Abstract: The influence of multi-stage far-infrared radiation-assisted heat pump drying versus heat pump drying on the textural characteristics, microstructure, browning degree, color, soluble sugar content, free amino acid contents and 5-hydroxymethylfurfural (5-HMF) content of longan was evaluated. The results indicated that the combined drying method could reduce the hardness, improve the springiness and chewiness (P < 0.05) and enhance the porosity of dried longan when compared with heat pump drying. The color change after drying mainly originated from the decrease of L* (turning darker) and the increase of a* (turning yellow), while far-infrared radiation at 60 ℃ coupled with heat pump drying could inhibit the browning effectively. Furthermore, the contents of soluble sugar, free amino acids and 5-HMF could affect the color of dried longan. Notably, when the far-infrared radiation temperature was 100 ℃, the contents of soluble sugar and free amino acids decreased, while the content of 5-HMF increased significantly (P < 0.05). The present study confirmed that multi-stage far-infrared radiation-assisted heat pump drying could improve the quality of dried longan, and the Maillard reaction was the major cause of the color change during the drying process.

Key words: longan; far-infrared radiation-assisted heat pump; multi-stage drying; quality