892
I N D U S T R IA L A ND E N G I N E E R I NG C H E M I ST R Y
better flavor after aging for 4 to 5 days a t 1" to 3" C. than after longer aging a t this temperature. These tests also show that fresh and quick-frozen steaks aged for the same periods a t 1" to 3" C. are equally juicy.
COWLUSION
''
steaks aged days at to 30 and then packaged, quick-frozen, and stored at -I8" for a month Or longer when thawed, are as tender and of better flavor than
Vol. 24, No. 8
adjacent steaks aged 6 or 7 days a t 1" to 3" C. and then tested immediately without freezing. LITER.4TCRE
CITED
(1) Treader, D. K., Birdseye, C., and Murray, W. T., ISD. ENG. CHEM.,24, 242 (1932). (2) Warner, K. F., Proc. Am. Soc. A n i m a l Production, 1928, 114-16. RECEIVED April 11, 1932. Presented before the Divisions of Biological and of Agricultural and Food Chemistry at the 83rd Meeting of the American Chemical Society, New Orleans, La., March 28 to April I , 1932.
New Curve of Thermal Behavior of Gelatin &I. BRIEFERAND J. H. COHEN,The Atlantic Gelatin Company, Inc., Woburn, Mass. A new curve of the thermal behavior of gelatin 'RIGHT ( l c ) , followtheless significant in view of the discussion of Wright's ining Arisz @),studied in solution is established showing the changes the effect of i n i t i a l teresting work. due to aging at different temperatures, the time The q u e s t i o n of v i s c o s i t y cooling temperatures of gelatin remaining constant. It is shown that at apsolutions and its r e l a t i o n to c h a n g e s of gelatin-water soluproximately 20" C. a solution of gelatin is in viscosity variation. I n this extions a n d g e l a t i n - i c e cream the most favorable condition to develop a structure periment with skim milk a temmixtures has occupied the attention of investigators over a perature range of 40" to 160" F. of m a x i m u m resistance to shearing stress. long period of time. Some of (4.4" to 71.1" C.) w a s u s e d . By means of curoes it is demonstrated that the more important references From these initial temperatures the structural changes as betrayed by jelly conthe gelatin solutions were slowly may be found in c e r t a i n absistency and viscosity measurements are similar cooled to 40" F. and the viscosistracts of l i t e r a t u r e (8). All ties determined a t 68" F. (20" f o r all types of gelatins irrespective of their of the published data are informative. Much of it, howC.). The rate of cooling, which previous history. T h i s thermal characteristic e v e r , is c l o u d e d b y extendwas different for each i n i t i a 1 m a y be considered a basic property of gelatin. ing t h e e x p e r i m e n t a l w o r k t e m p e r a t u r e , is not g i v e n . Gelatin incorporated in a n ice cream mixture beyond r e a s o n a b 1e temperaThe effect of the rate of cooling shows the same thermal behavior as pure gelatinture regions and by failure to for units of considerable volume observe t h e contributing water solutions. The maximum effect due to must be large, s i n c e u p o n it effects of s u b s t a n c e s mixed depends the average temperaaging is in the region of 20" C . f o r a n y approw i t h g e l a t i n , s u c h as milk ture to which the gelatin solupriate time period with a n y grade of gelatin, and cream. It will be unnecestion is subjected in the course irrespective of its origin or physical properties, sary to cite the m a n y o t h e r of the 24-hour aging p e r i o d . and in no case does it occur at a n y other temvariables which intrude upon inIncreasing the temperature from vestigators of this p r o b l e m . perature. 40" to 68" F. b e f o r e taking viscosity readings i n t r o d u c e s It is suggested that aging experiments be Mention should be made, however, of an excellent summary another variation. It will be conducted at 20" C . to insure comparable results given by Lucas ( I O ) . shown that reheating an aged in the hands of different inoestigators since in P r i m a r i l y , the o b s e r v e d gelatin s o l u t i o n r e s t o r e s its every case the m a x i m u m aging effect is obtained basic viscosity. effects due to aging a gelatin soat or near this temperature. l u t i o n a r e c h a n g e s of jelly Wright's c u r v e of i n i t i a l consistency and viscositv. The aging t e m p e r a t u r e shows a maximum in the region of 80" F. (26.7" C.) from which point authors ventured to find the specific relations that exist bethe curve is practically constant to 160" F. (71.1" (3.). I n tween viscosity, jelly consistency, time of aging, and temall probability the viscosities resulting from aging over this perature. The net results are expressed by a new curve of range of temperatures are affected both by the rate of cooling gelatin behavior which is designated as the characteristic and, a t the higher temperatures (100" to 160" F. or 37.8" to thermal curve of gelatin. The data are presented as a 71.1" C.), also by the growth or coagulation of the milk contribution t o the study of gelatin structure, thermal besolids. It will be shown that, when aging is restricted to havior, and thermal stability. In a previous communication ( 4 ) the authors have shown temperature effects only, the resultant viscosity curve takes a different form. I n Wright's experiments with ice cream that a t approximately 24" C. the viscosity of gelatin solutions mixtures the gelatin is added before pasteurizing and homog- increases with time of standing, if undisturbed. This enizing. Since hydrolysis of gelatin, owing to temperature observation is in agreement with the earlier work of Loeb and pressure, varies for different specimens, the results of (9). The authors, however, concluded that this viscosity such experiments may be open to question. Wright, how- increase was in reality incipient gelatin. I n view of the ever, anticipated in part the procedure followed in this study results obtained from the present experiments, it is apparent as being the more logical course to pursue. The studies of that this whole question of viscosity increase with time is Horrall (7) and of Anderson, Lyons, and Pierce ( I ) , while much more involved and that the forces operating in this not applying directly to the present investigation, are iiever- interesting phenomenon remain still undefined.
Mi
IN D USTR IA L AN D EN G INEER IN G CHEMISTR Y
August, 1932
THERMAL HEHAYIOR OF GELATIN Solutions of gelatin were held for 7 hours a t different temperatures from 5" to 35" C., inclusive, with increments of 5" C. The time was determined experimmtally to provide sufficient differentiation. Thus, a group of seven units was handled together. The gelatin concentrations varied with the grade according to the ratio of 200/Bloom grams. The average concentration was about 0.8 per cent. At the end of the 7-hour aging period, the entire group was placed in a 10" C. thermostatically controlled water bath and held
/
I S
10
16
AGING
ZO
25
90
35
893
It will be noticed that curve 1 approaches a straight line, parallel with the 5 axis. An ideal or perfect gelatin in so far as thermal behavior is concerned may be represented, in theory a t least, as having such a straight line function. It is conceivable that the observed variations of jelly consistency and viscosity may be predicated on particle size or size distribution, but the sharp differences in the thermal behavior of different specimens of gelatin and the fact that, irrespective of these differences, all specimens attain a maximum in the region of 20" C., would indicate that the forces a t work are of a more fundamental nature. The linkage of the group components of gelatin, their individual thermal stability, or the thermal balance of the group of amino acids composing any specimen of gelatin probably determines the gradients or differentials of its thermal curve. The magnitude of change varies with the time and temperature. T i t h i n the limits of the time-temperature periods of the experiments described, the jelly consistencies have changed more or less, and the observed effects found to be reversible. This phase of the problem is being studied to determine the time periods and temperature relations when hydrolysis or other changes take place coincident with irreversible viscosity or jelly consistency losses. Obviously, there must be a limit to the magnitude of any temperatureviscosity change with time. A static or equilibrium phase of limited duration may be predicted, whence reversal may he expected.
TEMPERATURE3 OC.
FIGLRE1. CHARACTERISTIC THERMAL BE:H.4\ DIFFEREAT SPECIJlETYS OF G E L A T J n
IOR O F
*-
at thih temperature for 16 hours. It was obviously necessary t o bring all units of the group to the same final temperature before testing, in order to observe what changes in jelly consistency the aging temperatures had effected. The volume of each unit was 120 cc. so that rapid adjustment of temperatures was deemed unnecessary, since the differences in cooling time (to 10" C.) for such small volumes were found negligible. At the end of the 16-hour chilling period the solutions had formed a gel of sufficient consistency to be measured by the penetration test previously published (5)--i. e., the penetration through the jelly of an 11-mg. lead shot dropped from a height of 50 em. The results are plotted as millimeters penetration. About thirty specimens of gelatin were examined in this way, including one of ash-free or standard gelatin ( 6 ) . Specimens included calfskin and pigskin gelatins, and gelatins extracted from acid- and alkali-conditioned stock, which, as has been shown (3, l a ) , have very different pH curve characteristics. Nevertheless, for rtll the various types of gelatins examined, their thermal curves were strikingly similar. I n each case the same maximum was obtained There were no exceptions. I n each case the maximum jelly consistency occurred a t an aging temperature in the region of 20" C. These results show that for all gelatins, irrespective of their origin or chemical history, there is a specific aging temperature and time a t which the gelatin structure attains its maximum viscosity and, when subsequently chilled, its maximum resistance to snearing stress. It is interesting to note that an aging temperature of 5" C. is as destructive to jelly consistency as is a n aging ternperature of 35" C. The magnitude of the jelly consistency maximum varies with the original physical values of the gelatin, but in general the thermal curve of any specimen will be practically coincident with one of the three curves shown in Figure 1. The three curves on this plate are composite curves of ten or more similar groups. The differences in each group were negligible, the curves being practically coincident.
3o
10
is
LO
LI
so
3s
AGING TEMPERATURES OC.
FIC~JRE 2. A.
JELLY
COXSISTENCY us. AGING TEMPERATURE
Jelly consistency with equal amounts of gelatin
B. Amount of gelatin required t o give equal jelly consistency
For the present these considerations are of interest scientifically. I n commercial practice the time periods covering the experiments will probably riot be exceeded except, perhaps, in the storage of ice cream. Since the maintenance of the volume of an ice cream depends, a t least in part, on the viscosity of the finished product, it follows that in all probability gelatins possessing the better thermal stability, such as are represented by the composite ciir\-e 1 (Figure l), will contribute to minimum shrinkage of ice cream with time of storage. The above experiment was duplicated in its significant part, using a typical ice cream mixture in place of water. The results, expressed as viscosity ratio, show the same thermal relation as the gelatin-water solutions. I n order to show the values of the changes resulting from the different aging temperatures in terms of percentage gelatin, duplicate thermal experiments m-ere made with vary-
1
"JI.
v
I) I1 s 'I_ II I 1 1.
A h 1)
li
uc
ing amounts of gelat,iii, so t,li:Lt i n t,lie eriil t l w jelly c m sistency of each iiiiit of the group \VIS eqiial to tlBe one aged at 200 c. Figure 2 sliows oiic uf tlicw ciirves, A , wiiertvis eiirve B shows tiie gelatin concentrations to which each tenrpernture et1 ti) equal tlic jelly consistency of the one unit aged a t 20" ('. Jn thin ease the differences arc sliorrii to be from aimit, 10 to 25 pw cent gelatin iiy wciglit ('Fables J and 11). It sIiould be einpliaaizeil that t,lii.sc Iiweeiitage ratios are sirriply indices of thermal liehnvior. T l w vi physical ~.:iliies of gelatin precliidt: the pwsi1dit.y of it ing any specific percent,age ratio.
N h: I?It 1 N
(;
