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The Relation between the Degree of Solidification of Fat in Cream and its Churning Time. II. The Physical Distribution of the Liquid-Solid Phases within the Globule
THE RELATION B E T W E E N T H E DEGREE OF SOLIDIFICATION OF FAT IN C R E A ~ AND ITS CHURNING TIME. II. THE PHYSICAL DISTRIBUTION OF THE LIQUID-SOLID PHASES W I T H I N THE GLOBULE J. ROBERT B R U N N E R
Michigan Agricultural Experiment Station1 AND E. L. J A C K
Division of Dairy Industry, University of California
The influence of variations in churning temperature of cream upon its churning time is common knowledge. As the temperature of cream is lowered, within the churning li~nits, the churning time is increased. I t is believed that some of the variations ia churning result from changes in the degree of solidification of the fat. This supposition is well supported by observation. Undoubtedly, lowering the temperature of the cream results in greater crystallization of the fat. The fact that satisfactory churning occurs when the temperature of the cream is within rather restricted limits suggests that, at churning temperatures, the degree of solidification in the fat is quite specific. The studies of van Dam and Burgers (4), van Dam (3), Rishoi and Sharp (9), Jack (6), Coulter and Combs (2) and others offer evidence that milk fat exists in a state of partial solidification, both in the cream at the churning temperature and in the churned butter. The extent of fat solidification is somewhat controllable by process temperature manipulation. Haglund et al. (5), Richardson and Abbott (8), Coulter and Combs (2), Wilster et al. (11) and others have emphasized that such measures often are necessary to produce a butter with desirable body and texture when made from milk fats that are abnormally hard or soft. The purpose of this study was to correlate the degree of solidification in the fat with the churning time of the cream and to determine to what extent processing procedures affect the solidification of fat in cream prepared for churning. EXPERIMEI~TAL METHODS
To determine the extent that processing procedures affect the degree of solidification in globular fat, the following experimental procedure was followed: One hundred pounds of freshly-separated cream testing 33 per cent fat was pasteurized at 66 ° C. for 30 rain. One-third of this cream, designated as lot A, was cooled and held at 0 ° C. for 18 hr. prior to churning. A second portion of the cream, lot B, was cooled and held at 10 ° C. for 18 hr. The remaining portion of the cream, lot C, was held at 60 ° C. while 5-lb. quantities were removed, cooled in ice-water to the churning temperature and immediately churned in a temperaReceived for publication Nov. 10, 1949. , Journal Article no. 1092 (n.s.), Michigan Agriculture Experiment Station. 267
268
J.
ROBERT BRUNNER
A N D E. L. J A C K
ture-controlled, motor-driven Daisy churn of an agitator type. Likewise, a f t e r cream lots A and B h a d been held for the designated 18-hr. holding period, 5-lb. portions were removed, adjusted to the churning t e m p e r a t u r e a n d immediately churned. A calorimetric technique, suitable for measuring the percentage of solid f a t in cream at the churning t e m p e r a t u r e , as used previously by J a c k and B r u n n e r (7), was employed to determine the degree of solidification. Churning t e m p e r a t u r e s were selected to cover the churning range. RESULTS
W h e n churning times of cream are plotted as ordinates against percentage of solidified f a t therein (degree of solidification) as abscissa, curves are obtained which, in the desirable churning r a n g e of 35 to 55 rain., are logarithmic (figure 1).
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PERGENTAGE of SOLIDIFIED FAT Fie. 1. The relation between the degree of solidification of milkfat at the churning temperature and the churning time of the cream (33.0% fat). Curve A---Cream held at 0 ° C. for 18 hr. and warmed to the churning temperature. Curve B--Cream held at 10° C. for 18 hr. and warmed to the churning temperature. Curve C---Cream pasteurized at 66° C. and cooled to the churning temperature. Below these time limits, churnings are incomplete and accompanied by excessive f a t losses in the buttermilk, while those above proceed with difficulty, if at all. Within the desirable churning range, creams f r o m lot C contained between 20 a n d 31 per cent solidified fat, while creams f r o m lots A and B contained between 48 a n d 68 per cent of the f a t in the solid state. Cream f r o m lot A contained slightly more crystallized f a t t h a n cream f r o m lot B, which would be expected, since more f a t should be solidified at 0 ° C. t h a n at 10 ° C. The more f a t t h a t is
269
LIQUID-SOLID PHASES IN FAT GLOBULES
in a solid state, the g r e a t e r is the time required for c h u r n i n g ; however, within the three lots studied, the churning time depends more u p o n the cooling procedure used t h a n on the percentage of solidified fat. F o r example, as shown in figure 1, cream f r o m lot A, held at 0 ° C. for 18 hr. p r i o r to churning, a n d lot C, churned immediately a f t e r cooling, containing a p p r o x i m a t e l y 40 per cent solidified f a t churned in 30 a n d n e a r l y 100 rain., respectively. The f a c t t h a t a wide variation exists between the percentage of solid f a t in churnings f r o m lots A and B and lot C indicates that the degree of solidification
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TEMPERATURE °G. FIG. 2. Apparent specific heat values over the temperature range studied of fat in cream held at 0 ° C. for 18 hr. and warmed to the churning temperature. Curve a--Control, cream held at 0 ° C. for 18 hr. Curve b---Cream churned at 11.10° C. Curve c--Cream churned at 13.90 ° C. occurring at the time of churning is not nearly as significant in the time required for churning as is the probable position of the solidified f a t in the globule. Some insight as to the general location of this solidified globular f a t m i g h t be obtained (a) by a p p l y i n g the principle of the second law of thermodynamics which states, essentially, t h a t heat is conducted f r o m an area of high heat level to an area of low heat level until a state of t e m p e r a t u r e equilibrium is reached and (b) f r o m an inspection of the a p p a r e n t specific heat curves of the fats involved, which, together with the work of v a n D a m (3) and Rishoi and S h a r p (10), show that f a t
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J. ROBERT BRUNNER AND E. L. J A C K
exhibits a considerable time lag in a t t a i n i n g t h e r m a l equilibrium throughout the f a t globule. The globular f a t in cream as p r e p a r e d f r o m lot A p r e s u m a b l y is in a state of nearly complete solidification. W h e n portions of this cream are w a r m e d to the churning temperature, heat is t r a n s m i t t e d to the continuous serum phase f r o m which it is t r a n s f e r r e d to the f a t phase. Due to the poor t h e r m a l conducting properties of fat, the heat t r a n s f e r process proceeds inward through the f a t at a relatively slow rate. This lag in attaining equilibrium probably results in the creation of a layer of partially liquid or softened fat on the outer surfaces of the f a t globule. To s u p p o r t this assumption it is necessary to demonstrate that the
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TEMPERATURE °C. Fin. 3. Apparent specific heat values over the temperature range studied of fat in cream pasteurized at 66 o C. and immediately cooled to the churning temperature. Curve a-~Control, cream held at 0 ° C. for 18 hr. Curve b--Cream churned at 10.60° C. Curve c--Cream churned at 19.10° C. r e m a i n i n g portion of solidified fat, supposedly located in the interior of the f a t globule, has not been liquified in the w a r m i n g process. I n figure 2, the a p p a r e n t specific heat curves of f a t f r o m churnings b and c, cream processed as in lot A, are compared with a similar curve of f a t in a state of nearly complete solidification (curve a). The partial curves b and c coincide f a i r l y well, in the r a n g e covered, with that of the control, curve a. Since ap-
LIQUID-SOLID ptt£SES IN FAT GLOBULES
271
parent specific heat measurements are made by determining the amount of heat necessary to change the temperature of the material, this indicates that the fat being liquefied from the churning temperature upward to a state of complete liquefaction is fat in the original state of equilibrium. Therefore, the heat applied to the cream system in raising the temperature from 0 ° C. to the churning temperature probably was manifested in melting or softening outer layers of the globular fat, rather than being distributed evenly throughout the globule, since the residual solid fat is still in the state of equilibrium existing at 0 ° C. Similar reasoning can be applied to creams prepared for churning as in lot C. When cooling cream from 60 ° C. to the churning temperature, heat is transferred from the serum to the cooling medium, then by a somewhat slower process from the fat globules to the serum. By this process, there probably is formed a layer of solid or liquid-solid fat surrounding the fat globules, leaving a large portion of the interior fat in a liquid state. In figure 3, the apparent specific heat curve a, like curve a in figure 2, is representative of globular fat in a state of nearly complete solidification, while curves b and c represent churning samples. At the time churning was started, immediately after the cream was cooled, and at the beginning of apparent specific heat determinations, very little of the fat had been solidified. However, after a period of 20 to 30 rain. in the calorimeter, some crystallization of the fat occurred, as indicated by the maxima in curves b and c. As assumed in the preceding case, this change in physical state occurred principally in the fat globule surface layers. The conjecture is made that the physical state of the globular fat on the surface layers is somewhat similar in creams that churn within desirable churning limits, regardless of the procedure used in preparing the creams for churning. The fat on the surface of the globule is neither totally liquid nor totally solid but in a soft semi-solid, sticky condition conducive to fat globule agglomeration. The fact that more solidified fat is formed in one method of processing than in another has little influence upon the churning time, but is manifested more significantly in the completeness of churning, as well as in the body and texture of the final butter. Richardson and Abbott (8) have indicated that cream cooled to churning temperature immediately after pasteurization should be churned at a lower temperature than cream that has been warmed to churning temperature. The data given in figure 4 show that cream samples prepared by cooling from the pasteurization temperature (lot C) required somewhat higher temperatures to effect satisfactory churnings in the same period of time than did creams warmed to the churning temperature (lots A and B). By cooling the cream with ice-water, it is possible that a layer of solidified fat was formed on the fat globule surface which interfered with fat coalescence beyond the effect of agitation, resulting in retarded churnings. This supposition is in harmony with the reasoning advanced in this report. In attempting to analyze the results obtained from a ehurning study of this nature the possibility of alteration in crystalline form should not be neglected as a means of explaining some of the variations in the churning behavior of
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J . ROBERT BRUNN~R AND E. L. JACK
cream. Clarkson and Malkin (1), employing X-ray diffraction technique and melting curves, were able to demonstrate in pure triglyeerides three distinct polymorphic crystal formations depending upon the rate of solidLfieation. Slow cooling results in the formation of the most stable form designated by them as beta and having the highest melting point. More rapid cooling results in the production of an intermediate alpha form, while extremely rapid chilling will produee the unstable gamma or " g l a s s " form. Upon aging or the application of heat, the unstable monotropic gamma and alpha forms will transform irreversibly into the more stable beta form.
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CHURNING TIME IN MINUTES Fin. 4. The relation between the churning time of cream and the churning temperature. Curve A---Cream held at 0° C. for 18 hr. and warmed to the churning temperature. CurveB-Cream held at 10° C. for 18 hr. and warmed to the churning temperature. Curve C--Cream pasteurized at 66° C. and cooled to the churning temperature.
X-ray diffraction technique and melting curve studies, representing the best methods available for observing polymorphie transitions in triglycerides, when used to study the pure and simple triglycerides give only qualitative data ; indeed, the difficulties encountered in trying to observe polymorphie transitions in a naturally occurring, mixed glyceride such as milk fat in the globular state would present a technical problem of some magnitude. Hence, certain facts obtainable from a study of the more simple glycerides are used to help explain phenomena occurring in the more complex natural fats. The temperature history of cream samples churned from lots A and B indicate that for all practical purposes the fat is in a state of stable equilibrium (Malkin's beta crystals) at the end of the 18-hr. tempering period (Rishoi and Sharp, 10). Upon warming the cream, the added heat is expended in melting a portion of the solidified fat and raising the over-all temperature to the desired churning temperature. The poss~ility of reverse polymorphic transition is precluded, since the gamma alpha beta transitions are monotropic. On the other hand, when
LIQUID-SOLID PHASES IN FAT GLOBULES
273
cream samples from lot C are prepared for churning by relatively quick cooling from 60 ° C., the opportunity exists for the formation of the intermediate alpha and possibly the unstable " g l a s s " types of crystallization. It is the authors' postulation, however, that if these polymorphic forms are crystallized their existence at the churning temperature would be relatively short-lived, since the transition of the unstable forms to the stable beta form is the normal transition pattern. Therefore, it is felt that the churning data obtained in this study are influenced primarily by the occurrence and distribution of solid and liquid fat in the globule as deduced from the specific heat curves shown herein. However, the possible role played by different polymorphie forms of crystalline milk fat should not be disregarded entirely, even though the data presented are not designed to demonstrate polymorphic transitions. SUMMARY
A calorimetric technique was used to determine the percentage of crystallized milk fat (degree of solidifieation) present in creams prepared for churning by the following methods: Lot A. Cream was cooled and held at 0 ° C. for 18 hr. prior to churning at selected temperatures. Lot B. Cream was cooled and held at 10 ° C. for 18 hr. prior to churning at selected temperatures. Lot C. Cream was cooled in ice-water from 60 ° C. to the desired temperature and immediately churned. The degree of solidification in creams from lots A and B, churning within 35 to 55 rain., ranged between 48 and 68 per cent solidified fat. Cream portions from lot C that churned within the same time range contained between 20 and 31 per cent solidified fat. Within any one lot of cream, the more solidified fat that is present at the churning temperature, the longer is the time required for churning. The experimental results indicate that the churning time is not entirely dependent upon the actual degree of solidification in the fat, but, to a greater extent, upon the distribution of the liquid and solid fat phases on the globular fat surfaces. Polymorphie transition of the crystallized fat has been considered as a possible explanation for the churning results obtained. REFERENCES (1) CI,ARKSON,C. E., AND MAI~KIN, W. Alternation in Long-chain Compounds. Part II. An X-Ray and Thermal Investigation of the Triglycerides. J. Chem. Soc., Pt. I, pp. 666-671. 1934. (2) COVLT~R,S. T., AWD COMBS, W . B . A Study of the Body and Texture of Butter. Minn. Agr. Expt. Sta. Tech. Bull. 115. 1936. (3) VAN DAM, W. The Importance of the Equilibria in the System Milk F a t in the Making of Butter. Proceedings of the Syracuse World's Dairy Cong., 2: 1004-1008. 1923. (4) VAN ]:)AM, W.~ AND BURGERS, W. G. X-Ray Investigation of the Micro-crystalline Structure of Butter Fat. J. Dairy Sci., 18: 45-50. 1936.
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(5) HAGLUN9, E.~ WeDs, G., AND OL,SSO~, T. Unders~kninger ~ver det Sve~ska Sm~rets Kensistens. Neddelande int. 387 fran Centralanstatten for Fers~ksuasendet pa Jordbrusksomradet. Mejeriavdelningen Nr. 41. 1930. (Cited by Coulter and Combs (1). Original not seen.) (6) JAck, ]~. L. The Influence of the Rate of Solidification on the Physical Structure of Milk Fat as Shown by Melting Rate Studies. Prec. 27th Ann. Meeting, Am. Dairy Sei. Assoc., Western Div., pp. 68-78. 1941. (7) JACK, ]~. L., AND B]aUNNER, J. R. The Relation Between the Degree of Solidification of Fat in Cream and its Churning Time. I. Measurement of the Degree of Solidification. J. Dairy ScL, 26: 169-177. 1943. (8) RICH~SO~, G. A., AN]) ABBO~, F. II. Prevention of the Defect in the Consistency of Butter Due to an Alfalfa Hay Ration. Prec. 21st Ann. Meeting, Am. Dairy Sci. Assoc., Western Div., pp. 63-69. 1935. (9) RISHOI, A. H., AND SHARP, P . F . Speci/]e Heat and the Physical State of Fat in Cream. J. Dairy SoL, 21: 399--405. 1938. (10) RISHOI, A. H., AND SHARp~ P . F . Volume Changes of Fat in Cooled Cream Held at Constant Temperature. J. Dairy Sci., 23: 593-595. 1940. (11) WILS~Ea, D. H., SWOUT,R. E., STzI~ R. W., H.~G, J. R., A~D JO~ES, I. R. Butter Making During Hay Feeding Season, ' %0--45-40 ,' Method. Oregon Agr. Expt. Sta. Bull. 414. 1942.
Michigan Agricultural Experiment Station1 AND E. L. J A C K
Division of Dairy Industry, University of California
The influence of variations in churning temperature of cream upon its churning time is common knowledge. As the temperature of cream is lowered, within the churning li~nits, the churning time is increased. I t is believed that some of the variations ia churning result from changes in the degree of solidification of the fat. This supposition is well supported by observation. Undoubtedly, lowering the temperature of the cream results in greater crystallization of the fat. The fact that satisfactory churning occurs when the temperature of the cream is within rather restricted limits suggests that, at churning temperatures, the degree of solidification in the fat is quite specific. The studies of van Dam and Burgers (4), van Dam (3), Rishoi and Sharp (9), Jack (6), Coulter and Combs (2) and others offer evidence that milk fat exists in a state of partial solidification, both in the cream at the churning temperature and in the churned butter. The extent of fat solidification is somewhat controllable by process temperature manipulation. Haglund et al. (5), Richardson and Abbott (8), Coulter and Combs (2), Wilster et al. (11) and others have emphasized that such measures often are necessary to produce a butter with desirable body and texture when made from milk fats that are abnormally hard or soft. The purpose of this study was to correlate the degree of solidification in the fat with the churning time of the cream and to determine to what extent processing procedures affect the solidification of fat in cream prepared for churning. EXPERIMEI~TAL METHODS
To determine the extent that processing procedures affect the degree of solidification in globular fat, the following experimental procedure was followed: One hundred pounds of freshly-separated cream testing 33 per cent fat was pasteurized at 66 ° C. for 30 rain. One-third of this cream, designated as lot A, was cooled and held at 0 ° C. for 18 hr. prior to churning. A second portion of the cream, lot B, was cooled and held at 10 ° C. for 18 hr. The remaining portion of the cream, lot C, was held at 60 ° C. while 5-lb. quantities were removed, cooled in ice-water to the churning temperature and immediately churned in a temperaReceived for publication Nov. 10, 1949. , Journal Article no. 1092 (n.s.), Michigan Agriculture Experiment Station. 267
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ROBERT BRUNNER
A N D E. L. J A C K
ture-controlled, motor-driven Daisy churn of an agitator type. Likewise, a f t e r cream lots A and B h a d been held for the designated 18-hr. holding period, 5-lb. portions were removed, adjusted to the churning t e m p e r a t u r e a n d immediately churned. A calorimetric technique, suitable for measuring the percentage of solid f a t in cream at the churning t e m p e r a t u r e , as used previously by J a c k and B r u n n e r (7), was employed to determine the degree of solidification. Churning t e m p e r a t u r e s were selected to cover the churning range. RESULTS
W h e n churning times of cream are plotted as ordinates against percentage of solidified f a t therein (degree of solidification) as abscissa, curves are obtained which, in the desirable churning r a n g e of 35 to 55 rain., are logarithmic (figure 1).
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PERGENTAGE of SOLIDIFIED FAT Fie. 1. The relation between the degree of solidification of milkfat at the churning temperature and the churning time of the cream (33.0% fat). Curve A---Cream held at 0 ° C. for 18 hr. and warmed to the churning temperature. Curve B--Cream held at 10° C. for 18 hr. and warmed to the churning temperature. Curve C---Cream pasteurized at 66° C. and cooled to the churning temperature. Below these time limits, churnings are incomplete and accompanied by excessive f a t losses in the buttermilk, while those above proceed with difficulty, if at all. Within the desirable churning range, creams f r o m lot C contained between 20 a n d 31 per cent solidified fat, while creams f r o m lots A and B contained between 48 a n d 68 per cent of the f a t in the solid state. Cream f r o m lot A contained slightly more crystallized f a t t h a n cream f r o m lot B, which would be expected, since more f a t should be solidified at 0 ° C. t h a n at 10 ° C. The more f a t t h a t is
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in a solid state, the g r e a t e r is the time required for c h u r n i n g ; however, within the three lots studied, the churning time depends more u p o n the cooling procedure used t h a n on the percentage of solidified fat. F o r example, as shown in figure 1, cream f r o m lot A, held at 0 ° C. for 18 hr. p r i o r to churning, a n d lot C, churned immediately a f t e r cooling, containing a p p r o x i m a t e l y 40 per cent solidified f a t churned in 30 a n d n e a r l y 100 rain., respectively. The f a c t t h a t a wide variation exists between the percentage of solid f a t in churnings f r o m lots A and B and lot C indicates that the degree of solidification
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TEMPERATURE °G. FIG. 2. Apparent specific heat values over the temperature range studied of fat in cream held at 0 ° C. for 18 hr. and warmed to the churning temperature. Curve a--Control, cream held at 0 ° C. for 18 hr. Curve b---Cream churned at 11.10° C. Curve c--Cream churned at 13.90 ° C. occurring at the time of churning is not nearly as significant in the time required for churning as is the probable position of the solidified f a t in the globule. Some insight as to the general location of this solidified globular f a t m i g h t be obtained (a) by a p p l y i n g the principle of the second law of thermodynamics which states, essentially, t h a t heat is conducted f r o m an area of high heat level to an area of low heat level until a state of t e m p e r a t u r e equilibrium is reached and (b) f r o m an inspection of the a p p a r e n t specific heat curves of the fats involved, which, together with the work of v a n D a m (3) and Rishoi and S h a r p (10), show that f a t
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J. ROBERT BRUNNER AND E. L. J A C K
exhibits a considerable time lag in a t t a i n i n g t h e r m a l equilibrium throughout the f a t globule. The globular f a t in cream as p r e p a r e d f r o m lot A p r e s u m a b l y is in a state of nearly complete solidification. W h e n portions of this cream are w a r m e d to the churning temperature, heat is t r a n s m i t t e d to the continuous serum phase f r o m which it is t r a n s f e r r e d to the f a t phase. Due to the poor t h e r m a l conducting properties of fat, the heat t r a n s f e r process proceeds inward through the f a t at a relatively slow rate. This lag in attaining equilibrium probably results in the creation of a layer of partially liquid or softened fat on the outer surfaces of the f a t globule. To s u p p o r t this assumption it is necessary to demonstrate that the
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TEMPERATURE °C. Fin. 3. Apparent specific heat values over the temperature range studied of fat in cream pasteurized at 66 o C. and immediately cooled to the churning temperature. Curve a-~Control, cream held at 0 ° C. for 18 hr. Curve b--Cream churned at 10.60° C. Curve c--Cream churned at 19.10° C. r e m a i n i n g portion of solidified fat, supposedly located in the interior of the f a t globule, has not been liquified in the w a r m i n g process. I n figure 2, the a p p a r e n t specific heat curves of f a t f r o m churnings b and c, cream processed as in lot A, are compared with a similar curve of f a t in a state of nearly complete solidification (curve a). The partial curves b and c coincide f a i r l y well, in the r a n g e covered, with that of the control, curve a. Since ap-
LIQUID-SOLID ptt£SES IN FAT GLOBULES
271
parent specific heat measurements are made by determining the amount of heat necessary to change the temperature of the material, this indicates that the fat being liquefied from the churning temperature upward to a state of complete liquefaction is fat in the original state of equilibrium. Therefore, the heat applied to the cream system in raising the temperature from 0 ° C. to the churning temperature probably was manifested in melting or softening outer layers of the globular fat, rather than being distributed evenly throughout the globule, since the residual solid fat is still in the state of equilibrium existing at 0 ° C. Similar reasoning can be applied to creams prepared for churning as in lot C. When cooling cream from 60 ° C. to the churning temperature, heat is transferred from the serum to the cooling medium, then by a somewhat slower process from the fat globules to the serum. By this process, there probably is formed a layer of solid or liquid-solid fat surrounding the fat globules, leaving a large portion of the interior fat in a liquid state. In figure 3, the apparent specific heat curve a, like curve a in figure 2, is representative of globular fat in a state of nearly complete solidification, while curves b and c represent churning samples. At the time churning was started, immediately after the cream was cooled, and at the beginning of apparent specific heat determinations, very little of the fat had been solidified. However, after a period of 20 to 30 rain. in the calorimeter, some crystallization of the fat occurred, as indicated by the maxima in curves b and c. As assumed in the preceding case, this change in physical state occurred principally in the fat globule surface layers. The conjecture is made that the physical state of the globular fat on the surface layers is somewhat similar in creams that churn within desirable churning limits, regardless of the procedure used in preparing the creams for churning. The fat on the surface of the globule is neither totally liquid nor totally solid but in a soft semi-solid, sticky condition conducive to fat globule agglomeration. The fact that more solidified fat is formed in one method of processing than in another has little influence upon the churning time, but is manifested more significantly in the completeness of churning, as well as in the body and texture of the final butter. Richardson and Abbott (8) have indicated that cream cooled to churning temperature immediately after pasteurization should be churned at a lower temperature than cream that has been warmed to churning temperature. The data given in figure 4 show that cream samples prepared by cooling from the pasteurization temperature (lot C) required somewhat higher temperatures to effect satisfactory churnings in the same period of time than did creams warmed to the churning temperature (lots A and B). By cooling the cream with ice-water, it is possible that a layer of solidified fat was formed on the fat globule surface which interfered with fat coalescence beyond the effect of agitation, resulting in retarded churnings. This supposition is in harmony with the reasoning advanced in this report. In attempting to analyze the results obtained from a ehurning study of this nature the possibility of alteration in crystalline form should not be neglected as a means of explaining some of the variations in the churning behavior of
272
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cream. Clarkson and Malkin (1), employing X-ray diffraction technique and melting curves, were able to demonstrate in pure triglyeerides three distinct polymorphic crystal formations depending upon the rate of solidLfieation. Slow cooling results in the formation of the most stable form designated by them as beta and having the highest melting point. More rapid cooling results in the production of an intermediate alpha form, while extremely rapid chilling will produee the unstable gamma or " g l a s s " form. Upon aging or the application of heat, the unstable monotropic gamma and alpha forms will transform irreversibly into the more stable beta form.
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CHURNING TIME IN MINUTES Fin. 4. The relation between the churning time of cream and the churning temperature. Curve A---Cream held at 0° C. for 18 hr. and warmed to the churning temperature. CurveB-Cream held at 10° C. for 18 hr. and warmed to the churning temperature. Curve C--Cream pasteurized at 66° C. and cooled to the churning temperature.
X-ray diffraction technique and melting curve studies, representing the best methods available for observing polymorphie transitions in triglycerides, when used to study the pure and simple triglycerides give only qualitative data ; indeed, the difficulties encountered in trying to observe polymorphie transitions in a naturally occurring, mixed glyceride such as milk fat in the globular state would present a technical problem of some magnitude. Hence, certain facts obtainable from a study of the more simple glycerides are used to help explain phenomena occurring in the more complex natural fats. The temperature history of cream samples churned from lots A and B indicate that for all practical purposes the fat is in a state of stable equilibrium (Malkin's beta crystals) at the end of the 18-hr. tempering period (Rishoi and Sharp, 10). Upon warming the cream, the added heat is expended in melting a portion of the solidified fat and raising the over-all temperature to the desired churning temperature. The poss~ility of reverse polymorphic transition is precluded, since the gamma alpha beta transitions are monotropic. On the other hand, when
LIQUID-SOLID PHASES IN FAT GLOBULES
273
cream samples from lot C are prepared for churning by relatively quick cooling from 60 ° C., the opportunity exists for the formation of the intermediate alpha and possibly the unstable " g l a s s " types of crystallization. It is the authors' postulation, however, that if these polymorphic forms are crystallized their existence at the churning temperature would be relatively short-lived, since the transition of the unstable forms to the stable beta form is the normal transition pattern. Therefore, it is felt that the churning data obtained in this study are influenced primarily by the occurrence and distribution of solid and liquid fat in the globule as deduced from the specific heat curves shown herein. However, the possible role played by different polymorphie forms of crystalline milk fat should not be disregarded entirely, even though the data presented are not designed to demonstrate polymorphic transitions. SUMMARY
A calorimetric technique was used to determine the percentage of crystallized milk fat (degree of solidifieation) present in creams prepared for churning by the following methods: Lot A. Cream was cooled and held at 0 ° C. for 18 hr. prior to churning at selected temperatures. Lot B. Cream was cooled and held at 10 ° C. for 18 hr. prior to churning at selected temperatures. Lot C. Cream was cooled in ice-water from 60 ° C. to the desired temperature and immediately churned. The degree of solidification in creams from lots A and B, churning within 35 to 55 rain., ranged between 48 and 68 per cent solidified fat. Cream portions from lot C that churned within the same time range contained between 20 and 31 per cent solidified fat. Within any one lot of cream, the more solidified fat that is present at the churning temperature, the longer is the time required for churning. The experimental results indicate that the churning time is not entirely dependent upon the actual degree of solidification in the fat, but, to a greater extent, upon the distribution of the liquid and solid fat phases on the globular fat surfaces. Polymorphie transition of the crystallized fat has been considered as a possible explanation for the churning results obtained. REFERENCES (1) CI,ARKSON,C. E., AND MAI~KIN, W. Alternation in Long-chain Compounds. Part II. An X-Ray and Thermal Investigation of the Triglycerides. J. Chem. Soc., Pt. I, pp. 666-671. 1934. (2) COVLT~R,S. T., AWD COMBS, W . B . A Study of the Body and Texture of Butter. Minn. Agr. Expt. Sta. Tech. Bull. 115. 1936. (3) VAN DAM, W. The Importance of the Equilibria in the System Milk F a t in the Making of Butter. Proceedings of the Syracuse World's Dairy Cong., 2: 1004-1008. 1923. (4) VAN ]:)AM, W.~ AND BURGERS, W. G. X-Ray Investigation of the Micro-crystalline Structure of Butter Fat. J. Dairy Sci., 18: 45-50. 1936.
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J. ROBERT BRUNNER AND E. L. JACK
(5) HAGLUN9, E.~ WeDs, G., AND OL,SSO~, T. Unders~kninger ~ver det Sve~ska Sm~rets Kensistens. Neddelande int. 387 fran Centralanstatten for Fers~ksuasendet pa Jordbrusksomradet. Mejeriavdelningen Nr. 41. 1930. (Cited by Coulter and Combs (1). Original not seen.) (6) JAck, ]~. L. The Influence of the Rate of Solidification on the Physical Structure of Milk Fat as Shown by Melting Rate Studies. Prec. 27th Ann. Meeting, Am. Dairy Sei. Assoc., Western Div., pp. 68-78. 1941. (7) JACK, ]~. L., AND B]aUNNER, J. R. The Relation Between the Degree of Solidification of Fat in Cream and its Churning Time. I. Measurement of the Degree of Solidification. J. Dairy ScL, 26: 169-177. 1943. (8) RICH~SO~, G. A., AN]) ABBO~, F. II. Prevention of the Defect in the Consistency of Butter Due to an Alfalfa Hay Ration. Prec. 21st Ann. Meeting, Am. Dairy Sci. Assoc., Western Div., pp. 63-69. 1935. (9) RISHOI, A. H., AND SHARP, P . F . Speci/]e Heat and the Physical State of Fat in Cream. J. Dairy SoL, 21: 399--405. 1938. (10) RISHOI, A. H., AND SHARp~ P . F . Volume Changes of Fat in Cooled Cream Held at Constant Temperature. J. Dairy Sci., 23: 593-595. 1940. (11) WILS~Ea, D. H., SWOUT,R. E., STzI~ R. W., H.~G, J. R., A~D JO~ES, I. R. Butter Making During Hay Feeding Season, ' %0--45-40 ,' Method. Oregon Agr. Expt. Sta. Bull. 414. 1942.