Roots of Morinda citrifolia (Noni or Yor in Thai) are the source of important compounds, anthraquinones, which have been proven to have anti-viral. anti-bacterial, anti-cancer activities. The most medicinally valuable anthraquinones in the roots of this plant is damnacanthal, which has been used for treatment of chronic diseases such as cancer and heart disease. In this study, subcrifical water extraction was investigated as a benign alternative for solvent extraction of damnacanthal from the dried root of Morinda citrifolia. The experiments were conducted in a continuous flow system at a pressure of 4 MPa at different temperature between 150 and 220 degrees C and water flow rates of 1.6, 2.4, 3.2 and 4 ml/min. The quantitative analysis of damnacanthal was performed using RP-HPLC with UV detection at 250 nm. The results of the study revealed that the highest amount of damnacanthal extracted with subcritical water was obtained at 170 degrees C. In addition to the effect of temperature, extractions were conducted at various flow rates and the data were fitted with mathematic models to determine the extraction mechanism. The results suggested that the overall extraction mechanism was influenced by solute partitioning equilibrium with external mass transfer through liquid film. Nevertheless, the desorption model could describe the extraction behavior of Morinda citrifolia reasonably at high flow rates. (c) 2007 Elsevier B.V. All rights reserved.