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Drying temperature, air velocity and humidity of drying medium are the three important parameters during hot air drying of fruits and vegetables. Among them, drying temperature and air velocity are positively related to drying efficiency. Relative humidity (RH) is usually used to reflect the size of the humidity or humidity content of the medium at the constant drying temperature and total pressure. However, it is still unclear on the influence mechanism of RH on drying. The manual regulation of relative humidity cannot fully meet the large-scale production, due to the low quality, drying efficiency, and high energy consumption. Therefore, this study aims to clarify the influence and control mechanism of RH on the heat and mass transfer during hot air drying of fruits and vegetables, in order to reduce the emission and loss for energy saving with high efficiency. The drying quality was optimized in the control of RH in four aspects. The low RH was used to increase the mass transfer coefficient and the evaporation of water on the surface of the material. While the high RH was to increase the convective heat transfer coefficient and the heating rate of the material. In the high RH, there was the increase in the temperature rise rate of the material, and the internal water migration, but there was the low evaporation of surface water. In the low RH, there was a low heating rate of the material and the internal water migration, but there was a high evaporation of surface water. As such, there was the coupled influence of RH on the heat and mass transfer. High RH was mainly reflected in the heat transfer, whereas, low RH was in the mass transfer. High RH was used to avoid crust formation on the surface of the material, in order to improve the rehydration for the less shrinkage rate. The honeycomb porous structure was formed and maintained on the surface of the material under the step-down dehumidification and multi-stage dehumidification drying, leading to high drying efficiency and quality. An optimal RH control was achieved using material temperature. The stage dehumidification and drying were attributed to: the relative magnitude of thermal resistance to heat transfer and internal heat conduction, and the relative magnitude of mass transfer resistance and internal mass transfer resistance during drying. The heat and mass transfer resistance depended mainly on the drying conditions, types and thicknesses of materials. The stage dehumidification and drying were suitable for the drying of this material at Bih>1 and Bim>0.1. The influence mechanism was given on the relative humidity during hot air drying of fruits and vegetables. The finding can also provide the theoretical basis and technical support to the influence and control mode of RH in the hot air drying of fruits and vegetables. [ABSTRACT FROM AUTHOR]

Copyright of Transactions of the Chinese Society of Agricultural Engineering is the property of Chinese Society of Agricultural Engineering and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Bulletin of the Chinese Ceramic Society is the property of Bulletin of the Chinese Ceramic Society Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A Biot number has been an excellent indicator for the heat or mass transfer performance during the agricultural production process. The difference between temperature and moisture can be determined between the inside and outside of the material, including heat transfer Bih and mass transfer Bim. Nowadays, a step-down relative humidity (RH) has been successfully applied to improve the drying efficiency and quality of some fruits and vegetables, such as carrots, American ginseng roots, and mushrooms. The high RH can be adopted in the step-down (RH) drying to pretreat the material, until the temperature increases to a high level. Afterward, the RH drying can be reduced to a low value for the surface moisture evaporation. The optimal angle of heat transfer can be selected for the high humidity in the early stage of step-down RH drying, leading to the rapid rise in the surface temperature. Once the surface temperature rises, the internal temperature increases in the way of heat conduction, and the migration and diffusion rate of water increases after the internal temperature rises. The mass transfer angle was also selected to inhibit the evaporation of water on the surface for the high humidity in the early stage, in order to prevent the surface from drying too fast and crust formation. Therefore, the difference between the surface and the internal temperature can be used to determine the dispersion and migration of the internal water into the surface in time, particularly for the effective dehumidification in step-down RH drying. The kind of fruit and vegetable materials can be the dominant factor in this case. It still remains unclear on step-down RH drying suitable for all fruit and vegetable materials. In this study, the applicability of step-down RH hot air drying was evaluated to clarify the drying characteristic, the effective moisture diffusion coefficient (Bih and Bim), color, rehydration ratio, and energy consumption of different thickness carrots (6, 12, and 18 mm) and logan under the step-down RH (first stage: 50% RH 30 min; second stage: 20% to end) and continuously dehumidification (RH<15%) with the drying temperature of 60 o C and the air velocity of 1.0 m/s. Results showed that the higher drying efficiency was achieved in the higher Deff value for the carrot and longan slices with the thickness of 6 mm in the continuous dehumidification, compared with stage one. By contrast, the stage dehumidification improved the drying efficiency with the higher Deff in the carrot slices of 12 mm or 18 mm. The Bih values of 6, 12, and 18 mm carrot slices were 0.582 7, 1.165 5, and 1.748 2, respectively. The maximum relative deviation was 2.7% between surface and internal temperature for the 6 mm carrot slices and Bim<1. The low RH greatly contributed to speeding up the drying rate, where the water inside the material was rapidly diffused to supply the surface. The maximum relative deviation between surface and internal temperature was 8.6% at the 12 mm carrot slices, and 13.8% at the 18 mm carrot slices, when Bim>1. Therefore, the step-down RH drying was necessary, due to the large water and temperature gradients on the surface and inside of the material. The Bim values of carrot slices with different thicknesses varied from 0.156 8 to 0.223 0. The Bim values of longan under the continuous dehumidification and step-down RH were 0.110 3 and 0.084 3, respectively. Furthermore, there was a small resistance of internal mass transfer in the longan (Bim<0.1), where the water quickly diffused to the flesh surface. Nevertheless, the flesh shrinkage, hard outer shell, and high RH outside caused great resistance to moisture migration during the drying process. The Bim values of carrot slices with different thicknesses were more than 0.1, indicating the large moisture gradient from the inside to the surface. Therefore, a high RH was adopted to reduce the evaporation rate of surface water for the high temperature. An excellent performance was achieved in the 6 mm carrot and longan under continuous dehumidification drying, indicating a better color, a higher rehydration ratio, and lower energy consumption. The carrot slices of 12 or 18 mm achieved better performance under step-down drying conditions. In conclusion, the applicability range of stage dehumidification drying was determined using heat and mass transfer Biot number, when Bih>1 and Bim>0.1 during step-down drying. The finding can provide theoretical and technical support for the different materials in the relative humidity control mode. [ABSTRACT FROM AUTHOR]

Copyright of Transactions of the Chinese Society of Agricultural Engineering is the property of Chinese Society of Agricultural Engineering and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Food & Machinery is the property of Food & Machinery Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Food & Machinery is the property of Food & Machinery Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Food & Machinery is the property of Food & Machinery Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Transactions of the Chinese Society of Agricultural Engineering is the property of Chinese Society of Agricultural Engineering and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Transactions of the Chinese Society of Agricultural Engineering is the property of Chinese Society of Agricultural Engineering and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The carbon fiber infrared plate has been used in fruits and vegetables drying as an innovative heating source, which has the advantages of higher heating rate, compact radiation distance, and simple structure. Apple is one of the most common drying materials. Whereas, the traditional natural open sun drying of apple cubes takes almost two days. In addition, during long drying time, products are very sensitive to microbial spoilage. Infrared radiation heating has several advantages such as high efficiency, low energy consumption, and high quality of dried products, and has been widely used in food industry. Many previous investigations indicate that vacuum pulsed drying can extensively enhance the drying rate and keep good quality of the dried products, such as bright color, high content of the heat sensitivity nutrients. In this work, based on carbon fiber infrared heating technology, a vacuum pulsed drying equipment was designed in order to combine the advantages of infrared heating and vacuum pulsed drying technology. The equipment was composed of a drying chamber, vacuum system, single drying unit and control system. For the convenience of analysis, the actual vacuum pulsed process was divided into four stages: The vacuum phase, the vacuum holding stage, the breaking stage, and the normal pressure stage. The automatic control scheme was realized based on MODBUS control protocol with the flow-process framework of the control system containing the touch screen and single chip microcomputer system. Sequential control system was realized for the continuous conversion of drying chamber from vacuum to atmospheric pressure. Combined with the feedback of the material internal temperature, drying temperature can be controlled effectively by monitoring temperature of the carbon fiber plate. The 20 mm×20 mm×5 mm apple cubes were dried to test this equipment. Result indicated that the equipment design and control system of the drying equipment were reliable, which can realize the continuous pulsing from vacuum to atmospheric pressure. When the internal temperature of the samples was kept at 31 , its drying time took about just 380 ℃ min, which was decreased about 30% compared with infrared hot air drying at the same drying temperature. Our results also showed that the most suitable power density of carbon fiber infrared heating plate was 1.1 kW/m2 and the distance between heating surface and the upper surface of material plate was only 3 cm. In addition, when the surface temperature of the carbon fiber infrared plate was 65 ℃, the internal temperature of apple cubes was 31 ℃ at the vacuum holding stage, and it would rise to 37 ℃ quickly at the normal pressure stage. The energy supply of carbon fiber infrared heating plate was excessive at the late stage of drying, and the surface temperature had a trend of fluctuating downward. The findings indicated that it was helpful to reduce the temperature of infrared plate, especially in the late drying stages. Moreover, the color attributes of the products dried under vacuum pulsed drying based on infrared heating were better that of the infrared hot air dried samples. The findings of the current work provide theoretical basis and technology references for the design of vacuum pulsed drying equipment based on carbon fiber infrared heating and its practical application in agricultural products’ drying. [ABSTRACT FROM AUTHOR]

Copyright of Transactions of the Chinese Society of Agricultural Engineering is the property of Chinese Society of Agricultural Engineering and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)