Pre-conditioning of parboiled rice kernels with different concentrations of NaCl and KCl: Its microstructure and microwave puffing characteristics

Abstract.

Pre-conditioned pressure-parboiled rice suitable for puffing was obtained with uniform heating (≈ 90°C) and drying of high moisture (30%wb) salt-infused (NaCl and KCl) kernels to around 10.5%db using fluid-bed dryer. Puffing was carried out using domestic microwave oven at 1000W for 30s. Effects of each salt (0 – 5% w/w levels) and moisture content (25 – 10.5% with 4% NaCl) on changes in microstructure of the surface and transverse section of the kernels, puffing characteristics and sensory quality of puffed rice were studied. No-salt rice kernels gave less-expanded and ripple-surface puffed products. Surface micro-structure of pre-conditioned kernel showed a structural transition from continuous fused mass (at 0% NaCl) to cluster mass with generation of porous body (at 1-2% NaCl), followed by interlocking and overlapping structure (at 3 – 5% NaCl). Changes in transverse section with salt beyond 3% salt level were found with distinct aggregation of mass and generation of porous structure. Expansion of the pre-conditioned kernel during puffing might be attributed to development impervious surface and aggregated-porous core. Similar phenomena have been observed for kernels containing KCl except less cluster formation at low concentration. Progressive moisture reduction increased cluster formation and voids in kernels. Granulation of mass was apparent in kernels at 10.5%(wb). Increase in salt concentration showed increase in percentage of kernel puffed and expansion ratio, reaching respective maxima at 4% level of either NaCl (80.87%; 7.6) or KCl 83.09%; 7.7%).