INDUSTRIAL AND ENGINEERING CHEMISTRY
1292
the plunger through the settled mass, through the use of a planimeter, a mathematical value for this property can be obtained. It is preferable that a decision regarding the settling tendencies of a paint be based not only on this mathematical value but on an examination of the curve. The curve offers additional definite and enlightening information regarding the condition of the settled material. This is illustrated in the three curves shown in Figure 5 , which are representative of three types of settling. Assuming that the same plunger was used and that the pendulum was equally weighted, a mathematical evaluation would show all three to be practically equal. Actually curve A represents a fairly deep layer of quite uniform material; curve B, a case where the major settling consists of a relatively thin, hard-packed layer such as is frequently encountered in nitrocellulose and oleoresinous enamels; curve C, the unusual condition in which a hard layer has formed in the upper portion of the settled mass. Settling such as is represented by curve A is not objectionable, and simple stirring will restore the homogeneity of the paint. Curve B represents an undesirable type of settling, and difficulty can be anticipated in reincorporating the caked material. Curve C usually indicates separation or gelation of part of the vehicle.
Vol. 25, No. 11
Curves cannot readily be compiled in the form of tables. Where tabulations are necessary, numerical values should be accompanied by descriptive data based on an inspection of the curves. The apparatus fo. reincorporating the settled paint, described in the previous paper, has proved satisfactory for determining the relative ease of remixing the settled paint to a uniform consistency.
CONCLUSION This paper is offered as a contribution to the general data on settling of paints, rather than as an integral piece of work. It is intended to develop a better understanding of the influence of storage conditions on settling, and of the simulation of these conditions in an accelerated test. A graphic method for determining the degree of settling is presented which greatly facilitates the interpretation of results. RECEIVEDMay 20, 1933. Presented before the Division of Paint and Varnish Chemiatry a t the 85th Meeting ot the American Chemical Society, Washington, D. C., March 26 t o 31, 1933
The Chemistry of Soft Rubber Vulcanization 11. The Function of Sulfur' B. S. GARVEY,JR., AKD G. THOMPSO~, The B. F. Goodrich Company, Akron, Ohio W h e n crude rubber is heated with sulfur, two reactions iake place, either of which can cause vulcanization: (1) the direct addition of sulfur to the double bond and (2) a reaction of the double bond in the hydrocarbon which does not involve a loss of unsaturation. I n the presence of accelerators, and possibly in their absence, sulfur acts as a catalyst for the second reaction. Accelerators affect both reactions. The relative effect on the two reactions probably depends on the accelerator, on the accelerator-
T
HE function of sulfur is a question of primary impor-
tance to any theory of vulcanization. I n spite of the large amount of work which has been reported on the reaction between rubber and sulfur and its relation to physical properties, there is still some uncertainty as to the nature of this relation. The work of a number of investigators has shown that sulfur dissolves in rubber to form a true solution. The solubility increases with rising temperature and also apparently with increasing coefficient of vulcanization. The solubility of sulfur in crude rubber a t 140" C. is at least 10 per cent and, according to Venable and Green (%), is approximately 20 per cent. The work of Dannenberg (9), Skellon (23), and Springer (26)is typical. The experimental work of Alexander (I), Axelrod ( 2 ) , Boiry (5),Hinricksen and Kindscher ( 1 4 , Kemp, Bishop, and Lackner ( l y ) , Lewis and McAdams (go), Spence and Scott (25), Bacon (9), and Weber (32) is excellent evidence that sulfur adds directly to the rubber with a loss in unsaturation equivalent to one carbon-carbon double bond for each atom 1
Part I appeared in IND.ENQ.C E I ~ M25, . , 1042 (1033).
sulfur ratio, and on the temperature of cure. In ordinary soft rubber cures with sulfur, both types of vulcanization come into play. The change in physical properties may be attributed to the building u p of a mechanical structure by two structure-forming reactions, and the properties of the vulcanizate depend to a considerable extent on the relative rates at which the two reactions take place. I n the high acceleratorlow sulfur type of compound the results depend almost solely on the nonsulfur reaction. of sulfur added. Under certain conditions there is also some substitution of hydrogen by sulfur which is accompanied by the evolution of hydrogen sulfide as has been demonstrated by van Heurn ( I S ) , Skellon (24),and H. P. and W. H. Stevens (28). Wolesensky (33) has shown that the rubber-sulfur addition compound evolves hydrogen sulfide very slowly at all the temperatures above 25" C. The work of Wolesensky and of H. P. and W. H. Stevens shows that the loss of hydrogen sulfide from the rubber-sulfur compound and the substitution accompanied by evolution of hydrogen sulfide are of negligible, or at least very minor, importance in ordinary soft rubber cures where the sulfur added is 10 per cent or less and the cure is never more than 8 hours a t 150' C. Kelly (16) showed that the acetone extract contains acetone-soluble sulfur compounds which probably come from nonrubber constituents. He also showed that some of the sulfur was combined with nonrubber constituents to form acetone-insoluble compounds. The latter form of combined sulfur involves a small correction for the coefficient of vulcanization. No chemical evidence has been found to show that there
November, 1933 Y
C
INDUSTRIAL AND ENGIKEERING CHEMISTRY
12993
is more than one type of sulfur addition to rubber. Kirchhof (16) on the basis of oxidation experiments concluded that sulfur combined as thioozone, leaving one atom of sulfur not bound to carbon. This view, however, is opposed by the findings of other investigators that there is a loss of unsaturation equivalent to one double bond for each sulfur atom added. Crude rubbers contain natural accelerators and age resisters varying in amount and probably also in nature as has been shown by Cummings and Sebrell(8), Martin and Elliott @ I ) , and van Rossem (22) among others. Cummings and Sebrell showed that these nonrubber constituents are not necessary for vulcanization but that they do affect the rate of vulcanization and the quality of the product. Purifiedrubber vulcanizes slowly and gives poor quality vulcanizates. The presence of these materials and the unknown changes they undergo during processing and vulcanization complicate considerably the problem of making deductions from rates of reaction. Except under specific conditions there is no relation between the coefficient of vulcanization and the state of vulcanization as judged by physical properties. This has been demonstrated by a number of investigations including those of Cranor (7), Endres (IO), Martin and Elliott ( $ I ) , and Weber (31). The coefficient of vulcanization has been defined by Weber (SO) as the number of grams of sulfur combined with 100 grams of rubber. Kirchhof (19) reported a well-cured compound containing only 0.55 per cent combined sulfur, some of which was probably combined with lead from the litharge in the batch. Bruni (6) by extrapolating from modulus data when plotted against combined sulfur obtained straight lines intercepting the combined sulfur axis a t 0.15 per cent. He thinks this may indicate that 0.15 per cent combined sulfur is the minimum necessary for vulcanization. Many rubber investigators believe that sulfur acts catalytically to cause some chemical reaction other than sulfur addition and that this other reaction is the primary reaction in soft rubber vulcanization. The sulfur addition is considered a secondary re-
INDUSTRIAL AND ENGINEERING CHEMISTRY
.. .. .. , ,
.. .,
: : : ..
c g *.
.. ..
bB
.0 0
e ' bid
mm
. i i . i i .33
u e
."
'00 ' 0 0
e a
e c
.lolo
.In10 'CIN
.lolo
lolo
lo10
GLQ
33
33
3-
."
:?? :?? :??
., .. ..
.. .. .. ..
mo
mm
00
33
33
3 d
Ig
%%% 2h.k 00
8
8
d
2
4
00000000000000000
E Il " rag,; 0000
5c m : ra- 6c
0000 000 000 000
2 E 1a
.'; k n
H 0000 0000 000 0 0 0 000
z
4-
23 0000 0000 000 000 000
a z
51m
??l $00
NN
m-3
a00
Vol. 25, No. 11
action which takesplace at the same time (11). Boggs and Blake ( 4 ) postulate two type,s of chemical unsaturation in rubber leading to two types of sulfur addition. While determinations of the minimum of sulfur necessary for vulcanization have been attempted, no experiment has been made to find the maximum combined sulfur which can be obtained without vulcanization. General knowledge of rubber t e c h n o l o g y suggests that well-vulcaniz e d compounds can be obtained with the same amount of combined sulfur as that in other compounds which are unvulcanized or which have undergone only incipient vulcanization. It is also known in a general way that the rate of sulfur addition and the rate of vulcanization as measured by physical properties can be varied independently, but definite experimental evidence is required. Certain types of compound revert so badly on long overcures that they appear to be unvulcanized. This suggests that the same chemically c o m b i n e d sulfur can be present both in vulcanized and in unvulcanized rubber. To explain reversion it is often said (4,16) that heat depolymerizes, or disaggregates, the rubber hydrocarbon and causes an effect which counteracts vulcanization. Experimental evidence to support this assumption is lacking.
EXPERIMENTAL PROCEDURH
M
.t:9
MIXINGAND TESTMethods of processing and testing are described in detail in the previous paper ( l a ) . The rubber used in all of these comING.
November, 1933
I N D U S T R I A 1, A N D E W G I N E E R I ?J G C H E M I S T R Y
pounds is part of the same batch used for the compounds discussed there. Because data reported in that paper are also used here, the compounds in the two papers are numbercd consecutively. The use of accelerators and zinc oxide introduces possible errors in the combined sulfur determinations since these materials may combine with sulfur to form acetone-insoluble products which would make the apparent coefficient of vulcanization higher than the true one. Correction for this would be difficult and uncertain. Such a correction would lower the coeficient of vulcanization of the well-vulcanized stocks with low combined sulfur and would thus accentuate the observed low values. It would tend, therefore, to confirm further the deductions drawn. I n several of the compounds there is 0.5 part of mercaptobenzothiazole, containing 0.19 part of sulfur. Most of this is probably extracted by acetone so that the error is undoubtedly small and constant. Even if it is assumed that all of the sulfur in the accelerator is extracted in compounds 17 and 18 and that none is extracted in compounds 15 and 16, the corrections would not be sufficient to alter the essential facts brought out in Table 11. For this reason no correction has been made. I n brief, there are several minor corrections which might be made on the coefficients of vulcanization as found. The possible errors appear to be constant for all the stocks compared or to be on the safe side with regard to the fundamental conclusions brought out by the work. The large amount of experimentation necessary to make accurate corrections is not, in the author's opinion, warranted by the small amount of added information to be obtained. The high accelerator-low sulLOW-SULFUR COMPOUNDS. fur type of compound was selected to obtain a high degree of vulcanization with low combined sulfur. While, in general, sulfur-bearing accelerators are most effective in this respect, they make uncertain the analytical determination of combined sulfur and were therefore not used. The recipes for the stocks used are as follows:
1295
The composition of the compounds is as follows: First latex crepe Zincoxide (BlackLabelKadox) Stearic acid Polybutyraldehyde aniline Mercaptobenaothiazole Diorthotolylguanidine Salicylic acid
19 100 1
20 100
5 I
...
... . . . . . . ... 4 1
. . . ... 102
110
21
22 100 5
.loo
23
100 5
.. . .. . . . . . . . . . . . . . . . . 0.5 0.5 0.5
. . . . . . . . .
. . . . . . 1 - 100.5 105.5 106.5
EFFECT OF HEAT. It was found that neither crude rubber nor certain types of vulcanized rubber show any material degradation as judged by physical properties after long press cures. Crude rubber was milled, sheeted out, and heated in the press for 360 minutes a t 142" C. It was then mixed as a regular batch with sulfur and given another cure of 15 and 30 minutes at 142". I n a similar fashion two o,ther compounds were mixed with everything but the sulfur and cured 360 minutes a t 142". The sulfur was then added on the mill and the complete compound cured 15 and 30 minutes at 142". The recipes are given below, and the data on the complete compounds in Table IV. The complete recipes 24, 25, and 26, carrying the heated rubber, correspond respectively to 12, 11, and 14: 24
First latex crepe (heated in press 360 min. at 142O C.) Polybutyraldehyde aniline Zinc oxide (Black Label Kadox) Sulfur ~~
100 0 5 1.0 1.0 102.5
25
First latex crepe Polybutyraldehyde aniline Zinc oxide (Black Label Kadox) Sulfur
lo! 1
{Heated in t h e 1 press 360 min. at 142' C.
102 0.5 102.5
26 First latex crepe Diorthotolylguanidine Zinc oxide (Black Label Kadox) Stearic acid Sulfur
100 4 5 1
Heated in {press 360 min.%/ 142' C.
110 0.8 c
110.8 11 12 100 100 1 1 0.5 1 1 0.5 ~~
First latex crepe Zina oxide (Black Label Kadox) Sulfur Polybutyraldehyde aniline Diorthoto!ylguanidine Stearic -.. acid
~~
13 100 5 0.6
14 100 5 . 0.8
. .. .. .. .. .. .41. . . . 4.1 . . ~ 102.5
102.5 110.6 110.8
To check the effect of heat degradation on vulcanized rubber, compound 27 was cured 15 and 360 minutes at 142'. The recipe is as follows:
-
27
First latex crepe Zinc oxide Tetramethylthiuram disulfide
100 5 4
109
Compound 11 is of particular interest because of the difference in the degree of vulcanization as shown by the physical Testing data are given in Table V. tests after the 15- and the 360-minute cures. The results of DISCUSSION OF RESULTS the physical tests and the combined sulfur determinations are given in Table I. COMBINEDSULFURAND VULCANIZATION. It is possible RATESOF VULCANIZATION AND OF SULFURCOMBINATION.to obtain a coefficient of vulcanization of 0.5 to 0.75 when the By using, with the same rubber-sulfur base, mercaptobenzo- physical tests show that the stock is not vulcanized beyond thiazole without zinc oxide (compounds 15 and 16), with zinc the stage of very slight scorch, as demonstrated by the data oxide (17), and with zinc oxide and salicylic acid (18), the assembled in Table VI. Table I shows that i t is also possible rates of vulcanization as measured by physical tests and of to obtain a high degree of vulcanization with a coefficient of sulfur addition were varied independently. The recipes are vulcanization of 0.5 to 0.8. It has been demonstrated by the as follows: results given in Table I1 that the rates of sulfur combination and of vulcanization as judged by physical tests can be varied 15 16 17 18 independently. These results show conclusively that the First latex crepe 100 100 100 100 0.5 0.5 0.5 0.5 Mercaptobenzothiazole amount of combined sulfur, as measured by the coefficient Sulfur 3.0 of vulcanization, is not the factor controlling vulcanization Zinc oxide ..2 ...0.. .. .3. ..0. . .3.0 5.0 5.0 . 1.0 Salicylic acid as measured by physical properties. There must be either - - 102.5 103.5 108.5 109.5 (a) two different kinds of sulfur addition or (b) sulfur addition and a second reaction which is not sulfur addition. With compound 11 (Table I) it is shown that the stock Table I1 gives the testing data. T'ithout sulfur none of these compounds will vulcanize as cured 15 minutes is well vulcanized whereas after a 360-minute is shown by the data of Table I11 on compounds 19 to 23, cure i t has the characteristics of a compound which is only inclusive. slightly vulcanized. The coefficient of vulcanization is the
INDUSTRIAL AND ENGINEERING CHEMISTRY
1296
same after both cures. It seems improbable that the sulfur wanders from one double bond to another during the heating; so that there is probably no change in the type of rubbersulfur union betweeh these two cures. In this case we have a stock with not only the same coefficient of vulcanization but actually the same combined sulfur having high and low degrees of vulcanization. The experiments on the effect of heat show that this difference cannot be attributed to a degrading action of heat on the rubber hydrocarbon. This result indicates that the alternative (b) above is correct and that during vulcanization two reactions take place, one of which is sulfur addition and the other a reaction of the hydrocarbon which is not combination with sulfur. This second, or nonsulfur reaction, must be a reaction of the double bond because when the double bond is saturated, as with hydrogen, the product cannot be vulcanized (6, 27). TABLEVI. COMBINED SULFURIN UNDERCURED RUBBER COM- CUREAT POUND
1 1 2 4 1 2 2 3
142'
COEFFICIENT OF
VUL-
c. C.4NIZATION
Min. 60 90 60
30 300 120 160 90
0.18 0.25 0.23
0.26 0.48
DEQREE OF VULCANIZATION BY
PHYBICAL TESTS"
No appreciable vulcanization Only the most sensitive tests show any evidence of vulcanization
0.44
0.53 0.49
a Verbal summaries of facts shown by the complete data given in other tables.
I n every case where the coefficient of vulcanization was one or more, there was definite evidence of vulcanization beyond the stage where the rubber could be processed. This is a good indication that sulfur addition itself causes vulcanization. In rubber-sulfur mixes a coefficient of vulcanization of at least 2 is always reached before good tensile properties are obtained. Crude rubber contains natural accelerators which promote the nonsulfur reaction. It is probable therefore that, if the second reaction were suppressed, a coefficient of considerably more than 2 would be necessary before good physical properties could be obtained. A coefficient of vulcanization of 2 means that 4.2 per cent of the double bonds in the rubber have reacted. Since vulcanization by sulfur addition alone requires, as a minimum, reaction a t 4.2 per cent of the double bonds, it is probable that this is also the case with the nonsulfur reaction. If this is true, the latter is not a reaction involving loss of unsaturation, since determinations on the unsaturation of vulcanized rubber show no loss of unsaturation not accounted for by combined sulfur, Determinations of unsaturation are believed to be accurate within one per cent. EFTECTOF HEAT. Investigators have often said (4, 16) that heat depolymerizes, or disaggregates, the rubber hydrocarbon and causes an effect which counteracts vulcanization. It has been found that heating unvulcanized rubber in the press for 6 hours a t 142' C. causes no material degrading of the rubber as shown by the physical properties of the resulting vulcanizates. It is also shown that some types of vulcanized rubber are not very much degraded under these conditions. Some other explanation must be found for various effects attributed to heat depolymerization. CHANGES DURING VULCANIZATION.I n the previous paper (12) it was pointed out that the different rates of change of physical properties of rubber-sulfur compounds suggest the building up of a mechanical structure.
Vol. 25, No. 11
When different types of stocks are compared, the various properties are found to develop at different rates. This can be seen by a comparison of Tables I and I1 and is further emphasized by comparing these results with those given in the previous paper on compounds 4, 6-10. No quantitative relation is apparent between the rates of change of the several properties. I n some cases all but one property may be completely changed while this one property remains almost unchanged-for example, the tendency of high accelerator-low sulfur stocks to freeze. The ultimate tensile strength seems to vary almost independently of all other criteria, including modulus. It is also noticeable that maximum values for different properties are seldom combined in the same stock. I n fact, maximum values for some characteristics are reached by only a few stocks. The fact that different characteristics may vary a t different rates suggests that in vulcanization at least two different types of structure are involved.
CONCLUSIONS The following conclusions concerning the function of sulfur during vulcanization are based on the work of previous investigators as well as on that reported here: Sulfur acts in a t least two ways during vulcanization: (1) It causes vulcanization directly by sulfur addition; (2) it acts as a catalyst for a second reaction which also causes vulcanization. This second reaction is a reaction a t the double bond of the hydrocarbon which does not involve a change in its unsaturation. Both of these reactions tend to build up a mechanical structure in the rubber. The actual structure depends on the composite effect of the two types of structure formation. The physical properties depend on the structure. Accelerators affect both the sulfur and the nonsulfur reactions. The relative effect on the two reactions probably depends on the accelerator, the accelerator-sulfur ratio, and the temperature of cure. I n ordinary soft rubber cures with sulfur, both types of vulcanization come into play. The physical properties of the vulcanizate depend to a considerable extent on the relative rates a t which the two reactions take place. I n the high accelerator-low sulfur type of compound the results depend almost solely on the nonsulfur reaction. I n ebonite, on the other hand, the product is formed primarily by the combination of rubber and sulfur.
LITERATURE CITED (1) Alexander, P., Chem.-Ztg., 36, 1289, 1340, 1358 (1912). (2) Axelrod, S., GummGZtg., 21, 1229 (1907). (3) Bacon, N., J. Phus. Chem., 32, 801 (1928). (4) Boggs, C. R., and Blake, J. T., IND.ENO.CHEM.,22, 748 (1930). (5) Boiry, F., Rev. g4n. caoutchouc, 1926, No. 23, 11; No. 24, 9 ;
No. 25, 9. ( 6 ) Bruni, G . , Ibid., 8, No. 75, 19 (1931). (7) Cranor, D. F., India Rubber W o r l d , 61, 137 (1919). (8) Cummings, A. D., and Sebrell, L. B., IND. ENG.CHEM.,21, 553 (1929). (9) Dannenberg, H . , Kautschuk, 1927, 104, 128. (10) Endres, H. A,, Caoutchouc & gutta-percha, 18, 11,089 (1921). (11) Fisher, H.L., Chem. Rev., 7, 127 (1930). (12) Garvey, B . S., and White, W. D., IND.ENG.CHEM.,25, 1042 (1933). (13) Heurn, F. C. van, Gummi-Ztg., 31, 746 (1917). (14) Hinricksen, F. W., and Kindsoher, E., Ber., 46, 1281 (1913). (15) Kelly, W. J., J IND.ENQ.CHEM.,12, 875 (1920). (16) Kelly, W. J., in "Survey of Rubber Chemistry," by Bedford and Winkelmann, p. 72, Chemical Catalog, 1923. (17) Kemp, A. R . , Bishop, W. S., and Lackner, T. d.. IND. ENQ CHEM.,20, 427 (1928). (18) Kirchhof, F., Kolloid-Z., 13, 49 (1913). (19) I b i d . , 26, 168 (1920). (20) Lewis, W. K., and McAdams, W. H., J. ISD. ENG.('HEM., 12, 673 (1920).
November, 1933
I N D U S T R I A L A N D E N G I X E E R I N G C €I E M I S T R Y
(21) Martin, G . , and Elliott, F. L., J. Soc. Chem. I n d . , 41, 225T (1922). (22) Rossem, A. van, I n d i a Rubber World, 59, 196, 251 (1919). (23) Skellon, H., I n d i a Rubber J.,46, 251 (1913). (24) Skellon, H., “Rubber Industry,” p. 172, Torrey and Manders, London, 1914. (25) Spenee, D., and Scott, J. H., Kolloid-Z., 8, 304 (1911). (26) Springer, G . , Ghmmi-Ztg., 15, 294 (1900). (27) Staudinger, H., and Fritschi, J., Helu. Chim. Acta, 5, 785 (1922). (28) Stevens, H. P., and Stevens, W. H., J. SOC.Chem. Ind., 48, 5 5 T (1929).
1297
(29) Venable, C. S., and Green, C. D., J. I N D .ESG.CHEM.,14, 319 (1922). (30) Weber, C. O., Gummi-Zfg.,17, 898 (1903): “Chemistry of India Rubber,” p. 283, Griffin and Co., London, 1909. (31) Weber, C. O., I n d i a Rubber J.,25, 276 (1903). (32) Weber, C. O., Kolloid-Z., 1, 33, 65 (1906). (33) Wolesensky, E., BUT.Standards J. Research, 4, 501 (1930).
March 30, 1933. Presented before the Division of Rubber Chemistry at the S5th Meeting of the American Chemical Society, Washington, D. C., March 26 t o 31, 1933. RECEIVED
Readjustment of Salts in Milk by Base Exchange Treatment Effect on Curd Tension J. F. LYMAN, E. H. BROWNE, AND H. E. OITING The Ohio State University and the M & R Dietetic Laboratories, Inc., Columbus, Ohio
T
HAT the milk of each species is adapted to the nutritive needs of its young has long been known (1). Man, the slowest growing species, produces milk of lowest protein and ash content; the rabbit, whose young make exceedingly rapid growth, produces a mammary secretion about six times as concentrated in protein and twelve times as rich in ash. The calf, whose normal rate of growth is intermediate between that of the child and the rabbit, is provided with maternal milk that contains about twice as much protein and three times as much ash as does human milk. Dilution of cow’s milk with water and the addition of lactose and fat in proper quantities bring the percentage of total solids, protein, fat, lactose, and ash into close agreement with that of human milk. However, the problem of bottle feeding of infants is not so simply solved. Differences in character as well as in concentration of the proteins, fats, and salts of human and cow’s milk have a most important effect upon the digestion and absorption by infants of the respective milk constituents (2). Cow’s milk, on reaching the human stomach, curdles in the form of large tough masses which digest slowly and lead to imperfect absorption of the milk constituents. Human milk, on the contrary, curdles under the influence of gastric juice in the form of small flocculent granules which digest rapidly with nearly complete absorption of the products of digestion. Three main factors are concerned with milk curdling in the stomach-casein, calcium ions, and the enzyme rennin. Modification of cow’s milk for infant feeding follows basic principles of altering the first two factors, such as (1) dilution with water, (2) dilution with a solution of a protective colloid such as barley gruel or gelatin, (3) precipitation of the calcium ions by adding alkali such as lime water, magnesium hydroxide, or potassium bicarbonate, (4) formation of a complex calcium ion by adding a soluble citrate, ( 5 ) partial precipitation of the calcium salts by heating the milk as by pasteurizing, cooking, or sterilizing, and (6) curdling the milk by adding acid before feeding, All of these methods are more or less successful in avoiding excessive curd formation in the stomach. Since calcium ions must be present for the curdling of milk by rennin, it should be possible to produce a soft-curd milk from normal hard-curd cow’s milk by removal of the calcium ions from the milk by treatment with base exchange silicates. The effect of zeolites on the inorganic constituents of cow’s inilk and on its curd tension are here reported.
The White House Conference on Child Health Protection for 1931 (4) reports: “Experience seems to have shown that an infant fed soft-curd milk may thrive, whereas fed toughcurd milk it may have digestive disturbances. This property applies also to older children and adults whose digestive powers are not vigorous.” The miters did not make records of curd tension after base-exchange treatment because they found that, when 20 per cent or more of the total calcium of cow’s milk is removed, there is no curd whatever formed by rennin action. Therefore they adjusted the various experimental factors, such as amount of base-exchange silicate and acidity of the milk, so as to remove a t least 20 per cent of the initial calcium of the milk. Pooled raw milk as delivered by farmers to a commercial plant was always used. The zeolites used were greensand and crystalite, which was kindly furnished by the International Filter Company of Chicago.
EFFECTO F DEGREEO F ACIDITYO F IhfILK ON REhlOVAL OF CALCIUMAND PHOSPHORUS BY BASE EXCHANGE SILICATES When cow’s milk with an acidity of about 0.16 per cent, calculated as lactic acid, is contacted with zeolites, the removal of calcium is comparatively small. Such milk, allowed to %owslowly through a bed of zeolite, under the conditions of the experiment, lost from 3 to 5 per cent of its initial calcium. Rennin formed dense curds in these treated milks. If the acid content of milk is raised to 0.3 per cent, calculated as lactic, by adding hydrochloric, citric or lactic acids, and the acidified milk then contacted with zeolite, removal of calcium is much larger as shorn in Table I. TABLE I. EFFE(TnF DEGREE OF ACIDITYOF MILKo s CALC IT-M .\ND PHOSPHORUS REMoV.4L BY ZEOLITES ACIDITYOF MILKA S WT.OF MOIST Ca RE- P REMILK LACTICACID ZEOLITE MOVED MOVED Cc. % Qrams % % 600 0.16 180 greensand 5 17 600 0.16 90 crystalite 3 14 180 greensand 15 24 600 0.30 90 crystalite 22 22 600 0.30
The sample from which 15 per cent of the initial calcium had been removed curdled with rennin while that from which 22 per cent of the initial calcium had been removed formed no
