July, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
Stabilization of Furfuryl Alcohol Many organic and inorganic bases were found to be effective in retarding the rate of change of the various properties of the alcohol, but only n-butyl amine and piperidine will be reported a t this time. To separate samples of furfuryl alcohol were added, by weight, 0.1,0.3,and 3.0 per cent n-butyl amine and piperidine, respectively. As was the case in the thermal stability study, approximately 40 cc. of each sample were then sealed in Pyrex glass tubes, placed in bombs, and heated in an oven a t 150' C . for the indicated period of time. The results obtained are given in Table 11. The differences between the physical properties of the stabilized and control samples, prior to heat treatment, are due to the presence of the stabilizer. It is evident from the data that both n-butyl amine and piperidine greatly retard the rate of intermolecular dehydration of furfuryl alcohol, as measured by the amount of water split out. When calculated (as in Figure 2) in terms of per cent furfuryl alcohol transformed to C10H1003, after 10.5 hours a t 150" C. the alcohol alone is transformed t o the extent of 33 per cent; in the presence of 0.1 per cent of the stabilizers, this value is only 0.4 per cent with n-butyl amine
817
and 1.1 per cent with piperidine. There is no advantage in employing these stabilizers in concentrations higher than 0.1 per cent of the furfuryl aleohol, and there is the disadvantage of discoloring the latter to a greater extent. In hydrogenating furfuryl alcohol to tetrahydrofurfuryl alcohol, a considerable amount of high-boiling residue is obtained. We believe that a sizable proportion of this residue results from the formation of furfuryl alcohol condensation products such as have been described. Use of stabilizers in the hydrogenation process may therefore lead to higher yields of tetrahydrofurfuryl alcohol.
Literature Cited Assoc. Official Agr. Chem., Official and Tentative Methods of Analysis, 3rd ed., p. 277 (1930). Dunlop, A. P., and Peters, F. N., Jr., IND.ENG.CHEM.,32, 1639
(1940). Erdmann, E., Ber., 35, 1855 (1902). Limpricht, H., Ann., 165,253,300 (1873). Pummerer, R.,and Gump, W., Ber., 56,999 (1923). Teunissen, H.P., Rec. trav. chim., 49, 784 (1930); dissertation, Leiden, 1929. Wissell, L.v., and Tollens, B., Ann., 272,291 (1892).
SOYBEAN PROTEIN ADHESIVE STRENGTH AND COLOR ALLAN K. SMITH AND HERBERT J. MAX U. S. Regional Soybean Industrial Products Laboratory', Urbana, 111.
The adhesive strength, hydrolytic cleavage with sodium hydroxide, and dispersion characteristicsof soybean protein are interrelated. Undenatured soybean protein gives low adhesive values when dispersed in alkaline salts but high adhesive values when dispersed in sodium hydroxide. A mild hydrolytic treatment of soybean protein with sodium hydroxide will change the dispersibility of soybean protein so that it will have good adhesive strength when dispersed in alkaline salts but not so good as the undenatured protein dispersed in sodium hydroxide. The dithionite salts (NaZStOd and ZnSSO4) are the best agents so far discovered for bleaching soybean protein. Coated paper has been prepared in the laboratory which shows the undenatured soybean protein to have higher adhesive strength and a lighter color than a good grade of commercial casein.
P
ROTEINS are used in industry for paper-coating adhesives, tub and beater sizing of paper, molded buttons and buckles, water paints, plywood and furniture glue, insecticide sprays, wax emulsions, leather dressings, fibers, films, and many other less important purposes. Of primary importance in the above uses are adhesive strength, water and oil resistance, color, dispersibility in
water, plasticity, and viscosity, all of which are related to particle or molecular dimensions and complicated structure of the proteins. Proteins from various sources are in competition with one another and, to a certain extent, with starch, gums, and the synthetic resins. The choice for a specific use depends upon the properties desired as well as the cost. The synthetic resins have a great advantage in their superior water resistance and are used where the glued article is exposed to severe moisture conditions. Perhaps it is worth while to point out also that until the recent overproduction of agricultural crops in this country, proteins had been so important in food economy that only waste or by-product proteins could be used economically for industrial purposes. For this reason and also because of their complicated chemistry, comparatively little scientific effort has been expended in their industrial development. While there is a great deal of literature on proteins, very little of i t pertains to industrial utilization or is even helpful a t the present state of development. Soybean protein, while relatively new in the field of proteins, is a primary industrial product. If the entire United States tonnage of soybean meal were considered as a source 1 A cooperative organization participated in by the Bureaus of Agricultural Chemistry a n d Engineering and of Plant Industry of the U. S. D e p a r t m e n t of Agriculture, a n d t h e Agricultural Experiment Stations of t h e North Central States of Illinojs, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, and Wisconsin.
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,INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 34, No. 1
pulled off. This serves as a comparative test for adhesive strength of the protein and is known as the wax pick test (2, 9). While it leaves much to be desired as a system of measurement, the results justify its use, and it has been adopted for the present purpose. Each of a series of soybean proPROTEIN PREPARATION. tein samples was given a different degree of alkali treatment. Considerable data on the preparation of soybean protein appeared in an earlier publication (7). One hundred grams of oil-free soybean meal ground to pass a 100-mesh sieve were stirred with 1000 ml. and 500 ml. of water, successively, the undispersed fraction being removed by centrifuging. A predetermined quantity of sodium hydroxide was added to the protein dispersion, making about 1300 ml. total volume, which was allowed to stand a t room temperature (25-30" C.) for 17 hours. The sodium hydroxide concentrations of the digesting solutions were 0, 2, 4, 6, 8, 10, 15, and 20 grams per liter, and the protein concentration was approximately 30 grams per liter. The protein was then precipitated with hydrochloric acid a t a pH of 4.2; the curd was removed in a centrifuge, washed twice with water, and dried below 50" C. The above protein samples were made up into paper coatings with a Martinson laboratory coater, using several different alkalies as 0 2 4 6 lo l4 l6 2o cutting or dispersing agents for the proteins ALKALI TREATMENT and clay as the coating pigment. Their relaFIGURE 1. RELATIVE CHANGES IN ADHESIVE STRENGTH O F SOYBEAN PROTEIN tive adhesive values were determined by the WITH HYDROLYTIC TREATMENT wax pick test, and the results are given in The carnauba-bayberry wax pick values are shown on the ordinate and the alkali treatment Figure 1. Theseresultsshow not only the effect of the protein on the abscissa. ofmild hydrolytic treatment on the relative adhesive strength of the proteins but also the effect of the dispersing agent; that is, these curves emphasize hesive strength are the factors which have held back its the fact that a certain degree of dispersion or reduction of the general acceptance by the paper-coating trade. The present protein micelles to smaller particle units is essential in order paper deals with these two factors. One of the few papers to get the maximum adhesive strength from soybean protein. containing information on the adhesive strength and color of RESULTS. It is apparent that a soybean protein prepared soybean protein is that of Roderick and Hughes (6) who from the aqueous meal extract with no alkali treatment has a evaluated calcium carbonate as a paper-coating pigment, using low adhesive strength when dispersed in alkaline salts, such various adhesives to show relationships between pigments as sodium carbonate, trisodium phosphate, or a combination and adhesives. They included commercial soybean protein, of ammonia and borax. This is explained on the basis of intogether with casein, animal glue, and various starches and complete dispersion of the protein by these salts. As the gums. Their work presented a fair picture of soybean protein amount of alkali treatment of the protein increases up to as a paper-coating adhesive a t the time of its publication. about 4-6 grams per liter, the adhesive strength of the protein I n addition they showed the influence of soybean protein on when dispersed in these salts increases to a maximum. On such characteristics of coated paper as brightness, opacity, the other hand, when sodium hydroxide is used as the disink resistance, smoothness, and gloss. Their work showed persing agent, the maximum adhesive value is obtained with the adhesive strength of commercial soybean protein to be soybean protein prepared with the least amount of alkali lower than that of casein but higher than that of other adtreatment. It is not expected that these results can be interhesives examined. Its brightness, however, is the lowest of preted in a strictly quantitative way because of the lack of the entire group of adhesives. precision in the method of measurement (2), but the recorded This paper presents data on the variation of the adhesive data are the average results of a number of determinations strength of soybean protein resulting from a mild hydrolytic and are considered relatively correct. They mere obtained treatment with sodium hydroxide and a new method of with the commercial waxes and also with the carnauba-baybleaching protein. While these data have been worked out berry waxes prepared in this laboratory, and the general by the techniques employed by paper coaters and may be of relation was further substantiated in a more practical way by immediate application by them, the results should also be printing ink tests in a paper mill laboratory. useful in other fields of soybean protein utilization. I n Figure 1 the ratio of protein to clay is fixed a t 12 grams of protein to 80 grams of clay. I n order to get a more comAdhesive Strength prehensive measurement: a series of tests were made in which the ratio of protein to clay was varied. The results are given METHOD.The paper-coating trade, which uses clay or in Figure 2. The amount of alkali used in preparing the other pigments in combination with protein adhesives, has coatings is based on the weight of dry protein. developed a system of picking the coating from the paper These results confirm those shown in Figure 1 and demonwith a series of wax sticks of different composition. One end strate further the fact known to some paper coaters that a of each stick is melted, quickly applied to the paper, and caustic dispersion of casein gives a somewhat better adhesive allowed to cool before the stick and adhering material are of the raw material, more than a billion pounds of the protein could be produced; this would be more than enough for all industrial uses now visible. Therefore, it can be stated that in soybeans there is available almost an unlimited source of supply of industrial protein. Soybean protein is one of the lowest priced proteins, but in order to take advantage of its economic possibilities, its properties must be modified so that it will be suitable and generally acceptable for most of the industrial needs. Heretofore the relatively low brightness or whiteness of soybean protein and the lack of knowledge as to how it should be prepared and dispersed t o obtain its maximum ad-
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1942
value than a sodium carbonate or sodium phosphate dispersion. The alkali requirement for dispersion of the unhydrolyzed soybean protein is 8.0 per cent, which is higher than those for the protein which has been modified by hydrolytic treatment. These results in practical terms mean that if a papercoating mill can use sodium hydroxide as a dispersing agent for coating mixtures, the most economical coatings are obtained with soybean protein which has had the minimum amount of preliminary chemical treatment. However, if the high alkalinity resulting from the use of sodium hydroxide is to be avoided, then the milder alkaline salts must be used as protein dispersing agents in combination with the protein modified by previous alkali treatment. These results further suggest that when various forms of soybean meal are used for adhesive purposes, the best results will be obtained by dispersion in sodium hydroxide rather than in alkaline salts. DISCUSSION. The ratio of alkali to protein used in the hydrolytic or degradation treatment was greater than that used for dispersing the protein for all samples except the concentration of 2 grams per liter, and the treatments of 2 and 4 grams per liter did not make a measurable change in the adhesive strength.
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this laboratory that the undegraded soybean protein molecule is very large, even in terms of protein chemistry, and that when sodium hydroxide is used as a dispersing agent, the type of dispersion is entirely dXerent from the dispersion in mild alkalies or of that resulting from the dispersion of the protein in soybean meal by water. The indications are that sodium hydroxide causes an initial dissociation of the protein, and that the action of stronger caustic and longer time of treatment results in a cleavage of the molecule to smaller “particle size” or molecular dimension. I n a study of the effect of calcium and phosphorus on the adhesive strength of casein by the same method, Gould and Whittier (1) showed that i t was lowered by these ions. I n the present study a corresponding effect was found also for soybean protein. Here again the explanation is based on the dispersing characteristics of calcium and phosphate ions which are known to hinder the dispersion of casein. The practice of paper coaters is to add small amounts of formaldehyde to coating colors to increase the water resistance of the coating and thus prevent the sheets from sticking together when wet or their smearing by wet hands, and to harden the coating for special purposes. Formaldehyde, even in small quantities, must be added to casein dispersions cautiously to prevent flocculation or gelation. Soybean protein, on the other hand, is much more tolerant to formaldehyde (8) which may be added in any quantity to coating mixtures as prepared above without fear of gelation. Large amounts, 10 to 20 per cent on the weight of the soybean protein, cause a . slight increase in viscosity but not enough to be serious, and the additon of formaldehyde does not measurably alter the adhesive strength of the protein.
Color and Bleaching
FIQURE
2.
WAX PICKTESTWITH VARIATIONS IN RATIO OF PROTEIN CLAY
CHANQES IN
I. Unhydrolyzed soybean protein dispersed in 8% NaOH. 11. Good grade of casein dispersed in 8% NaOH. 111. Commeroial soybean protein dispersed in 8% NaOH. IV. Casein dispersed in 8% NazCOa.
V.
Soybean protein extracted with 15 grams per liter of NaOH for 17 hours and redispersed with 6% NaOH.
One of the factors which makes difficult a quantitative interpretation of the results shown in Figure 1 is the claim of experienced paper coaters that the pick test will vary with the pH value of the coating mixture; that is, the mercerizing action of the alkali will cause an increase in the pick value which is confused with the effect of alkali on the protein. There have been no data in the literature showing the magnitude of this effect; but in Figure 2 the results for the dispersion of casein in both sodium carbonate and sodium hydroxide represent a maximum value which could be obtained. If one accepts, for purposes of argument, the difference between these two curves as a measure of the effect of caustic on the pick test, the general conclusions stated above are in no way changed. However, the authors do not believe the amount of alkali present causes much error through mercerization. There are not suflicient experimental data to explain these results in terms of fundamental chemistry. However, it would appear from this and other qualitative experiments in
The paper-coating trade has established certain standards of brightness for its product, and to be acceptable for paper coatings a protein must be light enough in color to meet these standards. Soybean protein was reported (4) to have a white color when prepared by Osborne’s salt extraction method, which calls for drying with alcohol and ether. These results are misleading in that a soybean protein dried with alcohol and ether has a false whiteness, due probably to its physical state. When this same protein is redispersed in dilute alkali, precipitated with acid, and dried in air, it will have a brown color. Some evidence has been found in this laboratory that a highly refined soybean protein will have a light color, and it may even be white. However, such a protein has never been prepared in substantial amounts. The soybean meal from which the protein is prepared is known to contain lecithin, cephalin, soluble and insoluble carbohydrates, saponins, plant pigments, lignin, probably tannin, and small amounts of several other substances. It seems possible that one of these materials or a combination of them is responsible for the color of the protein through coacervation, absorption, oxidation, or even chemical combination. The samples of proteins prepared for the adhesive strength tests were light brown in color, noticeably increasing in darkness with increasing alkali treatment. The color of a series of paper coatings, however, may or may not follow the order of color of the protein samples. According to brightness tests on coatings prepared from this series, the unhydrolyzed protein gave the darkest paper coating, and increasjng the hydrolytic treatment caused a small but steady improvement in color up to a maximum at about 1.0 per cent alkali. The color of the brightest of these coatings, however, was below that of a good grade of casein. While the cause of the brown color is unknown, empirical tests have resulted in a method for partial bleaching of the
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 34, No. 7
READINGS ON GENERAL ELECTRIC REFLECTANCE METEROF BLEACHED TABLEI. BRIGHTNESS AND UNBLEACHED PROTEIN BEFORE AND AFTER CALENDERIKG Protein Soybean Soybean Casein Soybean
Treatment Unhydrolyzed Commercial
Soybean
0.2%' hydrolyzed 0 . 1 yo hydrolyzed
Soybean Soybean
Unhydrolyzed Unhydrolyzed
"
Brightness Reading, Unbleached Uncalendered Calendered 74.3 75.6 76.0
Bleach,
7c
Method of Applicationa
Rleaching Agent, Sodium Dithionite 70.9 12 3 71.3 12 3 72.8 12 3
Brightness Reading, Bleachcd Uncalendered Calendered 76.5 78.0 78.4
73.2 25.3 r5.4
...
...
1
2
75.6
71.2
...
...
1
1
77.0
72.6
3 3
75.6 77.. 0
12.0 42.8
...
...
Bleaching .4gent, Zinc Dithionite ... 4.0 ... 6.0
a Method 1, dithionite added t o t h e protein dispersion after removal of t h e insolub1.e p a r t of t h e meal; method 2, dithionite added after concentration of t h e rotein curd (in methods 1 a n d 2 !he weight of t h e bleaching agent is based on t h e weight of t h e solution) : m e t h o i 3 , dithionite is added t o t h e protein a t t h e time of preparing t h e coating color, a n d t h e per cent of t h e bleach is based on t h e weight of t h e d r y protein.
salts were found also to bleach casein and linseed, peanut, and cottonseed proteins. The bleaching of soybean protein and casein a a s studied quantitatively by the somewhat indirect method of reflectance measurements on the brightness of paper coatings made with them. In using this method it must be remembered that the brightness of coated paper depends not only upon the color of the protein in the coating but also on the color of the pigment, the paper base stock, the weight of the coating, the method of dispersing the protein, and the calendering of the paper. Some of these factors have been ably evaluated and discussed in recent publications by Landes ( 3 ) ,Kirkpatrick ( 2 ) ,and Romland (6). The influence of the protein appears small in terms of brightness units, but to the paper trade it is an exceedingly important factor. In fact, its dark color has been the chief criticism of soybean protein by paper coaters. A number of hand sheets were prepared using clay as the pigment and observing all the known precautions to have them similar except for the protein. Bleached and unbleached soybean protein and casein were used as adhesives TABLE11. REFLECTANCE ON HARDYRECORDING SPECTRO- The brightness tests before and after calendering (four times PHOTOMETERn OF HAXD SHEETS WITH UNBLEACHED AND BLEACHED through) were made for this laboratory on the General ElecPROTEIN tric reflectance meter by two different paper companies, and (Pel cent of bleach based o n weight of protein; bleach added a t t h e time of the results are shown in Table I. Another series of measurepreparing t h e coating color) ments was taken on the Hardy recording spectrophotometer, Bleached and the data are given in Table 11. The data from the Sodium dithionite Zinc dithionite Protein Treatment Unbleached 493 8% 16% 8% Hardy curves were converted into per cent reflection of visible Soybean Unhydrolyeed 84.0 86.8 86.2 light for a tungsten lamp at 2848" K. by use of selected coSoybean Commercial 84.2 8i.7 85.3 8i.k 86.8 Casein ... 84.7 . . . 87.3 . . . ,.. ordinates. The results in Table I are the average for two or more sheets; those in Table I1 are for a single sheet. T h e H a r d y spectrophotometer i s owned b y t h e Electrical Engineering Department of t h e University of Illinois, and t h e readings were taken by Although both instruments were calibrated for visible light, J . 0. Kraehenbuehl. the Hardy instrument averaged six points higher in total reflectance for a given sheet of paper. However, both instruments gave relatively the same results. JVith experiments of When the protein is to be used for paper coating, it does not this type one must be satisfied with comparative data. require bleaching during the refining process as the application of the bleach a t the time of the preparation of the coating Literature Cited color is equally satisfactory. The amount of bleach required by the latter method for preparing soybean protein coatings (1) Gould, S. P., and Whittier, E. O., IND.EKG.CHEM.,24, 791 with brightness equal to casein coating is 4 to 5 per cent on (1932). the weight of the protein. When the bleach is applied during (2) Kirkpatrick, W. A., Paper Trade J., 109, No. 12, 36 (1939). (3) Landes, C. G., Ibid., 110, No. 11, 38 (1940). the refining process, the amount required for good results is (4) Osborne, T. B., and Campbell, G. F., J . Am. Chem. SOC.,20, 419 about 1 or 2 per cent based on the weight of the solution. If (1898). it is added to the solution after removal of the insoluble part ( 5 ) Roderick, H. F., and Hughes, A . E., Paper Trade J., 110, No. 8, of the meal, the decomposition of the dithionite will furnish 104 (1940). (6) Rowland, B. W., Ibid., 112,No. 26, 7.5 (1941). considerable acid to the solution so that the additional acid (7) Smith, A. K., and Circle, S. J., IND.ENG.CHEM.,31,1284 (1939). required for protein precipitation mill be somewhat reduced (8) Smith, A. K., and M a x , H. J., Ibid., 32,411 (1940). from the amount normally required. (9) Sutermeister, Edwin, "Chemistry of Pulp and Paper Making", I n addition to bleaching soybean protein, the dithionite 3rd ed., Chap. XIII, Kew York, John Wiley & S o n s , 1941.
protein to a light yellow. The paper coating made with the bleached protein is somewhat brighter than that normally obtained with casein. Early in these studies of soybean protein it was discovered that oxidizing agents such as chlorine and hypochlorite, when acting in an alkaline medium, cause a severe darkening of the protein, and that reducing agents such as sulfur dioxide gave a slightly lighter colored product. Further investigation showed that sodium dithionite (Na2S204)and zinc dithionite (ZnSn04),powerful reducing agents, were suitable as practical bleaches for soybean protein. The dithionite salts have been known also as hydrosulfites and are used by the textile trade for bleaching purposes. A dithionite treatment may be applied a t almost any step in the protein separation-e. g., after removal of the insoluble fraction of the meal or after concentrating the curd. I n general, it has been found to be most efficient toward the end of the refining procesR where there is less dilution.
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