Media components

 The media components usually are,

■       Water

■       Carbon source

■       Nitrogen source

■       Oxygen

■       Sources of P, S, minor and trace elements

■       Vitamins – Biotin, riboflavin etc

■       Buffers

■       Antifoam

■       Precursor

■       Inducer

■       Inhibitor

Most fermentations, except those involving solid substrates, require large quantities of water in which the medium is formulated. General media requirements include a carbon source, which provides both energy and carbon units for biosynthesis, and sources of nitrogen, phosphorus and sulphur. Other minor and trace elements must also be supplied, and some microorganisms require added vitamins, such as biotin and riboflavin. Aerobic fermentations require oxygen, and even some anaerobic fermentations require initial aeration of media, e.g. beer fermentations. As the fermentation proceeds different metabolic products are formed which may change the pH of the media and it can interfere with the growth or product formation, So, to adjust the pH, media should contain buffers or the pH is controlled by addition of acid/ alkali. Antifoam agents are also required to control foam production. Some processes require compounds like precursor, inducer or inhibitor which are added certain stages of the fermentation.

The composition of a fermentation medium may be simple to complex, depending on the particular microorganism and its fermentation.  Autotrophic microorganisms require only the simplest of inorganic media (inorganic salts, water, nitrogen source, carbon source is fulfilled by CO₂ or by carbonates) and are capable of synthesizing all the complex organic compounds required to sustain life.  Fastidious microorganisms on the other hand lack the ability to synthesize many of their sustenance and growth requirements.  They require the presence of many simple to complex preformed nutrients in the medium and must have an organic carbon supply to provide for synthesis of cell substances and release of metabolic energy.  So they require complex media.

Simple and complex media are further subdivided into two categories: synthetic and crude. In a synthetic medium, all the components are specifically defined and known compounds.  Each component is relatively pure and the exact concentrations are known.  Synthetic medium has defined components and concentrations and are expensive due to the relatively pure ingredients used. The concentration of one or several can be varied in order to determine the effect on cell growth and product yield. Individual components may be added or deleted as well. This has advantages in certain types of studies. However, yields derived from these media are relatively low.  Crude media contain crude factors or ill-defined sources of nutrients and growth.  They usually allow much higher yields.

The media adopted also depend on the scale of the fermentation. For small-scale laboratory fermentations pure chemicals are often used in well-defined media. However, this is not always possible due to cost, as media components may account for up to 60-80% of process expenditure. Industrial-scale fermentations primarily use cost-effective complex crude substrates, where many carbon and nitrogen sources are almost indefinable.  Most are derived from natural plant and animal materials, often by-products of other industries, with varied and variable composition.  The effects of such batch-to-batch variations must be determined.  Small scale trials are usually performed with each new batch of substrate, to examine the impact on product yield and product recovery.

            The main factors that affect the final choice of individual raw materials are as follows:

1 Cost and availability: ideally, materials should be inexpensive and of consistent quality and year round availability.

2 Ease of handling in solid or liquid forms, along with associated transport and storage costs, e.g., requirements for temperature control.

3 Sterilization requirements and any potential denaturation problems.

4 Formulation, mixing, complexing and viscosity characteristics that may influence agitation, aeration and foaming during fermentation and downstream processing stages.

5 The concentration of target product to be attained, its rate of formation and yield per gram of substrate utilized.

6 The levels and range of impurities and the potential for generating further undesired products during the process.

7 Overall health and safety implications.

Composition of a typical fermentation media

The composition of fermentation media is dependent on a number of factors characteristic of the particular fermentation.  The major ingredients of a typical fermentation media are:

■       Water

■       Carbon source

■       Nitrogen source

■       Sources of P, S, minor and trace elements

■       Vitamins – Biotic, riboflavin etc

■       Buffers

■       Antifoam

■       Precursor

■       Inducer

■       Inhibitor 

Water

Water is a major component of all fermentation media except solid-substrate fermentation. It also provides trace mineral elements. Water is also important for ancillary equipment and cleaning. Supply of large quantities of clean water, of consistent composition, is therefore essential. The quality of water in terms of pH, dissolved salts and effluent contamination is important.  The mineral content is important in brewing (mashing step) and has influenced the location of breweries and types of beer produced.  Before use, suspended solids, colloids and microorganisms should be removed. If the water is ‘hard’, it should be treated to remove salts such as calcium carbonate. Iron and chlorine should also be removed. For some fermentations, especially, plant and animal cell culture, the water must be highly purified. Since, water is becoming increasingly expensive, its recycle/reusage wherever possible should be encouraged. This minimizes water costs and reduces the volume requiring waste-water treatment.

References