Bidirectional hot air drying: an effective inhibitor of the browning of biomass similar to thick-layered honeysuckle

Abstract

Large amounts of Chinese medicinal materials (CMMs) should be quickly dried during the harvest period because improper drying leads to a browning reaction, which deteriorates the medicinal value of CMMs. In this study, a bidirectional alternating hot air drying system was designed for thick-layered CMMs. The drying capacity could be greatly improved by increasing the thickness of the material layer. The time-weighted average temperature (TWAT) was defined to determine the appropriate reverse time of the airflow direction. The two-stage air supply could reduce the moisture content of the sample to a certain degree at a low temperature, and then realizing the rapid drying of the sample at a high temperature. When the moisture content of honeysuckle reduces to 60%, the browning reaction would be effectively inhibited. Moreover, the drying efficiency of the thick layer could be improved by reducing the frequency of changes in the direction of hot air. An equal TWAT could be obtained in each layer by adjusting the ratio of the upward to downward airflow supply duration. When the TWAT of the upper layer was equal to that of the lower layer, the drying uniformity improved. The difference between the moisture content of the middle layer of the sample and that of its upper and lower layers could serve as the standard for drying uniformity; the smaller the difference is, the better the drying uniformity will be. The bidirectional alternating hot-air drying method was applied to thick layer drying of Chinese herbal medicine and grain. On the one hand, it reduces the drying energy consumption, on the other hand, it helps to improve the uniformity of drying for Chinese herbal medicine. Additionally, this drying method can be applied for grain drying.

GRAPHICAL ABSTRACT

The paper is to design and construct the bidirectional alternating hot air drying experimental facility for the biomass similar to thick-layered honeysuckle. The time-weighted average temperature (TWAT) was defined as evaluation criterion of drying uniformity. In the drying process, the hot air was supplied first in the upward direction and then switched to the downward direction. As depicted by the blue line in Figure 2, the air temperature at point B rapidly increased, after changing the hot air direction, whereas the air temperature at point A sharply declined to the temperature of point B and then increased along the trend. According to this phenomenon, the ratio of the upward to downward airflow supply duration is adjusted to make TWAT values at both points equal, which can effectively improve the drying uniformity.

KEYWORDS:

• Hot-air drying
• bidirectional alternating airflow
• Browning reaction
• drying uniformity
• drying efficiency
• two-stage drying
• thick layer

Disclosure statement

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

Additional information

Funding

The authors are grateful for the supports of the National Natural Science Foundation of China (Grant no. 51476009).