The acid treatment of unripe banana flour (UBF) was carried out to obtain a fiber-rich product, and some functional and physicochemical characteristics were tested. An experimental design with temperature, time, and acid concentration as independent variables produced 20 experiments that were studied with a response surface methodology to discover the effect of these variables on the total dietary fiber (TDF) content. UBF showed a TDF content of 17% and a total starch content of 73%. The response surface regression model fitted to experimental results of TDF showed a good determination coefficient (94%). The maximum TDF content that predicted the model was 61.0% with conditions of 38 degrees C, 11 days, and an acid concentration of 1.6M. During model validation for 99.5%. The thermal characterization of the fiber-rich products after the acid treatment [acid-treated unripe banana flour (ATUBF)] showed higher thermal stability than its parental sample. The water absorption index of ATUBF was lower but the water solubility of ATUBF was higher than its native counterpart. The preparation of a fiber-rich product with UBF may be important for the development of food and medical products. (C) 2008 Wiley Periodicals, Inc.