent 1,310,012 (July 15, 1919). (9) Gore, Reed, and Reese, U. S.Dept. Agr., Bull. 1158 (1923). (10) Krauss, Hawaii A g r . Rept., 1922, 55. (11) McDonnell, S. C. Agr. Expt. Sta., BUZZ. 136, 78 (1908). (12) Paine, Yearbook of Agriculture, p. 308, Washington, U. 8 . Govt. Printing Office, 1935. (13) Paine, Thurber, and Balch, IA-D. ENG.CHEM.,30, 1331 (1938). (14) Riemann, U. S. Patent 1,735,976 iNov. 19, 1930). (15) Ripperson, Hawaii Agr. Report, 1922, 38. (16) Thurber, IA-D.EA-G.CHEX.,25, 119 (1933). (17) Ibid., 25, 565 (1933). (18) Thurber, U. S. Patent 2,001,925 (May 21, 1935). , 567 (1934). (19) Thurber and Paine, ISD. E N G .C H E M .26,
NEW DISCOVERIES IN PNEUMATICKS . . . . . . BY
N o t h i n g was sacred to the famous English caricaturist James Gillray. Hence in 1802 there appeared his "Scientific Researches: New Discoveries in Pneumaticks, or An Experiment on the Powers of Air. May 23d, 1802."
This is a burlesque on the Royal Institution, which had been recently founded. Most of the figures are portraits of the more distinguished members of the Institution. The gentleman experimented upon is Sir J. C. Hippesley, M.P. The operator is Dr. Thomas Garnet. The bellows are held by Humphry Davy. To the extreme right Count Rumford is easily recognized, In the circle, begidning with him, are Benjamin D'Israeli (in spectacles), Earl Gower (afterwards Marquis of
VOL. 31, NO. 12
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Stafford), Lord Stanhope, Earl Pomfret, Sir Henry Englefield, Miss Lock, Mr. Sotheby, Mr. Denys with his little boy, his wife, Lady Charlotte Denys, Miss Denys, Mr. Tholdal, and others. The Chemists' Club of New York is very fortunate in possessing this original Gillray print, presented to it a few years ago by the late Sir William J. Pope. This is No. 108 in the Berolzheimer series of Alchemical and Historical Reproductions. Previously shown Gillray drawings are Nos. 27, 28, 69, and 74. D. D. BEROLZHEIMER 50 East 41st Street New York, N. Y.
A complete list of the first 96 reproductions appeared in our January, 1939, issue, page 124. additional reproduction appears each month.
An
