Starch is a natural polysaccharide that is used or exploited as a depressant/flocculant in the mineral processing industry. Native starch is typically digested with caustic soda or boiling water before use in such applications. How the digestion procedures affect the properties and functions of the starch has not been fully understood. In this study, zeta potential measurements, titration. Fourier transform infrared spectroscopic analysis, and turbidity measurements were utilized to detect and quantify the acidity of starch digested with different concentrations of caustic (sodium hydroxide or potassium hydroxide). The results revealed the formation of acidic groups on the starch after digestion by the alkali hydroxide. The generated acidic groups on starch seem to have contributed to the adsorption of starch on mineral surfaces. (C) 2012 Elsevier B.V. All rights reserved.