rshosted said:
Ok masta, What you have said made me wonder a few things.
Obviously yeast does need some oxygen... But I doubt they are like little turtles that need to swim to the top to gulp a few swallows of air and swim back down for a sugar feast and reproduce, but if the top of a carboy is deficient of O2, do they take oxygen from the water?
I've always used a bucket with an air tight lid and water lock, I figure there is no (limited) oxygen in there after a few days of fermenting. Typically I wait until it is done fermenting, then rack. Do you think this changes the wine in anyway?
I kind of assumed the idea was to get the wine off the gross lees to prevent spoilage from the pulp that is typically settled at the bottom, not really to prevent air from getting to the wine (though, I would conceed that is an added advantage and left after fermenting you are risking air getting into the container).
I also ferment almost all my kit wines in a sealed primary with airlock.The water added to your must does contain some dissolved oxygen and when you stir the must it adds more (another reason to stir the must well in addition to mixing the viscous concentrate completely with the water). I believe the amount of oxygen available for the pitched yeast to reach a healthy colony in a sealed primary that has been stirred well is more than enough for a completion and problem free fermentation.
Boiling water as done when making beer drives off oxygen so stirring the wort is even more critical to make sure there is enough oxygen for the yeast to get started.
Another factor toremember is that grape juice is the perfect fermentation medium since it contains all the proper nutrients, amino acids and other goodies the yeast thrive on.
A more detailed explanation of the life cycle of yeast:
1) Respiration -- the first stage in the life cycle is aerobic. When
yeast is added to an unfermented nutrient broth (called wort in brewing,
or must in winemaking), it utilizes free oxygen in the solution. No
alcohol is produced in this stage, and CO2 production is low. During
respiration, yeast stores energy in various chemical forms to be used
later during reproduction and fermentation. Aerobic respiration will
generally continue until most of the dissolved oxygen is exhausted.
2) Fermentation -- this is the stage during which most CO2 is
produced. When no oxygen is available, yeast will switch to an
alternate metabolic pathway utilizing sugars for energy and producing,
primarily, CO2 and ethanol. Yeast divides rapidly in this phase,
reaching its carrying capacity (about 50 million cells/ml) in the wort
or must, and remains suspended in solution in order to expose maximum
surface area to nutrients. Assuming no oxygen is added back to the
fermenting wort, yeast will continue fermentation until one of two
things happens; either alcohol concentration will exceed tolerance, or
the yeast run out of food.
3) Sedimentation -- once conditions are no longer amicable to
fermentation, yeast will stop dividing and start storing energy in the
form of glycogen, a polysaccharide. Yeast will flocculate and fall out
of solution, creating a cake of sludge on the bottom of your
fermentation vessel. At this point, the yeast are no longer
metabolically active, and await better conditions in a state of
dormancy.