DEHYDRATED SWEET POTATOES FOR ETHANOL PRODUCTION J. A. JUMP, A. I. ZAROW, The scarcity of grain for purposes other than food has caused the distilling industry to seek substitute material for the production of industrial alcohol. The sweet potato has received special consideration since i t is high in carbohydrates and is very productive in terms of tons per acre in certain sections of the South. Puerto Rico and L-4-5 dehydrated sweet potatoes were fermented in the laboratory to determine a practicable procedure for a plant trial. High alcohol yields were obtained when the final mash contained 33.370 by volume wheat-milo stillage. The L-4-5 potato gave somewhat better alcohol yields than the Puerto Rico. The use of sweet potato stillage, or the complete elimination of grain mash stillage, materially reduced the alcohol yield. The shredded, dehydrated potato could be mashed without milling if the fermenters were set at a concentration of 45 gallons of mash per bushel of grain. Pressure cooking of the potatoes failed to increase the alcohol yields significantly. A plant trial resulted in yields of 4.77 proof gallons per bushel with Puerto Rico and 5.44 with L-4-5 potatoes. The laboratory and plant work proved that dehydrated sweet potatoes can be mashed successfully in a grain distillery without change of equipment and without an admixture of grain other than barley malt for conversion purposes.
HE present shortage of corn and the possible scarcity of
T
wheat for purposes other than food have caused those engaged in the distilling industry to investigate promising substitutes (2). Two other factors of perhaps greater importance have stimulated the search for new sources of raw material. There is good reason to believe that postwar ethanol require ments may be far in excess of prewar needs. I n this event molasses will not be available in sufficient quantities. High grain prices do not favor continued industrial alcohol production from grain, However, grain will continue as the principal raw material for beverage alcohol production. For these reasons several organizations are reviewing the entire field of raw materials. They include, in addition to grains and molasses, waste sulfite liquor, wood hydrolyzates, Irish potatoes, and sweet potatoes. Sweet potatoes appear to offer most promise from the standpoint of carbohydrate yield per acre, possibility of crop expansion, and ease of handling and processing. Table I presents comparative data on sugar cane, sweet potatoes, and corn in terms of crop, carbohydrate, and alcohol yield per acre of cultivated land. Average national figures have not been employed since it would be illogical to cultivate these crops for market in areas incapable of good production. The sugar cane yield of 22 tons per acre is the Louisiana average for 1938 (4, reported to be a good year. This is lower than is generally obtained in Cuba and Puerto Rico. The 52-bushel-per-acre yield of corn is reported ct8 the Illinois average by the Department of Agriculture (4,and 400 bushels per acre for sweet potatoes is a conservative figure based on Kimbrough’s work (1) when scientific methods are employed. Kimbrough reports yields as high as 638 hushels per acre. A comparison of monetary returns is of interest. Based upon the data in Table I and assuming a molasses cost of 10 cents per gallon delivered (estimated postwar
AND
W. H. STARK
Joseph E. Seagram & Sons, Inc., Louisville, K y .
cost) and 2.5 gallons of molasses per gallon of alcohol produced, corn would deliver a t $0.708 per bushel to compete and whole sweet potatoes a t $0.311. Sweet potatoes for dehydration (assuming $5.00 per ton dehydration costs) would deliver a t $0.259 per bushel. The gross return t o the producer, less transportation and handling costs to the distillery per acre, would be for corn, whole sweet potatoes, and dehydrated sweet potatoes $36.80, $124.40, and $103.60, respectively. Variations in by-product credit and differences in processing methods, which are important, have not been taken into consideration since they are beyond the scope of this paper as are comparisons of cultivation costs. These calculations demonstrate the economic advantages of sweet potatoes. The dehydrated potato might well be able to compete with $0.06 molasses. There are obvious disadvantages to the use of the whole sweet potato. The most serious is the fact that it is a seasonal crop, with the exception of extremely limited areas in the far South where year-round cultivation is considered feasible. A second difficulty is the necessity of installing special equipment for the slicing or pulping of potatoes prior to cooking and new problems of conveying in distilleries designed for grain. Those considerations have indicated the desirability of dehydrating the crop, which reduces possibilities of spoilage and results in a product that may be stored or shipped economically and can be handled by a grain distillery with little or no additional equipment. Although this paper deals only with dehydrated sweet potatoes for industrial ethanol, some preliminary work has been done with the 2,3-butylene glycol fermentation process; obviously sweet potatoes should be considered as a raw material for other fermentation processes such as butanol. The studies reported here were made upon the yellow Puerto Rico variety, an important market type, upon a highly productive variety known as the L45,developed for starch processing
OF ALCOHOL YIELDPER ACREOF CORN, TABLE I. COMPARISON SUGARCANE,AND SWEET POTATO
Crop Corn Sugar cane Sweet potato
Locality Illinois Louisiana Louisiana
Crop Yield per Acre 52 bu. b 22 ton: 400 bu.
Carbohydrate Alcohol (Glucose). Wine Gal:/ Lb./Acre Acrea 2000
6300 6600
147 462 497
a 190° proof (90% fermentation effioiency assumed). b 56-lb. bushel. e
60-lh. bushel.
OF DEHYDRATED TABLE 11. STARCHAND MOISTURECONTENT SWEETPOTATOES Variety L-4-5 Puerto Rico Puerto Rico Puerto Rioan culla
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Moisture,
Form Shredded Shredded Diced Shredded
%
6.88 8.40
9.18 6.79
Starch (Dry Basis),
% 70.3 71.0
71.0
71.8
December, 1944
INDUSTRIAL AND ENGINEERING CHEMISTRY
and cattle feed but not suitable for food use, and upon Puerto Rican culls of the 1942 crop. The first two were from the 1943 crop. These investigations were made prior to a commercial trial in one of the small distilleries and were necessarily concerned with the establishment of the primary conditions for successful fermentations a t that plant rather than exhaustive studies of single variables to determine optimum conditions. LABORATORY FERMENTATIONS
All laboratory fermentations were made in accordance with the technique developed by Stark, Adams, Scalf, and Kolachov (3) with the exception of minor modifications in mashing methods, gallonage, etc., which will be noted. These modifications were introduced either to adapt the method to the requirements of the material being mashed or to similar conditions imposed by the equipment of the plant a t which the trial was to be conducted, With the exception of one lot of Puerto Ricos which were diced prior to dehydration, all of the potatoes were shredded so that the dehydrated product was in the form of small sticks about l/8 to inches in length. There were inch in diameter and up to occasional small roots and pieces of irregular shape. Milling tests revealed that these potatoes could be handled satisfactorily in three high-roller mills, but that there would be relatively high flour losses; for this reason it was deemed inadvisable to mill the potatoes, and the first laboratory experiments were made with unground potatoes. A number of trial cooks were first made to give an indication of the viscosity of the mash a t different concentrations and the total sugar present in the set fermenter under the various conditions. From this preliminary work it was determined that, when stillage was used in the cooking and as backset, it was advisable to cook and set with a mash of high gallonage or relatively low concentration. Except where noted, the cooks were made at 35 gallons of water and stillage per 50-pound bushel of potatoes, and the fermenters were set at a ratio of 45 gallons of mash per bushel of grain. The appearance of the mash was deceptive when it waa judged in comparison with a grain mash, as its thickness was custard- or gel-like rather than viscous or sticky. The starch content of the several types of potatoes was closely similar but the moisture varied as shown in Table 11. The Puerto Rico variety in particular was markedly hygroscopic in the dehydrated condition. This tendency would have to be controlled carefully by proper storage if it were desired to mill the potatoes. Thin stillage produced at the Louisville plant was used in the cooks of the first laboratory runs. This was believed advisable since it would be necessary to use stillage of this type for the plant scale experiment, as no sweet potato stillage would be available until the end of the plant run. This stillage was from a wheatmilo grain bill which was being currently mashed a t the plant. The first fermentations were made to determine the effect of milling the potatoes upon alcohol yield and to compare the yields obtained from L-4-5 and the Puerto Rican potatoes which were to be used in the plant trial. The two grinds, designated "coarse" and "fine", were equivalent to a corn grind and a malt grind, respectively. Table 111 presents comparative data. The mash bill consisted of 89% potato and 11%barley malt. Water and stillage at a 2 to 1 ratio were heated to 120' F. One per cent premalt and the potatoes were then added, and the temperature was raised to 206' F. in an hour and held between 202" and 208' F. for one hour and a half. The cook was cooled to 145' F. in 5 minutes and half (5%) of the conversion malt was added. -4fter a holding time of 10 minutes a t 145' F., the rest of the malt was added and the mash was cooled to setting temperature. Stillage was then added as backset to bring the total amount of stillage in the mash to 38%. The cooking pH was 5.3. The results of these fermentations &s shown in Table I V demonstrated the superiority of the G 4 5 potatoes under the conditions of the experiment. These data do not indicate that there is any marked improvement
1139
TABLE 111. SIEVEANALYSIS OF SWEET POTATOES, CORN,AND BARLEY MALT
Screen Siae On 12 On 16 Through 60
Sweet Potatoes Coarse Fine 24 3 36 8 4 10
Barley hlslt 2 4 18
Corn 19 60 4
TABLE IV. ALCOHOL YIELDFROM DEHYDRATED SWEET POTATOES, GROUND AND UNQROUNDS Final Datan
sugar
gram) Grind Balling 100 ml. Unground 0.6 0.72 0.6 0.80 Coarse 0:6 0.80 Fine Puerto Rico Unground 1.7 0.78 1.9 0.96 (shredded) Coarse Fine 1.8 0.86 Puerto Rioo Unground 1.2 0.76 Coarse 1.2 0.76 Fine 1.2 0.76 Represent the average of three fermenters. Potato L-4-6 (shredded)
Aloohol ield proof w b u : Wet Dry 6.06 6.60 6.16 6.61 6.61 6.16 6.76 6.27 6.32 6.70 5.76 6.27 6.76 6.33 6.88 6.47 6.89 6.48
'g!
ciency,
%
92.6 94.1 94.1 86.6 87.3 86.7 87 3 89.2 89.3
of yield when the potatoes are ground. It is anticipated that flour losses and milling costs would make i t uneconomical to grind the potatoes.
Examination of small particles of potatoes
remaining in the fermented mash showed that there was no starch even in the innermost parenchyma cells of the particles.
If a less dilute cook were made, it is possible that grinding might produce a better maah, but when fermenters are set a t 45 gallons per bushel, the unground potatoes should prove satisfactory. Since sweet potatoes are relatively low in nitrogen and certain minerals in comparison with wheat and milo, it could not be assumed that the results obtained with stillage from a sweet potato beer would be the same as those obtainable with sweet potato stillage. A series of fermentations were set up in which sweet potato stillage from laboratory fermentations was used. This series waa also designed to demonstrate the effect of pressure cooking compared with atmospheric cooking. Potatoes for these experiments were ground 6ne. The technique of mashing was the same except that 10% malt was used, of which 2% was premalt. The entire amount of conversion malt was added when the cook was cooled to 145' F. and was held for 60 minutes at this temperature. The preeaure cooks were made in exactly the same manner as the atmospheric except that, after cooking at 202208' F. for half an hour, the cook was autoclaved a t 22 pounds steam pressure for an hour. There was no significant difference between atmospheric and pressure cooking, but grain stillage was superior to a water mash (Table V).
TABLEV. YIELDS OBTAINEDWITH WHEAT-MILOSTILLAGE, SWEET POTATO STILLAGE, AND WATERS Final Data sugar, Cookin Ball- grams/ Methog Stillage rng 100 ml. Atm. Wheat-milo 0.5 0.68 Sweet potato 1 9 1.41 None 0.6 1.40 Pressure Sweetpotato 2.4 1.84 None 1.3 1.73 Sweet potatoes mashed (Puerto Rican culls).
Alcohol yield, proof eal./bu.
Wet 6.66 5.61 5.43 5.64 5.47
Dry 6.48 6.12 6.91 6.16 6 95
Plant ciency,
%
94 0 87 0 84 1 87.4 84.9
The increase in yield that is so marked when wheat-milo stillage is used suggests that the addition of growth substances or nutrient salts to a water mash or to a mash in which sweet potato stillage is used might prove worthy of investigation. Work has been directed along these lines, and recent data indicate that this problem has been solved without the use of stillage.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1140
RESULTS OF A PLANT RUN
The plant trial was run at the Midway, Ky., plant according to the procedure used in the first fermentations reported in this paper. Wheat-milo stillage was used, and two fermenters of the -5 and two of the Puerto Rico variety of sweet potatoes were set. The latter consisted almost entirely of the shredded form. There were two departuras from the initial specifications for the run, which were due to the low pH of the Midway plant stillage. It was not possible to put the specified amount of stillage by volume in the fermenters; therefore 6.0 to 10.0% was used. The cooking pH was between 5.00 and 5.40, although 5.30 to 5.60 was recommended. The bonded yield of the two fermenters of Puerto Rico was 4.77 and of the fermenters of G 4 5 waa 5.44 proof gallons per bushel. While these yields were not so high as might have been anticipated from the laboratory results, they were encouraging, especially since it had been necessary to depart from the stillage and pH specificationq Plant operations revealed mash pumping to be no problem, despite the deceptive appearance, this mash had the pumping characteristics of water. Therefore it is believed that fermenter concentrations of 32 to 35 gallons of mash per bushel are in line. These correspond to grain mash concentrations. Samples of sweet potato stillage were evaporated and dried on a laboratdry dryer. This material was analyzed for comparison with distillers’ dried solubles, a valuable poultry feed supplement. Comparative data (dry basis) follow: Distillers’ Dried Solublea (Corn) Protein. 5 Fat. % Ash 96 R i b h a v i n , y/qrani Pantothenic acid. s/pram
30
10-12 6 15-20
26-30
These data indicate potential value as a poultry feed supplement. The protein is low for dairy cattle feed. CONCLUSIONS
Dehydrated sweet potatoes may be cooked and Gonverted under conditions similar to those employed in grain processing. It is evident that less severe cooking conditions are required for sweet potato starch than for cornstarch. Dehydrated shredded sweet potatoes will yield up to 15-20% more alcohol per bushel than can be obtained from high-grade corn. Samples of the G 4 5 variety studied are superior to those of the Puerto Rican variety, on the basis of laboratory and plant alcohol yields. Dehydrated sweet potatoes need not be ground before cooking although milling does increase the alcohol yield slightly. Further research, now in progress, is necessary to obtain the maximum alcohol yield from dehydrated sweet potatoes. The by-product credit picture has been only partially studied, but it is apparent that the by-product may not be equivalent in value to grain alcohol by-products. The dried grains yield per bushel is little more than half that obtained from grain processing, and the protein content is 16% as compared with 30% in corn by-products. LITERATURE CITED (1)
Kimbrough, W. D., La. State Univ., Agr. Expt. Sta., La. RILZI. 348 (1942).
16.4 4.8
(2) Kolachov, Paul, Chemurgic Digest, 3, 24 (1944). (3) Stark, W. H., Adams, S. L., Scalf, R..E.,and Kolachov, Paul, IND. ENG.C ~ E MANAL. ., ED.,15, 443 (1943). (4) U. S. Dept. of Agr., Agricultural Statistics (1940). (5) Williamson, E’. S., La. State Univ., Agr. Expt. Sta., Circ. 25 (1942).
16:3 38.9
PRESENTED before the Division of Agriou!tural and Food Chemistry at the 108th Meeting of the A\rm~rc%v C H F x I c A L S o c r e ~ rNew , York, N. Y.
Sweet Potato Soluble8
PETROLEUM COKE Formation and Properties
T
Vol. 36, No. 12
H E trend of modern cracking processes is towards the use of distillate charging stocks rather than residues. This raises the problem of disposing of heavy residues, both straight-run and cracked. There are two general methods by which such residues may be converted into salable products; one is the addition of hydrogen, the other is the removal of carbon. The hydrogenation of heavy residues is a costly process which has not yet earned genuine profits in peacetime. Coking, on the other hand, is already a well-established refinery operrttion. Hence it appears that, in the immediate postwar period a t any rate, the economic importance of coking processes in the petroleum industry will increase, and larger quantities of petroleum coke will come into the market. Compared with other solid fuels, petroleum coke has received relatively little study. Descriptions have been published of various coking processes and plants, but the emphasis has usually been on the distillates. Mekler (,%‘O), Morrell and Egloff (88), Stroud (86),and others have given typical analyses of cokes from various sources. hlorrell and Egloff also give information on shatter strength, true density, cellular space, and the relative effect of various solvents.
A. G. V. BERRY AND R. EDGEWORTH- JOHNSTONE Trinidad Leaseholds Ltd., Pointe-a-Pierre, Trinidad, B. W. I.
The common meaning of the term “coke” is a cellular residue obtained by the pyrolysis of coal. I n the petroleum industry the word denotes a product similarly derived from oil. Chemists describe as coke any compact carbonaceous residue obtained by the destructive distillation of organic compounds. For the purposes of this paper, coke will be defined as a solid infusible residue obtained by the pyrolysis of organic compounds under conditions such that the residue passes through a plastic st age before becoming infusible. Coke in this sense can be prepared from a wide range of organic materials. The macrostructure of a coke does not depend upon the structure of the parent material, which is destroyed during the plastic stage, but upon the conditions under which pyrolysis is effected. It was formerly believed that coke consisted largely of “amorphous carbon” which was considered to be a separate allotrope. However, x-ray analysis has failed to reveal any such allotrope. On the contrary, it has shown that all forms of “amorphous” carbon, including carbon black, give interference figures similar to those of graphite (8, 3, 6,8-16, 84, 88). Cathode-ray diffraction studies ( d l ) , chemical tests (84), and observations with the electron microscope ( 1 1 ) combine to support the view that coke
