Metabolites and End Products

According to Gaden (1955, 1959), four types of product accumulation can be distinguished on the formal level based on the quantitative relationship 

Type 0. Type-0 production as a supplementary case occurs even in resting cells that use only a little substrate for their own metabolism. The microbial cells function only as enzyme carriers. Steroid transformation and vitamin E synthesis by Saccharomyces cerevisiae are examples. 

Type 1. Type-1 situations include processes in which product accumulation is directly associated with growth this is the case for primary metabolites, in which the formation of the product is linked to the energy metabolism. Examples include fermentation to produce alcohol and gluconic acid (Koga et al., 1967), and situations in biological waste water treatment. 

Type 2. Type-2 processes include fermentations in which there is no direct connection between growth and product formation and also no direct or indirect link to primary metabolism (secondary metabolites), for example, penicillin and streptomycin. 

Type 3. Type-3 processes include those having a partial association with growth and thus an indirect link to energy metabolism. Examples include citric acid and amino acid production. 

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Microorganisms have been identified and exploited for more than a century. The Babylonians and Sumerians used yeast to prepare alcohol. There is a great history beyond fermentation processes, which explains the applications of microbial processes that resulted in the production of food and beverages. In the mid-nineteenth century, Louis Pasteur understood the role of microorganisms in fermented food, wine, alcohols, beverages, cheese, milk, yoghurt and other dairy products, fuels, and fine chemical industries. He identified many microbial processes and discovered the first principal role of fermentation, which was that microbes required substrate to produce primary and secondary metabolites, and end products. 

Decomposition by different actions and in different media yields different metabolites and end-products. 

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