Light and Scanning Electron Microscope Studies on Dry Beans: Intracellular Gelatinization of Starch in Cotyledons of Large Lima Beans (Phaseolus Lunatus)

Light and scanning electron microscopy were employed to characterize intracellular configurational changes of starch granules during gelatinization as well as to estimate any apparent differences in gelatinization temperatures in standard and quick‐cooking Lima bean cotyledons. Because of restrictions imposed by intact cell walls at gelatinization temperatures, dilation and protrusion of starch granules were impeded. Intracellular gelatinization of starch was initiated at about 76°C for water‐soaked and at 85°C for salt‐soaked (quick‐cooking) beans and progressed successively through characteristic configurational changes until maximum expansion occurred at the boiling point (100°C). The stage of expansion was dependent only upon the temperature of the medium and independent of the time during which cells were held at any given temperature. Viewed under the microscope, each stage of gelatinization was populated by a distribution of granule configurations. At each temperature, water‐soaked beans had consistently larger proportions of advanced stages of granule expansion. Even at the boiling point the state of expansion of starch granules appeared greater for the water‐soaked than the salt‐soaked beans. Nominal but finite birefringence remained in the starch granules of salt‐soaked beans after cotyledons were held in boiling water for up to 90 min. In water soaked beans, birefringence of the starch granules was completely eliminated under the same conditions.