lium) [3 4], chestnut [5], line pepper [6], Elymus dahuricus seeds [7] and cherry tomato [8] ; nevertheless, because the main ingredient of poria coco

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1 Improving quality of Poria cocos using infrared radiation combined with air impingement drying Zhang Weipeng 1,2, Xiao Hongwei 1, Gao Zhenjiang 1, Zheng Zhian 1,2, Ju Haoyu 1, Zhang Ping 1,2, Fang Xiaoming 3 1. College of Engineering, China Agricultural University, Beijing , China; 2. China Research Center for Agricultural Mechanization Development, China Agricultural University, Beijing , China; 3. Bee Research Institute of Chinese Academy of Agricultural Sciences, Beijing , China Abstract: Poria cocos has a long history of medicinal use in China. It is a kind of edible and pharmaceutical mushroom. Drying process usually affects the chemical and physical properties of the extracts of Traditional Chinese Medicine (TCM). Quality control remains a big issue, affecting herbs, formulations, and even the practice of TCM. Poria cocos generally takes almost 7 days to be dried by traditional natural drying and is sensitive to microbial spoilage. Poria cocos blocks (15 mm 15 mm 15 mm) are also easily broken at the traditional hot air drying, such as oven drying. A promising solution to the problem is to take advantage of innovative process techniques including alternative drying methods in the pharmaceutical processing. In this work, poria cocos was dried by medium and short infrared wave drying combined with air impingement drying. Dincer's model was also applied to the drying process and the lag factor (G), drying coefficient (S), Biot number (Bi), moisture effective diffusivity velocity (Deff), mass transfer coefficient (k) were analyzed. The Deff was also calculated based on Weibull function and Fick's second law, and there was difference among them. Combined with the GB T-1992 Droping Test Method and Pharmacopoeia of People's Republic of China, the broken rate and the extractum of poria cocos under different drying conditions were tested. The main results were as follows: 1) Compared with air impingement drying, drying time was shortened by infrared radiation combined with air impingement drying technology, and It could reduce the broken rate by 18% and improve the extractum mass fraction by 1%; the drying process also occurred in the falling rate period; at the range of the testing parameters, the drying rate increased with the increase of temperature and wind speed, but there was not direct correlation between broken rate, extractum content and drying conditions. 2) The G values of different drying methods were between and , steady around 1. Drying coefficient was related to material drying speed; the higher temperature and wind speed, the faster drying ratio and drying coefficient was also larger. 3) The range of combined drying technology's Biot number was , lower than 0.1, which indicted the drying process was mainly influenced by external resistances. The range of mass transfer coefficient was m/s. 4) The Deff calculated by Fick's law, Weibull function and Dincer's model showed a certain regularity, and all of them increased with the increase of temperature and wind speed. Fick's law and Weibull function didn't have relation with external resistances; and Fick's law didn't require drying curve in exponential form, but only was applied in the falling rate drying process. Weibull function and Dincer's model had a broader application, but they needed drying curve must be exponential fitting. In summary, infrared radiation combined with air impingement drying technology can improve the quality of poria cocos. The results provide a reference for the application of Dincer's model on poria cocos drying, and help people to analyze drying process and gain the best drying method. 0 Introduction Keywords:drying; temperature; models; poria cocos; weibull function CLC number: TS255.1; TQ028.6 Poria cocos is the dried sclerotium of pore fungus poria cocos (Fam. Polyporaceae) and it is edible and pharmaceutical, which is considered as traditional Chinese medicine (TCM). However, it only can be used as medicine after being dried and the moisture content may not exceed 18% and Received: Supported by: National Natural Science Foundation of China ( ); Technical Improvement Project of Infinitus (China) Co., Ltd. First author: Zhang Weipeng, male (Han nationality), from Xihua, Henan Province, Ph. D., mainly interested in study on drying equipment and biological drying characteristics. zwp880905@126.com Corresponding author: Zheng Zhi'an, male (Han nationality), from Fuyu, Jilin province, associate professor, Ph. D., Ph. D. supervisor, mainly interested in studies on agricultural system engineering, agricultural engineering technology integration model, agricultural mechanization, etc. zhengza@cau.edu.cn the extractum mass fraction may not be less than 2.5% [1]. Reference [2] analyzed the drying characteristics of poria cocos under different drying conditions (i.e., natural drying, oven drying, air impingement drying and pulsed vacuum drying), through which it can be known that the drying time can be significantly shortened by air impingement drying. The air impingement drying technology has been widely used in the drying of American ginseng (Panax quinquefo

2 lium) [3 4], chestnut [5], line pepper [6], Elymus dahuricus seeds [7] and cherry tomato [8] ; nevertheless, because the main ingredient of poria cocos is starch polysaccharides, it may result in the nonuniform distribution of moisture and the stress concentration due to surface crusting of poria cocos blocks (15 mm 15 mm 15 mm), further contributing to the increasing of the broken rate and decrease of drying quality. At present, the medium and short wave infrared drying technology is the widely used drying technology for fruits and vegetables, and it possesses the advantages of strong heat penetration ability, rapid heating rate, direct coupling with moisture through penetrating materials and greater heating thickness. Besides, it combines hot water blanching process on enzyme deactivation with drying dehydration, which proves to be beneficial for the retention of effective ingredients through inhibiting the enzymatic reaction [9 10]. In view of this, in order to improve the drying quality, the infrared radiation combined with air impingement drying technology is proposed in this paper. The drying process of poria cocos blocks is related with some important indices such as moisture transfer, energy consumption and product quality. Through fitting the exponential form of drying curve, Weibull function, combined with scale parameter a and shape parameter β, can be used to effectively analyze the different drying methods and the heat and mass transfer processes [11]. Similarly, for the fitting of exponential form of drying curve, Dincer proposed the Dincer-Hussain model [12] (here referred as Dincer model ) based on the similarity between the temperature change curve during cooling process and the drying curve during drying process. In addition, the Dincer model exhibits good accuracy and conciseness in practical application. The drying process can be analyzed with the help of lag factor (G) and drying coefficient (S); moisture ratio curve can be accurately predicted according to the functional relationship between Biot number (B i) and G function; the water migration of regular materials (sphere, cylinder and flat plate) can be effectively quantified during the drying process [13]. This paper focuses on the effect of infrared radiation combined with air impingement drying on the drying characteristics and quality of poria cocos blocks. The Dincer model was used to analyze the drying process and further explore the change law of parameters of Dincer model; comparison with Weibull function was also carried out. Meanwhile, the broken rate and extractum mass fraction of poria cocos blocks was determined and analyzed, so as to provide theoretical basis for the application of combined drying technology on drying Chinese herbal medicine of poria cocos. Anhui province. According to the test requirements, several poria cocos (average quality of 3 5 kg), without mechanical damage and rot on surface, were randomly selected from the same batch of poria cocos. The sediment and skins were removed and then the poria cocos dicing machine was used to cut the materials into the cube of 15 mm 15 mm 15 mm. With the direct drying method in the national standard GB/T Determination of Moisture in Foods [14], the moisture content in wet basis of poria cocos block was confirmed as 51.0% ± 0.43%. Before the test, poria cocos blocks were sealed with polythene plastic, placed in cartons, and refrigerated at (5 ± 1) C. Then, the materials without broken edge were selected and used as raw materials after the removal of micro powder by brush. Single layer of poria cocos block was applied in the self-made supporting plate (400 mm 350 mm) made of 3 fine metal meshes, and the initial material mass in single tray is 300 ± 5 g during drying. There is a food grade silicone pad in the bottom of tray, so as to prevent the effect of fine particle crushing on weight during drying. 1.2 Main test device For medium and short wave infrared radiation combined with air impingement dryer (Jiangsu Taizhou Shengtai Infrared Technology Co., Ltd.), the power range of infrared heating tube is 0 2 kw, and the radiation distance range is between 80 and 120 mm. As shown in Fig. 1, the device mainly consists of main components (infrared heating tube, centrifugal blower) and the temperature automatic regulating system. Other equipment: YP-type electronic scale (Shanghai Jingke Scale), DHG-9140A-type electric heating constant temperature air dry oven (Shanghai Yiheng Technology Co., Ltd.), DZQ400/2D-type vacuum packaging machine (Beijing Tianyueyuan Packaging Machinery Co., Ltd.), several dryers and culture dishes. 1.3 Test method The common temperature for drying Chinese herbal 1 Materials and methods 1.1 Test materials The poria cocos used in this test comes from Jinzhai, Fig. 1 Schematic diagram of infrared drying equipment 1. Material tray 2. Temperature sensor 3. Wet discharging port 4. Spray nozzle 5. Air inlet port 6. Air inlet pipe 7. Centrifugal blower 8. Infrared heating tube 9. Touch screen 10. Power supply control switch -219-

3 piece is between 45 C and 65 C [15]. The preliminary tests indicated that the radiation distance range of mm showed no significant effect on drying poria cocos blocks when compared to wind speed and temperature. Therefore, according to the adjustable range of the obtained optimal parameter and the testing parameters in References [16] and [17], the equal space of drying temperature is divided into 45, 50, 55, 60 and 65 C. The radiation power is 2 kw and the radiation distance is 100 mm. The specific testing conditions are as shown in Table 1. The materials were weighed every 15 min in the early stage, and 30 min in later stage. When the change of material quality of 2 consecutive times is not over 1 g (moisture content in dry basis is less than 4%), the test is stopped; each test is repeated for 3 times, and the average value is taken as the result; the material is put into polyethylene plastic bag and sealed after being taken out and cooled. Table 1 Design for experiments with run conditions included Where W t is the total mass at any time t, g; W G is the mass of dry matter, g Fitting of drying curve by using Weibull function Weibull distribution function can be expressed as the following formula [21] : Where MR is moisture ratio, %; α is scale parameter, min, which represents the speed constant during drying, and it approximately equals to the time required for evaporating 63% moisture in material; β is shape parameter, which is related to the drying rate and moisture migration mechanism; t is drying time, min. The calculation formula of moisture diffusion coefficient D cal is as follows: Where D cal is the estimated moisture diffusion coefficient during the drying process, m 2 /s; r is the radius of cubic material volumn, set as m. The relationship between D* eff and D cal can be expressed as Formula (6) [22] below: Note: Radiation power is 2 kw, radiation distance is 100 mm. 1.4 Acquisition method for testing parameters Calculation method of moisture ratio (MR) 1) The curve which moisture ratio (MR) varies with drying time is adopted as the drying curve of poria cocos blocks during the drying process. The moisture ratio of poria cocos blocks at different drying time can be calculated with the simplified formula below [18] : Where M 0 is the initial moisture content of dry basis in material, g/g; M t is the moisture content of dry basis at time t, g/g. The calculation formula of drying rate [19] is expressed as: Where DR is the drying rate of materials between time t 1 and t 2 during the drying process, g/(g min); are the moisture contents dry basis at time t 1 and t 2, g/g. 2) The calculation formula [20] of moisture content in dry basis is: Where D* eff is the calculated moisture effective diffusivity coefficient based on Fick s second law [23], m 2 /s; R g is a parameter related with size [24], 18.6 for spherical materials, 9.5 for cylindrical materials and 13.1 for flat-plated materials; Poria cocos block is in cubic shape, not in flat-plated shape; so it is a kind of material in between, R g is then set in the range of Fitting of drying curve by using Dincer model The drying curve was fitted based on least square method, and the fitting result is shown as the form below [25] : Where G is lag factor, dimensionless constant, representing the internal and external resistances during heat and mass transfer; S is drying coefficient, representing the drying ability of material per unit time, 1/s, the larger S, the larger drying speed. Biot number B i can be calculated with the following formula [26] : The calculation formula for moisture effective diffusion coefficient [27] is: Where L is material thickness, set as mm here; μ 1 is -220-

4 roots for characteristic Formula (10), Formulas (11) and (12) [28]. Considering that poria cocos blocks are placed on tray in regular sequence, the problem can be simplified into flat plate problem and the characteristic root μ 1 is determined by Formula (10). For flat plate: For cylinder: For sphere: Based on Formulas (7) and (8), the mass transfer coefficient k (m/s) can be obtained; the calculation formula [29] is shown below: Determination methods for extractum mass fraction and broken rate The broken rate was tested in accordance with GB-T Packaging-Transport packages-vertical Impact Test Method by Dropping [30] and the dropping height was set as 1 ± 0.02 m. The calculation formula for broken rate η is: Where M is the total mass of test samples, g; m 0 is the mass of broken sample, g. The extractum were determined with the hot dipping method [31] in Chinese Pharmacopoeia, during which the dilute ethanol was chosen as solvent. The mass fraction of ethanol soluble extractum is calculated by analogy of Formula (14). 1.5 Data processing methods The function fitting software 1st Opt V1.5 was used to fit the drying curve and then the unknown parameters corresponding to Dincer model and Weibull function were obtained. SPSS 18.0 software was used to conduct variance analysis. 2 Results and analysis 2.1 Effects of drying temperature, wind speed on drying characteristics Fig. 2 shows the drying curves and drying rate curves of poria cocos blocks. From Fig. 2, it can be known that the infrared radiation combined with air impingement drying made the drying have falling rate, with relatively higher drying rate. The increase of temperature can effectively shorten the drying time. When the wind speed is 6 m/s, the drying times at 45, 50, 55, 60 and 65 C are respectively 350, 300, 250, 200 and 150 min. Under different wind speeds, the drying rates curves show also drying of falling rate, which is similar with the effect of temperature on drying rate; drying rate increases with the increase of wind speed, as shown in Fig. 2c. Under the same temperature and wind speed, the drying times for air impingement drying in Reference [2] are 415, 350, 305, 260 and 210 min. It can be observed that, under the radiation effect of medium and short wave infrared, the drying time of the combined drying technology is significantly shortened when compared with air impingement drying technology (p < 0.05). This may be because that the infrared radiation can penetrate the surface of poria cocos blocks and directly couple with the moisture in material, and then the temperature can be rapidly increased and the drying process is accelerated. In addition, as the decrease of moisture in poria cocos during drying process, a part of radiation energy is used to increase the temperature of material and then the internal and external temperature of material will be relatively consistent, which is favorable for the uniform migration of internal moisture into external environment and easing the stress concentration [32]. Fig. 2 Drying curves and drying rate curves of poria cocos Note: Air velocity of Figs. a, b is 6 m s 1, temperature of Fig. c is 55 C

5 2.2 Analysis and comparison of drying curve models Analysis of drying process based on scale parameter α and shape parameter β parameter a and shape parameter β The drying curve was fitted by Weibull function and by the aid of least square method. The fitting result is shown in Table 2, in which scale parameter α has relation with drying rate. Through analysis, it can be seen that value of α decreases with the increase of temperature and wind speed, while β value is less influenced by temperature and wind speed. When the shape parameter β is in the range of 0.3 1, it manifests that the drying process occurs in the falling rate drying period and is controlled by the diffusion of internal moisture; when β is larger than 1, it reveals that the drying rate tends to increase in the first stage of drying rates curves [33]. In this paper, β is in the range of When β is in the range of 0.3 1, it suggests that the drying process is controlled by the diffusion of internal moisture and the drying process occurs in falling rate drying period, which is consistent with the above description about drying rate. For the same dried materials, β value depends on drying method; besides, significant difference will occur with the change of material status [34]. β value of the infrared radiation combined with air impingement drying is close to that of air impingement drying in Reference [2] ( ), indicating that slight change may occur during drying process Table 2 but no significant changes will occur under the combined effect of air impingement and infrared radiation when compared with the independent action of air impingement technology. Bai [35] also drew the similar conclusion while studying the relationship among grape drying, blanching pretreatment and the structural morphology of grape. The calculated moisture diffusion coefficient D cal by Formula (5) is in the range of m 2 /s, showing similar change law with the calculated D* eff based on Fick s second law. The specific results are shown in Table 3 and D cal and D* eff both increase with the increase of temperature and wind speed. R g calculated by Formula (6) ranges from 9.53 to 11.36, indicating that it is feasible to set R g in the range of if the cubic poria cocos block is simplified into a shape which falls in between cylindrical and flat-plated material. Miranda et al. [36] studied the drying technology of carrot and they found that R g is a constant; besides, the R g values for spherical, cylindrical and flat-plated materials were calculated out, as shown in Formula (6). However, these researches mainly concentrated on the same material, as for poria cocos block, fission occurs during the drying process and the tissue morphology has been significantly changed, so it is inappropriate to simply regard the materials as the same material, and the conclusion is not in contradiction with this. For the same material, the structural morphology and physicochemical properties are different during different drying periods, requiring that we should regard it as a new material, which also acts as the theoretical basis of two-stage combined drying technology [37]. Fitting results of different models Note: G is lag factor, S is drying coefficient; α is scale parameter, β is shape parameter. Table 3 Results of different model parameters Note: a represents Fick s law of diffusion, b represents Weibull function model, c represents Dincer and Hussain s method

6 2.2.2 Analysis of drying process based on lag factor G and drying coefficient S The drying curve was again fitted by Dincer model and with the help of least square method. The G value and S value are shown in Table 2, from which it can be known that lag factor G is affected by drying conditions and there is a difference of G values between different drying methods. The G values are between and , steady around 1; S value increases with the increase of temperature or wind speed. Through the relevant analysis, the increase of drying temperature and wind speed is found to be conductive for drying. This finding indicates that the larger S, the larger drying speed under the same drying method, which is similar with the conclusion concerning the drying of garlic slices [38] drew by Mohammad. The obvious falling rate drying was found in medium and short wave infrared region, the moisture effective diffusion velocity (D* eff) can be calculated by directly using Fick s second law and the moisture diffusion coefficient D cal can be obtained with Formula (6). From Table 3, it can be seen that although the calculated D * eff based on Dincer model shows the same change law with the other two methods, namely, D* eff increases with the increase of temperature and wind speed, the results are significantly higher. This may be caused by the different calculation methods [39], that is, the calculation of D* eff based on Fick s second law or Weibull function neglects the effects of mass transfer and heat transfer; while Dincer model pays equal attention to the effects of internal heat resistance, thermal resistance of heat convection in boundary layer and mass transfer coefficient. In the calculation of moisture effective diffusivity velocity (D* eff) of broccoli [40], Vlatka calculated that it was in the range of m 2 /s by using Fick s second law; m 2 /s by using Dincer model. Mohammad carried out garlic slices tests by using convective drying technology, and the D* eff based on Fick s second law and Dincer model are and m 2 /s respectively. In spite of the different calculation methods, Fick s second law is the most common one because it is irrespective of the fitting of drying curve and it only applies to falling rate drying. With respect to Weibull function and Dincer model, although the change of drying rate is not considered, they need the drying curve to be in exponential fitting and the fitting precision is not higher, and finally the effective analysis will not be achieved. Weibull function and Dincer model show different fitting functions; therefore, the interpretations of drying process are also different, and the hidden mechanism of mathematical theory behind them needs to be further investigated Comparative analysis of mass transfer parameter As an important dimensionless parameter, Biot number B i represents the ratio between conductive heat resistance and the thermal resistance of heat convection in boundary layer [41]. Generally, under testing conditions, 0.1 < Bi < 100 means the change of material temperature which depends on not only the internal conductive heat resistance, but also the external resistance of heat convection, which is the most common in practical applications. Bi > 100 means that the change of material temperature are totally affected by the internal conductive heat resistance, which is regarded as a limiting case; 0 < Bi < 0.1 means that the material temperatures at all points are consistent at any time, and the change speed depends on the heat convection intensity on the surface of material. Bi in this paper is in the range of , as shown in Table 3, indicating that the change of internal temperature of poria cocos block is mainly affected by the heat convection intensity in boundary layer. Although the infrared radiation is helpful for the increase of internal temperature, the thermal resistance of heat convection in boundary layer still plays a leading role. In consideration of this, the increase of temperature and wind speed can improve the convective heat transfer coefficient and favorable for drying, which accords well with practical applications. At the range of testing parameters, the mass transfer coefficient k calculated by Formula (13) ranges from to m/s. 2.3 Analysis of broken rate and extractum mass fraction The early research shows that the pulsed vacuum drying method exhibits sound drying quality, but with longer drying time. Combing with the data in Table 4, each parameter was treated with analysis of variance, and the result is shown in Table 5. Under the significant level of 0.01, the broken rate and the extractum mass fraction F are and respectively, both much higher than F (0.01) = 3.89, indicting remarkable effect of the drying method. The average broken rate of infrared radiation combined with air impingement drying is 42.68%, higher than that of pulsed vacuum drying (3.37%), but lower than that of air impingement drying (61.2%). This finding reveals that the medium and short wave radiation can ease the stress concentration of poria cocos block during drying, reduce the broken by about 18%, which is consistent with the analysis of β of Weibull model. Under pulsed vacuum drying, the poria cocos blocks are in perfect condition and the broken rate is low. This may be because that the micro-channel in poria cocos blocks is frequently extruded, expanded and connected during the cyclic process of vacuum-normal pressure and then the structural stress during drying process can be eliminated, contributing to the intactness of poria cocos blocks [42]. The extractum mass fractions of pulsed vacuum drying, infrared radiation combined with air impingent drying and air impingent drying are 4.61%, 3.94% and 2.94%, respectively. The extractum mass fraction of combined drying is -223-

7 Table 4 Data of different drying methods Note: Data of impingent drying, pulsed vacuum comes from Reference [2]; The different letters a, b, c indicate significant difference (p < 0.01) of difference drying methods, the same letters indicates insignificant. Table 5 Analysis of variance of breakage rate, extractum mass fraction at different drying methods Note: *** means the results have a very significant impact on the factor with the level of % higher than that of air impingent drying, indicating that this technology is beneficial for the retention of effective ingredients. The drying time of pulsed vacuum drying is longer; nevertheless, the vacuum degree is higher during drying and the material is in pulsed hypoxia condition at most time and exposed to oxygen for a short time, oxygen consumption reaction can be effectively avoided; therefore, the extractum mass fraction is the highest. From above analysis, with combined drying technology, the quality of poria cocos blocks is not the best, but has been improved substantially compared with the independent action of air impingement technology. Therefore, combined drying technology should be taken into consideration for the drying of poria cocos. In view of the superior drying quality of pulsed vacuum method and the universality of air impingent drying, the two-stage combined drying of pulsed vacuum-air impingement needs to be verified so as to achieve the satisfactory drying speed and quality. 3 Conclusions 1) Compared with air impingement drying, drying time is shortened by infrared radiation combined with air impingement drying technology. And the combined drying technology can also reduce the broken rate and improve the extractum mass fraction. Besides, the increase of temperature and wind speed can effectively increase the drying rate; the average broken rate and extractum mass fraction are respectively 42.68% and 3.94%. 2) Fitting of drying curve by Weibull function and combing with scale parameter α and shape parameter β is favorable for the analysis of drying process. β value of combined drying method is in the range of , indicating the drying process occurs in the falling rate period; β value of air impingent drying is , indicating that the material state under combined drying method is similar with that under air impingent drying method; the broken rate is reduced but still in higher level, which accords well with the practical tests. 3) Lag factor G and drying coefficient S are used by Dincer model to analyze the drying process. The G values of different drying methods were between and S value increases with the increases of temperature and wind speed. The range of calculated Biot number (B i) is , which indicates that the drying process is mainly influenced by the thermal resistance of heat convection in boundary layer. At the range of testing parameters, the range of mass transfer coefficient k is m/s. 4) Moisture effective diffusion velocity (D eff) calculated by Fick s law, Weibull function and Dincer s model are m 2 /s, m 2 /s and m 2 /s, respectively, showing a certain regularity, and all of them increase with the increases of temperature and wind speed. Fick s second law and Weibull function do not have relation with external resistances; and Fick s second law does not require the drying curve in exponential form, but only is applied in the falling rate drying process. Weibull function and Dincer s model have broader application, but the flaw lies in that they need drying curve to be exponential fitting. References [1] State Administration of Traditional Chinese Medicine. Chinese herbal medicine [M]. Shanghai: Shanghai Science and Technology Press, (in Chinese) [2]Zhang Weipeng, Gao Zhenjiang, Xiao Hongwei, et al.drying characteristics of poria cocos with different drying methods based on Weibull distribution[j].transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE), 2015, 31(5): (in Chinese with English abstract) [3]Xiao Hongwei, Bai Junwen, Xie Long, et al.thin-layer air impingement drying enhances drying rate of American ginseng(panax quinquefolium L.)slices with quality attributes considered[j].food and Bioproducts Processing, 2015, 94(2): [4]Xiao Hongwei, Law Chunglim, Sun Dawen, et al.color change kinetics of American ginseng(panax quinquefolium)slices during air impingement drying[j].drying Technology, 2014, 32(4): [5]Lou Zheng, Gao Zhenjiang, Xiao Hongwei, et al.air impingement drying characteristics and process optimization of chestnut[j].transactions of the Chinese Society of Agricultural Engineering(Transactions of the -224-

8 CSAE), 2010, 26(11): (in Chinese with English abstract) [6]Zhang Qian, Xiao Hongwei, Yang Xuhai, et al.effects of pretreatment on air impingement drying characteristics and product color for line pepper[j].transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE), 2012, 28(1): (in Chinese with English abstract) [7]Yao Xuedong, Gao Zhenjiang, Lin Hai, et al.air-impingement rotary drying experiments of Elymus dahuricus seeds[j].transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE), 2011, 27(8): (in Chinese with English abstract) [8]Wang Lihong, Gao Zhengjiang, Xiao hongwei, et al.air impingement drying kinetics of cherry tomato[j].journal of Jiangsu university(natural Science Edition), 2011, 32(5): (in Chinese with English abstract) [9]Ma Haile, Wang juan, Liu Bin, et al.experiment and dynamics of dehydration and inactivation of enzyme of potato slices by simultaneous infrared dry-blanching[j].transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE), 2015, 31(7): (in Chinese with English abstract) [10]Ju Haoyu, Xiao Hongwei, Bai Junwen, et al.medium and short wave infrared drying characteristics and color changing of apple slices[j].transactions of the Chinese Society for Agricultural Machinery, 2013, 44(Supp.2): (in Chinese with English abstract) [11]Xiao Hongwei, Pang Changle, Wang Lihong, et al.drying kinetics and quality of Monukka Seedless grapes dried in an air-impingement jet dryer[j].biosystems Engineering, 2010, 105(2): [12]Dincer I, Dost S.A modeling study for moisture diffusivities and moisture transfer coefficients in drying of solid objects[j].international Journal of Energy Research, 1996, 20(6): [13]Kavak E, Dincer I.Moisture transfer models for slabs drying[j].international Communication in Heat and Mass Transfer, 2005, 32(1): [14]GB/T National standards for food safety: determination of moisture in food [S]. (in Chinese) [15] Pan Yongkang, Wang Xizhong, Liu Xiangdong. Modern drying technology [M]. Beijing: Chemical Industry Press, 2006: (in Chinese) [16]Liu Wensan.Studies on orgin processing methods of Poria cocos Wolf and analysis of the major harmful substances[d].changsha:hunan University of Traditional Chinese Medicine, 2009:20-25.(in Chinese with English abstract) [17] Zhu Wenxue. Traditional Chinese medicine drying principle and technology [M]. Beijing: Chemical Industry Press, 2001: (in Chinese) [18]Li Changyou, Mai Zhiwei, Fang Zhuangdong.Analytical study of grain moisture binding energy and hot air drying dynamics[j].transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE), 2014, 30(7): (in Chinese with English abstract) [19]Zhang Xukun, Sun Ruichen, Wang Xuecheng, et al.drying models and characteristics of thin layer sludge in superheated steam drying[j].transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE), 2014, 30(14): (in Chinese with English abstract) [20]Li Changyou, Zhao Yikun, Ma Xingzao.Model analytical and verification of heat and moisture characteristics in litchi drying[j].transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE), 2014, 30(15): (in Chinese with English abstract) [21]Bai Junwen, Gao Zhenjiang, Xiao Hongwei, et al.polyphenol oxidase inactivation and vitamin C degradation kinetics of Fuji apple quarters by high humidity air impingement blanching[j].international Journal of Food Science and Technology, 2013, 48(6): [22]Dai Jianwu, Rao Junquan, Wang Dong, et al.process-based drying temperature and humidity integration control enhances drying kinetics of apricot halves[j].drying Technology, 2015, 33(3): [23]Xiao Hongwei, Gao Zhenjiang, Lin Hai, et al.air impingement drying characteristics and quality of carrot cubes[j].journal of Food Process Engineering, 2010, 33(5): [24]Bai Junwen, Sun Dawen, Xiao Hongwei, et al.novel high humidity hot air impingement blanching(hhaib)pretreatment enhance drying kinetics and color attributes of seedless grapes[j].innovative Food Science and Emerging Technologies Food Science and Technology, 2013, 20(4): [25]Dincer I, Mc Minn WAM.Development of a new drying correlation for practical applications[j].international Journal of energy research, 2002;26(3): [26]Dincer I.Moisture Loss from Wood Products During Drying-Part I:Moisture Diffusivities and Moisture Transfer Coefficients[J].Energy Sources, 2007, 20(1): [27]Dincer I, Hussain MM.Development of a new Bi-Di correlation for solids drying[j].international Journal of Heat and Mass Transfer, 2002, 45(15): [28]Dincer I, Hussain MM.Development of a new Biot number and lag factor correlation for drying applications[j].international Journal of Heat and Mass Transfer, 2004, 47(4): [29]Mc Minn WAM.Prediction of moisture transfer parameters for microwave drying of lactose powder using Bi-G drying correlation[j].food Research International, 2004, 37(10): [30]GB-T , Test method of package dropping in packaging transportation [S]. (in Chinese) [31] National Pharmacopoeia Committee.Pharmacopoeia of the People's Republic of China [M]. Beijing: Chinese Medical Science and Technology Press, 2010: (in Chinese) [32]Zheng Xia, Xiao Hongwei, Wang Lihong, et al.shorting drying time of Hami-melon slice using infrared radiation combined with air impingement drying[j].transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE), 2014, 30(1): (in Chinese with English abstract) [33]Uribe E, Vega-Gálvez A, Scala K D, et al.characteristics of Convective Drying of Pepino Fruit(Solanum muricatum Ait.):Application of Weibull Distribution[J].Food&Bioprocess Technology, 2011, 4(8): [34]Bai Junwen, Wang Jiliang, Xiao Hongwei, et al.weibull distribution for modeling drying of grapes and its application[j].transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE), 2013, 29(16): (in Chinese with English abstract) [35]Bai Junwen.Drying Kinetics and Anti-Browning Mechanism of Thompson Seedless Grapes[D].Beijing:China Agricultural University, 2014:41-45.(in Chinese with English abstract) [36]Miranda M, Vega-Gálvez A, García P, et al.effect of temperature on structural properties of Aloe vera(aloe barbadensis Miller)gel and Weibull distribution for modeling drying process[j].food and Bio products Processing, 2010, 88(2): [37]Huang Qingde, Deng Qianchun, Xu Jiqu, et al.processing technology combining degumming with oil pressing for flaxseed[j].transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE), 2015, 31(7): (in Chinese with English abstract). [38]Rahman M S, Al-Shamsi Q H, Bengtsson G B, et al.drying kinetics and allicin potential in garlic slices during different methods of drying.[j].drying Technology, 2009, 27(3): [39]Tomislav Jurendić, Branko Tripalo.Biot number-lag factor(bi-g)correlation for tunnel drying of baby food[j].african Journal of Biotechnology, 2011, 10(59): [40]Vlatka Mrkic, Marko Ukrainczyk, Branko Tripalo.Applicability of moisture transfer Bi-Di correlation for convective drying of broccoli[j].journal of Food Engineering, 2007, 79(2): [41]Dincer I.Moisture transfer analysis during drying of slab woods[j].heat and Mass Transfer, 1998, 34(4): [42]Chua K J, Chou SK.On the experimental study of a pressure regulatory system for bioproducts dehydration[j].journal of Food Engineering, 2004, 62(2):

9 第 31 卷 第 10 期 农业工程学报 Vol.31 No 年 5 月 Transactions of the Chinese Society of Agricultural Engineering May 农产品加工工程 中短波红外联合气体射流干燥提高茯苓品质 张卫鹏 1,2, 肖红伟 1, 高振江 1, 郑志安 1,2, 巨浩羽 1, 张平 1,2, 方小明 (1. 中国农业大学工学院, 北京 ; 2. 中国农业大学中国农业机械化发展研究中心, 北京 ; 3. 中国农业科学院蜜蜂研究所, 北京,100093) 3 摘要 : 为探索茯苓的干燥特性, 改善茯苓干燥品质, 该文将中短波红外联合气体射流干燥技术应用于茯苓块的干燥 利用 Dincer 模型拟合茯苓块干燥曲线, 结合滞后因子 干燥系数分析干燥过程, 并估算其水分有效扩散系数 给出 Dincer 模型的具体应用方法, 求出并分析不同干燥条件下的毕渥数 水分有效扩散系数 传质系数 测定干燥后茯苓块的破碎率, 及茯苓块浸出物的质量分数 对比分析 Fick 第二定律 Weibull 函数 Dincer 模型的优缺点 结果表明 :1) 与气体射流干燥相比, 中短波红外联合气体射流干燥可缩短干燥时间, 降低破碎率约 18%, 提高浸出物质量分数约 1%; 联合干燥过程亦为降速干燥 ; 试验参数范围内, 提高温度 风速均可提高干燥速率 ;2) 滞后因子范围为 ~1.0202, 且温度 风速越高, 干燥速度越快, 干燥系数越大 ;3) 联合干燥技术的的毕渥数为 ~0.0982, 小于 0.1, 表明干燥过程与边界的对流换热热阻有关 传质系数的范围为 ~ m/s 4) 基于 Fick 第二定律 Weibull 分布函数 Dincer 模型计算的水分有效扩散系数变化趋势一致, 均随温度 风速的升高而增加 Fick 第二定律不要求干燥曲线呈 指数 形式, 但仅适用于降速干燥 Weibull 分布函数不考虑边界的对流换热热阻 Weibull 分布函数 Dincer 模型均可应用于非降速干燥, 但二者的缺陷是干燥曲线需呈 指数式 拟合 综上所述, 中短波红外联合气体射流干燥技术可提高茯苓品质, 借助于 Weibull 函数 Dincer 模型可从不同角度更全面地解读干燥过程 研究结果可为 Dincer 模型在茯苓生产加工过程中联合干燥技术的应用提供参考 关键词 : 干燥 ; 温度 ; 模型 ; 茯苓 ;Weibull 函数 doi: /j.issn 中图分类号 :TS255.1; TQ028.6 文献标志码 :A 文章编号 : (2015) 张卫鹏, 肖红伟, 高振江, 等. 中短波红外联合气体射流干燥提高茯苓品质 [J]. 农业工程学报,2015,31(10): doi: /j.issn Zhang Weipeng, Xiao Hongwei, Gao Zhenjiang, et al. Improving quality of Poria cocos using infrared radiation combined with air impingement drying[j]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2015, 31(10): (in Chinese with English abstract) doi: /j.issn 引言 茯苓 (Poria cocos) 为多孔真菌茯苓的干燥菌核, 可药食两用, 是中国传统的中药材 茯苓必须经过干燥后方可入药, 且含水率不得超过 18%, 浸出物质量分数不得少于 2.5% [1] 文献[2] 中对不同干燥方式下 ( 自然晾晒 普通热风 气体射流 真空脉动 ) 的茯苓干燥特性进行了系统分析, 可知气体射流能明显缩短干燥时间 尽管其已被广泛应用于西洋参 [3-4] 板栗 [5] 线辣椒 [6] 披碱草种子 [7] [8] 圣女果的干燥, 但茯苓主要成分为淀粉多糖, 气体射流干燥易导致茯苓块 (15 mm 15 mm 15 mm) 内外水分不均 表面结壳引起应力集中, 进而导致破碎率增高, 降低干燥品质 中短波红外干燥技术是目前常用 收稿日期 : 修订日期 : 基金项目 : 国家自然科学基金 ( ); 无限极 ( 中国 ) 有限公司技改项目作者简介 : 张卫鹏, 男 ( 汉 ), 河南西华人, 博士生, 主要从事干燥装备与生物干燥特性的研究 北京中国农业大学工学院, zwp880905@126.com 通信作者 : 郑志安, 男 ( 汉 ), 吉林扶余人, 副教授, 博士, 博士生导师, 主要从事农业系统工程 农业工程技术集成模式 农业机械化等方面的研究 北京中国农业大学工学院, zhengza@cau.edu.cn 的果蔬干燥技术, 具有加热穿透能力强 升温速度快, 能量能够穿透物料直接与水分耦合, 对物料加热厚度深等优势, 能够将漂烫灭酶与干燥脱水合二为一, 遏制酶促反应, 利于有效成分的保留 [9-10] 为此, 本文将中短波红外联合气体射流干燥技术应用于茯苓干燥过程中, 以期能够提高其干燥品质 茯苓块的干燥过程关系到水分传递 能源消耗 产品品质等重要指标 通过拟合物料干燥曲线的 指数形式,Weibull 函数结合尺度参数 α 形状参数 β 可对不同干燥方式 传热传质过程进行有效分析 [11] 然而, 同样是对干燥曲线 指数形式 的拟合,Dincer 基于冷却过程温度变化曲线和干燥过程干燥曲线的相似性, 提出的 Dincer-Hussain 模型 [12] ( 此处简称 Dincer 模型 ), 在实际生产应用中亦较为准确和简洁 而且, 可通过滞后因子 G(lag factor) 和干燥系数 S(drying coefficient) 对干燥过程进行分析 ; 借助毕渥数 Bi 和 G 函数关系, 能准确预测水分比曲线 ; 并可有效量化规则物料 ( 球形 圆柱形 平板形 ) 干燥过程中的水分迁移规律 [13] 本文重点研究中短波红外联合气体射流干燥工艺对茯苓块干燥特性和品质的影响 利用 Dincer 模型对干燥过程进行分析, 探究 Dincer 模型中参数的变化规律 ; 并

10 270 农业工程学报 ( 年 与 Weibull 函数进行比较 同时, 测定 分析该方式下茯苓块的破碎率 浸出物质量分数, 为联合干燥技术在中草药茯苓干燥中的应用提供理论依据 1 材料与方法 1.1 试验材料试验所用的茯苓, 采自安徽金寨 根据试验要求, 从同一批次的茯苓中, 随机挑选若干表面无机械损伤 无腐烂的茯苓作为试验材料, 平均质量为 3~5 kg 去除泥沙和外皮后, 再经茯苓切丁机切制成 15 mm 15 mm 15 mm 的立方体 结合国家标准 GB/T 食品中水分的测定 [14] 中的直接干燥法, 测定茯苓块的湿基含水率为 51.0%±0.43% 试验前将茯苓块用聚乙烯塑料密封包装后, 置于纸箱中, 在 (5±1) 条件下冷藏 试验前, 挑选边角无破损的物料, 用毛刷去除表面的细微粉末后作为试验原料 将茯苓块单层平铺在自制的 3 个细金属网做成的托板 (400 mm 350 mm) 中, 干燥过程中单个托盘中初始物料质量为 (300±5) g 托盘底部有食用级硅胶垫, 以防止干燥过程中细小颗粒破碎对称量造成的影响 1.2 主要试验装置中短波红外联合气体射流冲击干燥机 ( 江苏泰州圣泰红外科技有限公司 ), 红外加热管功率范围 0~2 kw; 辐射距离范围 80~120 mm 结构如图 1 所示, 主要由主体装置 ( 红外加热管 离心风机 ) 以及温度自动调节系统组成 1. 物料托盘 2. 温度传感器 3. 排湿口 4. 喷嘴 5. 进风口 6. 进风管道 7. 离心风机 8. 红外加热管 9. 触摸屏 10. 电源控制按钮 1.Material tray 2.Temperature sensor 3.Wet discharging port 4.Spray nozzle 5.Air inlet port 6.Air inlet pipe 7.Centrifugal blower 8.Infrared heating tube 9.Touch screen 10.Power supply control switch 图 1 中短波红外干燥箱示意图 Fig.1 Schematic diagram of infrared drying equipment 其他仪器设备 :YP 型电子秤 ( 上海精科天平 ), DHG-9140A 型电加热恒温鼓风干燥箱 ( 上海一恒科技有限公司 ),DZQ400/2D 型真空包装机 ( 北京市天月缘包装机械有限公司 ), 干燥器 培养皿若干 1.3 试验方法中药饮片干燥常用温度 45~65 [15] 前期预试验结果表明, 相对于风速和温度而言, 辐射距离 80~120 mm 范围, 对茯苓块干燥过程的影响并不明显 因此, 结合得到的较佳参数可调范围, 及文献 [16] [17] 试验参数 ; 将干燥温度等间距划分为 辐射功率为 2 kw, 辐射距离为 100 mm, 选取具体的试验条件如表 1 所示 前期每隔 15 min, 后期每隔 30 min 称量一次, 连续 2 次物料质量变化不超过 1 g( 干基含水率小于 4%) 时停止试验 ; 每组试验重复 3 次, 取平均值作为结果 ; 取出 冷却后装入聚乙烯塑料袋, 密封包装 表 1 试验设计和试验参数 Table 1 Design for experiments with run conditions included 干燥方式 Drying methods 中短波红外 + 气体射流干燥 Medium and short infrared wave + impingent drying 温度 Temperature/ 风速 Velocity/(m s -1 ) 注 : 辐射功率为 2 kw 辐射距离为 100 mm Note:Radiation power is 2 kw, radiation distance is 100 mm. 1.4 试验参数的获取方法 水分比 (MR) 的计算方法 1) 茯苓块干燥过程中的干燥曲线采用水分比 (MR, moisture ratio) 随干燥时间变化的曲线 不同干燥时间茯苓块水分比的计算可简化为公式 [18] : M t MR= (1) M 0 式中 :M 0 为物料初始干基含水率,g/g;M t 为物料在 t 时刻的干基含水率,g/g [19] 干燥速率计算公式表示为 : M t M 1 t2 DR= (2) t2 t1 式中 :DR 为干燥过程中时间在 t 1 和 t 2 之间的物料的干燥速率,g/(g min); M t 1 和 M t 2 为干燥过程中时间为 t 1 和 t 2 时物料的干基含水率,g/g [20] 2) 干基含水率计算公式为 : Wt WG M t = (3) WG 式中 :W t 为在任意干燥 t 时刻的总质量,g;W G 为干物质质量,g 利用 Weibull 函数拟合干燥曲线 Weibull 分布函数表达式如下式所示 [21] : β t MR= exp (4) α 式中 :MR 为水分比,%;α 为尺度参数 (scale parameter), min, 表示干燥过程中的速率常数, 约等于物料内的水分蒸发 63% 所需要的时间 ;β 为形状参数 (shape parameter), 与物料干燥过程中的干燥速率和水分迁移机理有关 ;t 为干燥时间,min 水分扩散系数 D cal (moisture diffusion coefficient) 的估算公式如下 :

11 第 10 期张卫鹏等 : 中短波红外联合气体射流干燥提高茯苓品质 r Dcal = (5) α 式中 :D cal 为干燥过程中估算的水分扩散系数,m 2 /s;r 表示立方体物料的体积半径, 此处值为 m D * [22] eff 与 D cal 二者由如下公式表示 : 2 * Dcal r Deff = = (6) Rg α Rg 式中 :D * [23] eff 为基于菲克第二定律得到的水分有效扩散系数,m 2 /s;r g 是一个与尺寸有关的参数 [24], 球形物料为 18.6, 圆柱形物料为 9.5, 平板形物料为 13.1; 由于茯苓块为立方体, 而非单纯的平板 ; 因此, 认为其介于圆柱形物料与平板形物料之间,R g 取 9.5~ 利用 Dincer 模型拟合干燥曲线基于最小二乘法对干燥曲线进行拟合, 可拟合为如下形式 [25] : MR= G exp( St) (7) 式中 :G 为滞后因子 (lag factor), 无量纲常数, 表征干燥传热传质过程中受到的内部和外部阻力 ;S 为干燥系数 (drying coefficient), 表示单位时间内物料的干燥能力, 1/s,S 越大, 物料干燥速度越快 毕渥数 Bi 可由下式估算 [26] : Bi 有效水分扩散系数计算公式 [27] : 2 SL Deff = (9) 2 μ 26.7 = G (8) 式中 :L 为物料厚度, 此处取 mm;μ 1 为特征式 (10) 式(11) 式(12) 的根 [28], 由于茯苓块规则排列于料盘中, 可简化为平板问题, 此处特征根 μ 1 由式 (10) 确定 对于平板 : μ 1 = G G G + (10) G 对于圆柱体 : μ 1 = G G 68.43G + (11) G 对于球体 : μ 1 = G G G + (12) G 结合式 (7) 式(8) 可得传质系数 (mass transfer [29] coefficient)k,m/s; 计算公式如下 : 2D eff Bi k = (13) L 浸出物质量分数 破损率的测定方法破碎率按 GB-T 包装运输件 跌落试验方法 [30] 进行试验, 跌落高度为 (1±0.02)m, 破碎率 η 的计算公式如下 : m η = 0 100% (14) M 式中 :M 为测试样品的总质量,g;m 0 为测试时破碎样品的质量,g 浸出物的测定按照 中国药典 的热浸法 [31], 用稀乙醇作溶剂, 类比公式 (14) 计算醇溶性浸出物的质量分数 1.5 数据处理方法通过函数拟合软件工具包 1stOpt V1.5 对干燥曲线进行拟合, 分别求出 Dincer 模型,Weibull 函数对应的未知参数 采用 SPSS 18.0 软件对数据进行方差分析 2 结果与分析 2.1 干燥温度 风速对干燥特性的影响茯苓块的干燥曲线 干燥速率曲线如图 2 所示 从图中可以看出, 中短波红外联合气体射流干燥茯苓块呈降速干燥, 且干燥过程较快 提高温度可有效缩短干燥时间, 风速 6 m/s, 条件下, 干燥时间分别约为 min 不同风速下, 干燥速率曲线亦成降速干燥, 且与温度对干燥速率的影响类似, 干燥速率随风速增加而增加, 如图 2c 所示 温度 风速相同条件下, 文献 [2] 中气体射流干燥则需约 min 注 : 图 a b 风速为 6 m s -1, 图 c 温度为 55 Note: Air velocity of figure a, b is 6 m s -1, temperature of figure c is 55. 图 2 茯苓块干燥曲线与干燥速率曲线 Fig.2 Drying curves and drying rate curves of poria cocos

12 272 农业工程学报 ( 年 可见, 在中短波红外的辐射作用下, 干燥时间比气体射流单独作用显著减少 (p<0.05) 这可能是因为在中短波红外辐射联合气体射流冲击干燥中, 红外辐射可穿透茯苓块表面, 直接与物料中的水分耦合, 使温度迅速升高, 加速了干燥过程 而且, 随着干燥过程茯苓块水分的减少, 所吸收的辐射能一部分用于提高物料自身温度, 内外温度梯度较一致, 有利于内部水分向外均匀迁移, 利于缓解干燥过程的应力集中现象 [32] 2.2 干燥曲线模型分析比较 基于尺度参数 α 形状参数 β 对干燥过程的分析借助最小二乘法, 利用 Weibull 函数拟合干燥曲线 具体结果见表 2, 尺度参数 α 与干燥速率有关 分析可知, 该干燥方式下,α 值随温度的升高, 风速的增加而减小 而温度 风速对 β 值的影响较小 形状参数 β 在 0.3~1 时, 表示干燥过程为内部水分扩散控制的降速干燥 ;β>1 时, 表示干燥速率曲线前期呈现上升态势 [33] 本研究中形状参数 β 值的范围为 ~0.9951, 在 0.3~1 之间, 表明干燥过程受内部水分扩散控制, 全程为降速干燥 这与上述对干燥速率的描述相一致 对同一种干燥物料而言, 形状参数 β 值与干燥方式有关, 也会随着物料状态的变化而产生显著性差异 [34] 红外联合气体射流干燥 的形状参数 β 值与文献 [2] 中气体射流干燥的 β 值 (0.877~ 0.980) 比较, 取值范围接近, 说明在气体射流与红外辐射联合作用下, 与气体射流单独作用相比, 干燥过程茯苓块的状态可能会有细微改变, 但不会发生显著的变化 [35] 白俊文在研究葡萄干燥, 烫漂预处理和葡萄组织结构形态之间的关系时, 也发现了类似结论 公式 (5 ) 所估算的水分扩散系数范围 D cal 为 ~ m 2 /s, 与基于 Fick 第二定律计算的水分有效扩散系数 D * eff 变化规律类似, 具体结果见表 3, 二者均随温度 风速的增大而增大 由公式 (6) 计算的 R g 值范围 9.53~11.36 说明将茯苓块立方体简化为介于圆柱形和平板型物料之间, 认为其 R g 值介于 9.5~ [36] 13.1 之间是可行的 Miranda 等研究胡萝卜的干燥时, 发现 R g 是一个常量, 并计算出球形 圆柱形 平板型物料的 R g 值, 如公式 (6) 中的内容所示 但其针对的是同一种物料, 而茯苓块在干燥过程中裂变, 组织形态已经发生显著变化, 不能简单地认为其是同一种物料, 因此结论并不与其相矛盾 同一物料, 不同干燥阶段, 物料的结构形态 理化性质也不尽相同, 应将其当做全新的物料来看待, 这也是 分段式联合干燥技术 的理论基础 [37] 干燥方式 Drying method 中短波红外 + 气体射流干燥 Medium and short infrared wave + impingent drying 干燥条件 Drying conditions 表 2 不同模型的拟合结果 Table 2 Fitting results of different models Dincer 模型 Dincer model G S(10-5 S 决定系数 -1 ) Coefficient of α/min β determination R 2 Weibull 函数模型 Weibull function model 决定系数 Coefficient of determination R 2 45 ; 6 m s ; 6 m s ; 6 m s ; 6 m s ; 6 m s ; 4 m s ; 8 m s 注 : G 为滞后因子, S 为干燥系数 ; α 为尺度参数, β 为形状参数 Note: G is lag factor, S is drying coefficient; α is scale parameter, β is shape parameter. 干燥方式 Drying method 中短波红外 + 气体射流 Medium and short infrared wave + impingent drying 干燥条件 Drying conditions a 水分有效扩散系数 Moisture effective diffusion coefficient D * eff /(10-10 m 2 s -1 ) 表 3 不同模型参数的计算结果 Table 3 Results of different model parameters b 估算的水分扩散系数 Calculated moisture diffusion coefficient D cal / (10-9 m 2 s -1 ) 几何参数 Geometric factor R g c 水分有效扩散系数 Moisture effective diffusion coefficient D eff /(10-7 m 2 s -1 ) 毕渥数 Biot number Bi 特征根传质系数 Characteristic Mass transfer k/ root μ 1 (10-6 m s -1 ) 45 ; 6 m s ; 6 m s ; 6 m s ; 6 m s ; 6 m s ; 4 m s ; 8 m s 注 : a 表示 Fick 第二定律的计算结果, b 表示 Weibull 函数的计算结果, c 表示 Dincer 模型的计算结果 Note: a represents Fick s law of diffusion, b represents Weibull function model, c represents Dincer and Hussain s method 基于滞后因子 G 干燥系数 S 对干燥过程的分析再次借助于最小二乘法, 采用 Dincer 模型拟合干燥曲线,G S 值结果如表 2 所示 可知, 滞后因子 G 值受 干燥条件的影响, 且不同干燥方式间的 G 值会存在差异 为 ~1.0202, 均在 1 附近 ; 且 S 值随温度或风速的上升而增大 由干燥曲线的分析可知, 提高干燥温度

13 第 10 期张卫鹏等 : 中短波红外联合气体射流干燥提高茯苓品质 273 风速, 有利于干燥的进行 表明同种干燥方式下,S 值越大, 物料的干燥速度越快 这也与 Mohammad 在大蒜片 [38] 干燥中的研究结论相类似 中短波红外为明显的降速干燥, 水分有效扩散系数 D * eff 可直接运用 Fick 第二定律计算, 并由式 (6) 求出估算的水分扩散系数 D cal 由表 3 数据可看出, 基于 Dincer 模型计算的水分有效扩散系数, 尽管变化规律与其他 2 种方式相同, 也随温度 风速升高而增大, 但结果却明显偏高 这是因为基于 Fick 第二定律 Weibull 函数对水分有效扩散系数的计算, 均忽略了质量传递和热量传递的影响 ; 而 Dincer 模型综合衡量了物料内部导入热阻 边界对流换热热阻和传质系数的影响, 三者的计算方法存在差异造成的 [39] [40] Vlatka 在西兰花的水分有效扩散系数计算中, 相同条件下采用 Fick 第二定律计算范围为 ~ m 2 /s; 而采用 Dincer 模型的计算结果范围为 ~ m 2 /s Mohammad 在对流干燥大蒜片试验中, 采用 Fick 定律 Dincer 模型的计算结果分别为 ~ ~ m 2 /s 尽管计算方法各异, 但 Fick 第二定律因不考虑干燥曲线的拟合而最为常见, 且仅适用于降速干燥 Weibull 函数模型和 Dincer 模型虽不考虑干燥速率的变化, 但对干燥曲线 指数式 的函数拟合也有其局限性, 拟合精度较低, 无法进行有效分析 二者对曲线的拟合函数不同, 因此对干燥过程的解读角度也不同, 其隐藏的数学理论机理需要进一步探索 传质参数的比较分析毕渥数 Bi 是重要的无量纲参数, 表示物体内部的导热热阻与边界处对流换热热阻之比 [41] 一般而言, 试验条件下,0.1<Bi<100, 表示物料温度的变化, 即取决于物料内部的导热热阻, 也取决于物料外部的对流换热热阻, 在实际应用中也最为常见 Bi>100 表明物料内部温度变化完全取决于内部导热热阻, 是一种极限情况 ;0<Bi<0.1 表明物料内部各点温度在任一时刻都趋于均匀一致, 且变化快慢取决于物料表面的对流换热强度 本文 Bi 的变化范围为 ~0.0982, 见表 3 说明茯苓块内部温度变化主要由边界的对流换热强度决定 尽管红外辐射有助于其内部升温, 但边界的对流换热热阻依然起主导作用 因此, 升高温度 风速都可提高对流换热系数, 都有助于干燥的进行, 这也与实际应用相符合 试验参数范围内, 由公式 (13) 计算的传质系数 k 的范围为 ~ m/s 2.3 破碎率 浸出物质量分数的分析前期结果表明, 茯苓块的真空脉动方式干燥品质最佳, 但干燥时间较长 结合表 4 数据对各参数进行方差分析, 结果如表 5 所示 在显著性水平 0.01 条件下, 破碎率 浸出物质量分数 F 值分别为 , 均远大于 F (0.01) =3.89, 说明干燥方式对二者的影响非常显著 中短波红外联合气体射流的破碎率均值为 42.68%, 高于真空脉动的 3.37%, 但低于气体射流的 61.2% 说明中短波红外辐射, 能缓解茯苓块干燥过程中应力集中现象, 破 碎率比单独依靠气体射流降低约 18%, 与 Weibull 模型中对 β 的论述相一致 真空脉动方式下茯苓块外观完整 破碎率低, 可能的原因是在 真空 - 常压 的循环过程中, 茯苓块内部的微观孔道不断地被挤压 扩张 连通, 消除了干燥过程中的组织应力, 利于茯苓块外观的保持 [42] 干燥方式 Drying method 中短波红外 + 气体射流干燥 Medium and short infrared wave + impingent drying 真空脉动干燥 Pulsed vacuum 表 4 不同干燥方式的试验测试数据 Table 4 Data of different drying methods 破碎率浸出物质量分数干燥条件 Breakage Extractum mass Drying conditions rate η/% fraction/% 45 ; 6 m s a 3.69 a 50 ; 6 m s a 4.01 a 55 ; 6 m s a 4.07 a 60 ; 6 m s a 3.88 a 65 ; 6 m s a 3.99 a 55 ; 4 m s a 3.98 a 55 ; 8 m s a 3.99 a b 4.55 b b 4.75 b drying b 4.54 b 气体射流干燥 Impingent drying 45 ; 8 m s c 3.16 c 55 ; 8 m s c 2.63 c 65 ; 8 m s c 3.02 c 55 ; 4 m s c 2.73 c 55 ; 6 m s c 3.15 c 注 : 气体射流 真空脉动试验数据由文献 [2] 获得 ; 不同字母 a b c 表示不同干燥方式差异性显著 (p<0.01), 相同字母表示差异性不显著 Note: Data of impingent drying, pulsed vacuum comes from reference[2]; The different letters a b c indicate significant difference (p<0.01) of difference drying methods, the same letters indicates insignificant. 表 5 不同干燥方式下破碎率 浸出物质量分数方差分析表 Table 5 Analysis of variance of breakage rate, extractum mass fraction at different drying methods 指标 Index 破碎率 Breakage rate 项目 Items 离差平方和自由度 SS df 均方差 MS F F Value P 值 P Value 组间 组内 总计 显著性 *** 组间 浸出物质量分数 组内 Extractum mass 总计 fraction 显著性 *** 注 :*** 表示显著性水平 0.01 条件下因素对结果影响非常显著 Note: *** means the results have a very significant impact on the factor with the level of 真空脉动 中短波红外联合气体射流 气体射流浸出物质量分数均值分别为 4.61% 3.94% 2.94% 联合干燥的浸出物质量分数高于气体射流约 1% 说明该干燥方式有利于茯苓块有效成分的保持 尽管真空脉动干燥时间较长, 但干燥过程真空度高, 大部分时间处于脉动低氧状态, 与氧气接触时间最短, 抑制干燥过程耗氧反应的发生 因此, 浸出物质量分数最高 由上述分析可知, 联合干燥方式下, 尽管茯苓块物料品质不是最佳, 但相对于气体射流干燥单独作用而言, 品质有较大改善 因此, 针对茯苓的干燥, 可以考虑采用联合干燥的方式 考虑到真空脉动的干燥品质较优, 以及气体射流应用的普遍性, 也可对 真空脉动 气体