how does yeast fermentation effect pH levels?!

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In any fermentation that I am aware of pH drops.

Here is a site of two on the subject:

"pH of a fermentation will go more acidic as the ferment progresses due to yeast metabolism byproducts being excreted into the fermenting solution.

For this reason, we in the beverage brewing/distilling world start our fermentations at the range of

pH 5 - pH 5.5 so that when finished, the pH will still be around pH 4 which ensures we don't stop or retard the yeast action too soon ( i.e. before all the sugars are converted to ethanol & CO2 )."

http://gillesenergies.webs.com/phinferme…
http://www.makemead.net/adjusting-ph-and…