The fermentation of new preparations of modified starches (wheat, potato, pea) by Bifidobacterium monocultures or microflora of rat caecum was evaluated on the basis of changes in the microstructure as well as bacterial basic end-products in comparison to their initial native starches. The growth and acidifying activity of the selected strains: B. pseudolongum KSI9, B. breve KN14 and B. animalis KS20al, were higher in fermented modified starches compared to their native forms, however lower than those obtained upon glucose as the control. Diversified effects of the strains examined on native and modified starches were shown in images of microstructure after 24-h fermentation, including formation of globular and flat lamellar structures. As a result of caecal digesta fermentation, a tendency for the formation of low molecular short-chain fatty acids, i.e. acetic acid, and lactic acid, compared to the high molecular ones, e.g. propionic and butyric acids, was indicated. (c) 2006 Elsevier Inc. All rights reserved.