Fouling and cleaning of solid surfaces. The influence of surface characteristics and operating conditions
Sammanfattning: The severe fouling common in food processing puts high demands on equipment cleaning. Adsorption of the protein betalactoglobulin to solid surfaces and its subsequent removal were followed using in situ ellipsometry. Most experiments were done in a stirred cuvette. Additionally, a flow cell was designed, which was employed in some work. The characteristics of the surface were shown to influence protein adsorption and removal. While differences between chromium oxide and stainless steel were small, methylated silica showed significantly different adsorption, desorption during rinsing and cleanability. Near the betalactoglobulin denaturation temperature, rapid multilayer build-up was observed, and the importance of the bare surface properties appeared to decrease. On metal oxides, a lag phase preceded the build-up, while on methylated silica an initial phase of slower adsorption was seen. Prolonged adsorption times gave less desorption during rinsing from metal oxides. For methylated silica, increasing the betalactoglobulin residence time on the surface led to lower removal by the anionic surfactant SDS. Increasing the NaOH concentration gave enhanced cleaning rates and better cleanability of the stainless steel. Combining SDS with NaOH resulted in improved cleaning compared to if using the individual components. Adsorption and removal at high temperature resulted in lower cleanability compared to at room temperature, due to heat-induced alterations in the protein. No correlation was found between the bulk composition or the surface finish of the stainless steel and the fouling tendency or the cleanability. Differences in NaOH-induced protein removal were striking between surfaces subjected to different pretreatments. Passivated surfaces were less cleanable than surfaces pretreated with strong alkali or plasma-cleaned. After repeated fouling and cleaning without renewed pretreatment, differences decreased. While the cleanability of alkali-precleaned surfaces decreased when reused, changes for passivated surfaces were smaller. The initial betalactoglobulin adsorption in turbulent flow was found to be kinetically controlled. At the laminar flow rate, results indicated effects of both mass transport limitations and adsorption kinetics. No significant influence on rinsing or cleaning was evident, and effects on the amounts adsorbed were small. All NaOH concentrations tested caused significant protein removal, and the performance of the two highest concentrations was comparable.
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