Flocculation of yeast

The liquid was then evaporated in vacuo to a small volume and two volumes of 95% ethanol were added. A small amount of flocculent precipitate, probably consisting of yeast polysaccharides, was removed by centrifugation, and the solution was concentrated to a sirup in a vacuum oven at 40°. This colorless sirup, when treated with hot absolute alcohol and stirred, set to a crystalline mass. The crystals were filtered, washed with alcohol and ether and dried in vacuo at 70°. The yield was 2.8 g. 

Schorr-Galindo, S., Ghommidh, C., and Guiraud, J.P., Influence of yeast flocculation on the rate of Jerusalem artichoke extract fermentation, Curr. Microbiol., 41, 89-95, 2000. 

Ge, X.M. and Bai, F.W. (2006) Intrinsic kinetics of continuous growth and ethanol production of a flocculating fusant yeast strain SPSCOl. J Biotechnol, 124 (2), 363-372. 

Matsumoto T, Fukuda H, Ueda M, et al. (2002). Construction of yeast strains with high cell surface lipase activity by using novel display systems based on the Flolp flocculation functional domain. Appl. Environ. Microbiol. 68 4517-4522. 

As demonstrated by the good agreement between experiments and theory, the time variation of the different species in the fermentation medium can be adequately described by a relatively simple model based on yeast metabolism and the physical processes of yeast flocculation, CO2 desorption and CO2 mixing. Essential in the model is the introduction of a yeast to sugar yield depending on the measurable ethanol concentration to represent the loss of cellular viability. 

Several investigations were carried out to remove toxic heavy metal ions from waste water by biosorption. Microbial cells loaded with heavy metals were recovered by flotation, e.g. Streptomyces griseus and S clavuUgerus loaded with Pb [108] and Streptomyces pilosus loaded with Cd [109]. In these flotation processes the microbial cells were dead therefore, they are not considered here. The removal of pyritic sulfur from coal slurries such as coal/water mixtures by Thiobacillus ferrooxidans and recovery of this iron-oxidizing bacterium by flotation is a special technique in the presence of high concentrations of solid particles (see e.g. [110]). The flotation of colloid gas aphrons was used for the recovery of yeast in continuous operation [ 111 ] for the recovery of micro algae, and in the presence of flocculants in batch operation [112]. These special techniques are not discussed here. 

The distribution of yeast throughout the fermentation depends on (/) the behaviour of the yeast strain in relation to sedimentation, flocculation, and yeast-head formation (ii) the agitation provided by convection currents, carbon-dioxide evolution, rousing, and stirring, and (Hi) the size and geometry of the fermentation vessel. In connection with yeast strain, six types of behaviour have been recognized [7] ... 

Other approaches to continuous fermentation involve the entrapment of yeast so that wort can be passed through a reactor of yeast and be fermented. Immobilization on calcium alginate, wood and other materials has been achieved for a wide variety of yeast strains, including non-flocculent ones. Rather more attention has been paid to the construction of a yeast reactor containing both yeast and kieselguhr [72]. Such a system can be used either... 

Fig. 41. Flocculation of Na yeast nucleate with some salts. ...

One functional definition of flocculation is that it describes the ability of yeast strains to clump together and fall out of solution. Near the end of fermentation, single cells aggregate into clumps of thousands of cells. Different strains of yeast have different flocculation characteristics. Some strains flocculate earlier during fermentation and subsequently do not attenuate (i.e. finish the fermentation) normally. Flocculating too early results in a beer that is under attenuated and sweet however, when yeast fails to flocculate entirely, it results in a beer that is cloudy with a yeasty flavour (Speers, 2012). 

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