Yeast pellets

For ethanol production a substantial improvement over the packed bed reactor is offered by the gas-solid fluidized bed fermenter which uses a liquid instead of a solid substrate and replaces the packed bed of substrate or micro-organisms with a fluidized bed of yeast pellets (Smith et al, 1997). This system can be viewed as a standard fermentation against which the performance of different bioreactors may be assessed. 

A summary of the factors which are known to influence ethanol production from glucose in a gas-solid fluidized bed fermenter, or which may have an influence based on observations with submerged fermentations, is shown in Figure 6.1. In anaerobic beds, the key factors are the fermentation temperature and ethanol inhibition, both of which have a dramatic effect on the specific rafe of ethanol production. Bed dehydration and its influence on yeast pellet moisture content is also important, since a failure of fermentation may occur if the pellets become too dry (Bauer, 1986). 

Bauer (1986) recommended that the moisture content of pressed baker s yeast pellets should be maintained between 55% and 65% (wwb) for satisfactory fermentation of the feed glucose. The original moisture content of the yeast is about 70% (wwb) and any decline in moisture content in the bed is due to a mismatch between the drying rate in the bed and the rate at which water is added with the feed solufion. Such a mismatch occurred in work reported by Moebus and Teuber (1984) who obtained ethanol yields of 0.36 and 0.21 at final yeasf moisture contents of 55.9% and 32.6% respectively. 

The production of ethanol from cooked rice starch (Moebus and Teuber, 1985) differs from the normal process of spraying the carbon source into the bed since all of the carbon source is made available at the start of the run, subject only to the breakdown of starch to glucose (and maltose) by amylases. The starch (0.3 mm particles), amylases and yeast pellets were mixed in the bed and water sprayed in to maintain the fermentative activity of the yeast. The fermentation was carried out at 31.5°C. 

Bauer and co-workers (Rottenbacher, 1985 Rottenbacher et ah, 1987) carried out anaerobic fermentation under nitrogen gas at 18-20°C in a gas-tight closed circuit with a partial condenser followed by a gas adsorber for ethanol and water vapour recovery. The highest rate of ethanol production was about 50gh with a bed of 2.4 kg of extruded cylindrical baker s yeast pellets of diamefer 800 rm. This rate is less... 

Table 6.2 Properties (Hayes, 1998). of yeast pellets used by Hayes and co-workers...

Figure 6.3 Effect of fermentation medium composition on the rate of ethanol production from glucose using extruded yeast pellets. Reproduced from Hayes (1998) with permission.

The presence of solutes other than ethanol might be expected to reduce the mole fractions of ethanol and water and influence the nonideality of the ethanol-water system. However, both Williams (1983), who modelled a batch wine fermentation, and Rottenbacher (1985), in ethanol sorption experiments with yeast pellets in a fluidized bed, established that the ethanol-water-yeast system behaves as if the water and ethanol content of the pellets were a simple ethanol-water solution supported by a solid matrix which influences neither mole fractions nor activity coefficients. 

Ten microcuries of C -labeled inulin were added to 15.0 ml of a yeast suspension. The suspension was then centrifuged and the supernatant fluid carefully drawn off. The pellet of packed yeast occupied 0.2 ml and contained 10,000 CPM. The counting efficiency was 25%. Calculate the proportion of the packed yeast pellet that is interstitial space, assuming that the yeast cells were completely impermeable to the inulin and that the inulin did not adsorb to the cell surface. 

The specific activity of the original suspension is 10 /i.Ci/15 ml or 0.667 /iCi/ml. Because the yeast cells occupy such a small proportion of the total volume, we could assume that each milliliter of extracellular fluid contains 0.667 /rCi, However, for a more exact calculation, we can assume that the volume of the suspension occupied by the yeast cells is at least the same volume as the packed yeast pellet. The specific activity of the extracellular fluid then becomes ... 

Add 750-1000 jxI of Tris buffer (pH 7.8) that contains 1 mM PMSF (see Appendix 5) to your thawed yeast pellet and transfer to a 1.5 ml centrifuge tube. 

Yeast was suspended in 90 ml of 10 mM phosphate buffer at pH 7.5 and well stirred. Fluorophores were then added. Table 8.1 gives details of the fluorophores. The suspension (now 100 ml in volume) was gently stirred for 30 min to allow the yeast cells to absorb the fluorophores at 20+2°C. The suspension was then washed to remove excess fluorophores and EPS. A washing procedure was used similar to that reported by others [7]. The suspension was placed into two 50 ml centrifuge tubes, and centrifuged at 2500 rpm for 10 min. The supernatant was removed and the yeast pellet was resuspended with buffer. The process was performed three times after which the yeast suspension was further diluted to the required concentration using the same buffer. In all experiments fresh yeast suspensions were prepared on the same day and used within a few hours. 

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