We developed an in vitro assay system that mimics the physiological conditions for starch digestion. In this system all undigested starch was termed resistant starch (RS). The richest sources of RS were firm bananas and roasted chickpeas. Different food-processing techniques produced different amounts of RS. RS in uncooked oats (0.161 g/g dry wt) and firm bananas (0.247 g/g dry wt) was reduced by cooking to 0.028 and 0.032 g/g dry wt, respectively. Cooling boiled potato overnight at 4 degrees C produced a 2.8-fold increase in the amounts of RS. Whole rice contained more RS than did ground rice whereas grinding did not affect amounts of RS in roasted chickpeas. Amounts of RS in food decreased with increased chewing, indicating that chewing can also affect the amount of starch escaping digestion. This assay may be useful in predicting which foods and processing techniques result in high amounts of starch escaping digestion in the small intestine.