Gaden classified fermentation processes into three types. In type 1 process the production is due to result of primary metabolism. The rate of product formation is directly related to the rate of substrate consumption and also to the rate of cell mass produced. Examples of aerobic systems that follow this classification are acetic acid (using Gluconobacter suboxidans), single-cell protein (using Candida utilis, S. cerevisiae) and baker's yeast (using S. cerevisiae). The first system is operated under batch mode. The second under batch or fed batch mode, while the third system is operated at fed batch mode. Examples of anaerobic processes are ethanol (S. cerevisiae, Zymomonas mobilis), acetone/butanol (C. acetobutylicum), lactic acid (Lactobacillus bul-garicus) and propionic acid (Propionibacterium shermanii). The first two products are run in continuous mode of operation and the next two in batch mode.
Type 2 production process is due to primary metabolisms, but the reaction rates could be complex. The product may be an intermediate and not the end product of a metabolic pathway. It has something to do with catabolism/energy metabolism. The production phase can be distinguished from the growth phase. Examples of type 2 classifications are alkaline protease (Bacillus lycheniformis), amylase (Aspergillus oryzae), pectinase (Aspergillus niger), cellulase (Trichoderma resii), citric acid (Aspergillus niger), amino acids (Corynebacterium glutamicum), and riboflavin (Eremothecium ashbyi). All these processes are aerobic and are operated in batch mode. The first four systems are operated in fed batch mode as well. One example of an anaerobic system is vitamin B12 (Pseudomonas nitrificans), which is operated in batch mode.
Type 3 process is a nongrowth-associated production. Sometimes production only begins when the main carbon source is exhausted and a secondary carbon source is used. The product is not connected to catabolism or energy metabolism, e.g., production of secondary metabolites like antibiotics, vitamins. There are distinct growth and production phases, with negative specific growth rate. Examples of aerobic processes in this category are alkaloids (using Claviceps paspali), immunoglobulins (using hybridoma cells), interferons (mammalian cells), penicillin (Penicillium chrysogenum), cephalosporin (Acremonium chrysogenum), tetracycline (streptomyces strains). The first example is operated in the batch mode, the second two in the continuous mode and the last three in the fed batch mode.
Deindoerfer proposed another type of classification, which relates product formation with nutrient consumption during the fermentation process. This classification is useful for designing batch fermentors.
1. Simple type, where nutrient is converted to products with fixed stoichometry as in the case of glucose getting converted to gluconic acid:
where CA and CB are concentrations of the two species. At t = 0, Ca = CA0 and CB = 0.
2. Simultaneous: The nutrient is converted to two products in variable stoichometry without the formation of intermediates. Formation of cell mass and yeast from glucose is an example of this type.
rAB and rAC represent the rate corresponding to the conversion of A to B and A to C, respectively. The rate constants for the forward and backward reaction between A and B are larger than those relating A and C.
3. Sequential: Also known as consecutive reaction, where the formation of final product is superceded by an intermediate. An accumulation in intermediate is observed here.
Here —dCA/dt = rA, dCB/dt = rA — rB, and dCC/dt = rB-rA and rB are the rate of reactions.
An example of sequential reaction is the conversion of glucose to glucolactone and then to gluconic acid in the presence of the enzyme Pseudomona ovalis.
4. Stepwise: Here nutrients get completely converted to intermediate first, before this intermediate gets converted to the final product. As the nutrient is fully consumed, the biocell adapts itself to use the intermediate as the new nutrient. The reaction sequence here is similar to sequential type, but the formation of C will occur only after complete exhaustion of A. Two examples of stepwise production are (1) diauxic growth of E. Coli in glucose-sorbitol medium and (2) growth of Acetobacter suboxydans in glucose. In the second example the biocell first grows on glucose and as soon as it is consumed on gluconic acid, which is formed from glucose. During this phase, gluconic acid gets converted to 5-ketogluconic acid.
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