Date of Award
Master of Science (MS)
M. Serpil Kislalioglu
In the first paper of this work, lambda and kappa carrageenan, guar gum, Water Locks A-100 and DD-223 and Carbopol 971 were selected based on their rheological properties to study of the effects of gel concentration, osmotic pressure and rheological properties as determined by oscillatory viscometry on the growth rate of Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus and Candida albicans. The object of this study was to determine the contributions of gel rheology on the growth of microorganisms that commonly contaminate such gels, and assess the influence of the rheology on their self-preserving properties. The rheological properties of the gels were determined by oscillatory viscometry at a stress range of 0 - 100 Pa and a frequency of 0.05 Hz using a 1 mm gap. Their viscoelastic properties were determined by applying a stress range of 0 - lOOPa and deformation of the gels were observed until the elasticity dissipated. The rheological parameters measured were the elastic modulus (G'), the viscous modulus (G"), the complex viscosity (ri*) and the phase angle (a). The parameters used to determine any influence of the rheological properties on the microbial growth rate were G' and G" at the critical region, the critical stress ( a'c) at G' and G" , the time to reach crc at G' and G". The microbial growth rates were determined by following division during a 24 hour period taking measurements at 0, 6, 12 and 24 hours. By the use of multiple regression analysis, the growth rates were correlated with the aforementioned parameters. The growth rates of S. aureus and E. coli were found to be influenced by the rheological parameters described earlier, whereas a trend was visible for the growth rate of P. aeruginosa. The growth rate of C. albicans was not affected by these parameters. In the second paper, the rheological properties of nine different gels, namely carrageenan, guar gum, pectin, sodium carboxymethylcellulose, methylcellulose, hydroxypropyl methyl cellulose, Carbopol 971, Water Lock and bentonite were studied. Oscillatory viscometry was used to study the elastic modulus, viscous modulus and the phase angle in the linear and critical regions at a stress range of 0 - 100 Pa and a frequency of 0.05 Hz. The gel macrostructures included long linear chains of the cellulose derivatives; natural gels forming helix and ribbon structures such as carrageenans, guar gum and pectin; cross-linked gels such as Carbopol 971; grafted ones like Water Locks and suspended particles like bentonite. Their flow behavior followed either shear thinning or thickening properties. Five concentration ranges used varied from 0.3% to 7.0% with no less than a 3 fold increment in the concentration range. The critical stress ( crc), where the elasticity of the gel begins to dissipate was also determined with the intention of using it as a parameter to describe the gel strength. Within the concentration ranges studied, the critical stress was found to be a linear function of the concentration within 95% confidence. The model proposed is applicable to gels of different chemical structure, molecular weight, molecular weight distribution, chain structure and viscosity types. It is also considered as a universal model to describe gel strength and offers a practical and simple description useful for the formulation scientists. In addition, "concentration sensitivity", another parameter described, may enable formulators to replace one gel with another to make necessary concentration-gel strength adjustments to food, cosmetics and pharmaceuticals.
Mendoza, Jorge A., "A STUDY OF THE RHEOLOGICAL PROPERTIES OF SOME OF THE GELS COMMONLY USED IN THE PHARMACEUTICAL, FOOD AND COSMETIC INDUSTRIES AND THEIR INFLUENCE ON MICROBIAL GROWTH" (1998). Open Access Master's Theses. Paper 268.