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Alpha Lactose and Crystallization Rate
Alpha Ladose and Crystallization Rate T. A. NICKERSON and E. E. MOORE Department of Food Science and Technology University of California, Davis 95616 Abstract
hydrate but published nothing further on the subject. Nickerson and co-workers (1, 8) reported that mutarotation, which produces fllactose, limited cystallization rate only in special circumstances where crystallization was exceptionally fast. They found no indication that the beta form was inhibitory. On the other hand, van Kreveld (3) found that even small amounts of fl-lactose in solutions of stable anhydrous a-lactose markedly retarded the growth of needle and seed crystals. This conflicted with observations where a-hydrate crystals grew faster in the presence of /3-lactose (5). Therefore, this study of crystal growth was planned to delineate the roles of a- and /3-lactose more completely.
The effect of a- and fl-lactose on crystal growth was measured by weight gain of crystal aggregates in solutions differing in alpha-beta content. Solutions were prepared from stable anhydrous alpha-lactose; the beta content was controlled by direct addition and by mutarotation. In 5-h tests, aggregates of alphahydrate crystals grew more if fi-lactose was present whereas for short test periods (5 min) fl-lactose made no difference. The growth of regular a-hydrate crystals depended on the amount of a-lactose in solution and was independent of the amount of beta. Growth in a-stable solutions slows rapidly because of the rapid decrease in a-lactose by mutarotaticm. Conversely, increased supersaturation of the solution increased the rate of crystal growth because the increased alpha was not decreased by mutarotation but was sustained. It also explains why large amounts of fl-lactose in highly supersaturated solutions do not slow crystallization. Nucleation and growth of needle crystals were inhibited by fi-lactose. Thus, the effect of beta on growth rate depended on the type of alpha-hydrate crystal being produced in the solution. We suggest that beta influences the type of crystal normally formed. Inhibiting the growth of the needle or prism form allows mutarotation time to reduce the amount of a-lactose, thereby reducing the crystallization pressure and allowing the more commonly observed lactose crystal types to develop.
Materials and Methods
Introduction
Reports that fl-lactose inhibits lactose crystallization are fragmentary and incomplete, leaving the subject unsettled. Herrington (2) reported inconclusive experiments indicating that fl-lactose may retard crystallization o f a Received September 10, 1973.
Stable anhydrous a-lactose was produced by crystallization from methanol (4), which produces relatively large quantities with high purity (100% a-stable within the limitations of analytical procedures). The product is a fluffy white powder with bulk density 34.2 g/100 ml, compared with 83.8 g/100 ml for the a-hydrate powder from which it was produced. The alternative method of Sharp (6) limits both the quantity and the purity (93 to 94% a-stable) of the product (3). Generally, crystal growth was measured by the gain in weight of large clumps of crystals, but selected experiments used other crystals as described. Following the growth period the crystals were removed from the solution, allowed to drain onto an absorbent paper towel, and dried in a forced-air oven at 75 C for 1.5 h before being reweighed. Ten flasks were used, each containing 40 crystal aggregates approximately 1 to 1.5 cm in diameter (5) to evaluate the effect of solution composition on crystal growth. Only the solutions were changed in the various experiments. Optical rotations were measured at 25 C on a Perkin-Elmer automatic polarirneter (model 141), allowing continuous monitoring of mutarotation. A number of solutions were prepared containing 13 to 30 g stable a-lactose/100 ml water. Lower concentrations could not support growth over a reasonable period whereas
160
ALPHA
161
LACTOSE
TABLE i, Crystal growth in presence or absence of beta lactose. Crystal lot
Crystallization time (h) 20 a-S
1 2 3 4 5 6 7 8 9 10
1 1 2 2 3 3 4 4 5 5
.40 .36 .4O .53 .31 .30 .16 0 --.07 --.11
Solution composition (g/100 ml H.~O) 20 20 a-S+5~ a-S .48 .59 .58 .67 .43 .60 ,57 .54 .73 .59
Crystallization time
Solution composition (g/100 ml H.~O) 20 20 a-S a-S+5//
20 a-S+53
(g gain during test period) .33 .60 15 min .31 .65 15 min .38 .82 30 rain .32 .87 30 rnin .29 .68 1 h .45 .94 1 h .23 .80 2 h .30 .79 2 h .23 .89 3 h .15 .77 3 h
higher ones gave immediate spontaneeus crystallization and erratic growth. Most experiments, therefore, used 20 g a-stable/100 ml water which gave satisfactory growth for the 5-h tests. In early experiments spontaneous crystallization of the a-stable solution produced many extraneous crystals. Adding/i-lactose to the solution inhibited this spontaneous crystallization. We believed at first that the crystals grew faster in the presence of beta because of the inhibition of spontaneous crystallization, with more supersaturated lactose consequently available for crystal growth. In later experiments, however, the solutions were stabilized by filtering immediately after the a-stable was dissolved. This removed the initial nuclei so that the solution remained stable during the test. Only filtered solutions were used for this study.
.45 .44 .51 .61 .53 .60 .54 .49 .62 .58
.43 .44 .57 .65 .59 .74 .70 .77 .81 .74
added to the crystals and when removed. From these results and the rotation when the solution reached equilibrium (measured the following day), the quantities of the two stereo-isomers were calculated (7). This procedure was repeated with various amounts of a-stable, both with and without added fi-laetose. The quantity of ~-]aetose formed with change in time (Fig. 1) is similar in shape to van Kreveld's
14.0 i
i'I 12,0~
Results
Growth of crystal aggregates. Table 1 shows the effect of solution composition on gains in weight of the 10 lots of crystals. Growth was always greater when /t-lactose was added before the a-stable lactose. This unexpected conflict with results of van Kreveld (3) led to similar experiments repeated many times, but the crystals always made greater gains when /i-lactose was already present. To obtain more definitive data on the effects of the two optical isomers, crystal growth was measured in conjunction with mutarotation of the solutions. The solutions were prepared, rapidly filtered with a Biichner funnel using S & S 410 at 63.5 cm vacuum, and 100 ml were added to each lot of crystals. Optical rotation was measured both when the solution was
N t:l
{0.0
8"0I i I I/2
I
|
{
I
I
I
TIME (H)
I 2
I
I
I
l $
FIG. 1. Effect of mutarotation on a level in lactose solution with time. Legend: • = 20 g astable/100 ml H:O; A _-- a-stable + 5 g 3/100 ml H20; • = a-stable + 10 g 3/100 ml H~O. JOURNAL
OF
DAIRY
SCIENCE
VOL.
57,
No.
2
]6~
NICKERSON AND MOORE
TABLE 2. Relation of solution composition to growth rate. Crystal- a-stable lactose" lizing a/100 Crystal time ml growth (min) 0 5 10 15 20 30
(g) 15.3 14.4 13.7 13.1 12.4 11.8
(g) .31,'.33 .42, .48 .45, .51 .46, .49 .39, .48
a-stable + /~ lactoseb a/100 Crystal ml growth (g) 15.0 14.4 •3.9 13.4 12.7 12.2
(g) .32",'.32 .41, .50 .37, .56 .55, .56 .60, .60
.4
Z
°~
(Jl) \ z •
> ,,~
•
0
"20 g a-stable lactose/10O ml H...O. b 20 g a-stable + 10 g ~ lactose/100 ml H,,O. curves. The gain in weight of crystals with time is in Table 2. A relation between growth and a-lactose concentration was suggested by a surprisingly rapid growth during the 1st h that the crystals were in the solutions and a great decrease in the quantity of a-lactose in the solution during that period (Fig. 2). The effects of the two isomers were tested again with growth intervals less than an hour. Crystal lots were duplicate. The data again show that at the beginning growth was rapid while a-lactose content was high, and later growth dropped off rapidly as mutarotation decreased the a-lactose. In these many cases /3-lactose never slowed the crystallization of regular lactose crystals in solutions of a-stable. During the longer tests,
°~
I
....
13.0
J
I
14.0
AV
I
15.0
gcX/IOOml
FIG. 3. Relation of growth at 5-inin intervals to a-lactose content of the sohition. Legend: • -= 20 g a-stable:/1O0 ml H.~O; • = a-stable + 1O g /~/10O ml H:O.
beta increased growth by retarding mutarotation, thereby keeping the a-lactose content of the solutions high (Fig. 1). Weight gain per unit of time decreased rapidly as a-lactose decreased in the solution. For a 5-h test, about 8.5 g a/100 ml are needed to sustain crystal growth (Fig. 2). A rapid drop in growth rate occurred as the alpha content decreased (Fig. 3). The solution needed to have at least 13.7 g a/100 ml solution to get a measurable growth in 5 min. Growth of a-hydrate crystals is governed by the amount of a-lactose in solution. Although mutarotation is faster in solutions prepared .40 from only a-stable, the alpha content was essentially the same for short intervals in solutions with added fl-lactose (Fig. 1). The growth of lactose crystals is the same in both .20 solutions, being related to the amount of alpha All and independent of the amount of beta. • =6 •. • Crystal growth is greatly accelerated by ino creasing supersaturation. As supersaturation is increased, not only is the concentration of total lactose greater, but the amount of alpha in so- ,2o • = i l I I I I L I lution is also greater. Since the solution is in 6.o el• I0.0 12.0 '4.0 ~6.0 alpha-beta equilibrium, the alpha concentraAV Ooc/lOOml tion is decreased only by crystallization, not FIG. 2. Relation of growth during 5-h test period to a-lactose content of the solution. Legend: • ---- by mutarotation. Consequently, rapid growth 20 g a-stable/100 ml H~O; A = a-stable + 5 g can continue longer because the amount of ~/100 ml H20; • = a-stable + 10 g /~/100 ml alpha remains high in supersaturated solutions. H~O. Growth of needle crystals. It was not possiJOURNAL OF DAIRY SCIENCE VOL. 57, NO. 2
A L P H A LACTOSE
ble to explain why our results differed from resuits of van Kreveld (3). Most of his work had used needle crystals (those formed with over 20 to 25 g a-stable/100 g water), and he verified his conclusions with growth of a-hydrate seed crystals. During preparation of the solutions for the growth studies previously described, needle crystals had a much greater tendency to form spontaneously in solutions without added /3lactose. More quantitative tests were conducted with solutions prepared with 20 or 25 g astable lactose/100 ml water with and without addition of 5 g/3-lactose before addition of the a-stable. The solutions were seeded with a small amount of needle crystals after the lactose was dissolved and were stirred meehanically for 15 rain before the crystals were collected by filtration, dried, and weighed. Thus, from 20 g a-stable, .18 g crystal was recovered; from 20 g a-stable + 5 g/3, .11 g crystal was reexwered; from 25 g a-stable, 1.31 g crystal was recovered; and from 25 g a-stable + 5 g/3, .38 g crystal was recovered. More crystalline lactose was recovered without added beta, and microscopically the needle crystals were much larger. The greater recovery of lactose from solutions of only a-stable therefore could not be attributed to more crystals being produced spontaneously but was from greater total growth of crystals. This verifies observations and conclusions of van Kreveld (3) that/3-lactose inhibits the growth of needle crystals. Discussion
Van Kreveld (3) concluded that /3-lactose inhibited crystallization velocity in general. His solutions were prepared by varying lactose concentrations in solutions which were in aft equilibrium and by adding stable anhydrous a-lactose to yield a final concentration of 4.70 g in 20 g water. Thus, the solutions had the same total lactose but differed only in the amounts of alpha and beta. Consequently, as /3-lactose was increased, alpha decreased. Van Kreveld (3) concluded that both needle and seed crystals were inhibited in growth by fl-lactose. Our data verify that /3lactose inhibits the growth of needle crystals. The growth of regular a-hydrate crystals, however, is not inhibited by the/3-form. Van Kreveld's conclusions probably stemmed from his method of preparing solutions for growth of the seed crystals. Since total lactose was constant, decreased alpha concurrently increased beta, and since beta was inhibitory to needle crystal growth, he assumed that it was inhibi-
163
tory to the regular seed crystals. Our data show that crystallization of the usual form of a-hydrate crystals depends on the a-lactose content; alpha was decreased in van Kreveld's solutions as beta was increased, which effectively slowed crystallization by reducing the supersaturation of the crystallizing form. Our results indicate that the role of/3-lactose in determining growth rates varies with the type of alpha-hydrate crystal being produced in the solution. Herrington (2) showed that prisms (needles) are formed when lactose crystallization is rapidly forced. As crystallization pressure becomes less, the needles become shorter and broader. A further decrease leads to diamond-shaped plates, and then the more common crystal types appear. Van Kreveld's (3) solutions were highly supersaturated with a-lactose, a condition that favors the growth of needle-shaped crystals. Our data confirm that /3-lactose inhibits the growth of needle crystals. That beta does not inhibit the growth of regular lactose crystals leads to the conclusion that/3-lactose influences the type of crystal produced. Inhibiting growth of the needle or prism form, for example, allows mutarotation time to reduce the amount of a-lactose present, thereby reducing the crystallization pressure and allowing the more commonly observed crystal forms to develop. Under conditions usually encountered in the processing of milk products, the quantity of a-lactose is a major determinant of the nature and degree of crystallization; /3-Iactose seems to be important only under exceptional conditions. Van Kreveld also observed that the growth rates of all crystal faces in equilibrium solutions were about twice as high at 170% supersaturation as at 120% supersaturation. Our data indicate that this is due to the increased amount of a-lactose in solution. It also explains why an even greater amount of fl-lactose in more highly supersaturated solutions does not slow crystallization. Acknowledgments
This research was supported in part by grants-in-aid from the Foremost-McKesson Foundation, Inc., San Francisco, California. We are grateful to Dr. R. E. Feeney for use of the automatic polarimeter. References
(1) Haase, G., and T. A. Nickerson. 1966. Kinetic reactions of alpha and beta lactose. II. Crystallization. ]. Dairy Sei. 49:757. JOURNAL OF DAIRY SCIENCE VOL. 57, NO, 2
164
N I C K E R S O N AND MOORE
(2) Herrington, B. L. 1934. Some physieo-chemical properties of lactose. II. Factors influencing the crystalline habit of lactose. J. Dairy Sci. 17:533. (3) Kreveld, A. van. 1969. Growth rates of lactose crystals in solutions of stable anhydrous alactose. Neth. Milk Dairy J. 23:258. (4) Lira, S. G., and T. A. Niekerson. 1973. Effect of methanol on the various forms of lactose. J. Dairy Sci. 56:843. (5) Nickerson, T. A. 1973. Factors influencing
JOURNAL OF DAIRY SCIENCE VOL. 57, NO. 2
lactose crystallization rate. J. Dairy Sci. 56: 636. (Abstr.) (6) Sharp, P. F. 1943. Stable crystalline anhydrous alpha-lactose product and process. U. S. Pat. 2,319,562. (7) Sharp, P. F., and H. Doob, Jr. 1941. Quantitative determination of alpha and beta lactose in dried milk and dried whey. J. Dairy Sci. 24:589. (8) Twieg, W. (3., and T. A. Nickerson. 1968. Kinetics of lactose crystallization. J. Dairy Sei. 51:1720.
hydrate but published nothing further on the subject. Nickerson and co-workers (1, 8) reported that mutarotation, which produces fllactose, limited cystallization rate only in special circumstances where crystallization was exceptionally fast. They found no indication that the beta form was inhibitory. On the other hand, van Kreveld (3) found that even small amounts of fl-lactose in solutions of stable anhydrous a-lactose markedly retarded the growth of needle and seed crystals. This conflicted with observations where a-hydrate crystals grew faster in the presence of /3-lactose (5). Therefore, this study of crystal growth was planned to delineate the roles of a- and /3-lactose more completely.
The effect of a- and fl-lactose on crystal growth was measured by weight gain of crystal aggregates in solutions differing in alpha-beta content. Solutions were prepared from stable anhydrous alpha-lactose; the beta content was controlled by direct addition and by mutarotation. In 5-h tests, aggregates of alphahydrate crystals grew more if fi-lactose was present whereas for short test periods (5 min) fl-lactose made no difference. The growth of regular a-hydrate crystals depended on the amount of a-lactose in solution and was independent of the amount of beta. Growth in a-stable solutions slows rapidly because of the rapid decrease in a-lactose by mutarotaticm. Conversely, increased supersaturation of the solution increased the rate of crystal growth because the increased alpha was not decreased by mutarotation but was sustained. It also explains why large amounts of fl-lactose in highly supersaturated solutions do not slow crystallization. Nucleation and growth of needle crystals were inhibited by fi-lactose. Thus, the effect of beta on growth rate depended on the type of alpha-hydrate crystal being produced in the solution. We suggest that beta influences the type of crystal normally formed. Inhibiting the growth of the needle or prism form allows mutarotation time to reduce the amount of a-lactose, thereby reducing the crystallization pressure and allowing the more commonly observed lactose crystal types to develop.
Materials and Methods
Introduction
Reports that fl-lactose inhibits lactose crystallization are fragmentary and incomplete, leaving the subject unsettled. Herrington (2) reported inconclusive experiments indicating that fl-lactose may retard crystallization o f a Received September 10, 1973.
Stable anhydrous a-lactose was produced by crystallization from methanol (4), which produces relatively large quantities with high purity (100% a-stable within the limitations of analytical procedures). The product is a fluffy white powder with bulk density 34.2 g/100 ml, compared with 83.8 g/100 ml for the a-hydrate powder from which it was produced. The alternative method of Sharp (6) limits both the quantity and the purity (93 to 94% a-stable) of the product (3). Generally, crystal growth was measured by the gain in weight of large clumps of crystals, but selected experiments used other crystals as described. Following the growth period the crystals were removed from the solution, allowed to drain onto an absorbent paper towel, and dried in a forced-air oven at 75 C for 1.5 h before being reweighed. Ten flasks were used, each containing 40 crystal aggregates approximately 1 to 1.5 cm in diameter (5) to evaluate the effect of solution composition on crystal growth. Only the solutions were changed in the various experiments. Optical rotations were measured at 25 C on a Perkin-Elmer automatic polarirneter (model 141), allowing continuous monitoring of mutarotation. A number of solutions were prepared containing 13 to 30 g stable a-lactose/100 ml water. Lower concentrations could not support growth over a reasonable period whereas
160
ALPHA
161
LACTOSE
TABLE i, Crystal growth in presence or absence of beta lactose. Crystal lot
Crystallization time (h) 20 a-S
1 2 3 4 5 6 7 8 9 10
1 1 2 2 3 3 4 4 5 5
.40 .36 .4O .53 .31 .30 .16 0 --.07 --.11
Solution composition (g/100 ml H.~O) 20 20 a-S+5~ a-S .48 .59 .58 .67 .43 .60 ,57 .54 .73 .59
Crystallization time
Solution composition (g/100 ml H.~O) 20 20 a-S a-S+5//
20 a-S+53
(g gain during test period) .33 .60 15 min .31 .65 15 min .38 .82 30 rain .32 .87 30 rnin .29 .68 1 h .45 .94 1 h .23 .80 2 h .30 .79 2 h .23 .89 3 h .15 .77 3 h
higher ones gave immediate spontaneeus crystallization and erratic growth. Most experiments, therefore, used 20 g a-stable/100 ml water which gave satisfactory growth for the 5-h tests. In early experiments spontaneous crystallization of the a-stable solution produced many extraneous crystals. Adding/i-lactose to the solution inhibited this spontaneous crystallization. We believed at first that the crystals grew faster in the presence of beta because of the inhibition of spontaneous crystallization, with more supersaturated lactose consequently available for crystal growth. In later experiments, however, the solutions were stabilized by filtering immediately after the a-stable was dissolved. This removed the initial nuclei so that the solution remained stable during the test. Only filtered solutions were used for this study.
.45 .44 .51 .61 .53 .60 .54 .49 .62 .58
.43 .44 .57 .65 .59 .74 .70 .77 .81 .74
added to the crystals and when removed. From these results and the rotation when the solution reached equilibrium (measured the following day), the quantities of the two stereo-isomers were calculated (7). This procedure was repeated with various amounts of a-stable, both with and without added fi-laetose. The quantity of ~-]aetose formed with change in time (Fig. 1) is similar in shape to van Kreveld's
14.0 i
i'I 12,0~
Results
Growth of crystal aggregates. Table 1 shows the effect of solution composition on gains in weight of the 10 lots of crystals. Growth was always greater when /t-lactose was added before the a-stable lactose. This unexpected conflict with results of van Kreveld (3) led to similar experiments repeated many times, but the crystals always made greater gains when /i-lactose was already present. To obtain more definitive data on the effects of the two optical isomers, crystal growth was measured in conjunction with mutarotation of the solutions. The solutions were prepared, rapidly filtered with a Biichner funnel using S & S 410 at 63.5 cm vacuum, and 100 ml were added to each lot of crystals. Optical rotation was measured both when the solution was
N t:l
{0.0
8"0I i I I/2
I
|
{
I
I
I
TIME (H)
I 2
I
I
I
l $
FIG. 1. Effect of mutarotation on a level in lactose solution with time. Legend: • = 20 g astable/100 ml H:O; A _-- a-stable + 5 g 3/100 ml H20; • = a-stable + 10 g 3/100 ml H~O. JOURNAL
OF
DAIRY
SCIENCE
VOL.
57,
No.
2
]6~
NICKERSON AND MOORE
TABLE 2. Relation of solution composition to growth rate. Crystal- a-stable lactose" lizing a/100 Crystal time ml growth (min) 0 5 10 15 20 30
(g) 15.3 14.4 13.7 13.1 12.4 11.8
(g) .31,'.33 .42, .48 .45, .51 .46, .49 .39, .48
a-stable + /~ lactoseb a/100 Crystal ml growth (g) 15.0 14.4 •3.9 13.4 12.7 12.2
(g) .32",'.32 .41, .50 .37, .56 .55, .56 .60, .60
.4
Z
°~
(Jl) \ z •
> ,,~
•
0
"20 g a-stable lactose/10O ml H...O. b 20 g a-stable + 10 g ~ lactose/100 ml H,,O. curves. The gain in weight of crystals with time is in Table 2. A relation between growth and a-lactose concentration was suggested by a surprisingly rapid growth during the 1st h that the crystals were in the solutions and a great decrease in the quantity of a-lactose in the solution during that period (Fig. 2). The effects of the two isomers were tested again with growth intervals less than an hour. Crystal lots were duplicate. The data again show that at the beginning growth was rapid while a-lactose content was high, and later growth dropped off rapidly as mutarotation decreased the a-lactose. In these many cases /3-lactose never slowed the crystallization of regular lactose crystals in solutions of a-stable. During the longer tests,
°~
I
....
13.0
J
I
14.0
AV
I
15.0
gcX/IOOml
FIG. 3. Relation of growth at 5-inin intervals to a-lactose content of the sohition. Legend: • -= 20 g a-stable:/1O0 ml H.~O; • = a-stable + 1O g /~/10O ml H:O.
beta increased growth by retarding mutarotation, thereby keeping the a-lactose content of the solutions high (Fig. 1). Weight gain per unit of time decreased rapidly as a-lactose decreased in the solution. For a 5-h test, about 8.5 g a/100 ml are needed to sustain crystal growth (Fig. 2). A rapid drop in growth rate occurred as the alpha content decreased (Fig. 3). The solution needed to have at least 13.7 g a/100 ml solution to get a measurable growth in 5 min. Growth of a-hydrate crystals is governed by the amount of a-lactose in solution. Although mutarotation is faster in solutions prepared .40 from only a-stable, the alpha content was essentially the same for short intervals in solutions with added fl-lactose (Fig. 1). The growth of lactose crystals is the same in both .20 solutions, being related to the amount of alpha All and independent of the amount of beta. • =6 •. • Crystal growth is greatly accelerated by ino creasing supersaturation. As supersaturation is increased, not only is the concentration of total lactose greater, but the amount of alpha in so- ,2o • = i l I I I I L I lution is also greater. Since the solution is in 6.o el• I0.0 12.0 '4.0 ~6.0 alpha-beta equilibrium, the alpha concentraAV Ooc/lOOml tion is decreased only by crystallization, not FIG. 2. Relation of growth during 5-h test period to a-lactose content of the solution. Legend: • ---- by mutarotation. Consequently, rapid growth 20 g a-stable/100 ml H~O; A = a-stable + 5 g can continue longer because the amount of ~/100 ml H20; • = a-stable + 10 g /~/100 ml alpha remains high in supersaturated solutions. H~O. Growth of needle crystals. It was not possiJOURNAL OF DAIRY SCIENCE VOL. 57, NO. 2
A L P H A LACTOSE
ble to explain why our results differed from resuits of van Kreveld (3). Most of his work had used needle crystals (those formed with over 20 to 25 g a-stable/100 g water), and he verified his conclusions with growth of a-hydrate seed crystals. During preparation of the solutions for the growth studies previously described, needle crystals had a much greater tendency to form spontaneously in solutions without added /3lactose. More quantitative tests were conducted with solutions prepared with 20 or 25 g astable lactose/100 ml water with and without addition of 5 g/3-lactose before addition of the a-stable. The solutions were seeded with a small amount of needle crystals after the lactose was dissolved and were stirred meehanically for 15 rain before the crystals were collected by filtration, dried, and weighed. Thus, from 20 g a-stable, .18 g crystal was recovered; from 20 g a-stable + 5 g/3, .11 g crystal was reexwered; from 25 g a-stable, 1.31 g crystal was recovered; and from 25 g a-stable + 5 g/3, .38 g crystal was recovered. More crystalline lactose was recovered without added beta, and microscopically the needle crystals were much larger. The greater recovery of lactose from solutions of only a-stable therefore could not be attributed to more crystals being produced spontaneously but was from greater total growth of crystals. This verifies observations and conclusions of van Kreveld (3) that/3-lactose inhibits the growth of needle crystals. Discussion
Van Kreveld (3) concluded that /3-lactose inhibited crystallization velocity in general. His solutions were prepared by varying lactose concentrations in solutions which were in aft equilibrium and by adding stable anhydrous a-lactose to yield a final concentration of 4.70 g in 20 g water. Thus, the solutions had the same total lactose but differed only in the amounts of alpha and beta. Consequently, as /3-lactose was increased, alpha decreased. Van Kreveld (3) concluded that both needle and seed crystals were inhibited in growth by fl-lactose. Our data verify that /3lactose inhibits the growth of needle crystals. The growth of regular a-hydrate crystals, however, is not inhibited by the/3-form. Van Kreveld's conclusions probably stemmed from his method of preparing solutions for growth of the seed crystals. Since total lactose was constant, decreased alpha concurrently increased beta, and since beta was inhibitory to needle crystal growth, he assumed that it was inhibi-
163
tory to the regular seed crystals. Our data show that crystallization of the usual form of a-hydrate crystals depends on the a-lactose content; alpha was decreased in van Kreveld's solutions as beta was increased, which effectively slowed crystallization by reducing the supersaturation of the crystallizing form. Our results indicate that the role of/3-lactose in determining growth rates varies with the type of alpha-hydrate crystal being produced in the solution. Herrington (2) showed that prisms (needles) are formed when lactose crystallization is rapidly forced. As crystallization pressure becomes less, the needles become shorter and broader. A further decrease leads to diamond-shaped plates, and then the more common crystal types appear. Van Kreveld's (3) solutions were highly supersaturated with a-lactose, a condition that favors the growth of needle-shaped crystals. Our data confirm that /3-lactose inhibits the growth of needle crystals. That beta does not inhibit the growth of regular lactose crystals leads to the conclusion that/3-lactose influences the type of crystal produced. Inhibiting growth of the needle or prism form, for example, allows mutarotation time to reduce the amount of a-lactose present, thereby reducing the crystallization pressure and allowing the more commonly observed crystal forms to develop. Under conditions usually encountered in the processing of milk products, the quantity of a-lactose is a major determinant of the nature and degree of crystallization; /3-Iactose seems to be important only under exceptional conditions. Van Kreveld also observed that the growth rates of all crystal faces in equilibrium solutions were about twice as high at 170% supersaturation as at 120% supersaturation. Our data indicate that this is due to the increased amount of a-lactose in solution. It also explains why an even greater amount of fl-lactose in more highly supersaturated solutions does not slow crystallization. Acknowledgments
This research was supported in part by grants-in-aid from the Foremost-McKesson Foundation, Inc., San Francisco, California. We are grateful to Dr. R. E. Feeney for use of the automatic polarimeter. References
(1) Haase, G., and T. A. Nickerson. 1966. Kinetic reactions of alpha and beta lactose. II. Crystallization. ]. Dairy Sei. 49:757. JOURNAL OF DAIRY SCIENCE VOL. 57, NO, 2
164
N I C K E R S O N AND MOORE
(2) Herrington, B. L. 1934. Some physieo-chemical properties of lactose. II. Factors influencing the crystalline habit of lactose. J. Dairy Sci. 17:533. (3) Kreveld, A. van. 1969. Growth rates of lactose crystals in solutions of stable anhydrous alactose. Neth. Milk Dairy J. 23:258. (4) Lira, S. G., and T. A. Niekerson. 1973. Effect of methanol on the various forms of lactose. J. Dairy Sci. 56:843. (5) Nickerson, T. A. 1973. Factors influencing
JOURNAL OF DAIRY SCIENCE VOL. 57, NO. 2
lactose crystallization rate. J. Dairy Sci. 56: 636. (Abstr.) (6) Sharp, P. F. 1943. Stable crystalline anhydrous alpha-lactose product and process. U. S. Pat. 2,319,562. (7) Sharp, P. F., and H. Doob, Jr. 1941. Quantitative determination of alpha and beta lactose in dried milk and dried whey. J. Dairy Sci. 24:589. (8) Twieg, W. (3., and T. A. Nickerson. 1968. Kinetics of lactose crystallization. J. Dairy Sei. 51:1720.