gmr044@leah.Albany.Edu (Gregg Recer) (08/25/89)
For the rec.food.*'s: Sorry if this seems to be getting a bit far flung. Perhaps the discussion should move to sci.bio or perhaps we need a new group: rec.food.yeast.physiology 8-). For the sci.bio's, _The Story So Far_: Basically several posts have discussed the possibility, or lack thereof, of generating alcohol in batches of homemade rootbeer. The discussion has drifted towards the ability of _Sachromyces cervesiea_ (i.e., yeasties) to produce ethanol from various substrates. In article <10600@fluke.COM> inc@tc.fluke.COM (Gary Benson) writes: ....stuff about different strains of yeast deleted... > My second point is that the amount of alcohol a yeast will produce is >directly proportional to the amount of *fermentable* sugars available to it. >Regular old white sugar like you're going to add to the root beer will not >make ANY alcolohol. True, the yeast will ingest it, and yes, even live a >while on it, but the only thing they're going to excrete is C02. I'm not a home-brewer so I can't say from that kind of experience what a batch of root beer might generate in terms of alcohol. I do, however, have some experience growing _Sachromyces_ and various fermenting bacteria under laboratory conditions. Yeast will grow quite nicely on pure glucose or sucrose (which is a dimer that upon hydrolysis produces one glucose molecule and one fructose molecule which is a glucose isomer). Whether or not ethanol is produced in such as culture depends on the aeration of the culture. When Oxygen is present _Sachromyces_ will produce a complete electron transport chain so that oxidative phosphorylation can take place. Glucose, or whatever is being metabolized, will then be completely oxidized to carbon dioxide and water (assuming a perfectly efficient system which probably doesn't exist in the biological world). Under anaerobic conditions, the yeast cells will only metabolize sugars via glycolysis resulting in a lot of pyruvic acid which is then reduced (after a decarboxylation that we'll only gloss over) in a non-energy producing side reaction to generate ethanol. This reaction also, conveniently for the yeasties, frees up a limited enzyme co-factor (NAD+). Well then, biochemistry suggests that alcohol might or might not result from the homemade rootbeer recipe suggested in an earlier post. I would guess, in practice, that little or none is generated in the relatively short time suggested by the earlier recipe. This assumes that the brew is mixed in some kind of large, open container [like a big pot or the kitchen sink :^) ]. As long as the mix is open to the air, and occasionally mixed, little or no fermentation should occur. We had an exercise in wine-making in the bacteriology class I teach. Those cultures were stationary (no mixing) and were closed, except for a vent for the CO2. Under these conditions plenty of alcohol was produced. We used welches grape juice as the substrate (ooooh, the French are shaking in their boots). I don't know the complete breakdown of constituents of white grape juice but fructose is a common sugar found in a lot of fruit and I would guess that this is a major substrate in this fermentation. Gregg "In future you should delete the words crunchy frog and replace them with the legend crunchy raw unboned real dead frog!!" -- Inspector Bradshaw, The Hygiene Division
martenm@cb.ecn.purdue.edu (The Yeastmaster) (08/28/89)
S. cerevisiae can and do produce ethanol under aerobic conditions. This phenomena is called the "Crabtree" effect and is more pronounced at higher substrate levels. This presents a problem to those attempting to maximize biomass production as the substate is made into a byproduct, (EtOH) as opposed to the desirable primary product (yeast biomass or some particular protein). In our lab we study the localization of secreted protein from recombinant S. cerevisiae and we constantly struggle with this problem. -- Mark Marten | "He is no fool to give what he can not Purdue University | keep to gain what he can not lose." martenm@cb.ecn.purdue.edu | -- Jim Elliot