Moisture flow characteristics and variation patterns of dielectric constants in bitter gourd slices during microwave hot-airflow vibrating drying

Abstract

The uniformity of microwave-dried bitter gourd slices was poor, and a microwave hot-airflow vibrating drying (MHAVD) apparatus was used to carry out an in-depth elucidation of moisture flow characteristics underlying the variation in dielectric characteristics. Low-field nuclear magnetic resonance and imaging (LF-NMR/MRI) was used to measure the moisture state and distribution during different drying stages, and the relationship model between moisture semaphore and moisture ratio were analyzed. A vector network analyzer was used to measure the patterns in the variation of dielectric characteristics in bitter gourd slices at different drying stages. As a result, the temperature and moisture distribution of bitter gourd slices during MHAVD was uniform caused by hot air and vibration, and the water semaphore in NMR gradually decreased, but the bound water semaphore increased during certain drying stages. There is a significant linear relationship between the total semaphore A₂ and the moisture ratio M_R in bitter gourd slices per unit dry basis mass in the NMR spectrum (R² = 0.99). As moisture decreases, the ability of the material to store microwaves and convert electromagnetic energy to heat energy decreases, the ability of the material to absorb microwaves decreases, and the microwave penetration significantly increases. The moisture flow characteristics in bitter gourd slices is related to the variation of dielectric characteristics during MHAVD, and drying uniformity was improved.

Keywords:

• Microwave drying
• vibrating
• uniformity
• NMR
• dielectric properties

Acknowledgments

We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.

Additional information

Funding

The financial support from the Research and Development of High-quality Energy-saving Drying Process and Equipment for Special Agricultural Products, National Key R&D Program of China (2018YFD0700201) and Research and Development of Key Technologies and Equipment for Damage Reduction in Fruit and Vegetable Drying, National Key R&D Program of China (2017YFD400900).

Funding

National Key Research and Development Program of China (2018YFD0700201; 2017YFD400900), which enabled us to carry out this study.