| The ethanol tolerance of a self-flocculating yeast strain SPSC01 was investigated in an oxygen-limited fed-batch bioreactor. Employing Focused Beam Reflectance Measurement (FBRM) on-line monitoring system, four yeast floc populations with the average size ranging from 100 to 400μm were obtained. When subjected to 20% (V/V) ethanol shock for 6 h at 30℃, the remained viability was measured to be 3.5%, 26.7%, 48.8% and 37.6% for the aforementioned four floc populations.The contents of trehalose and ergosterol (including free ergosterol and total ergosterol) were found to be positively correlated with the average size distributions from 100 to 300μm; however, in the 400μm flocs, the content of trehalose and ergosterol decreased, which coincided with its reduced ethanol tolerance comparing to that of the 300μm flocs. Examination of the membrane composition of different floc populations revealed that the plasma membrane composition of the floc populations was significantly different in the contents of phosphatidylinositol, as well as phospholipid palmitoleic acid. What's more, higher plasma membrane ATPase activities were detected in the floc populations with higher ethanol tolerance. Furthermore, when subjected to 15% (V/V) ethanol shock at 30℃, the equilibrium nucleotide concentration and plasma membrane permeability coefficient (P') of the 300μm flocs accounted for only 43% and 52% respectively of those of the 100μm and 200μm populations, indicating that the effect of floc size distribution on the ethanol tolerance of the self-flocculating yeast strain SPSC01 is closely related to plasma membrane permeability.The results indicate that the average size distribution of the floc populations exerted great influence on the physiological status of yeast cells during the ethanol production process, leading to the changes in plasma membrane composition that contributed to improved ethanol tolerance in self-flocculating yeast SPSC01.
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