Velissariou, Maria (1992)
Ph.D. thesis, University of Birmingham.
The aim of the present work was to study the effects of molecular interactions between proteins upon foam quality in batch and continuous operations, and to investigate foam fractionation as a method of protein recovery from real biological feedstocks. Foam stability studies of model protein solutions acted as engineering indicators and highlighted the importance of electrostatic interactions between basic (lysozyme and cytochrome-c) and acidic proteins (BSA) upon foamability and foam stability. Batch and continuous foam operations were employed in the study of electrostatic interactions between BSA and lysozyme and their effects upon foam quality. Batch foam production at pH 8.0 strongly demonstrated the importance of molecular stoichiometry upon individual protein partition into foam. Maximal lysozyme recovery was achieved at equimolar conditions and coincided with minimal fractionation between the two proteins. Continuous foaming demonstrated that foam positive proteins such as BSA (collector) can function as mobile ion-exchangers for poor foaming agents such as lysozyme (colligend). Experimentation with real biological systems such as brewer's yeast extract indicated that complex protein systems appear to behave like single protein solutions in terms of the effects of operating parameters upon foam performance. Such behaviour was confirmed by preliminary studies with bovine heart muscle homogenate. "Dry" foams, in continuous foam processing of brewer's yeast extract, were associated with dilute feedstocks, low gas flowrate and prolonged liquid residence times. They were characterised by high protein enrichment, low recovery, enhanced protein fractionation from RNA, but suffered extensive protein precipitation. Studies of dynamic changes in foam bubbles showed that such phenomena are associated with extensive coalescence. It was concluded that, although single protein model systems can offer valuable information on foam fractionation, direct comparisons with real biological solutions should be treated with care. A need for further study on the role of protein interactions upon foaming was also identified. Finally, the currently held view of rejecting foaming as a source of protein precipitation should be reviewed and advancements in foam fractionation as a preliminary step in downstream processing should employ the development of semi-empirical predictive models.
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