Effects of time, ultrasonic treatment and pH during extraction on l-theanine and caffeine yields from white tea leaves

L-theanine and caffeine are extracted simultaneously from tea. Extraction protocols to minimize caffeine can help prepare low-caffeine tea-based products. The ultrasound (US) application and the solution's pH are factors that could affect the L-theanine and caffeine's relative extraction. In this study, white tea was extracted under low temperatures (∼10ºC) for 5min and 20min, with and without applying US (20kHz), to determine whether the extraction of L-theanine and caffeine could be differentiated. A time-dependent increase in L-theanine and caffeine levels in both treatments was observed. The highest levels of L-theanine (0.81±0.06mg/ml) and caffeine (1.34±0.07mg/ml) were obtained by 20min US treatment. Although US-treated samples had higher amounts of these compounds, the 5min US-untreated sample was the only treatment with a higher level of L-theanine (0.21±0.03mg/ml) than caffeine (0.16±0.06mg/ml), hence, the lowest caffeine-to-L-theanine ratio (0.75±0.16). This ratio was lower in the shorter treatments regardless of US application. The second experiment investigated the impact of pH on extraction efficiency. Lowering the extraction pH from 5.17 to 2.79 by lemon juice did not significantly change the L-theanine and caffeine levels in tea samples. These data may suggest that conventional infusion of tea is preferable to US-assisted extraction (UAE) for the preparation of low-caffeine tea drinks.