I feel a bit like a nucleation site and responsible for trigging the bursts of parallel/serial issues lately. I also confess being a yeast philosopher :)

The harder questions you ask, the more important does language, terminology and the proper construction of the question become. IMO, in real life more often than not, many questions are indeed fuzzy. While a computer would simply reject it as syntax error, a human philosopher will not do so, that is too easy. Once a question is properly posed and technically well defined answering it is often principally a matter of "computing" and thus from a philosophers point of view - trivial. I find the learning*process* itself intriguing.

Sometimes I find the mechanism of concluding something more interesting that then conclusion itself. Facts are like matter, while the "why" is the essence of life. My experience is that many real problems, and almost *all* of the most profound ones, are fuzzy questions, and does require a "philosophical aspect" to solve them. You can not (currently) ask a computer something like, at what point of complexity does matter turn into life? It is a fuzzy question, yet I find it quite relevant and most humans does get the basic quest, but we lack logic to deal with it. This is were a computer fails, but a philosopher will invent new logic as needed. IMO one of the differences between a computing device and real intelligence is the capability to invent new logic, and not turn into syntax error halts.

What I am trying to say is that some of this discussions look to me a bit like we are computers here. I think everyone agree the basic (fuzzy) question, we want to know how yeast behaves. Look at something like this (*just in principle*, beware for typos)

x = the entire set of variables, yeast, wort and fermenter (except time!)

dglucose/dt = - fg(x)*glucose*yeast_active +
+ 0.5 * [ - fs1(x)*sucrose*invertase_wort -
fs2(x)*sucrose*invertase_surfacebound ]

dfructose/dt = - ff(x)*fructose*yeast_active +
+ 0.5 * [ - fs1(x)*sucrose*invertase_wort -
fs2(x)*sucrose*invertase_surfacebound ]

dsucrose/dt = - fs1(x)*sucrose*invertase_wort
- fs2(x)*sucrose*invertase_surfacebound

dmaltose/dt = - fm(x)*maltose*yeast_active
dmaltotriose/dt = - fmt(x)*maltotriose*yeast_active

Note that the equations account for the active yeast. The dormant yeast does not consume wort sugars. So once the yeast go dormant, the remaining sugars will be left as residuals.

f are different regulations. Let's assume some typical enzyme kinetics (michaelis-menten) this might be something like:

fg(x) = ag(x) / { glucose + kg(x) }
ff(x) = af(x) / { fructose + kf(x) }

..etc

where ag() and kg() are further regulations to find out, so using knowledge, I will keep expanding these regulators, until I reach a level where it's reasonable to think that are constants, then these will be my model parameters, and of course each parameters should be intrinsic to yeast, wort or fermentor. There must not be parameters that depend on everything.

The problem would be to find these regulations. The model will be *crowded* with these regulation functions, and they must all satisfy balancing of energy, redox, carbon, nitrogen etc.

So training the model is to keep tweaking the regulation *functions* and also to tune in the parameters.

On thing I failed to find is if the invertase enzymes are cellbound or expelled freely into wort, or both??? I know that in theory there are both options but for brewing yeast I don't know which is relevant? I think I asked this question before on here long time ago but I got no comments.

Worst case I have to account for both, like above. Any ideas?

Where is the location of action of invertase splitting sucrose? I wonder if it is all expelled into the wort, or if there is also relevant activity on the cell surface bound invertase? I remember reading some cells do have cellbound sucrose enzymes, what is that the case for S.C?

- Fredrik

RESPONSE:

Hello again,

Almost all invertase activity is located in the periplasm. This is a thin cell wall-associated region external to the plasma membrane and internal to the cell wall. The invertase is unable to permeate the cell wall. Sucrose can permeate the cell wall but can not cross the cell membrane. It is hydrolyzed in the periplasm into glucose and fructose which then can cross the cell membrane into the cell. (Walker, G.: Yeast Physiology and Biotechnology, Wiley & Sons, Chichester, 1998, p 21)

Tobias & Forbes