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Characterisation of microencapsulation process in Saccharomyces cerevisiae
[Thesis]. Manchester, UK: The University of Manchester; 2011.
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Abstract
Since the 1970’s there has been industrial interest in using microorganisms as microcapsules. The encapsulation of actives (e.g. flavours, drugs, perfumes) is a necessary process for pharmaceutical and food companies because the precious and often expensive ingredients must be protected from degradation and also released in a specific site or under a specific stimulus. Saccharomyces cerevisiae, baker’s yeast, represents a first choice microorganism for the encapsulation of active ingredients. It is biodegradable and biocompatible with human digestion and skin, and can be produced in an easy and cheap way. A major part of this project has been dedicated to the development of robust methods of extraction and quantification of hydrophobic substances loaded inside yeast cells, which have been subsequently combined with an indirect, fluorescence-based method for the evaluation of the rate of loading of hydrophobic substances in the same cells. In particular, it has been found that this process reaches a limit in the maximal loading capacity of intact yeast cells, most likely reflecting the maximal volume of the lipid droplet organelles in which loaded hydrophobes accumulate. With the new on-line (fluorescence-based) and off-line (chromatography-based) methods developed here it has been established that the loading process fundamentally follows a diffusion model, in which the solubility in water determines the permeation of substances through the cell wall and ultimately their uptake by yeast cells. However, treating yeast cells with organic solvents like DMSO - a new approach introduced in Prof. Tirelli’s lab to enhance the encapsulation of hydrophobes – completely changes the chemical-physical parameters of the encapsulation process. In DMSO-treated cells, substances are loaded fundamentally in response to their hydrophobicity. Conversely, once loaded, the same substances are released with a rate that is inversely proportional to their hydrophobicity, as observed by applying a novel approach to measure the release of hydrophobes encapsulated in yeast cells, either in the absence of presence of DMSO-treatment. In conclusion, the new evidence reported here clarifies basic aspects of hydrophobe encapsulation in intact yeast cells and will thus help improving future applications of these microcapsules as a valid, inexpensive and biocompatible drug delivery system.