Info

Xf = final cell concentration, tT = turn-around time (washing, sterilizing filling with media), tD = delay time until inoculation, tL = lag time after inoculation.

The overall productivity P is given by:

It will be possible from this equation to determine the effects of process changes on the overall productivity. A larger initial inoculum would increase X0 and shorten the process time. Actively growing inocula would reduce the lag time tL. Aspects of this problem have been discussed in Chapter 6. It is also worthwhile to isolate faster-growing organisms and/or higher yielding strains (Chapter 3). In many processes the growth phase and/or the production phase has been extended by the use of fed-batch or continuous feed (Chapter 2) with improved productivity.

Richards (1968a,b), Geysen and Gray (1972) and Stowell and Bateson (1984) have given details for determining maximum production at minimum cost and the optimum time for harvesting (Fig. 12.1).

In fermentations with short growth cycles such as bakers' yeast (14 to 24 hours), the turnaround time will be as important as the time between inoculation and

Higher level of raw materials and energy Lower level of raw materials and energy

Productivity tonnes/vol.

Installed capacity/

week

Higher level of raw materials and energy Lower level of raw materials and energy

Productivity tonnes/vol.

Installed capacity/

week

Fig. 12.1. Fed-batch process—effect of running time on productivity and product cost (Stowell and Bateson, 1984). Fermentation should be terminated at I for maximum productivity, II for minimum cost. Productivity is defined as the units of product generated per unit of fermenter volume in a given time.

where /jum = maximum specific growth rate,

Xn initial cell concentration,

Fig. 12.1. Fed-batch process—effect of running time on productivity and product cost (Stowell and Bateson, 1984). Fermentation should be terminated at I for maximum productivity, II for minimum cost. Productivity is defined as the units of product generated per unit of fermenter volume in a given time.

harvesting. When the production cycle is long, as with penicillin (6 to 7 days), a few extra hours for turnaround will have little influence on overall productivity.

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