The growth medium meant for the cultivation of a production strain and subsequent production of either microbial cells or a biochemical product is referred to as a production or fermentation industry is to design a suitable production medium. This is done by a trial and error method. In a batch fermentation process, the fermentor is filled with the production medium leaving the head space. The production strain is added in a particular way according to the type of fermentation process (e.g. surface culture or submerged culture method). Usually, production media are used in the liquid state to facilitate the fermentation process.
Characteristics of an ideal production medium
No single production medium is the best or the ideal one for any and all purpose. This is not surprising in view of the variety of conditions under which media may be used, differences in chemical composition and the many types of products to be manufactured had there been an ideal production medium: it would have to possess a formidable array of characteristics. A single production medium possessing these properties may never be found. Nevertheless, the specification described below can be aimed at in the design of new production media and they should be considered in the evaluation of the media made available for practical use.
i) Chemical composition
The production medium must have a suitable chemical composition. Generally, the medium should contain a source of carbon, a source of nitrogen, growth factors and mineral salts.
In certain fermentations (e.g.penicillin fermentation), the medium should supply the required precursor for better yields of a desirable product.
Maintenance of the pH in the optimum range is necessary for making the process successful, since acidic and or basic compounds depending on the nature of the fermentation process accumulate during the progress of the fermentation. In order to control the pH of the medium, buffers(e.g.CaCO3) should be added to the medium. Media containing considerable quantities of proteins, peptides and amino acids possess good buffering capacity in the pH range near neutrality. Additional buffering capacity in this pH range is also provided by phosphates (e.g. mono-and dihydrogen potassium or sodium phosphates).
Avoidance of foaming
Foaming is a serious problem in a fermentation industry, since it may aid in contaminating the fermentation medium and possibly cause other problems for the fermentation. Hence, defoamers (e.g. lard oil mixed with octadecanol for pencillin fermentations) should be used for controlling foam. These defoamers may be added at the stage of media make-up (before sterilization), or they may be incorporated after sterilization or as and when called for during the fermentation.
Being an ideal producing medium, it should be free from any toxic effect on culture or product formation.
In aerobic fermentations, it is necessary to supply sterile air into the medium. Under such circumstances, liquid media allow the diffusion of air throughout the medium under agitation. Thus, this explains why most of the fermentation processes make use of liquid media. In addition to this, fermentation media should not be viscous, since the viscous nature of the medium creates difficulty in the penetration of the air interior of the medium. In other words, air is not easily absorbed by the liquid medium.
Certain condition of the production medium are helpful to check the contamination for example, low pH_ values in citric acid production using Aspergillusniger keep the contamination under check. Moreover, media having low pH values may be sterilized at low temperatures.
Recovery of the desired product is an important operation affecting the final cost of the product. Therefore, components of the medium should be such that separation and extraction of product becomes easy and cheaper.
Availability of raw materials
The raw materials required for designating of the production medium should be freely available in large quantities at a reasonable price. This explains why non-synthetic media,(or crude media) find wide application in large-scale production processes. Synthetic media on the other hand are limited for research studies since they are expensive and low yields are produced. But these media are reproducible, allowing the researcher to study the effect of different components of a medium on the culture and product formation. Again, media may be simple or complex in their composition.
ii) Raw materials
Many different types of raw material are used in different types of industrial fermentation processes. Usually, crude nutritive sources are preferred, since they are economical. Mostly agricultural products are utilized as a source of raw material in fermentation industries. We is interested in agricultural wastes for the following reasons:
i. Agricultural wastes are produced in large quantities for example: nearly two billion tons of animal wastes alone are produced annually in the United States.
ii. Most of these wastes have a high biological oxygen demand (B.O.D) and, as such, are a potential pollution hazard. It is very expensive to dispose of these wastes within the federal environmental standards.
iii. These wastes contain a large amount of potentially usable proteins and the expected rise in the proteins and carbohydrate materials present in the wastes.
iv. The caloric value of wastes is recoverable as biogas and/or ethanol.
v. Most of the agricultural wastes are renewable and, hence there is no shortage of fermentable substrate.
vi. Often, there is little cost involved in recovering the wastes.
vii. In nearly all cases, agricultural wastes are not a part of the human food chain and any process for recycling the wastes will not compete with man’s food sources.
iii) Sterilization of production media
Usually, production media are sterilized before they are inoculated with the desired fermentative pure culture. Sterilization of these media is decided by the chemical composition of a particular medium. For instance, media containing sugars cannot be sterilized by prolonged heating, since sugars react with phosphates. Therefore, either the sugar or the phosphate is to be sterilized separately before adding it to the sterilized production medium.
Sterilization of production media may be done by one of the following three methods -by boiling, by passing live steam, orc)by subjecting the medium to steam under pressure (i.e. autoclaving).
It is to be kept in mind that heat treatment should be sufficient only to sterilize. Avoiding overcooking of the medium. Overcooked media demonstrate poor growth or low yields as compared to properly sterilized media. The classical technique of making the medium sterile by the use of steam may be carried out in two ways:
(i) Batch sterilization (ii) Continuous sterilization.
(i) Batch sterilization
This is the simplest method of sterilizing production media. The vessel is equipped with a coil or jacket for heating and cooling. Also the agitator may be fitted to aid heat exchange. It is needed to raise the temperature of the whole system (i.e. both vessel and medium) to 120°C, with steam and to maintain this for a period of 20 minutes before cooling the system. There is an interconnecting pipeline between a batch cooker and fermentor. Therefore, the interconnecting pipeline must also be sterilized before making use of it for transfer purpose.
The batch cooker method saves the production time, since the fermentor is unoccupied between two fermentation runs.
- It occupies increased plant space.
- It involves higher cost of the additional equipment required and
- It involves increased steam usage.
- Most of these wastes have a high biological oxygen demand (BOD) and, as such, are a potential pollution hazard. It is very expensive to dispose of these wastes within the federal environmental standards. These wastes contain a large amount of potentially usable proteins and the expected rise in the proteins and carbohydrate materials present in the wastes.
- The caloric value of wastes is recoverable as biogas and/or ethanol.
- Most of the agricultural wastes are renewable and, hence there is no shortage of fermentable substrate.
- Often, there is little cost involved in recovering the wastes.
- In nearly all cases, agricultural wastes are not a part of the human food chain and any process for recycling the wastes will not compete with man’s food sources.
(ii) Continuous sterilization
The continuous sterilization offers more flexibility in the choice of time-temperature conditions to which the medium is exposed. Usually, this method involves passing of production medium through a heat exchanger. a holding coil and a cooler. The temperature of the medium under-going sterilization is raised to the desired level in the heat exchanger. The medium then passes on to a holding coil, where it is maintained at the sterilizing temperature for a predetermined time period. Finally, the medium is rapidly cooled by counter circulating it in the exchanger against the cool input medium and then against cold water. The possibility of short holding time allows the use of sterilization temperatures higher than 120°C without any adverse effects on the nutritional qualities of the culture medium (Table 1).
A slightly modified system involves the injection of high-temperature steam into the medium. Here, the required temperature (<140°C) is reached in microseconds. The cooling of the medium is carried out in the flash cooler in a vacuum chamber.
A continuous retention tube sterilizer is also available. It is widely employed in the antibiotic field.
- It saves both production time and plant space.
- It gives improved quality of the medium for instance an increase of 160% in the yield has been secured in the manufacture of vitamin B12.
- It involves some economy in steam costs, provided the heat-exchange principle is applied
- It allows the use of lower sterilizing temperature or shorter holding periods provided the medium has a low pH or some other characteristic aiding the sterilization.