The study of surface reactions in biological systems by ellipsometry

Effects of pH adjustment on physicochemical properties of microfiltration retentates of skim milk and rehydration of resulting micellar casein concentrate (MCC) powders were investigated. Aliquots of retentate (pH 6.9) were adjusted to pH 7.3, 7.6 or 7.6 followed by readjustment to pH 6.9 (6.9R) prior to powder preparation. The retentates with pH 6.9, 7.3, and 7.6 had casein micelle size of 179, 189 and 197 nm, respectively, while sample 6.9R had size of 183 nm, similar to retentate at pH 6.9. Higher retentate pH resulted in lower ionic calcium and higher conductivity, with sample 6.9R having higher values for both parameters than the pH 6.9 sample. The MCC powders displayed poorer wettability and enhanced dispersibility with increasing retentate pH. Interestingly, the 6.9R powder had the best wettability and dispersibility. This study demonstrated that pH-mediated modifications of the physicochemical properties of retentates improve the rehydration properties of resultant MCC powders.

This study investigated the physicochemical properties of milk protein isolate (MPI) suspensions when sodium caseinate (NaCAS) was incorporated by wet- or dry-blending. The studied MPI had a solubility of only 89.6% when it was the sole contributor to total protein. In dry-blended samples, solubility was increased to a maximum of 97.5% when NaCAS contributed 15% to total protein. High volumes of micellar casein (73.2%) and low volumes of primary powder particles (27.8%) were measured in dry-blended systems with 15.0% NaCAS addition, corresponding to high suspension stability. Wet blending only improved the solubility slightly to 92.3% and resulted in a large volume of primary powder particles. Differences in physicochemical properties between dry and wet-blending might be attributed to the strong tendency of NaCAS to promote dissociation of casein micelles under dry-blending conditions. The results indicate the dry-blending approach represents an effective strategy for obtaining MPI/NaCAS suspensions with high solubility.

Static light scattering (SLS) was applied to monitor the rehydration process of milk protein concentrate (MPC) powder. The size distribution and volume concentration of the suspended powder particles were measured to quantify the dissolution kinetics of MPC powder. The results obtained showed that the low solubility index reported for MPC85 (85% protein) powder at room temperature was the consequence of slow dissolution kinetics rather than the presence of a large amount of insoluble material in the rehydrated powder. The rehydration process of MPC85 powder occurs in two overlapping steps: the disruption of agglomerated particles into primary powder particles and, simultaneously, the release of material from the powder particles into the surrounding aqueous phase. The latter process appeared to be the rate-limiting step of dissolution of MPC85 and was accelerated by an increase of the solvent temperature.

The adsorption of lipase to methylated silica and its subsequent removal upon sequential addition of surfactants were measured in situ by ellipsometry. In addition, the specific activity of adsorbed lipase without, and after, exposure to surfactants was determined. Two lipases from Humicola lanuginosa were studied, the wildtype (WT) and a mutant which was modified to increase the hydrophobicity of the active-site region. The surfactants used were SDS, C₁₂E₅ and sodium dodecanoate and they were all studied in the concentration range⩽their critical micelle concentration (cmc). In all experiments a non-desorbable fraction remained at the surface. This fraction was larger, however, for the WT compared to the mutant. The concentration, expressed as a fraction of the cmc, of surfactant inducing the most pronounced desorption was found to vary among the surfactants. Desorption occurred at increasing fractions of the cmc in the order C₁₂E₅, sodium dodecanoate and SDS. This was found for both the WT and the mutant. No significant effect of C₁₂E₅ or SDS on the specific activities could be observed while the exposure to sodium dodecanoate markedly increased the specific activity of both lipases.

In situ ellipsometry was used to study the adsorption/desorption of highly purified lipase from Humicola lanuginosa in mixtures with surfactants, at the solid/liquid interface. The effect of the surfactant was studied both when it was allowed to adsorb in mixture with lipase and when added after lipase adsorption. Silica surfaces, totally or partially methylated, were used and the surfactants were SDS (anionic), C₁₂E₅ and a commercial alcohol ethoxylate (AE) (both nonionic). The experiments were carried out so as to simulate a laundry process and the pH throughout the study was kept at 9 by means of Tris-HCl buffer.

From the results it was shown that the lipase did not adsorb until the diluted, that is during the rinsing period. This was found for all the lipase-surfactant mixtures in the study. However, the amount of lipase adsorbed was larger after rinsing in mixture with SDS, compared with C₁₂E₅. The results from addition of surfactant after lipase adsorption indicated that the lipase was replaced by surfactant. This was found for SDS and C₁₂E₅ at both the hydrophobic surface and the surface with intermediate hydrophobicity.