INDUSTRIAL A N D EhTGIiVEERING CHEMISTRY
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VOl. 15, No. 9
Formation of Maltose in Sweet Potatoes on Cooking’ By H.C. Gore FRUITA N D VEGETABLE UTILIZATION LABORATORY, BUREAU OF CHEMISTRY, \vASHINGTON, D.
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KTIL very receatly the finished product. Again, The presence of maltose in cooked sweet potatoes is shown by the W. E. Stone2 was i t is known that as a conagreemenf of ihe quantify of sugars defermined by copper reduction with the quantities measured by polarization and by its isolation sequence Of the autolysis, the only chemist who had reported 011 the sweet potatoes require far in crystalline form. changes in composition of less malt in preparing them Sugar is notformed at the boilingpoint. I t is formed oery rapidly, sweet Potatoes which Occur alihough not instantaneously, during the initial siages of the digesfor SirUP-making, or for during cooking. He showed tioe fermenting and distilling, that dextrin was formed, than other like materials. This fact will cheapen and that, while it was possible to isolate sucrose from raw sweet potatoes by allowing methods of utilizing sweet potatoes industrially as sources it to crystallize from the alcohol extract, sucrose could not of sirup, alcohol, vinegar, or other products requiring the be thus prepared from baked sweet potatoes. digestion of the starch through the use of malt. Thus, I t has been shown by the ~ r i t e r that ~ , ~ sweet potatoes in sirup-making6 a trifling amount of barley malt only, from are rich in diastase and that upon slowly cooking Porto Rico 0.2 to 0.1 per cent of the weight of the potatoes, is required, potatoes nearly all of the.starch changes into dextrin and Since upon cooking sweet pot?toes at least half, and fremaltose. Magoon and Culpepper5found that similar changes quently nearly all, of the starch present becomes converted occur in sweet potatoes during canning. into sugar, our concept of the sweet potato as a starchy food The work described below shows that maltose, the sugar should be revised, since when consumed by man it really always formed in the saccharification of starch by diastase, is saccharine rather than starchy. forms in large quantity during cooking. The increases in It is possible too that, like barley, the sweet potato may sugars calculated as maltose, determined by copper reduction, become an important industrial source of diastase. It has agree substantially with the increases in sugar determined already been shown3 that raw sweet potato tissue retains its by polarization when similarly calculated. Moreover, it is diastase during drying, and that the meal or flour produced possible to isolate crystalline maltose in quantity from the by grinding has a diastatic power at least twice that of barley alcohol extract of cooked sweet potatoes. The sacchari- malt. I n the sweet potato the diastase occurs ready formed, fication is shown not to occur at the boiling point. The evi- and does not have to be developed by the malting process. dence that the formation of the maltose in the sweet potato CHANGESI N COMPOSITION U P O N COOKING is due to diastase is thus fairly conclusive. It is shown AS I N SIRUP-MAKING also that sugar formation in the initial stages of the action of the diastase on the starch of sweet potato tissue, while A satisfactory heat treatment for potatoes in the production not instantaneous, is extremely rapid-e. g., slices of sweet of sweet potato sirup consists of cooking in live steam at potato dropped one by one into boiling water developed about 7 per cent of sugar calculated as maltose during the atmospheric pressure for an hour and a half, allowing the brief period when they were passing through the range a t condensate to run to waste during at least the first hour. which diastase is active, this phenomenon indicating the In the experiment described below Southern Queen sweet close association of diastase with starch in the cells of the potatoes, grown and stored a t the Arlington Experimental were used. They were washed, trimmed by cutting sweet potato. away all dried tissue resulting from healed bruises and withThe foregoing facts have many practical bearings. So far as is now known, they are unique and peculiar to the ered ends of roots, and sampled by cutting sections from a sweet potato. It is evident that the formation of maltose number of potatoes taken a t random. The sample of raw is of paramount importance in the production of delicious potato was ground by forcing through the die of a special cooked sweet potatoes as we know them. The extremely sampling press of Austrian make, known as a “Herles” rapid action of the diastase explains how it is that sweet press.8 The weight of trimmed potatoes was 18 kg. They potatoes develop a fair amount of maltose even in the tissues were placed in a 25-gallon, steam-jacketed copper kettle near the outer surfaces when quickly heated in steam or in and cooked for an hour and a half in a current of live steam rapidly boiling water. Differences in diastatic power be- delivered from a pipe which passed through a slot, in the cover tween different varieties of sweet potatoes, and between of the kettle and terminated near the bottom. During the sweet potatoes grown under different cultural conditions cooking the condensed water flowed away continuously or kept under different conditions of storage, may be related through an opening in the bottom of the kettle. After to such factors as table or canning quality, and thus serve cooking the weight was 17.8 kg. The potatoes were reduced as a guide in the further development of sweet potatoes to a pulp by stirring, and a sample was taken for analysis. The Methods of Analysis of the Association of Official from the horticultural standpoint. It is likely, too, that the information will be of value as a guide in working out the Agricultural Chemists were used. Solids were determined best methods of cooking or canning sweet potatoes, especially by drying to constant weight i n vacuo a t 70” C. Indeterwhen the highest practicable content of sugar is desired in mining carbohydrates, samples of 26 grams were taken. Each sample was washed into a beaker with 95 per cent 1 Presented before the Division of Agricultural and Food Chemistry alcohol and heated to boiling. It was then thrown on a a t the 05th Meeting of the American Chemical Society, New Haven, Conn.,
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April 2 t o 7, 1923. 2 Ber., 28, 1406 (1890). J . Bid. Chem., 44, 19 (1920). 4 Chem. Age, 29, 151 (1921). 5 U.S. DePL Agr , Bull. 1041 (1922).
*
Gore, Reese, and Reed, U.S. Dept. Agr., Bull. 1168 (1923). Thanks are due J. H. Beattie, of the Office of Horticulture and Pomology of this department, for the generous supplies of typically cured sweet potatoes used in this work and in all the subsequent experiments. 8 A similar press is described by Clark, THISJOURNAL, 9, 788 (1917). 6
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INDUSTRIAL AhTDENGI NEERING CHEMISTRY
September, 1923
suction filter, the pulp on the filter washed with alcohol and returned to the beaker. Here it was heated with a fresh portion of alcohol and the extraction process repeated several times. The extracts were combined, a little calcium carbonate added, and the whole was evaporated on the steam bath to a thin sirup. This was made up nearly to 100 cc. with water, a little lead acetate solution added, and the volume completed to 100 cc. After filtering, the excess of lead was removed by dry potassium oxalate. Reducing sugar before and after inversion with hydrochloric acid was determined by the method of Munson and Walker. Starch (together with dextrin insoluble in 95 per cent alcohol) was determined by heating the exhausted pulp with hydrochloric acid by the official method and estimating the reducing sugars formed. The results are given in Table I. TABLEI-SUGAR FORMATION ON COOKING SWEETPOTATOES ReducINCREASEIN Ing SUGARCALCUSu- Sugar LATED AS MALcrose Calcd. TOSE DETERby as MINED B Y POLARIZATIONS Polari- MalReduc- PolarizaDirect Invert zation tose tion tion DESCRIPTION V. V. % % % % Southern Queen Rawa 2 851 0 00 2 1.5 0 23 Cookedin steamb 26.702 2 3 . 3 2 . 5 6 11.93 1i:iO l i : 4 6 Nancy Hall Raw 1.858 0.39 4.52 0.39 ... . .. Baked No. I C 18.98e 1 6 . 4 5 5 . 0 6 19.69 ... ... Baked No. 2c 16.2e 14.2 4 . 0 4 17.57 .. ... Baked No. Id 15.60 13.5 4 . 1 6 16.19 15.8 17.2 Baked N o . 2d 12.5e 11.0 3 . 1 2 13.56 13.17 1 3 . 3 Southern Queen Raw 2.851 0.23 Whole boiled 32.W 15.02 14:79 14:4 Sliced boiled 17.21 ... .. 7 51 7.28 6.9 Southern Queen Cubed / I in., raw 1.5e , , 0.34 ... . . Cubed 3/1ein., heated 7.2e ,. 8.32 7.98 7.13 5 minutes .. 7.82 7.48 6.94 Cubed 8/16 in., heated 7.056 1 hour Nancy Hall ... Cubed 1/6 in., raw 1.96 . .. ... ... 3.25 Cubed '/e in.. heated 4 . 5 6 .. .. ... ... 1 minute Cubed 1 / 6 in., ground 2 35, *. 0.56 and added to boiling water a Solids 33.90. starch 22.85 per cent. b Solids: 35.303 starch'and dextrin, 11.77 per cent. o On basis of sample as analyzed. d On basis of raw weight. e 10-gram sample in 100 cc. f 26-gram sample in 100 cc.
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.... .. .. .. . .
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... ...
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A large proportion of the starch was digested, the percentage changing from 22.85 to 11.77 per cent. As a considerable proportion of dextrin was included with the starch, the actual quantity of starch was lower than indicated by analysis. The sucrose content did not change materially. A large increase in reducing sugars estimated as maltose (11.70 per cent) occurred. The increase in polarization calculated as maltose was 11.46 per cent. According to B r ~ w n e the ,~ normal weight of maltose-that is, the amount of maltose which if dissolved in water and made up to 100 cc. would polarize a t 100" V. in a 2-dm. tube-is 12.506 grams. Accordingly, a solution containing 1 gram of maltose in 100 cc. will polarize a t 8" V. The increase in polarization observed-23.85 degrees corresponds to 2.98 grams per 100 cc., or to 2.98 X 100 divided by 26, or 11.46 per cent.
BAKING Three large, typically shaped Xancy Hall potatoes were used. One was used for analysis in the raw state. Samples of tissue were cut from various portions, reduced to pulp in the sampling press, and sugars determined in 10-gram samples by the method described above. The other two potatoes were each fitted with a thermometer reaching to the center of the potato. The baking was done in an electric baking oven previously heated to 200" C. and maintained at this temperature during the baking. At the end of 23
* "Handbook
of Sugar Analysis," 1912, p. 197.
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minutes the temperatures recorded were 48" and 49" C., respectively. At the end of an hour the temperatures were 92" and 94' C. At the end of 77minutes thetemperatures were 99" and 100' C., when the potatoes wereremovedand allowed to cool. The raw potato used for analysis weighed 441 grams. The two baked potatoes weighed 450 and 396.5 grams, respectively, before baking, and 370 and 306 grams after baking and cooling. None of the tissues became carbonized. Each baked potato was ground for analysis by passing it through a meat chopper, and lo-gram samples were then taken. The sugars were extracted by the alcohol exhaustion method already described. The analyses of the baked potatoes are given both in terms of the sample as analyzed and calculated to the basis of the original weight before baking. The results are shown in Table I. The sucrose content remained substantially unchanged. A very large increase in percentage of reducing sugar reckoned as maltose occurred, amounting to 15.8 and 13.17, respectively. These values agree fairly well with the figures for the increase in the percentage of maltose determined by polarization, 17.20 and 13.3, respectively.
COOKING OF WHOLE AND SLICED SWEETPOTATOES One kilogram of pared Southern Queen potatoes, consisting of three large roots, was covered with water in a sauce pan, and heated. Nine minutes were required for the water surrounding the potatoes to come to the boiling point. They were boiled for an hour and a half, occasionally replacing the water which evaporated with fresh portions of boiling water, thus keeping them covered a t all times. The water and potatoes were then mashed together to form a smooth pulp and weighed. Samples weighing 45.77 grams, corresponding to 26 grams of the original potato, were taken, and reducing sugar was determined by the methods described above. Another lot of cooked potatoes was prepared at the same time by washing and paring three large potatoes and slicing inch thick in a slicing machine. One kilogram from l / ~to of the slices was used. They were cooked by dropping them one by one into boiling water in a sauce pan, kept boiling while the slices were being added. Four minutes were required for adding the slices. They were then boiled for an hour and a half. The cooked slices were reduced to pulp in the water in which they were cooked, weighed, sampled, and analyzed, the sample weight used being 54.34 grams, corresponding to 26 grams of the original potatoes. The results are shown in Table I. The potatoes cooked whole contained 15.02 per cent of reducing sugar calculated as maltose, but half as much reducing sugar, 7.51 per cent, was found in the cooked sliced potato. The higher sugar content in the potatoes cooked whole can be ascribed to the fact that the potato tissues were kept for longer periods within the temperatures a t which the diastase was active than in case of the sliced potato dropped into boiling water. The most striking feature, however, is the formation of so large an amount of reducing sugar during the few moments required to heat the slices as they were added to the boiling water. Thus, 7.51 per cent of reducing sugar as maltose was found in the cooked slices, but 0.23 per cent being present in the raw potato before cooking. There remained the possibility, however, that more or less sugar formation might have occurred during the boiling period. Accordingly, the experiments described below were made.
COOKINGFINELYDIVIDEDSWEETPOTATO Southern Queen sweet potatoes were cut into small cubes and 10-gram samples were heated for 5 and 60-minute periods
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IATDUXTRIAL A N D ENGINEERING CHEMISTRY
in boiling water. The cubes were prepared in a slicing mainch in each dimension. chine and were approximately Determination of the sugars present in the raw cubes was made by finely crushing 10-gram samples in a portion of water with a little alcohol, washing into a beager, and boiling with alcohol, throwing on a suction filter and thoroughly exhausting the residue by returning it to the beaker and boiling with alcohol, again filtering, and repeating the exhaustion of the potatoes. The alcohol extracts were combined and the determination was finished as already described, the de-alcoholized solutions being diluted to 100 cc. for polarization. In the heating tests, 10-gram samples of the cubes TGere transferred to 400-cc. beakers. Each portion was then treated with 50 cc. of boiling water, set immediately on a burner and quickly heated to boiling, from 50 seconds to 1 minute being required. The boiling was then continued for 4 minutes longer in one test. I n the other the total heating period was 1 hour, during which time the volume of water was kept nearly constant by the occasional addition of boiling water. At the end of the heating period, 2 volumes of 95 per cent alcohol were added to each beaker, the liquid was decanted off , and the cubes were transferred to a mortar and finely crushed. Liquid and pulp were then returned to the beaker, boiled, the liquid filtered off, and the pulp extracted twice by boiling with alcohol. The extracts were collected, evaporated, and sugars determined by methods already described. The raw potatoes contained 0.34 per cent of reducing sugar calculated as maltose, the cubes cooked for 5 minutes in boiling water contained 8.32 per cent, and the cubes cooked for 1 hour, 7.82 per cent. Thus, no sugar formation occurred at the boiling point. The experiment again demonstrated the extremely rapid saccharification which occurs when sweet potato tissue is heated through the temperature range a t which the diastase acts.
INSTANTANEOUS HEATIXG OF SWEETPOTATOES Very small cubes, about '/e inch in each dimension, of Nancy Hall potato were prepared. Sugar was determined in 10-gram portions of the raw cubes by crushing in a mortar in alcohol and completing the determination in the manner already described. Two heating tests were made. I n one, a 10-gram portion was added to 25 cc. of boiling water contained in a beaker, set over a burner, and strongly heated. Thirty seconds were required for the resumption of boiling after the 10-gram portion had been added. The boiling was continued for 30 seconds longer, when the contents of the beaker was transferred to a mortar and 50 cc. of 95 per cent alcohol added. The cubes were then crushed, the sugars extracted and estimated, as previously described. In the other test, a portion of the fresh cubes was forced through the Herles press. Ten grams of the resulting pulp were mixed with 25 cc. of water and slowly poured with constant stirring into a beaker containing 50 cc. of actively boiling water. Two volumes of 95 per cent alcohol were then added. The sugars were then extracted and estimated in the usual manner, the polarizations only being determined. The results are shown in Table I. During the heat treatment lasting for 1 minute, 3.2 per cent of reducing sugar calculated as maltose formed in the potato tissue. When the rate of heating to boiling was increased still further by adding the suspended pulp slowly to actively boiling water, but 0.56 per cent of reducing sugar similarly calculated was formed. PREPARATION OF CRYSTALLINE MALTOSE Three carefully washed and trimmed Nancy Hall sweet potatoes, weighing 1 kg., were baked for 11/*hours in an electric baking oven at about 200" C. They were then removed
VOl. 15, No. 9
and allowed to cool. They then weighed 781 grams. They were reduced to pulp and 200 grams were exhausted with four successive portions of alcohol. I n making the first extraction 1 liter of 95 per cent alcohol was used. Five hundred cubic centimeters of 90 per cent alcohol were used in each of the three subsequent extractions. In making each extraction the alcohol and pulp were digested in the steam bath under a return condenser for an hour, allowed to cool, and then filtered on a Buchner funnel with suction. Each extract was evaporated to a thin sirup, transferred to a crystallizing dish, and allowed to stand with occasional stirring in a desiccator over sulfuric acid. Ten days later a very fine, white, crystalline deposit had formed in the second extract. It slowly increased in amount during the next two days, when it was filtered off with suction, and the crystals, after sucking as free from mother liquor as possible, were washed on the filter with alcohol until the washings were colorless. They were then spread out on a watch glass and dried to constant weight over sulfuric acid. The filtrate and washings were evaporated to a thin sirup and placed over sulfuric acid. During the next few days another crop of crystals gradually formed. This was recovered as described above. Crystals began to separate from the first extract 16 days from the time it was placed in the desiccator, and slowly increased in quantity until 3 days later, when the crop was filtered off, washed, and dried as described above. The third and fourth extracts were similarly treated. The filtrates and washings from these three extracts were combined, evaporated to a thjn sirup, and another crop of crystals was subsequently recovered. The total weights of crystals obtained from the 200-gram portion 6f baked sweet potato and the specific rotatory power of each of the crops are given below. DESCRIPTION First extract.. Second extract.. ....................... Third extract., ........................ Fourth extract.. ....................... Mother liquor of second extract.. Mother liquor of remaining extracts, .....
.........................
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Weight G.
Specific Rotatory Powera
9.437 1,451 0.831 1.172 0.282 3.999
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I a1
130.3 131.4 130.2 108.6 128.5 97.9
TOTAL .......................... 17.172 a Determined in a micropolariscope tube, 1 dc. long, the solutions being heated t o the boiling point t o destroy mutarotation and cooled before making up t o t h e final weights. T h e specific rotatory power of crystalline maltose is [aID = 131.34 (calculated from data given by Browne, LOG.d . ) .
Thus, 8.59 per cent by weight of the baked sweet potato, equal to 6.71 per cent of the weight of the fresh potatoes taken, was recovered in the form of crude, crystalline maltose. The reducing power of crystals from the first extract was determined by the Munson and Walker method. Two 200-mg. portions gave 0.2397 and 0.2401 gram of cuprous oxide, corresponding to crystalline maltose of purity 98.8 and 99.0 per cent, respectively. The maltose was recrystallized from alcohol and compared by Edgar T. Wherry, of this bureau, with a sample of pure maltose made from starch by 0. S. Keener of this bureau. The refractive indices determined by the oil immersion method and other indications were found to be identical, the indices being as follows: y axis - 1.550 CY axis - 1.515
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