Solid or Semi-solid Culture
Varying amount of agar amended with nutrient media gives solid or semi-solid phase. For the research purpose, this type of media are used but they are generally avoided for microbial products, because such media occupy space and create difficulty during harvesting. However, for the production of amylase from Aspergillus oryzae such media are used. After the growth of A. oryzae for several days at 30°C mycelia are harvested, dried and ground which results in crude preparation of amylase.
This is the simplest type of culture in which micro-organisms grow in a vessel, known as fermenter (Fig. 14.1) or bioreactor. A fermenter is a vessel designed to carry out fermentation process i.e. biological reactions under the controlled conditions. Hence it is also known as bio-reactor; while designing a fermenter, several criteria which help in maximizing the yield, are taken into account. These are (a) long term operation in aseptic condition, (b) adequate aeration and agitation, (c) pH control system, (d) sampling facility, (e) minimum labor in operation, harvesting, cleaning and maintenance, (f) temperature control system, (g) minimum evaporation losses from fermenter, (h) suitable for a variety of processes. Fermenter is provided with limited amount of medium containing all the nutrients at optium environmental conditions.
During the log phase of culture, growth rate of the micro-organisms reaches to its maximum (max). However, after depletion of a substrate, growth rate decreases and finally is ceased. Monod (1942) has demonstrated the relationship of growth rate and concentration of the rate-limiting substrates by the following formula :
m = Growth rate constant,
[s] = Concentration of limiting substrate,
Ks = Saturation constant value of limiting substrate [s] at which the growth rate is half of the maximum growth rate (m max).
A continuous culture is that where a steady exponential phase for growth of culture retards due to depletion of nutrients, rather than by accumulation of toxic products; it is prevented by addition of fresh medium to the fermenter and removal of spent medium and microbial biomass from it as a result of which the exponontial phase of culture is prolonged.
Basically it is the batch culture which is fed continuously with fresh medium without removal of the original culture medium from the fermenter. It results in continuous increase in volume of medium in the fermenter.
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