Sodium Bicarbonate and the Microstructure, Expansion and Color of Extruded Black Beans

Seed microstructure provides complementary information to physicochemical determinations. Scanning electron micrographs of cross sections of black bean extrudates illustrated the increase in volume expansion, with an increase of sodium bicarbonate (NaHCO 3 ) in the extrudate. This increase in volume expansion is attributed to the increase in air cell size and corresponding decrease in air cell wall thickness with additional concentrations of NaHCO 3 . The measured increase in diameter and expansion ratio of extrudates with NaHCO 3 addition may be explained by the increase in number of air cells within the extrudate and the increase in pores in the gelatinized starch matrix of air cell walls. Extrusion conditions, which involved the use of heat and moisture, provided the necessary conditions for the release of CO 2 from NaHCO 3 during processing. Expansion ratio increases between the control extrudate, and extrudate with 0.5% NaHCO 3 addition were twofold at the node and 1.8-fold at the area between the nodes. Statistical analysis of color data exhibited no significant change in L*, hue or chroma across concentrations of NaHCO 3 for nonextruded flours. A simple linear regression adequately described changes in L* and hue for extruded flours. However, a curvilinear relationship was needed to explain changes in chroma versus NaHCO 3 for extruded flours, making changes in chroma measurements less easy to interpret. The color data in this study serve as primary information for future establishment of cut-off values of color for the development of an acceptable legume snack.