A mathematical model based on the specific interfacial area as a key independent variable and titratable acidity content as a final goal was proposed for scaling-up Kombucha fermentation. The model was derived by processing data obtained from the experiments performed using the local tea fungus in sweetened black tea inoculated with 10% of fermentation broth from the previous process. The fermentation was conducted at 28 +/- 1 degrees C in six reactors of the different liquid phase volumes, i.e. having various specific interfacial areas. The fermentation broth samples were collected during the process to measure their pH values and titratable acidity. Validity of the scale-up model was verified by comparing the fermentation duration calculated from the model and duration of the real process in a large reactor, and a high agreement was found. Also, the model was successfully applied to extremely small volumes, proving its wide-span applicability. (c) 2007 Elsevier Ltd. All rights reserved.