A Gelometer for Starch Pastes R. M. HIXON AND BERNADINE BRIMHALL, Iowa Agricultural Experiment Station, Ames, Iowa
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H E P P A R D (8) and Bogue (2) have reviewed the methods for measuring jelly strength. These fall generauy into two classes-those in which the jelly is broken, as in the Delaware jelly strength tester (1, 11), and those in which i t is deformed only a specified amount, as exemplified by the Bloom gelometer (4), yeither type of measurement is entirely satie factory because the breaking strength of a gel is not always an indication of its resistance to deformation ( 7 ) . Consequently, there has come into favor a third class of instrument which takes both these properties into consideration. Thus, the methods of Sheppaid (23) and of Sax1 ( 5 , 6 ,7 ) give a more complete picture of the rheological properties of a gel than was possible by the older methods. The apparatus described in this paper was designed especially for use with 6 to 9 per cent starch pastes, and will measure on them the same properties that Saxl's gelometer measures on more resistant materials.
Description of Apparatus The gelometer shown in Figure 1 consists of a regulator, B, to give constant head of suction; expansion chamber, c; screw clamp, D, to regulate rate of flow of air through the system;
in suction are convenient and allow ample time to make accurate readings. When Plotted (Figure 2) the readings of cm. suction us. ml. deformation show a straight-line relationship in the range where the gel is elastic. The breaking point of the gel is indicated by a fairly sharp break in this line. In order to obtain hysteresis curves, the three-way sto cock below H is opened before suction has reached the elastic Timit, and readings are made as the force on the gel is gradually reduced. From these three quantities-elasticity, breaking point, and hysteresis-may be calculated the gel factor, Bloom point, elastic recovery, and other values which have been used as expressions Of gel strength @)* The slope of the suction-deformation line (indicative of elasticity) is roughly proportional t o the area of the hole over which the gel is placed. The amount of suction (centimeters of water) necessary to break the gel, since i t is exerted equally on each unit area, is relatively independent of hole size except in cases where the gel is deformed by its own weight. Starch pastes are extremely sensitive t o variations in the method of preparation; before starting routine measurements on this instrument, it is essential t h a t t h e method of preparation be checked as to duplicability of the product.
Principles The combination of principles which differentiates this gelometer from others described in the literature may be summarized as follows: 1. Suction is used to deform the gel (9). The force is thus transmitted uniformly to all parts of the gel exposed t o it. Since the deformed portion is in contact only with water, such factors as abrasion due to mechanical plungers and inconveniences due to the use of elastic membranes are eliminated. 2. The volume of deformation is measured by hydrostatic means (10). Using a hole size of 2.6-em. diameter and a 2-ml. pipet, this volume may easily be measured t o 0.02 ml., corresponding to a deformation of 0.04 mm. by instruments which measure depth directly. 3. The deforming force changes slowly, uniformly, and a t a constant rate. Having started the flow of water from K , the operator may
FIGURE 1. DIAGRAM OF GELOMETER 30
water manometer, E , to read the amount of suction applied to the paste as determined by the height of water column in H; brass platform, J, with hole in center over which starch paste is placed; graduated pipet, F,for reading volume of water displaced by the gel as it is deformed by suction; and constant-head capillary-overflow, K , to provide constant rate of increase in suction applied to the paste.
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Procedure The weighed sample mixed with 150 ml. of water is pasted by heating at 99' C. for 0.5 hour in a water bath. In order to prevent excessive evaporation, the tubes are covered by rubber s t o p pers containing a hole in which a stirring rod is placed. The pastes are stirred by hand from time to time to allow for uniformity in heating. Violent mechanical agitation is avoided, since it increases evaporation losses and may injure the granules. The paste is poured into a 400-ml. beaker, covered with a watch lass, and allowed to stand for 15 hours at room temperature. &he gel may then be removed from the beaker and placed bottom down on the brass platform, J. Water is turned on at K and suction applied. As water drips from the capillary into the funnel, the level in H rises at a constant rate. During this time, readings are taken on the manometer, E, and pipet, F . Rates of 1 to 4 cm. per minute increase
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the method of preparation would have to be altered by inTABLE I. COMPARISON OF BREAKING STRENGTH AND ELASTICITY creasing the concentration and time of setting. Such gels OF 6.5 PERCENTSTARCH GELS would be expected to exhibit plasticity and hysteresis as the Starch Breaking Point Slope Rigidity X 10-1 ratio of gelling to nongelling components becomes increasingly Cm. Hz0 Cm./ml. Dynes/sq. cm. large. 15.0 660 Yellow Creole Popcorn Maridan Commercial corn Hegari sorghum Modified A Mod/fied B Modified C Country Gentleman
18.8 17.5 12.5 10.5 7.8 9.5 7.0 4.5 3.5
13.8
10.7 11.0 10.3 9.5 9.0 7.5 7.5
590 440 370 378 320 270 205 175
750
650
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concentrate on reading the manometer and pipet with no further attention to the mechanics of increasing the load. This uniform change of applied force is especially valuable in measuring properties where the time factor is important and stepwise addition of load would be likely t o introduce error. 4. Skin effects and the influence of the walls of the containin vessel are eliminated by removing the mold from the beaker an! placing it bottom down on the brass platform. The dimensions of the gel in relation to the area to which suction is applied should be large enough so that deformation occurs chiefly in the adjacent to the hole without causing the rest of the mol!% change its shape appreciably.
Applications Figure 2 indicates the type of information which may be obtained on this apparatus. Curve 1 is typical for starch pastes. The amount of deformation proceeds in direct proportion to the increasing suction up to the point where the gel breaks, with a corresponding sharp break in the straight line (at 23.5 cm. in this case). Such behavior tends to show that starch gels of this concentration (7 per cent) are almost perfectly elastic, since they do not undergo a period of yield before actual shearing; otherwise the straight-line relationship would not hold at pressures close to the breaking point. The fact that the gel is broken may further be confirmed by direct observation upon removing the mold from the platform. A circular cut is usually produced; sometimes a hemispherical piece is torn from the center. The dotted portion of curve 1, representing gradual reduction of suction, has no significance since the gel has been broken. It is included for comparison with curve 2 where suction on the paste was increased up t o 20 cm. (just below the breaking point) and then reduced to its original value. The difference between the ascending and descending portions of curve 2 (0.05 ml.) indicates that hysteresis is present to only a limited extent, about 2 per cent of the total deformation. Further evidence supporting this view was obtained by subjecting a starch paste with breaking point a t 18 cm. to a constant suction of 16 cm. for 3 hours. The paste yielded only 0.01 ml. (or 0.002 cm.) per hour, which is probably beyond the range of accuracy of the apparatus. Typical results obtained on starch pastes using this instrument are listed in Table I. The figures in the last column represent rigidities determined by the method described in a previous publication (3). Both rigidity and slope are a measure of the force required to deform or stretch the gel a given amount. This relationship is evident from Figure 3. Rigidity values show greater differentiation, and when the concentration of paste can be kept below 6.5 per cent the rigidometer method is to be preferred. However, with concentrations of 6.5 to 8.5 per cent, rigidity determinations present difficulty because of the high consistency of the paste. It is in this range that the gelometer is useful, not only for values of slope, but for breaking point data. This paper deals primarily with raw starches and slightly modiiied products. With more highly converted starches,
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