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.2024 Aug 24;13(17):2672.
doi: 10.3390/foods13172672.

Radio Frequency Vacuum Drying Study on the Drying Characteristics and Quality of Cistanche Slices and Analysis of Heating Uniformity

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Radio Frequency Vacuum Drying Study on the Drying Characteristics and Quality of Cistanche Slices and Analysis of Heating Uniformity

Ao Chen et al. Foods..

Abstract

To fully leverage the advantages of both hot air drying and radio frequency vacuum drying, a segmented combination drying technique was applied to post-harvestCistanche. This new drying method involves using hot air drying in the initial stage to remove the majority of free water, followed by radio frequency vacuum drying in the later stage to remove the remaining small amount of free water and bound water. During the radio frequency vacuum drying (RFV) phase, the effects of temperature (45, 55, and 65 °C), vacuum pressure (0.020, 0.030, and 0.040 MPa), plate spacing (65, 75, and 85 mm), and slice thickness (4, 5, and 6 mm) on the drying characteristics, quality, and microstructure ofCistanche slices were investigated. Additionally, infrared thermal imaging technology was used to examine the surface temperature distribution of the material during the drying process. The results showed that compared to radio frequency vacuum drying alone, the hot air-radio frequency combined drying significantly shortened the drying time. Under conditions of lower vacuum pressure (0.020 MPa), plate spacing (65 mm), and higher temperature (65 °C), the drying time was reduced and the drying rate increased. Infrared thermal imaging revealed that in the early stages of hot air-radio frequency vacuum combined drying, the center temperature ofCistanche was higher than the edge temperature. As drying progressed, the internal moisture of the material diffused from the inside out, resulting in higher edge temperatures compared to the center and the formation of overheating zones. Compared to natural air drying, the hot air-radio frequency vacuum combined drying effectively preserved the content of active components such as polysaccharides (275.56 mg/g), total phenols (38.62 mg/g), total flavonoids (70.35 mg/g), phenylethanoid glycosides, and iridoids. Scanning electron microscopy observed that this combined drying method reduced surface collapse and cracking of the material. This study provides theoretical references for future drying processes ofCistanche.

Keywords: drying characteristics; drying uniformity; hot air–radio frequency vacuum combined drying; microstructure; quality analysis.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
The test device of radio frequency vacuum medium heating.
Figure 2
Figure 2
FreshCistanche slices.
Figure 3
Figure 3
Surface mean temperatures of RFV-driedCistanche slices at different time intervals.
Figure 4
Figure 4
Moisture ratio and drying rate of RFV-driedCistanche slices at different time periods.
Figure 5
Figure 5
The green line is the characteristic curve of RFV drying ofCistanche at a temperature of 55 °C, a vacuum pressure of 0.030 MPa, and a pole-plate spacing of 75 mm. The black, red, and blue lines are the characteristic curves of HA-RFV drying ofCistanche at different temperatures (in which the 0–120 min overlapping line part of the drying condition is hot air at 55 °C). The purple line is the drying characteristic curve ofCistanche hot air (55 °C).
Figure 6
Figure 6
The green line is the characteristic curve of RFV drying ofCistanche at a temperature of 55 °C, a vacuum pressure of 0.030 MPa, and a pole-plate spacing of 75 mm. The black, red, and blue lines are the HA-RFV drying characteristic curves ofCistanche under different spacings of pole plates (in which the 0–120 min overlapping line part of the drying condition is hot air at 55 °C). The purple line is the drying characteristic curve ofCistanche hot air (55 °C).
Figure 7
Figure 7
The green line is the characteristic curve of RFV drying ofCistanche at a temperature of 55 °C, a vacuum pressure of 0.030 MPa, and a pole-plate spacing of 75 mm. The black, red, and blue lines are the characteristic curves ofCistanche under different HA-RFV degrees (in which the 0–120 min overlapping line part of the drying condition is hot air at 55 °C). The purple line is the drying characteristic curve ofCistanche hot air (55 °C).
Figure 8
Figure 8
The green line is the characteristic curve of RFV drying ofCistanche at a temperature of 55 °C, a vacuum pressure of 0.030 MPa, and a pole-plate spacing of 75 mm. The black, red, and blue lines show the HA-RFV drying characteristic curve ofCistanche under different thicknesses of slices (in which the 0–120 min overlapping line part of the drying condition is hot air at 55 °C). The purple line is the drying characteristic curve ofCistanche hot air (55 °C).
Figure 9
Figure 9
Infrared thermogram ofCistanche slices dried under HA-RFV drying conditions for the 3rd hour (i.e., surface temperature distribution ofCistanche slices 1 h after transferring from HA to RFV drying).
Figure 10
Figure 10
Infrared thermography ofCistanchis slices under the conditions of HA-RFV drying (55 °C, 0.030 MPa, 75 mm, 5 mm) for different time periods.
Figure 10
Figure 10
Infrared thermography ofCistanchis slices under the conditions of HA-RFV drying (55 °C, 0.030 MPa, 75 mm, 5 mm) for different time periods.
Figure 11
Figure 11
Effects of polysaccharides (a), polyphenols (b), total flavonoids (c), and oxidization resistance (d) ofCistanche slices under different drying conditions. Note: A different lowercase letter after each column indicates a significant difference (p < 0.05).
Figure 11
Figure 11
Effects of polysaccharides (a), polyphenols (b), total flavonoids (c), and oxidization resistance (d) ofCistanche slices under different drying conditions. Note: A different lowercase letter after each column indicates a significant difference (p < 0.05).
Figure 12
Figure 12
HPLC chromatogram of phenylethanol glycosides and cyclic ether terpene contents ofCistanche slices under different drying conditions.
Figure 13
Figure 13
Sample plot and scanning electron microscope diagram ofCistanche sections under different HA-RFV drying conditions.
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References

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