英文文献引用中文文献格式

Transactions of the Chinese Society of Agricultural Engineering

No. 12

? 2015 China Academic Journals (CD Edition) Electronic Publishing House Co., Ltd.

______________________________________

Received: 2015-03-15

Supported by: National Natural Science Foundation of China (Grant No. 31201436)

First author: Ju Haoyu, male, Ph. D. student. His research mainly focuses on equipment and technologies for drying agricultural products. College of Engineering, China Agricultural University, Beijing 100083. Email: ju56238@http://www.wendangku.net/doc/b9e887622a160b4e767f5acfa1c7aa00b52a9db7.html

Corresponding author: Gao Zhenjiang, male, professor, Ph. D., doctoral supervisor. His research mainly focuses on the processing tech-nology and equipment for agriculture products (food). College of Engineering, China Agricultural University, Beijing 100083. Email: zjgao@http://www.wendangku.net/doc/b9e887622a160b4e767f5acfa1c7aa00b52a9db7.html

DOI: 10.11975/j.issn.1002-6819.2015.12.031

Design and experiment of vacuum-steam pulsed blancher for fruits and vegetables

Ju Haoyu 1, Xiao Hongwei 1, Fang Xiaoming 2, Liu Yanhong 1, Zhang WeiPeng 1, Cheng Peng 1, Gao Zhenjiang 1

1. College of Engineering, China Agricultural University, Beijing 100083, China;

2. Bee Research Institute of Chinese Academy of Agricultural Sciences, Beijing 100093, China

Abstract: Blanching is an essential thermal processing using hot water or steam to treat fruits and vegetables, which is often carried out prior to the preservation process like drying, canning and freezing. Blanching can effec-tively inactivate enzymes in products such as polyphenol oxidase (PPO) and peroxidase (POD) enzymes, which cause deterioration reactions, off-flavor and undesirable changes in color. Under the condition of blanching, fruits and vegetables can keep their original colors, flavors and nutritional ingredients. Hot water and steam are by far the most widely used blanching methods. However, the main disadvantage of hot water blanching is that it causes nutri-tional substances especially sugar, proteins, carbohydrates, vitamins and minerals lost into water because of leach-ing and diffusion. What"s more, hot water blanching can also cause environmental pollution due to the release of waste water containing considerable amount of nutrients. On the other hand, the main problems of steam blanching are blanching uniformity and small load. It is reported that solid foods are surrounded by thin-layer air and water, and steam cannot pass through these barriers of air and water, which act as insulation against the steam. The nonu-niform blanching is possibly due to the existing of thermal resistance between material and steam. The thermal re-sistance prevents steam from transferring heat to the material. In addition, the steam will be condensed due to the air of low temperature. Considering those reasons, the pulsed vacuum-steam blanching machine is designed. This ma-chine consists of vacuum system, steam system, automatic control system and blanching body system. The cold air and water around the material and useless steam can be wiped out by vacuum system in time. Therefore, thermal re-sistance is removed and heat can be transferred to material directly. In order to ensure the uniformity of the internal flow field, the flow field of inner blanching body was simulated by computational fluid dynamic (CFD) software Fluent, and the steam jet structure was optimally designed by adding interceptor in the bottom of the blanching body. The result showed that the velocity magnitude seemed to be equivalent and flow filed presented an-ti-clockwise in the xoz plane. Automatic control system used LCD12864 to show real-time temperature, pressure and working condition in the blanching body and switch the working condition between vacuum and steam blanch-ing regularly through controlling the electromagnetic valve. The median filtering was applied in the control system to eliminate accidental factors which influenced temperature and pressure signal. Lily was adopted to test the per-formance of pulsed vacuum-steam blanching equipment. One group of experiment was that lily slice was blanched to validate its uniformity and another was single factor experiment designed by pulsed vacuum-steam blanching. In the first group, the drying time tended to be equivalent. It could be concluded that the thermal resistance was re-moved after the vacuum processing and steam could contact with every piece of lily sufficiently. Additionally, the strategy of adding interceptor in the bottom of the blanching body was feasible. The comparison of color values of dried lily slices also showed the uniformity of pulsed vacuum-steam blanching machine. The single factor experi-ment results showed that drying time would decrease and then increase as the increment of blanching time and cycle times. Besides, drying time showed a decrease tendency with the decreasing of vacuum degree. In this paper, the designed vacuum-steam pulsed blancher for fruits and vegetables has improved blanching loading capacity and uniformity, which has provided theoretical foundation and technical support for its popularization and application. Keywords: mechanization; computational fluid dynamics; design; blanching CLC number: S126

0 Introduction

Blanching is the thermal processing using hot water or steam to treat fresh fruits and vegetables. Blanching can effectively inactivate enzymes in products such as poly-phenol oxidase (PPO) and peroxidase (POD) enzymes, which cause deterioration reactions, off-flavor and undesir-able changes in color. Under the condition of blanching, fruits and vegetables can keep their original colors, flavors and nutritional ingredients [1–2]. Additionally, blanching can

induce protoplasm denaturation within fruit and vegetable cells, increase the penetrability of cytomembrane, and pro-mote the migration and evaporation of water [3]. Hot water and steam are by far the most widely used blanching meth-ods. However, the main disadvantage of hot water blanch-ing is that it causes nutritional substances especially sugar, proteins, carbohydrates, vitamins and minerals lost into wa-ter because of infiltration and diffusion [4–9].

Steam blanching solved such problems as the loss of soluble nutritional substances and liquid waste-induced pollution, which are caused by hot water blanching. How-ever, this type of blanching is limited by inhomogeneous blanching and small loading capacity. Du [10]designed a high-temperature and high-humidity steam percussion blancher, which inactivates enzymes in products by heating them to the enzyme-deactivation temperature in a short time. Nevertheless, this equipment is of small loading ca-pacity. Furthermore, materials that are on the back of jet cannot contact with the steam and thus the blanching is of poor homogeneity. Chen [11]designed a ribbon-type high-temperature and high-humidity steam percussion blancher. This equipment employs a continuous material conveyor belt, which improves the loading capacity, and a steam allocation room, which improves the homogeneity. However, this machine is only suitable for blanching single layer materials. In addition, due to the poor tightness be-tween the material inlet and outlet, a large amount of steam is leaked. Wang [12]designed a tumbling-type continuous steam blancher, which improves the blanching homogeneity by turning over materials with spiral-plates. This equipment is likely to cause scratch to material tissure. Additionally, the spiral-plates and spiral axis body are comparatively large, which limits the loading capacity. As a result, it is highly necessary to solve the homogeneity and loading ca-pacity issue.

Research on steam sterilization suggests that temperature in vacuum sterilization room can quickly rise to the required threshold for sterilization, making the center and surround-ing of the material rise to a comparatively high temperature as well [13–14]. Michael et al. [15–16]found that steam could hardly penetrate air and water and directly pass heat to the material. When the material is conducted with vacu-um-steam-vacuum processing, steam-condensing liquid drop, the heat-transition rate of which is 618 times of liquid membrane, appears on the material surface. The cold air is expelled before the blanching and then steam is put into the blanching room, making the high-temperature steam direct-ly contact with the material and the temperature within the blanching room rise quickly and homogeneously. When the steam is kept for a while, the temperature and effect of ma-terial blanching decreases [17]. Then the steam is excluded by vacuuming and high-temperature stem is brought into again for blanching. In this case, the adoption of vacuum pulsation technology in steam blanching equipment is use-ful for enhancing the blanching homogeneity and produc-tion efficiency.

According to the small loading capacity and inhomoge-neous blanching, this research added vacuum pulsation technology [18–21]to the steam blanching equipment and proposed one vacuum-steam pulsed blancher. Blanching experiments were conducted using lily as the material to test the efficiency of the new machine in enhancing blanching loading capacity and homogeneity.

1The overall design of vacuum-steam pulsed blancher

1.1Schematic of vacuum-steam pulsed blancher

The overall structure of vacuum-steam pulsed blancher is displayed as Fig. 1. This machine constitutes of vacuum generator, steam generator, blanching body and automatic

control system.

Fig. 1 Schematic of vacuum-steam pulsed blancher. 1. Contain-ment envelope 2. Steam generator 3. Manual ball valve 1 4. Nut connecting pipe joints 5. Steam valve 6. Flowmeter 7. Pressure sensor 8. Union elbow 9. Teperature sensor 10. Blanching body 11. Manual ball valve 2 12. Pedestal 13. Bolt 14. Vacuum valve 15. Vacuum pump 16. Universal wheel

Vacuum system constitutes of water ring vacuum pump (Beijing HCHD Vacuum & Air compressing Equipment Co., LTD, 2BV-2601 type), vacuum cooling equipment (Foshan Kewely refrigeration equipment Co., Ltd., FNF-2.8/13 type) and vacuum valve (Shanghai Wilton Valve Manufacturing Co., Ltd., ZCV-20). Steam system consists of steam generator (Shanghai HuaZheng Special Boiler Manufacture Co., LTD, LDR0.013-0.7), steam deliv-ery line and steam valve (Shanghai Wilton Valve Manufac-turing Co., Ltd., ZCV-20). Steam enters into the blanching body through ball value (Shanghai Wilton Valve Manufac-turing Co., Ltd., ZCV-20, GU-20(G)type), steam valve (Shanghai Wilton Valve Manufacturing Co., Ltd., ZCV-20), flowmeter (Kaifeng Zhongyi flowmeter Co., Ltd, LUGB) and steam line. Blanching body is a cylindrical body, which is made from 304 stainless steel. The lower side of the

? 2015 China Academic Journals (CD Edition) Electronic Publishing House Co., Ltd.

blanching body is equipped with a material basket hold, jet, and turbulent flow plate. The material basket is a cylin-der-shaped stainless steel net support, which is of 490 mm diameter and 320 mm height and placed on the material basket hold. The jet and turbulent flow plate is placed on the bottom of the material basket. Steam is distributed all over the blanching body through steam line, turbulent flow hole and turbulent flow plate. Automatic control system consists of temperature sensor (Beijing YouPuSi technology Center, Pt 100 type), pressure transmitter (Beijing Micro Sensor Co., Ltd. HT-801 type) and control box. This blancher can adjust the vacuum degree, vacuum keeping time, steam pressure and blanching time. The possible stacking of mate-rials increases the loading capacity and thus is suitable for blanching a diversity of materials. Main technical parame-ters for the vacuum-steam pulsed blancher are displayed as Table 1.

Table 1 Main technical parameters of vacuum-steam pulsed

blancher

1.2Working principle

The working principle of vacuum-steam pulsed blancher: Firstly, place the material in the blanching body and then turn on the blancher. When the vacuum degree reaches to the set value, the control system starts to count down ac-cording to given vacuum keeping time. Meanwhile, the cold area on the material surface is expelled. When the vacuum keeping time is over, turn over the vacuum valve and turn on the steam valve. Import some steam and enable the pressure to reach set value and then turn off the steam valve. Meanwhile, the control system starts count down according to set blanching time. During the blanching, steam directly contacts with the material and the temperature of the mate-rial rises quickly, which works efficiently for enzyme deac-tivation. When the blanching is over, the vacuum stage starts again. By repeating this process, the vacuum-steam pulsed blanching is realized. Comparatively, continuous steam blancher [10–11], does not expel the air in the blanching body and thus inhaled steam will condensate and release heat when it comes across cold air. Next, the steam will condensate to water drop and cool down, reducing the blanching effect. Furthermore, continuous blancher is only suitable for single layer material. The vacuum-steam pulsed blancher expels the cold air by vacuuming the blanching body. In this case, the loading capacity of each blanching is enhanced and the blanching is more homogeneous, which improves the efficiency of the steam. 2The design of key components

2.1The design of blanching body

Blanching body is the processing unit for blanching ma-terials. To improve the loading capacity of single blanching, the blanching body is designed as a cylinder-shape body with 500 mm in diameter and 330 mm in height. To ensure the blanching body unchanged under pressure, the wall thickness needs to be checked. The pressure within the blanching body varies between 0.01 and 0.1 MPa and the blanching body receives the largest pressure when it is vac-uum. The equation for calculating wall thickness under pressure is demonstrated as follows [22]

:

Where: δ is calculated wall thickness, mm; p is outer pre s-sure, i.e., 0.1 MPa; D i is the internal diameter of the body, i.e., 500 mm; σ is the allowable pressure for 304 stainless steel under normal temperature to 150 °C, i. e., 137 MPa; n is the security coefficient, herein take n = 10; Through calculation, δ is 1.8 mm and 2 mm after rounding off. To r educe the heat loss from the blanching body, the outer wall of the blanching body is wrapped up by 10 mm thick heat insulation cotton and supported by 1 mm stainless steel outside.

2.2The air exhaust rate of vacuum pump and the design of steam generator

The vacuum pump is the key for ensuring the stable work of vacuum generation system. Water-ring vacuum pump can enable the extreme vacuum reach to 2–4 kPa of coarse vac-uum. The blanching pressure in the experiment is 10 kPa, and thus the water-ring vacuum pump can meet the re-quirement. In addition, selected vacuum pump should ena-ble the pressure within the blanching body to reach the set value in a short time, so air exhaust velocity is an important parameter for selecting vacuum pump. The air exhaust time for the vacuum pump is calculated as [23]

:

Where: t is air exhaust time, s; K is correction coefficient, and is 1.25 when the vacuum degree is required as 10 kPa; V is vacuum equipment capacity, i.e., 0.065 m3; S p is the actual exhaust rate, m3/s; P1is the initial vacuum degree, kPa; P2 is required vacuum degree, kPa.

The actual exhaust rate of the vacuum pump is decided by the effective exhaust rate S of the vacuum pump and the connection pipeline conductance between the vacuum pump and blanching room through conductance series equation [23]. Conductance is decided by Equation (4) [23]

.

? 2015 China Academic Journals (CD Edition) Electronic Publishing House Co., Ltd.

? 2015 China Academic Journals (CD Edition) Electronic Publishing House Co., Ltd.

Where U is the connection pipeline conductance, m 3

/s;

is

the average pressure within the pipeline, i.e., 1000 Pa. D is the diameter of the vacuum pipeline, m; S is the actual ex-haust rate, m 3/s; μ is the viscosity, i.e., 1.25 × 10?5 Pa·s; L is the equivalent length of the pipeline, i.e., 0.7 m.

When the internal diameter of vacuum pipeline is 0.02 m, if the vacuum pump make the pressure within the blanching body decrease from 0.1 MPa to 0.01 MPa in 15 s, then sim-ultaneously calculate Equation 24 and the air exhaust rate of the vacuum pump S obtained is 0.0124 m 3/s, namely, 0.74 m 3/s. In this case, the selected vacuum pump is the 2BV-2061 type (Beijing HCHD Vacuum & Air compressing Equipment Co., LTD). Its main technical parameters are listed as Table 2.

Table 2

Main technical parameters of vacuum pump

When steam pressure is 0.11 MPa, the density is 0.6601

kg/m 3 and its volume is 0.065 m-3

. The steam mass m is

calculated using Equation (5).

Where ρ is the steam density, kg/m 3

; V is the volume of the blanching body, m 3. Thus, the steam mass is calculated to be 0.0429 kg. If the blanching is conducted for three times, then 0.128 kg steam is totally required. The selected LDR 0.0130.7 electric heating steam generator (Shanghai Huazheng Special Boiler Manufacture Co. LTD.), which generates steam at the velocity of 13 kg/h and can meet the requirement of this experiment, in actual work, this genera-tor can produce enough steam for multiple vacuum-steam pulsed blanching.

2.3 Fluent simulation and optimization of the jet module

2.3.1 The air control formula and the setting of its boundary conditions

Steam within the blanching is stable viscous flow and corresponds to standard k-ε model, and the control formula consists of continuous formula, motion formula, k (turbu-lence energy) formula and ε (dissipation rating) formula, which all comply with the following general forms [24]

.

Where φ is general coefficient; Γ is diffusion coefficient; S is source item; t is time, s; u is velocity at x direction, m/s; v is velocity at y direction, m/s; w is velocity at z direction, m/s. Entrance conditions: take the velocity entrance as the boundary condition, presumably the entrance direction is vertical to the boundary and is homogeneously distributed, the measured value is 58.3 m/s (determined by RHAT-301 anemograph with precision of 0.1 m, Tsinghua Tongfang), and the turbulence energy value was estimated as 3.8% using empirical formula [25–26]. The outlet condition is set as free effluent and all steam from the entrance will be expelled from the outlet. Solid wall employs non-slip condition and uses standard wall function method [25] for correction.

2.3.2 The optimization of jet structure

The faster the steam is, the larger the convective heat transfer coefficient is [27–29]. In this case, the velocity distri-bution of steam within blanching body will influence the homogeneity of blanching. Steam allocation plays a role in optimizing the distribution of flow field and equally allo-cating jet flow velocity [30]. The jet, located 150 mm above the bottom of the blanching body, is made from DN20 304 seamless steel tube. The single jet structure is demonstrated as Fig. 2. The distribution of internal flow field at z = 0, xoy surface and y = 0.05, xoz surface (Fig. 3) is calculated ac-cording to Fluent analysis. According to Fig. 3, for single jet structure, when the steam moves upward through the bot-tom of blanching body, anticlockwise circulation is formed at the xoy surface. Within the body, circumferential velocity is comparatively large while the velocity at the center is

comparatively small.

Fig. 2 Schematic diagram of signal jet. 1. Steam pipeline 2.

Steam jet 3. Condensate water outlet 4. Vacuum pipeline

Fig. 3 Internal flow field with single jet.

Note: In Fig. 3a, z = 0. In Fig. 3b, y = 0.05. The same as below.

To enhance the homogeneity of internal flow field and reduce the hollow-center effects, six trapezoidal turbulent flow plates are added homogeneously at the bottom of the blanching body. The structure is demonstrated as Fig. 4. The distribution of internal flow field at z = 0, xoy surface and y = 0.05, xoz surface is calculated according to Fluent analy-

sis.

Fig. 4 Schematic diagram of jet and turbulent plate. 1. Steam pipeline 2. Steam jet 3. Turbulent flow hole 4. Turbulent flow plate 5. Condensate water outlet 6. Vacuum pipeline

Fig. 5 is the distribution of internal flow field when the turbulent flow plates are fixed. According to Fig. 5, when the turbulent flow plates are added, the steam sprays to-wards the bottom of the blanching body from the pipeline and distributes equally upward after passing the turbulent flow plates. Then the steam forms anticlockwise circulation at the xoz surface. Although the steam velocity within the blanching body is generally equally distributed, the velocity at the center remains comparatively small, causing the con-vective heat transfer coefficient at the center is smaller than

its surrounding.

Fig. 5 Internal flow field with turbulent flow plate According to above two situations, to solve the problem that the velocity at the center is small, it is required to gen-erate steam moving upward following y = x path at the steam outlet, which works as turbulence to steam back from the blanching body. Thus a turbulent flow hole is added to the bend pipeline of the jet (as shown in Fig. 4). The diame-ter of the turbulent flow hole is 10 mm and the angle be-tween the section and the horizontal line is 45°. The distribution of internal flow field at z = 0, xoy surface and y = 0.05, xoz surface is calculated according to Fluent analy-sis and Fig. 6 demonstrates the internal distribution of flow field when the turbulent flow hole and turbulent flow plates have been added. According to Fig. 6, due to the effect of turbulent flow hole, steam within blanching body moves irregularly and the steam velocity is unified, which offset the hollow-center effect and make the steam full of the blanching body. In this case, the homogeneity of the blanching is enhanced. Among three circumstances, the calculation demonstrates that the mean velocities are 1.30, 0.92 and 0.31 m/s respectively, indicating that the turbulent flow hole and turbulent flow plates increase the flow re-

sistance and thus reduce the mean velocity.

Fig. 6 Internal flow field with turbulent flow plate and turbulent flow hole

In this case, this vacuum-steam pulsed blancher is de-signed with turbulent flow hole and turbulent flow plates. By employing equal area method, the section at z = 0, xoy surface is divided into several small rectangular with the same area [24]. Next, the steam flow velocity is measured at the center of each rectangular respectively. As shown in Fig. 7a, three points are selected at the x axis and three points are selected at the y axis and a mean value is acquired by measuring three times at each point. The measurement and calculation results are demonstrated as Fig. 7b. The result indicates that the calculated and actual values are generally close and the largest relative error is 5.2%.

2.4The design of control system

2.4.1The hardware design of the control system

The control system is used for collecting, displaying and controlling the temperature and pressure data within the blancher. This control system includes following modules:

(1) Main control module: This module mainly accom-plishes signal sampling, data processing, and displaying and controlling the measurement results. This system employs the P89V51RD2FN (NXP Company) as the core to com-plete signal sampling, data processing, result displaying and other tasks.

? 2015 China Academic Journals (CD Edition) Electronic Publishing House Co., Ltd.

? 2015 China Academic Journals (CD Edition) Electronic Publishing House Co., Ltd.

Fig. 7 Measurement positions for fluent velocities and velocity magnitude comparison between calculated and experimental

(2) A/D conversion module: Through ADC chip TLC2543, which is of 12 bit resolution, acquired pressure and temperature analog signals (0–5 V) are converted to digital signals.

(3) Output control module: The single chip processor in-puts digital signals to the input terminal of solid-state relay (SSR) (Zhejiang Xinda electronic Co., Ltd., HHG1-1/ 032F-28 10Z type) through I/O pin and internal timer. The output terminal of the SSR then controls the vacuum pump, valve and alarm light.

(4) Button and display module: The control panel sets the times, vacuum degree and cycle time of the vacuum steam blanching through buttons. The working status is displayed on the LCD12864 screen.

2.4.2 The design of control system software

The software for the single chip processor is coded using

C language and packed using modularized program. This software including modules for collecting temperature and pressure data, watchdog reset, output and LCD12864 dis-play. For collected pressure, temperature data, the single chip processor employs median filter [31], to effectively off-set the influences of accidental factors on slowly changing signals. The data is updated every 2 s. The result indicates, the measurement data is stable. The program procedure of the control system software for the blancher is demonstrated

as Fig. 8.

Fig. 8 Vacuum-steam pulsed blanching program procedure

3 Performance examination experiment

3.1 Experiment materials and conditions

Fresh lily is usually processed through blanching to pre-vent enzymatic browning [32]. This experiment selected lily as materials. Fresh lily was preprocessed by cutting roots, breaking into slices and cleaning.

Homogeneity experiments: The material basket was equally divided into inner, middle and outsider regions in radial direction (0.490 m) and three regions in lengthwise direction. The division of regions is demonstrated as Fig. 9. The homogeneity was verified at both radial and lengthwise directions. Lily slices were put in three regions, and the weight of lily slices for each region was about (6 ± 0.5) kg. Set the vacuum degree as 10 kPa, vacuum keeping time as 6 s, blanching time as 30 s and cycle time as 3.

Experiments on the influence of blanching conditions on blanching effect: Through single factor experiment con-cerning vacuum degree, blanching time and cycle time, the influence of the three factors on blanching effect was ex-plored. The experiments were arranged according to Table 3: the vacuum keeping time was set as 5 s for all experi-ments and the steam temperature was set as (120 ± 5) °C.

? 2015 China Academic Journals (CD Edition) Electronic Publishing House Co., Ltd.

Fig. 9 Regional division schematic Table 3

Single factor experiment design

Note: Vacuum keeping time is 5 s, and steam temperature is (120 ± 5) °C

After blanching, lily slices were taken from three regions and the weight of samples was (180 ± 5) g. The lily samples were put on the tray of the electric heat drum wind drying oven (Shanghai Yiheng technology Co., LTD) and dried at 60 °C until the wet basis moisture content reached 13%. The drying time after lily blanching, color, specifically, L *(lightness), a * (red green), b *(blue yellow) of dried lily were used as evaluation criteria. The color was examined through SMY-2000SF type colorimeter (Beijing SMY Sci-ence & technology Co., Ltd). For each experiment, three measurements were conducted and the mean value was em-ployed.

3.2 Results and analysis

3.2.1 Homogeneity experiment result and analysis

Moisture Ratio (MR) [33–34] and drying time t curves for lily slices at the internal, middle and outer layers, and the upper, middle, down positions, within the vacuum-steam pulsed blancher are demonstrated in Fig. 10 respectively. The wet basis moisture content of blanched lily was 72% ± 1% (oven drying method, 105 °C, 24 h). According to Fig. 10, it all required 11 h for drying lily slices at different loca-tions. This was because vacuum environment removed thermal resistance between the steam and the materials. In addition, imported steam did not condensate and release heat. The steam could quickly enter into the material basket and fully contact with every lily slice and make the temper-ature of lily slices rise homogeneously. Furthermore, the

Fig. 10 Drying curves of lily slices of different positions with pulsed vacuum-steam blanching

influence of the difference between steam velocity within the blanching body on the homogeneity of the blanching was very limited, indicating that added turbulent flow hole and turbulent flow plates worked effectively.

The color examination result for dried lily is shown in Table 4. According to Table 4, the color of lily slices dried by the vacuum-steam pulsed blancher at different locations was generally the same. According to Table 5, the signifi-cance levels p were all larger than 0.05, and the color is of no notable difference, indicating vacuum-steam pulsed blanching can make lily at different (internal and outer) lo-cations reach the similar effect of enzyme deactivation with the same time.

Table 4 Color value of dried lily under clearance cycle steam

vacuum blanching

Note: Lightness L 0*, red green a 0*, and blue yellow b 0* of fresh lily slices are 88.67, 0.72 and 14.75.

? 2015 China Academic Journals (CD Edition) Electronic Publishing House Co., Ltd.

Table 5

Analysis of variance of color value at different location

3.2.2 Influence of different blanching conditions on the blanching effect.

Drying time and color value of lily slices under differ-ent blanching conditions are demonstrated as Table 6. Ac-cording to experiments 1, 2, 3 and 4, with increasing steam time, the drying time decreased firstly and then increased. A short blanching time could not increase the permeability of water on lily surface and were not suitable for the migration of internal water. Additionally, the variation of color was large. A long blanching time was likely to cause the loss of surface water and lead to crusting effects. According to ex-periments 5, 2, 6 and 7, with increasing cycle time, the dry-ing time firstly decreased and then increased and cycle time as well led to under-blanching and over-blanching. Accord-ing to experiments 8, 2 and 9, lower vacuum extent led to more complete blanching and shorter drying time. Higher vacuum extent led to incomplete expelling cold or low temperature steam, causing low heat transfer efficiency be-tween the materials and following imported steam. This would further result in incomplete blanching and longer drying time. When the vacuum degree was 10 kPa, the vac-uum keeping time was 5 s, blanching time was 30 s and cycle time was 3, the drying time was the shortest and the color variation was comparatively small.

Table 6 Drying time and color value of lily slice under different

blanching conditions

4 Conclusions

This paper proposes a vacuum-steam pulsed blancher, which mainly consists of vacuum system, steam system, blanching body and automatic control system.

1) According to Fluent simulation, the distribution of in-ternal flow field is homogeneous with added turbulent flow hole and turbulent flow plates. The calculated result is gen-erally the same to the actual result and the largest relative error is 5.2%.

2) Automatic control system can set and control vacuum degree, blanching time and cycle time. Meanwhile, it can display real-time temperature, pressure and working status within the blanching body.

3) Performance examination experiments for the blanch-er are conducted using lily. The result suggests that vacu-um-steam pulsed blanching is homogeneous and the drying time is 11 h under the condition of 60 °C hot wind and the color does not vary notably after drying. With increasing blanching time and cycle time, the drying time firstly de-creases and then increases. With increasing vacuum degree, the drying time increases as well. Among these blanching conditions, when vacuum extent is 10 kPa, vacuum keeping time is 5 s, blanching time is 30 s and cycle time is 3, the drying time is the shortest and the color variation is com-paratively small.

This vacuum-steam pulsed blancher increases the loading capacity and blanching homogeneity, thus, provides theo-retical evidence and technical support for its wide applica-tion.

References

[1]Shao Xiaolong, Li Y unfei.Effects of blanching on water distribution and water status in sweet corn investigated by using MRI and NMR[J].Transaction of the Chinese Society of Agricultural Engineer-ing(Transactions of the CASE), 2009, 25(10):302-306.(in Chinese with English abstract)

[2]Qin Shan, Wen Xuesen, Shen Tao, et al.Thin layer drying characteristics and quality evaluation of steam blanched chrysanthemum[J].Transaction of the Chinese Society of Agricultural Engineering(Transactions of the CASE), 2011, 27(6):357-364.(in English with Chinese abstract)

[3]Bai Junwen, Wang Jiliang, Xiao Hongwei, et al.Weibull distribution for modeling drying of grapes and its application[J].Transaction of the Chi-nese Society of Agricultural Engineering(Transactions of the CASE), 2013, 29(16):278-285.(in English with Chinese abstract)

[4]Chen Junchen, Zhou Xuehua, Lai Pufu, et al.Optimization of processing nutrient powder from blanching liquid of stropharia rugoso-annulata with spray drying technology[J].Transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE), 2012, 28(21):272-279.(in Chinese with English abstract)

[5]Xiao Hongwei, Bai Junwen, Sun Dawen, et al.The application of super-heated steam impingement blanching(SSIB)in agricultural products processing-A review[J].Journal of Food Engineering, 2014, 132(1):39-47.

[6]Xiao Hongwei, Yao Xuedong, Lin Hai, et al.Effect of SSB(superheated steam blanching)time and drying temperature on hot air impingement drying kinetics and quality attributes of yam slices[J].Journal of Food Process Engineering, 2012, 35(3):370-390.

[7]Bai Junwen, Sun Dawen, Xiao Hongwei, et al.Novel high-humidity hot air impingement blanching(HHAIB)pretreatment enhances drying ki-netics and color attributes of seedless grapes[J].Innovative Food Sci-

ence&Emerging Technologies, 2013, 20(4):230-237.

[8]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):1135-1141.

[9]Xiao Hongwei, Lin Hai, Yao Xuedong, et al.Effects of different pre-

treatments on drying kinetics and quality of sweet potato bars undergo-ing air impingement drying[J].International Journal of Food Engineering, 2009, 5(5):64-67.

[10]Du Zhilong.Experimental Study of Blanching and Drying in Fruits and

Vegetables on the Air-jet Impingement Oven[D].Beijing:China Agricul-tural University, 2007.(in Chinese with English abstract)

[11]Chen Tianming.Design and Experiment Investigation of Hot and hu-

mid Gas Jet Impingement Blancher for Fruits and Vegeta-bles[D].Beijing:China Agricultural University, 2012.(in Chinese with English abstract)

[12]Wang Fengliang.A type of continuous steam blancher[P].Chinese pa-

tent:2737165, 2004-07-08.(in Chinese)

[13]Sui Ruirong.Pulsating vacuum pressure steam sterilizer[J].Chinese

Journal of Disinfection, 1988, 5(4):238-241.(in Chinese with English abstract)

[14]Chen Jianfang.The operational principle of pulsating vacuum pressure

steam sterilizer[J].Giude of Chinese Medicine, 2010, 8(33):165-166.(in Chinese with English abstract)

This line does not need translation.Chen Jianfang.The operational principle of pulsating vacuum pressure steam sterilizer[J].Giude of Chinese Med-icine, 2010, 8(33):165-166.(in Chinese with English ab-stract)[15]Michael K, Neil G, James C.The vacuum/steam/vacuum process[J].Innovative Food Science&Emerging Technologies, 2003, 57(12):30-33.

[16]Michael K, Radewonuk E R, Scullen O J, et al.Application of the vac-

uum/steam/vacuum surface intervention process to reduce bacteria on the surface of fruits and vegetables[J].Innovative Food Science and Emerging Technologies, 2002, 3(10):63-72

[17]Xu Juan, Zhang Hong, Sun Yanlin, et al.Effect of high-temperature,

short-time steam blanching(HTSTSB)on peroxidase activity of maca[J].Food Science, 2013, 34(2):457-478.(in Chinese with English abstract)

[18]Gao Zhenjiang, Wu Dingwei, Zhang Shuge, et al.Design of pulsed

vacuum drum dryer[J].Transactions of the Chinese Society for Agricul-tural Machinery, 2010, 41(3):113-116.(in Chinese with English abstract) [19]Cao Zhixiang, Gao Zhenjiang, Lin Hai.Experimental investigation of

roller vacuum pulsating pressure carrots drying[J].Food Science and Technology, 2009, 34(3):81-85.(in Chinese with English abstract) [20]Maache-Rezzoug Z, Rezzoug S A, Allaf K.Kinetics of drying and

hydration of the scleroglucan polymer.Acomparative study of two con-ventional drying methods with a new drying process:Dehydration by successive pressure drops[J].Drying Technology, 2001, 19(8):1961-1974.

[21]Haddad J, Juhel F, Louka N, et al.A study of dehydration of fish using

successive pressure drops(DDS)and controlled instantaneous pressure drop(DIC)[J].Drying Technology, 2004, 22(3):457-478. [22]Xie Chunliang, Luo Ligang.The thickness design calculation should be

paid attention to during the manufacture of glass fiber reinforced plastic tank[J].Chemical Technology Equipment, 2014, 35(6):12-14.(in Chinese with English abstract)

[23]Zhang Chao, Miao Bu.Calculation of vacuum system in chemical and

pharmaceutical industries[J].Chemical and Pharmaceutical Engineering, 2015, 36(1):19-22.(in Chinese with English abstract)

[24]Wang Juan, Wang Chunguang, Wang Fang.Numerical simulation on

three-dimensional turbulence air flow of 9R-40rubbing and breaking machine based on Fluent software[J].Transactions of the Chinese Soci-ety of Agricultural Engineering(Transactions of the CSAE), 2010, 26(2):165-169.(in Chinese with English abstract)

[25]Han Zhanzhong.Fluent --fluid engineering simulation and analysis of

examples[M].Beijing: Beijing Institute of Technology Press, 2009.(in Chinese)

[26]Zhang Zhe, Tian Jinjin.Numerical research on flow fieldin a refriger-

ated truck[J].Refrigeration, 2009, 37(8):59-61.(in Chinese with English abstract)

[27]Wang Xibo, Hu Qiong, Xiao Bo, et al.Modeling simulation of com-

bined convective and infrared radiation in rice drying pro-cess[J].Transactions of the Chinese Society for Agricultural Machinery, 2013, 44(9):145-151.(in Chinese with English abstract)

[28]Kaya A, Ayd?n O, Dincer I.Experimental and numerical investigation

of heat and mass transfer during drying of Hayward kiwi fruits:Actinidia deliciosa planch[J].Journal of Food Engineering, 2008, 88(3):323-330.

[29]Du Zhilong, Gao Zhenjiang, Zhang Shixiang.Research on convective

heat transfer coefficient with air jet impinging[J].Transactions of the Chinese Society of Agricultural Engineering(Transactions of the CSAE), 2006, 22(2):1-4.(in Chinese with English abstract)

[30]Dai Jianwu, Xiao Hongwei, Bai Junwen, et al.Numerical simulation

and optimum design on airflow distribution chamber of air-impingement jet dryer[J].Transactions of the Chinese Society of Agricultural Engi-neering(Transactions of the CSAE), 2013, 29(3):69-76.(in Chinese with English abstract)

[31]Wang Dong, Lin Hai, Xiao Hongwei, et al.Design of online monitoring

system for material moisture content in air-impingement drying pro-cess[J].Transactions of the Chinese Society of Agricultural Engineer-ing(Transactions of the CSAE), 2014, 30(19):316-324.(in Chinese with English abstract).

[32]Li Xia, Li Yongcai, Bi Yang, et al.Formula optimization of non-sulfur

color-protective agents for dried lanzhou Lily by response surface methodology[J].Food Science, 2014, 35(4):16-20.(in Chinese with Eng-lish abstract).

[33]Ju Haoyu, Xiao Hongwei, Bai Junwen, et al.Medium and short wave

infrared drying characteristics and color changing of apple slic-es[J].Transactions of the Chinese Society for Agricultural Machinery, 2013, 44(Supp.2):186-191.(in Chinese with English abstract)

[34]Xiao Hongwei, Bai Junwen, Xie Long, et al.Thin-layer air impinge-

ment drying enhances drying rate of American ginseng(Panax quinque-folium L.)slices with quality attributes considered[J].Food and Bioproducts Processing, 2015, 94(2):581-591.

(Translated by CHEN Z)

? 2015 China Academic Journals (CD Edition) Electronic Publishing House Co., Ltd.