Lag phase and glycogen:
1) If there is basically no metabolic activity during the lag phase - is the glycogen level not an issue during the lag phase? I had the impression that low glycogen levels (like in stored yeast?) would cause an extended lag due to some "free energy limitation"? I thought that until yeast start to uptake and metabolize sugars, glycogen (and some others) was the energy supply for whatever activities that is going on at this time? It this wrong? Are all the adaptations in the lag phase of passive nature, like equalizing gradients? Requiring no metabolic energy to be spent? I understand that choice of strain may impact the lag time, but for any given strain, what would be the most important factors and variables be that determines the length of the lag?
2) From your answer about sugar utilization, to make sure I got it right, did I understand things correct if I think there is no regulation of sucrose uptake and invertase activity due to external glucose? so they would both start at the same time and go on in parallel? If, as like I would guess(?) that glucose uptake is a lot quicker than sucrose uptake, would it be possible to observe a small dip in CO2 production activity at the point where the external glucose is consumed or reached some threshold? I did observer such a dip in a test I did. It sure is possible that ambient conditions cause this, but in this case for some reason I think not, and the dip coincidently occurred at about 6% depletion of fermentables. I would like to ask you about your opinion if you think that detecting such a dip (about 1.5 hours long and the "dip" was lowering the CO2 rate by some 15%) is possible? Or do you think I should rule this explanation out and start looking for another explanation of the dip?
(1) You are correct in your impressions. Perhaps I overstated the lack of metabolic activity during the lag phase. While it is acclimating itself to its new environment, it is still very much alive and requires energy. This energy does come from the glycogen and trehalose reserve present in the yeast from the last fermentation cycle and left over from the refrigerated storage period. If this carbohydrate reserve has been consumed during poor and or extended storage, there will be an extended lag phase.
(2) Perhaps I should explain the sucrose step a little better. Sucrose itself does not pass through the cell wall. Invertase is an external enzyme that is present at the beginning of the fermentation. It begins immediately to convert sucrose to glucose which joins with the initial glucose and inters through the cell wall together via glucose permease. There is no separate stage in which the initial glucose is used up, then the invertase begins to convert the sucrose to glucose.
Glucose is a maltose inhibitor. Neither the permeases or the maltase are produced in the presence of >0.4% glucose. There is probably a short period before these enzymes get into full production. Also, the maltose inters the cell at a slower rate than glucose, so you would probably note a noticeable slowing down or dip in CO2 production for a short period of time. Yeast strain, wort composition, and fermentation temperature can effect this transition period. You were very observant. Keep in touch with future observations.
Dr. Clayton Cone