Ultrasonic Pretreatment With dl-Malic Acid–Water Media: An Innovative Strategy to Enhance Infrared-Assisted Spouted Bed Drying Efficiency and Product Quality of Chinese Yam

This study employed ultrasound (US) pretreatment with different media—water (W), ethanol (EtOH), dl -malic acid aqueous solution (DLMA-W), and dl -malic acid ethanol solution (DLMA–EtOH)—on Chinese yam ( Dioscorea opposita ), with no US as control, to systematically investigate the effects of pretreatment methods on infrared-assisted spouted bed drying (IRSBD) characteristics and product quality. The results showed that all pretreatments reduced the drying time and specific energy consumption to varying degrees. However, only US + W and US + DLMA-W treatments significantly increased the moisture diffusion coefficient in IRSBD. Among all pretreatments, US + DLMA-W effectively enhanced water gain (which facilitated the opening of natural pores in the yam matrix) and reduced bound water content, resulting in a 40.07% reduction in drying time for IRSBD compared to control. The treated samples also exhibited the highest rehydration ratio (RR) (1.79 ± 0.03) and total phenol content (91.43 ± 7.84 mg GAE/100 g DW), as well as superior 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging capacity (35.58 ± 1.27 mg VCE/100 g DW). In contrast, EtOH-associated pretreatments (EtOH, DLMA–EtOH, US + EtOH) showed no significant improvement in RR, total phenolic content, or DPPH scavenging capacity compared to the control group ( p > 0.05). Therefore, US + DLMA-W pretreatment emerges as the optimal approach, offering a promising solution for industrial-scale agricultural product drying that simultaneously improves processing efficiency and end-product quality.