HPLC mobile phase for acids determination was prepared with KH2PO4 (Riedel-de Hae¨ n, Germany) and HPLC-grade methanol (Panreac, Spain). For caffeine determination, the mobile phase was prepared with triethylamine (Riedel-de Hae¨ n, Germany) phosphate buffer and HPLC-grade acetonitrile (Carlo Erba, Italy). Caffeine standard was obtained from Fluka (Germany).