Changes of Drying Characteristics and Raisin Quality in the Process of Drying Grape by Far-infrared Combined Heat Pumps

This study investigated moisture migration and quality changes in grape during the far-infrared combined heat pump drying process, utilizing low-field nuclear magnetic resonance and imaging techniques to analyze drying characteristics and proton density imaging. The research further examined the microstructure and nutritional quality of raisins subjected to heat pump temperatures of 60 ℃, 80 ℃, and 100 ℃. The results revealed that the drying endpoint was reached in 600 min, 540 min, and 480 min at temperatures of 60 ℃, 80 ℃, and 100 ℃, respectively. Under varying heat pump temperatures, the low-field NMR T₂ spectra of raisins exhibited a consistent leftward shift, accompanied by a gradual decrease in signal amplitude. The NMR results suggested that the free water on the surface of the raisins was removed first, while internal moisture migrated outward, with an increased migration rate at higher temperatures. Microstructural observations demonstrated that increased heat pump temperatures correlated with increased structural damage to grape cells. At a heat pump temperature of 80 ℃, the L^* value of raisins increased by 11.23% and 4.68% compared to 60 ℃ and 100 ℃, respectively; the total soluble solids (TSS) content rose by 31.42% and 33.45%; and the titratable acid (TA) content decreased by 13.09% and 6.59%. These findings suggested that treatment at 80 ℃ was more effective in preserving the color, total soluble solids content, and in mitigating the degradation of organic acids in raisins. Furthermore, at this temperature, the contents of Vitamin C (Vc), total phenols, total flavonoids, and anthocyanins decreased by 78.28%, 66.01%, 40.59%, and 77.14%, respectively, compared to their levels before drying. The nutritional loss of raisins was minimal at 80 ℃ when compared to 60 ℃ and 100 ℃. In conclusion, 80 ℃ is identified as the optimal temperature for far-infrared combined heat pump drying, serving as a key process parameter for enhancing the quality of grape products.