Effect of berry size and sodium hydroxide pretreatment on the drying characteristics of blueberries under infrared radiation heating.

This research studied the effect of berry size and dipping pretreatment in hot sodium hydroxide (NaOH) solution on the drying characteristics of blueberries under infrared radiation (IR) heating. Changes in the microstructure and diffusion coefficient of the berries after the NaOH pretreatment were also determined using scanning electronic microscopy and dynamic vapor sorption (DVS), respectively. To quantify the effect of berry size, non-pretreated bulk blueberries were sorted into 6 groups based on their diameters and dried at 70 degrees C. To determine the effectiveness of NaOH pretreatment in improving drying characteristics, bulk blueberries of different sizes, both nonpretreated and NaOH pretreated, were dried at constant temperatures of 80 and 90 degrees C, and variable temperatures of 70 degrees C for 50 min followed by 90 degrees C for 50 min. The NaOH pretreatment dipped blueberries in 0.1% NaOH solution with fruit to solution ratio 1:1 (w/v) at initial temperature of 93 degrees C for 5 s. Results showed that the drying rate increased with decreased berry size. Average moisture diffusivity was in the range of 5.89 to 8.13 m2/s at 70 degrees C. The NaOH pretreatment increased drying rate and moisture diffusivity and reduced the number of broken berries, especially at high drying temperatures. Results from SEM observation and DVS showed that the increase in diffusivity coefficients of berry coat and loss of intact microstructure in coat and tissue cells might contribute to the effect of NaOH pretreatment on the IR drying of blueberries.