Air drying characteristics of osmotically concentrated diced sweet potato (Ipomoea batatas L.)

The sorption and air drying characteristics of fresh and osmotically concentrated diced sweet potato were studied. Diced sweet potato (l-cm cubes) were concentrated in 50 and 60% (w/w) corn syrup solutions of 42 DE at 25°C for 2 and 5 h. Fresh and osmotically concentrated diced sweet potato were air dried in a laboratory tray dryer at 55°C with an air velocity of 1.5 m/s for approximately 12 h.

The sorption characteristics of osmotically concentrated diced sweet potato in the a_{w range of 0.9 to 0.97 were found to be different from that of fresh diced sweet potato. Considerable overlapping of the sorption data of fresh and osmotically concentrated diced sweet potato was observed outside the aw range of 0.9 to 0.97. The Oswin equation was found to fit sorption isotherms well. Parameters of the Oswin equation were estimated in the aw range of 0.6 to 0.99 with R² values of 0.89 and 0.91 for osmotically concentrated and fresh diced sweet potatoes, respectively.}

The drying rate curves of fresh and osmotically concentrated diced sweet potato exhibited non-linear falling rate periods with no constant rate drying period. Moisture transport during the falling rate drying period was modeled using Pick's second law. The effective moisture diffusivities for osmotically concentrated diced sweet potato were slightly greater than that of fresh diced sweet potato (3.88x10^-10 m²/s for fresh and between 3.69x10^-10 m²/s to 4.35x10^-10 m² for osmotically concentrated diced sweet potato). Although some differences were observed between the drying rates of fresh and osmotically concentrated diced sweet potato, the differences among the diffusivities were not significant.

The sorption data and moisture diffusivities obtained through this study provide useful information for the design and development of a sweet potato dehydration process.