An Active Whipping Substance from Soybean Flour J
BETTY M. WATTS AND DORIS ULRICH The substance responsible for the whipping of undenatured, solvent-extracted soybean flour is not glycinin. The whipping substance is extracted from the flour at pH 5.0, the isoelectric point of glycinin. This extract whips more readily and to a much greater volume than suspensions of the original flour; it has a better flavor and can successfully be used in place of egg white in many food preparations. About 37 per cent of the weight of flour is obtained as soluble extract. The residue may be used for the preparation of glycinin.
University of California, Berkeley, Calif.
R
ECENTLY it was shown (5) that solvent-extracted, undenatured soybean flour suspended in water could be whipped up to a foam greatly resembling egg white. Attempts to use this flour in its original form as a substitute for egg white have not proved highly successful. The foam structure is not sufficiently stable to withstand the necessary mixing and heating in cooked products; even in cold whips the bitterness of the soybean flour is often detectable, and its use is thus undesirable. It has now been found possible to prepare from the soybean flour an extract containing the substance responsible for the foaming. This extract whips more readily than the original flour and to a much greater volume. It is much less bitter and can successfully be used in a number of prepared foods.
Preparation of Fat-Free Flour The soybean flour used in the majority of these experiments was prepared by grinding in a steel ball mill yellow-green soybeans, believed to be of the Tokyo variety and grown in South America. About 60 per cent of the flour passed through a 100-mesh screen and 95 per cent through a 60-mesh screen. That which failed to pass the 60-mesh screen was discarded. The whole bean flour was then extracted with petroleum ether as previously described ( 5 ) . Untreated, whole bean flours prepared from several varieties of soybeans grown in the Middle West were also extracted and found to be identical in whipping properties with the variety mentioned above, although some flavor differences existed in the finished product. With one exception, none of the commercial solvent-extracted flours tried could be used for preparation of the whipping material, presumably because of protein denaturation during the extraction process. The exception noted was a special product claimed by the manufacturer to have a much higher protein solubility than the regular commercial solvent-extracted flour. It was found to be only slightly inferior to the flour prepared as described. Identification of Whipping Substance
A water extract of the fat-free flour was prepared by soaking 16 grams of flour in 100 ml. of water for 30 minutes. The insoluble material was centrifuged down and the extract tested for whipping ability as previously described (6). A volume increase of 1250 per cent was attained as compared with a maximum volume increase of 820 per cent previously obtained with the flour. The extract reached its maximum volume in approximately 8 minutes; the foam obtained was thick, moist, and heavy, and resembled whipped cream. A second portion of this water extract was then brought to pH 5.0, the isoelectric point of glycinin (2), by the addition of 5 per cent hydrochloric acid, pH changes being followed by means of the quinhydrone electrode. The precipitated gly-
chin was centrifuged down. The supernatant liquid, containing presumably no proteins other than the small amounts of legumelin and proteose reported present in the soybean (S), was found to whip slightly better than the original unacidified extract; it attained a foam volume of approximately 1320 per cent of the liquid volume. The behavior of this extract on whipping resembled that of egg white rather than whipped cream; i. e., a large volume of coarse foam was obtained during the first minute, and subsequent whipping resulted in a division of the coarse air cells. The optimum foam, obtained after approximately 6 minutes of whipping, was stiff and dry, and resembled whipped egg white. Portions of the acidified (glycinin-free) extract were then heated to various temperatures over a water bath. Turbidity began to appear a t approximately 55" C.; as the temperature was increased, a heavy flocculent precipitate (legumelin) settled out and left a water-clear solution. When whipped, the foam gradually decreased as the temperature increased ; but even the clear solution obtained after heating to the boiling point increased in volume 820 per cent on whipping, although the foam was very coarse and unstable. It appears from these observations that neither the glycinin nor the legumelin can be considered the active foaming substance. The presence of glycinin in the extract actually retards the whipping slightly, probably because of the increased viscosity. It is impossible from the evidence available to name the whipping substance. I n this connection it is interesting to note that the whipping ability of skim milk has been ascribed to very small amounts of an unknown foaming substance of a nitrogenous character that can be obtained from milk from which the casein, lactalbumin, and lactoglobulin have been removed ( I ) .
Preparation of Whipping Extract The method finally adopted for preparing the whipping material consists briefly of extraction of the fat-free flour a t pH 5.0, followed by neutralization and drying of the extract. 1282
OCTOBER, 1939
1283
INDUSTRIAL AND ENGINEERING CHEMISTRY
One hundred and sixty grams of fat-free EXTRACTION. flour are used per liter of water. The p H is brought to 5.0by the addition of 5 per cent (by weight) of hydrochloric acid (28.2cc.) to the water. The amount of acid required varies with the kind of flour used. The extraction is continued for 30 minutes. The suspended material is then centrifuged for 10 minutes. Approximately 750 cc. of extract are recovered which on subsequent drying yield 45 grams of solid material. A second extraction on the same material is carried out by adding the amount of water removed (750 cc.) plus sufficient acid to bring the p H again to 5.0 (3.0cc. of 5 per cent hydrochloric acid). The extraction is continued for 10 minutes and centrifuged for a n equal time. The 700 cc. of extract obtained contain about 14 grams of dried material. Thus in the two extractions about 37 per cent of the weight of flour taken is recovered as soluble extract. The period during which the flour is in contact with the acid should not be extended beyond that outlined above; with longer extraction periods (up to 6 hours) the extract becomes darker in color and more hygroscopic in character, probably as a result of a gradual hydrolysis of the sucrose to invert sugar, followed by partial caramelization during the drying period. The extraction is reasonably complete in the half-hour period if the flour is ground fine enough to pass a 60-mesh screen. Coarser flour is less completely extracted, and finer flour forms lumps during the addition of the acid which makes mixing difficult. The residue from this extraction should make excellent material for the preparation of purified glycinin for technical purposes. The glycinin (practically insoluble a t the pH of the extraction) is undenatured and is freed from water-soluble proteins and carbohydrates. Satow (4), in fact, recommends such a preliminary acid washing in the preparation of glycinin to be used as a plastic. NEUTRALIZATION. Neutralization of the acid filtrate is desirable for two reasons. First it improves the flavor. Secondly, it retards the formation of invert sugar during the subsequent drying. For the kind of flour used in these experiments, the first extract requires 1.66 cc. of 5 per cent sodium hydroxide per 100 cc. to bring the pH to 7.0. The second requires 0.52 cc. per 100 cc. The amount of alkali required would probably vary with the kind of flour used. A turbidity begins to develop during the neutralization a t about pH 6.4and increases with further addition of alkali. No attempt has been made to determine the nature of this precipitate. The p H adjustment has little effect on the whipping ability; approximately the same foam volume and stability are obtained a t all p H values between 5.0 and 7.0, but the foam is somewhat drier and more brittle in character a t the lower pH. DRYING. The neutralized extract is spread out on porcelain plates in layers 5 to 10 mm. thick and dried under reduced pressure in an electrically heated vacuum oven. Under these conditions the drying is completed in 4 to 8 hours without significant loss in foaming ability. Higher temperatures result in partial loss of whipping ability and caramelization of the sugar. Lower temperatures are equally undesirable, since during the longer drying period required fermentative changes often occur which impair both flavor and whipping ability. The dried material has a tendency to stick to the container and can be cracked and scraped off only with difficulty, although it dissolves freely on the addition of water. It seems likely that the difficulties of drying could be eliminated and a more desirable product obtained by use of the spray-drying technique now widely employed in the preparation of dried milk, eggs, etc. Unfortunately facilities were not available for adequately testing the efficiency of this method with the soy extract.
Characteristics and Uses of Dried Extract The moisture-free extract analyzes as follows: protein (total nitrogen X 6.25),32.2 per cent; ash, 14.3; nitrogenfree extract, 46.5; sucrose, 28.2; reducing sugar as invert, 0.017. The material is light yellow in color. It has both a sweet and a salty taste and also a slight burning sensation on the tongue, characteristic of many of the salts of potassium. The typical bitterness of the original flour is not apparent in the extract. It is somewhat hygroscopic in character; if allowed to remain exposed to damp air, it becomes sticky within a few hours. However, it may be kept indefinitely in a tightly closed container. The whipping ability of various concentrations of the dried extract is as follows: Concn. of Soybean Extract, 5 7.5
10
15
Volume Increase,
%
1220
1310 1360 1480
Cc. of Drainage in 20 Min. 10 8
7 5.5
The whipping time was 6 minutes. The foam volume and drainage were measured as previously described (6). Because of the difficulty of preparing large amounts of the extract under laboratory conditions, the material has been tested only in a few type recipes. Solutions containing 10 to 15 per cent of the dried extract were successfully substituted for egg white in standard recipes for hard meringues (meringue shells or kisses), candies of the divinity type, and soufflbs. The products obtained were practically indistinguishable in texture and flavor from those prepared in the same manner with fresh egg white. The extract was unsuccessful in the preparation of angel cake; the resulting cake was coarse, bready in texture, and poor in volume, and resembled the cakes which the authors have invariably obtained when commercial dried whites were substituted for fresh egg white. In cold whips, toppings, frozen desserts, etc., where it is not necessary or desirable to obtain the maximum foam volume, the concentration of the soybean extract can be cut down to 4 or 5 per cent, and in most cases the dried extract can be dissolved and whipped directly in the fruit juice or sugar sirup.
Literature Cited (1) Ansbacher, Flanigan, and Supplee, J. Dairy Sci., 17, 723 (1934). (2) Hartman and Cheng, J. Phys. Chem., 40,453 (1936). (3) Osborne and Campbell, J. Am. Chem. SOC.,20,419 (1898). (4) Satow, Tahoku Imp. Univ., Tech. Repts. 2, 41 (1921). (5) Watts, IND,ENQ.CHEM.,29, 1009 (1937).
Courtesy, Bromborough P o r t Estate
