Dear Dr. Brew,What will cause sweet tea to have a sour taste that has only been brewed for several hours?
Fresh brewed tea, sweetened or unsweetened should keep a "fresh" taste for more than the two hours you mention. According to the Centers for Disease Control and Prevention (CDC) and the tea industry, iced tea should be brewed at > 195°F for 3-5 minutes, that tea should be stored for no longer than 8 hours and the tea brewer, storage dispenser and faucet should be cleaned daily.
My first response would be to examine the dispensing devise and assure cleaning procedures have been followed. I have attached a link to a cleaning procedure card. This may not match your dispenser, although will supply you with a recommended cleaning procedure.