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3.4 Use of immobilized yeast cells in alcohol fermentation's

3.4.1 Preparation of immobilized yeast cells
3.4.2 Continuous plant operation using immobilized yeast cells
3.4.3 Fermentation processes used in ethanol production
3.4.4 Flash fermentation using immobilized yeast cells

Alcohol fermentation is a well established technology which has long been practiced throughout the world. Continuous alcohol production with the use of immobilized yeasts, which give rise to more efficient fermentation, is described in this Section. A fermentation process incorporating the flash method which is seen as a means of improving alcohol productivity using yeasts with poor alcohol resistance is also discussed here.

3.4.1 Preparation of immobilized yeast cells

Of the various methods used for continuous fermentation, that employing the use of immobilized yeast cells was selected. Methods of yeast immobilization evaluated included collagen casting, acetic acid-lactic acid cellulose microencapsulation, chitosan/glutar-aldehyde molding, carrageenan-entrapping method, and calcium alginate gel-entrapping. Of these, the calcium alginate gel-entrapping method was preferred because of its high enzymatic activity, simple manner of preparation, and stability.

Preparation of a uniform calcium alginate gel, necessitated maintaining the viscosity of the mixture of calcium alginate and yeast cells between 1000 and 2000 cps. The addition of a nonionic surfactant and an unsaturated fatty acid at the time of gelling was also found to improve cell retention and enzyme activity. Preparation of immobilized yeast cells is outlined in Fig 3-15.

Figure 3.15 - Immobilization of yeast cells

3.4.2 Continuous plant operation using immobilized yeast cells

The spherical gel method was employed for the preparation of calcium alginate gels since this method does not require the use of specialized equipment. Spherical gels are readily obtained by adding sodium alginate solution to calcium chloride solution using a nozzle. No special granulation apparatus was used when the equipment was assembled, but a gel-dropping nozzle was provided at the top of the fermentor. The fermentor was filled with a calcium chloride solution prior to fermentation, and sodium alginate solution was added dropwise to form granules. The culture medium was then supplied to the fermentor to initiate the fermentation. This procedure was found to simplify the gel preparation process.

Continuous fermentation tests were conducted using two 1-kL fermentors arranged in series. The results are shown in Fig. 3-16. Continuous production of alcohol was carried out stably for 4000 hours while maintaining high enzymatic activity (8).

3.4.3 Fermentation processes used in ethanol production

Alcohol productivity has been greatly improved through the development of bioreactors. Alcohol production by the batch method, immobilized continuous, and immobilized flash methods are compared in Table 3-9.

3.4.4 Flash fermentation using immobilized yeast cells

Alcohol inhibition of yeasts, does not pose any problems in alcohol fermentation by the batch method, since yeasts are used only once in batch processes. In such processes, yeast damage due to alcoholic inhibition is of relatively little significance. However, where immobilized continuous methods of alcohol production are used, stable yeast activity needs to be maintained, and the alcohol concentration in the fermentor must be relatively low. The relationship between alcohol concentration and fermentation rate, using an immobilized yeast fermentor is presented in Figure 3-17. Several methods of continuously removing alcohol from fermentors as a means of avoiding alcoholic inhibition of yeasts have been proposed. The fermentor used in our study was equipped with a distillation column (operable either under atmospheric or reduced pressure), as shown in Fig. 3-18. Circulation of the fermentation broth through this distillation column, enabled the removal of alcohol, and sugar thereby facilitating the maintainance of low alcohol concentrations within the fermentor. Results obtained using a 200-L scale pilot fermentation are shown in Fig. 3-19. Alcoholic inhibition of yeast was reduced while the fermentation rate increased. In addition, this method enabled raw material to be used at a high concentration, reducing the volume of waste liquor. However, the use of raw material at a high concentration necessitates a relatively longer retention time within the fermentor, thus increasing the accumulation of by-products, which inevitably reduces the fermentation rate due to by-product inhibition.

Figure 3.16 - Time profile of continuous fermentation using immobilized yeast cells

Table 3-9 Comparison of Fermentation Methods Used in Alcohol Production

Fermentation Method

Fermentation Rate-

Alcohol Cone.(v/v%)

Properties Required of Strain

Complexity of Equipment

Batch Method




Not so complicated

Immobilized Method



Resistance to alcohol Resistance to contamination

Somewhat complicated

Immobilized Flash Method

20 to 30

15 to 25

Resistance to contamination (Resistance to alcohol and sugar)

Greatly complicated

Fermentation Rate: Expressed relative to batch method designated as 1

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