An Examination Of Cell Wall Properties Affecting Brewing Yeast Flocculation
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Flocculation, the process whereby yeast cells attach in groups and sediment to the top or bottom of a fermenter, is industrially important in many fermentation batch operations. These batch operations include wine, distilled spirits, cider, bio-ethanol and production of commercial yeast metabolites. In the case of brewing yeast, it has been determined that flocculation occurs due to three forces called hydrophobic interactions, zymolectin binding and to a lesser extent, surface charge neutralization. This project sought to more closely study hydrophobic interactions and zymolectin binding. Earlier studies had shown that certain hydrophobic carboxylic acids, 3-OH oxylipins, formed in brewing yeast at flocculation onset. Therefore, these compounds showed potential as an indicator of overall cell surface hydrophobicity, and it was believed that flocculation level, cell surface hydrophobicity and oxylipin level would increase in unison in the yeast cell during brewing fermentations. During lab scale fermentations in shaker flasks and in a miniature fermentation assay setup, both flocculation level and cell surface hydrophobicity increased coincidently. However, 3-OH oxylipins could not be isolated from whole cells or cell wall isolates grown in the shaker flasks or whole cells grown in the miniature fermentation assay at detection limits approximated as 50 ng/0.5 g wet yeast. Due to their minute levels in brewing yeast cells, it was proposed that 3-OH oxylipins may mediate flocculation and aggregation via a quorum sensing mechanism instead of by increasing cell surface hydrophobicity. A disagreement exists in the literature where certain researchers believe zymolectin activity is induced, while others believe it is constitutive. The second part of this study attempted to address this by measuring zymolectin density during lab scale fermentations with a flow cytometer. Because of flow cytometry’s capacity for multiparametric analysis, large amounts of data were produced which gave information on not only zymolectin density, but also cell size and cellular complexity. Upon statistical analysis of the data, it was not possible to either refute or confirm the claim that zymolectin activity is induced or constitutive. However, the results suggested there could have been a population of cells with fewer zymolectins, and this certainly warrants further investigation. During the lab scale fermentations, cell size measured by a flow cytometer appeared to be correlated with manual measures of cell size. Furthermore, cell size tended towards uniformity during the fermentation which has also been observed in similar studies employing flow cytometry. Conversely, the cellular complexity of the yeast in this study did not change as in other studies by this may have been due either to strain differences or the methods used herein.