Study of the water vapour sorption of freeze dried

Study of the water vapour sorption of freeze dried carrots
using micro-CT
Martin Koster, Gerard van Dalen
UNILEVER Research, Olivier van Noortlaan 120, 3133 AT Vlaardingen, The Netherlands
ABSTRACT
We studied the microstructural changes in freeze dried carrots due to water vapour sorption, including
the influence of different drying treatments. Small cylindrical freeze dried samples were stored for
prolonged time in a desiccator at 22 °C and 100% relative humidity. The moisture uptake was quantified
by weighing. The microstructure was visualised by X-ray microtomography (µCT) using a SkyScan 1172
bench-top system and also the facilities of the ID19 beam-line of the European Synchrotron Radiation
Facility (ESRF) in Grenoble. Morphological changes caused by water sorption were examined by imaging
the samples at different times. High resolution Synchrotron µCT images revealed the swelling of the pore
walls. The diffusion of water vapour into freeze dried carrots is slow. It takes about 15 days to reach
equilibrium. Within 1 day, sorption of water resulted in swelling of the pore walls. After longer sorption
times large pores remain unhydrated. The water uptake results in shrinkage of the samples which can be
attributed to viscoelastic relaxation. No significant differences were observed between different drying
treatments.
1
Introduction
Dried vegetables are used in instant food products which are easy to prepare by adding hot water. Fast
rehydration of these pieces is desirable. It is believed that the degree of rehydration can be related to the amount
of cellular and structural damage that is introduced by the freeze drying process (1). The aim of this study is to
obtain information about microstructural changes in freeze dried carrots due to water vapour sorption. Also the
influence of blanching and different drying treatments used for the vegetables on their absorption behaviour (2)
was evaluated. It is known that blanching disrupts cell membranes and hence, has an effect on the
microstructure (3). Also the sugars will disperse over the tissue (4). When fruits and vegetables are being dried,
changes to sensory characteristics, colour and size (shrinkage) occur. Often these changes are irreversible and
result in loss of quality of the products. The work is part of a larger study where we looked into the change in
microstructure when fruits and vegetables are being dried, in particular broccoli, carrots, bell peppers and
mushrooms (5).
2
Material and Methods
Small cylindrical samples with a diameter of 8 mm and a length of 10 mm were cut from a batch of fresh
winter-carrots from the cortical tissue (Figure 1). One portion was pre-treated by blanching for one minute in
boiling water whilst the others remained untreated (raw). Next, the pieces were freeze dried at 2 different
temperatures yielding 4 different lots of carrot samples (Table 1).
Fig. 1 Fresh winter-carrots are being cut (left) and then cylindrical samples (middle) were bored from the
cortical tissue. These were then then freeze dried (right)
InsideFood Symposium, 9-12 April 2013, Leuven, Belgium
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Table 1. Four lots were taken from a large batch and pre-treated using the different conditions.
Sample
1
2
3
4
Pre-treatment
Raw
Raw
Blanched
Blanched
Drying Method
Freeze dried -28°C
Freeze dried -150°C
Freeze dried -28°C
Freeze dried -150°C
The freeze dried samples were approximately 5 mm in diameter and were stored under controlled
conditions (22°C and 100% relative humidity) on a perforated mesh in a desiccator (see Figure 2). The bottom
of the desiccator was filled with water. The temperature and RH were monitored with a data logger. The
moisture uptake was quantified by weighing using an analytical balance. The water adsorbed (g) divided by the
dry sample weight (g) is expressed as the rehydration
ratio. The samples were marked with a small ink-spot
and imaged at different time intervals. At each time
step a sample was removed from the desiccator and
mounted at approximately the same location (using the
marker) in a closed sample holder (capped
polycarbonate tube). To maintain a high RH% during a
scan, a small piece of wet cotton-wool was also placed
inside the tube.
Fig. 2 Desiccator with sample and data logger
One set of samples was stored for a period of 24 days and imaged with a SkyScan 1172 desktop microCT system at intervals of approximately one week for a period of three weeks. Images of 4000x4000 pixels
were acquired with a pixel size of 2 µm using 60kV/167µA, 180° rotation, 2x frame averaging and a step size of
0.2 degrees. A three-part oversized scan was required to image a complete cylinder. Reconstruction was done
with Skyscan NRECON software with smoothing set to 1, ring artefact correction to 16, and beam hardening
correction to 60%
Another set, was monitored on a single day to
study the uptake of water in the early hours. From two
of these samples (raw, freeze dried at -150°C and
blanched, freeze dried at -28°C) images were
acquired. This was done at the facilities of the ID19
beam-line of the European Synchrotron Radiation
Facility (ESRF) in Grenoble which allows very short
scan times (Figure 3) and pixel sizes down to 0.56 µm.
Such high resolution comes at the cost of a reduced
field of view, so that only smaller parts of the carrot
could be investigated.
Images of 2048 * 2048 pixels were made using
pixel sizes of 2.80 µm and 0.56 µm (a FReLoN
camera was used through objectives with
magnification 2x and 10x) resulting in field of views
of 5.7 mm² and 1.1mm². An energy of 17.6 keV was Fig. 3 Stage at ESRF beam line ID-19 (left) with mounted
test tube (right)
selected to yield sufficient contrast.
A novel 3D volume reconstruction was used (Paganin’s single distance phase retrieval) which requires
images taken at only a single distance and yields phase contrast in the final images (6). Such images can be
effectively segmented using local thresholding. The algorithm is implemented in ESRF’s reconstruction
software and makes use of GPU’s (7).
For comparison of the microstructure at the different time intervals, we needed to retrieve the same
spatial location before and after storage. 3D alignment by affine registration was used (Avizo fire V7.1, from
VSG) which worked reasonably well for lower resolution scans (2 and 2.8 µm), but it was not always possible
to locate the exact same spot in the high resolution (0.56 µm) images. This was partly due to the fact that the
samples showed shrinkage after moisture uptake, but also small differences in the position of the sample in the
holder (due to sample transfer) makes it nearly impossible to scan at exactly the same place.
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3
Results
The moisture adsorption curves of freeze
dried carrots at 100%RH and 22°C that were
imaged with the Skyscan 1172 are shown in
Figure 4. These curves were obtained for the
single cylindrical pieces that were also used for
micro-CT imaging.
The graph shows that the diffusion of
water vapour is relatively slow. It took about 15
days to reach equilibrium. The moisture content
reached a plateau at about 80% (wet basis). All
samples showed shrinkage caused by the uptake
of water. No significant differences were
observed between raw or blanched samples or
between the different drying temperatures. Fig. 4 Moisture sorption of freeze dried carrot pieces stored at
Representative Skyscan µCT images of the freeze 100%RH
dried carrot samples before (A) and after (B) storage for 2 weeks at 100 %RH and 22°C are shown in Figures 5
and 6. The water content of the wet samples corresponds to the equilibrium values of the moisture curves shown
in Figure 4. Water gives a higher X-ray absorption (dark grey) than the dry material. The images of the wet
samples clearly show the presence of non-hydrated pores. Note that the cross sections shown were not obtained
at the same spatial location.
Fig. 5 Vertical (A) cross sections of Skyscan µCT images of freeze dried carrot samples, before and after
storage for 2 weeks at 100 %RH and 22°C (pixel size is 2.0µm).
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Fig. 6 Horizontal (B) cross sections (slice) of Skyscan µCT images of freeze dried carrot samples, before and
after storage for 2 weeks at 100 %RH and 22°C (pixel size is 2.0µm).
To compare the microstructure at the same spatial location before and after water-uptake, a 3D alignment
by affine registration was used. An example for raw freeze dried carrot sample (-150°C) is show in Figure 6.
Some of the larger pores were not filled with water. The images also reveal that the water sorption causes some
shrinkage. Such behaviour is also observed when a sample is rehydrated by submerging into water.
Fig. 6 Vertical and horizontal cross section of Skyscan µCT images of a raw freeze dried carrot sample (-150°C) efore and
after storage for 2 weeks at 100 %RH and 22°C
Synchrotron µCT images were made from two freeze dried carrot samples stored at 100%RH and 22°C
for maximal 14 hours. The images were acquired at low (2x) and high (10x) magnification. Although the lower
magnification (2x) images could be effectively aligned in 3D space they are not in exact the same location
because of shrinkage of the sample after moisture uptake. At higher magnification (10x), however, the same
location could not be retrieved. Because the field of view is very small, it is much more difficult to scan
precisely the same space.
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An example of images of a raw freeze carrot stored at 100% RH and 22°C, at a magnification of 2x, is
shown in Figure 7. The increase in grey level in these images is mainly due to an increase in water content
(black is low and white is higher X-ray absorption).
Fig. 7 Synchrotron-µCT images (2x magnification with pixel size 2.8 µm) of horizontal slices (A) covering an area of 5.3 x
5.3 mm and corresponding vertical cross sections (B) of a piece of raw freeze dried (-150°C) carrot tissue stored at 100% RH
and 22°C for 0, 2⅓ and 14 hours (from left to right).
These results are already very encouraging, but still much detail of the pore walls is lost. At higher
magnification (10x) the pixel size can be as small as 0.56 µm (Figure 8), which boosts the resolution of the
images significantly. Also the high contrast allows reliable segmentation, so the images can be further studied
using image analysis techniques. The data showed that connectivity and porosity decreases over time (Figure 9).
Fig. 8 Synchrotron-µCT images (10x magnification with pixel size is 0.56 µm) of horizontal cross sections of a piece of raw
freeze dried (-150°C) carrot tissue. Image size is 1.15mm x 1.15mm (top) and 0.25mm x 0.25mm (bottom). Storage was at
100% RH and 22°C for 0, 2⅓ and 14 hours (from left to right).
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Fig. 9 3D render impressions (top) of pore walls created from Synchrotron-µCT images (10x magnification with pixel size is
0.56 µm), and corresponding connected pores (middle). The graphs (bottom) represent the distribution of connected
segments. The number of segments increases over time due to swelling of the pore walls, and hence, porosity also decreases.
Carrot sample stored at 100% RH and 22°C for 0, 2⅓ and 14 hours (from left to right).
4
Conclusions
The diffusion of water vapour into the carrots is slow. It takes about 15 days to reach equilibrium. The
moisture content at equilibrium ranges from 81-84 % (wet basis). The water uptake results in shrinkage of the
samples. No significant differences were observed between different drying treatments. Images of wetted
samples revealed the presence of non-hydrated pores.
Micro-CT can be used to visualise and analyse microstructural changes during water sorption of freeze
dried carrot. Morphological changes caused by water sorption can be examined by visually comparing micro-CT
images taken at different time intervals. Ideally, these images should be in exact the same spatial location. For
Images having pixel sizes around 3µm (representing a sample size of approximately 5-6 mm in diameter) this is
achievable by marking the sample and carefully positioning it during subsequent scans. Exact aligning is then
done with affine registration. Smaller pixel sizes make it hard, if not impossible to scan at exactly the same
position during subsequent scans. The quality the high resolution images (pixel size 0.56 µm), however, is
significantly better. The phase contrast and low noise results in good image analysis results, which did not work
so well on the lower resolution images.
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Acknowledgements
We like to thank the following people for their support; Elodie Boller from ESRF France, for her support
during our experiments at ESRF; John van Duynhoven, Seddik Khalloufi, Jaap Nijsse and Adrian Voda from
Unilever Research, which all participated in the project; Arno Duijster from TU Delft University, who prepared
novel image analysis techniques; Ruud van der Sman from the University of Wageningen for deriving sorption
models; Henk van As, also from the University of Wageningen, responsible for NMR/MRI experiments.
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