21st Century Guidebook to Fungi, SECOND EDITION, by David Moore, Geoffrey D. Robson and Anthony P. J. Trinci

Table of Contents

1. Chapter 16: Fungi as pathogens of animals, including man
2. Chapter 17: Whole organism biotechnology
   1. Fungal fermentations in submerged liquid cultures
   2. Culturing fungi
   3. Oxygen demand and supply
   4. Fermenter engineering
   5. Fungal growth in liquid cultures
   6. Fermenter growth kinetics
   7. Growth yield
   8. Stationary phase
   9. Growth as pellets
   10. Beyond the batch culture
   11. Chemostats and turbidostats
   12. Uses of submerged fermentations
   13. Alcoholic fermentations
   14. Citric acid biotechnology
   15. Penicillin and other pharmaceuticals
   16. Enzymes for fabric conditioning and processing, and food processing
   17. Steroids and use of fungi to make chemical transformations
   18. The Quorn fermentation and evolution in fermenters
   19. Production of spores and other inocula
   20. Natural digestive fermentations in herbivores
   21. Solid state fermentations
   22. Digestion of lignocellulosic residues
   23. Bread; the other side of the alcoholic fermentation equation
   24. Cheese and salami manufacture
   25. Soy sauce, tempeh and other food products
   26. Chapter 17 References and further reading
   27. Buy a PDF of Chapter 17
3. Chapter 18: Molecular biotechnology

17.10 Beyond the batch culture

Although we have occasionally mentioned other culture systems in previous pages, most of the discussion so far has concentrated on batch cultures, so we now want to turn to other types of culture system.

The term fed-batch culture (or simply, ‘fed-batch’) is used to describe a batch culture which is fed with medium either intermittently or continuously. Nothing is removed until final harvest, so the volume of the culture increases with time (Fig. 23A). Fed-batch is a way of obtaining very slow growth rates resulting from substrate limitation; the penicillin fermentation is the classic fed-batch industrial process, but it is an industrial process that is likely to become important for production of fine chemicals for life sciences; health care, agrochemicals, and nutraceuticals (Blamey et al., 2017).

Fig. 23. A, sketch diagram of a fed-batch culture. The right-hand panel (B) records the basic kinetics of biomass concentration (x), substrate (glucose) concentration (S) and dilution rate (D) in such a fed-batch culture.

Although the total biomass in the culture increases with time, biomass concentration remains virtually constant because medium is added and the volume of the culture increases with time. Providing the dilution rate (D) is less than µ_max and K_s is much smaller than substrate concentration in the inflowing medium, a quasi-steady state may be achieved. However, unlike a continuous culture, D (and therefore µ) decreases with time, because the tank has a finite volume and as nothing is removed feeding with medium must stop at some stage in the cultivation (it was stopped at the 6 hour mark in the culture illustrated in Fig. 23B).

The dilution rate of a fed-batch culture is given by:

Where, D = dilution rate, V₀ = original volume of culture, and F = flow rate of medium into the culture.

The next development of the fed-batch culture is to remove some of the culture at intervals but to keep feeding the culture with more medium; this is a repeated fed-batch culture. In repeated fed-batch cultures the culture volume and consequently the dilution rate and specific growth rate undergo cyclical variations as parts of the culture are removed.

Because it is technically easier to obtain substrate limited growth in fed-batch culture than in chemostat culture, this type of culture is used for many industrial fermentations. In particular, feeding a batch culture with fresh medium can be used to:

• relieve catabolite repression when this is an issue in a production process; and
• avoid toxic effects of some medium components, that may be required for particular production processes, on the growth of the biomass; for example sodium phenylacetate, a toxic precursor of penicillin, can be kept at sub-inhibitory concentration by feeding it gradually to the culture.

 

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Updated July, 2019