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

17.1 Fungal fermentations in submerged liquid cultures

It was the brewer who invented industrial fermentation; using yeast in a pot to make beer. Today, fermentation engineers manufacture, separate and purify products that serve many different sectors of industry, ranging from chemical feedstocks, pharmaceuticals, foods, and biofuels, as well as beverages. The industry is continuing to refine (and redefine) itself for production of heterologous proteins, vaccines, hormones, urgently-needed novel antibiotics, as well as animal cell culture, bioprocessing and post-genomics metabolic engineering. In this Chapter we will give you a good foundation to all this, but if you want to venture further into the topic you need to read these books: Bioprocess Engineering Principles (Doran, 2012); Fermentation and Biochemical Engineering Handbook (Vogel & Todaro, 2014); and Principles of Fermentation Technology (Stanbury et al., 2016).

There are two basic fermenter culture systems: closed (‘batch’) and open (‘continuous’) systems although there are several ways of operating batch cultures, which may be stirred (that is, agitated by being shaken or mixed with an impeller) or unstirred, and there are several ways of controlling continuous systems. Leaving the variations aside for the moment, the main features of these two basic fermenter culture systems are summarised in Table 1.

A batch culture is so called because it is a single batch (or quantity) of medium that is inoculated with the organism to produce the new culture. It will be incubated at the best temperature (determined by experiment), but after inoculation no further change or addition is made to the culture until it is harvested at the end of the incubation period (at a time chosen by the experimenter). A batch culture may be stationary, in which case the microorganism may grow in suspension within the fluid medium, or may float and grow as a surface culture. Batch cultures may alternatively be agitated in some way, the agitation being intended to improve gas exchange (most usually uptake of oxygen into the medium and release of carbon dioxide from the medium). Small, laboratory scale cultures are most often cultivated in conical flasks, agitated on shakers that may be reciprocating (a back and forth linear motion) or orbital (a circular motion that swirls the fluid in the vessel).

Larger cultures, including those on industrial scales of up to hundreds of cubic metres capacity, will be cultivated in what are known as stirred tank reactors (STRs) in which the fluid is stirred by a centrally-placed impeller (an impeller is a bladed rotor). In industrial fermenters the ratio of tank height to diameter averages 1.8 (that is, they tend to be tall cylinders); the tanks are filled to about 70% of their absolute volume; impeller rotors extend over about 40% of the tank diameter; agitation power varies between 2 and 6 kW m-3; and impeller tip speed averages 5.5 m s-1. Such impeller speeds fragment mycelia and loosely aggregated masses of mycelia (called flocs), which is an advantage overall as it improves nutrient supply, including oxygen, to the biomass.

We will discuss variations on the batch process, the various continuous culture processes, and aspects of fermenter design below; for the moment we want to concentrate on the practicalities of batch culture and the behaviour of the fungi, especially filamentous fungi, in laboratory scale cultures. So in the following paragraphs think in terms of cultures of the sort you might make up yourself in conical flasks for incubation on a shaker in an incubator.

Updated July, 2019