INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1930
most of the water in the ice-cream mix is frozen. The size of the ice crystals and resulting texture of the ice cream will be influenced by the rate of freezing. Soluble Constituents
It is interesting to consider what becomes of the soluble constituents of the mix, as salts, milk sugar, and cane sugar. Thermal measurements made in the bureau laboratories on the rate of warming of frozen ice cream indicate that the sugars and salts are present in supercooled solutions, although exceptions exist. An interesting field of work is still open here. Conclusions
We have thus seen how physico-chemical methods may be applied to obtain a better understanding of the “why” of the ordinary processes for manufacturing ice cream, with the idea that improvement of these processes is still possible. The paper has also shonn how physical measurements may be applied in classifying ice cream, although it is most emphatically not a plea for a standard ice cream. Popular
51
tastes will always vary. It should be possible scientifically t o adapt the product to variations in popular preference. Literature Cited (1) Bancroft, “Applied Colloid Chemistry,” p. 270, McGraw-Hill, 1921. (2) Clayton, “Theory of Emulsions and Their Technical Treatment,” p. 87, Blakiston, 1928. (3) Dahlberg, New York Agr. Expt. Sta., Tech. Bull. 111 (1925). (4) Dahlherg, Carpenter, and Hening, IND. END. CHBM.,20, 516 (1928). (5) Elliott and Sheppard, Ibid, 13, 699 (1921). (6) Grewe and Holm, Cereal Chem., 5, 461 (1928). (7) Hening, Ice Cream Trade J.,24, No. 10, 53 (1928). (8) Hening. Unpublished paper read a t 1929 meeting of American Dairy Science Assocn. (9) Kurtz, J . Phys. Chem., 33, 1489 (1929). (10) Leighton, J . Dairy Sci., 10, 300 (1927). (11) Leighton and Peter, Proc. World’s Dairy Congress, Vol. I, p. 477, (1923). (12) Leighton and Williams, J . Phys. Chem., 31, 596 (1927). (13) Leighton and Williams, Ibid., 33, 1481 (1929). (14) Nugent, Trans. Faraday Soc., 17, 703 (1922). (15) Sommer, Ice Cream Trade J.. 26, No. 7, 41 (1929). (16) Sommer and Young, IND. ENG.CHEX.. 18, 866 (1926). (17) Turnbow, Private communication. (18) Zoller, Ice Cream Trade J., 20, No. 6, 53 (1924).
Some Methods of Preparing Quickly Soluble Lactose‘ R. W. Bell BCREAUOF DAIRYINDUSTRY, WASHINGTON, D . C.
ACTOSE is the sugar of milk and, until its recent synthesis (7), the milk of mammals was its only source. Searly two-fifths of the solids of cow’s milk are present as lactose. The annual production of lactose from cow’s milk is from three to four million pounds, which is but a small part of what could be produced if there were a greater demand for it. Lactose is not used to a greater extent for several reasons, among which may be mentioned its low solubility and small degree of sweetness. It will be pointed out in this paper that these drawbacks can be partially overcome by the application to commercial practice of facts which have long been known regarding lactose in its various forms. Estimates (8) based on the known production of milk show that over one-third as much lactose as cane sugar is consumed annually. Of the amount which is actually isolated from milk, about 25 per cent is used for establishing and maintaining the proper intestinal flora. It would be much easier to increase the consumption of lactose if it were more soluble and sweeter. Methods for making such a product in the laboratory have long been known as the result of the work of Schmoeger (9), Erdmann ( I ) , Hvdson and Brown (6),and others. These experimenters were interested chiefly in the study of the chemical and physical properties of the forms of milk sugar rather than in an investigation of their commercial possibilities. We have found that the possibility of producing a sweeter and more soluble lactose is of interest to the industry.
L
Forms of Milk Sugar
Three forms of milk sugar have been isolated. These are known as alpha anhydride, alpha hydrate, and beta anhydride. Mixtures of these forms have also been prepared. According to Hudson ( 5 ) , the following reaction occurs in solutions of milk sugar: ClzHzzOii
+ Hz0 1
(aanhydride)
1
C12Hz4012
2 Hz0
+
C12H22011
(hydrate) 2 ( p anhydnde)
Received October 3, 1929.
Equilibrium 1 is quickly established. Equilibrium 2, on the other hand, is slowly established and is the cause of mutarotation which lactose solutions exhibit when not in equilibrium. A different view is taken by Gillis ( 2 ) , who claims that in an aqueous solution of lactose the following equilibria tend to become established :
+ HgO 2 a hydrate 11 p anhydride + HzO p hydrate a
anhydride
11
$
The hydration equilibria become established almost instantaneously, so that the real cause of mutarotation is in the establishment of the equilibria: 01
anhydride
11
p anhydride
01
and
hydrate
11
t3 hydrate
The fact that no solid beta hydrate has been isolated merely proves, in the opinion of Gillis, that its solubility is greater than that of the beta anhydride (or alpha hydrate). I n other words, its isotherms would fall entirely in the supersaturated region. Alpha-anhydride lactose may be prepared by dehydration in the solid state of the alpha hydrate form a t 120’ C. in vacuum. Made in this way it is white and odorless. It has never been obtained by crystallization from solution. Alpha-hydrate lactose may be obtained pure by recrystallization below 90” C. from its aqueous solution. This is the milk sugar of commerce. It is a hard, sparingly soluble sugar, having a flat and but slightly sweet taste. It is nonhygroscopic, has good wetting properties, and keeps well. It is stable below 93.5” C. The method of preparation proves this, as does the change of both alpha and beta anhydride below 93” C. into the hydrate in the presence of water,
[CY]? = 88.0’.
Beta-anhydride lactose may be obtained by crystallization from solution above 95” C. (16). It is stable above 93.5” C., as proved by its method of preparation. The
INDUSTRIAL AND ENGINEERING CHEMISTRY
52
temperature 93.5" C. is both a transition ( a to 0) and a dehydration (hydrate to anhydride) point, [a]': = 35.5". Schmoeger (9), who discovered beta-anhydride lactose independently in 1880, states that it is stable in air, and the work of other investigators previous to that of Hudson (S), gives no evidence to the contrary. That it really adsorbs water from the air was shown by him. When any form of lactose is dissolved in water an equilibrium condition among the forms is finally obtained. The rate at which equilibrium is attained, other things being equal, depends upon the temperature. At temperatures above about 75" C. it is nearly instantaneous; at temperatures below 75" C. a time factor is involved which is about 90 hours at 0", 20 hours a t 15", and 10 hours at 25" C. These figures apply to all three forms. The equilibrium ratio between alpha lactose and beta lactose in solution changes only very slightly with the temperature between 0" and 100' C. (2, 6). The approximate ratio of alpha to beta in a stable solution is 40: 60, or 0.666.
7.=-Mas+z4?rw=.~ Figure 1-Solubility
Relations of Milk Sugar
Methods of Recovering Milk Sugar in a Sweet and Quickly Soluble Form from Its Solution
Several methods for preparing beta-anhydride lactose have been described. The methods of Erdmann (f), Tanret (If), and Hudson (4) give the anhydride mixed with considerable alpha hydrate. Hudson and Brown ( 6 ) later published directions for obtaining the beta anhydride in large pure crystals. Sharp (IO) has modified Hudson's method so as to obtain large yields and has shown that a sweet and quickly soluble lactose can be made commercially. If either form of lactose be made to crystallize from solution, the ratio of alpha to beta in solution tends to remain constant, as the unsaturated form changes to the supersaturated form on removal from solution of the supersaturated form. The yield of alpha or beta which may be obtained by crystallization, therefore, is chiefly a function of the concentration and temperature of the solution. Hudson (5) has published a figure (Figure 1) showing the solubility relations of milk sugar. I n this figure curve CH represents the concentration of the hydrate in equilibrium with a saturated solution of C-St, the anhydride. D-St is equal to CE C-St, or the final solubility of the beta anhydride, and is a curve marking the maximum solubility of the anhydride under the conditions of the experiment. Its solutions are all saturated, and unstable only below 93.5" C. Curve CA represents the concentration of the anhydride in equilibrium with a saturated solution of A-Sf, the hydrate. B-St is equal to C A A-SE, or the final solubility of the hydrate in its saturated solution. All solutions on this curve are saturated, and stable below 93.5" C.
+
+
Vol. 22, No. 1
It is clear that the initial solubility (C-Sc) of the beta lactose is much greater than the initial solubility (A-8:) of the hydrate. This greater initial solubility results in an apparently greater sweetness, and these two properties add to the commercial possibilities of milk sugar. It is not known whether the forms differ intrinsically in sweetness. It should be borne in mind, however, that, regardless of the form of milk sugar which is put into solution at any temperature, the final concentration of the dissolved sugar is the same for that particular temperature, that alpha hydrate is the stable solid form below 93.5" and beta anhydride is the stable solid form above 93.5", and that therefore the line B to 8 : marks the greatest concentration of equilibrated solutions of lactose that can be attained without supersaturation and the separating out of either alpha below 93.5" or beta above 93.5" C. The greater solubility of the beta is, then, only a temporary advantage, which depends mainly on the temperature and the reaction of the solution, as these factors govern the time required for equilibrium to be attained. The curves in Figure 1 are reproduced to facilitate the explanation of the solubility relations of the forms of lactose. The interpretation of these curves is believed to be correct, but their accuracy is questionable, as Hudson and Brown (6) state that there is only "slightly greater hydration a t the higher temperatures." This being the case, the ratio of beta to alpha at the lower temperatures is somewhat greater than indicated. It has been stated that lactose solutions come to equilibrium nearly instantaneously above about 75" C. and more slowly at lower temperatures. If the concentration of the solution a t this and other temperatures below 93.5" C. is such as to exceed the solubility of the form with respect to which the solution is more quickly saturated, the alpha form will separate out, some beta will change to alpha, and the ratio of alpha to beta will be reestablished immediately or gradually as conditions permit. This should hold true as long as it is possible for crystals of the form with respect to which the solution is supersaturated to form and is the explanation for the fact that nearly pure alpha-hydrate or beta-anhydride lactose (above 93.5" C.) can be obtained by evaporating solutions of any form of lactose to dryness. If the beta is being deposited some alpha will change to beta to maintain the equilibrium ratio. When lactose is obtained by crystal1izat;on and centrifuging, the solute is separated from the solution. This results in a low yield, 30-40 per cent of the lactose always remaining in solution. This dissolved sugar is, however, not a total loss, as it can be added to the next batch of sirup. If the solvent is removed from the solute practically all of the solute may be recovered. This was done as follows: (1) by evaporating to dryness with stirring a t atmospheric pressure; and (2) by drying solutions of lactose by the drum and spray-drying systems. Lactose Made by Evaporating Its Solutions to Dryness
Several methods for preparing beta-anhydride lactose have been published. The first was probably by Erdmann ( I ) , who boiled a solution of any form of milk sugar to dryness. Others (6, 10) have changed the method in several ways. In this work 700 cc. of hot water was added to 1200 grams of c. P. lactose and the mixture was heated to boiling in a paraffin bath with stirring. All of the sugar dissolved. Evaporation of the water was continued to dryness with agitation. The resulting sugar was white and crystalline. il solubility determination made as herein described gave 34.0 per cent as the total solids content of the filtrate. It
January, 1930
I N D U S T R I A L A N D EXGi'XEERING CHEMISTRY
had a specific rotation to the right with sodium light, a t 25" C., of 37". Hence it was composed of approximately 95.0 per cent of the beta and 5.0 per cent of the alpha form. Other experiments of somewhat similar nature were carried out with comparable results as to the appearance, solubility, and rotation of the product. I n one experiment commercial lactose was dissolved in twice its weight of water and concentrated under a high vacuum to 60.0 per cent total solids, after which it was heated to its boiling point a t atmospheric pressure in a paraffin bath. seeded with pure beta, and concentrated to dryness. When this method was employed with a good grade of crude sugar instead of the c. P. product there was considerable foaming during boiling a t atmospheric pressure and the dry material was rather dark in color. If solutions of crude sugar are decolorized in the usual manner with charcoal, phosphoric acid, and limewater, the decolorized sirup can be dried as described to yield a white product. Spray and Drum Drying of Lactose Solutions
Ten to 20 per cent solutions of commercial milk sugar were forewarmed to temperatures of from 60" to 100" C. and dried by the Gray-Jensen spray process. Very little trouble was experienced in carrying out the operations. To some judges the sugar thus obtained had a more pleasing taste than the pure crystalline beta form and in some cases it had a greater initial solubility. That the high solubility was due to its amorphous condition can be shown by comparing it with crystalline lactose made by precipitation from an equilibrated solution with alcohol and ether (3, 11). Solutions of the two products failed to show mutarotation. Each had a specific rotation at 20' C. to the right with sodium light of 55.0". This is proof that they were mixtures.of alpha and beta lactose in the proportion in which these forms exist in equilibrium, since the specific rotation of an equilibrated lactose solution is [ D ] ? = 55.0'. To test the relative initial solubilities of those products, 10 cc. of distilled water a t 27" C. was added to 6.0 grams of the amorphous material and to 5.0 grams of the crystalline precipitate. The mixtures were stirred for 2 minutes and quickly filtered. The filtrate had a total solids concentration of 34.0 per cent for the amorphous spray-dried powder and 26.5 per cent for the precipitated crystalline material. When the spray-dried lactose was examined with the aid of a microscope it was found to consist of small, spherical particles that contained one or more air holes. This increased its rate of solution over the larger crystalline product by increasing the surface per unit of solid in contact with the solvent. The spray-dried sugar was a mixture of the beta and alpha forms of lactose in the ratio in which they occur in an equilibrated solution, because the water was evaporated so quickly that the alpha form did not have time to change to the beta modification, or vice versa, as the solute was precipitated from solution. There must be a rate of evaporation of water which is just slow enough, provided other conditions are suitable, to permit the conversion of practically all of the unsaturated form to the supersaturated form, with conversion to the solid phase of the latter before all of the water has evaporated. The spray-drying system can be less readily adapted to changing this rate of evaporation than the drum-drier type. There was no indication in any of the many trials made with the spray drying units that the ratio of solid beta to solid alpha lactose could be increased in favor of the beta form. Experiments were conducted on vacuum drum and atmospheric double drum driers to find out whether pure or relatively pure beta lactose could be obtained by a correct correlation of such factors as steam pressure in the drums, revolu-
53
tions per minute of the drums, concentration of the solution being dried, and thickness of the film on the drum. The results of these experiments are summarized in Table I. The moisture determinations were made by the BidwellSterling toluene-distillation method (IS). These results, together with the specific rotations obtained, prove that nearly pure beta lactose can be made by desiccating equilibrated milk sugar solutions on either the vacuum drum or atmospheric double drum driers. I n determining the percentage of beta lactose present the specific rotation of beta-anhydride lactose was considered to be 35.5" dextro a t 20" C. and that of alpha-hydrate lactose 88.0" in the same direction. The angular rotation of the solutions was obtained as soon as possible after dissolving the sugar and filtering. Table I-Influence of Different Factors on Percentage of Beta Lactose Obtained by Desiccating Lactose S o l u t i o n s on D r u m Driers MomTURE
CONCN. OF
RuNSOLN.
%
ROTA- IN STEAM TION DRIEDSPECIFICBETA PRESOF PRODROTA- LACSURE DRUM UCT TION TOSE Pounds R . 9 . m. % Degrees %
REMARKS
LABORATORY SIZE VACUUM DRUM DRIER
1 2 3
20 25 20
0-5 15-20
4
30
55
3
0.71
40 3
90.9
5
30
55
5
0 83
41.6
88 4
6
30
65
5
0.50
38 7
93.9
7
30
65
7
0.73
41.0
89.5
8
30
65
5
0.88
41.8
88.0
10
3 3 3
1.50 0.30 0.30
50.0 38.0 37.0
72.4 95.3 97.1
Easily dried Easily dried Easilydried
SMALL ATMOSPHERIC DOUBLE DRUM DRIER
Coating o n d r u m thin Coating thin, irregular C o a t i n g even; fine fluffy product Sugar less fine and fluffy than No. R
9
30
75
5
0.20
36.0
99.0
10
30
75
7
0.40
38.4
94.6
C i e a F a n c e of drums reduced Sugar fine and fluffy Sugar c o a r s e r than No. 9
As the method used for determining moisture gives the total water in the sugar, that is, both the bound and free, it is apparent why the values for moisture noted are greater than the theoretical amounts that can easily be calculated from the percentage of alpha hydrate present as determined from the specific rotation of the samples. The factors controlling the percentage of beta in the dried sugar are not entirely clear from a study of Table I. The solutions used were free of crystals, so that a clear solution was delivered to the drum, The interesting thing about runs 2 and 3 is that nearly pure beta lactose wm obtained, even though the film on the drum was in a vacuum of approximately 26 inches. The film, being heated indirectly by the steam inside the drum, must have been hot enough to exceed 93.5" C., the transition dehydration temperature, and evaporation of the water must have been slow enough to permit deposition of chiefly the beta form, or the usual equilibriuni ratio between the forms of sugar in solution in the film on the drum was increased in favor of the beta form. The latter condition is highly improbable. The solutions dried in runs 4 to 8, inclusive, were free of crystals when delivered to the drums by the trough type feed. Suitable drying did not take place until the boiling material between the drums had reached a concentration of approximately 80 per cent total solids. The boiling material then between the drums and in the films when first formed on the drums was well seeded with crystalline beta lactose. The steam pressure in the drums and the revolutions per minute of the drums seem to be the most important factors governing the amount of beta lactose in the dry material. These factors should be studied for each drying unit.
INDUSTRIAL AND ENGINEERING CHEMISTRY
54
Discussion
One of the objections to the recovery of lactose by the desiccation processes here outlined is the fact that substances in the sirup other than the sugar appear in the dried product. For most purposes, such as for use in modified milk and for correcting the intestinal flora, these so-called impurities would not be detrimental. Because spray-dried amorphous lactose is hygroscopic and has poor wetting properties it is of doubtful commercial value. Also its saturated and highly concentrated solutions soon begin to deposit crystalline alpha because they are supersaturated with respect to it. Theoretically it should be possible to obtain highly concentrated solutions of this and other products which are mixtures of alpha and beta lactose, since the solubilities of individual substances are additive. The drum-dried products were largely amorphous, but more nearly crystalline in appearance than the spray-dried products. They were nonhygroscopic under ordinary room temperature and humidity conditions and had good wetting properties. The cost of the drying operation is variable, depending among other things, on the size of the unit used, the revolutions per minute of the drum, and the concentration of the solution. It has been estimated that the cost of dry-
Vol. 22, No. 1
ing enough 27.7 per cent lactose solution to make a pound of beta lactose is $0.008 when a 32 by 90 inch atmospheric double drum drier is employed. The cost of manufacturing the sweeter and more quickly soluble beta-anhydride lactose under the conditions just described should not exceed that of the alpha-hydrate lactose of commerce. Acknowledgment
The writer is indebted to the Douthitt Engineering Company, Chicago, Ill., and to the Buffalo Foundry and Machine Company, Buffalo, N. Y., for assistance in conducting some of the experiments discussed in this paper. Literature Cited Erdmann, Ber., 19, 2180 (1880). Gillis, Rec. fruu. chim., 99, 88 (1920). Hudson, Princeton University, Bull. 19 (4), 63 (1902). Hudson, Z. fihysik. Chem., 44, 488 (1903). Hudson, J . Am. Chem. Soc., 30, 1767 (1908). (6) Hudson and Brown, I b i d . , SO, 960 (1908). (7) Pictet and Vogel, Comfit. rend., 185, 332 (1927). (8) Pirtle, Private communication. (9) Schmoeger, Ber., 13, 1915, 2130 (1880); 14, 2121 (1881). (10) Sharp, Unpublished data. (11) Tanret, Bull. SOL. chim., 3, 15, 349 (1896). (12) Verschuur, Rec. Lruv. chim., 47, 123 (1928). (13) Wright, J . Dairy Sci., 11, 240 (1928).
(1) (2) (3) (4) (5)
Manufacture of Casein and Lactose from Skim Milk' Fred P. Nabenhauer SMITH, KLINE8. FRENCH LABORATORIES, PHILADELPHIA, PA.
ROM a practical standpoint skim milk is considered to be composed of casein 3 per cent, albumin 0.7 per cent, lactose 4.5 to 5.0 per cent, and mineral constituents about 1 per cent. The casein content varies from 2.7 per cent in the spring, or when the flow of milk is greatest, to 3.3 per cent in the winter. The other constituents do not vary so widely and depend on such factors as the breed of cows supplying the milk and the care and rapidity with which the milk is handled. These are especially important to the sugar yield. The casein is precipitated from the milk by the addition of either rennet or acids. Although this paper deals principally with muriatic casein and its whey, it will be necessary for comparison to describe briefly the steps in the various methods and t o point out the differences in the products obtained. These methods are all described in greater detail by Sutermeister ( 2 ) .
F
Preparation of Rennet Casein
I n the preparation of rennet casein the milk is warmed to 96" F. and treated very much as in the manufacture of cheese, with sufficient rennet to bring about curdling within 15 to 20 minutes, during which time it is stirred and gradually warmed t o produce a curd of suitable size for washing. After settling it is drained of the whey, washed with warm water to remove all the soluble impurities, and then pressed and dried. As it is used almost exclusively in the manufacture of plastics, rennet casein must be very carefully prepared and must possess the following properties: (I) high ash content and low fat; (2) freedom from mechanical dirt; and (3) nearly white color, free from colored particles. These qualifications can only be attained by (1) starting with very fresh and well-skimmed milk (any acidity increases 1 Received
October 19, 1929.
the solubility of the calcium phosphate in the milk); (2) cleanliness throughout all steps; (3) thorough washing and pressing; and (4)drying as rapidly and a t as low a temperature as possible. Preparation of Self-soured Casein
The acid casein used most extensively for technical purposes is lactic or natural soured casein. The milk, warmed to about 100" F. is allowed to undergo spontaneous souring or is aided by addition of sour whey from a previous batch. After a few hours the casein separates as a soft curd, which is brought together by direct heating with steam jets. The operator can judge when souring has reached the proper point and just how hot to cook the curd. If insufficient acidity has developed, the separation is incomplete and the curd is soft and stringy when heated. If too acid, it does not hold together and is liable to break up further on heating. The curd is pushed back from the gate with a screen and the clear whey drained off as thoroughly as possible, some of it being saved for the next run. A stream of cold water is run through the curd to remove excess whey, but this washing seems rather superficial and does not greatly reduce the ash content of the casein. After draining it is shoveled into hydraulic presses and gradually freed of as much moisture as possible. This type of curd is usually soft and soggy and cannot be pressed very dry. Often it is allowed to drain after pressing. It is then put through a shredding machine, which breaks it into small grains and spreads it on trays, which are placed in tunnel driers. After drying it is ground and sacked very much as muriatic casein described later. There are many other methods of heating and of using soured whey, the handling a t various steps being slightly different but the product being about the same in each case.
