Industrial Microbiology

FERMENTATION

Written by MicroDok

The phrase fermentation is derived from the Latin word “fevere”which means to boil”. It was first coined and used by Louis Pasteur – who used it to describe the bubbling of sugar solutions especially due to the microbial activity occurring in them. The bubbling action of the sugar (a substrate) was due to the conversion of the substrate to carbondioxide gas and alcohol. Pasteur’s association of microbes (i.e. yeast) to fermentative activities made scientists to relate fermentation to microorganisms. In this Chapter, it will be emphasized that the term fermentation means different thing to different disciplines in the biological sciences; and the use of the word fermentation in industrial microbiology is usually based on the activities carried out by microbes on several substrates to produce products that are of economic importance. In order words, there are several definitions for the word fermentation. However, the definition used depends on the context of the task at hand. Several definitions of fermentation are succinctly elucidated in this section.

Fermentation is defined as the microbial process in which microbes are cultivated on a large scale especially in a fermenter/fermenter to produce products of industrial importance such as penicillin and alcoholic beverages. Fermentation can also be defined as the metabolic activity in which energy is generated via substrate-level phosphorylation when a carbon-compound is degraded or broken down by microbial action. Fermentation is also defined as an energy-yielding metabolism (i.e. catabolism) in which an energy substrate is oxidized without an exogenous electron acceptor. In such processes, organic molecules usually serve as both the electron acceptors and electron donors. Fermentation is the mass controlled culture of microorganisms that is predominantly occurring in an anaerobic condition with available substrates under a controlled environmental factor requirement. Fermentation is the energy-yielding anaerobic breakdown of a nutrient (substrate) molecule. It is also worthy of note that fermentation can occur in both aerobic and anaerobic processes. Fermentation can also occur under microaerobic conditions (i.e. in environments where there is minimal or small amount of oxygen) in which microaerophilic microbes are known to dominate the reaction process. To the Food Microbiologist, fermentation is defined as the microbial process in which microbes are employed in the production of fermented foods such as cheese, yoghurt and bread. The Food Microbiologist also sees fermentation as a process that helps in food preservation and improvement in food quality in terms of taste and flavour. Fermentation is the chemical transformation of organic substances into simpler compounds by the action of enzymes, complex organic catalysts, which are produced by microorganisms such as moulds, yeasts, or bacteria. Most fermentation processes are activated by microbes including moulds, yeasts, or bacteria that either work on their own or in conjunction with other microbes; and these organisms are important in fermentation processes because they drive fermentation via the enzymes that they produce and also through the metabolites that they secrete in the process.

In industrial microbiology, the phrase fermentation is generally used to describe a wide range of metabolic processes carried out by microbes regardless of whether fermentative or respiratory metabolism is involved. Fermentation is the process in which microbes catalyze the conversion of a suitable substrate molecule to a desired end-product. Fermentation uses microorganisms to convert raw materials (generally known as substrates) to useful end-products. Microbes unlike plants and other photosynthetic organisms cannot make their own carbohydrates (glucose) from carbon dioxide, water, and sunlight. And thus microorganisms need a substrate to feed and grow on. In industrial microbiology, these substrates are provided for in the form of fermentation media or culture media – that allows optimal growth of microbes in a fermenter. It is noteworthy that most fermentation processes require liquid culture/growth media (known as broth) for growth during a fermentation process. Solid culture media can also be used for some fermentation process. But it is important to note that the major essence or rationale for providing a culture media for fermentation process is to satisfy all the nutritional requirements of the microbe(s) used for the fermentation process.

Growth media used in fermentation processes must satisfy and meet all the nutritional requirements of the industrial microorganism; and the culture media or nutrients should be formulated in such a way that they support the production of the desired metabolite by the microorganism. The fermentation media must also promote the synthesis of cell biomass and a given product of industrial importance as aforementioned. As microorganisms utilize their growth nutrients in their immediate environment (such as is obtainable in a bioreactor or fermenter), they produce several products of industrial importance including but not limited to fermented foods, drugs, alcoholic beverages, alcohols, wine, beer, pharmaceutical products and enzymes that are of industrial importance. The rate of fermentation depends on several factors including the concentration of microorganisms used for the process, pH, pressure, temperature, and enzymes.

In a typical fermentation process, the fermenter (bioreactor) is filled with a suitable sterile nutrient substrate that supports the growth of a given microbe or groups of microbes. After which the microorganism that drives the fermentation process is then inoculated or introduced into the bioreactor. Oxygen is usually introduced into the fermenter in the form of air. Antifoaming agents are also introduced to control the formation of foam during the process. Other environmental parameters such as temperature and pH are also controlled in order to ensure a smooth and unperturbed fermentation process. The pH of the fermentation process is usually controlled by the introduction of an acidic or basic substance. The fermentation process is allowed to run; and during this process the organism utilizes the nutrients provided by the fermentation media to grow and synthesize its metabolite or desired product. Microorganisms usually exhibit different phases of growth in a fermenter including the lag phase of growth (where no growth is observed); exponential/log phase of growth (in which there is an increased level of growth); stationary phase of growth (in which the organism ceases to grow); and death or decline phase (in which the organism dies).

Microbial products or metabolites may be excreted into the liquid media (broth) whether or not the products are excreted by the organism. And microbial product recovery always involves the concentration of the dilute solution. As the fermentation is harvested of its end-products (metabolites), the fermenter is usually cleaned out and made ready for the next round of fermentation process. Fermentation processes is usually carried out in the laboratory in a smaller reactor (that measures between 100-1000 ml) before it is scaled-up to a large fermenter/bioreactor (that measures several thousand liters in size). An important element for industrial fermentations is scale up (scaling up).

Scaling up is defined as the processes involved in the increasing scale of operation culminating in the production plant or a bioreactor. It is the conversion of a laboratory procedure to an industrial process – in which desired products are produced in large quantities. Usually, a typical fermentation process in an industrial or biotechnological plant is first carried out in a series of flasks (ranging from 10-20 liters) where the productivity of the microorganism is first tested before being transferred to a pilot plant. The major function of the pilot plant is to stimulate the conditions and structures of the production plant as well as determine the proficiency or viability of the microorganism’s used for the fermentation process in a larger vessel. Pilot plants or bioreactors usually range from 100-500 liters. From the pilot plant, the fermentation process progresses to a production plant or reactor (that ranges from 10,000-200,000 liters or more). These whole processes that set the stage for the fermentation process from the conical flasks to the pilot plant and then to the production plant is generally known as scaling up.

FERMENTATION PROCESSES USED IN INDUSTRIAL/BIOTECHNOLOGICAL PRODUCTIONS

Fermentation processes are broadly divided into two parts viz: liquid fermentation and solid fermentation. In liquid fermentation, the microbial cells are suspended in an aqueous nutrient medium. But in solid fermentation, the microbial cells are adsorbed to a solid and nutrient rich material that supports the growth of the organism. The volume of free liquid or aqueous medium is minimal in solid fermentation processes unlike in liquid fermentation processes where the free liquid is large. Only liquid fermentation processes will be elucidated in this section – since they are mostly applied in many fermentation activities. Fermentation occurs when microbes including yeasts, actinomycetes, moulds and bacteria consume, breakdown or metabolize organic substrate molecules as part of their own metabolic process; and in the process produce desired end-products or metabolites that are of great economic importance.

The growth of microbes in food is considered a problem especially when such microbial activities result in disease outbreak (as is obtainable in food borne diseases) and food spoilage. Nevertheless, some fermentative processes spurred by microbial activities are important to man, animals and the environment because they result in the production of desirable end-products including fermented foods, beverages, drugs and alcohols. These fermentative activities are usually carried out in bioreactors that support the growth of microbes via nutrient supplementation or addition. Fermentation processes are usually classified according to the ways or manner in which the substrate or fermentation nutrients are added and the desired end-product recovered or harvested from the process. To this end, there are usually different types of fermentation processes used in industrial/biotechnological productions especially those industrial processes that has to do with the production of foods, alcoholic beverages, drugs and pharmaceutical products. Batch fermentation, fed-batch fermentation, continuous fermentation and semi-continuous fermentation are usually the major types of liquid fermentation processes used in industrial microbiology productions.

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