Product Formation

A classic study by Luedeking and Piret (1959) considered the relationship of cell growth to product formation. When the cells or some constituent of cells that is proportional to cell mass is the product, the rate of formation of product directly relates to the rate of growth. Many other products are known that have zero or a small rate of formation until growth ceases. It is useful to think of this as the cell using resources to grow until some important nutrient such as the sugar falls to a low concentration. For several years, lactose was thought to be essential for the penicillin fermentation. It was found that lactose (glucose and galactose linked to form a disaccharide) was hydrolyzed at a rate that supplied sugar for the synthesis of the antibiotic but did not provide concentrations that stimulated growth. Dramatic improvements in titer were realized by abandoning lactose and feeding glucose fast enough for optimum synthesis of penicillin while avoiding concentrations that gave growth instead. Actual situations, then, range from entirely growth-associated product formation, through cases where product formation is partially growth-associated, to cases where product formation is an alternative to growth.

Luedeking and Piret proposed the following simple equation:

When tested with the microbial process for producing lactic acid, this equation fit actual data very well. This was somewhat fortuitous because the agreement is not so good with some other processes. Probably alpha and beta do not remain constant as the physiological age of the cells or the conditions of the process change. Nevertheless, the concept of distinguishing the relationship of product formation to growth rate is sound.
Simulation exercise for graphing a batch process

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    This page last updated by in February 2016