EVIDENCE OF STRUCTURE I N GELATIN GELS BY AKSEL G. O L S E S *
In 1 9 2 2 Gortner and Hoffman‘ published data which l o them appeared to “indicate that gelatin gels have a structure and that this structure is more or less fixed a t the time that gelation takes place.” These authors visualize “a crystal structure where the crystals melt or soften at the gelation temperature. The micelles would then be formed by the solidification of crystals and later when micelle touched micelle the hardened surface of the crystal would prevent cohesion.” Sheppard and Elliott2 however, preferred to explain these differences as due to case-hardening and bearing no relation to a crystal structure of the gelatin gels. More recently, however, the view that gelatin gels owe their properties to some form of internal structure has become quite widely accepted. Observations which we have made lead us to postulate that the structure of gelatin gels is that of chains of molecules interlacing in a brushheap fashion, and that in setting of a gel two processes occur, (a) solidification in the usual sense of loss of mobility, and (b) the rather slow building up of chains of molecules. The latter process presupposes some degree of motility of the molecules which are rearranged and on this basis we should expect that gelatin gels set quickly would be composed of rather short chains, only slightly interlaced and therefore more quickly dispersed upon melting than a similar gel solidified very slowly with a maximum opportunity for internal orientation and building up of such structure. On this basis a major difference between weak and strong gelatins may lie in their ability to form long chains. Now, if these postulates are true we should expect a gel caused to set rapidly by immersion in an ice bath to melt more quickly than a duplicate allowed to set slowly by being held at more elevated temperature. Our observations confirm this view. Also we should expect that a series of jellies made up to the same jelly strength by using proportional amounts of gelatin of different strengths would melt in very different times, depending upon the ability of each particular gelatin to form long interlaced chains of molecules. This also was confirmed by our experiments. In our observations we have made use of two very simple methods for determining melting and setting time. Incidentally it should be pointed out that melting and setting temperatures of gelatin jellies are rather misnomers as our experience indicates that setting temperature is entirely a matter of rate of cooling and only by having that rate infinitely slow can a true setting temperature be observed. The same may be said of the melting temperature, and no doubt the differences observed in these two would approach zero as the rates of cooling and heating respectively approached infinity. ‘Research Department of General Foods Corporation, Battle Creek, Mich. R o c . Soc. Expt. Biol. Med., 19, 257-264 (1922). * J. Am. Chem. Soc., 44,373-379 (1922).
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AKSEL G. OLSEN
Setting Time The gelatin solution is placed in test tubes (15 mm. internal diameter) and these immersed in a bath a t 50'C. for 15-20 minutes. They are then at once placed in a bath of the desired temperature and observed from time to time. When they can be inverted and show a firm meniscus not deformed by this inversion the gel is considered set. I t is well to have more than one tube and to permit the one used for the final observation to remain undisturbed until practically set. This allows of a rather sharp determination. Melting Time For melting time observation these same tubes are stoppered, inverted and mounted on a rack immersed in a water bath held at the desired temperature. The meniscus may be observed to gradually rise and become deeply rounded as the jelly softens until finally an air bubble passes up through the jelly. The time interval from immersion until this air bubble rises is recorded as melting time. Both of these methods are of course empirical and only relative. But using standardized conditions and test tubes of uniform bore, gelatins may be evaluated in a definite numerical way.
Effect of Temperature on Setting and Melting Time The influence of temperature upon the setting and melting times of gelatin solutions is given in Table I. For these observations a commercial acidified gelatin dessert preparation was used. For melting time observations the tubes of jelly are kept a t 14-15' C. for about 18 hours, before being placed in the melting bath. The results listed in Table I indicate that for both setting and melting point observations the temperature should be maintained within very close limits if comparisons are to be between series tested a t different times. It was observed that the time interval between setting and melting materially affects the latter, that is, the melting time of jellies set a t 14-15'C. was found to increase with the age of the jelly. However, this maturing did not occur in a jelly set a t O'C. unless the temperature was subsequently raised. These findings are illustrated in Table I1 which give typical results. Similar results have been obtained repeatedly with jellies prepared with other samples of gelatin. Evidently a t 14-15' there is opportunity for a gradual building up of a complex structure which is broken up again with considerable difficulty, while at 0'-I'C. the setting is so rapid and the jelly becomes so firm that the maturzng effect, or the gradual molecular orientation, evident a t the higher temperature is interfered with. There is then a t oO-1' no complex structure formed, hence the jelly while firm has no permanency, but melts almost at once when it is subjected to melting temperatures.
EVIDEKCE O F STRC'CTURE I N G E L A T I S GELS
53 1
TABLE I The Setting and Melting Times of Gelatin Jellies at Different Temperatures Temqerature
C
2 IO
Jelly B* Setting time Melting time Minutes Minutes
Jelly A' Setting time Melting time Minutes Minutes
9
I3 35 I20
I8 65
2 40
10;
I4 16 17
400
I75
I9
Not firmly
20
set after 24 hours Very soupy after 2 4 hours
Firm after 2 4 hours Firm after 24 hours 2 60
22
I20
23
48
72
24
25
25
I4 5
34 I7 7 3
27
I$
30
*Jellies A and B prepared with same gelatin: a blended edible gelatin of medium strength. Difference in setting and melting times are due to our varying of other conditions, such aa acidity. Conditions most favorable to setting (Jelly B) increase the melting time.
TABLE I1 Influence of Time and Temperature upon the Maturing of Gelatin Jellies Series No.
(All tubes stoppered after setting to prevent drying of surface film)
Setting time Minutes
Melting time (at 22.5OC.)after 6 24 48 72 hrs. hrs. hrs. hrs.
Min. Min.
Min.
II*
13*
17
j4
165
220
I2
96
231
303
I.
Set a t 0-IOC. Kept in ice bath for duration of experiment *
25
5
2.
Set a t O-IOC. After 60 min. Changed to 14.5OC.
25
3.
Set a t 14.5'C. Kept, at same temperature for duration of experiment
2j
o
Min. -
*The temperature of the bath would rise several degrees over night, which accounts for the gradual rise in melting time.
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AKSEL G. OLSEN
It should be of interest to test other temperatures, however, the series here presented indicates a rearrangement or orientation of the gelatin molecules which can occur a t 14-15Oc. but not a t all or a t least a t an extremely slow rate, at o°C. It is also evident that this structure is not a part of the actual solidification or gelation of the gelatin solution but an independent change for, while not actually measured it was evident to the observer that the gelatin set at oo was very firm, notwithstanding its very low permanency a t higher temperatures. It should be of interest to observe the rate of orientation a t different temperatures from oo up to the upper limit for the setting of gelatin. Particularly would it be of interest to observe whether a jelly held a t that upper limit for some period of time and then quickly set at o°C. would show that molecular orientation had taken place. Some of our observations included different grades of gelatin prepared so as to give jellies of approximately equal jelly strength. Typical comparisons of two such gelatins are given in Table I11 No. R-60 is an average strength gelatin, while M-61 is a superior gelatin. An attempt was made to prepare the jellies with such amounts of gelatin as would result in equal setting time.
TABLE I11 Influence of Time on the maturing of Acidified Gelatin Jellies (pH about 3 . 0 ) prepared with Proportional Amounts of Different Grade Gelatins and kept a t 14-15OC. Gelatin
Series
No.
Setting tirne Minutes
(Melting time at 22.5"C.) after 6hra. 16hrs. 2 2 hrs. 48 hrs.
Grna.
No.
I.
9.6
M-61
2 50
16
75
120
270
2.
11.2
R-60
2 50
12
45
70
161
These results indicate that the stronger gelatin has a much stronger tendency towards orientation and gradual building up of a complex jelly structure than has the weaker gelatin, although the latter was used in proportionally larger amounts. summary
Evidence is presented which indicates that two separate processes may occur when gelatin solutions solidify: I ) Solidification, which may be quite rapid at low temperatures. 2) Orientation of molecules into an internal structure. This change is retarded by too low temperatures. When acidified gelatin solutions are rapidly cooled to o°C. solidification quickly occurs and orientation is largely prevented. Such a gel has little permanency at melting temperatures.
EVIDENCE OF STRUCTURE IN GELATIN GELS
533
When acidified gelatin solutions are kept at 14-15'C. solidification occurs more slowly and is accompanied by molecular orientation which may continue for days, resulting in a gel with increasing resistance to melting temperatures. To melt a gelatin requires both melting per se and destruction of the structure composed of linked gelatin molecules. The first of these is rapid, while at temperatures not much above the melting point the latter process is slow. High-strength gelatin shows a greater tendency towards orientation of molecules than does low-strength gelatin.
