SOLVENTS FOR ZEIN PRIMARY SOLVENTS CYRIL D. EVANS' AND RALPH H. MANLEY' Armour Research Foundation, Illinois Institute of Technology, Chicago, Ill.
Primary organic solvents for zein are eiLher hydroxy compounds, amines, amides, or acids. The stability of zein in single solvents is, in general, much better than i n binary solvents containing water. Twenty per cent solutions of zein in diethylene glycol, triethylene glycol, propylene glycol, and dipropylene glycol have remained clear and fluid for two years when stored at 27' C., while a similar concentration of zein in 85 per cent aqueous ethyl alcohol stored at the same temperature usually gels in about 30 days.
HE growing commercial importance of zein is reflected in the increasing number of articles appearing in the literature, particularly in the large number of patents issued during the past few years. Since two of its greatest fields of potential usefulness seem to be in the disperse form as in spirit varnishes and lacquers, this series of articles deals with solvents for zein. Zein belongs to the class of proteins called "prolamines", which are characterized by their solubility in aqueous alcohol and by the fact that on hydrolysis they yield a relatively large amount of proline and amide nitrogen. Zein is the alcohol-soluble fraction of the protein in the corn endosperm. It is commercially available by extraction from the gluten obtained in the corn wet-milling process. It has long been characterized by its ready solubility in aqueous solutions of the lower aliphatic alcohols, by its insolubility in water, and by its stability toward the hydrolyzing action of dilute acids and alkalies. Cohn and his associates (8, 3) showed that zein is one of the most inert proteins, and Dill (4) observed that it is not amphoteric inasmuch as it does not combine with aqueous solutions of the mineral acids. Gorham (6) named and first described zein in 1821. Osborne (7) reported in 1897 that zein could be readily dispersed in 85 to 95 per cent ethyl alcohol, in 95 per cent methyl alcohol, in aqueous propyl alcohol, in glacial acetic acid, and in phenol. Osborne also reported the complete solubility of zein in glycerol at 150" C. and noted that such a solution could be heated to 200" C. without apparent denaturation of the protein. Critical peptization temperature is commonly defined as the temperature a t which a protein is dispersible in a solvent in all proportions and below which the protein is dispersible only to the extent of a few per cent or less. This property as exhibited by zein was probably first observed by Chittenden and Osborne (1) in 1892. In this paper they described the peptization temperature phenomenon for an alcoholic solution of zein, but apparently did not recognize it to be a definite temperature for each solvent or solvent mixture. In 1908 Galeotti and Giampalmo (6) observed a "discontinuity point" for zein in a ternary solvent mixture of alcohol, water, and ether, and also reported that this temperature was independent of the concentration of zein in the solvent. Dill (4) thoroughly investigated the peptization temperatures for
T
1 Present address, Northern Regional Research Laboratory, U. S.Department of Agriculture, Peoria, Ill.
zein in aqueous alcohols and showed that these values are independent of the concentration of zein as long as the amount of zein present is greater than about 5 per cent, but that they may vary widely at lower concentrations. He advanced the theory of a freezing point phenomenon, proposing that a protein-alcohol-water complex is formed which is completely miscible with the solvent above the peptization temperature, but below which the complex is a solid with very limited solubility. SINCE the usefulness of zein dispersions for such purposes as paper coatings, spirit varnishes, and lacquers depends in part upon the volatility of the solvent and its critical peptization temperature for zein, a study has been made of the types of molecular substituents which must be present in primary, binary, and ternary mixtures in order to peptize this protein. Because small amounts of a second component can greatly alter the critical peptization temperature of the zein in a solvent, great care has been necessary in collecting these data. For example, the monobutyl ether of ethylene glycol (butyl Cellosolve) will not peptize zein even a t its boiling point (171" C. a t 760 mm.) but if 5 to 10 per cent water is added, the zein will peptize immediately to form a clear homogeneous system. The present studies were made on a commercial sample of zein dried to constant weight over phosphorus pentoxide a t room temperature, and all solvents have been specially dried and redistilled. Since the conditions under which zein is extracted may have a marked effect upon its properties, all of the present observations mere conducted on the same sample. The present data concern the critical peptization temperature of such dried zein in single solvents. Subsequent articles will report the behavior of zein in binary and ternary solvents in which water may be one component. T o obtain the critical peptization temperature of zein in a solvent, a gram of zein was added to 10 grams of solvent in a test tube and then allowed to stand with some agitation until the mixture became clear. Then the dispersion was slowly cooled in ice water or solid carbon dioxide in acetone while being stirred with a thermometer. The temperature a t which the liquid showed the first signs of cloudiness was taken as the critical peptization temperature. I n instances where the solution temperature was found to be above room temperature, the mixture was slowly heated to a point high enough to produce a clear liquid, and then slowly cooled with stirring 1416
November, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLE I. ZEIN SOLVENTS Solvent Acetamide Acetic acid 2-Amino-2-ethyl-1 3-propanediola 2-Amino-2-methyl:l-propanol" 2-Amino-2-methyl- 1.3-propsnedioP o-Amipophenol b m-Aminophenolb
Critical Peptization Temp., O C. 82
14 38 24 > l o 9 (m. p.) >173 (m. p.) >122 (m. p.) Gels 18
