Relationship Between Microstructure and in Vitro Digestibility of Starch in Precooked Leguminous Seed Flours

Abstract

Precooked flours (PCFs) were prepared by milling boiled and freeze-dried red kidney beans, white beans and lentils. As demonstrated by scanning electron microscopy, PCFs were rich in relatively large particles which contained cell structures filled with starch. In contrast, flours from raw seeds contained a large number of free starch granules. The in vitro a-amylolysis rate ofPCFs was remarkably low, but increased after physical and chemical treatments of the flours. Homogenization resulted in the largest increase of hydrolysis rate. The susceptibility to enzymatic hydrolysis was also enhanced when PCFs were preincubated with pepsin or submitted to additional boiling. These treatments promoted evident alterations in the microscopic appearance of the cotyledon cell walls of the PCFs, changes that ranged from an apparently thinner surface (pepsin effect), to an almost complete disruption (homogenization effect). A flour prepared from boiled and vacuum-dried red beans showed Jess structural integrity and greater rate of amylolysis than the corresponding PCF, indicating that the drying procedure may influence the microstructural and digestibility features of precooked legumi nous materials. Neither cell walls nor starch granules were observed after suspending PCFs in 2N KOH, giving support to the use of alkaline pre-treatment for the evaluation of to tal starch content of PCFs by enzymatic procedures. The present results suggest the persistence of starch granules enclosed in cotyledon cells as a primary reason for the limited enzymatic availability of starch in PCFs.