We have a discussion with our colleagues. We do remember that is not possible to calculate microbial product yields during non-stationary conditions. But we do not remember why.
We do remember that this is the case in continuous fermentations, where we wait for the steady state to calculate the yields.
Cheers,
Dear Juan-Rodrigo,
The third phase of growth is the stationary phase. The stationary phase in a batch culture can be defined as a state of no net growth, which can be expressed by the following equation:
dX/dt = 0
Although there is no net growth in stationary phase, cells still grow and divide. Growth is simply balanced by an equal number of cells dying. There are several reasons why a batch culture may reach stationary phase. One common reason is that the carbon and energy source or an essential nutrient becomes completely
used up. When a carbon source is used up it does not necessarily mean that all growth stops. This is because dying cells can lyse and provide a source of nutrients.
Growth on dead cells is called endogenous metabolism. Endogenous metabolism occurs throughout the growth cycle, but it can be best observed during stationary phase when growth is measured in terms of oxygen uptake or evolution of carbon dioxide. Thus, in many growth curves the stationary phase actually shows a small amount of growth. Again, this growth occurs after the substrate has been utilized and reflects the use of dead cells as a source of carbon and energy.
A second reason that stationary phase may be observed is that waste products build up to a point where they begin to inhibit cell growth or are toxic to cells. This generally occurs only in cultures with high cell density. Regardless of the reason why cells enter stationary phase, growth in the stationary phase is unbalanced because it is easier for the cells to synthesize some components than others. As
some components become more and more limiting, cells will still keep growing and dividing as long as possible. As a result of this nutrient stress, stationary phase cells are generally smaller and rounder than cells in the exponential phase .
For more details, please use the following link:
http://booksite.elsevier.com/samplechapters/9780123705198/Sample_Chapters/04~Chapter_3.pdf
Hoping this will be helpful,
Rafik
You can calculate product yields in a chemostat setup by simply dividing the steady-state product concentration (assuming that there is no product in the feed medium) by the decrease in substrate concentration (S_feed - S_residual). During exponential growth (e.g in shake flasks or batch fermentations) one can assume pseudo-steady state and one can plot the increase in product over the decrease in substrate. The resulting slope corresponds to the product yield. This works only when product formation is constant and occurs in stoichiometric proportion to the substrate uptake. When products are simultaneously produced and consumed at varying ratios or growth rate and substrate uptake are not constant over time, steady-state can no longer be assumed and the calculation fails. One could provide maximum product concentrations and net carbon uptake rates instead.
I don't think it is always impossible to calculate product yields from dynamic experiments. Provided that (i) culture volume is constant and (ii) there is no product formation from storage materials or other biomass components, the product yield on substrate (YP/S) equals dCP/dCS in each time point. When CP and CS can both be measured with a sufficiently high time resolution, dCP and dCS under dynamic conditions can be calculated by, for example, fitting and subsequently differentiating a spline function through the data points. Obviously, this approach is not suited for very subtle or extremely fast fluctuations. I am not an engineer by training, so if specialist colleagues think this is too simplistic an answer I'll be happy to learn from their comments!
