THE MASHING OF CORN Effect of pH and Pressure Cooking R. C. ERNST, C. E. BROWN, AND J. B. TEPE University of Louisville, Louisville, Ky.
In American practice the mash-tub method of mashing is by far the most common. The operation is carried out in a mash tun, sometimes called the tub or key, which is a circular. steel or copper vessel provided with heating and cooling coils and scraper and stirring arms. This tub is operated at atmospheric pressure, and the treatment consists of cooking
NE manufacture of alcoholic beverages is a very old
T
industry. The reactions accompanying the production of alcohol by bacteriological fermentation have been practiced for centuries. Ilowever, the successful economic production of spirits depends largely upon the accurate control of chemical and physical conditions. The essential steps in the production of Adustrial alcohol by fermentation are cleaning and grinding of the raw grains, cooking and mashing, bacteriologicat fermentation of the sugars proMASH TUB (right) duced in mashing to alcom o FERMENTERS (behol, and distillation of the Imu) IN A MODERN fermented mash to sepaDISTILLERY rate the ethyl alcohol from other undesirable products and residue. Maltingand cultivation of yeasts are carried on separately. Mashing is the operation by which the starch of the grains is made solnMe; it involves an extraction of the starch from the grain and a conversion of the starch into sugars. After cleaning, grinding, a n d cooking, conversion by hydrolysis is brought about by enayinic action. Cooking the grain causes the starch g r a n u l e s t o swell and makes the starch available to the subsequent action of the enzymes present in the malt. These enzymes have the catalytic eEect in the conversion of starch to sugar. The extent of enzyme action is governed hy time, temperature, and hvdrogen-ion concintratioi. The conversion of starch into sugar by the action of malt is represented bs the fcilluaing scheme: Starch soluble starch maltose + dextrin (diastase) Msltose dextrin and + dextrin (diastase)* Maltose dextrose (maltase)
-+ -
,,ralt, and under varying conditions of time and temmrature.. dependinrr . I w.o n the mains used. The cooker-mash method is carried out in a pressure cooker and has the advantage of saccharifying more starch than the open mash-tub methods. The grain and water mixture is treaded in a cylindrical steel plate vessel provided with both a ,,liXt,,rP "f
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1248
steam supply and vacuum pump. The temperature is raised to over 250" F. with 50-60 pounds pressure and then the temperature is reduced by means Of the vacuum to about 153' F. The starch granules burst under this treatment and form a pasty mass which is readily converted when cooled to 148' F. and when malt is added. The hand-process method Of mashing is Of Only secondary importance; hand agitation, lower temPeratUreS, and longer periods of time are required for reaction, The methods of mashing described are fundamental, but variations and modifications are in use. Fermentation and distillation of mashes containing the same amount of corn but subjected to different degrees of conversion, yield quantities of alcohol which are dependent upon the final amount of sugar in the mash.
minutes and filtered, and the amounts of dextrose and dextrin p r $ ~ \ ~ ' ~ ~ . d emilliliters ~ ~ ~ ~of ~thev efiltered solution were diluted with distilled water t o 250 ml., and the resulting solution was titrated against 25 ml. of standard Benedict's solution. This indicated the amount of dextrose present. TOTAL DISSOLVED CARBOHYDRATES. A 25 ml. sample of the su ar solution was diluted with distilled water to 100 ml. and refuxed for 30 minutes with 3 ml. of concentrated hydrochloric acid. This converted the dextrins to dextrose. The dextrose solution was then diluted to 500 ml. with distilled water and titrated against 25 ml. of Benedict's solution. This indicated the amount of dextrose and other carbohydrates originally present. Grain was analyzed by the method of Leach.'
Data The effect of varying the p H on the conversion of starch is shown in Table I and Figure 1.
TABLEI. EFFECTO F Run No. PH Carbohydrates, grams CnHioOs per 100 ml. of mash: Total Soluble Convertedtodextrpse Convertedtodextrin 3
4 pH
5 6 VALUE
r
VOL. 31, NO. 10
VARYING THE
1 1.3@
... 0:644
...
2 3.16
PH
3 4.04
ON THE
4 5.61
CONVERSION 5 5.87
6 6.40
OF
STARCH
7 7.00
8 8.13
14.10 14.10 14.10 14.10 14.10 14.10 14.10 10.70 10.76 11.55 11.20 9.90 9.82 8.69 5.00 5 . 1 5 5.65 5.48 5.31 4.83 2.80 5.70 5.61 5.90 5.72 4.59 4.99 5.89
The solution was no4 filterable, and soluble carbohydrates could not be determined: dextrose was determined in the presence of the starch. a
8
This investigation was undertaken to determine the optimum pH for the diastase conversion of a corn mash and the effect of pressure cooking the corn prior to mashing. The p H of the mashing medium was varied from 1.30 to 8.13, and the pressures of cooking studied were from atmospheric to 50 pounds gage. The percentage of reducing sugars and the total soluble extract resulting from mashing under each condition were determined.
Procedure In determining the optimum pH for the diastase conversion of a corn mash, mixtures of the following composition were treated: 157.5 grams ground corn, 17.5 grams malt, and 700.0 ml. distilled water. The distilled water was boiled to expel the dissolved carbon dioxide, and a calculated quantity of 0.1 N hydrochloric acid or 0.1 N sodium hydroxide was added t o give approximately the desired pH. The exact pH was determined by a quinhydrone electrode. The water was heated to boiling and the corn then added with constant stirring. The mixture was maintained at the boiling temperature for 30 minutes with continuous agitation. The resulting paste was then cooled to 65" C. and the malt was added. A temperature of 62-65' C. was maintained for 30 minutes, and the mixture was cooled in a cold water bath for 15 minutes. The soluble dextrins and dextrose were separated from the insoluble material by filtration. During one run the pH value was determined every 10 minutes after the corn was added and every 10 minutes during mashing. A t the start of the run the pH was 5.1. Thirty minutes after the corn was added, the value had increased to 5.30. After melt was added, the pH remained at 5.70 during the remainder of the run. In determining the effect of pressure cooking the corn prior to mashing, the following charge was treated: 8.25 pounds (3.74 kg.) corn, 0.92 pound (0.42 kg.) malt, and 36.50 pounds (16.56 kg.) city water. The corn was added t o 31.5 pounds (14.29 kg.) of water in the pressure cooker with constant agitation. Steam was introduced into the jacket, and cooking was carried out at different temperatures for various lengths of time. After cooking was completed, the steam was shut off and cold water was run through the jacket until the temperature of the mixture reached 70" C. At this point the malt, mixed with 5 pounds (2.27 kg.) of water, was added and a temperature of 62-65" C. was maintained for 30 minute&. The mash was cooled for 5
There is no change in the amount of conversion as the pH is raised from 3.16 to 5.00. The amount of carbohydrates in solution increases rapidly when the pH is raised above 5.1. An almost constant value of carbohydrates in solution occurs between 3.16 and 5.00, and a maximum is reached at 5.6. At this point the soluble carbohydrates change from 10.70 to 11.55 grams per 100 ml., the dextrose from 5.10 to 5.65, and the dextrin from 5.65 to 5.90. As the p H value increases from 5.6 to 6.4, the amount of soluble carbohydrates decreases rapidly while the amount of dextrose decreases slowly. The amount of dextrin remaining in solution reaches a minimum of 4.59 grams per 100 ml. at a pH of 6.4; the amount of dextrose is 5.31 grams per 100 ml. When the pH rises above 6.4, the amount of dextrose decreases much more rapidly than the amount of soluble carbohydrates until, at a p H of 7.0, the amount of dextrin (4.99 grams per 100 ml.) is slightly higher than that of dextrose (4.83 grams per 100 ml.). At a pH of 8.13 there are 8.69 grams of soluble carbohydrates per 100 ml.; the amount of dextrose has decreased to 2.8 grams per 100 ml., leaving 5.89 grams of dextrin per 100 ml. The effect of pressure cooking the corn prior to mashing is shown in Table I1 and the results are plotted on Figures 2, 3, and 4. Within the pressure range studied, the amount of soluble carbohydrates produced is a straight-line function of the length of time that the corn has been cooked. The concentration varied from 3.92 (cooked a t atmospheric pressure for 10 minutes) to 12.6 grams per 100 ml. (cooked a t 50 pounds pressure for 40 minutes). When cooked from 10 to 40 minutes a t atmospheric pressure, the dextrin and dextrose varied from 2.23 t o 4.62 and from 1.69 to 3.63 grams per 100 ml., respectively, and did not show any regularity. When cooked a t pressures of 15, 30, and 40 pounds per square inch, the dextrose approaches a maximum of 3.55, 4.32, and 5.70 grams per 100 ml. a t the respective pressures. 1 "Food Inspection and Analysis," New York, John Wiley & Sons, 4th ed., 1920.
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TABLE 11. EFFECTOF PRESSURE COOKING OF CORNPRIOR TO MASHING Run No.: 9 10 11 12 13 14 15 16 17 18 19 Pressure 30 30 15 30 15 15 Lb./sq. in. {Atm. Atm. Atm. Kg./aq. om. A t m . ) lf,l 1.1 1.1 1.1 2.2 2.2 2.2 Time of cooking, 20 30 40 15 20 30 40 10 min. 10 20 30 Carbohydrates, grams CsHioOa per 100 ml. of mash: 14.1 14.1 14.1 14.1 14.1 14.1 14.1 14.1 14.1 14.1 Total 14.1 Soluble 3.92 5.50 7.00 8.25 6.45 6.50 7.83 9.15 6.48 7.49 9.06 Converted t o 1.69 2.91 4.60 3.63 2.50 3.30 3.45 3.55 3.24 4.00 4 35 dextrose Converted t,o dextrin 2.23 2.59 3.00 4.62 2.95 3.20 4.38 6.60 3.24 3.49 4.71
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0
f 4
I&
0
t
0
21
22
23
24
25
40 2.8
40 2.8
40 2.8
40 2.8
50 3.5
40
10
20
30
40
40
14.1 14.1 14.1 14.1 10.50 8.55 10.00 11.10
14.1 14.1 12.35 12.6
4.32
3.21
5,35
5.47
5.70
5.95
6.18
5.34
4.65
5.63
6.65
6.65
its maximum value even though the concentration of dextrin, from which dextrose is formed, is at a minimum. When the pH of the water is approximately 1.3, the production of dextrose has nearly ceased. The optimum pH of the water for the conversion of starch to sugar lies between 5.4 and 5.8. The pH of the mash remains constant after the malt has been well mixed with the paste.
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FIG.3. CONVERSIONOF STARCH TO DEXTRIN
210
I5
OF C O O K I N G
30 IMINUTES)
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The dextrin approaches a straight-line function of time after 20 to 25 minutes of cooking a t 15, 30, and 40 pounds per square inch. The advantage of mashing for 60 instead of 30 minutes is shown in Table 111. The amount of dextrose was increased from 3.30 to 4.53 and from 2.91 to 4.12 grams per 100 ml.; the amount of soluble carbohydrates was increased from 6.5 to 8.05 and from 5.5 to 7.33 grams per 100 ml. in the two runs studied.
TABLE111. EFFECTOF
MASHINGON CORN
60-MINUTE
VERSION OF
Run No. 10 Time of mashing, min. 30 Carbohydrates, grams CaHloOs er ml of mash: goluble 5.5 Converted to dextrose 2.91 Converted to dextrin 2.59
10 60 7.33 4.12 3.21
14 30 6.5 3.3 3.2
THE CON-
14 60 8.05 4.53 3.53
Variation of pH The activity of both enzymes, maltase and diastase, increases as the acidity of the mash is increased from a slightly basic solution (pH 8.0) to a neutral one. The activity of the enzyme maltase increases more rapidly than that of diastase, as shown by the decrease in the amount of dextrin present. The activity of diastase begins to increase rapidly after a pH of 6.5 and reaches its optimum a t about 5.5. At this pH the concentration of dextrose is also a t a maximum. The optimum pH for the enzyme maltase is somewhat higher. At a pH of 6.4 the concentration of the dextrose approaches
Pressure Cooking When corn is cooked a t constant pressure for various periods, the amount of starch that can be converted by allowing diastase to act upon the paste for 30 minutes is a straightline function of the time the corn has been cooked, so long as cooking has not proceeded longer than the time required to convert all of the starch. On the other hand, the dextrose reaches a maximum after about 25 minutes of cooking at the pressure studied and seems to be limited by the activity of the enzyme. Considering time as a constant, t h e a m o u n t of starch converted and the amount of dextrose produced increase rapidly up to a pressure of about 40 pounds per square inch. Increasing t h e pressure from 30 to 40 p o u n d s p e r square inch, and cooking f o r 20 minutes in each case, b r o u g h t about a 37.5 per cent increase in the concentration of 3510 IOI 20 30 40 dextrose. When TIME OF C O O K I N G I M I N U T E S ) the pressure was increased t o 50 pounds per square inch and cooking was carried on for 40 minutes, the concentration of dextrose increased only 9.3 per cent above the amount obtained by cooking the corn for 20 minutes a t 40 pounds per square inch. Therefore, pressure cooking the corn a t about 40 pounds per square inch for 20 or 25 minutes would be most economical. Allowing the action of the enzymes to continue for one hour gives a marked increase in dextrose production and starch conversion. Acknowledgment The authors wish to thank J. L. Herin and M. Mattingly for assistance in obtaining the data reported, and F. M. Shipman of the Brown-Forman Distilling Company for the donation of materials used in experimental work, pictures presented, and his kind suggestions offered concerning this work.
