INDUSTRIAL AND ENGINEERING CHEMISTRY
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bility increased, the 4.6 pH value representing the maximum hydrogen-ion concentration obtained. The mixture containing butter appeared least viscous a t a p H of about 7.2; it was least stable a t 6.3. There was a close relation between stability and viscosity, as shown in Figure 1. Similar results were obtained when butter oil was used as the source of fat; however, on account of the higher breaking temperature the stability and viscosity values were of different magnitude although of the same order as those obtained when butter was used. The results with hydrogenated fat differed greatly from those with butter or butter oil. The emulsions containing hydrogenated fat were most stable a t the higher acidity, as in butter, but became less stable with increasing amounts of alkali (higher pH). oa6
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FIGURE8. EFFECTOF HYDROGEX-IOX CONCENTRATION ON SPECIFICGRAVITYOF FAT-SUGAR-EGG EMULSIONS (20-MINUTE AGITATION)
All three emulsions were most stable a t hydrogen-ion concentrations of approximately 4.8 pH. This was probably due to the effect of the egg protein. At pH 7.3 the albumen is soluble in water and has a tendency to form an oil-in-water emulsion. The isoelectric point of egg albumen is approximately a t a pH of 4.8. As the albumen approaches its isoelectric point, the protein tends to precipitate. As a rule, emulsion stabilizers which are soluble in water will produce emulsions of the water-in-oil type. It seems probable that the increase in stability with increase in hydrogen-ion concentration is due to the fact that the hydrogen-ion concentration is nearing the isoelectric point. This lower range in p H values for obtaining the most stable emulsion corresponds closely with the isoelectric point of the other protein constituents-for example, casein, which is least soluble a t or near pH 4.6. The cause for the tendency of the butter and butter oil mixtures to form more stable emulsions a t lower hydrogenion concentrations than pH 6.5 is not understood. Since the mixtures containing hydrogenated fat gave different results from those containing butter and butter oil, it may be that the lower melting point of these fats is a factor. Effect of Hydrogen-Ion Concentration on Specific Gravity
It was thought that the measure of specific gravity might be important as a measure of the leavening or foaminess of the emulsion. The determinations of specific gravity showed less change than would be expected from the importance of this factor in cake making. I n order to determine the effect of variation in hydrogenion concentration on the specific gravity of the emulsion, sodium bicarbonate or acid potassium tartrate was added in varying amounts to mixtures containing one of the three fats.
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The specific gravities of the mixtures are shown in Tables 11, 111,and IV, and in Figure 5. The tests were made following al5-minute heating period. Reading from left to right on the curve, it will be observed that the emulsions containing butter decreased in specific gravity with decreasing pH values (increasing hydrogen-ion concentrations), and that the emulsions containing hydrogenated f a t increased in specific gravity with decreasing p H values. When the specific gravity of the mixtures containing butter oil is plotted against hydrogen-ion concentration, practically a horizontal line is obtained.
Summary The effect of change in hydrogen-ion concentration on the stability, viscosity, and specific gravity of fat-sugar-egg emulsions, based on the formula of 200 grams butter, 400 grams sugar, and 200 grams egg, was studied. This formula gives an emulsion of about average or medium fat content in the practical range of fatty emulsions used in the manufacture of cake batter. I n this formula 180 grams of butter oil or hydrogenated fat were also used. Changes in hydrogen-ion concentrations were effected within a pH range of 8.5 and 4.5 by adding varying amounts of sodium bicarbonate or acid potassium tartrate. The hydrogen-ion concentration of the emulsions, without the addition of either chemical, was about pH 7.3. When butter or butter oil was used, the stability and viscosity increased a t the lower hydrogen-ion concentrations (higher pH values), As the hydrogen-ion concentration increased, the stability decreased slightly to a concentration of about 6.3 pH and then increased with increasing acidity (lower pH values). The viscosity, which was lowest a t pH 7.2, also increased with increasing acidity. The most stable emulsions containing butter or butter oil as the fat were produced a t pH 4.8. The emulsion containing butter oil had a slightly higher breaking temperature (30 C.) than the emulsion containing butter (25” C.), owing partly to the conditioning that the butter oil received when it was prepared. The trends of the changes in viscosity and stability are similar to those for butter. When hydrogenated fat was used, the emulsion became less stable as the hydrogen-ion concentration decreased (pH value increased) and more stable as the hydrogen-ion concentration increased (pH value decreased). The viscosity increased very slightly in either direction from that a t a pH value of 7.0 to 7.3. As the pH value of the emulsions was changed from the higher to the lower ranges, those containing butter decreased slightly in specific gravity, those containing butter oil decreased very slightly, and those containing hydrogenated fat increased slightly. The relatively minor changes in specific gravity indicate that the change in hydrogen-ion concentration had very little effect on the lightness of the foamy emulsion. R ~ C E I V EDecember D 23, 1937. Presented before the Division of hgricultural and Food Chemistry a t the 85th Meeting of the Amerioan Chemical Society, Washington, D . C., March 26 to 31, 1933.
Vapor Pressures of Solvents (Addendum)
Since the publication of my nomographs on “Vapor Pressures of Solvents” in the April and May, 1938, issues of INDUSTRIAL AND ENGINEERING CHEMISTRY, my attention has been called t o a number of nomographs for a similar purpose by A. J. V. Underwood [Trans.Znst. Chem. Engrs. (London), 10,112 (1932)l. D. H. KILLEFFER
