The effects of acid treatments (HCl or organic acids) and drying processes (oven or sun) on the physico-chemical and functional properties of cassava starch have been studied. The objective was to obtain modified starches with expansion properties similar to those of sun dried fermented cassava starch (polvilho azedo) as produced by an empirical process. Expansion was evaluated using a baking test. Independent of the drying method, organic acid-hydrolysed cassava starch presented the same X-ray diffraction patterns (changes from C to A) and similar intrinsic viscosity values. SEM observations showed no differences between granule surfaces of sun dried and oven dried samples. However sun dried organic acid-hydrolysed starches presented lower paste consistency values at 30 degrees C when compared with oven dried ones. HCl-modified cassava starches were oven or sun dried, giving biscuits with low specific volumes (2.5-3.1 ml g(-1)). Similar behaviour was obtained using oven dried organic acid modified cassava starches. When submitted to sun drying, organic acid modified starches showed great improvements in biscuit expansion (5-10 ml g(-1)). The highest average specific volumes were obtained using lactic acid modified starches. (C) 1998 SCI.771103108