Colloidal examination of yeast and wort associated with premature yeast flocculation.

摘要

Premature Yeast Flocculation (PYF) is a problem in the brewing industry with serious economic implications. This research reports on a study of PYF phenomena from a colloidal perspective. A control wort inducing normal flocculation as well as wort causing PYF was fermented for 120 h in 'tall' tube fermentors. A lager yeast strain 'OSMA' was used to ferment a control wort and a PYF wort and subsequently analyzed for cell wall properties: zeta potential (zeta), cell separation force (Fr) and orthokinetic capture coefficient (alpha0). After 72 h, upon microscopic inspection, yeast collected from the PYF wort exhibited more flocculation than the yeast from the control wort. Yeast zeta potential was measured in control and PYF fermented worts after filtration through a 0.2 mum filter. The PYF fermentation yielded a yeast with less negative surface charge compared to the control yeast at 48 h of fermentation and thereafter (p0.001). When both yeasts were resuspended in the same 120 h wort (filtered through a 10 kDa molecular weight cut off filter), there was no difference in surface charge of the control or PYF fermented yeast (p>0.05). This implied that wort colloids/trub larger than 10 kDa caused a reduction in surface charge. Thus, there were less repulsive forces between PYF fermented yeasts presumably encouraging PYF. Interestingly, zeta potentials of trub from control and PYF worts were similar when suspended in an acetate buffer. A mechanism of yeast flocculation has been proposed involving electrostatic interaction between trub particles (positively charged towards end of fermentation) and yeast (negatively charged through fermentation).; A separation force was estimated by examination of the floc breakup of control and PYF fermented yeast after a passage through a capillary (0.45 mm x 75 mm). The PYF yeast flocs showed higher apparent separation forces compared to control yeast flocs at 96 h of fermentation. At 120 h of fermentation, the control and PYF yeast separation forces in wort were 3.24 x 10 -9 N and 1.52 x 10-8 N, respectively. When suspended in a pH 4.0 sodium acetate buffer the control and PYF separation forces were 5.87 x 10-9 N and 1.26 x 10-8 N, respectively. A higher separation force of PYF yeast implies a higher adhesion strength and a more tightly bound floc. Repeated washing reduced the separation forces of both the control and PYF yeast. Results indicated the PYF yeast cell surface was altered and, the PYF binding factors can not be completely washed off.; The orthokinetic capture coefficients of control and PYF yeast were measured in fermented wort as a function of shear rate (18.7--465 s-1). There was a significant difference in capture coefficient values between both the yeast suspensions, with the PYF capture coefficient values being higher from 72 h of fermentation (p0.05). Furthermore, the capture coefficients of these suspensions were directly proportional to the inverse of shear rate.