OIL-IN-WATER CREAM STABILITY 221 formation, promoted firmer consistency, and eliminated the bleed potential of the cream. Storage at 5øC for 3 to 6 days inhibited or delayed gel formation, promoted a softer consistency, and increased the bleed potential of the cream. Storage at 24øC for 3 to 6 days gradually increased the consistency firmness and decreased the bleed potential of the cream. Microscopic observations were made in studying the effects of storage temperature on the consistency and bleed potential of the bulk cream. Initially after manufacture, the cream had the appearance of discrete microscopic spheres of internal phase, closely packed, but floating freely in the aqueous external phase. The spheres were not fused together and a rigid three-dimensional gel structure was not observed, which accounted for the soft cream consistency and bleed propensity. The presence of uniformly distrib- uted droplets of clear aqueous phase loosely entrapped throughout the internal oil-wax phase also signified an increased bleed potential. Microscopic examination of the cream sample stored at 33øC showed that the internal phase spheres were fused together and that the aqueous external phase was securely entrapped in the matrix. Through the use of polarizing optical microscopy it was con- cluded that the gel structure was liquid crystalline in nature. These properties yield firm cream consistency without the potential for the cream to bleed. The 5øC-stored sample of the cream was similar microscopically to the sample stored at 24øC. The oil-wax spheres were packed more closely due to shrinkage of the wax struc- ture at 5øC, but after the sample equilibrated at 24 ø __ 3øC, it was observed that the spheres were not fused and a rigid gel structure was not present. The physical shrinkage of the wax structure at 5øC apparently expelled or "squeezed out" a larger portion of the aqueous external phase from the loosely bound matrix, which resulted in a greater amount of bleed after 6 days storage at 5øC. EFFECT OF INDIVIDUAL U.K. EXCIPIENTS ON CREAM CONSISTENCY AND BLEED POTENTIAL A series of 20-kg cream batches was prepared using a 10-gallon Groen mixer. The batches consisted of the U.S.-sourced excipients and a systematic substitution by U.K.- sourced excipients. Penetrometer and bleed testing indicated that the cream base containing U.K.-sourced glyceryl monostearate (with all other excipients U.S.-sourced) demonstrated softer con- sistency and extensive bleed. U.K.-sourced cetyl alcohol, Syncrowax ERL-C, Tween 60, and Amerchol L-101 did not promote bleed in the cream bases prepared using the Groen mixer (Table II). The results of this study led to a focus on the differences between U.S.- and U.K.-sourced glyceryl monostearate. ANALYTICAL AND THERMAL ANALYSIS TESTING OF U.S.- AND U.K.-SOURCED GLYCERYL MONOSTEARATE Thermal analysis, including DSC and powder x-ray diffraction measurements, were determined for each cream excipient. No significant differences were observed between U.S. and U.K. excipients except for glyceryl monostearate. Differential scanning calorimetry was used as the first characterization tool for glyceryl monostearate since slight variations in chemical composition could yield variations in

222 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table II Initial Consistency and Bleed Potential in Cream Bases Containing Individually Substituted U.K.- Sourced Excipients Substituted Penetrometer Bleed U.K. excipient consistency (% w/w) Glyceryl monostearate 299 9-10 Cetyl alcohol 281 0 Syncrowax ERL-C 284 0 Amerchol L- 101 282 0 Tween 60 270 0 the glyceryl monostearate melting points. Furthermore, since the processing used in the manufacture of the cream involved melting and congealing of glyceryl monostearate, the DSC thermogram of melted and congealed glyceryl monostearate was also obtained. The DSC profile of U.S.-sourced glyceryl monostearate, as it was received, is shown in Figure 3, as is the DSC thermogram obtained on glyceryl monostearate which had been melted and allowed to solidify. It is evident from the data that glyceryl monostearate does not solidify back into its original physical state. Most samples were found to exhibit an initial melt around 60øC, and after the melting/congealing process they (+) (-) Temperature (oC) Figure 3. Differential scanning calorimetry thermograms obtained for U.S.-sourced glyceryl monostearate as received (solid trace) and after being melted and congealed (dashed trace).