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Three-dimensional microscopic MRI of maize kernels during drying
J. agric. Engng Res. (1992) 53, 51-69
Three-dimensional
Microscopic MRI of Maize Kernels during Drying
H. P. SONG;* J. B. LITCHFIELD;*
* Departmentof AgriculturalEngineering,
University
H. D. MORRIS~
of Illinois,
Urbana,
IL
61801,
USA
I’ BiomedicalMagneticResonance Laboratory,Universityof Illinois,Urbana,IL 61801,USA (Received
17 February
1991; accepted
in revised
form
7 March
1992)
Three-dimensional(3D), transient moisture transfer in solid food particles during drying is important information for the evaluation of existing drying theories and the optimization of the drying process.Such information has not yet been obtained. Microscopic magnetic resonanceimaging (MRI) was usedto non-destructively measurethe transient moisture transfer in individual maize kernels for two drying conditions. A 3D Fourier transform technique was applied to collect the MRI data. Moisture transfer was analysed from sequential 3D magnetic resonance(MR) imagesof maize kernels obtained during drying. There were two primary routes for moisture transfer in a maize kernel during drying, through the glandular layer of the scutellum and through the pericarp. 1. Introduction
1.1. Problem identification Drying is one of the most important processes in food production and preservation. The moisture transfer and associated structural changes during drying processes are of
fundamental importance in both process control and quality control. There are two primary methods to determine moisture profiles in food materials: (a) destructive, and (b) non-destructive. Destructive measurement methods for obtaining moisture profiles are inherently inaccurate (Song and Litchfield’). Non-destructive experimental techniques have been used in obtaining moisture profiles in some food materials. Chiang and Petersen’ measured two-dimensional (2D) moisture profiles with a gamma ray attenuation technique during drying of an apple. The linear resolution of their result was 2000 ,um. Perez et a1.3 applied MRI for non-destructive measurements of one-dimensional (1D) moisture profiles during the drying of an apple. They obtained a linear resolution of 1000 ,um. Two-dimensional and 3D moisture transfer in an ear of maize during drying has been measured by MRI with a volume element (voxel) size of 234 x 234 x 1100 pm (Song and Litchfieldls4). From these studies, MRI proved to be a powerful tool for 3D, non-destructive, high resolution measurements of moisture transfer. However, the resolution used in previous investigations was not high enough for rigorous study of mass transfer within small food particles during drying. Such information is valuable for the evaluation of existing drying theories and the optimization of the drying process. The aim of this study was to investigate 3D moisture transfer in individual yellow-dent maize kernels during drying at two temperatures. 51
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Notation
G N(H) S t TE TR
T
magnetic field gradient, Tcm-’ proton density signal intensity time, h echo time, ms repetition time, ms
spin-lattice
relaxation time, s relaxation time, s effective spin-spin relaxation time, s magnetogryic ratio spatial resolution, pm
T2 spin-spin
I; y 6x
1.2. MRI techniques 1.2.1. Basic principles of MRI Certain atomic nuclei, such as the hydrogen nucleus, possess a property known as nuclear spin. If a strong static magnetic field (B,) is applied to a sample containing hydrogen nuclei, the magnetic field lifts the degeneracy of the nuclear spin energy levels, and the spins precess about the magnetic field at the Larmour frequency (w) given by: w = yBO
(1)
where y is the magnetogyric ratio and BO is the magnetic field strength. When an oscillating linear electromagnetic field, B,, is applied to the sample, and its frequency exactly matches the frequency given by the energy difference between the nuclear spin energy levels, a resonance occurs. A plot of total spin intensity versus frequency, but no spatial information, can be obtained. This is the basic principle of conventional nuclear magnetic resonance spectroscopy (NMR). Using the Larmor equation, spatial information can be obtained by imposing a known magnetic field gradient on the sample so that different regions in space experience slightly different magnetic fields.5 This causes them to resonate at different frequencies (Fig. 1). For example, in the y-direction 4~) = r(Bo + GyY) (2) The total field strength at a point Y is the sum of the static field, Bo, and the gradient field, GyY. Where Gy is the magnetic field gradient in the y-direction and Y is the spatial coordinate. Each point is associated with a unique field strength. Hence, position is encoded as frequency of precession. A radio frequency (RF) pulse is applied which will excite only the protons in its frequency spectrum. By applying an RF pulse with very narrow frequency range to the sample, NMR signals can be obtained from a thin slice at which the field strength is resonant with this frequency range. The thickness of the slice depends upon the strength of the magnetic field gradient in the perpendicular direction of the slice and the bandwidth of the RF pulse (Fig. 2). The slice thickness can be reduced by increasing the gradient strength. By successively changing the RF pulse frequency range, continued NMR signals can be collected from sequential slices. Then a 1D NMR image can be resolved from these signals by using one of several image-reconstruction techniques. This is the basic principle of MRI. Two-dimensional or 3D MR images can be obtained by simultaneously applying magnetic field gradients in two or three directions. 1.2.2. Three-dimensional MRI techniques Three MRI techniques, the three-dimensional dimensional projection reconstruction (3DPR),
Fourier transform (3DFT), the threeand multislice two-dimensional Fourier
H.
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AL
(a)0
0
-#
Fourier
transform h
-\ Y gradient.
(h) 0
Cs = 0
0
Frequency
Fourier
transfom
A-A
ALinear
Y gradient,
Frequency
Cy > 0
Fig. 1. Schematic diagram illustrating the basic principles of (A) absence of a field gradient, (G, = 0), the water samples experience resultant NMR spectrum consists of only one resonance peak; (b) gradient, (G, > 0), the samples no longer experience the same field, consists of two resonance peaks
NMR, and (B) MRI. (a) In the the same magnetic field, and the In the presence of a linear field and the resultant NMR spectrum
transform (2DFT), are widely used for 3D images. With the 3DFT techniques, a sequence of three switched magnetic field gradients is applied systematically. Three orthogonal gradients, two for phase encoding (phase-encoding gradients) and one for MR data collection (read out gradient), are applied in three orthogonal directions, respectively. In each imaging cycle, a transverse magnetization is created by means of an RF pulse (F&.3). The two phase encoding gradients are then applied to encode spatial information in two directions. Then a spin-echo is observed as the read out gradient is applied. The signals are stored and Fourier-transformed in three dimensions.
y coordinate
of slice
Fig. 2. Determinants of slice thickness and effect of magnetic gradient strength. At a given bandwidth Ao of the RF frequency, a strong magnetic gradient (steep line a) selects a thin slice of width AY.. At lower gradient strength (line b), the width of the selected slice AY, increases
54
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MAIZE
KERNELS
I
G read OUI
MR
OF
signal
T, 12
T,/2 Acquisition
4
TR
k-+i
I
.
Fig. 3. Timing diagram of the three-dimensional Fourier transform imaging sequence. Where 90” and 180” represent 90degree and 180degree RF pluses. G,, and G,, are programmable phase-encoding magnetic field gradients, and Grcad (,U, b a frequency encoding magnetic field gradient applied in the read out direction. The three gradients are applied in the three orthogonal directions. The sequence starts with the 9Odegree pulse, then the process is followed by G,,, and G,, in two directions. Also during this period, the gradient Gread ,,,,[ is applied along the read out direction. This gradient has the purpose of compensating for the phase imparted by the read out gradient during MR signal acquirition. After the applications of the aboue gradients, the 180degree pulse is applied, generating the spin-echo sample in the data acquisition period. During this sampling period, the read out gradient is applied, causing the spins to be frequency encoded along the read out axis.
1.2.3. Relaxation times and their effects on MRI signal intensity Application of an RF pulse creates magnetization in a sample. After releasing the RF pulse applied to the sample, information is obtained about the system by monitoring the rate of return of magnetization to the original equilibrium state. The spin-lattice relaxation time (T,) and the spin-spin (transverse) relaxation time (T’) are used to describe the time required for the effects of a perturbation of an RF pulse to decay to the equilibrium state. The spin-lattice relaxation time is a measure of the time required for the sample to return to its original state by releasing the absorbed RF energy to the environment or lattice. The spin-spin relaxation time reflects the rate of similar nuclear moments returning to their original random orientations. 1.2.4. Pulse sequence A series of RF pulses are often used to acquire desired results. A pulse sequence usually consists of two or more pulses with designed time intervals between consecutive pulses. The sequence is repeated once for each voxel during image data acquisition. For example, the widely used Hahn spin-echo pulse sequence is expressed as 90”t lSO”r, where 90” and 180” represent 90 and 180 degree rectangular RF pulses, and r is the time interval between the pulses. The MR signal (an echo) is recorded at the end of the sequence. 1.2.5. Contrast of the MR image The contrast in MR images arises from two main sources. One is the amount of water (proton density) present in a particular sample. The other source of contrast is related to
H.
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55
the relaxation times, T, and T,, of the water nuclei. For the Hahn spin-echo sequence, the MR signal intensity, S, from a voxe17 is (3) where N(H) is the proton density, TE is echo time, the time between the 90” RF pulse and the formation of the spin echo, TE = 2t. TR is the sequence repetition time, the total sequence performing time. TE and TR are specified by the user. If TR >> TE (the situation used in this study), Eqn (3) can be simplified as s = N(H)[l _ e-w-l] e-wT’ (4) According to Eqn (4), S is affected by three factors, (1) proton density N(H), (2) T, and, (3) T2. By varying the two user-selectable delay times, the echo time (TE) and the sequence repetition time ( TR), the spin-echo sequence can be used to highlight T, , T,, or proton density effects. For example, if TR/T, > 5 and TE/TZ+O, both expressions 1 - exp( - T,/T,) and exp(- TE/TZ), are near unity. The proton density images can be obtained. If T,/T,+= 0 but T,/T, < 5, then signal intensity is contributed by both proton density and the factor 1 - eXp( - TR/Ti) and T, weighted images are obtained. Since the collection of proton density images requires a long time ( TR must be at least five times T,), which is usually too long in practice, and T,/T, cannot be set to near zero for short TZ materials such as a maize kernel (Tz =4ms at 36% m.c.w.b), both T, and TZ weighted images are often acquired. 1.2.6. Spatial resoltuion and magnetic field gradient The spatial resolution of an image is determined by the size of the voxel. The size of each individual voxel is determined by three factors, field of view (FOV), matrix size, and slice thickness (Fig. 4). Each of these is selected by the operator before an MRI experiment. Spatial resolution can be improved by reducing voxel size which needs a
I
I Slice
I
I Matrix (64, 128.256
size
d=
FOV Matrix
size
voxels)
Fig. 4. Factors that determine voxel size. In plane resolution of view (FOV) and matrix size. In this example, pixel
of the field
thickness
is determined by the x and y dimensions d is the horizontal dimension of a single
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small FOV, a large matrix size, and a large magnetic field gradient (for thin slice thickness). In order for an imaging system to achieve the required spatial resolution, it is necessary that the separation of each picture element (pixel) in the frequency domain dominate the line and spectral widths of the material contributing to that pixel. This, therefore, imposes on the magnetic field gradient, G, the condition7 that:
where 6~ is the required degree of resolution, y is the magnetogyric ratio, and Tz is the effective spin-spin (transverse) relaxation time (which is always shorter than the true T2). It is generally undesirable to increase the gradient strength more than is necessary to achieve the required resolution as this spreads the signal information over a wider bandwidth and degrades the signal-to-noise ratio (SNR) in the final image. The final resolution of the image in a direction is determined by either FOV/matrix size or by Eqn (5), whichever is larger. 1.2.7. Signal-to-noise ratio (SNR) The MR image is a map of RF signals emitted by the sample. The brightness of each image voxel is proportional to the intensity of the RF signal emitted by the corresponding sample voxel. Random RF emissions from samples produce variations in voxel signal intensity or “noise” is also visible in the image. The noise is generally undesirable and reduces the visibility of low-contrast structures. In order to obtain a high signal strength in relation to a specific noise level, a certain minimum voxel size must be used. This limits spatial resolution. 1.3. Maize structure Fig. 5 shows the structure of a typical yellow-dent maize kernel (Brooker er al.*). It includes a germ, an endosperm, and a pericarp. The germ is composed of a plumule, a radicle and a scutellum. The scutellum, which functions as a nutritive organ for the plumule, makes up lo-12% of the kernel dry weight. The endosperm constitutes 82-84% of the kernel dry weight and is 86-89% starch by weight (Earle et af.‘). Endosperm cells are filled mainly with starch. Starchy endosperm is of two types, floury and vitreous. The opacity of floury endosperm is due to light refraction from minute air pockets around starch granules, which result from the tearing of the thin protein matrix as it shrinks during drying. In the vitreous endosperm, the protein matrix is thicker and remains intact during drying. During drying, the plastic starch granules in the vitreous endosperm are compressed into polyhedral shapes. Endosperm cells become progressively smaller from the central endosperm to the outer endosperm. The pericarp (seed coat) is the transformed ovary wall, which covers the kernel and furnishes protection for the interior parts. The pericarp is composed of several layers. One layer, called the cross-cell layer, is very loose and open, and has a great deal of intercellular space. These pits and open areas in the cross-cell layer provide capillary interconnections between all cells (Watson”). The pericarp extends to the tip cap. Inside the tip cap are spongy cells connected only by the ends of the branches, thus forming an open structure continuous with the cross-cell layer (Wolf et al.“).
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ET AL Hull
Pericarp Mesocarp Cross cells Tube cells Seed coat (test) Aleurone layer Vitreous endosperm Floury endosperm Cells filled with starch go in protein matrix Walls of cells Scutellum Plumule or rudimentaT shoot & leaves Glandular layer of scutellum
\
Radicle
or primary
root
Tip cap
Fig. 5. Structure of a yellow-dent maize kernel (Brooker
et al.“)
2. Materials and methods 2.1. Sample preparation
A yellow-dent maize hybrid (FR27 x Mo17) was used for the drying tests. The maize kernels were hand-shelled from the central portion of an ear. A single regular-shaped kernel was selected for each drying experiment. The initial average moisture content of the hand-shelled kernels was 36% (w.b.) as determined by the air oven method.” 2.2. Equipment
A 4.7 Tesla MRI instrument (Spectroscopy Imaging System, Co., USA) with a 12 mm MRI probe (Doty Scientific, Co., USA) was used for data collection. The RF coil region in the probe was 17 mm long. The longitudinal axis of the probe was defined as the z-direction (Fig. 6). The maize kernel samples were suspended in the centre of the probe during experiments with loops of nylon thread. A custom designed drying apparatus was used to supply conditioned drying air. 2.3. Drying experiments
Two drying experiments were conducted. The first one was drying at 27°C for 3 h and then cooling at 20°C for 1 h. The second experiment was drying at 49°C for 1 h and then cooling at 20°C for 1 h. The same drying temperatures had been used for previous thin-layer drying experiments (Li and Morey13). The drying and cooling periods were selected according to drying speed of each drying process and to ensure that clear images could be obtained at the end of the drying and cooling experiments. The air velocity was 5 m/s for both experiments. This velocity was found to be high enough to ensure that the internal, not external, resistance to mass transfer was limiting (Litchfield and Okos14).
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hemcl
Fig. 6. Schematic diagram
of the MRI probe and the system co-ordinates
The relative humidity of the air was 36% for the 27°C experiment (54% for 20°C cooling) and 11% for the 49°C experiment (54% at 20°C). The effect of kernel orientation on the overall drying rate was tested at 49°C. The overall drying rates of a kernel with tip cap up and with tip cap down were measured every 0.5 h during 2 h of drying. There was no significant difference in drying rate between the two kernels. For both experiments, the samples were held vertically in the RF coil region with the tip down so air flowed from the crown of the maize to the tip. Image data were collected at 0.25 h intervals during drying. This time interval was found to be appropriate to observe moisture transfer changes through consecutive images within 49°C drying. 2.4.
The MRI
parameters
The 3DFI technique with the Hahn spin-echo pulse sequence was used to acquire image data. The MR experiment parameters for the drying experiments were selected based on three criteria. The first criteria is image acquisition time. Because drying is a dynamic process, the allowable acquisition time for each image cannot be too long. Five minutes (O-08 h) was set as the limitation for acquiring one set of 3D data. The second criteria is to obtain a sufficient SNR to enhance the moisture transfer information during drying. For this purpose, the shortest possible echo time (TE) and the longest repetition time (TR) were chosen. The third criteria is to achieve a high spatial resolution (small voxel size) to investigate the detailed moisture transfer inside the maize kernels. This requires more data points, smaller FOV, and higher magnetic field gradients. To achieve a higher SNR with a fixed number of data points, however, a longer acquisition time is required which conflicts with the short acquisition time criteria. Also, the smaller the voxel size and the shorter the repetition time, TR, the lower the SNR. Based upon the above criteria, the echo time, TE, was set as short as 8 ms (limited by the switching time of the gradient amplifier) and the repetition time, T,, was set as long as 73 ms to obtain highest possible SNR. The read out gradient, GY, was set to 7.5 x 10e4T/cm. The phase encoding gradients G, and G, were set to 7.6 x 10P6Tcm-‘/step with 32 increments and 5.7 X lo-“Tcm-‘/step with 128 increments, respectively. These gradient settings resulted in a voxel size of 93 x 156 x 312 pm”.
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2.5.
Data analysis
For the Hahn spin-echo pulse sequence, the signal intensity of a voxel (S) was determined according to Eqn (4). For measured S, T, and T2 of the voxel, and experimental parameters TE and TR, the proton density N(H) of the voxel can be determined as
In practice, because of limitations in data acquisition time for each image, short TR and non-zero TE were set. Such settings resulted in T, and TZ weighted images. To obtain proton-density images which related to moisture content in the maize kernel, detail of how T, and T2 in each voxel varied with drying time is needed. This information, unfortunately, is impossible to obtain under the current experimental conditions. As an alternative, the T, and T2 of the whole maize kernel were measured during the 27°C and the 49°C drying and cooling experiments. The relationship of T, and TZ versus drying time was applied to Eqn (6) to estimate the change of proton density N(H) during drying. Then the moisture transfer in the maize kernel was analysed qualitatively. The moisture transfer in maize kernels was analysed from the moisture distribution changes during drying. The moisture transfer was examined in three orthogonal directions, back-to-front (germ side), top (crown)-to-tip cap, and side-to-side (Fig. 7). In each direction, a central 2D image slice was selected which shows the moisture changes in both the endosperm and the germ. The slices from the same position but from different drying times were displayed together. The sequential images present the moisture distribution in the 2D slice and show the moisture transfer in the 2D plane during the drying and cooling processes. Maize kernels contain approximately 4.5% oil, about 85% of which is located in the germ (Weber15). Since drying at 22 to 93°C does not change the oil content (Weller et al.“), the decrease of signal intensity in the kernel during drying was mainly caused by moisture loss. To examine moisture transfer and to eliminate the oil signal influence on moisture transfer, an image subtraction technique was used for the sequential 2D images during drying and cooling. The image at each drying time was subtracted from the initial image and the difference represented the cumulative moisture loss.
‘slice
Fig, 7. Three orthogonal
planes for moisture
transfer analysis in maize kerneLF
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3. Results
OF
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and discussion
3.1. Three-dimensional
images
The 3D MR images of a maize kernel (36% m.c, w.b.) can be inspected from various perspectives (Fig. 8). Sections of the image were removed from the three orthogonal planes to expose the inside detail. The brightness of the image is proportional to the signal intensity. The brighter the image, the stronger the signal. The image was T, and TZ weighted. The T, was 472 ms for the floury endosperm and 404 ms for the vitreous endosperm. The T2 was 4 ms for both the floury endosperm and the vitreous endosperm. With the TI and T2 values and the TR and TE settings, the factors [l/(1 - exp in Eqn (6) for the floury endosperm and the vitreous endo(-T,IT,)l[llexp(-TEIT,)l sperm, were 0.019 and O-022 respectively. Therefore, the MR signal (S) distribution in the images corresponded to the proton density [N(H)] distribution because the factor was nearly the same throughout the endosperm. [l/(1 - exp(-TR/T,)J[l/exp(-T,/T,)l Since there is a correlation between the proton density and moisture content (Song and
Fig. 8. Threedimensional proton density image top
of end
a FR27 view
x
Ma1 7 maize kernel; (a) tip end view (b)
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Litchfield4), the MR signal distribution represented the moisture distribution in the kernel. From the tip view of the kernel (Fig. 8a), a brighter image was observed in the germ than in the endosperm. Part of the reason is that the oil in the germ also contributed to the MR signal. From the top view of the same image (Fig. 8b), the MR signal was stronger in the tissue near the pericarp and in the endosperm close to the pericarp than in the central region of the kernel. This means that the moisture content in the above areas were higher than that of the central area of the kernel. 3.2. Experiments
at 27°C
The 3D MR data of the kernel before drying at 27°C is displayed as a series of 2D images in three orthogonal directions (Fig. 9). These images approximately represented the initial moisture distribution in the maize kernel. The brighter the image, the higher the moisture content. Fig. 9a shows the back-to-front (back-to-germ side) view of the kernel. The very bright triangle areas (slices 7 to 25) at the bottom of the images were the images of the germ. The white outline was the pericarp. The black area within the pericarp but outside the bright image area was the vitreous endosperm. The bright area between the vitreous endosperm and the germ was the image of the floury endosperm. The moisture content in the floury endosperm was nearly uniform at the centre, higher around the germ, and gradually decreased from the centre to the edge of the kernel (slices 2, and 6-16). There was a dark area in the centre of slices 3 to 7 caused by a cavity at that location which was confirmed by physically separating the kernel after the drying experiment. In the side-to-side view (Fig. 96), the very bright triangle-shaped areas at the right side on the bottom (slices 5-15) was the germ. The black zones at the left top and the left bottom (slices 5-15) and the wide vertical black slot at the left side (slices 3, 4, and 16-19) were the vitreous endosperm. The narrow black slot in slices 7 to 15 at the left centre of the image was a cavity. The white image between the vitreous endosperm and the germ was the floury endosperm. The moisture distribution in the floury endosperm was not uniform, but higher near the germ and lower near the vitreous endosperm. The moisture content in the pericarp was also high. In the top-to-tip cap view (Fig. 96c), a very bright spot on the right side was the image of the germ (slices 21-46). The left black vertical band (slices 3-17) and the triangular black areas at the top and the bottom (slices 18-43) were the images of the vitreous endosperm. The white area between the germ and the vitreous endosperm was the floury endosperm. The narrow vertical black slot at the left side centre in slices 20 to 32 was the cavity mentioned above. From all three views, moisture content was higher in the floury endosperm and was lower in the vitreous endosperm. The moisture content of the pericarp was also high, especially higher than the adjacent vitreous endosperm. There was a moisture gradient from the floury endosperm to the vitreous endosperm as well as in each type of endosperm at an equilibrium state. The spin-lattice and the spin-spin relaxation times of maize kernels during the 27-20°C and 49-20°C drying-cooling processes were measured. Figs 10 and 11 show the variation of 7” as a function of drying time during the 27-20°C and the 49-20°C processes. During the 27°C drying, T, was almost 460 ms during the 3 h of drying and decreased to 430 ms during the 1 h of cooling (Fig. IO). During the 1 h of drying at 49”C, T, changed from 560 to 535 ms (Fig. 11). During the following hour of cooling at 2o”C, T, reduced to 491 ms. The T2 of the kernel was 4 ms during drying at 27°C and was 5 ms during drying at 49°C.
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Fig. 9. The 3D proton density image of a FR27 x Ma1 7 kernel before drying at 27°C displayed series of 20 images; (a) back-to-front view (b) side-to-side view (c) top-to-tip cap view
as a
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I
0
2 Drying
Fig. 10. Spin-lattice
relaxation
4
3 time,
h
time of a FR27x Mo17 process
kernel
during
a 27-20°C
drying-cooling
According to the above results, the factor [l/(1 - exp(-T,/T,)][l/exp(-T,/T,)] in the Eqn (6) was almost the same during drying at 27°C and only changed from 0.0247 to 0.0257 during drying at 49°C. So the factor could be considered constant during 49°C drying. The changes of the MR signal intensity in the images were therefore caused by the proton density changes (moisture loss), so they are proportional to the moisture content changes. Moisture transfer in the maize kernel was investigated by examining the changes of moisture distribution of one central slice in each direction (Fig. 12). The central slice involved both the endosperm and the germ so that thorough moisture transfer information could be examined. The images were arranged according to the order of data collection. All images for each direction were scaled together, so the images acquired from different times were comparable. The slice numbers were the same as those in Fig. 9. The time interval between two sequential images was 0.5 h. During drying, two areas were subjected to quick moisture loss, the pericarp and the floury endosperm above the top of the germ. From the side-to-side view (Fig. I,&), the 600
: ;
550
c
500
-
4.50
-
400
-
-
300
-
250
-
200 0
0.25
0.5
0.75 Drying
Fig. 11. Spin-lattice
relaxation
Cooling
*
I I I I I I I I I
-
350
<
.Y I I
Drying
I
I 25 time.
time of a FR27x Mo17 process
I.5
I .75
2
h
kernel
during
a 49-20°C
drying-cooling
64
Fig. 12. Moisture distribution FR27xMo17 kernel during
changes of three orthogonal 20 drying at 270°C; (a) back-to-front top-to-tip cap view
images through the centre of a view (b) side-to-side view (c)
moisture content of the pericarp was higher than that of the adjacent endosperm before drying, but lower than the same area after drying for 1 h. The moisture content of the floury endosperm near the pericarp decreased quickly while the centre of the kernel was still wet. This indicated that the pericarp was a major moisture transfer route. As described by Watson,” the cross-cell layer underneath the pericarp is very loose and open, and has a great deal of intercellular space. This layer also extends to the tip cap connected through some spongy cells, thus forming an open structure. This may be the reason why this tissue holds more moisture before drying and transfers moisture during drying. The moisture content of the floury endosperm above the top of the germ was fairly high (appeared very bright) before drying. After 1 h of drying, however, a low moisture content zone (dark area) appeared in that area while the moisture content of the surrounding endosperm was still high. This indicated that the moisture loss from this area was not through the pericarp but through another route. This low moisture content area expanded quickly during the drying. The same observations were also made in the side-to-side view (Fig. 126). Fig. 13 shows the results of image subtraction of the slices in Fig. 12. The images show the location and the relative amount of moisture loss. For example, the second image in Fig. 13a shows that the pericarp, and area in the centre of the floury endosperm, and the area surrounding the germ lost moisture quickly (white pixels) after drying for 1 h. The
65
Fig. 13. Cumulative moisture decrease of three orthogonal FR27 x Mo17 kernel during drying at 27°C; (a) back-to-front cap view
20 images through the centre of a view (b) side-to-side view (c) top-to-tip
image also shows that moisture was lost through two major routes; (1) through the pericarp, and (2) through the glandular layer of the scutellum (the tissue between the germ and the floury endosperm) and the tip cap. The glandular layer route apparently is the path for moisture in the centre of the kernel transferring to the outside of the kernel. As drying continued, more moisture in the centre of the kernel was transferred by this route. The moisture in the endosperm near the pericarp, however, transferred through the pericarp. The side-to-side view images (Fig. 136) again show the two moisture transfer routes. The images clearly show that moisture transfer was not uniform in the kernel. Some areas in the centre of the kernel lost moisture quickly while adjacent areas near the surface lost moisture slowly. The moisture lost from the centre transferred through the glandular layer of the scutellum and the tip cap but not through the shortest path which was through the vitreous endosperm and the pericarp in the radial direction. This transfer phenomenon may be explained because the density of the vitreous endosperm is higher than that of the floury endosperm and the glandular layer of the scutellum and the tip cap is less resistant to moisture transfer than the vitreous endosperm. During drying, the moisture in the centre of the kernel transferred through the least resistant path. A similar description was advanced by Le Bras.” The information obtained from the top-to-tip cap view of the 3D. data supported this analysis (Fig. 13c).
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3.3.
Experiments
OF
MAIZE
KERNELS
at 49°C
The 3D image of a maize kernel (36% m.c., w.b.) before the 49°C drying is displayed as a series of 2D images in the three orthogonal planes (Fig. 14). The images demonstrate a similar moisture distribution in the kernel as for the kernel before the 27°C drying experiment (Fig. 9). Fig. 15 presents the moisture distribution changes in a central slice in each direction during drying and cooling processes. The time interval between two sequential images is
(c) Fig. 14. The 30 proton density image of a FR27 x Mel 7 kernel before drying at 49°C displayed view (b) side-to-side uiew (c) top-to-tip cap view series of 2D images; (a) back-to-front
as a
67
Cc) Fig. 15. Moisture FR27 x Mo17 20
distribution changes kernel during drying
of the orthogonal 20 images through the centre of a at 49°C; (a) back-to-front view (b) side-to-side view (c) top-to-tip cap view
0.25 h. The images showed that the moisture transfer was much faster than during the 27°C drying process, especially during the first 0.25 h of drying. With an identical shape and size maize kernel dried under the same drying-cooling conditions. It took 2 h at 27°C but 0.9 h at 49°C to dry the kernel from 36 to 26% m.c.w.b. Before drying, the moisture content in the floury endosperm was nearly uniform (slice t = 0). After 0.25 h of drying, however, the moisture content in the floury endosperm around the top of the germ was lower than the moisture content in the other area of the floury endosperm (Fig. 151, slice t = 0.25 h). The moisture content in this area continuously decreased during drying while the moisture content of its outer area (near the pericarp) was higher (Fig. Ha, slices f = 0.5 h to t = 1 h). At the same time, the moisture content in the endosperm near the surface also continuously decreased. The low moisture zone gradually moved to the interior of the kernel along the radial direction. This again demonstrated that the moisture in the centre and near the surface transferred through two routes, the glandular layer of the scutellum and the pericarp. The moisture distribution changes during drying in the side-to-side and in the top-to-tip cap views showed that moisture content at the back of the kernel decreased quickly (Figs 15b and c,
68
(b)
Cc) Fig. 16. Cumulative moisture decrease of three orthogonal FR27 x Mol7 kernel during drying at 49°C; (a) back-to-front cap view
20 images through the centre of a view (b) side-to-side view (c) top-to-tip
slice t = O-25 h). The moisture in the central area near the germ-also lost moisture quickly (Fig. 156, slice f = 0.75 h). The image subtraction results for the 49°C drying experiment (Fig. 16) shows some different phenomena as compared to the 27°C image subtraction results (Eg. 13). First, the moisture loss in the floury endosperm was much faster than that at 27°C during the first 0.25 h of drying. Second, the embryo also lost moisture from the beginning of the drying process (Figs 16a and c, slice t = 0.25 h, the white image at the centre of the germ). Finally, the germ also lost moisture (Fig. 16, slices t = 0.75 h and 1= 1 h). 4. Conclusions
Moisture was distributed non-uniformly in the maize kernel at an equilibrium state. There are two primary moisture transfer routes in maize kernels during drying, through the glandulary layer of the scutellum and through the pericarp. The moisture in the centre of the kernel transferred through the glandular layer of the scutellum and the tip cap, while the moisture in the endosperm near the surface of the kernel transferred through the pericarp. The floury endosperm lost moisture faster than the vitreous endosperm, and in the germ, the embryo lost moisture first. At a higher temperature (49°C) drying, the germ also lost moisture.
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Acknowledgement This study was funded by the National Science Foundation, NSF grant CBT-8808748. Appreciation is expressed to the Biomedical Magnetic Resonance Laboratory at the University of Illinois, Dr Paul Lauterbur, Director, for the use of its equipment and facilities. Thanks are also extended to Mr Clinton Potter, NCSA, for his help with the image processing. References ’ Song, H.; Litchfield, J. B. Nondestructive measurement of transient moisture profiles in ear corn during drying using NMR imaging. Transactions of the ASAE 1990, 33(4): 1286-1290. * Chiang, W. C.; Petersen, J. N. Experimental measurement of temperature and moisture profiles during apple drying. Drying Technology 1987, 5: 25-49 3 Perez, E.; Kauten, R.; McCarthy, M. J. Non-invasive measurement of moisture profiles during the drying of an apple. In: Drying ‘89, (A. S. Mujumdar, ed.) Washington, D.C.: Hemisphere Publishing Co., 1990 * Song, H.; Litchfield, J. B. Nuclear magnetic resonance imaging of transient three-dimensional moisture distribution in an ear of corn during drying. Cereal Chemistry 1990, 67: 580-584 ’ Lauterhur, P. C. Image formation by induced local interactions: examples employing nuclear magnetic resonance. Nature 1973, 242: 190-191 ’ Hahn, E. L. Spin echoes. Physics Review 1950, 80: 580-594 ’ Mansfield, P.; Morris, P. G. NMR imaging in biomedicine. Supplement 2: Advances in magnetic resonance. New York: Academic Press, 1982 ’ Brooker, D. B.; Bakker-Arkema, F. W.; Hall, C. W. Drying Cereal Grains. Westport, m. The AVI Publishing Company, Inc., 1974 a Earle, F. R.; Curtis, J. J.; Hubbard, J. E. Composition of the component parts of the corn kernel. Cereal Chemistry 1946, 23: 504-511 lo Watson, S. A. Structure and Composition. In: Corn Chemistry and Technology. Watson, S. A.; Ramstad, P. E., eds). St Paul, MN: American Association of Cereal Chemists, Inc., 1987, pp. 53-82 ” Wolf, M. J.; Buzan, C. L.; MaeMasters, M. M.; Rist, C. E. Structure of the mature corn kernel. II. Microscopic structure of pericarp, seed coat, and hilar layer of dent corn. Cereal Chemistry 1952, 29: 334-348 ” ASAE Standards S352.2. Moisture measurement-Unground grain and seeds. St Joseph, MI: American Society of Agricultural Engineers, 1989 ” Li, H.; Morey, R. V. Thin-layer drying of yellow dent corn. Transactions of the ASAE 1984, 27: 581-585 l4 Litchfield, J. B.; Okos, M. R. Mass transfer during the drying of extruded durum semolina. Journal of Food Engineering, 1991 (in press) l5 Weber, E. J. Lipids in corn, In: Corn Chemistry and Technology. (Watson, S. A.; Ramstad, P. E., eds). St Paul, MN: American Association of Cereal Chemists, Inc., 1987 ‘* Weller, C. L.; Paulsen, M. R.; Mbuvi, S. Germ weight, germ oil content, and estimated oil yield for wet-milled yellow dent corn as affected by moisture content at harvest and temperature of drying air. Cereal Chemistry 1989, 66: 273-275 ” Le Bras, A. Maize drying conditions and its resulting quality for wet-milling industry, In: Maize: Recent Progress in Chemistry and Technology. New York, NY: Academic Press, 1982
Three-dimensional
Microscopic MRI of Maize Kernels during Drying
H. P. SONG;* J. B. LITCHFIELD;*
* Departmentof AgriculturalEngineering,
University
H. D. MORRIS~
of Illinois,
Urbana,
IL
61801,
USA
I’ BiomedicalMagneticResonance Laboratory,Universityof Illinois,Urbana,IL 61801,USA (Received
17 February
1991; accepted
in revised
form
7 March
1992)
Three-dimensional(3D), transient moisture transfer in solid food particles during drying is important information for the evaluation of existing drying theories and the optimization of the drying process.Such information has not yet been obtained. Microscopic magnetic resonanceimaging (MRI) was usedto non-destructively measurethe transient moisture transfer in individual maize kernels for two drying conditions. A 3D Fourier transform technique was applied to collect the MRI data. Moisture transfer was analysed from sequential 3D magnetic resonance(MR) imagesof maize kernels obtained during drying. There were two primary routes for moisture transfer in a maize kernel during drying, through the glandular layer of the scutellum and through the pericarp. 1. Introduction
1.1. Problem identification Drying is one of the most important processes in food production and preservation. The moisture transfer and associated structural changes during drying processes are of
fundamental importance in both process control and quality control. There are two primary methods to determine moisture profiles in food materials: (a) destructive, and (b) non-destructive. Destructive measurement methods for obtaining moisture profiles are inherently inaccurate (Song and Litchfield’). Non-destructive experimental techniques have been used in obtaining moisture profiles in some food materials. Chiang and Petersen’ measured two-dimensional (2D) moisture profiles with a gamma ray attenuation technique during drying of an apple. The linear resolution of their result was 2000 ,um. Perez et a1.3 applied MRI for non-destructive measurements of one-dimensional (1D) moisture profiles during the drying of an apple. They obtained a linear resolution of 1000 ,um. Two-dimensional and 3D moisture transfer in an ear of maize during drying has been measured by MRI with a volume element (voxel) size of 234 x 234 x 1100 pm (Song and Litchfieldls4). From these studies, MRI proved to be a powerful tool for 3D, non-destructive, high resolution measurements of moisture transfer. However, the resolution used in previous investigations was not high enough for rigorous study of mass transfer within small food particles during drying. Such information is valuable for the evaluation of existing drying theories and the optimization of the drying process. The aim of this study was to investigate 3D moisture transfer in individual yellow-dent maize kernels during drying at two temperatures. 51
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Notation
G N(H) S t TE TR
T
magnetic field gradient, Tcm-’ proton density signal intensity time, h echo time, ms repetition time, ms
spin-lattice
relaxation time, s relaxation time, s effective spin-spin relaxation time, s magnetogryic ratio spatial resolution, pm
T2 spin-spin
I; y 6x
1.2. MRI techniques 1.2.1. Basic principles of MRI Certain atomic nuclei, such as the hydrogen nucleus, possess a property known as nuclear spin. If a strong static magnetic field (B,) is applied to a sample containing hydrogen nuclei, the magnetic field lifts the degeneracy of the nuclear spin energy levels, and the spins precess about the magnetic field at the Larmour frequency (w) given by: w = yBO
(1)
where y is the magnetogyric ratio and BO is the magnetic field strength. When an oscillating linear electromagnetic field, B,, is applied to the sample, and its frequency exactly matches the frequency given by the energy difference between the nuclear spin energy levels, a resonance occurs. A plot of total spin intensity versus frequency, but no spatial information, can be obtained. This is the basic principle of conventional nuclear magnetic resonance spectroscopy (NMR). Using the Larmor equation, spatial information can be obtained by imposing a known magnetic field gradient on the sample so that different regions in space experience slightly different magnetic fields.5 This causes them to resonate at different frequencies (Fig. 1). For example, in the y-direction 4~) = r(Bo + GyY) (2) The total field strength at a point Y is the sum of the static field, Bo, and the gradient field, GyY. Where Gy is the magnetic field gradient in the y-direction and Y is the spatial coordinate. Each point is associated with a unique field strength. Hence, position is encoded as frequency of precession. A radio frequency (RF) pulse is applied which will excite only the protons in its frequency spectrum. By applying an RF pulse with very narrow frequency range to the sample, NMR signals can be obtained from a thin slice at which the field strength is resonant with this frequency range. The thickness of the slice depends upon the strength of the magnetic field gradient in the perpendicular direction of the slice and the bandwidth of the RF pulse (Fig. 2). The slice thickness can be reduced by increasing the gradient strength. By successively changing the RF pulse frequency range, continued NMR signals can be collected from sequential slices. Then a 1D NMR image can be resolved from these signals by using one of several image-reconstruction techniques. This is the basic principle of MRI. Two-dimensional or 3D MR images can be obtained by simultaneously applying magnetic field gradients in two or three directions. 1.2.2. Three-dimensional MRI techniques Three MRI techniques, the three-dimensional dimensional projection reconstruction (3DPR),
Fourier transform (3DFT), the threeand multislice two-dimensional Fourier
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(a)0
0
-#
Fourier
transform h
-\ Y gradient.
(h) 0
Cs = 0
0
Frequency
Fourier
transfom
A-A
ALinear
Y gradient,
Frequency
Cy > 0
Fig. 1. Schematic diagram illustrating the basic principles of (A) absence of a field gradient, (G, = 0), the water samples experience resultant NMR spectrum consists of only one resonance peak; (b) gradient, (G, > 0), the samples no longer experience the same field, consists of two resonance peaks
NMR, and (B) MRI. (a) In the the same magnetic field, and the In the presence of a linear field and the resultant NMR spectrum
transform (2DFT), are widely used for 3D images. With the 3DFT techniques, a sequence of three switched magnetic field gradients is applied systematically. Three orthogonal gradients, two for phase encoding (phase-encoding gradients) and one for MR data collection (read out gradient), are applied in three orthogonal directions, respectively. In each imaging cycle, a transverse magnetization is created by means of an RF pulse (F&.3). The two phase encoding gradients are then applied to encode spatial information in two directions. Then a spin-echo is observed as the read out gradient is applied. The signals are stored and Fourier-transformed in three dimensions.
y coordinate
of slice
Fig. 2. Determinants of slice thickness and effect of magnetic gradient strength. At a given bandwidth Ao of the RF frequency, a strong magnetic gradient (steep line a) selects a thin slice of width AY.. At lower gradient strength (line b), the width of the selected slice AY, increases
54
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I
G read OUI
MR
OF
signal
T, 12
T,/2 Acquisition
4
TR
k-+i
I
.
Fig. 3. Timing diagram of the three-dimensional Fourier transform imaging sequence. Where 90” and 180” represent 90degree and 180degree RF pluses. G,, and G,, are programmable phase-encoding magnetic field gradients, and Grcad (,U, b a frequency encoding magnetic field gradient applied in the read out direction. The three gradients are applied in the three orthogonal directions. The sequence starts with the 9Odegree pulse, then the process is followed by G,,, and G,, in two directions. Also during this period, the gradient Gread ,,,,[ is applied along the read out direction. This gradient has the purpose of compensating for the phase imparted by the read out gradient during MR signal acquirition. After the applications of the aboue gradients, the 180degree pulse is applied, generating the spin-echo sample in the data acquisition period. During this sampling period, the read out gradient is applied, causing the spins to be frequency encoded along the read out axis.
1.2.3. Relaxation times and their effects on MRI signal intensity Application of an RF pulse creates magnetization in a sample. After releasing the RF pulse applied to the sample, information is obtained about the system by monitoring the rate of return of magnetization to the original equilibrium state. The spin-lattice relaxation time (T,) and the spin-spin (transverse) relaxation time (T’) are used to describe the time required for the effects of a perturbation of an RF pulse to decay to the equilibrium state. The spin-lattice relaxation time is a measure of the time required for the sample to return to its original state by releasing the absorbed RF energy to the environment or lattice. The spin-spin relaxation time reflects the rate of similar nuclear moments returning to their original random orientations. 1.2.4. Pulse sequence A series of RF pulses are often used to acquire desired results. A pulse sequence usually consists of two or more pulses with designed time intervals between consecutive pulses. The sequence is repeated once for each voxel during image data acquisition. For example, the widely used Hahn spin-echo pulse sequence is expressed as 90”t lSO”r, where 90” and 180” represent 90 and 180 degree rectangular RF pulses, and r is the time interval between the pulses. The MR signal (an echo) is recorded at the end of the sequence. 1.2.5. Contrast of the MR image The contrast in MR images arises from two main sources. One is the amount of water (proton density) present in a particular sample. The other source of contrast is related to
H.
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55
the relaxation times, T, and T,, of the water nuclei. For the Hahn spin-echo sequence, the MR signal intensity, S, from a voxe17 is (3) where N(H) is the proton density, TE is echo time, the time between the 90” RF pulse and the formation of the spin echo, TE = 2t. TR is the sequence repetition time, the total sequence performing time. TE and TR are specified by the user. If TR >> TE (the situation used in this study), Eqn (3) can be simplified as s = N(H)[l _ e-w-l] e-wT’ (4) According to Eqn (4), S is affected by three factors, (1) proton density N(H), (2) T, and, (3) T2. By varying the two user-selectable delay times, the echo time (TE) and the sequence repetition time ( TR), the spin-echo sequence can be used to highlight T, , T,, or proton density effects. For example, if TR/T, > 5 and TE/TZ+O, both expressions 1 - exp( - T,/T,) and exp(- TE/TZ), are near unity. The proton density images can be obtained. If T,/T,+= 0 but T,/T, < 5, then signal intensity is contributed by both proton density and the factor 1 - eXp( - TR/Ti) and T, weighted images are obtained. Since the collection of proton density images requires a long time ( TR must be at least five times T,), which is usually too long in practice, and T,/T, cannot be set to near zero for short TZ materials such as a maize kernel (Tz =4ms at 36% m.c.w.b), both T, and TZ weighted images are often acquired. 1.2.6. Spatial resoltuion and magnetic field gradient The spatial resolution of an image is determined by the size of the voxel. The size of each individual voxel is determined by three factors, field of view (FOV), matrix size, and slice thickness (Fig. 4). Each of these is selected by the operator before an MRI experiment. Spatial resolution can be improved by reducing voxel size which needs a
I
I Slice
I
I Matrix (64, 128.256
size
d=
FOV Matrix
size
voxels)
Fig. 4. Factors that determine voxel size. In plane resolution of view (FOV) and matrix size. In this example, pixel
of the field
thickness
is determined by the x and y dimensions d is the horizontal dimension of a single
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small FOV, a large matrix size, and a large magnetic field gradient (for thin slice thickness). In order for an imaging system to achieve the required spatial resolution, it is necessary that the separation of each picture element (pixel) in the frequency domain dominate the line and spectral widths of the material contributing to that pixel. This, therefore, imposes on the magnetic field gradient, G, the condition7 that:
where 6~ is the required degree of resolution, y is the magnetogyric ratio, and Tz is the effective spin-spin (transverse) relaxation time (which is always shorter than the true T2). It is generally undesirable to increase the gradient strength more than is necessary to achieve the required resolution as this spreads the signal information over a wider bandwidth and degrades the signal-to-noise ratio (SNR) in the final image. The final resolution of the image in a direction is determined by either FOV/matrix size or by Eqn (5), whichever is larger. 1.2.7. Signal-to-noise ratio (SNR) The MR image is a map of RF signals emitted by the sample. The brightness of each image voxel is proportional to the intensity of the RF signal emitted by the corresponding sample voxel. Random RF emissions from samples produce variations in voxel signal intensity or “noise” is also visible in the image. The noise is generally undesirable and reduces the visibility of low-contrast structures. In order to obtain a high signal strength in relation to a specific noise level, a certain minimum voxel size must be used. This limits spatial resolution. 1.3. Maize structure Fig. 5 shows the structure of a typical yellow-dent maize kernel (Brooker er al.*). It includes a germ, an endosperm, and a pericarp. The germ is composed of a plumule, a radicle and a scutellum. The scutellum, which functions as a nutritive organ for the plumule, makes up lo-12% of the kernel dry weight. The endosperm constitutes 82-84% of the kernel dry weight and is 86-89% starch by weight (Earle et af.‘). Endosperm cells are filled mainly with starch. Starchy endosperm is of two types, floury and vitreous. The opacity of floury endosperm is due to light refraction from minute air pockets around starch granules, which result from the tearing of the thin protein matrix as it shrinks during drying. In the vitreous endosperm, the protein matrix is thicker and remains intact during drying. During drying, the plastic starch granules in the vitreous endosperm are compressed into polyhedral shapes. Endosperm cells become progressively smaller from the central endosperm to the outer endosperm. The pericarp (seed coat) is the transformed ovary wall, which covers the kernel and furnishes protection for the interior parts. The pericarp is composed of several layers. One layer, called the cross-cell layer, is very loose and open, and has a great deal of intercellular space. These pits and open areas in the cross-cell layer provide capillary interconnections between all cells (Watson”). The pericarp extends to the tip cap. Inside the tip cap are spongy cells connected only by the ends of the branches, thus forming an open structure continuous with the cross-cell layer (Wolf et al.“).
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Pericarp Mesocarp Cross cells Tube cells Seed coat (test) Aleurone layer Vitreous endosperm Floury endosperm Cells filled with starch go in protein matrix Walls of cells Scutellum Plumule or rudimentaT shoot & leaves Glandular layer of scutellum
\
Radicle
or primary
root
Tip cap
Fig. 5. Structure of a yellow-dent maize kernel (Brooker
et al.“)
2. Materials and methods 2.1. Sample preparation
A yellow-dent maize hybrid (FR27 x Mo17) was used for the drying tests. The maize kernels were hand-shelled from the central portion of an ear. A single regular-shaped kernel was selected for each drying experiment. The initial average moisture content of the hand-shelled kernels was 36% (w.b.) as determined by the air oven method.” 2.2. Equipment
A 4.7 Tesla MRI instrument (Spectroscopy Imaging System, Co., USA) with a 12 mm MRI probe (Doty Scientific, Co., USA) was used for data collection. The RF coil region in the probe was 17 mm long. The longitudinal axis of the probe was defined as the z-direction (Fig. 6). The maize kernel samples were suspended in the centre of the probe during experiments with loops of nylon thread. A custom designed drying apparatus was used to supply conditioned drying air. 2.3. Drying experiments
Two drying experiments were conducted. The first one was drying at 27°C for 3 h and then cooling at 20°C for 1 h. The second experiment was drying at 49°C for 1 h and then cooling at 20°C for 1 h. The same drying temperatures had been used for previous thin-layer drying experiments (Li and Morey13). The drying and cooling periods were selected according to drying speed of each drying process and to ensure that clear images could be obtained at the end of the drying and cooling experiments. The air velocity was 5 m/s for both experiments. This velocity was found to be high enough to ensure that the internal, not external, resistance to mass transfer was limiting (Litchfield and Okos14).
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hemcl
Fig. 6. Schematic diagram
of the MRI probe and the system co-ordinates
The relative humidity of the air was 36% for the 27°C experiment (54% for 20°C cooling) and 11% for the 49°C experiment (54% at 20°C). The effect of kernel orientation on the overall drying rate was tested at 49°C. The overall drying rates of a kernel with tip cap up and with tip cap down were measured every 0.5 h during 2 h of drying. There was no significant difference in drying rate between the two kernels. For both experiments, the samples were held vertically in the RF coil region with the tip down so air flowed from the crown of the maize to the tip. Image data were collected at 0.25 h intervals during drying. This time interval was found to be appropriate to observe moisture transfer changes through consecutive images within 49°C drying. 2.4.
The MRI
parameters
The 3DFI technique with the Hahn spin-echo pulse sequence was used to acquire image data. The MR experiment parameters for the drying experiments were selected based on three criteria. The first criteria is image acquisition time. Because drying is a dynamic process, the allowable acquisition time for each image cannot be too long. Five minutes (O-08 h) was set as the limitation for acquiring one set of 3D data. The second criteria is to obtain a sufficient SNR to enhance the moisture transfer information during drying. For this purpose, the shortest possible echo time (TE) and the longest repetition time (TR) were chosen. The third criteria is to achieve a high spatial resolution (small voxel size) to investigate the detailed moisture transfer inside the maize kernels. This requires more data points, smaller FOV, and higher magnetic field gradients. To achieve a higher SNR with a fixed number of data points, however, a longer acquisition time is required which conflicts with the short acquisition time criteria. Also, the smaller the voxel size and the shorter the repetition time, TR, the lower the SNR. Based upon the above criteria, the echo time, TE, was set as short as 8 ms (limited by the switching time of the gradient amplifier) and the repetition time, T,, was set as long as 73 ms to obtain highest possible SNR. The read out gradient, GY, was set to 7.5 x 10e4T/cm. The phase encoding gradients G, and G, were set to 7.6 x 10P6Tcm-‘/step with 32 increments and 5.7 X lo-“Tcm-‘/step with 128 increments, respectively. These gradient settings resulted in a voxel size of 93 x 156 x 312 pm”.
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2.5.
Data analysis
For the Hahn spin-echo pulse sequence, the signal intensity of a voxel (S) was determined according to Eqn (4). For measured S, T, and T2 of the voxel, and experimental parameters TE and TR, the proton density N(H) of the voxel can be determined as
In practice, because of limitations in data acquisition time for each image, short TR and non-zero TE were set. Such settings resulted in T, and TZ weighted images. To obtain proton-density images which related to moisture content in the maize kernel, detail of how T, and T2 in each voxel varied with drying time is needed. This information, unfortunately, is impossible to obtain under the current experimental conditions. As an alternative, the T, and T2 of the whole maize kernel were measured during the 27°C and the 49°C drying and cooling experiments. The relationship of T, and TZ versus drying time was applied to Eqn (6) to estimate the change of proton density N(H) during drying. Then the moisture transfer in the maize kernel was analysed qualitatively. The moisture transfer in maize kernels was analysed from the moisture distribution changes during drying. The moisture transfer was examined in three orthogonal directions, back-to-front (germ side), top (crown)-to-tip cap, and side-to-side (Fig. 7). In each direction, a central 2D image slice was selected which shows the moisture changes in both the endosperm and the germ. The slices from the same position but from different drying times were displayed together. The sequential images present the moisture distribution in the 2D slice and show the moisture transfer in the 2D plane during the drying and cooling processes. Maize kernels contain approximately 4.5% oil, about 85% of which is located in the germ (Weber15). Since drying at 22 to 93°C does not change the oil content (Weller et al.“), the decrease of signal intensity in the kernel during drying was mainly caused by moisture loss. To examine moisture transfer and to eliminate the oil signal influence on moisture transfer, an image subtraction technique was used for the sequential 2D images during drying and cooling. The image at each drying time was subtracted from the initial image and the difference represented the cumulative moisture loss.
‘slice
Fig, 7. Three orthogonal
planes for moisture
transfer analysis in maize kerneLF
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3. Results
OF
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and discussion
3.1. Three-dimensional
images
The 3D MR images of a maize kernel (36% m.c, w.b.) can be inspected from various perspectives (Fig. 8). Sections of the image were removed from the three orthogonal planes to expose the inside detail. The brightness of the image is proportional to the signal intensity. The brighter the image, the stronger the signal. The image was T, and TZ weighted. The T, was 472 ms for the floury endosperm and 404 ms for the vitreous endosperm. The T2 was 4 ms for both the floury endosperm and the vitreous endosperm. With the TI and T2 values and the TR and TE settings, the factors [l/(1 - exp in Eqn (6) for the floury endosperm and the vitreous endo(-T,IT,)l[llexp(-TEIT,)l sperm, were 0.019 and O-022 respectively. Therefore, the MR signal (S) distribution in the images corresponded to the proton density [N(H)] distribution because the factor was nearly the same throughout the endosperm. [l/(1 - exp(-TR/T,)J[l/exp(-T,/T,)l Since there is a correlation between the proton density and moisture content (Song and
Fig. 8. Threedimensional proton density image top
of end
a FR27 view
x
Ma1 7 maize kernel; (a) tip end view (b)
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Litchfield4), the MR signal distribution represented the moisture distribution in the kernel. From the tip view of the kernel (Fig. 8a), a brighter image was observed in the germ than in the endosperm. Part of the reason is that the oil in the germ also contributed to the MR signal. From the top view of the same image (Fig. 8b), the MR signal was stronger in the tissue near the pericarp and in the endosperm close to the pericarp than in the central region of the kernel. This means that the moisture content in the above areas were higher than that of the central area of the kernel. 3.2. Experiments
at 27°C
The 3D MR data of the kernel before drying at 27°C is displayed as a series of 2D images in three orthogonal directions (Fig. 9). These images approximately represented the initial moisture distribution in the maize kernel. The brighter the image, the higher the moisture content. Fig. 9a shows the back-to-front (back-to-germ side) view of the kernel. The very bright triangle areas (slices 7 to 25) at the bottom of the images were the images of the germ. The white outline was the pericarp. The black area within the pericarp but outside the bright image area was the vitreous endosperm. The bright area between the vitreous endosperm and the germ was the image of the floury endosperm. The moisture content in the floury endosperm was nearly uniform at the centre, higher around the germ, and gradually decreased from the centre to the edge of the kernel (slices 2, and 6-16). There was a dark area in the centre of slices 3 to 7 caused by a cavity at that location which was confirmed by physically separating the kernel after the drying experiment. In the side-to-side view (Fig. 96), the very bright triangle-shaped areas at the right side on the bottom (slices 5-15) was the germ. The black zones at the left top and the left bottom (slices 5-15) and the wide vertical black slot at the left side (slices 3, 4, and 16-19) were the vitreous endosperm. The narrow black slot in slices 7 to 15 at the left centre of the image was a cavity. The white image between the vitreous endosperm and the germ was the floury endosperm. The moisture distribution in the floury endosperm was not uniform, but higher near the germ and lower near the vitreous endosperm. The moisture content in the pericarp was also high. In the top-to-tip cap view (Fig. 96c), a very bright spot on the right side was the image of the germ (slices 21-46). The left black vertical band (slices 3-17) and the triangular black areas at the top and the bottom (slices 18-43) were the images of the vitreous endosperm. The white area between the germ and the vitreous endosperm was the floury endosperm. The narrow vertical black slot at the left side centre in slices 20 to 32 was the cavity mentioned above. From all three views, moisture content was higher in the floury endosperm and was lower in the vitreous endosperm. The moisture content of the pericarp was also high, especially higher than the adjacent vitreous endosperm. There was a moisture gradient from the floury endosperm to the vitreous endosperm as well as in each type of endosperm at an equilibrium state. The spin-lattice and the spin-spin relaxation times of maize kernels during the 27-20°C and 49-20°C drying-cooling processes were measured. Figs 10 and 11 show the variation of 7” as a function of drying time during the 27-20°C and the 49-20°C processes. During the 27°C drying, T, was almost 460 ms during the 3 h of drying and decreased to 430 ms during the 1 h of cooling (Fig. IO). During the 1 h of drying at 49”C, T, changed from 560 to 535 ms (Fig. 11). During the following hour of cooling at 2o”C, T, reduced to 491 ms. The T2 of the kernel was 4 ms during drying at 27°C and was 5 ms during drying at 49°C.
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KERNELS
Fig. 9. The 3D proton density image of a FR27 x Ma1 7 kernel before drying at 27°C displayed series of 20 images; (a) back-to-front view (b) side-to-side view (c) top-to-tip cap view
as a
H.
P. SONG
ET
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AL.
I
0
2 Drying
Fig. 10. Spin-lattice
relaxation
4
3 time,
h
time of a FR27x Mo17 process
kernel
during
a 27-20°C
drying-cooling
According to the above results, the factor [l/(1 - exp(-T,/T,)][l/exp(-T,/T,)] in the Eqn (6) was almost the same during drying at 27°C and only changed from 0.0247 to 0.0257 during drying at 49°C. So the factor could be considered constant during 49°C drying. The changes of the MR signal intensity in the images were therefore caused by the proton density changes (moisture loss), so they are proportional to the moisture content changes. Moisture transfer in the maize kernel was investigated by examining the changes of moisture distribution of one central slice in each direction (Fig. 12). The central slice involved both the endosperm and the germ so that thorough moisture transfer information could be examined. The images were arranged according to the order of data collection. All images for each direction were scaled together, so the images acquired from different times were comparable. The slice numbers were the same as those in Fig. 9. The time interval between two sequential images was 0.5 h. During drying, two areas were subjected to quick moisture loss, the pericarp and the floury endosperm above the top of the germ. From the side-to-side view (Fig. I,&), the 600
: ;
550
c
500
-
4.50
-
400
-
-
300
-
250
-
200 0
0.25
0.5
0.75 Drying
Fig. 11. Spin-lattice
relaxation
Cooling
*
I I I I I I I I I
-
350
<
.Y I I
Drying
I
I 25 time.
time of a FR27x Mo17 process
I.5
I .75
2
h
kernel
during
a 49-20°C
drying-cooling
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Fig. 12. Moisture distribution FR27xMo17 kernel during
changes of three orthogonal 20 drying at 270°C; (a) back-to-front top-to-tip cap view
images through the centre of a view (b) side-to-side view (c)
moisture content of the pericarp was higher than that of the adjacent endosperm before drying, but lower than the same area after drying for 1 h. The moisture content of the floury endosperm near the pericarp decreased quickly while the centre of the kernel was still wet. This indicated that the pericarp was a major moisture transfer route. As described by Watson,” the cross-cell layer underneath the pericarp is very loose and open, and has a great deal of intercellular space. This layer also extends to the tip cap connected through some spongy cells, thus forming an open structure. This may be the reason why this tissue holds more moisture before drying and transfers moisture during drying. The moisture content of the floury endosperm above the top of the germ was fairly high (appeared very bright) before drying. After 1 h of drying, however, a low moisture content zone (dark area) appeared in that area while the moisture content of the surrounding endosperm was still high. This indicated that the moisture loss from this area was not through the pericarp but through another route. This low moisture content area expanded quickly during the drying. The same observations were also made in the side-to-side view (Fig. 126). Fig. 13 shows the results of image subtraction of the slices in Fig. 12. The images show the location and the relative amount of moisture loss. For example, the second image in Fig. 13a shows that the pericarp, and area in the centre of the floury endosperm, and the area surrounding the germ lost moisture quickly (white pixels) after drying for 1 h. The
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Fig. 13. Cumulative moisture decrease of three orthogonal FR27 x Mo17 kernel during drying at 27°C; (a) back-to-front cap view
20 images through the centre of a view (b) side-to-side view (c) top-to-tip
image also shows that moisture was lost through two major routes; (1) through the pericarp, and (2) through the glandular layer of the scutellum (the tissue between the germ and the floury endosperm) and the tip cap. The glandular layer route apparently is the path for moisture in the centre of the kernel transferring to the outside of the kernel. As drying continued, more moisture in the centre of the kernel was transferred by this route. The moisture in the endosperm near the pericarp, however, transferred through the pericarp. The side-to-side view images (Fig. 136) again show the two moisture transfer routes. The images clearly show that moisture transfer was not uniform in the kernel. Some areas in the centre of the kernel lost moisture quickly while adjacent areas near the surface lost moisture slowly. The moisture lost from the centre transferred through the glandular layer of the scutellum and the tip cap but not through the shortest path which was through the vitreous endosperm and the pericarp in the radial direction. This transfer phenomenon may be explained because the density of the vitreous endosperm is higher than that of the floury endosperm and the glandular layer of the scutellum and the tip cap is less resistant to moisture transfer than the vitreous endosperm. During drying, the moisture in the centre of the kernel transferred through the least resistant path. A similar description was advanced by Le Bras.” The information obtained from the top-to-tip cap view of the 3D. data supported this analysis (Fig. 13c).
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MRI
3.3.
Experiments
OF
MAIZE
KERNELS
at 49°C
The 3D image of a maize kernel (36% m.c., w.b.) before the 49°C drying is displayed as a series of 2D images in the three orthogonal planes (Fig. 14). The images demonstrate a similar moisture distribution in the kernel as for the kernel before the 27°C drying experiment (Fig. 9). Fig. 15 presents the moisture distribution changes in a central slice in each direction during drying and cooling processes. The time interval between two sequential images is
(c) Fig. 14. The 30 proton density image of a FR27 x Mel 7 kernel before drying at 49°C displayed view (b) side-to-side uiew (c) top-to-tip cap view series of 2D images; (a) back-to-front
as a
67
Cc) Fig. 15. Moisture FR27 x Mo17 20
distribution changes kernel during drying
of the orthogonal 20 images through the centre of a at 49°C; (a) back-to-front view (b) side-to-side view (c) top-to-tip cap view
0.25 h. The images showed that the moisture transfer was much faster than during the 27°C drying process, especially during the first 0.25 h of drying. With an identical shape and size maize kernel dried under the same drying-cooling conditions. It took 2 h at 27°C but 0.9 h at 49°C to dry the kernel from 36 to 26% m.c.w.b. Before drying, the moisture content in the floury endosperm was nearly uniform (slice t = 0). After 0.25 h of drying, however, the moisture content in the floury endosperm around the top of the germ was lower than the moisture content in the other area of the floury endosperm (Fig. 151, slice t = 0.25 h). The moisture content in this area continuously decreased during drying while the moisture content of its outer area (near the pericarp) was higher (Fig. Ha, slices f = 0.5 h to t = 1 h). At the same time, the moisture content in the endosperm near the surface also continuously decreased. The low moisture zone gradually moved to the interior of the kernel along the radial direction. This again demonstrated that the moisture in the centre and near the surface transferred through two routes, the glandular layer of the scutellum and the pericarp. The moisture distribution changes during drying in the side-to-side and in the top-to-tip cap views showed that moisture content at the back of the kernel decreased quickly (Figs 15b and c,
68
(b)
Cc) Fig. 16. Cumulative moisture decrease of three orthogonal FR27 x Mol7 kernel during drying at 49°C; (a) back-to-front cap view
20 images through the centre of a view (b) side-to-side view (c) top-to-tip
slice t = O-25 h). The moisture in the central area near the germ-also lost moisture quickly (Fig. 156, slice f = 0.75 h). The image subtraction results for the 49°C drying experiment (Fig. 16) shows some different phenomena as compared to the 27°C image subtraction results (Eg. 13). First, the moisture loss in the floury endosperm was much faster than that at 27°C during the first 0.25 h of drying. Second, the embryo also lost moisture from the beginning of the drying process (Figs 16a and c, slice t = 0.25 h, the white image at the centre of the germ). Finally, the germ also lost moisture (Fig. 16, slices t = 0.75 h and 1= 1 h). 4. Conclusions
Moisture was distributed non-uniformly in the maize kernel at an equilibrium state. There are two primary moisture transfer routes in maize kernels during drying, through the glandulary layer of the scutellum and through the pericarp. The moisture in the centre of the kernel transferred through the glandular layer of the scutellum and the tip cap, while the moisture in the endosperm near the surface of the kernel transferred through the pericarp. The floury endosperm lost moisture faster than the vitreous endosperm, and in the germ, the embryo lost moisture first. At a higher temperature (49°C) drying, the germ also lost moisture.
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P. SONG
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Acknowledgement This study was funded by the National Science Foundation, NSF grant CBT-8808748. Appreciation is expressed to the Biomedical Magnetic Resonance Laboratory at the University of Illinois, Dr Paul Lauterbur, Director, for the use of its equipment and facilities. Thanks are also extended to Mr Clinton Potter, NCSA, for his help with the image processing. References ’ Song, H.; Litchfield, J. B. Nondestructive measurement of transient moisture profiles in ear corn during drying using NMR imaging. Transactions of the ASAE 1990, 33(4): 1286-1290. * Chiang, W. C.; Petersen, J. N. Experimental measurement of temperature and moisture profiles during apple drying. Drying Technology 1987, 5: 25-49 3 Perez, E.; Kauten, R.; McCarthy, M. J. Non-invasive measurement of moisture profiles during the drying of an apple. In: Drying ‘89, (A. S. Mujumdar, ed.) Washington, D.C.: Hemisphere Publishing Co., 1990 * Song, H.; Litchfield, J. B. Nuclear magnetic resonance imaging of transient three-dimensional moisture distribution in an ear of corn during drying. Cereal Chemistry 1990, 67: 580-584 ’ Lauterhur, P. C. Image formation by induced local interactions: examples employing nuclear magnetic resonance. Nature 1973, 242: 190-191 ’ Hahn, E. L. Spin echoes. Physics Review 1950, 80: 580-594 ’ Mansfield, P.; Morris, P. G. NMR imaging in biomedicine. Supplement 2: Advances in magnetic resonance. New York: Academic Press, 1982 ’ Brooker, D. B.; Bakker-Arkema, F. W.; Hall, C. W. Drying Cereal Grains. Westport, m. The AVI Publishing Company, Inc., 1974 a Earle, F. R.; Curtis, J. J.; Hubbard, J. E. Composition of the component parts of the corn kernel. Cereal Chemistry 1946, 23: 504-511 lo Watson, S. A. Structure and Composition. In: Corn Chemistry and Technology. Watson, S. A.; Ramstad, P. E., eds). St Paul, MN: American Association of Cereal Chemists, Inc., 1987, pp. 53-82 ” Wolf, M. J.; Buzan, C. L.; MaeMasters, M. M.; Rist, C. E. Structure of the mature corn kernel. II. Microscopic structure of pericarp, seed coat, and hilar layer of dent corn. Cereal Chemistry 1952, 29: 334-348 ” ASAE Standards S352.2. Moisture measurement-Unground grain and seeds. St Joseph, MI: American Society of Agricultural Engineers, 1989 ” Li, H.; Morey, R. V. Thin-layer drying of yellow dent corn. Transactions of the ASAE 1984, 27: 581-585 l4 Litchfield, J. B.; Okos, M. R. Mass transfer during the drying of extruded durum semolina. Journal of Food Engineering, 1991 (in press) l5 Weber, E. J. Lipids in corn, In: Corn Chemistry and Technology. (Watson, S. A.; Ramstad, P. E., eds). St Paul, MN: American Association of Cereal Chemists, Inc., 1987 ‘* Weller, C. L.; Paulsen, M. R.; Mbuvi, S. Germ weight, germ oil content, and estimated oil yield for wet-milled yellow dent corn as affected by moisture content at harvest and temperature of drying air. Cereal Chemistry 1989, 66: 273-275 ” Le Bras, A. Maize drying conditions and its resulting quality for wet-milling industry, In: Maize: Recent Progress in Chemistry and Technology. New York, NY: Academic Press, 1982