Theoretical Simulation and Experimental Study on Effect of Vacuum Pre-Cooling for Postharvest Leaf Lettuce

Veiw figure View Figure

Theoretical analysis and practice were combined in this study. Samples of leaf lettuce were shaved to form a bundle with diameter close to 4 centimeter and length close to 20 centimeter respectively. The physical model used in the study is illustrated in Figure 1. Considering the internal heat conduction and the heat and mass transfer on the leaf surface, the following assumptions were made for the model calculations: (1) the physical properties of the leaf lettuce were constant during vacuum cooling; (2) heat transfer from respiration, convection and radiation during vacuum pre-cooling were not considered; and (3) based on the refrigeration system, water diffusion inside the lettuce was ignored during vacuum pre-cooling.

Based on these assumptions, the two-dimensional cylindrical heat transfer equations for an unstable state were as follows:

(1)

Initial condition: t=0, (0＜r＜R);

Boundary condition: r=0, (t＞0);

(2)

The applicable properties of leaf lettuce were expressed as: , density (kg m^-³); , specific heat capacity (J [kg^-1·K^-1]);, coefficient of thermal conductivity (W ([m^-1·K^-1]); ,, cylindrical coordinate system; , temperature (K); , , surface temperature of leaf lettuce and ambient temperature, respectively (K); , time (s); , heat transfer coefficient (W [m^-2·K^-1]); and , inner heat source (W m^-³), where is calculated as:

(3)

is the latent heat (kJ kg^-1); is the volume (m³); and is the mass loss rate (kg s^-1).

Convection and radiation heat transfer were neglected in the low-pressure environment. We used a VFDS-2000 vacuum pre-cooling dry dual-use machine (Shanghai in China Co., Ltd.) with a vacuum volume of 0.1 m³. The optimum conditions were initial temperature, 24C; final temperature, 5C; initial pre-cooling pressure, 600Pa; and final pressure, 1500 Pa. We analyzed the temperature changes and weight loss of the leaf lettuce under variable pressure during the vacuum pre-cooling process. A detailed description of the model, devised to reduce error resulting from the use of different lettuce samples during the pre-cooling test, is provided below.

(4)

(5)

2.2. Model Solution

According to guidance for fresh fruits and vegetables, =0.61 W (m^-1·K^-1) and =4.11 J (kg^-1·K^-1). For the MATLAB calculations, two groups of experimental materials were selected: group I (40.47, 41.75, 38.28 and 52.02 g), and group II (35.43, 42.95 and 56.59 g).

According to the basic theory of mass transfer coefficients, the weight loss of the leaf lettuce during vacuum cooling storage can be written as:

(6)

Where is the mass transfer coefficient; is the proportionality constant (0.62); is the vacuum chamber pressure, Pa; and is the saturation pressure corresponding to temperature T, (Pa), the equation can be written as:

(7)

Temperature calculation of the leaf lettuce during vacuum pre-cooling process is provided below.

(8)

(9)

3. Experimental Verification

An unsteady computational model was constructed based on heat and mass transfer mechanisms to analyze the factors affecting leaf lettuce during vacuum pre-cooling, and the model was verified experimentally. Changes in the vacuum chamber pressure, temperature and quality of leaf lettuce over time were analyzed and compared.

3.1. Experimental Apparatus

Vacuum pre-cooling equipment used Shanghai in China Co., Ltd. VFD-2000-type vacuum cooling test machine, mainly by the refrigeration system, vacuum system and the acquisition and control system. As shown in figure 2, the vacuum system used a 2XZ-4 rotary vane vacuum pump with a pumping rate of 4L s-1 and a speed of 1400 rpm. Four thermocouple temperature measurements were established in the vacuum chamber and a vacuum pressure sensor was installed to collect data. The control system consisted of a programmable logic controller, personal computer and electrical components, with system parameters set for timely data collection, storage and processing. Figure 2 presents experimental table for the vacuum pre-cooling test machine.

Figure 2. Experimental table for the vacuum pre-cooling test machine

Download as

Veiw figure View Figure

3.2. Experimental Material and Test Method

Screening and binding into cylindrical rolls, sub-samples of lettuce were taken to the Mao Zhuang Science and Technology Park, Henan Agricultural University for analysis. The samples were weighed on an LT502E electronic balance with maximum load of 500 g, sensitivity of 0.01 g, and level-III precision. Group-I (40.47, 41.75, 38.28 and 52.02 g) and group-II (35.43, 42.95 and 56.59 g) samples were placed in the vacuum chamber on racks for the comparative test. After pre-cooling the samples were weighed and the test was repeated with different control parameters. Having selected leaf lettuce, after simple treatment, we shut down the experimental machine door in vacuum tank, start the computer test program and vacuum system.

1) Temperature Test

During the test, in accordance with the test requirements of the thermocouple arranged in the geometric center of the cylindrical sample, with the edge of the measuring points of the test sample arranged as shown in Figure 3. The vacuum chamber door lock open test device, the collection test data. Pre-wet processing leaf lettuce pre-cooling mass loss, as shown in Table 1.

2) Pressure test

In order to test the effects of the pressure in vacuum container on vacuum pre-cooling for lettuce, a pressure sensor was placed in the vacuum chamber pressure-recording line (600, 1000 and 1500 Pa, data collection interval=30 s), and Microsoft Excel was used for editing. Changes in the material and temperature were measured. The weight loss of leaf lettuce under variable pressures is shown in Table 2.

Table 1. Pre-wet processing leaf lettuce pre-cooling mass loss

Download as

View current table in a new window

Tables index

Veiw figure View Table

View next table

Table 2. Weight loss of leaf lettuce under the variable pressure

Download as

View current table in a new window

Tables index

Veiw figure View Table

View previous table

3.3. Analysis and Discussion

The simulation and experimental analyses examines the effects of the leaf lettuce submersion on weight loss: the lower the vacuum pressure, the more rapid the water loss and the greater the cooling rate. The theoretical results and measured values matched well, and the differences between them were small. Heat and mass transfer mechanisms based on vacuum cooling, pressure control during cooling theoretical modeling and experimental testing of the water loss rate and sample temperature are presented in Figure 4 using the values shown in Figure 8. The computational model of leaf lettuce was fitted and it was confirmed by the experimental data.

Figure 3. The measurement points arrangement

Download as

Veiw figure View Figure

Figure 4. Temperature change in vacuum pre-cooling process under different pressures

Download as

Veiw figure View Figure

Figure 5. Temperature changes during vacuum pre-cooling of unsubmerged lettuce (Sample 1).

Download as

Veiw figure View Figure

Figure 6. Temperature changes during vacuum pre-cooling of lettuce sprayed with 1.76 g water (Sample 2)

Download as

Veiw figure View Figure

Figure 7. Temperature changes during vacuum pre-cooling of lettuce sprayed with 1.97 g water (Sample 3).

Download as

Veiw figure View Figure

Figure 8. Temperature changes during vacuum pre-cooling of lettuce sprayed with 3.12 g water (Sample 4)

Download as

Veiw figure View Figure

Based on the thermodynamic calculation, under lower vacuum pressure, the velocity of dehydration of leaf lettuce increased, while surface temperature decreased rapidly (Figure 4, Table 2). During the vacuum pre-cooling process, the mass loss and speed of the evaporation showed that the lower the pressure, the faster the mass transfer process, and the more volatile the surface of the water. Comparing measured and simulated values, when samples were pre-stabilized, the medium gradually became faster than the analogue values of the pressure in the vacuum chamber: in the latter case, when the pressure in the vacuum chamber had stabilized, the pressure was higher than the measured analogue value.

Figure 5- Figure 8 and Table 1 show that the measured and the analogue values were substantially similar, and the general trends were similar in Figure 5 and 6. Rapid changes in the surface and internal temperatures occur in leaf lettuce during vacuum pre-cooling because of internal thermal and moisture diffusion. The processing time for pre-wetted samples was 10 min less than that of the control samples (Sample 1 in Table 1); when the quantity of water was increased the the pre-cooling time was reduced. The amount of water added to the test samples corresponded 4.211- 5.977% of the total sample mass, and to a mass loss of 1.987- 2.873%. Under pre-cooling pressure of 600, 1000, and 1500 Pa, the mass loss was 2.758, 2.701 and 1.929%. The optimum percentage of spray water was 5% of the total sample mass, which corresponded to the best efficiency and optimum vacuum pre-cooling parameters for leaf lettuce.

3.4. The Quantities of Capture Water and Cooling Capacity Calculation

The compressor and vacuum pump are the main energy-consuming parts in vacuum pre-cooling. Here, we provide an analysis of an energy-saving method could be obtained based on controlling the load of compressor and start-stop of vacuum pump by the temperature or pressure. According to the above analysis and vapor condensation theory, the quantities of capture water of the pre-cooling unit is given by

(10)

In formula,is Knudsen Number, is the pipe diameter, m, is Mach number, is Reynolds number, is fouling factor, ,is free path, and it can be expressed as

(11)

Where is Coefficient parameters, is diameter, m, is Vacuum pressure, Pa, and is temperature, C.

The quantities of capture water

(12)

In formula, is latent heat, kJ/kg, is the quantities of capture water, is heat transfer, W, and according to the basic theory of mass transfer coefficients, the quantities of capture water of the leaf lettuce during vacuum pre-cooling can be written as:

(13)

Where is steam consumption, g, and is water consumption in evaporation, g.

(14)

Where is the cooling capacity, W, is time, min.

Water-catcher was a key component, vacuum extracted with water vapor in the air and subject to capture water, trapping the water down, otherwise it would reduce the pump's performance affects the entire vacuum pre-cooling device performance. Based on the experimental data and the records of the pre-cooling unit, and following from equations (10) through (14), the calculated the quantities of capture water was 1.607–2.567 g, and the cooling capacity of the total sample was 3.722–5.946 W.

Figure 9. The quantities of capture water during vacuum pre-cooling

Download as

Veiw figure View Figure

Figure 10. The curves of pressure changing with time

Download as

Veiw figure View Figure