Abstract: Objective To explore the effects of constant relative humidity and stage dehumidification on the drying characteristics and quality of panacis quinquefolii Radix roots, so as to provide basis for improving the drying efficiency and quality of panacis quinquefolii roots. Methods the effects of constant relative humidity (20%, 30%, 40%) and stage dehumidification (40%) on the drying characteristics, water effective diffusion coefficient (deff), rehydration ratio, shrinkage ratio, total saponin content and microstructure of American ginseng were studied. Results under the condition of constant relative humidity, the lower the relative humidity is, the greater the drying rate is, and the drying time is 6.8% shorter when the relative humidity is 20% than 40%; under the condition of stage dehumidification, the relative humidity is kept at 40% for 5% The drying time is shortened by 3.4% compared with the continuous dehumidification drying mode, and the drying process has a short speed-up stage; the moisture deff is in the range of 1.49 × 10? 10 ~ 2.50 × 10? 10 M2 / S; rehydration is mainly affected by the degree of damage to the cell structure of the material and the moisture content before rehydration, and decreases with the increase of relative humidity. The shrinkage ratio increases with the increase of relative humidity. High humidity is conducive to the retention and transformation of total saponin content. The microstructure observation shows that the surface of Panax quinquefolium roots hardens during continuous dehumidification drying, so the drying time and rehydration are prolonged At the same time, when the relative humidity of 40% was kept for 5 hours and then reduced to 20%, the highest comprehensive score was 0.61 ± 0.01, which was the best drying technology. At this time, the water recovery ratio was 2.23 ± 0.12, the shrinkage ratio was 0.26 ± 0.06, and the total saponin content was (5.01 ± 0.04)%. Conclusion the stage dehumidification drying method can improve the drying efficiency and quality of American ginseng roots, and provide theoretical basis and technical support for how to control relative humidity in the process of hot air drying American ginseng roots.
American ginseng is the dried root of Panax quinquefolium L., a perennial herb of Panax quinquefolium in Acanthopanax family. It is a kind of precious Chinese medicinal material and widely used in the field of food and medicine [1]. Chemical composition research shows that the main active ingredients of Panax quinquefolium are ginsenosides. Modern pharmacological research shows that Panax quinquefolium has the functions of deep tonic, strong body, nourishing blood, promoting body fluid, calming the mind, nourishing qi and Yin, reducing fire and heat, tonifying five internal organs, and anticancer [2-3]. The moisture content of fresh Panax quinquefolium on wet basis is about 70%, which needs to be dried and processed in time. Otherwise, it will rot and deteriorate, and the effective ingredients will be decomposed and lost, thus losing the medicinal value [4].
The drying methods of American ginseng include hot air drying, microwave drying, vacuum freeze drying, air jet impact drying and so on [2,5-8]. Lu Guosheng [2] studied the vacuum freeze-drying technology of American ginseng. The results showed that the slice thickness of American ginseng was 3 mm, the pre freezing time was 4 h, the sublimation time was 8 h, the analytical drying temperature and drying time were 30 ℃ and 6 h, and the mass fraction of ginsenoside Rb1 could reach 2.41%. Xiao et al. [7] applied the air jet impingement technology to the drying process of American ginseng slices. The results showed that when the drying temperature was 45 ℃, the wind speed was 6 m / s and the slice thickness was 2 mm, the drying efficiency could be improved and the ginsenoside could be retained to the greatest extent. Wang et al. [8] found that the drying temperature had a significant effect on the drying time in the study of air jet impingement drying of American ginseng tablets. Davidson et al. [5] found that the drying time of American ginseng roots varied from 70 to 190 h when the temperature was 30-50 ℃ and the wind speed was 1.0 m / s. Among many drying methods, hot air drying technology is widely used in American ginseng drying process because of its simple equipment, convenient operation and wide applicability [9]. In the process of hot air drying, the relative humidity of drying medium, as an important parameter of drying medium, will also affect the drying characteristics and quality of American ginseng roots. Martynenko et al. [10] found that when the drying temperature is 38-50 ℃ and the relative humidity is 12% - 24%, the drying time decreases with the increase of the drying temperature, and increases with the increase of the relative humidity. Wilhelm [11] found that when the drying temperature was 23-40 ℃ and the relative humidity was 70% - 90%, the relative humidity had no significant effect on the drying time. Xiao et al. [7] showed that when the drying temperature was higher than 45 ℃, the content of Ginsenoside Rg1 and re decreased sharply with the increase of drying temperature. Du et al. [12] also found that the content of total saponins decreased with the increase of temperature when the drying temperature was 40-70 ℃.
Stage dehumidification drying technology refers to the use of high relative humidity to preheat the material, so that the temperature and humidity of its center and surface tend to be the same; then reduce the relative humidity of the medium, which is conducive to accelerate drying and reduce the phenomenon of crusting [13]. It has been applied to the drying processing of carrots, yam slices, apricots and other materials [13-15]. From the enthalpy and humidity diagram of wet air, it can be seen that the drying medium with low temperature, high relative humidity and high temperature, low relative humidity can have the same enthalpy value and achieve the same effect of heating materials. Therefore, it can reduce the ginsenoside degradation of American ginseng due to high temperature drying to some extent. At present, there are few studies on improving drying efficiency and quality of American ginseng by regulating relative humidity. Therefore, this experiment is to study the drying characteristics and effective diffusion coefficient (deff) of Panax quinquefolium under constant relative humidity and stage dehumidification drying conditions, and analyze and compare the rehydration, shrinkage ratio, total saponin content and microstructure of the products under different drying conditions. This study aims to improve the drying efficiency and quality of American ginseng by regulating the relative humidity, and provide theoretical basis and technical support for the processing of American ginseng roots by hot air drying.
1 instruments and materials
The box type hot air drying experimental device based on temperature and humidity control (self-made by agricultural product processing technology and equipment laboratory of China Agricultural University of Technology) is shown in Figure 1. It is mainly composed of electric heating pipe, drying chamber, dehumidification fan, humidification curtain, internal circulation fan and automatic control part. When the temperature of the drying chamber is lower than the preset value, the electric heating tube works; when the temperature of the drying chamber is higher than the preset value, the electric heating tube stops working. When the relative humidity of the drying chamber is less than the set range value, the humidification solenoid valve is opened, and the wet curtain humidifies the drying chamber; when the relative humidity is greater than the set range, the dehumidification centrifugal fan is opened, and the drying chamber is dehumidified.
4-year-old fresh American ginseng was purchased from Changbai Mountain, Jilin Province. It was identified as the dried root of Panax quinquefolium L., a perennial herbaceous plant of Acanthopanax family, by Xiao Hongwei, associate professor of Engineering College of China Agricultural University. The roots of Panax quinquefolium with a mass of (45 ± 5) g / branch, an average diameter of (1.8 ± 0.2) cm, a length of (12 ± 1) cm, no damage to the epidermis, no mildew and normal skin color were selected as the test materials. The initial moisture content was (68.0 ± 0.5)% (drying method, 105 ℃, 24 h). Before the experiment, the roots of Panax quinquefolium were stored in a refrigerator at (4 ± 1) ℃.
2 methods and results
2.1 calculation of drying parameters of American ginseng
2.1.1 the calculation of moisture ratio (MR) of Panax quinquefolium roots at different times can be simplified as formula (1) calculation [16-17].
MR＝Mt/M0 （1）
MT is the moisture content of dry basis at the time of T, and M0 is the initial moisture content of dry basis
The water content of dry basis is calculated according to formula (2) [15].
Mt＝(Wt－G)/G （2）
WT is the total mass at any time of drying, G is the absolute dry matter mass
2.1.2 drying rate (DR) of American ginseng is calculated according to formula (3) [16].
DR＝(Mt1－Mt2)/(t2－t1) （3）
T1 and T2 are the drying time, Dr is the drying rate between T1 and T2, MT1 and MT2 are the dry basis moisture content at T1 and T2
The drying characteristic curve is reflected by the change of moisture content of American ginseng with drying time.
2.1.3 the water deff is usually calculated by the simplified Fick's second law [17].
MR＝Mt/M0≈8 e?π2Defft/L2)β/π2 （4）
L is the thickness of the material, t is the drying time
2.1.4 determination of rehydration ratio (RR) put a qualified sample into 40 ℃ constant temperature distilled water, take it out and drain it for 20 min after 30 min, dry the surface water with absorbent paper, weigh the mass after rehydration, and calculate according to the formula [13].
RR＝m2/m1 （5）
RR is rehydration, M2 is the mass after rehydration, M1 is the mass before rehydration
2.1.5 the volume of Panax quinquefolium was measured by solid displacement method [18], and the medium was glass beads with a diameter of 200 μ M. Assuming that the density of water is 1.0g/cm3, the volume of beaker (VV) can be calculated by formula (6) if the beaker with proper volume is selected.
VV＝(WW－WV)/ρ （6）
WV is the mass of the empty beaker, WW is the total mass of the beaker filled with water
After obtaining the volume of the beaker, the volume density ρ B of the glass bead can be calculated by formula (7).
ρB＝(WB－WV)/VV （7）
WV is the mass of the empty beaker, WB is the total mass of the beaker filled with glass beads, VV is the volume of the beaker
Put the sample to be measured into the beaker, fill the beaker with glass beads, level the surface of the beaker with glass rods, and weigh the mass. Then the volume vs of the sample can be calculated by formula (8).
VS＝(WB－WT＋WS)/ρB （8）
WS is the mass of the sample, WT is the total mass after the sample is put into the beaker and filled with glass beads, ρ B is the bulk density of glass beads
The shrinkage ratio (SR) is calculated from the following formula. The larger the shrinkage ratio is, the larger the volume of American ginseng will be after drying.
SR＝(1－VS/V) （9）
Vs is the volume of dried American ginseng, V is the initial volume of American ginseng
2.2 drying of American ginseng
Set the temperature and humidity in advance to control the drying temperature and relative humidity of the dryer according to the experimental requirements. When the dryer reaches the preset drying parameters, take out the American ginseng root from the refrigerator and lay it on a 30 cm × 20 cm flat plate and place it in the dryer. Determine the mass change of the sample every 0.5 h until the moisture content of the wet basis reaches 10%. After drying, put it into a fresh-keeping bag and store it in a drying dish. The specific experimental settings are shown in Table 1. The temperature of hot air is 55 ℃ and the wind speed is 3.0 m / S; continuous dehumidification drying refers to that the dehumidification fan (9 in Figure 1) in the dehumidification control dryer is always kept open during the drying process, so that the evaporated water can be removed in time and the relative humidity is kept low (7% - 10%). The drying experiment was repeated three times each time, and the average value was taken as the result.
2.2.1 the effect of relative humidity on the root drying characteristics of American ginseng is as shown in Fig. 2 and Fig. 3 when the constant drying temperature is 55 ℃ and the relative humidity is 20%, 30% and 40%. When the relative humidity is 20%, 30% and 40%, the drying time is 27.4, 28.7 and 29.4 hours respectively. When the relative humidity is 20%, the drying time is shortened by 6.8% compared with 40%. Reducing the relative humidity is beneficial to shorten the drying time and improve the drying efficiency. This is consistent with the conclusion of DAR? C? Et al. [19] when studying the effect of different relative humidity on the drying characteristics of kiwifruit. In addition, when the relative humidity is 40%, the final moisture content of Panax quinquefolium is about 24.9%, which is because the higher the relative humidity of the drying medium is, the greater the final moisture content of the balance of Panax quinquefolium roots is [20]. The high relative humidity dried Panax quinquefolium could not reach the safe moisture content and could not be stored for a long time.
It can be seen from Figure 3 that under different relative humidity, the whole drying process of American ginseng root did not increase or maintain speed