Objective: This science fair project consists of an investigation of several tea types (black, green, oolong and white tea) that have been commercially decaffeinated for the presences of detectable traces of caffeine using a tannic acid assay. The longer the tea is brewed, the more caffeine will be removed but so will other compounds in the tea leaves. 2  Carefully pour 2 ml of each tea infusion into five test tubes (or pill bottles if used instead of test tubes) and allow the tea infusions to cool.