17.1 Fungal fermentations in submerged liquid cultures

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.

Table 1. Comparison of the main features of the two basic fermenter culture systems, batch and continuous
 
Type of culture system
Feature
Closed system
Batch culture
Open system
Continuous culture
Nutrients
NOT replenished
Replenished
Products of growth
NOT removed
Removed
Environmental conditions
NOT constant
Constant
Exponential growth
Lasts only a few generations
Lasts indefinitely

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 December 17, 2016