Clayton Cone has explained that if have more than 0.2% glucose in our starters that we cannot expect any cell wall improvement even in the presence of oxygen, since the mechanism changes from a Pasteur Effect to a Crabtree effect.. Presumably the addition of oxygen to our wort at the beginning of the fermentation will likewise make no sense. Does this mean we need to wait until the end of the fermentation? And why do it at all?

http://www.biotech.kth.se/courses/3A1308/Downloads/Overflowmetabolism.pdf

Does this affect your explanation?

Keep on Brewin'
Dave Burley

RESPONSE:

I am glad that you asked this question because I did not wish to imply that there would be no cell wall improvement even in the presence of oxygen at >0.2% sugar. When the wort has more than >0.2% sugar, the function of the O2 is to assist the yeast in producing lipids. The lipids in the cell wall act as a growth factor by keeping the cell wall fluid, allowing buds to form. The production of these lipids require trace amounts of oxygen to move the squalene to the lipid stage. With out O2 the mother cell cannot produce any lipids and must shares her lipids with her daughter cell. This can occur for about 3 - 4 cycles before the cell wall becomes leathery and will not allow a new bud to form. Active Dry Beer Yeast initially contain enough lipids in their cell wall for 3 - 4 growth cycles. This is enough to complete most beer fermentations. Recycled yeast usually do not contain enough lipids for 3 - 4 growth cycles. The lipids must be replenished with fresh additions of Oxygen.

In low gravity brewing, oxygen at the beginning of the fermentation is usually adequate. It is when you get into hi gravity brewing that additional oxygen is helpful after 12 to 24 hours of fermentation. Dr. Mike Ingledew has found that aeration at about the 14th hours is optimum for high gravity brewing. We have found that aeration after 36 hours is optimum for wine fermentations.

Dr. Clayton Cone
 

Q- So we need to develop method of stirring our starters which will keep the glucose concentration low, but allow yeast to grow more cell wall fatty acid containing substances. Since few of us have the metering and measuring equipment to do this in a straightforward process control way, perhaps there is another way to do it.

RESPONSE:

One of the functions of a batch starter culture is to produce alcohol and a healthy cell mass free! free! free! of any contamination. Alcohol produced at this stage at the sacrifice of cell mass is welcome because it minimizes spoilage organisms. Mechanical stirring such as a magnetic stirrer with out an airlock is great. However, any attempt to add nutrients or sugar during the starter stage to keep a low level of sugar, lends itself to contamination.

Dr. Clayton Cone
 

Q- Any idea about how fast sucrose would be inverted e.g. if we started with a 1% sucrose solution with appropriate nutrient levels in a stirred, oxygenated starter , is there any chance that the rate of inversion would be slower than the rate of consumption (I guess this would be yeast concentration dependent) , so we can keep the glucose (does Crabtree also work for fructose?) concentration down and permit good cell wall development in an oxygenated starter?

RESPONSES:

I would doubt that there is any benefit to doing this. Invertase activity will be reduced by the presence of glucose....thus as the invertase cleaves sucrose it is producing glucose which, if the glucose accumulated, would eventually shut down the invertase activity. In general the invertase activity never seems to lead to this situation, so you would have to assume that invertase activity is modulated to prevent glucose accumulation. You are probably better using malt extract/ malt as you normally do. The presence of glucose is very low (about 10% of the sugar) and will be consumed quickly. This then means that maltose become the sugar of choice for the yeast. Maltose does not initiate the Crabtree effect in the same way; in fact it is a very weak activator in yeast. This is probably due to the fact that maltose is actively transported prior to being cleaved into two glucose units and this will bypass the major glucose signaling systems in the cell membrane.

Forbes

The invertase enzyme produced by the yeast converts the sucrose to glucose much faster than the yeast can metabolize the glucose. I believe that the Crabtree Effect is nearly the same for fructose.

Dr. Clayton Cone

Addition of air/oxygen at the start of the fermentation allows the yeast to consume glycogen to produce sterols and unsaturated fatty acids......it is a different situation to respiration. Production of these compounds has an absolute requirement for oxygen. The aeration you do at the start of the fermentation will be consumed very quickly (30 mins.) but this is normally enough. In aerobic propagations with high sugar (as you normally would be doing) you will have some effect on the cell membrane but it will not be as good as growing the yeast at low sugar. Under aerobic conditions the yeast has to make sterols and fatty acids as both these can have trouble being transported in the presence of air. However, under anaerobic conditions both these substances are easily transported but not produced by the yeast.

Forbes
 

How about the Overflow Metabolism effect?

http://www.biotech.kth.se/courses/3A1308/Downloads/Overflowmetabolism.pdf

Does this affect your explanation?

RESPONSE:

Overflow metabolism or the pyruvate dehydrogenase (PDH) bypass is a reasonably well understood mechanism that is only truly relevant for cultures with controlled growth rates/glucose feeding. There is really no overflow involved in the batch propagation you mentioned. If you were trying to control the propagation then the PDH bypass would only be in play if you were trying to grow the yeast at a growth rate that is very close to the limit of its respiratory capacity. At this point pyruvate metabolism into TCA is saturated and pyruvate starts to spill over towards Ethanol but (and this is perhaps simplistic) the amounts are still insufficient to commit to alcoholic fermentation so you get the acetic acid-acetyl Co-A part of the pathway continuing to supply acetyl Co-A. All of this is well presented in the internet link that you provided...many thanks, much more eloquent in graphic form.

Forbes