• Control of continuous fermentations
• Continuous fermentations - benefits and limitations
• What are continuous fermentations?
Continuous fermentations are those in which fresh nutrient medium is added either continuously or intermittently to the fermentation vessel, accompanied by a corresponding continuous or intermittent withdrawal of a portion of the medium for recovery of cells or fermentation products. That is, on one hand, there is a continuous addition of nutrients and on the other; there is a continuous removal of fermented broth. This is in contrast to a batch fermentation process in which a large volume of nutrient medium is inoculated, and growth and biochemical synthesis are allowed to proceed only until maximum yields have been obtained. At this point, the batch fermentation is stopped for product recovery, the fermentor is cleaned and resterilized, and a new fermentation is started up.
At first glance, the continuous fermentation would appear to be the better of the two procedures, because the fermentation equipment is in constant usage with little shutdown time and, theoretically at least, after the initial inoculation, further production of inoculum is not required. However, as we shall see, the inherent problems associated with a continuous fermentation process often do not allow the achievement of this goal.
• How are continuous fermentations carried out?
A continuous fermentation can be conducted in various ways. It can be carried out as a “single stage” in which a single fermentor is inoculated, then kept in continuous operation by balancing the input and output of nutrient solution and harvested culture respectively. In a “recycle” continuous fermentation, a portion of the withdrawn culture, or of the residual unused substrate- ‘plus the withdrawn culture, is recycled to the fermentation vessel. For example, the immiscible hydrocarbon substrate of hydrocarbon fermentation can be recycled for further microbial attack. A portion of the organisms being produced during a continuous fermentation also can be recycled in certain instances in which the actual available substrate level in the nutrient solution for microbial growth is quite low. An example of this type of substrate is sulfite waste liquor with .its low available carbohydrate content; in this instance, the recycling of cells provides a higher population ‘of cells in the fermentor and, hence, a greater productivity. Multistage ethanol production with recycling of yeast is another such example.
A third type of continuous fermentation, the “multiple-stage continuous fermentation. It involves two or more stages with fermentor being operated in sequence. The multiple-stage
continuous fermentation is particularly applicable to those fermentations in which the growth and synthetic activities of the cells are not simultaneous; that is, synthesis is not growth- related but occurs after the cell-multiplication rate has slowed. Table Representive Chemical Products form Continuous Fermentation
Growth-Associated Not Growth Associated Acetic acid Acetone
Butanediol Butanol
Ethanol Glycogen
Gluconic acid Subtilin
Hydrogen sulfide Chloramphenicol Lactic acid PenicillinStreptomycin
Vitamin B12
• How do we control the microbial activity in continuous fermentations?
There are several possible means by which microbial activity in continuous culture can be controlled, although only two of these approaches, the “turbidostat” and the “chemostat”. In the turbidostat, the total cell population is held constant by employing a device that measures the culture turbidity so as to regulate both the nutrient feed rate to the fermentor and the culture withdrawal rate from the fermentor. If the population numbers rise above a predetermined level, a greater amount of fresh medium is added to the fermentor so as to dilute the cell Concentration. Thus, there is no limiting nutrient consciously imposed with this process so that the cell growth rate should always be maximal. However, the .growth must be maintained in the logarithmic growth phase or very close to it. This factor is a disadvantage in that the fermentation must be operated at a lower maximum cell population than is possible with a chemostat, and this causes a greater residual of unused nutrient to be lost from the fermentation with the withdrawn harvested culture.
• Ok, and how does the chemostat work?
In contrast to the turbidostat, a chemostat maintains the nutrient feed and culture withdrawal rates at constant values, but always less than that which allows a maximum growth rate. The growth rate is controlled by supplying only a limiting amount of a critical growth nutrient in the feed solution. Thus, cell multiplication cannot proceed at a rate greater than that allowed by the availability of this critical nutrient. The controlling factor for growth, however, does not necessarily have to be a limiting nutrient; it can also be a relatively high concentration of a toxic product of the fermentation, the pH value, or even temperature. The chemostat concept of continuous fermentation is employed more often then the turbidostat, because fewer mechanical problems are
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encountered, and because of the occurrence of less residual unused nutrient in the harvested culture.
• Tell me, is it possible to convert all batch type fermentations into continuous ones?
Again, theoretically yes. Many fermentation processes have been investigated, at least on the pilot plant scale, for their possible conversion to a continuous fermentation process. The following table presents a list of representative microorganisms investigated for their possible use in continuous fermentations.
Table Representative Genera of Organisms Grown in Continuous Culture Organisms Genera Actinomycetes Streptomyces Algae Chorella Euglena Scennedesmus Bacteria Aerobacter Azotobacter Bacillus Brucella Clostridium Salmonella Fungi Ophiostoma Penicillium Protozoa Tetrahymena Yeast Saccharomyces
From among these potential continuous fermentation processes, only the production of beer, fodder yeast (from sulfite paper mill waste), vinegar, and baker’s yeast (from molasses) have found commercial application. However, the activated sludge system for the processing of waste waters also may be considered as a commercialized continuous
fermentation, differing from the more conventional fermentations only in that it deals with mixed microbial population acting on a heterogeneous substrate and also in that the products of these fermentations are not having as much commercial importance.
• It is said that the productivity of a continuous
fermentation is greater than that for batch fermentation. If this is true, then why have so few batch fermentations been successfully converted to a continuous
fermentation process?
There are several answers. A successful continuous
fermentation requires a thorough knowledge of the dynamic aspects of microbial behavior and growth, knowledge that is lacking or deficient for most industrial fermentation processes because of the complexities of the growth and synthetic patterns of the organisms. Also, contamination and mutation present a distinct problem for the
development of a successful continuous fermentation process. The prolonged incubation periods associated with continuous fermentations can allow contaminating microorganisms the time that they require for gaining
ascendancy in the culture, although certain fermentations, such as that for Torula yeast on sulfite waste liquor, provide built-in contamination control, in this instance, the presence of sulfite and a low pH. As regards the contamination problem, suggestions have been made that antibiotics or other chemicals be added to continuous fermentations to hold down the level of contaminant growth. Mutation of the fermentation organism becomes a problem only if the resulting mutant cells have a selective growth advantage during prolonged incubation and, at the same time, produce less of the desired fermentation product. It has been proposed that the answer to the question of mutation is to use multistage continuous fermentations, with the first fermentor of the sequence being periodically reinoculated. Nevertheless, the real overall solution to both contamination and mutation is to reduce their rates of occurrence so that the offending cells will be flushed from the fermentors before they have a chance to multiply
• So there are some drawbacks of continuous fermentations, is it?
There are. Continuous fermentations often waste nutrient substrate. Thus, the fermentation broth as it is continuously withdrawn for product recovery contains a certain amount of the residual unused nutrients of the medium as well as a portion of the fresh nutrient constituents being
continuously added to the fermentation. Certain
fermentation media are rather viscous and require that strong mixing activity be employed in the fermentor to equally distribute the incoming fresh medium to all parts of the broth already in the fermentor. Obtaining adequate mixing also is a problem when slow feed rates of fresh nutrient are employed, regardless of the viscosity of the medium. Antibiotic fermentations are included in this group of complex fermentation processes for which a continuous fermentation would seem to difficult to accomplish. In this regard, the feasibility of a single stage continuous
fermentation for chloramphenicol and penicillin investigated by Bartlett and Gerhardt (1959) (Figure 13.2). These workers reported that, at dilution rates of 1and 0.5 volume changes of per day, respectively, they were able to obtain yields from 1/4 to 1/2 of per day, respectively, of the maximum observed in batch fermentations, and that these rates were maintained in a steady state for more than two weeks.
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Learning Objectives
In this lecture, you will learn Production of Microbial Biomass The Bel ProcessIn most of the fermentation processes already discussed, the conversion of a proportion of the substrate to biomass is somewhat incidental, or for some purposes biomass formation may be actively suppressed. The main aim has been the
conversion of substrate into a useful primary or secondary metabolic product, such as antibiotics, ethanol and organic acids. In such cases, once the optimal amount of target product has been achieved, the organisms produced are often merely waste materials that have to be disposed of safely and at a cost, or are simply used as a cheap source of animal feed. However, in dedicated biomass production, the cells produced during the fermentation process are the products. Consequently, the fermentation is optimized for the production of a maximum concentration of microbial cells.