Abstract: In this study, potato-oat composite noodles were dried by a new combined heat pump-hot air drying technique. Low-field nuclear magnetic resonance (LF-NMR) and scanning electron microscopy were used to analyze the drying characteristics and the heat transfer characteristics during the drying process. The results showed that higher heat pump temperature, water content at the conversion point or hot air temperature could result in higher drying efficiency and increased diffusion of moisture. The Midilli model was suitable for describing the drying behavior of noodles. Under different drying conditions, the effective water diffusion coefficient was 3.82 × 10-10–5.12 × 10-10 m2/s. The results of LF-NMR showed that the total water content decreased continuously during the drying process, and the peak of weakly bound water shifted to the left, indicating that the content of weakly bound water decreased to the greatest extent, accompanied by a simultaneous decrease in the degree of freedom; weakly bound water was mostly transformed to free water along with tightly bound water. When the equilibrium moisture was reached, bound water was relatively more abundant than free water. The moisture content on a dry basis exhibited a significantly positive correlation with peak area (P < 0.01). Magnetic resonance imaging showed that the density of protons in the middle of noodles was the largest, and protons migrated outside as the drying progressed. The microstructure became denser, and tightly bound water more tightly combined with macromolecules, thus resulting in the formation of a more intact gluten network structure.

Key words: composite noodles, combined heat pump-hot air drying, drying characteristics, low-field nuclear magnetic resonance, moisture mobility