Abstract:

Comparative drying of Chinese jujubes by three different methods, stagewise hot-air drying, intermittent
microwave heating and microwave-hot air treatment was studied to reveal the effect of the dyring methods on the quality of
dried products. Meanwhile, we optimized the microwave-hot air drying process by response surface analysis. The results of
comparative studies indicated that total VC content was increased by 99.53% and 30.99%, and the browning index A420 was
much decreased, but total flavonoid content had no obvious difference through the use of microwave-hot air drying intead of
stagewise hot-air drying and intermittent microwave heating. The optimum microwave-hot air drying conditions were 305 g
of material loading, microwave drying at 119 W for 12 min with 7 intermittent intervals for 4 min each followed by hot air
drying at 55 ℃ for 9 h and at 50 ℃ for 12 h. The total VC content, total flavonoid content and A420 of dried products obtained
under these conditions were 36.22 mg/100 g, 35.53 mg/100 g and 0.3079, respectively. From the present study, we conclude
that microwave-hot air drying deserves to be popularized for drying jujubes.

Key words: Chinese jujube, microwave, hot-air, combined drying, technology optimization