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<p>To investigate the enhancement effect of dual-frequency ultrasound (DFUS) on vacuum drying (VD), purple sweet potato slices were subjected to dual-frequency ultrasound combined vacuum drying (DUVD). Low-field nuclear magnetic resonance, scanning electron microscopy, and X-ray micro-computed tomography were employed to analyze the effects of different DFUS power and frequency combinations on drying characteristics, moisture state, moisture migration behavior, and microstructural mass transfer pathways. The findings indicated that, relative to VD alone, DUVD shortened the drying duration by 23.81%-66.67%, while enhancing the mean drying rate by 26.01%-189.03%. As DFUS power increased, the moisture migration rate improved significantly, and the (28 + 40) kHz DFUS mode exhibited superior mass transfer performance. Low-field nuclear magnetic resonance demonstrated that free water was preferentially removed throughout the drying process, with its removal efficiency increasing at higher DFUS power. Semi-bound water showed a rise-then-fall pattern, whereas bound water remained relatively stable. Scanning electron microscopy observations revealed that DFUS power of 40&#xa0;W facilitated the formation of a well-connected porous network structure. However, increasing DFUS power to 60&#xa0;W caused rupture of the microporous structure and pore coalescence. Micro-computed tomography (µCT) imaging and three-dimensional reconstructed models indicated that the total porosity of purple sweet potato slices initially increased and then slightly decreased as DFUS power rose. The equivalent pore diameter distribution revealed that, under the (28 + 28), (28 + 40), and (40 + 40) kHz DFUS frequency combinations, increasing DFUS power shifted the distribution from predominantly 0–50&#xa0;μm micro-pores toward larger pore size distributions (50–100, 100–150, and &gt; 150&#xa0;μm), accompanied by the expansion and evolution of micro-pores into larger pore channels. Consequently, DUVD effectively enhanced internal moisture migration and optimized the micro-scale mass transfer pore network in purple sweet potato slices.</p> Graphical Abstract <p></p>

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Moisture migration and pore characteristics of purple sweet potato slices during dual-frequency ultrasound combined vacuum drying

  • 期刊论文

  • 发表日期：

    2026年6月22日

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  • Linxin Li,
  • Keyu Qian,
  • Han Wu,
  • Yuchen Wang,
  • Yunhong Liu

摘要

To investigate the enhancement effect of dual-frequency ultrasound (DFUS) on vacuum drying (VD), purple sweet potato slices were subjected to dual-frequency ultrasound combined vacuum drying (DUVD). Low-field nuclear magnetic resonance, scanning electron microscopy, and X-ray micro-computed tomography were employed to analyze the effects of different DFUS power and frequency combinations on drying characteristics, moisture state, moisture migration behavior, and microstructural mass transfer pathways. The findings indicated that, relative to VD alone, DUVD shortened the drying duration by 23.81%-66.67%, while enhancing the mean drying rate by 26.01%-189.03%. As DFUS power increased, the moisture migration rate improved significantly, and the (28 + 40) kHz DFUS mode exhibited superior mass transfer performance. Low-field nuclear magnetic resonance demonstrated that free water was preferentially removed throughout the drying process, with its removal efficiency increasing at higher DFUS power. Semi-bound water showed a rise-then-fall pattern, whereas bound water remained relatively stable. Scanning electron microscopy observations revealed that DFUS power of 40 W facilitated the formation of a well-connected porous network structure. However, increasing DFUS power to 60 W caused rupture of the microporous structure and pore coalescence. Micro-computed tomography (µCT) imaging and three-dimensional reconstructed models indicated that the total porosity of purple sweet potato slices initially increased and then slightly decreased as DFUS power rose. The equivalent pore diameter distribution revealed that, under the (28 + 28), (28 + 40), and (40 + 40) kHz DFUS frequency combinations, increasing DFUS power shifted the distribution from predominantly 0–50 μm micro-pores toward larger pore size distributions (50–100, 100–150, and > 150 μm), accompanied by the expansion and evolution of micro-pores into larger pore channels. Consequently, DUVD effectively enhanced internal moisture migration and optimized the micro-scale mass transfer pore network in purple sweet potato slices.

Graphical Abstract