Consistency Changes in Starch Pastes Tapioca, Corn, Wheat, Potato, and Sweet Potato
FIGURE1. CONSISTOMETER
G. V. CAESAR AND E. E. MOORE Stein, Hall & Company, Inc., New York, iV. Y.
A
PREVIOUS contribution (1) described a
new method for obtaining a graphical record of the viscous and plastic changes taking place during the cooking and cooling of relatively concentrated starch pastes. The apparatus by which these changes were recorded was called a consistometer. The consiqtometer consists, in brief, of an electrically heated, jacketed cooker in which the power input to a motor actuating a stirring mechanism a t constant speed against the friction of a paste is recorded as a function of temperature. The present paper describes a greatly improved design of the original model. With this new instrument it is hoped to investigate not only the pasting properties of the commoner starches but as many as possible of the rarer varieties as well. The present paper treats of high-grade commercial sample. of tapioca, corn, wheat, potato, and sweet potato.
Improved Consistometer The new instrument (Figure 1) retains the shape and dimensions of the old : Its well-lagged w a t e r j a c k e t has a capacity,
with t h e i n n e r container in place, of about 5 liters, and is heated from the bottom by a 750-watt heater. X direct-current '/*-horsepower compoundwound motor is mounted horizontally upon a platform projecting from the top of the outer jacket. The motor can be screwed forward to engage a horizontal bevel-gear drive on the shaft of the agitator, or backwards to permit of removal and cleaning of the inner container. Into the hollow and latticed agitator shaft is inserted a Leeds & Northrup resistance thermometer, the leads of which extend to
a Wheatstone bridge calibrated in F. The paste passes freely through the latticed shaft and its average temperature is determined with accuracy. Manual regulation of motor and heater voltage is eliminated. Direct-current voltage across the motor and heater is regulated to about 0.5 per cent by means of a General Electric voltage regulator. The power input required to agitate a paste at constant speed is indicated by a recording wattmeter sensitive to 0.5 watt. The agitator rotates a t 100 r. p. m., a drop of approximately 1 per cent being observed at peak load. As in the original model, the paddles are streamlined. Surprisingly little air embolism has been observed even in very heavy and stringy pastes. A 20-gallon tank provides water at constant temperature for cooling of the paste, the flow being adjusted to a reasonably constant value.
General Procedure For the testing of starch pastes, modified and unmodified, a concentration of 20 per cent was found to be best. At this concentration small differences in the character of the pastes are sufficiently amplified. Starches of unusual swelling power, such as potato, can probably be run with reasonable results a t concentrations as low as 5 per cent; but as a rule a 10 per cent concentration represents roughly the lower limit. At very thin consistencies the consistometer is not as
A new and improved consistometer and technic for the study of the pasting phenomena of starches are described. Charts give the consistency record of pastes of tapioca flour, heavy- and thin-boiling corn, potato, and wheat starches. The usual concentration is 20 per cent, but for tapioca, concentrations of 10, 20, and 30 per cent are given. The temperature range extends from the zone where starch and water constitute a milk, to boiling and back to room temperature. The record covers, therefore, the whole paste history of a starch from cooking to cooling. The degree of degeneration of a starch is sensitively revealed by the form of the curves, where paste temperature is plotted against the net power in watts required to maintain a constant speed of agitation. Each starch thus assumes a characteristic curve form, and a classification into types exhibiting similar characteristics may readily be made. The scientific and practical interest of the method is described. 1447
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INDUSTRIAL AND ENGINEERING CHEMISTRY
sensitive as a properly d e s i g n e d v i s cometer. It c o u l d probably be made so, but the c o s t w o u l d be high. I t s p r i n c i p a l function is t o measure the viscous and plastic c h a n g e s in starch p a s t e s a t any temperature and a t r e l a t i v e l y high concentrations. All tests were run upon bottled samples whose moisture content had been determined. A moisture value of 15 per cent ( b a s e d on bone-dry weight) was adopted as standard. T h e a m o u n t of s t a r c h Paste Temp - De9.F weighed o u t was FIGURE 2. COKSISTENCY RECORD OF TAPIOCA-FLOUR PASTESAT 10 m a d e equivalent to standard m o i s t u r e TO 30 PER CENT CONCENTRATIONS c o n t e n t . A “milk” of definite concentrations by weight was prepared, and one liter was added to the inner container. It is obviously important that the no-load power input to the motor be stabilized and maintained at an approximately constant value throughout the operation of a test. That is, changes in power input must be associated only with changes in paste consistency, not with changes in the motor’s noload requirements. Control of motor temperature is very important, and a standardized procedure was evolved which yielded satisfactory results. Nevertheless, a greater degree of simplicity in no-load control is desirable, and some very practical suggestions to this effect have already been received. During preliminary heating to the major gelatinization temperature zone, the variations in time required for any starch milk to attain a temperature short of the gelatinization zone were less than 1per cent. Above the major gelatinization zone these times differed slightly owing to variable heat conductivity of pastes of different character and consistency; but in no case was the difference more than 3 or 4 minutes in heating to 200” F. The average time required to attain this temperature was 85 minutes. After about 40 minutes, depending upon the major gelatinization temperature, the wattmeter and temperature indicator were closely observed. I n undegraded starches (and in many thin-boiling types also) the first evidence of the begin-
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ning of gelatinization was revealed by a halt in the movement of the galvanometer needle of the Wheatstone bridge. This is an interesting and extremely sensitive phenomenon. It immediately precedes the rise in power input caused by the thickening of the paste and marks what may fairly be called the “gelatinization temperature.” This term has been variously defined with wide discrepancies. Actually, there is no such thing as a definitely fixed and specific temperature of gelatinization for all the granules of a starch. The process may take place over a considerable range of temperature, although it is true that the majority of starch granules usually do gelatinize within a narrow range. Certain of the larger granules, and also those which may have suffered some mechanical disruption, possibly from grinding, may even hydrate slowly a t room temperature. Time is an important factor since the temperature range of gelatinization will vary with time of heating. The writers agree with Ostwald (4) that the lowest temperature in the gelatinization range should be accepted as the gelatinization point of the starch under given conditions of cooking and concentration. (The process of manufacture of a starch exerts a n important influence.) All further references to gelatinization temperature will imply this. The rate of heating in these consistometer tests reasonably approaches commercial conditions of jacket cooking. As soon as a paste commenced to gelatinize, temperature and wattmeter readings were taken a t suitable intervals, the wattmeter chart subsequently affording reference in the event of a recheck of observations. When cooking characteristics only were being studied, observations were carried to 160-200” F. For a complete consistency record, 30-minUte heating was allowed after the temperature of the paste had attained 200’ F. At the end of 30 minutes when the temperature had risen to approximately 210 O F., the heater switch was opened and cooling water a t 70” F. was circulated through the jacket; this water rapidly displaced the hot water and reduced the paste temperature to 85-80” F. within approximately one-half hour, depending upon the nature of the paste and its heat conductivity. Since the speed of agitation and the rate of flow and temperature of the cooling water were constant, time differences required to cool to a given temperature were strictly related to differences in character of the pastes themselves. Consecutive runs on any given starch checked closely in all respects (Figure 3). Cooling times for different starch pastes did not vary more than 5 per cent. Whenever a paste upon cooling became either too “short” or too L“rubbery,” plastic flow was interrupted and the paste would “break.” This was revealed by a wobbly and uneven wattmeter curve, rising and falling in sudden steps and holding more or less to a constant level. The term “rubbery” best describes a semi-elastic condition characteristic of the pastes of certain starches. The terms “short” and rrlong,’ras applied to starch pastes, warrant
20
V 2.
2.h, w
‘0
5
u 0
RECORD OF 20 PER CENTTAPIOCA-FLOUR PASTES FIGURE3. COKSISTENCY 1 Cooling curve alter cooking to 210’ F.; 2. after copking to 180” F.: 3 after cooking to 160’ F.; 4. from eame sample as pastes 1 to 3, but subjected t o disorganization durmg cooking.
DECEMBER, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY 10 Per Cent
a brief attempt a t definition. Chain length of the amyloses @), as expressed in size of the bundles or oriented groups (micelles), is probably the controlling factor in paste length. I n general, the cereal starches consist of shorter chains than the tuber starches ( 5 ) ,and their*pastes are short. Degeneration (through hydrolytic scission) increases shortness.
Tapioca 1 Tapioca 2
142.6
20 Per Cent
139.04F. 140.0
An interesting comparison of the gelatinization characteristics of two high-grade tapioca flours is shown in Figure 2. I n this series of tests, heating or cooking of the pastes was carried only to 160' F. Comparative swelling power and disruption of granules were the object of this study. The concentrations selected were 10, 20, and 30 per cent by %.eight. Abscissas represent paste temperatures. Ordinates represent consistency in the form of power in watts required t o maintain a constant speed of agitation of approximately 100 r. p. m. Zero watts signifies the no-load of the motori. e., the power required to agibate the starch milk; .power in excess of this no-load value represents the thickening effect caused by swelling of the granules. This applies equally to Figures 3 to 6. At concentrations of 10 per cent the forms of the curves of tapiocas 1 and 2 are practically identical; tapioca 2 begins to gelatinize a t a temperature 0.8" F. higher than tapioca 1. At concentrations of 20 per cent a marked difference occurs in form and in peak body; a t 30 per cent these differences become very pronounced. Also, the temperature of the peaks decreases with increasing concentration. The higher the concentration of solids in a paste, the greater the sensitivity to fundamental differences in the character of the starches. The case is somewhat analogous to the action of a ball mill. Within limits, the larger the number of balls, the more intensive is their abrasion. I n concentrated starch pastes, the swollen granules (cells) tend to grind each other into fragments, an effect particularly enhanced by mechanical agitation. The swollen cells are the balls of the mill. The tougher they are, the slower is their disruption. -4t comparatively low concentrations, such as 10 per cent, the cells are not sufficiently crowded to reveal their true properties; but a t 20 and 30 per cent concentrations any unusual degree of elastic strength and resistance to rupture becomes apparent. For example, the 20 and 30 per cent curves of tapioca 1 rise very steeply, but the curves of tapioca 2 deviate more from the vertical and near their peaks show a pronounced inflection significant of cell rupture upon an ever-increasing scale. I n other words, the latter are definitely more fragile, a fact which might prove to be of considerable interest and importance. Differences in gelatinization temperature (represented on Figure 2 by the point a t which the curve leaves the horizontal) are worthy of comment:
20
y I
*
s 0
Paste
Temperofurr
De?.
F
FIGURE 4. CONSISTENCY RECORD OF 20 PER CENTCORNSTARCH PASTES 1.
Heavy boiling
2, 3.
30 Per Cent 137.4' F. 138.5
The gelatinization point appears to fall as the paste becomes more concentrated. Recollecting that these temperatures represent the point a t which initial creaming occurs, the explanation is more apparent. I n a. 10 per cent milk the thickening effect produced by this creaming or initial hydration is insufficient to promote the latent heat effect resulting a t higher concentrations. There is an average difference of approximately 1" F. in the gelatinization temperatures of the two tapiocas. Variations of as much as 3" or 4" F. may occur between different lots of the same starches. For the purest of high-grade tapioca flours, a gelatinization temperature of about 137" F., assuming a moderately slow heating rate, may be reasonably assumed. At very rapid rates of heating, such as would apply in live-steam cooking, gelatinization temperatures are raised, owing to the time factor. Real differences in temperature of gelatinization may exert a very considerable effect on account of selective water-absorption and the resultant tendency of one starch to rob another. This should be particularly true a t extremely high concentrations. Figure 3 shows the behavior of 20 per cent tapioca pastes variously treated. Four tests were run, all from the same sample. The principal purpose was to study the effect of cooking to different temperatures prior to cooling. Pastes 1 to 3 passed through the same initial cooking stages; cooling was begun on paste 3 a t 160" F., a t 180" on paste 2, and a t 210" on paste 1. Curve 4 shows the effect of the presence of an oxidizing agent, heating being carried to 200" F. prior to cooling. Figure 2 shows the behavior of tapioca pastes to 160" F. Above this temperature a rather abrupt change of slope occurs; the curve shows a marked tendency to flatten until somewhat above 200°, a t which point its slope again increases. Microscopic evidence indicates that this flattening is brought about by approximate completion of the cell-rupturing process; the increasing slope above 200" F. appears to mark the beginnings of a more profound "disorganization." Thus we seem to have two types of thinning in the simple cooking of an undegraded tapioca paste-that produced by cell rupture and that caused by the beginnings of micellar dissociation. This will be further discussed. The terms "degeneration," "disorganization," and "dispersion," as applied to amyloses in organized granule form, may be subject to confusion. Degeneration refers to hydrolytic or chemical scission of the amylose chains (3); dispersion refers to the physical effect upon the micellar aggregates; disorganization refers to the sum total effect. Under acid treatment, degeneration and dispersion go hand in hand , the former promoting the latter; but in chlorination, notably on the alkaline side, there are indications that the purely physical effect may take precedence over the chemical effect.
Tapiocas
'
141.S0F.
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Acid-treated
4.
Chlorinated
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INDUSTRIAL AKD ENGINEERING CHEMISTRY
Paste
FIGURE5 .
COrVSISTENCY
Emp.
-
De3.F
RECORDOF 20
P E R C E P T W H E I T 4ND SWEET-POTATO P A S T E S
S\teet potato
1
I90
FIGURE 6.
COVSISTENCY
1
Wheat
Cornstarches Figure 4 shows the pasting characteristics of four types of pure commercial cornstarches tested in 20 per cent concentration. Curve 1 represents the characteristic form of a heavy-boiling starch; curves 2 and 3 are acid-treated, thinboiling types; curve 4 is a chlorinated starch. In the heating zone various states of disorganization are revealed. The original, more or less undegraded amyloses in organized granule form, of which curve 1 is representative, may be converted by acid treatment into amyloses whose chains are more or less cut by hydrolytic scission (3) of the glucosidic oxygen linkings (curves 2 to 4). If the progress of granule disorganization were followed microscopically, the swollen or balloon-like cells so prominent in paste 1 would appear fragile and disrupted in paste 2, reveal a high degree of disorganization in paste 3, and have exploded to fragments in paste 4. Profound differences are revealed by the consistometer in the pasting behavior of these four starches, differences which could not even be remotely inferred from the usual viscosity tests taken a t or near the boiling point. Differences in the gelatinization temperature of Figure 4 cannot be discussed, owing to the fact that the samples did not have a common source.
1PO
210
paste &nf
2
little rethickening or association occurs until the temperature has fallen to about 150' F., the rise in consistency being again a linear function of temperature, though with a steeper slope than in curve 1. Thus a paste which has been subjected to more than a certain degree of disorganization may exhibit an increased associative tendency in cooling; that is, its tendency when left to cool undisturbed is to form a gel. This will be further discussed under sweet potato and wheat starches.
Examination of the cooling curves shows that the curve of paste 3 rises rapidly to about 120" F. and then inflects rather abruptly, that the average consistency of paste 2 is softer throughout and its curve inflection less abrupt, and that paste 1 exhibits a free-flowing viscous character, consistency varying as a linear function of temperature. In paste 3, which has been cooked only t o 160" F., dispersion of the original cellular organization is in its early stages. Owing to partial disorganization and to the nature of the starch, the paste is tough and rubbery; these properties are augmented by the associative influences of falling temperature until plastic flow is interrupted by a certain degree of discontinuity or "breakage," indicated by a more or less abrupt curve inflection. Paste 2, having been more thoroughly cooked, exhibits only a minor degree of rubberiness; it is more truly plastic. I n paste 1 the granule disorganization brought about by cooking t o the boiling point has been enough to permit (under the powerful agitation to which all these pastes were continuously subjected) a sufficient state of dispersion for the development of viscous flow. In weighing consistometer data it should be kept in mind that thorough and efficient mechanical agitation has a very powerful influence upon the properties of starch pastes. Its effect varies with the concentration and is apparent both in the heating and cooling zones. I n the commercial preparation of starch pastes, inefficient agitation is all too common. The importance of efficient agitation can hardly be exaggerated. Curve 4 illustrates the effect of a suitable oxidizing agent upon a 20 per cent concentration of tapioca from the sample used for pastes 1 to 3. Cell rupture takes place rapidly and completely, followed by a considerable degree of further disorganization, probably both physical and chemical. Very
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170
130
De9 F
RECORDOF 20 ImDorted
P E R C E V T POTATO-STARCH P I S T E S
2
Domestic
110
90
,
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IKDUSTRIAL AND ENGINEERING CHEMISTRY
I n the cooling zone of Figure 4 the consistency curve of the undegraded starch, I, rise. rapidly until the paste has cooled to a temperature of about 110" F. Below this temperature the combined effect of shortness and heavy body induces a discontinuity, revealed by an abrupt inflection. Curve 2 parallels curve 1 approximately to 110" F.; but since its paste is not as short as that of curve 1, no inflection occurs until the temperature has fallen another 10" or more. Curve 3 likewise parallels curves 1 and 2 down to about 100" F. when it too inflects sharply, the paste having become very short. Thus the form and location of these three curves are indicative of the relative characters of their pastes. Paste 1 is a typical cornstarch paste, stiffly plaqtic and short; paste 2 is also heavy but definitely more plastic than 1; paste 3 is of a comparatively dry, brittle nature. If the properties of dried films are reflected by these curves, starch 2 should be the toughest and most flexible. Warp-sizing results tend to confirm such deductions. Starch 4, as shown by the cooling curve, is comparatively thin and fluid a t room temperature.
Sweet-Potato and Wheat Starches The consistency record of 20 per cent pastes of sweet-potato and wheat starches is shown in Figure 5. The sample of sweet-potato starch was made by the Government. The gelatinization point is 145.2" F.-that is, between tapioca and corn. Its curve rises to a peak, in the normal manner, followed by a fall in consistency as the cells rupture. In the cooling zone a certain rubberiness develops, causing an interruption of plastic flow between 120" and 110" F. Curve 2 of wheat starch is especially interesting. It is wholly unlike t h e normal gelatinization curve of the other starches. Gelatinization no longer takes place within the usual range of 8" to 10" F. For a 20 per cent concentration of high-grade wheat starch it extends over a range of about 43" F., commencing a t 142.5" and rising slowly and steadily to a peak at about 185". Microscopic examination of the paste a t its peak shows a characteristic mass of large and small spherical cells practically undeformed by the very severe shearing stresses to which they have been subjected. Their resistance to rupture is remarkable. At about 185" F., however, an abrupt change occurs. The cells begin to shatter rapidly and profoundly (as if an explosive were tossed into their midst), and the effect on consistency is revealed by the sharp downward plunge of the curve. The reasons for this remarkably slow rate of gelatinization combined with high resistance to granule disorganization are not definitely known. The granule size variation in wheat starch is pronounced; the small granules are of the order of size of rice starch. It has been observed that, in a given starch, the larger granules generally hydrate more readily than the smaller ones ( 8 ) , and this may be a contributing factor to the peculiar gelatinization curve of wheat. The small cells also appear to be more resistant to rupture. But there is another characteristic of wheat starch which may well constitute a factor in its unusual behavior: .kcording to Taylor (?) its alpha-amylose content is exceptionally highnamely, 24 per cent. Tapioca and corn have about 15 per cent. Alpha-amylose is very resistant to hydration and, when present in excess, might well retard gelatinization. In the cooling zone, curve 2 lies intermediate between sweet-potato and undegraded cornstarch (curre 1, Figure 4). The slope is less than for corn but the paste is shorter, since the consistency is lower: If the cooling curves of two pastes differ noticeably and both show an abrupt inflection indicative of discontinuity, then the paste having the lowest consistency will have become ( a ) shorter or ( b ) more rubbery than the heavier paste. When a paste is merely plastic and short, a much heavier consistency will be required to cause
1451
discontinuity under agitation than if its nature were rubbery. Thus, the paste of curve 4, Figure 3, though a tapioca and very soft a t 80" F., is fairly short; it has suffered some degeneration and, if allowed to cool unagitated, would trend toward a gel. On the other hand, the paste of curve 4, Figure 4 (a cornstarch), though short, exhibits little inclination to form a gel. The disorganization has been so drastic that its power of a3sociation a t 20 per cent concentration has almost disappeared.
Potato Starches Figure 6 shows the consistency record of 20 per cent pastes of imported and domestic starches. In the heating zone a difference of 7.0" F. is shown in the gelatinization points, the imported "opening" a t 142.6", the domestic a t 135.6". The latter represents the 1owe.t gelatinization temperature which the authors have so far discovered at 20 per cent concentrations. The tremendous peak bodies of these potato-starch pastes reflect unusual swelling power. The relative thinning effect is also pronounced. As shown in curve 1, the fall in consistency from the peak a t 150" F. to the low a t 210.4" is 24.5 watts; in tapioca (Figure 3) the drop is 15 watts. The different character of these two samples is exhibited also in their cooling curves. Curve 1 rises rapidly, and then a t some temperature not far below 170" F., a rubbery toughness develops to such an extent that the paste will no longer flow. Curve 2 shows rubberiness developing much slower. Kevertheless, it is still sufficient to check the plastic flow a t some temperature not far belom- 130" F. For potato starches, reasonably viscous pastes can be obtained only at concentrations lower than 20 per cent. The imported starch obviously would require more water than the domestic sample to yield a paste of similar characteristics. It is interesting to note, however, that a t approximately the boiling point, the consistencies of theqe two samples differ but little. Quite apart from the fact that the usual viscometric measurements must be made a t very low concentrations (this is particularly true for potato starch), the paucity of information afforded by such tests is here especially obviouq. Figure 6 thus provides another illustration of the limitation of viscometric methods when applied to starches. It also shows the variation possible between different sources of the same kind of starch. In conclusion, the difference in form between the curves obtained by the new consistometer and by the old should perhaps be explained. With the old instrument, jacket temperatures only could be accurately measured. I n the original charts ( I ) the curves are laterally distorted. Kevertheless their characteristic forms are so far unchanged that they may readily be identified from familiarity with the truer forms as shown in our present studies.
Literature Cited (1) Caesar, IYD. ENG.CHEM.,24, 1432 (1932). (2) Haworth, DiriBme Conference de 1'Union Intern. de Chimie, 1930, 33-58. (3) Hirst, Plant, and Wilkinson, J . Chem. SOC.,1932, 2375. (4) Ostwald, Trans. Faraday Soc., 9, 34 (1913). (5) Taylor, T. C., private communications. (6) Taylor and Iddle, ISD. END CHEM.,18, 713 (1926). (7) Taylor and Walton, J . Am. Chem. Soc., 51, 3431 (1929). (8) Whymper, J. SOC.Chem. Id.,28, 806 (1910).
RECEIVEDkpril 23, 1935 Presented as part of the Symposium on Starch before the Division of Agrlcultural and Food Chemistry at the 89th Meeting of the American Chemical Society, Ntw York. N. Y , April 22 t o 26, 1935
