INDUSTRIAL A N D ENGINEERING CHEMISTRY
48
in addition contain a part of the vitamin C of the original raw milk (14). Although the digestibility of sweetened condensed milk has not been fully investigated, it is probable that for infant feeding it is not quite the equal of evaporated milk, since the type of curd precipitation is less favorable. Certain pediatricians have claimed that the ratio of sugar to milk solids is too wide for infant feeding, whereas others have claimed they have obtained satisfactory results with it. The fat of evaporated milk, being in a more finely divided condition, is a little more readily split by the digestive enzymes than the fat of sweetened condensed milk. Plain Condensed Milk
This product is prepared for manufacturers of ice cream, candy, and other users of milk in bulk. It is usually skim milk. Concentrated in the vacuum pan in the proportions of 3 or 4 parts of raw milk to 1 of concentrated, it is then usually put through a process known as superheating, which consists in injecting live steam directly into the milk while i t is still in the pan. Increased viscosity, or body, is thus given the milk, a property which makes it more valuable for the ice-cream and candy maker. The high viscosity is apparently the result of an incipient coagulation of the casein, for if the superheating process is allowed to go too far large curds appear in the milk. Literature Cited (1) (2) (3) (4) (5) (6)
Benton and Albery, J . Bid. Chem., 68, 251 (1926). Brennemann, “Abt’s Pediatrics,” Vol. 11, p. 669. Brennemann, J . A m . Med. Assocn., 92, 364 (1929). Courtney, Can. Med. Assocn. J., 17, 919 (1927). Cutler, J . A m . Med. Assocn., 92, 964 (1929). Daniels and Brooks, Proc. Soc. Erpti. Bioi. Med., 26, 161 (1927).
Vol. 22, No. 1
.
Daniels and Loughlin, J. B i d . Chem., 44, 381 (1920). de Dominicis and La Rotonda, A n n . chim. afipircata, 16, 294 (1926). Dutcher, Pa. Expt. Sta., Bull. 181, 18 (1923). Dutcher, Francis, and Combs, J . Dairy Sci., 9, 379 (1926). Grosser, Biochem. Z., 48, 427 (1913). Hammer, “Dairy Bacteriology.” Hart, Steenbock, Waddell, and Elvehjem, J . Biol. Chem., 77, 797 (1929). Hume, Biochem. J . , 16, 163 (1921). Jackson and Rothera, Ibid., 8, 1 (1914). Johnson and Norton, Food and Health Education, June, 1921, p. 89. Kerley, “Practice of Pediatrics.” Kohman, N a f l . Canners Assocn., Bull. 19-L (1927). Kramer, Latzke, and Shaw, J . Biol. Chem., 79, 283 (1928). Lane-Claypon, Rept. Local Gou. Board (GI. Britain) Pub. Health Med. Subjects, [N. S.], No. 6 3 (1912). (21) Leighton and Mudge, J. Biol. Chem., 66, 53 (1923). (22) Lowenburg, Med. Times, 67, 84 (1929). (23) McCollum, “Newer Knowledge of Nutrition,” p. 136 (1918). (24) McLean and Fales, “Scientific Nutrition in Infancy and Early Childhood.” (25) Marriott and Schoenthal, Arch. Pediatrics, 46, 135 (1929). (26) Miller, Analyst, 37, 49 (1912). (27) Mojonnier, Med. News, 87, 877 (1905). (28) Orr, Crichton, Crichton, Haldane, and Middleton, Scottish J . Agr., 9, 377 (1926). (29) Palmer. Proc. SOC.Ex5tl. B i d . Med.. 19, 137 (1921). Rice, Arch. Pediafrics, 44, 758 (1927). . Rice, J . Dairy Sci., 9, 293 (1926). Rice, Ibid., 9, 459 (1926). Rice and Downs, Ibid., 6, 532 (1923). Rice and Markley, Ibid., 6, 64 (1922). Rice and Miscoll, Ibid., 4, 261 (1923). Rogers, Canner, 62, 165 (1921). Rogers, Dahlberg, and Evans, J. Dairy Sci., 6, 64 (1922). Rogers, Deysher, and Evans, Ibid., S, 468 (1920). Rogers, Deysher, and Evans, Ibid., 4, 294 (1921). Schloss, A m . J . Diseases Children, 19, 450 (1920). Sommer and Hart, J. Biol. Chem., 40, 137 (1919). Sommer and Hart, Wis. Expt. Sta., Bull. 67 (1926). Sterling, Med. J . Record, 129, 612 (1929). Wiegner, Kolloid-Z., 16, 105 (1914). Willard and Blunt, J . Bid. Chem., 76, 251 (1927). (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20)
Application of Physical Chemistry to Ice Cream’ Alan Leighton BUREAUOF DAIRYINDUSTRY, WASHINGTON, D . C.
A
N ATTEMPT is made to show how physico-chemical methods may be applied to ice-cream problems as
yet untouched, and also to give in passing such pertinent information as may already have been acquired by the writers and others on the various physical properties of ice cream. Physically ice cream is probably one of the most complex dairy products. Its complexity is due, not only to the fact that flavorings, fruits or nuts, and sugar have been added to the dairy products which go into its make-up, but also to the fact that the physical effects of homogenization, aging, whipping, and freezing have in the finished product been superimposed upon the effects produced by such other factors as composition and the preliminary treatment of the materials from which it has been compounded. As might be surmised in the case of such a complex product, the methods of ice-cream manufacture have been developed largely from manufacturing experience. Some good research has been done to solve problems that have arisen in manufacture, but these investigations have not been closely linked together. It is the writer’s belief that further improvement of the product and more economical methods of manufacture can come about only through painstaking research dealing with fundamentals, to the end that a complete theory of ice cream may be developed. 1
Received November 19, 1929.
Adequate methods f or expressing or describing the physical properties and the flavor of ice cream are almost wholly lacking. It is known that a smooth, small-crystalled, moderately whipped, well-flavored product appeals to the taste, and experts can judge these qualities very satisfactorily; but for scientific research it is as yet impossible, after varying some process of manufacture, to express in figures the effect of this variation upon the properties of the finished product. For the solution of this problem the microscopical methods, thermal analyses, and physical tests which have been developed for the examination of metallic alloys offer great possibilities for adaptation to ice cream. For research purposes, a t least, it is necessary to be able to measure such physical properties as elasticity, tensile strength, ductility, hardness, resistance to shear, yield value, and density, together with the thermal properties such as specific heat, heat resistivity, etc. ; also degree of crystallization, hydration, and air-cell structure and size. When this can be done, it will be possible to make direct physical comparison of ice creams and to express numerically the characteristics of each. One can readily perceive the difference between water ice, milk sherbet, and ice cream, and can probably describe these points of difference; but exact evaluation of these differences is another matter. At the present time certain forms of penetrometers are in existence for measuring the so-called hardness of ice cream. It is easy to measure density, and by means of.the microscope
January, 1930
INDUSTRIAL AND ENGIXEERIKG CHEMISTRY
a great deal of knowledge can be obtained concerning structure. Very little has yet been done toward measuring the other properties. The fact must never be lost sight of that these properties will be of further value if they can be correlated with varying popular taste. They will then serve two purposes-to measure the effect of variations in experimentation, and to express differences in the palatability of the finished product, apart, of course, from the matter of flavor. Commercial ice cream is usually compounded from condensed or evaporated skim or whole milk or dry skim milk, cream, sugar, flavoring materials, and possibly gelatin or other stabilizers. Comparatively little attention has been paid to the physical properties or previous history of the ingredients ordinarily used in ice cream, although it is a matter of common knowledge that the characteristics of the finished product vary when evaporated milk, condensed milk, or dry skim milk is used as the source of milk solids not fat. Forewarming Temperature
The forewarming temperature greatly affects the stability of concentrated milks to heat. It has been shown that the forewarming temperature to which dry skim milk has been subjected is a factor in baking (6). Experiments are being carried out in this bureau to determine whether there is also an optimum temperature of forewarming for dry skim milk that is to be used in ice cream. One set of data seems to show definitely that ice cream containing dry skim milk, made from skim milk forewarmed a t 83" C., is of better quality than ice cream containing powdered milk previously subjected to other forewarming temperatures. Other experiments, in which gelatin was purposely omitted from the mix in order that differences in the powders would be more apparent, so far have shown no choice. The question is therefore still unsettled. It is quite possible that gelatin may react differently to the different powders, but this subject has not yet been investigated. Steps in Manufacture
It is customary to combine the ingredients of the mix and then to pasteurize for about 30 minutes a t 63" C. The mix is homogenized or viscolized while hot. It is then allowed to age or ripen by being held a t a temperature slightly above freezing for a few hours to a day. In the freezer not more than half the water in the mix is usually frozen while air is being beaten into the mass. Zoller (18) from calorimetric measurements indicates that about 20 per cent of the water is usually frozen as the mass is drawn from the freezer. Data given by Leighton (IO), based upon the measurements of actual freezing temperatures made upon concentrated icecream mixes, indicate that it is a more common practice to draw the mix when about half the water is frozen. After the ice cream is drawn from the freezer, the freezing is finished in the hardening room a t a temperature which usually is a t least 17" C. below zero. Freezing Temperature
The temperature a t which freezing of the mix begins depends mostly upon the amount of sugar and milk salts present, the milk fat and protein exerting a negligible effect. As the mix is usually proportioned commercially, ice starts to separate at from -2" t o -3" C. The mix cannot freeze completely until the eutectic for cane sugar is reached a t about -12" C. EI-en here it is unusual for cane sugar to separate in any quantity except occasionally in water ice. I n ice cream ice separation is probably never complete. Although Dahlberg ( 3 ) has shown that the presence of sugar actually im-
49
proves the texture of ice cream by decreasing the amount of ice that is formed, excess sugar should be avoided, as mixtures of too low freezing temperature are more unstable when removed from refrigerating atmospheres and it is more difficult to freeze such mixtures. An excess of milk solids not fat should be avoided also, not only because of the freezing point lowering, but also because milk sugar if present in too great quantities may separate, producing the so-called sandy ice cream. The unfrozen portion of ice cream is always highly supersaturated to lactose. A study of the solubility relationships of lactose (11 ) shows that in supersaturated solution it exhibits an extensive labile area in which crystallization is not easily initiated. This probably explains why all ice cream is not sandy. Pasteurization Temperature
Although 63 " C. is the usual temperature of pasteurization, Hening (7') has shown that a temperature 10 degrees higher may be used without markedly affecting the quality of the finished product. A temperature 20 degrees higher gave a cooked taste to the ice cream. Processing
Processing-that is, homogenization or viscolization of the mix-is an important step in the manufacture of ice cream. During this operation the fat particles are broken up so that their surface areas are greatly increased, and the equilibrium between their surfaces and the other constituents of the mix is greatly altered. A great deal of study should be given to this surface equilibrium. The viscosity of the mix is increased by processing, usually in proportion to the pressure used. High temperatures of homogenization usually lower the viscosity. The frozen ice cream is much smoother and of better quality when the mix has been homogenized. The processing markedly affects the texture of the finished ice cream. Dahlberg ( 3 ) has shown that the effect of homogenization upon the texture of ice cream is dependent upon the presence of milk fat and solids not fat. Homogenization reduced the percentage of gelatin required to form a gel in ice-cream mixtures. The smoothing effect of gelatin due to gel formation was increased by homogenization because a firmer gel was formed. He also points out that air cells in the ice cream are reduced in size about 50 per cent by homogenization, although no direct relation between texture and the size of these air cells can be shown. These data are in accord with those of other investigators. Aging
The process of aging is one which calls for further investigation. It is frequently the custom, after homogenization, to cool the mix to a temperature slightly above freezing and to hold it a t this temperature for a t least a day. An aged icecream mix usually whips more rapidly than an unaged mix, and the frozen ice cream made from it is of finer texture. During this aging process there is a thickening of the mix, and a t one time it was the theory that the beneficial effects of aging came from this viscosity increase. A few years ago the writer with hlr. Williams (12) showed that an ice-cream mix exhibits the property of basic viscosity. I n other words, if the mix is stirred after standing, the viscosity of the mix decreases t o a point below which it does not drop with further agitation. Since it was shown that the structural viscosity acquired by a mix during aging is destroyed by the beaters in the freezer, this work led t o the discarding of the theory that the beneficial effects of aging were brought about by this increase of structural viscosity. At the time the experiments on basic viscosity were published, it was shown that the basic
50
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
viscosity of a mix aged one day was about double that of a fresh mix, but that the basic viscosity did not increase with further standing. This phenomenon was not investigated further a t that time. Recently Hening (8) has shown that the effects of aging are marked in as short a time as 2 to 4 hours; in fact, that this short period produces almost the same beneficial results as the 24-hour period. A brief series of experiments in the Bureau of Dairy Industry laboratories seems to show that the basic viscosity of the mix practically reaches its maximum value in about 2 to 4 hours. This is in accord with the work of Turnbow ( I ? ) who showed that the basic viscosity of mixes as measured by the McMichael viscometer did not change materially after 2 hours’ standing. It is thus evident that the components of the ice-cream mix as it leaves the cooler after homogenization are not in equilibrium. Sommer (15) has recently pointed out that aging is apparently intimately connected with the surface phenomena and equilibria of the proteins and fat particles. From the foregoing basic viscosity measurements it appears that the aging process simply allows time for the equilibrium of the f a t and protein particles with the surrounding medium to become established. The exact nature of this equilibrium is yet to be determined, but it must be pointed out that the fat particles are in a melted condition as they leave the homogenizer and cannot acquire true surface equilibrium until they have had time to solidify, and that if no fat is present in the mix this basic viscosity increase does not take place even if gelatin is present. There is no marked change of the surface tension of a mix with age as measured with the du Notiy tensimeter. Much information will probably be gained by studying aging from the angle of surface equilibrium. This becomes more apparent on considering some work done on gelatin in the laboratories of the bureau by Mr. Kurtz (9). The methods in current use for the testing of gelatin for ice-cream work are really empirical. Plasticity measurements and basic viscosity determinations on ripened ice-cream mixes containing no gelatin and different grades of gelatin promised to give interesting information concerning the action of gelatin in the mix and a t the same time afford a measure in absolute units which could be used in standardizing the gelatin. It has been found that the basic viscosity of the mix is increased by the presence of gelatin, the degree depending upon the quality of the gelatin. Plasticity measurements indicate that in most instances the consistency value is practically identical with the basic viscosity value, which means that the yield value is usually a measure in absolute units of the structural strength of the gel. This substantiates the statement of Dahlberg (4) that the initial viscosity of a ripened ice-cream mix is due to the setting of the gelatin in the mix and that stirring breaks up the structure of the gel with lowering viscosity until the basic viscosity value is reached. This gel structure is destroyed in the freezer. A number of measurements have indicated that the tendency of the gel structure to re-form a t normal concentration is very slight. This is not in accord with data given by Dahlberg (4) and coworkers, who found upon experimentation that the gel structure did actually re-form after the ice-cream mix had been stirred, and who concluded that this re-gelation is responsible for the beneficial effect of gelatin in ice cream. It is apparent, therefore, that the conditions under which this gel structure may or may not re-form need careful study. An effect upon the surfaces of the fat and protein particles by the gelatin is also indicated and the setting of this surface gel may be significant. There is yet much work to be done on this problem. The same general idea has been advanced by Clayton (2). I n referring to work of Nugent ( I d ) , he says that Nugent finds that the “protective action of gelatin on a benzene
Vol. 22, No. 1
emulsion increases with the age of the gelatin,” whereas in contradiction “Elliott and Sheppard (6) found that the gold number of gelatin-water solutions increases with the age of the solutions; that is, the protective power of the gelatin gradually decreases, probably owing to an agglomeration of amicrons to form larger particles whose protective power is less. It may be,” continues Clayton, “that the thickening of the adsorbed layer of gelatin around the benzene globules in Nugent’s emulsions more than balances the lesser protective action of the gelatin due to aging.” I n most of the gelatins used in the Bureau of Dairy Industry experiments the Bloom test values paralleled the structural strength and basic viscosity measurements. I n the case of one gelatin apparently treated with alum to increase its gel strength, no direct relationship between yield value, basic viscosity, or Bloom test could be found. Work is still in progress on this problem, with observations on as wide a variety of gelatins as possible. Whipping
The whipping of air into the ice-cream mix during the freezing process, or more particularly the capacity of the mix to whip, is an important factor in ice-cream freezing concerning which much has been written. Theoretically, a t least, a low surface tension and high viscosity should be favorable to whipping, but this does not seem to be true in practice. Under certain quite limited conditions in which the viscosity of the mixes was varied by altering the temperature of homogenization, experiments in the laboratories have shown that whipping capacity is proportionally less the higher the basic viscosity ( I S ) . The data indicate that, because factors other than basic viscosity affect whipping capacity, the basic viscosity value is no direct indication of whipping capacity. Bancroft ( I ) has pointed out that to obtain a foam the only essential is a surface film; in other words, the concentration in the surface layer shall differ perceptibly from that in the mass of the liquid. All colloidal solutions will foam if the colloid concentrates in the interface or if it is driven away. Ice-cream mix will whip, but is not stable unless ice has formed. The interfacial relationships of ice-cream mixes have yet to be investigated, but it is evident that stabilization of the whipped mass can come from two possible factors; either the small ice particles go into the interface of the air bubbles or the foam is stabilized by the increase of protein-fat concentration due to the separation of the ice. Work needs to be done on these relationships. The whipping capacity of ice-cream mixes is markedly affected by the milk salts, as is shown by Sommer and Young ( l e ) ,who in their summary say: This preliminary report points out the importance of the milk salts as a factor in determining the whipping ability of ice-cream mixes, a factor that has hitherto been overlooked. These results indicate an explanation for variations in the whipping ability of mixes that are otherwise uniform in their composition, variations which always have been very puzzling and up to the present time inexplicable.
There is also a field for research in the matter of freezer design. A great deal of work dealing with emulsification has shown that each combination of materials seems to require for the best emulsification a specific speed and time of beating, and that the size and shape of the container are factors in determining this optimum speed and time. This suggests the construction of freezers of various types for different mixes. With the present freezers the ice-cream maker’s skill lies in his ability to recognize when the best whipping conditions are attained and when the ice cream is a t the best stage for drawing. In the hardening room the temperatures are so low that
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 CY
anhydride
11
$
The hydration equilibria become established almost instantaneously, so that the real cause of mutarotation is in the establishment of the equilibria: CY
anhydride
11
p anhydride
CY
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
