Influence of Thickness on Ice Crystal Formation in Strawberry during Freeze-drying
A.M. Mimi Sakinah
This study demonstrates the capability of X-ray Micro-computed Tomography (XMT) technique to characterize the internal ice crystal microstructure of fresh strawberry after freezing. The method requires freeze-drying of the sample to remove frozen water before scanning to indicate ice crystal and internal structure of the material. Results are presented for the 2-D ice crystals formed within strawberry frozen at different rates. The dendrite spacing of ice crystals has been related to the freezing conditions of the material. Moreover ice crystal parameters such as size, volume and width can be measured by using the technique. The overall results indicate that the ice crystal distribution within strawberries diverse with the axial distance of the material from its cooling surface and thickness of the samples. In the latter stage the ice crystal size is bigger when the material is far from the cooling surface while the smallest ice crystal size will be formed from the thickness of the materials.
Improving methods of food preservation is an important technique needed by
all of the food industry. Achieving the right preservation technique is very
important in result of the changes of nutritional and sensory qualities of food.
Freezing is a conventional food preservation technique in which the temperature
of food is reduced below its freezing point and a proportion of the water undergoes
a change in state to form ice crystals. A quick versus slow freezing mechanism
that caused towards ice crystal formation is shown in Fig. 1.
The immobilization of water to ice and resulting concentration of dissolved
solutes in unfrozen water lower the water activity of the food (Fellows,
2000). This method often results in substantial textural damage caused by
the growth of ice crystals within the delicates structure either present naturally
or created during processing (Mousavi et al., 2005).
Recent studies visualized the ice crystal structures formed during freezing
of a number of foods has been applied using X-ray micro-CT. An understanding
of the relationship between the freezing conditions and the size of ice crystal
formed is critical in controlling product quality and texture (Miri
et al., 2007). The observation of the ice crystal size can be direct
or indirect. Direct observation of ice crystal can be done by cryo-scanning
electron microscope (Russell et al., 1999), cold
microscopy (Donhowe et al., 1991) and confocal
laser scanning microscopy (Evans et al., 1996).
|| Quick versus slow freezing mechanisms
Indirect method such as freeze substitution (Bevilacqua
et al., 1979; Martino and Zaritzky, 1998),
freeze fixation (Miyawaki et al., 1992) and freeze
drying techniques (Fayadi et al., 2001; Woinet
et al., 1998a, b) which followed by the sectioning
had been used. Freeze drying is well established process for the indirect method
to observe ice crystal formation.
However, the indirect methods assume the original morphology is maintained
during the sectioning into thin enough layers to allow microscopic and quantitative
information about ice crystal formation of these strawberries in thick samples
(Mousavi et al., 2005). X-ray micro-CT is relatively
a new technique method to be used. Previously a number of research efforts have
been reported that take into account the ice crystal formation in frozen food
(such as fish, meat and mycoprotein) may effect their textural, microstructural
and qualitative changes. The quality of freeze dried strawberry pieces had been
studied and an experiment on a thick layer of strawberry pieces in different
operating condition of freeze drying had been done. The working pressure and
heating plate temperature during freeze drying were the most important factors
affecting the criteria of final product quality in terms of its appearance,
shape, colour, texture and dehydration ratio. The optimal conditions for freeze
drying process for strawberry is at 30Pa, 50°C and the time ranged for freeze
drying is from 60 to 65 h (Hammami and Rene, 1997).
Although, the freeze drying time, appearance and colour of the freeze dried
strawberries had been investigated, there is no qualitative which has found
potential applications in food science research and quality evaluation (Mendoza
et al., 2006). An axial and lateral resolution down to a few micrometers
and without sample preparation and chemical fixation of the architecture of
cellular materials can be visualize and analyze through this microCT technique
(Van Dalven et al., 2003).
Using X-ray micro-CT, the study had been done on various field such as an internal
microstructure study for ice crystal visualization of mycoprotein, carrot, meat,
fish, chicken, potato and cheese (Mousavi et al.,
2005; Miri et al., 2007), detected internal
quality changes in peaches, investigated core breakdown disorder in Conference
pears based on their mass density variations during storage and quantitatively
analyze and characterize apple tissue to micrometer resolution (Mendoza
et al., 2006). X-rays are short wave radiations, which can penetrate
through fruit. The level of transmission of these rays depends mainly on the
mass density and mass adsorption coefficient of the material. The density of
many fruits increases with maturity (Mendoza et al.,
2006). The motivation of those studies was found in the necessity to extract
realistic and statistical 2-D internal data for the ice crystal formation in
frozen strawberry at micron resolution. Observing the ice crystal formation
of the internal microstructure of food is important as it will help in preserved
the texture quality of the frozen and freeze-dried strawberry.
MATERIALS AND METHODS
Experimental system: Strawberry fruits were collected from an orchard
in Cameron Highlands, Pahang, Malaysia in October 2008. Fresh strawberries were
transported to the Analytical Lab of Chemical Engineering Department of University
Malaysia Pahang on the same day of recollection and were immediately cut into
various thicknesses and frozen -20°C before undergone drying treatment.
||Schematic drawing of strawberry slicing in different thickness
The average moisture content of the fresh and dried strawberries was 89.6
and 10.8%, respectively. Three batches of fruits were sampled, dried and analyzed.
Drying process of strawberries was conducted according to the method modified
from previous work (Hammami and Rene, 1997). For freeze
drying of strawberries, 250 g sample was initially sorted at about 5, 10 and
15 mm thickness as shown in Fig. 2 and placed on a stainless
steel tray. The trays were placed in a freeze dryer (BioTron model Cleanvac
12S) and the blast-freezing process was started at -40°C for 120 min. Under
0.1 Torr constant pressure, the drying process of strawberries required 72 h
to reach a moisture content of 10.8%. Dried strawberries was then stored at
4°C until analyzed within a week.
X-ray micro-computed tomography: The white painted metal sheet of Skyscan
1072 X-ray microtomograph is being used in the present study. Figure
3 is illustrated the schematic diagram of X-ray micro-computed tomography
technique. It is includes a chamber where the sample is being placed throughout
the analysis. The lead shield outside the chamber is kept closed in order to
prevent any X-rays to escape from it. It is using the electricity powered and
appropriate software attached from a computer drive. It has to be connected
to the computer during operation because an exchanges data between both is needed
during the analysis period. It is only respond to the command when it is being
connected to each other. A flashing red light at the back of this apparatus
will be lit automatically when it detects that X-rays are generated from this
equipment. The sample is placed on the sample holder and it is being fixed securely
into the right position before the operation can take place. The X-rays is being
produced by the apparatus and directly spot onto the sample. The sample is perpendicularly
rotated about its long axis and the images are directly shown as grey scale
image on the computer. The photographs of the sample at different positions
are taken and being saved to the computer drive.
||Schematic diagram of X-ray micro-computed tomography technique
The images then being reconstructed using software (Nrecon) and a plane image
obtained is further used in the analysis of the sample that performed by the
analysis software (CT analysis).
RESULTS AND DISCUSSION
Ice crystal quantification: Cooling rates lead to the development of
a dendrite ice crystal structure and the changes is from the bottom to the top
of the samples can be clearly seen by the X-ray (Mousavi
et al., 2005). Figure 4a-c also
compared clearly that the shape of ice crystals changed from sample base to
its top and thus creating the different width distribution of ice crystals in
the samples. The width distributions range between 0.06 to 0.39 mm. As seen,
ice crystal width increased from the bottom to the top of the samples. These
happen due to the top of the sample is far from the cooling surface and leading
to larger ice crystals, while the bottom sample is near to the cooling surface
and thus creating much smaller ice crystals throughout the samples. The rate
of cooling decreasing with increasing distance from cooling surface and these
leading to larger ice crystals which is possible to influence the microstructure
by changing the freezing conditions (Mousavi et al.,
2005). Freezing of different thickness of strawberry samples produced similar
shapes of ice crystal as shown in Fig. 5. Similar shapes of
ice crystal for each sample but the size is different neither in each axial
position nor the thickness itself. Figure 6 has been noted
the trend that thicker sample gave smaller ice crystal distribution measurement
compared to the less thick of the samples. Figure 6 shows
the width distribution of the ice crystal measurement for the different thickness
of the samples that using the same freezing technique. The relationship between
microstructure and ice crystals is clearer when these ice crystals structure
formed in different axial position and thickness of the strawberry.
||Tomographic image of freeze-dried strawberry: (I) Typical
side view of X-ray image; (a) 7.435 mm from cooling surface (b) 9.700 mm
from cooling surface and © 12.945 mm from cooling surface
||Microstructure of different thickness of freeze-dried blast
||Average width distribution of the ice crystal measurement
for the different thickness of the freeze-dried strawberry
As figured, ice crystal in the central zone of the strawberry are larger than
upper and bottom of the samples. The stronger cell wall in the intermediate
of the sample might cause these larger ice crystals. It was clear here that
the size or thickness and direction or position of the samples mostly affect
the size of ice crystal. However, the shape of the ice crystal and structure
under this freezing condition is not much different because the strawberry samples
have similar microstructure at either direction.
Ice crystals in frozen strawberries are qualitatively and quantitatively investigated using X-ray micro-CT. The X-ray technique could visualise and differentiate the effect of ice crystal growth in frozen and fresh strawberry and the freezing rate condition of the strawberries regarding the distances of the frozen strawberries from the cooling surface and thickness of the strawberries. Ice crystals in thin samples for frozen strawberries have the smallest ice crystal size and the samples that isolated from the cooling surface have the slowest freezing rate thus creating the biggest ice crystals in frozen strawberry. Larger ice crystals and broader crystals size distributions were seen at the top of the sample than the centre, while at the bottom of the strawberry samples, the ice crystals formed are much smaller. The result suggests that the freezing rate affect on microstructures of these frozen strawberries, the interrelationship between ice crystal and strawberry microstructure in each case and freeze dried is a good technique to visualise the microstructure of food samples.
This research has been carried out with the support of Universiti Malaysia Pahang (UMP) under the Graduate Research Scheme, grant GRS 080105. The author would like to acknowledge Malaysian Ministry of Science, Technology and Innovation (MOSTI), for the National Science Fellowship (NSF) scholarship.
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