Although beer production is one of the oldest biotechnologies in the world, a
major constraint in brewing remains controlling flocculation. Evidence points
towards a possible role of lipids, associated with the cell surfaces, as a major
factor responsible for flocculation. Therefore, the aim in this study became to
evaluate the contribution of lipids, especially oxylipins, in the flocculation of
Saccharomyces cerevisiae UOFS Y-2330.
Saccharomyces cerevisiae UOFS Y-2330 was selected as a model, since it was
found to demonstrate both Flo1 and NewFlo phenotype flocculation behaviour,
when cultivated in different media. In a defined medium with glucose as a sole
carbon source, this strain immediately flocculated strongly and lost this ability
before stationary phase was reached. In a complex medium containing glucose,
this yeast strongly flocculated towards the stationary growth phase without losing
this ability during this phase. This inverse pattern may be ascribed to a switch in
sensitivity of the yeast to flocculate in the presence of glucose as well as pH
level, which may, in turn, influence the availability of calcium ions. In both media,
matured cells produced protuberances upon flocculation as observed by electron
and immunofluorescence microscopy, which may be involved in cell adhesion.
This was followed by further investigations into the role of lipids over the growth
cycle of this yeast. Here, it was uncovered that Sacch. cerevisiae UOFS Y-2330
does not only demonstrate inverse flocculation, but is also characterised by two
different lipid turnover patterns. During Flo1 phenotype flocculation, this yeast
showed two neutral lipid accumulating stages (i.e. at 8 h and from 12 h). This is
probably triggered by flocculation, which may be regarded as a survival
mechanism where cells accumulate especially neutral lipids as reserve energy
source - a similar mechanism is probably operative when cells enter stationary growth. Contrary to Flo1 behaviour, this strain in NewFlo phenotype mode
demonstrates only a single lipid accumulation phase i.e. when cells enter
stationary growth, which coincides with the increase in flocculation. In addition,
an increase in phospholipids was experienced during active growth in both
flocculation behaviours, probably as a result of active membrane production.
These results prompted us to investigate the possible role of oxylipins present on
the cell surfaces during the flocculation process. It was found that some strains
of Sacch. cerevisiae (include strains used in fermentation processes) produce
short chain (mainly 8 carbon) oxylipins and not potent inflammatory long chain
(20 carbon) oxylipins such as prostaglandins. When aspirin was added to
cultures of Sacch. cerevisiae UOFS Y-2330, flocculation was significantly
inhibited as well as the production of 3-hydroxy (OH) 8:0 thereby linking
flocculation and this oxylipin. Furthermore, no traces of 3-OH 8:0 could be
detected before flocculation onset in this yeast. Next, the involvement of these
oxylipins in co-flocculation was assessed. According to the lectin-theory, the
yeast Schizosaccharomyces pombe lacks the specific receptors necessary to
facilitate co-flocculation with Sacch. cerevisiae species. In this study we
demonstrate oxylipin associated co-flocculation between Sacch. cerevisiae UOFS
Y-2330 and S. pombe strains using differential cell staining, immunofluorescence
and ultrastructural studies. Using a 3-OH oxylipin specific antibody coupled to a
fluorescing compound, 3-OH oxylipins were found to be present on the cell
surfaces of Sacch. cerevisiae and S. pombe. The presence of 3-OH oxylipins
was confirmed using gas chromatography-mass spectrometry. Whether these 3-
OH oxylipins play a role in affecting co-flocculation of Sacch. cerevisiae with S.
pombe cells through possibly entropic-based hydrophobic interactions and/or
hydrogen bonds still needs to be verified. Studies on the physiological, genetic
as well as colloidal aspects of flocculation using this model strain may lead to
important new insights in this fascinating phenomenon as well as applications in
industry.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-09122006-114856 |
| Date | 12 September 2006 |
| Creators | Strauss, Catharina J |
| Contributors | Dr EJ Lodolo, Prof PWJ van Wyk, Prof JLF Kock |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf, video/x-msvideo |
| Source | http://etd.uovs.ac.za//theses/available/etd-09122006-114856/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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