The 8thAsia-Pacific Drying Conference (ADC 2015)
Kuala Lumpur, Malaysia, 10-12August 2015
EFFECT OF PRE-FREEZING METHODS ON POROUS STRUCTURE AND THERMAL PROPERTIES OF FREEZE-DRIED POTATO CUBES
Lijuan Zhao 12, Xiaokui Geng1, Kai Chen1, Zhonghua Wu12*,
1College of Mechanical Engineering, TianjinUniversity of Science and Technology
1083 Dagulan Road, Hexi Distrist, Tianjin, 300222, China
E-mail:
2Tianjin Key Laboratory of Integrated Design and On-line Monitoring for Light Industry & Food Machinery and Equipment, Tianjin, 300222, China
Abstract: In this paper, a porosity determination method was proposed for the freeze-dried potato based on the image processing method. The potato was first frozen using different methods and then was vacuum-dried. The Scanning Electron Microscope (SEM) photographs were taken for the sliced dried potato and then were converted into the black-white images by image processing. By accumulating the area ratio of the black pixel in the image, the porosity of the freeze-dried potato can be determined. By applying this image processing method, the sample size plays a important role in the porosity in this experiment. it was found that the porosity first increased and then decreased as the size increased, and compared with the results of the mercury intrusion method tests found that the error is within the range of 10% -13%.The results showed that the lyophilized potato blocks of 10mm by slowing pre-freeze to -30 ℃ had maximum porosity of about 61%.
Keywords:Potato, Freeze-drying, Image processing, Porosity, Conduction coefficient
INTRODUCTION
The porous medium is filled with the single-phase or multi-phase medium in its skeleton solid porosity. Solid matrix is all over the volume of space occupied by the porous media. it is necessary to know the porosity and the pore distribution, and it is very important to study its heat conduction coefficient and the heat and mass transfer rate. The block lyophilized potato plant belongs to the porous medium, and there are a large number of pore in internal.
The common used methods to measure the material porosity are, gas absorption method, etc. However, the gas absorption method is effective in measuring the meso pores (2-50 nm) and the Mercury Intrusion Method (MIM) is effective in measuring macro pores (>50 nm). Both methods have the limitations to measure porosity of the freeze dried materials which have meso and macro pores.
In recent years, Photoshop and Matlab have been widely used, SEM image processing method for determining the porosity of the samples has been used in Nano fiber membrane(Zhou and Wang, 2012), clay (Zhou et al, 2009), rocks (Wang and Xu, 2009; Wang and Zhou, 2009), sediment (Xu and Liu,2011) mesoporous carbon , coal (Qi and Wang, 2012), etc. Different thickness and different pre-freezing low-temperature vacuum freeze-dried potato block as the research object, through the use of two methods of Photoshop and Matlab, we get internal porosity by SEM image method and mercury porosimetry, he basis for its internal heat and mass transfer.
Thus, in this paper, the image process method is applied to determine the porosity of freeze dried potato which were pre-frozen using various freezing method, and provides a basis for further study of its internal heat and mass transfer
EXPERIMENTAL MATERAIALS AND METHOD
Experimental Materials
The potatoes were purchased from the local supermarket. They are washed, peeled and then cut into the cube samples with sizes of 5, 10, and 15mm. After blanching in the 95 ℃for 2 minutes, the potato cubes were air-dried for 30 minutes to remove surface moisture, wrapped with plastic wrap, placed in the refrigerator at 5℃ for later use.
Instruments and Equipments
The following instruments and equipments were used in the experiments: LGJ-18 freeze-dryer (Beijing Songyuan Huaxin freezing equipment company, Beijing, China), SC-15 CNC super bath; electronic analytical balance (precision 0.0001g), GJR-II eutectic, eutectic temperature tester, scanning electric microscopy (SU-1510, Japan's Hitachi), AutoPore IV of 9500 pressure mercury instrument (American Mike), LFA447 Nanoflash thermal conductivity analyzer (German resistance Chi) DSC200F3 differential scanning calorimeter thermal analyzer (German netzsch).
Experimental Methods
Sample Pre-freezing
The potato samples are then frozen in three methods: slow freezing, fast freezing, liquid nitrogen freezing.(1) the samples were put into the freezing chamber of a LGJ-18 type freeze dryer (Beijing Songyuan Huaxin freezing equipment company, Beijing, China), which is kept at room temperature. Then, the sample temperature is reduced to settling temperature, and we called this method as “slow freezing". According to the 7th reference，the eutectic point of potato is -22.5℃ and the common melting point is -16.8℃，so we settled the pre-freezing temperature as -30℃,the total pre-freezing time of potato was 4 hours. (2)the samples were put into the freezing chamber of a LGJ-18 type freeze dryer,which is kept at a settling temperature in advance,and we called this method as “fast freezing”. In order to ensure the pre-freezing time is same as slow cooling, also keeping the pre-freezing time as 4 hour at -30℃.(3)the samples were immersed into the liquid nitrogen (atmospheric pressure, -196 ℃),and the freezing processing ends when the liquid nitrogen evaporated over, and we call it as “liquid nitrogen freezing”. In this paper, the freezing time was about 5min when the liquid nitrogen evaporated over.
After pre-frozen, putting the potato cubes into the LGJ-18 type freeze dryer, the drying temperature is -30℃ and the pressure is maintained below 10Pa. During the drying process, the internal ice of potato has been sublimated into water vapor diffusion, the space occupied by the ice crystals will become pores after all the ice crystal was evaporated. As the drying temperature was the same as the material temperature, and the drying driving force is only the vapor pressure difference inside and outside the potato block, the drying time was long (approximately 80h); The drying stress of slow freezing is small, dried potato block deformation is small, the pore structure formed by the pre-frozen remains better.
Pore Structure Characterization
In this paper ,SEM image method and the mercury intrusion method were used to measure the porous structure of the freeze-dried potato block.The mercury intrusion method is gradually applied pressure within a certain range by placing a sample in the mercury porosimeter, the mercury was injected into the hole of the potato and recorded the volume of mercury in the pressing process, and we can ger the curve between the volume of the injected mercury and the pressure. A series of parameters which characterizing the pore characteristic of the sample can be obtained by analyzing of the curve. The AutoPore IV 9500 mercury porosimeter were used in the experiment,its pressure range of 0 ~ 228 MPa, and the the aperture measurement range is 0.005 to 1000 μm. The instrument has two low-voltage stations and a high-voltage stations, low-voltage stations measure 0 ~ 228 kPa, and high-voltage station measuring 0.1 to 228.0 MPa.
In recent years, using SEM image processing method to get the porosity of the measurement sample is applied to nano-fiber membranes, clay, rock, sand, macroporous carbon, coal, etc. The principle of an image processing method, Firs we sliced the sample sprayed with gold in the surface of the sample slice in a vacuum state, and then use a scanning electron microscope observing the morphology of the slice surface, obtained SEM grayscale image. The SEM grayscale image transmission was changed into black and white image with threshold segmentation method, and then calculated the area ratio of the black pixels points,that is the sample porosity.
Thermal conductivity
The thermal conductivity property of lyophilized porous potato block were measured by laser flash method with the LFA447 Nanoflash flash thermal conductivity tester. The principle of the laser flash method for measuring thermal conductivity is that under a certain set temperature T (constant temperature conditions), emitting a beam of light pulses by the laser source at the moment, and uniformly illuminated in the sample lower surface, and the temperature of lower surface increases after absorbing the light energy, then the heat was transfer to the cold end(the up surface) as one-dimensional heat conduction propagation. Using infrared detector continuously measured the surface of the central portion of the corresponding temperature rise of the process, and then processing on the temperature rise curve and sequentially computing the samples thermal diffusion coefficient, thermal conductivity. The specific heat of potato was measured by differential scanning calorimetry(DSC) method, the instrument of DSC200F3 differential scanning calorimeter was used.
RESUTLS AND DISCUSSION
Effect of Pre-freezing Methods on Porous Structure f Freeze-dried Potato Cubes
The porosity of the freeze-dried potato samples in different sizes and different freezing state was obtained by SEM image processing method. The the porosity of 5mm potato in different refrigeration mode was obtained by the mercury intrusion method, as shown in Table 2. It showed that the porosity obtained by SEM image processing method, the slow pre-frozen potato block porosity was greater than frozen in liquid nitrogen, and is greater than fast pre-frozen, and the porosity of 10mm potato block is maximum in the same pre-freezing conditions. The porosity of 10mm potato block reaches the maximum, approximately 60%,which was slowly pre-freezing at -30℃
A1：5mm，slow freezing /
B1: 10 mm, slow freezing
A2：5 mm, fast freezing /
B2：10 mm, fast freezing
A3： 5mm, liquid nitrogen freezing /
B3： 10mm, liquid nitrogen freezing
Fig.1 The SEM photograph of freezing dried potato in different size and different pre-freezing methods（A: 5mm,B: 10mm)
Tab.1 The porosity of dried potato obtained by SEM image processing and mercury intrusion method
Sample Size / Pre-freezing method / Porosity（%） / Error（%）Imaging Processing / Mercury intrusion method
5mm / slow freezing / 52.82 / 47.16 / 10.72
fast freezing / 39.88 / 35.67 / 10.56
liquid nitrogen freezing / 43.83 / 38.30 / 12.62
10mm / slow freezing / 61.22 / ______/ ______
fast freezing / 46.50 / ______/ ______
liquid nitrogen freezing / 48.60 / ______/ ______
15mm / slow freezing / 28.32 / ______/ ______
fast freezing / 26.81 / ______/ ______
liquid nitrogen freezing / 32.05 / ______/ ______
Compared SEM image processing method with mercury intrusion method ,the measurement data of porosity by the SEM image processing method is greater than the data by the pressure mercury penetration method, the error range ranges 10% -13%. Because the holes size in the porous material, ranging from nanometers to hundreds of microns and has a certain distribution. There are various types of holes, the hole of the opening, also has a closed hole. Only surface holes was seen by image method, not necessarily through holes. But the pressure measured by the mercury method can only computed the opening holes, that is a through hole, so the porosity obtained by SEM image is greater than that measured by the mercury intrusion method .
In the pre-freezing process, the potato block was continuously frozen through the heat transfer process and the water in the potato cube began crystallized when the temperature drops to a certain degree of sub cooling.At first, a large number of minute crystal nuclei were formed, then nuclei slowly grew up to the formation of ice crystals in contact with each other, and the crystallization process ends when the ice crystals grew up together.
When the samples were slowly freezing, its temperature was gradually decreased from the room temperature 25℃ to the settled temperature of -30℃ .When the temperature was lowered to a certain degree of subcooling, the internal moisture began slowly crystallized, and formed small nuclei. As the temperature was slowly decreased, the crystal has sufficient time for growth and therefore the final lyophilized potato block has a bigger internal porosity and more macroporous.
In the fast pre-freezing process, the potato was placed directly into the constant temperature the freezer of -30 ℃, the potato cubes were sharp cold and the crystallization rate of the water inside is very quick, and forming a large number of small nuclei, and each small nuclei rapidly grew in a short period of time, and then meet each other and the crystal stopped growing. Thus a large number of the small size of the crystals, therefore the holes by fast pre-frozen at -30 ℃ were more and the size was smaller.
The liquid nitrogen freezing is a special kind of fast pre-frozen. The liquid nitrogen has a extremely low temperature (atmospheric temperature of liquid nitrogen, -196℃) and the potato cubes were fast frozen in liquid nitrogen, and the crystallization rate of the water in the potato is very fast, and rapidly formed of a large number of small nuclei. However, the temperature is so low that the nucleus growth was relatively slow, leading to the last generated ice crystals are very small, so the formation holes was smallest.
In the slow freezing process of potato, low temperature is conducive to the formation of nuclei, and high temperature is conducive to nucleus growth. Therefore, the holes were few but bigger when the potato was slow pre-frozen at -30 ℃, the holes were more but smaller when the potato was pre-frozen in liquid nitrogen freezing.
Effect of Pre-freezing Methods on Thermal Properties of Freeze-dried Potato Cubes
Figure 2 is the specific heat curve of dried potato after quick-freezing with a temperature of -30 OC measured by DSC method. The specific heat of lyophilized potato varies with temperature curve. With the temperature increasing, the specific heat rise slowly. Table 2 compares the different pretreatment methods on the thermophysical parameters of freeze-dried potato. As seen from Table 2, the specific heat of fresh potato is the biggest, the potato rapidly pre-frozen dried potato was followed, and there was little difference among the slow pre-freezing , rapid pre-freezing, and liquid nitrogen frozen potato.
Fig.2 The specific heat curve of dried potato after quick-freezing with a temperature of -30 OC
Effect of pre-freezing methods on drying process of potato cubes
The potato block of freeze-drying time is shown in Table 3, seen from the table in the case of the same size, the drying time of fast pre-freezing was longer than the slow freezing, and the required time of liquid nitrogen drying was shortest. More pores of the potato within a block, the larger the pore size, and quicker the rate of heat and mass transfer, and accordingly will accelerate the rate of drying, the drying time was short. Correspondingly, fewer pores, the smaller the pore size, and slower the rate of heat and mass transfer , the drying rate was slow, the drying time was long. The drying time by liquid nitrogen freezing was the shortest, due to the extremely low temperature of liquid nitrogen, the potato block was sharp cold, resulting in surface cracks in the potato block , and even split in the fastest lyophilized indoor and media contact area, heat and mass transfer. However, this approach is not suitable for freeze-dried potato block production practice, this article, it was studied as a special theoretical case .
Tab.2 The thermal properties of dried potatoes after various pre-freezing processing
Pre-freezing method / Densityg/cm3 / specific heat
J/g×K / thermal diffusion coefficient
mm2/s / coefficient of heat conductivity
W/m×K
Fresh potato / 1.0344 / 3.506 / 0.113 / 0.163
-30OC slow freezing / 0.405 / 1.803 / 0.114 / 0.083
-30 OC fast freezing / 0.413 / 1.931 / 0.114 / 0.091
liquid nitrogen freezing / 0.207？ / 1.772 / 0.145 / 0.053
Tab.3 the drying time of freeze dried potato samples with various sizes and freezing methods
Drying time（h）Pre-freezing method / Potato size（mm）
5 / 10 / 15
slow freezing / 8 / 14 / 22
fast freezing / 14 / 24 / 30
liquid nitrogen freezing / 4 / 8 / 13
CONCLUSIONS
In this paper, the use of an SEM image processing method to obtain freeze-dried potato block porosity method is feasible, and it can determine of the porosity of the freeze-dried potato block effectively and rapidly. However, image processing, through Photoshop segmentation threshold and Matlab image calculated by the two procedures, so that the results with the actual situation had some error. Further, the image processing method obtained the porosity of the slice sectional potato block, can not be completely reacted potato block layers inside the pores as well as the overall porosity, which is the image processing method of the inadequacies. The mercury intrusion method can be used to measure the porosity of potato cubes .
ACKNOWLEDGEMENTS
This work was sponsored by grants from the TianjinMunicipal Science and Technology Commission(Project Num: 12ZCZDGX4630-2013), Natural Science Foundation of China (Project Num: 31471618)
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