Grading by Fruit Density: An Effective way to Control the Drying Characteristics and Qualities of Mulberry ( Morus nigra L.)

To enhance the product quality, manufacturers of dried fruits are dedicated to developing reliable methods for assessing ripeness. In this study, a densimetric flotation method was employed to categorize mulberries into five distinct ripening stages (D1–D5, 0.905–1.055 g/cm³). The influence of ripening on mulberry water status, distribution, microstructure, drying characteristics, and product quality was examined. Heat pump drying, utilizing a temperature of 60 °C, relative humidity of 15%, and air velocity of 2 m/s, was employed as the drying technique for mulberries. As ripening progressed, the water binding capability initially decreased and then increased. Simultaneously, water distribution and cell microstructure shifted, resulting in an initial reduction followed by an extension in drying time. The drying time initially decreased and then increased with increasing maturity, with D4 fruits displaying the shortest drying time (13.9 h). This was 35.05%, 24.04%, 20.14%, and 6.47% shorter than the drying times observed in stages D1, D2, D3, and D5, respectively. Ripeness significantly affected the volatile composition, color, texture, and sensory attributes of dried fruits. D1 fruits were not suitable for drying due to their intact cellular structure, lengthy drying time, and poor sensory quality of the resulting product. Conversely, D4 and D5 fruits exhibited shorter drying times and their products possessed a robust fruity aroma, improved palatability, and overall acceptance. This rendered them ideal candidates for the drying process. The findings of this research have practical implications for effective identification and drying procedures during mulberry ripening.