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Some Physical Properties of Milk. III. Effects of Homogenization Pressures on the Viscosity of Whole Milk1
SOME P H Y S I C A L P R O P E R T I E S
OF MILK.
III.
EFFECTS
OF
H O M O G E N I Z A T I O N P R E S S U R E S ON T H E V I S C O S I T Y OF W H O L E M I L K 1 C. H. WHITNAH,~ W. D. RUTZ,~ AND H. C. FRYER ~
Kansa.~ State College, Ma~dtattaT~
The c h a n g e s o c c u r r i n g in the v i s c o s i t y o f 4% f a t milk h o m o g e n i z e d at p r e s s u r e s o f 15 to 3,500 p.s.i, w e r e m e a s u r e d at t e m p e r a t u r e s r a n g i n g f r o m 4 ° to 49 ° C. w i t h an O s t w a l d v i s c o m e t e r . Editor.
The effects of homogenization upon the viscosity of pasteurized whole milk have been studied by m a n y investigators (1-5, 8, 10, 12-15). Wiegner (16) and Buglia (2) were among the first to report an increase in milk viscosity as a result of homogenization. I n 1924, Evenson and Ferris (5) reported : " H o m o g enization of whole milk at 3,500 p.s.i, increases considerably the viscosity of milk. At 1,200 p.s.i, the increase in viscosity was to a small extent o n l y . " According to Bateman and Sharp (1), who studied two samples of milk, the viscosity of whole milk was materially increased by homogenization. Later Doan (4), Tretsven (12) and others reported similar results. However, Trout ct al. ( l i ) found that homogenization decreased the viscosity of pasteurized whole milk. Some investigators (4, 12) found the fat content of the milk to be an important factor, and homogenization temperature was reported by Doan (4) to be a factor of some importance. Differences in the temperatures at which viscosities were measured appear not to have been considered as a cause of conflicting results. The data on viscosity of milk were obtained with m a n y types of viscometers, some of which lacked the precision to detect small variations in viscosity. The complicating effects of age or time of storage of milk have usually been ignored, and frequently precise temperature control of milk in the viscometer was lacking. A f t e r reviewing the literature pertaining to the viscosity of homogenized milk, Trout (13) stated: " S o m e differences of opinion seem to prevail among authorities as to the effect of homogenization upon tim viscosity of milk. This is not surprising, considering the many possible temperature and pressure relationships which may exist in processing m i l k . " I n solutions as complex as milk, viscosity changes may be the result of m a n y factors, some of which were discussed by Moore ct al. (9) and by Hermans (7). The purpose Iteeeived for publication March 14, 1956. 1Contribution No. 535 Department of Chemistry, No. 240 Department of Dairy Husbandry, and No. 21 Statistical Laboratory; Kans,qs Agrieultur~d Experiment Station, Manhattan. "~Department of Chemistry. :~Department of Dairy Husbandry. ' Statistical Laboratory. 150O
SOME P H Y S I C A L P R O P E R T I E S OF 5 [ I L K .
III.
1501
of this p a p e r is to report the effects of homogenization pressures of 15 to 3,500 p.s.i, on the viscosity of whole, pasteurized milk at t e m p e r a t u r e s r a n g i n g front 4 ° to 49 ° C. PROCEDURE
Mixed milk f r o m 100 cows of six d a i r y breeds was used in these experiments. The milk contained 4'~ f a t and 13~., total solids. A f t e r pasteurization in a 200-gal. vat at 62 ° C. for 30 minutes, 30 gal. of milk was immediately divided into 20 samples. One unhomogenized sample was quickly cooled in a bath of ice water to 4 ° C. and stored until viscosities were measured. Ninteen samples were processed in homogenizers at 59 ° C. according to the following p r o c e d u r e : Each of nine samples was put through a l a b o r a t o r y homogenizer previously described (10) at one of the following pressures: 15, 25, 35, 50, 100, 175, 200, 250, or 300 p.s.i. E a c h of the remaining ten samples was processed in one stage of a commercial 125 gal. per hour homogenizer at one of the following pressures: 300, 500, 700, 1,000, 1,200, 1,500, 2,000, 2,500, 3,000, or 3,500, p.s.i. A f t e r homogenization, each sample was cooled as described above to 4 ° C. W h e n t h a t t e m p e r a t u r e was reached, the time was recorded as zero age. Measured viscosity values were adjusted to age 50 hours (15). Viscosities were determined with an Ostwald viscometer, modified as previously described (15). The flow times of each milk sample were measured at two viscometer pressures because milk is a non-Newtonian liquid. The different pressures were obtained b y placing the viscometer in a vertical and an inclined position. There was a three-fold increase in the flow time for water when the viseometer was inclined. Statistical t r e a t m e n t of the viscosity data consisted of multiple linear regression analysis of the linear, quadratic, and cubic trends of viscosity with homogenization pressure. Two types of homogenizers, two positions of the viScometer, a n d nine t e m p e r a t u r e s at which viscosities were measured, were involved in this analysis. RESULTS
As honlogenization pressures were increased above 300 p.s.i, with a commercial sized homogenizer, there were obvious increases in the viscosity of the whole pasteurized milk at all t e m p e r a t u r e s at which viscosities were measured ( F i g u r e 1). The numerical increases in viscosity as a result of increasing homogenization pressures were greater when viscosities were measured at low temperatures. The relative increases, however, as determined by dividing the initial viscosity into the increase were not m a t e r i a l l y different at 49 ° C. f r o m those at 4 ° C. ttomogenization at pressures of 1,000, 1,500, 2,000, 3,000, and 3,500 p.s.i, increased viscosity as compared with the unhomogenized milk by respective averages of 7.1, 9.2, 11.9, 13.7, and 15.0~-. More complex effects on viscosity were f o u n d when the pasteurized whole milk was treated at lower pressures in the l a b o r a t o r y homoge,izer. At all tern-
1502
C . H . WHITNAH l~T AL
peratures, and when measured with the vertical viscometer, viscosities of the sample of milk that had been subjected to a homogenization pressure of 25 p.s.i. were lower t h a n for the unhomogenized milk. Differences between the viscosities obtained with the vertical and inclined viscometer were greater a f t e r this pressure t h a n for samples treated at any other homogenization pressure. Homogenization pressures, a f t e r which viscosities were least, varied when viscosities were measured with the inclined viscometer. At each t e m p e r a t u r e , and for some increase of homogenization pressure in the range 15 to 300 p.s.i., the viscosity decreased. This decrease in viscosity sometimes followed a previous increase at lower pressures. No reason is known for these more complex viscosity differences at low homogenization pressures. The fact that curves of viscosity versus homogenization pressure were usually nearly continuous at 300 p.s.i. indicates that similar homogenization effects were obtained f r o m the commercial type and laboratory type of homogenizer. TABLE
1
Statistical a~talysis ~ of factors affecting the viscosity of mille P o s i t i o n of viseometer
Vcrt.
Homogenizer component of tren d. L
Q
Temp . (¢C) 4 8 18 22.7 29 34 39 44 49
0 () 0 0 0 0 0 0 0
0 0 0 0 2 2 2 1 2
Incl. lmb. ]5 to 300 p.s.i. C L Q
0 3 1 ] 2 1 0 3 0
0 0 0 0 2 0 3 3 3
Vert.
C
L
Q
Degree of s i g n i f i c a n c e h 0 2 4 2 0 2 4 0 0 1 4 2 0 0 4 0 0 2 4 0 0 0 4 l 0 0 4 2 0 (~ 4 ] 0 0 4 1
Inc l .
Confl. 300 to 3,500 p.s.i. C L Q
0 0 0 2 0 0 0 0 0
4 4 4 4 4 4 4 4 4
2 0 3 3 3 2 2 2 2
C
0 0 0 0 0 0 0 0 0
" S i g n i f i c a n c e of linear, L, q u a d r a t i c , Q, and cubic, C, c o m p o n e n t s of t r e n d s of v i s c o s i t y of m i l k w i t h h o m o g e n i z a t i o n p r e s s u r e s i n c r e a s i n g f r o m 15 to 3,500 p.s.i. T e s t e d a t nine temp e r a t u r e s , u s i n g a l a b o r a t o r y or a c o m m e r c i a l h o m o g e n i z e r , a nd u s i n g t he v i s c o m e t c r both v e r t i c a l a n d i n c l i n e d enough to give a t h r e e - f o l d i n c r e a s e in flow t i me for w a t e r . b 0 = ] 7 w a s above 1 0 % l e v e l = N S ; I=F in the 5 to l(,¢/e l e v e l - - b o r d e r line N S ; 2 = F in th e 1 to 5 % l e v e l = * ; 3 = F in the 0.1 to 1 % l e v e l = * * ; 4 = F ~ 0 . 1 % level=~*.
Statistical analysis (Table 1) indicates that tlle linear increase of viscosity with homogenization pressures over 300 p.s.i, was always significant at the 0.1% level (scored 4). This is the m a j o r over-all trend. The quadratic statistical trends, indicating some c u r v a t u r e in the general linear increase of viscosity with homogenization pressure, are somewhat obscured in F i g u r e 1 by the logarithmic scale of pressures. However, they were significant (scored 2 or 3) in eight of the nine tests with the inclined viscometer. These trends were less pronounced for the vertical viscometer. The scores of 1 or 2 in six of the nine tests still a p p e a r to differ, however, from the scores of 0 in quadratic trends which were consistently found at pressures below 300 p.s.i. The existence of some significant cubic trends indicated a tendency for a curve to be concave
1503
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HOMOGENIZATIOhl PRESSURE, P,$,I
|1 , ~ . . . . . . I i . ,.. i ,.~ 0 15 25 50 I00 300500 10002000"3500 HOMOGENIZATION PRESSURE wP..~L
~IG. 1. E f f e c t s of h o m o g e n i z a t i o n p r e s s u r e on viscosities of milk. All viscosities a d j u s t e d ±o a g e 50 h o u r s , a n d m e a s u r e d w i t h v i s c o m e t e r v e r t i c a l ( ) or i n c l i n e d ( . . . . . ). B r e a k s at 300 p.s.i, a r e at c h a n g e f r o m l a b o r a t o r y to c o m m e r c i a l h o m o g e n i z e r .
u p w a r d over one pressure range but concave downward over another range. A t pressures over 300 p.s.i, such cubic trends were not significant even at a 10% level (scored 0) in all tests with the inclined viseometer and in eight of the nine tests with the vertical viseometer. W h e n milk was homogenized at pressures below 300 p.s.i., the only significant viscosity-pressure treuds at temperatures below 23 ° C. were cubic. At higher temperatures the linear t r e n d was more consistent with the vertical viscometer (scored 1 or 2) but was usually more pronounced (scored 0, 2, 3) with the inclined viscometer. Quadratic trends at pressures below 300 p.s.i, continued to score 0 at the higher temperatures. The significant cubic statistical
1504
c.H.
W H I T N A H ]gT AL
trends at some t e m p e r a t u r e s usually indicated a tendency for viscosity to increase at pressures n e a r 300 p.s.i, a f t e r a decrease beginning at pressures n e a r 15 p.s.i. DISCUSSION
The increase in the viscosity of pasteurized whole milk with homogenization pressure, reported here, substantiates and extends the range of some studies previously reported (1, 5). The lower homogenization pressures used in this s t u d y are not enough to significantly disintegrate fat globules. Effects of suspended inert particles on viscosity depend on the total volume of particles r a t h e r than on the size of individual particles. The exact causes of changes in the viscosity of milk as a result of homogenization are therefore obscure. They m a y be entirely independent of the changes in fat particle size used to define homogenization. They Inay include indirect effects of decreased particle size on the volume and composition of globule membranes. Increases in the viscosity of colloids are sometimes ascribed (a) to changes in particle shape (e.g., uncoiling of chains or unfolding of surfaces in protein particles) ; (b) to t r a n s f e r of material from the continuous phase to the disperse phase (e.g., the complexing of proteins with water, sugar, fat, or enzymes) ; or (c) to electroviscous effects. Electroviseous effects m a y involve electric charges or zwitterions on the surface of protein particles and possibly also charges buried in the folds or coils of the long chains of a globular protein. Effects similar to electroviscous uiay also involve Van der W a a l ' s type of interaction between particles (1l) or degrees of hydrogen bonding (e.g., hydration) not strong enough to be recognized as stable complexes (6). Effects of t e m p e r a t u r e on viscosity m a y more clearly indicate causes of these differences in viscosity. The pre(;ision of several theoretical and empirical equations relating the t e m p e r a t u r e and viscosity of milk is being studied. Effects of such factors as previous heat treatment, mineral content or nature of mineral combinations, amount of numerous minor organic constituents, as well as coneentration of m a j o r constituents, are a wide field for numerous studies. Differences in viscosity not only directly affect the stability and attractiveness of some fluid milk products but also may be i m p o r t a n t in the processing of milk products for use in other foods. SUMMARY
D a t a on effects of homogenization pressures from 15 to 3,500 p.s.i, on the viscosity of whole pasteurized milk at temperatures from 4 ° to 49 ° C. are re-ported. As homogenization pressures were increased above 300 p.s.i, there were highly significant linear increases in viscosity when measured at 9 temperatures. Pressures of 1,000, 1,500, 2,00,0, 3,000, and 3,500 p.s.i, increased viscosity as compared with the unhomogcnized milk by respective averages of 7.1, 9.2, 11.9, 1:3.7, and 15.0%. Quadratic trends were also present. At pressures below ;300 p.s.i, and t e m p e r a t u r e s above 23 ° C., linear trends were usually significant. At
SOME PHYSICAL PROPERTIES OF MILK. III.
lower temperatures
there were no significant linear or quadratic
1505 trends.
Cubic
t r e n d s s o m e t i n I e s i n d i c a t e d ali i n c r e a s e i n v i s c o s i t y a t p r e s s u r e s n e a r 300 p . s . i . after a decrease beginning
a t p r e s s u r e s n e a r 15 p.s.i.
REFERENCES (1) BATEI~IAN, G. F., AND SHARP, P. F. A Study of the A p p a r e n t Viscosity of Milk as Influenced by Some Physical Factors. J. Agr. Research, 35: 647. 1928. (2) BUOLIA, G. Ueber einige physikalish-chemische Merkmale der honiogenisierten Milch. Kolloid Z., 2: 353. 1908. (3) CAFFYt¢, J. E. The Viscosity Temperature Coefficient of Homogenized Milk. J. Dairy Research, 18: 95. 1951. (4) DOAN, F. J. Problems Related to Homogenized Milk. Milk Technol., 1 (6) : 20. 1938. (5) EVENSON, O. L., A]ZD FE~RIS, L. W. A Quantitative Determination of the Ammonia, Amino Nitrogen, Lactose, Total Acid, and Volatile Acid Content of Cow's Milk. J. Dairy Sci., 7: 75. 1924. (6) GLASSTONE, SAI~:UEL, KIETH, J. L., AND EYRING, HENRY. The Theory of Rate Processes. McGraw-Hill Book Co., New York. 1941. (7) HERI~ANS, J. J. Plow Properties of Disperse Syste,ms. Interscience Publishers, Inc., New York. 1953. (8) LEVITON, A., AND LEIGHTON, A. Viscosity Relationships in Emulsions Containing Milk Fat. J. Phys: Che~n., 40: 71. 1936. (9) MOORE,R. T., GIBBS, PETER, AND EYRING, HENRY. Structure of Liquid State and Viscosity of Hydrocarbons. J. Phys. Che,m., 57: 1025. 1953. (10) l~cTz, WM. D., WHITNAH, C. I~., AND BAETZ, G. D. Some Physical Properties of Milk. I. Density. J. Dairy Sci., 38: 1312. 1955. (11) SUTHE~LAN]), W~i. The Viscosity of Gases and Molecular Force. Phil. Mag., 36: 507. 1893. (12) TF~ETSVEN, W. I. Homogenization of Milk and Properties of tIomogenized Milk. Mast e r ' s Thesis, Univ. of Tenn., Knoxville. 1939. (13) TROUT, G. M. Ho~,ogenized Milk. Michigan State College Press, East Lansing. 1950. (14) T~OUT, G. M., HAL~O~AN, C. P., AN])GOULd), I. A. The Effect of Homogenization on Some Physical and Chemical Properties of Milk. Mich. Agr. Expt. Sta., Tech. B~dl. 145. 1935. (15) WHITNAH, C. H., RUTZ, WM. D., AN]) FRYER, H. C. Some Physical Properties of Milk II. Effects of Age upon the Viscosity of Pasteurized Whole Milk. J. Dairy Sci. 39: 356. 1956. (16) WIEGNER, GEORG. Ueber die Anderung einiger physikalishcr Eigenschaften der Kuhmilch mit der Zerteilung ihrer dispersen Phasen. Kolloid Z., 15: 105. 1914.
OF MILK.
III.
EFFECTS
OF
H O M O G E N I Z A T I O N P R E S S U R E S ON T H E V I S C O S I T Y OF W H O L E M I L K 1 C. H. WHITNAH,~ W. D. RUTZ,~ AND H. C. FRYER ~
Kansa.~ State College, Ma~dtattaT~
The c h a n g e s o c c u r r i n g in the v i s c o s i t y o f 4% f a t milk h o m o g e n i z e d at p r e s s u r e s o f 15 to 3,500 p.s.i, w e r e m e a s u r e d at t e m p e r a t u r e s r a n g i n g f r o m 4 ° to 49 ° C. w i t h an O s t w a l d v i s c o m e t e r . Editor.
The effects of homogenization upon the viscosity of pasteurized whole milk have been studied by m a n y investigators (1-5, 8, 10, 12-15). Wiegner (16) and Buglia (2) were among the first to report an increase in milk viscosity as a result of homogenization. I n 1924, Evenson and Ferris (5) reported : " H o m o g enization of whole milk at 3,500 p.s.i, increases considerably the viscosity of milk. At 1,200 p.s.i, the increase in viscosity was to a small extent o n l y . " According to Bateman and Sharp (1), who studied two samples of milk, the viscosity of whole milk was materially increased by homogenization. Later Doan (4), Tretsven (12) and others reported similar results. However, Trout ct al. ( l i ) found that homogenization decreased the viscosity of pasteurized whole milk. Some investigators (4, 12) found the fat content of the milk to be an important factor, and homogenization temperature was reported by Doan (4) to be a factor of some importance. Differences in the temperatures at which viscosities were measured appear not to have been considered as a cause of conflicting results. The data on viscosity of milk were obtained with m a n y types of viscometers, some of which lacked the precision to detect small variations in viscosity. The complicating effects of age or time of storage of milk have usually been ignored, and frequently precise temperature control of milk in the viscometer was lacking. A f t e r reviewing the literature pertaining to the viscosity of homogenized milk, Trout (13) stated: " S o m e differences of opinion seem to prevail among authorities as to the effect of homogenization upon tim viscosity of milk. This is not surprising, considering the many possible temperature and pressure relationships which may exist in processing m i l k . " I n solutions as complex as milk, viscosity changes may be the result of m a n y factors, some of which were discussed by Moore ct al. (9) and by Hermans (7). The purpose Iteeeived for publication March 14, 1956. 1Contribution No. 535 Department of Chemistry, No. 240 Department of Dairy Husbandry, and No. 21 Statistical Laboratory; Kans,qs Agrieultur~d Experiment Station, Manhattan. "~Department of Chemistry. :~Department of Dairy Husbandry. ' Statistical Laboratory. 150O
SOME P H Y S I C A L P R O P E R T I E S OF 5 [ I L K .
III.
1501
of this p a p e r is to report the effects of homogenization pressures of 15 to 3,500 p.s.i, on the viscosity of whole, pasteurized milk at t e m p e r a t u r e s r a n g i n g front 4 ° to 49 ° C. PROCEDURE
Mixed milk f r o m 100 cows of six d a i r y breeds was used in these experiments. The milk contained 4'~ f a t and 13~., total solids. A f t e r pasteurization in a 200-gal. vat at 62 ° C. for 30 minutes, 30 gal. of milk was immediately divided into 20 samples. One unhomogenized sample was quickly cooled in a bath of ice water to 4 ° C. and stored until viscosities were measured. Ninteen samples were processed in homogenizers at 59 ° C. according to the following p r o c e d u r e : Each of nine samples was put through a l a b o r a t o r y homogenizer previously described (10) at one of the following pressures: 15, 25, 35, 50, 100, 175, 200, 250, or 300 p.s.i. E a c h of the remaining ten samples was processed in one stage of a commercial 125 gal. per hour homogenizer at one of the following pressures: 300, 500, 700, 1,000, 1,200, 1,500, 2,000, 2,500, 3,000, or 3,500, p.s.i. A f t e r homogenization, each sample was cooled as described above to 4 ° C. W h e n t h a t t e m p e r a t u r e was reached, the time was recorded as zero age. Measured viscosity values were adjusted to age 50 hours (15). Viscosities were determined with an Ostwald viscometer, modified as previously described (15). The flow times of each milk sample were measured at two viscometer pressures because milk is a non-Newtonian liquid. The different pressures were obtained b y placing the viscometer in a vertical and an inclined position. There was a three-fold increase in the flow time for water when the viseometer was inclined. Statistical t r e a t m e n t of the viscosity data consisted of multiple linear regression analysis of the linear, quadratic, and cubic trends of viscosity with homogenization pressure. Two types of homogenizers, two positions of the viScometer, a n d nine t e m p e r a t u r e s at which viscosities were measured, were involved in this analysis. RESULTS
As honlogenization pressures were increased above 300 p.s.i, with a commercial sized homogenizer, there were obvious increases in the viscosity of the whole pasteurized milk at all t e m p e r a t u r e s at which viscosities were measured ( F i g u r e 1). The numerical increases in viscosity as a result of increasing homogenization pressures were greater when viscosities were measured at low temperatures. The relative increases, however, as determined by dividing the initial viscosity into the increase were not m a t e r i a l l y different at 49 ° C. f r o m those at 4 ° C. ttomogenization at pressures of 1,000, 1,500, 2,000, 3,000, and 3,500 p.s.i, increased viscosity as compared with the unhomogenized milk by respective averages of 7.1, 9.2, 11.9, 13.7, and 15.0~-. More complex effects on viscosity were f o u n d when the pasteurized whole milk was treated at lower pressures in the l a b o r a t o r y homoge,izer. At all tern-
1502
C . H . WHITNAH l~T AL
peratures, and when measured with the vertical viscometer, viscosities of the sample of milk that had been subjected to a homogenization pressure of 25 p.s.i. were lower t h a n for the unhomogenized milk. Differences between the viscosities obtained with the vertical and inclined viscometer were greater a f t e r this pressure t h a n for samples treated at any other homogenization pressure. Homogenization pressures, a f t e r which viscosities were least, varied when viscosities were measured with the inclined viscometer. At each t e m p e r a t u r e , and for some increase of homogenization pressure in the range 15 to 300 p.s.i., the viscosity decreased. This decrease in viscosity sometimes followed a previous increase at lower pressures. No reason is known for these more complex viscosity differences at low homogenization pressures. The fact that curves of viscosity versus homogenization pressure were usually nearly continuous at 300 p.s.i. indicates that similar homogenization effects were obtained f r o m the commercial type and laboratory type of homogenizer. TABLE
1
Statistical a~talysis ~ of factors affecting the viscosity of mille P o s i t i o n of viseometer
Vcrt.
Homogenizer component of tren d. L
Q
Temp . (¢C) 4 8 18 22.7 29 34 39 44 49
0 () 0 0 0 0 0 0 0
0 0 0 0 2 2 2 1 2
Incl. lmb. ]5 to 300 p.s.i. C L Q
0 3 1 ] 2 1 0 3 0
0 0 0 0 2 0 3 3 3
Vert.
C
L
Q
Degree of s i g n i f i c a n c e h 0 2 4 2 0 2 4 0 0 1 4 2 0 0 4 0 0 2 4 0 0 0 4 l 0 0 4 2 0 (~ 4 ] 0 0 4 1
Inc l .
Confl. 300 to 3,500 p.s.i. C L Q
0 0 0 2 0 0 0 0 0
4 4 4 4 4 4 4 4 4
2 0 3 3 3 2 2 2 2
C
0 0 0 0 0 0 0 0 0
" S i g n i f i c a n c e of linear, L, q u a d r a t i c , Q, and cubic, C, c o m p o n e n t s of t r e n d s of v i s c o s i t y of m i l k w i t h h o m o g e n i z a t i o n p r e s s u r e s i n c r e a s i n g f r o m 15 to 3,500 p.s.i. T e s t e d a t nine temp e r a t u r e s , u s i n g a l a b o r a t o r y or a c o m m e r c i a l h o m o g e n i z e r , a nd u s i n g t he v i s c o m e t c r both v e r t i c a l a n d i n c l i n e d enough to give a t h r e e - f o l d i n c r e a s e in flow t i me for w a t e r . b 0 = ] 7 w a s above 1 0 % l e v e l = N S ; I=F in the 5 to l(,¢/e l e v e l - - b o r d e r line N S ; 2 = F in th e 1 to 5 % l e v e l = * ; 3 = F in the 0.1 to 1 % l e v e l = * * ; 4 = F ~ 0 . 1 % level=~*.
Statistical analysis (Table 1) indicates that tlle linear increase of viscosity with homogenization pressures over 300 p.s.i, was always significant at the 0.1% level (scored 4). This is the m a j o r over-all trend. The quadratic statistical trends, indicating some c u r v a t u r e in the general linear increase of viscosity with homogenization pressure, are somewhat obscured in F i g u r e 1 by the logarithmic scale of pressures. However, they were significant (scored 2 or 3) in eight of the nine tests with the inclined viscometer. These trends were less pronounced for the vertical viscometer. The scores of 1 or 2 in six of the nine tests still a p p e a r to differ, however, from the scores of 0 in quadratic trends which were consistently found at pressures below 300 p.s.i. The existence of some significant cubic trends indicated a tendency for a curve to be concave
1503
SOME PHYSICAL Id~OPERTIES OF MILK. I I I .
f' 240(
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~200
200.0
........
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HOMOGENIZATIOhl PRESSURE, P,$,I
|1 , ~ . . . . . . I i . ,.. i ,.~ 0 15 25 50 I00 300500 10002000"3500 HOMOGENIZATION PRESSURE wP..~L
~IG. 1. E f f e c t s of h o m o g e n i z a t i o n p r e s s u r e on viscosities of milk. All viscosities a d j u s t e d ±o a g e 50 h o u r s , a n d m e a s u r e d w i t h v i s c o m e t e r v e r t i c a l ( ) or i n c l i n e d ( . . . . . ). B r e a k s at 300 p.s.i, a r e at c h a n g e f r o m l a b o r a t o r y to c o m m e r c i a l h o m o g e n i z e r .
u p w a r d over one pressure range but concave downward over another range. A t pressures over 300 p.s.i, such cubic trends were not significant even at a 10% level (scored 0) in all tests with the inclined viseometer and in eight of the nine tests with the vertical viseometer. W h e n milk was homogenized at pressures below 300 p.s.i., the only significant viscosity-pressure treuds at temperatures below 23 ° C. were cubic. At higher temperatures the linear t r e n d was more consistent with the vertical viscometer (scored 1 or 2) but was usually more pronounced (scored 0, 2, 3) with the inclined viscometer. Quadratic trends at pressures below 300 p.s.i, continued to score 0 at the higher temperatures. The significant cubic statistical
1504
c.H.
W H I T N A H ]gT AL
trends at some t e m p e r a t u r e s usually indicated a tendency for viscosity to increase at pressures n e a r 300 p.s.i, a f t e r a decrease beginning at pressures n e a r 15 p.s.i. DISCUSSION
The increase in the viscosity of pasteurized whole milk with homogenization pressure, reported here, substantiates and extends the range of some studies previously reported (1, 5). The lower homogenization pressures used in this s t u d y are not enough to significantly disintegrate fat globules. Effects of suspended inert particles on viscosity depend on the total volume of particles r a t h e r than on the size of individual particles. The exact causes of changes in the viscosity of milk as a result of homogenization are therefore obscure. They m a y be entirely independent of the changes in fat particle size used to define homogenization. They Inay include indirect effects of decreased particle size on the volume and composition of globule membranes. Increases in the viscosity of colloids are sometimes ascribed (a) to changes in particle shape (e.g., uncoiling of chains or unfolding of surfaces in protein particles) ; (b) to t r a n s f e r of material from the continuous phase to the disperse phase (e.g., the complexing of proteins with water, sugar, fat, or enzymes) ; or (c) to electroviscous effects. Electroviseous effects m a y involve electric charges or zwitterions on the surface of protein particles and possibly also charges buried in the folds or coils of the long chains of a globular protein. Effects similar to electroviscous uiay also involve Van der W a a l ' s type of interaction between particles (1l) or degrees of hydrogen bonding (e.g., hydration) not strong enough to be recognized as stable complexes (6). Effects of t e m p e r a t u r e on viscosity m a y more clearly indicate causes of these differences in viscosity. The pre(;ision of several theoretical and empirical equations relating the t e m p e r a t u r e and viscosity of milk is being studied. Effects of such factors as previous heat treatment, mineral content or nature of mineral combinations, amount of numerous minor organic constituents, as well as coneentration of m a j o r constituents, are a wide field for numerous studies. Differences in viscosity not only directly affect the stability and attractiveness of some fluid milk products but also may be i m p o r t a n t in the processing of milk products for use in other foods. SUMMARY
D a t a on effects of homogenization pressures from 15 to 3,500 p.s.i, on the viscosity of whole pasteurized milk at temperatures from 4 ° to 49 ° C. are re-ported. As homogenization pressures were increased above 300 p.s.i, there were highly significant linear increases in viscosity when measured at 9 temperatures. Pressures of 1,000, 1,500, 2,00,0, 3,000, and 3,500 p.s.i, increased viscosity as compared with the unhomogcnized milk by respective averages of 7.1, 9.2, 11.9, 1:3.7, and 15.0%. Quadratic trends were also present. At pressures below ;300 p.s.i, and t e m p e r a t u r e s above 23 ° C., linear trends were usually significant. At
SOME PHYSICAL PROPERTIES OF MILK. III.
lower temperatures
there were no significant linear or quadratic
1505 trends.
Cubic
t r e n d s s o m e t i n I e s i n d i c a t e d ali i n c r e a s e i n v i s c o s i t y a t p r e s s u r e s n e a r 300 p . s . i . after a decrease beginning
a t p r e s s u r e s n e a r 15 p.s.i.
REFERENCES (1) BATEI~IAN, G. F., AND SHARP, P. F. A Study of the A p p a r e n t Viscosity of Milk as Influenced by Some Physical Factors. J. Agr. Research, 35: 647. 1928. (2) BUOLIA, G. Ueber einige physikalish-chemische Merkmale der honiogenisierten Milch. Kolloid Z., 2: 353. 1908. (3) CAFFYt¢, J. E. The Viscosity Temperature Coefficient of Homogenized Milk. J. Dairy Research, 18: 95. 1951. (4) DOAN, F. J. Problems Related to Homogenized Milk. Milk Technol., 1 (6) : 20. 1938. (5) EVENSON, O. L., A]ZD FE~RIS, L. W. A Quantitative Determination of the Ammonia, Amino Nitrogen, Lactose, Total Acid, and Volatile Acid Content of Cow's Milk. J. Dairy Sci., 7: 75. 1924. (6) GLASSTONE, SAI~:UEL, KIETH, J. L., AND EYRING, HENRY. The Theory of Rate Processes. McGraw-Hill Book Co., New York. 1941. (7) HERI~ANS, J. J. Plow Properties of Disperse Syste,ms. Interscience Publishers, Inc., New York. 1953. (8) LEVITON, A., AND LEIGHTON, A. Viscosity Relationships in Emulsions Containing Milk Fat. J. Phys: Che~n., 40: 71. 1936. (9) MOORE,R. T., GIBBS, PETER, AND EYRING, HENRY. Structure of Liquid State and Viscosity of Hydrocarbons. J. Phys. Che,m., 57: 1025. 1953. (10) l~cTz, WM. D., WHITNAH, C. I~., AND BAETZ, G. D. Some Physical Properties of Milk. I. Density. J. Dairy Sci., 38: 1312. 1955. (11) SUTHE~LAN]), W~i. The Viscosity of Gases and Molecular Force. Phil. Mag., 36: 507. 1893. (12) TF~ETSVEN, W. I. Homogenization of Milk and Properties of tIomogenized Milk. Mast e r ' s Thesis, Univ. of Tenn., Knoxville. 1939. (13) TROUT, G. M. Ho~,ogenized Milk. Michigan State College Press, East Lansing. 1950. (14) T~OUT, G. M., HAL~O~AN, C. P., AN])GOULd), I. A. The Effect of Homogenization on Some Physical and Chemical Properties of Milk. Mich. Agr. Expt. Sta., Tech. B~dl. 145. 1935. (15) WHITNAH, C. H., RUTZ, WM. D., AN]) FRYER, H. C. Some Physical Properties of Milk II. Effects of Age upon the Viscosity of Pasteurized Whole Milk. J. Dairy Sci. 39: 356. 1956. (16) WIEGNER, GEORG. Ueber die Anderung einiger physikalishcr Eigenschaften der Kuhmilch mit der Zerteilung ihrer dispersen Phasen. Kolloid Z., 15: 105. 1914.