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'The Fermentation Bifurcation' - Season 9, Episode 22
The gang goes to a wine bar and runs into Penny's old boyfriend, Zack. Also, Bernadette gets stuck spending the evening with Sheldon when she can't go wine tasting with the group, and things don't go as planned when Koothrappali introduces his new girlfriend, Claire.
Air Date: April 28, 2016.