Aging Effects on Gelatin Dispersions W. S. MUE:LLER, Department of D a i r y I n d u s t r y , M a s s a c h u s e t t s State College, Amherst, Mass.
T
HE purpose of this investigation' was to determine what
tively weak gels, the diameter of the plunger was increarccl to 1.25 inches (3.18 em.), and the metal cup receiving the shot was replaced with a light paper cup weighing 2.1 grains. TITO or more gel strength determinations were made a t 2.2' C., after an aging period of 24 hours. DETERJIIKATION OF BASICVISCOSITY. The term "basic viscosity" in this paper is defined as the viscosity which persists in an ice cream mixture after being stirred by ordinary means until there is no further reduction in the consistency. After completing the gel strength determinations, the same samples were used for determining the basic viscosity by the 1laclIichael viscometer. Before agitation the samples w r e tempered in a constant-temperature bath a t 20" C. $ g i b tion \vas accomplished by a motor-driven paddle consisting of a 0.25-inch (0.635-cm.) mesh wire screen. The deyice was arranged so as to prevent the incorporation of air and so that the samples could be agitated while remaining in the constaiittemperature bath. A constant agitation time of 10 minutes was used because a preliminary experiment showed no decrease in consistency after 10 to 30 minutes of agitation. The disk bob was used a t a speed of 15 r. p. m., with 125 cc. liquid a t 20" C. Duplicate or triplicate determinations n-ere made a t a fixed time (3 minutes) after the samples were agitated. All results are reported in degrees h9aciVichael for KO.30 gage wire. Klien it was necessary to use other sizes of wire, the angular deflection was multiplied by a factor in order to transpose to KO.30 gage wire. These factors were obtained by calibrating the various wires 1%-henusing mineral oil and standard castor oil.
effects variouf initial aging temperatures have, when used for definite periods of time, on certain physicochemical properties of gelatin dispersions. A gelatin-ice cream mixture, pure gelatin-water solution, and other gelatin dispersions, such as cane sugar, butter oil, and milk plasma solids, werk used. Gel strength and basic viscosity m6,asurements were employed as indexes in determining the initial aging temperature effects, mainly for tn-o reasons: (1) The consistency of a gelatin changes with changing temperature and time; and (2) the practical utility of gelatin in ice cream is dependent upon its power to form a gel. Thus this study should be of practical value to tlie commercial ice cream industry. The fact that the structural strength of metals is greatly affected by the rate of cooling the molten metal suggested the possibility of increasing the gel strength of gelatin by subjecting it to various temperatures while the transition from sol t o gel was taking place. This analogy between gelatin arid metals may a t first be surprising but appears logical in viev- of von Weimarn's theory (5)that the process of gelatination is identical with the process of crystallization.
HISTORICAL Much experimental work has been reported regarding the influence of thermal history upon the consistent). of gelatin systems. I l u c h of the experimental work on the effect of aging temperatures has been done when only one temperature mas used throughout the entire aging period; only a few inrestigators have toui7hed the subject of aging temperatures when the total aging period was split into two or more temperatures for definite periods of time. Therefore, a systematic study regarding the latter method of aging is lacking. Arisz ( 1 ) studied the effect of various temperatures on the consistency of a gelatin-glycerol dispersion. H e did not split the aging period into various temperatures but used one temperature continuously throughout the aging period. Kraemer (3) used a combination of tm-o temperatures in studying the influence of previous history on gel formation. H e conjectured that the colloidal behavior of tlie already much-qtudied gelatin dispersions may be entirely different from what would be expected on the basis of current theories. Depev- ( 2 ) made a preliminary study on the effect of rate of cooling on the viscosity of a gelatin-ice cream mixture. The results from four trials indicated that the rate of cooling the mixture after homogenization did not materially affect its viqcosity after being aged. Kright (6) qtudied the effect of initial cooling temperatures on the behavior of gelatin in an ice cream mixture aiid in qkim milk. His n-ork slion-s t h a t the rate of cooling gre,ltly influences the behavior of the gelatin. EXPEItIJfESTSL
~IETHODS
DETERMISATIOSOF GEL STRESGTH. The Bloom gelometer was used for gel strength determinations. However, the gel strength of a commercial ice cream mixture or even one containing 0.85 per cent (175 grains Bloom) gelatin is too weak to be measured by the standard Bloom gelometer method. I n order to adapt the Bloom gelometer to rela1
Since t h e preparation of this paper, Briefer and Cohen [Isn.ESC. CHEM.,
24, 892 (1932)l published results on the effect of aging temperatures on gelatin-water s o l u t i o n s .
GELATIS-ICE CRE.IhI l \ I I S T U R E A butter-skim milk powder mixture containing 10 per cent fat, 10 per cent serum solids, 15 per cent sugar, and 0.85 per cent gelatin was used. This mixture was subjected for definite periods of time to the following initial aging teniperatures: lo", 15",20', 25", 30', 40", and 65" C.; i t was then cooled to 2.2" and held for a total of 24 hours. The gelatin was a composite eample containing the three sources of gelatin-calf skin, pork rind, and ossein. Ten-pound (4.5-kg.) samples of approximately 175 Bloom strength were obtained from nine different gelatin manufacturers and thoroughly mixed. The final test on the composite sample was 176 grams Bloom with p H 5.25. The gelatin-ice cream mixture w s pasteurized a t 62.7" C. for 30 minutes and homogenized a t this temperature a t 2500 pounds per square inch (175.8 kg. per sq. cm.) pressure. Immediately after homogenization, portions of the mixture were placed in T o . 1 tin cans, holding about 11 ounces (312.4 grams), and set in a n ice-salt water mixture. By slowly stirring, the ice cream mixtures were cooled to their respective initial aging temperatures within 3 minutes. Each initial aging temperature v a s used for one-hour intervals u p to 6 hours. The initial aging was carried out in a thermostatically controlled m-ater bath. The high initial aging temperature period was always followed by a low aging temperature (2.2" C.) period of such length that the total aging period was equal to 24 hours. At the close of the high initial aging period the ice cream mixture was transferred into standard gel teqt jars n-hich were set in cold hrine, thus cooling to 2.2' C. within 5 minutes. The samples were aged a t 2.2" C. in a n electric refrigerator. An identical mixture aged only a t 2.2" C. for 24 hours served
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narrow temperature range on both sides of 20" is not definitely shown in Figure 1, since the temperature interval on either side of 20" is 5" C. However, the curves show that, if the maximum increase does not occur a t one temperature only, its temperature range is narrow and very near t o 20". EFFECT OF INITIAL AGINGTIME. Although data a t hour intervals of initial aging time are given in Table I, only 2-, 4-,and 6-hour periods are plotted in Figure 1, in order to simplify the chart. The curves show that the increases and decreases are magnified with an increase in the initial aging time. However, the increase is not uniform with time increments. The greatest increase occurred during the first 2 hours of initial aging. In case of the 20" C. initial aging temperatpre, of the total increase occurring during the 6-hour initial aging period, approximately 71 per cent occurred during the first 2-hour period, 16 per cent during the next 2 hours, and 15 per cent during the last 2 hours. OF INITIAL AGINGTEMPERATURE3 AND TABLE I. EFFECT PERIODS ON BARICVISCOSITY O F GEL.4TIN-ICE CREAM MIXTURE
INITIAL AGING TEMPERATURE IN *C.
FIGURE1. RELATIONSHIP BETWEEN INITIAL AGING TEMPERATURE, INITI.4L AGINGTIME, AND BASIC VISCOSITY OF GELATIN-ICECREAM MIXTURE as the control. As it was impossible to use portions of the same mixture for all the various initial aging temperatures used, mixtures were made up a t different times. In order to eliminate the unavoidable viscosity and gel strength variations which naturally occur in identical mixtures when made up a t different times, a control was used with each high initial aging temperature and the results reported as increases or decreases over the control. However, the variation in the control mixtures was not great, as the average deviation from the mean for basic viscosity was 8.6" MacMichael, and for gel strength 3.1 grams. BASICVISCOSITY.Data on viscosity are given in full in Table I and, in part, graphically in Figure 1 which shows the relationship between initial aging temperature, initial aging time, and basic viscosity. It is obvious that the initial aging temperature greatly influences the basic viscosity after a 24hour total aging period, when compared to an identical mixture aged only a t one temperature (2.2" (3.). The zero line represents the control mixtures. Curves above the zero line show the increase, and those below show the decrease in basic viscosity for the various initial aging temperatures. The curves show that, as the initial aging temperature is raised from 10" C., the magnitude of the basic viscosity increase grows larger until a maximum is reached a t 20" C., after which the viscosity increase declines with further raising of the initial aging temperature. Near 37.7" C. the basic viscosity remains unchanged. Raising the initial aging temperature from 37.7" to 65" C. results in a decrease in basic viscosity when compared to a mixture aged only a t 2.2" C. There are two major points of interest in the curves: (1) The temperature a t which the curves cross the zero line is shown-e. g., the initial aging temperature which has no effect on basic viscosity. This temperature corresponds closely with the sol-gel transition temperature (38.03O C.) given by Oaks and Davis (4). Comments on the significance of this observation cannot be made until data have been collected on the cause for the behavior of gelatin when subjected t o high initial aging temperatures. (2) The maximum increase in basic viscosity is a t 20" C. Whether this maximum increase in basic viscosity occurs definitely a t 20" or includes a
INITl.4L AGING TEMPER.4TURE
AGEDAT EFFECT O F TIMEHELDAT INlTI4L .kGING TEMPERA2 2' TURE BEFORE COOLING TO 2.2' 136O F.) ON BASIC ONLY
c.
c.
1 hr. eC.(°F.)oM. 10 (50) 156.6 15 (59) 207.0 20 (68) 215.5 25 (77) 155.2 30 (86) 138.0 40 (104) 115.5 65 (149) 111.5
2 hr.
VISCOSITY" 3 hr. 4 hr. ~~~.
O
O
M
.
M
.
O M ,
5 hr.
6 hr.
O
O M .
M
.
(COXTROLI
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.
156.0
162.5 120.5 157.0 158.0 162.5 216.; 225.5 230.0 240.0 235.5 137.2 224.0 235.6 243.2 252.7 258.5 140.0 115.5 162.0 179.7 175.5 189.2 209.7 145.7 150.5 152.5 143.0 147.0 130.0 115.5 113.8 114.5 112.0 111.9 112.2 104.5 99.7 98.5 87.5 88.0 130.5 Degrees MacMichsel viscometer for No. 30 gage wire after total aging period of 24 hours.
GEL STRENGTH.Data on this property are given in Table I1 and shown in part graphically in Figure 2. A general parallelism of the gel strength and basic viscosity curves is a t once apparent. The maximum gel strength was obtained when the aging was initially carried out a t 20" C. The gel strength curves are not as smooth or as accurate as the viscosity curves, because the Bloom gelometer test (even when
INITIAL AGING TEMPERATURE IN '6.
FIGURE2. RELATIONSHIPBETWEEN INITIAL AGINGTEMPERATURE, INITIALAGING TIME,AND GEL STRENGTH OF GELATIN-ICECREAMMIXTURE
modified) on weak gels is less sensitive than basic viscosity measurements. Kevertheless, the gel strength data show conclusively that the use of the various initial aging temperatures affects the gel strength and basic viscosity in a similar manner.
PUREGELATI?*'-WATER SOLUTION A one per cent gelatin (225 Bloom grams) solution was subjected t o the following initial aging temperatures: lo", Z O O ,
I N D U ST R I A L A N D E N G I N E E R I N G C H E M I S T R Y
June, 1933
70 9
perature (20" C,). This belief has remained unquestioned for a long time, probably for two reasons: (1) The fact that a gel sets more rapidly at 2" than a t 20" C. would lead to t h e belief that the lower temperature would produce the greater TABLE11. EFFECTOF INITIALAGING TEMPERATURES gel strength, unless further investigation was made; ( 2 ) t h e AND PERIODS ON GEL STRENGTH OF GELdTIN-ICE CREAM fact that a long aging period is required to secure the desired MIXTURE gel strength when aging at a high temperature (20" C.) only AQEDA T would make such an aging procedure impracticable, because INITIAL EFFECT O F TIMEHELDA T I N ~ T ~AQINQ A L TEMPER.4- 2.2' c . of the possibility of bacterial spoilage. The data which have TO 2.2O C. ON GELSTRENQTH" ONLY AQINQ TCRE BEFORE COOLINQ TEMP. 1 hr. 2 hr. 3 hr. 4hr. 5hr. 6 hr. (CONTROL) been presented show that initially aging a gelatin dispersion C. Grams Grams Grams Grama Grams Grama Grams a t 20" C. produces the maximum increase in basic viscosity 45.5 52.5 50.1 49.6 48.4 49.3 10 53.0 49.8 71.9 72.4 70.8 71.4 69.1 15 71.1 and gel strength when compared t o a solution aged only a t a 50.8 90.0 88.2 79.6 80.2 80.7 85.6 20 low temperature (2.2"C.). The significance of these findings 50.9 81.2 83.3 78.6 70.6 62.8 77.6 25 54.7 60.3 61.2 59.1 62.2 59.3 59.1 30 is that the efficiency of the gelatin was increased by using a 42.3 46.1 45.2 42.0 42.8 42.8 42.9 40 51.8 44.5 48.2 48.0 50.1 48.8 49.2 65 short 20" C. aging temperature in conjunction with a low Modified Bloom gelometer method using 1.25-inch (3.18.cm.) plunger aging temperature, thus avoiding the long-time high aging and light paper cup (2 1 grama); deiermmatlons made after total aging period, Possibilities for the practical application of such a n period of 24 hours. aging method which increases the efficiency of the gelatin are The results are given in Table I11 and show t h a t both the a t once apparent from an economic viewpoint. A prelimigel strength and basic viscosity of a pure gelatin-water solu- nary experiment showed t h a t the benefits from high initial tion are affected by initial aging temperatures in much the aging temperature (20" C.) were lost after reheating near t h e same way as when the gelatin is incorporated in a n ice cream melting temperature region. Therefore, although gelatin mixture. has many uses, a 20" C. initial aging temperature would be applicable only when the product is kept below room temperaOF INITIAL AGINGTEMPERaTURES ON BASIC TABLE 111 EFFECT VISCOSITY AND GELSTRENGTH OF GELATIN-WATER SOLUTION ture from the time i t is made until it is consumed. Since ice cream meets this requirement to the fullest degree, exINITIAL INITIAL perimental work is now in progress on the use of 20" C.iniAQINQ BASIC GEL AQINQ BASIC GEL TEMP. VISCOSITY STRENQTE TEMP. VISCOSITY STRENQTH tial aging temperature in the manufacture of ice cream. M. Grams c.
30", and 40" C. The experimental procedure was similar to that used for the gelatin-ice cream mixture, with the exception that the initial aging time remained constant a t 4 hours.
Grams C. M. 36.0 43.3 10 45.8 54.1 20 30 24.1 39.6 a Aged at 2.2' C. only. O
.
20.8 23.0
40 Control5
33.3 33.5
1. When using a gelatin-ice cream mixture:
OTHERGELATIISDISPERSIONS I n order to determine what influence various ice cream constituents have on the initial aging temperature effects already observed, various gelatin dispersions were initially aged a t 20" C. for 4 hours, followed by a 20-hour aging period a t 2.2". A 200-gram Bloom test gelatin was used. I n each dispersion the gelatin concentration was one per cent of the water portion of the mixture. The experimental procedure was similar to that used for the gelatin-ice cream mixture, except that only basic viscosity determinations n-ere made, and these were measured by the Edible Gelatin Research Society's standard riscobity pipet after a total aging period of 24 hours. All of the dispersions were pasteurized and homogenized. The following results were obtained:
DISPERSION
INCREASE I N BASICV I S C O ~ ~ OF ITY DISPERSIONS ILITIALLY AGEDAT 20' C. OVER THOSEAQEDAT 2.2' C. ONLY
% Gelatin-skim milk G elatin-sugar Gelatin-butter oil
SUMMARY
231.6 276.0 118.2
Although these results have not been sufficiently checked to be conclusive, they indicate that the magnitude of the basic viscosity increase, which is due t o the use of a 20" C. initial aging temperature, is greatly altered by the various ice cream constituents. I n all probability some of these differences in viscosity values are due to unavoidable experimental errors in obtaining a n equal gelatin concentration in the aqueous portion of the various mixtures used. However, these experimental errors would not be great enough to account for the observed differences in percentage increase for basic viscosity.
DISCUSSION OF RESULTS It is a current belief that a low aging temperature (2' C.) will produce a &mer gel than one aged at a much higher tem-
Initially aging an ice cream mix for 1 to 6 hours at temperature range of 10" to, 30" C., followed by a low aging temperature, increases the basic viscosity and gel strength over those mixtures aged a t the low temperature, 2.2' C. only. As the initial aging temperature is raised from l o o , this increase in basic viscosity and gel strength reaches a maximum at 20" C . , then decreases with further increase of the initial aging temperature. Initially aging an ice cream mixture at or near the transition temperature of gelatin (38.03' C.), as given by Oaks and Davis, has no marked effect on basic viscosity and gel strength. Initially aging an ice cream mixture at temperatures above the transition point of gelatin decreases the basic viscosity and gel strength when compared to mixtures aged at the low temperature (2.2"C.) only. The magnitude of the basic viscosity and gel strength changes increase with an increase in the initial aging temperature period. However, the first 2 hours produce the greatest change.
2. An initial aging temperature of 20" C. has a similar effect on pure gelatin-water solution to that occurring when the gelatin is incorporated in an ice cream mixture. 3. Sugar, plasma solids, and butter oil have a marked influence on the magnitude of the basic viscosity increase owing to the use of a 20" C. initial aging temperature. LITERATURE CITXD (1) Arisz, Kolloidchem. Beihcjte, 7, 17 ( 1 9 1 5 ) . ( 2 ) Depew, N. H. Agr. Expt. Sta., Tech. BuZI. 38, 12 (1928). (3) Kraomer, Colloid Symposium Monograph, IV,120 (1926). (4) Oaks a n d Davis, J . Am. Chem. SOC.,44, 4 6 9 ( 1 9 2 2 ) . (5) W e i m a m , von, Repts. Imp. Ind. Research Inst. Osaka, Japan, 9, I53 ( 1 9 2 8 ) . (6) Wright, J . Dairu Sci., 13,406 (1930). RECEIVED December 20, 1932. This is part of a paper presented at the 11th Annual Meeting of the Eastern Division, American Dairy Science Association, Springfield, Mass., September 19, 1932. Contribution 158 of the Massachusetts Agricultural Experiment Station.
