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Journal of Food Engineering 85 (2008) 222–231
www.elsevier.com/locate/jfoodeng
Microstructure affects the rate of chemical, physical and color
changes during storage of dried apple discs
Nuria C. Acevedo a, Vilbett Briones b, Pilar Buera a,*,1, José M. Aguilera b
a
Departamento de Industrias, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria (1428),
Ciudad de Buenos Aires, Argentina
b
Department of Chemical and Bioprocess Engineering, Pontificia Universidad Católica de Chile, Santiago, Chile
Received 16 April 2007; received in revised form 19 June 2007; accepted 28 June 2007
Available online 8 August 2007
Abstract
Blanching, freezing and drying induce major changes in the physical properties of processed foods. Microstructural changes induced
by these processes in apple discs were related to the degree and kinetics of browning and to fracture mechanics after drying and later
storage at 70 °C under a wide range of relative humidity (RH). Blanched and unblanched apple discs were dehydrated by vacuum drying
or freeze-drying to induce the formation of different structures, then equilibrated from 33% to 75% RH and stored at 70 °C in order to
promote browning. Color changes on the surface of apple discs were analyzed non-invasively by image analysis using a computerized
vision system. Pre-treatments and drying conditions modified the structural characteristics of apple discs, which in turn, changed sorption properties, texture hardness and browning development. Microstructural changes (e.g., loss of cellular integrity) promoted higher
browning rates, the rate and degree of browning was analyzed.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Apple; Microstructure; Browning; Kinetics; Hardness; Freeze-drying; Vacuum drying; Color
1. Introduction
Food materials science attempts to develop structure–
property relationships in foods. In particular, it has been
demonstrated that many desirable traits of food such as
texture, color, or flavour, depend on the way foods are
structured (Aguilera, 2005). In the case of plant food tissues (e.g., fruits and vegetables) pre-treatment such as
blanching and processing (canning, freezing, dehydration,
etc.) induce significant changes in the microstructure whose
effects on product properties are starting to be elucidated.
Browning in dehydrated fruits could be caused by
enzyme action, taking place in early stages of processing,
previously to polyphenoloxidase inactivation, or by Maillard reactions or non-enzymatic browning (NEB), during
*
1
Corresponding author. Tel./fax: +54 11 4576 3366.
E-mail address: [email protected] (P. Buera).
Member of CONICET, Argentina.
0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jfoodeng.2007.06.037
drying and the later storage. Browning discoloration lowers the quality value of products. Some of the deleterious
effects that these reactions have in foods are the undesirable
brown discoloration, textural changes, off-flavours,
decreased solubility and loss of nutritional value. The effect
of temperature and relative humidity (RH) on NEB rate
has been extensively investigated in blanched and dehydrated systems. Mechanisms and factors influencing NEB
have been reviewed by Labuza and Baisier (1992).
Although the effect of phase changes and transitions on
NEB kinetics has been reported (Buera & Karel, 1995),
the impact of structural characteristics has not yet been
analysed.
The quantitation of brown coloration appearance is
usually made spectrophotometrically after extraction of
pigments in a solvent (Lievonen & Roos, 2002), or instrumentally by hand-held colorimeters (Krokida, Maroulis, &
Saravacos, 2001; Lopez et al., 1997). These methods provide an average color value which is inappropriate to
N.C. Acevedo et al. / Journal of Food Engineering 85 (2008) 222–231
discriminate the changes of color patterns in foods such as
sliced apples (Hatcher, Symons, & Manivannan, 2004).
Alternatively, computer vision is becoming an established
tool to assess quality traits of foods based on its simplicity,
analysis of the whole image and non-destructive nature.
Changes in average color and color patterns of whole apple
slices during dehydration were effectively determined by
this technique (Fernández, Castillero, & Aguilera, 2005).
The objective of the present work was to assess if the
changes induced on tissue structure by different pre-treatments and drying methods were reflected in the development and kinetics of brown discoloration and in hardness
changes during the later storage of dehydrated apple discs
under different conditions.
2. Materials and methods
2.1. Materials
Granny Smith apples were purchased from a local market (Santiago, Chile) and stored at 4 °C until the moment
of the experiment.
2.2. Sample preparation
Apple discs (22 mm diameter and 0.5 mm thickness)
were obtained from a cylinder cut parallel to the main axis
of the fruit using a cork borer and sliced by two parallel
knives. Cellular debris on cut surfaces were removed by
rinsing with distilled water. In order to avoid enzymatic
browning the cut materials were submerged in a 1% (w/
w) ascorbic acid solution until blanching or freezing
(approx. 1 min). In order to induce structural changes in
apple tissue, apple discs with different pre-treatments and
dehydration procedures were studied, leading to the following types of samples and nomenclature:
Vacuum-dried (VD)
Blanched/vacuum-dried (B/VD)
Fast-frozen/freeze-dried (FF/FD)
Slow-frozen/freeze-dried (SF/FD)
A fifth group of samples (FD/P) was included for comparative purposes in the form of a powder by grinding
slow-frozen freeze-dried discs in a mortar before thermal
treatment (data from Acevedo, Schebor, & Buera, 2006).
Steam treatment in B/VD samples had the objective of
denaturing cell membranes and also this pre-treatment
inactivated the enzymes causing enzymatic browning. It
was accomplished by holding apple discs over a metal grid,
exposing them for 3 min to live steam and immediately
after submerging the samples in cold water until the dehydration process.
The freeze-drying process requires freezing the sample as
a previous procedure. Two freezing rates (fast and slow
freezing, FF and SF, respectively) were used to freeze
unblanched apple discs (samples FF/FD and SF/FD) in
223
order to induce changes in apple structural characteristics.
Samples FF/FD were frozen by immersion in liquid nitrogen immediately before the freeze-drying process, while
samples SF/FD were frozen at approximately 20 °C in
a home freezer for 12 h and then freeze-dried.
Vacuum drying was performed in a vacuum oven
(model 5831, Napco, Winchester, VA) over desiccant (silica
gel) at 50 °C and 25 in Hg vacuum for 4 h. Residual water
(approx. 2–3%) was removed by incubation over P2O5 in
vacuum desiccators at 20 °C for 12 h. The freeze-drying
process lasted 48 h. A Labconco 4.5 (Kansas, USA)
freeze-dryer was used which operated at 70 °C, at a
chamber pressure of 6 103 mbar. After drying, apple
discs (15 samples per treatment) were equilibrated at
20 °C over appropriate saturated salt solutions in vacuum
desiccators. Saturated solutions of magnesium chloride
(MgCl2), magnesium nitrate (MgNO3), sodium bromide
(NaBr), sodium chloride (NaCl) salts were used for 33%,
52%, 64% and 75% relative humidity, respectively (Greenspan, 1977).
2.3. Methods
The water content was determined by difference in
weight before and after vacuum drying over desiccant (silica gel) at 70 °C for 24 h or 48 h. The remaining water in
the samples was removed in vacuum desiccators over desiccant (P2O5) at 20 °C for 12 h. Water content was expressed
as mass of water per mass of solids (in dry basis, d.b.), as
the average of two measurements.
Soluble solids, measured as sugar, were determined in
apple juice using a refractometer (ATAGO Co. Ltd., model
NAR-3T, Japan) and expressed as °Brix. The pH of apple
juice was measured with a pHmeter (model 320, Hanna
Instruments, Portugal). Titratable acidity was determined
by titration against a 0.1 N NaOH solution to pH 8.1
and the acidity was expressed as percentage citric acid
(AOAC, 1984).
2.4. Environmental scanning electronic microscopy (ESEM)
The microstructure of the cross-section of samples from
all four pre-treatments was examined using ESEM (LEO
1420VP, Carl Zeiss SMT AG, Oberkochen, Germany)
without metal coating. At least duplicate specimens were
viewed at different magnifications and images of representative areas saved for further analysis.
2.5. Computer vision system (CVS)
The CVS used consisted of three elements: a lighting system, a digital camera and a personal computer.
The lighting system included four D65 lamps inside a
dark chamber. This is particularly important because the
color of the sample will depend on the light spectrum generated by the source of illumination. The angle between the
camera axis and the light source was 45° in order to capture
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N.C. Acevedo et al. / Journal of Food Engineering 85 (2008) 222–231
the fuzzy reflection responsible for color which is produced
at this angle (Papadakis, Abdul-Malek, Kamden, & Yam,
2000).
The digital camera, a Power Shot A70 (Canon Inc.,
Tokyo, Japan), is positioned above the base on which the
samples are placed. Samples were transferred from the
forced air oven to the dark box and images acquired every
2 h during the whole storage period. The iris was operated
in manual mode, with the lens aperture at f = 8 and speed
1/6 (no zoom, no flash) to achieve high uniformity and
repeatability. The calibration of the camera and the parameters used for image capture are described in Briones and
Aguilera (2005).
Images of apple discs had a resolution of 2048 1536
pixels. Images were saved on a PC (Pentium III, 30 GB,
300 MHz) in uncompressed JPG format using Canon’s
Remote Capture program (Canon Inc., USA).
2.6. Storage and color measurement
Apple discs equilibrated to different RH were placed
inside rubber o-rings which in turn were sandwiched
between two glass plates held hermetically with metal
clamps to avoid water loss, as depicted in Fig. 1. The glass
sample holders were then placed in an air-convection oven
kept at 70 ± 1 °C to observe changes during high temperature storage at constant water content. The glass plates
containing six discs were removed every 2 h to acquire
images and placed back in the oven. CVS permitted acquiring information for the whole pieces directly inside the
glass plates and then calculate an average color for the
entire image. Color images of apple disc were digitized into
pixels of 24 bits containing levels of the three primary colors: red, green and blue. Then RGB values were converted
to the L, a and b color values using a program Adobe
PhotoshopÒ 7.0 (Adobe Systems Inc., San Jose, CA). L,
a and b are not standard color values however they can
be converted to CIELab (or L*, a*, b* space) using mathematical formulas described by Papadakis et al. (2000). The
CIELab space is commonly used in research and quality
control of foods. This color space is device-independent,
creating consistent colors regardless of the device used to
acquire the image. L* is the luminance or lightness component, which ranges from 0 to 100 from black to white, while
Glass
plates
Apple
disc
clip
Glass plates
o-rings
clip
o-ring
Fig. 1. Sample holder for apple disc thermal treatment and color measurement.
Entrance of probe head
Screw system
Apple disc
Fixed to TA.XT2
Fig. 2. Scheme of test cell for measurements of maximum force at fracture (hardness).
N.C. Acevedo et al. / Journal of Food Engineering 85 (2008) 222–231
a* (green to red) and b* (blue to yellow) are the two chromatic components. The color function selected to follow
browning changes was DL*, which is defined as L0 L ,
where L0 is the lightness before the high temperature storage and L* is the corresponding value at a given storage
time. DL* is sometimes referred to as browning index.
2.7. Force at fracture (hardness) measurements
Force at fracture was measured in apple discs before and
after storage at 70 °C by puncture test using a texture analyzer TA.XT2 (Texture Technologies Corp., Scarsdale,
NY). A special test cell (Fig. 2) was designed so that apple
discs (five samples per treatment) were tightly held by a
bolted metal ring while penetrated through the center by
a cylindrical probe of 2 mm diameter. Crosshead speed
was 1 mm/s and force–displacement curves were recorded
as the plunger descended through the apple disc to the
point of fracture and the maximum force recorded as hardness. The reported values of hardness correspond to the
average of the individual measurements of five apple discs
corresponding to each of the four sample types and the
respective RH. The calculated confidence interval ranged
between 5% and 10%.
3. Results and discussion
3.1. Effect of pre-treatment and drying
Fresh Granny Smith apples had 12.6 ± 0.2 °Brix and
84.4 ± 0.2% water (wet basis) at the initial time of the
experiment. Fig. 3 shows the water adsorption isotherms
(20 °C) for dehydrated apple discs equilibrated at different
relative humidities. Samples belonging to groups VD, B/
VD and SF/FD showed similar water sorption isotherms,
which were also comparable to those of the powdered
freeze-dried (P/FD) apple samples analyzed in a previous
work (Acevedo et al., 2006). FF/FD samples (fast-frozen/
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freeze-dried), however, were more hygroscopic and
adsorbed more water than the rest of the samples, particularly at RH > 33%. It is known that fast-frozen, freezedried materials have higher porosity (more and smaller
pores) than similar slow-frozen materials and consequently
more effective sites for water adsorption (Aguilera & Stanley, 1999), thus, a high hygroscopicity in these samples was
to be expected. The water adsorption isotherms shown in
Fig. 3 were fitted to the GAB equation using the least
square method for minimizing the absolute differences
between measured and calculated water content values,
and the water content at the monolayer (mm) was calculated (van den Berg & Bruin, 1981). The following equation
(Eq. (1)) was used to evaluate the sorption surface area
(Gregg & Sing, 1982):
A ¼ mm Na=M
ð1Þ
where A is the surface area of adsorbent, N is the Avogadro’s number (6.02 1023 molecules/mol); M is the molar
mass of water (18 g/mol); a is the area of one water molecule (10.6 1020 m2). The specific surface area of adsorption places plays an important role in determining the
water binding properties of materials (Tolaba, Peltzer,
Enriquez, & Pollio, 2004), and the obtained values are
shown in Table 1. The powdered (P/FD) samples had the
highest monolayer value, and correspondingly, the highest
surface area, followed by the FF/FD samples. The rest of
the samples had lower effective surface for adsorption.
During fast-freezing many small water crystals are rapidly formed which leave a very porous material after water
sublimation, hence freeze-dried fruit products have more
surface area for water adsorption (Tsami, Krokida, &
Drouzas, 1998). Therefore, after freeze-drying the apple
matrices FF/FD presented larger surface/volume ratio
than those SF/FD, which explains their higher capacity
to adsorb water.
Structural changes after drying were observed by ESEM
and shown in Fig. 4a–d. A higher cellular collapse and tissue disruption were observed in vacuum-dried apple discs
Table 1
Sorption parameters corresponding to the different apple samples
Fig. 3. Water adsorption isotherm (20 °C) for dehydrated apple discs
equilibrated at different relative humidity. The results from freeze-dried
and powdered samples are shown in dotted lines.
System
mma (g water/g solids)
Ab (m2/g)
%Ec
VD
B/VD
FF/FD
SF/FD
FD/Pd
0.0624
0.0669
0.0715
0.0658
0.0836
220.90
236.83
253.1
232.93
295.94
3.4
2.7
2.7
5.7
2.5
a
mm is the monolayer water content calculated through the GAB
equation.
b
A is the effective surface for water sorption calculated as A = mmNa/
M.
c
%E is the goodness of fit as applied to the experimental adsorption
P
isotherms of apple systems, calculated as %E = 100/n (mi mi*)/ mi,
where n is the number of experimental points, mi is the measured water
content and mi is the calculated content.
d
Results from freeze-dried and powdered samples (FD/P) were taken
from Acevedo et al. (2006).
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N.C. Acevedo et al. / Journal of Food Engineering 85 (2008) 222–231
Fig. 4. ESEM micrographs of cross-sections of apple discs. (a–d) After drying: (a) vacuum-dried, VD samples, (b) blanched and vacuum dried, B/VD
samples, (c) fast-frozen and freeze-dried, FF/FD samples and (d) slowly frozen and freeze-dried, SF/FD samples. (e–h) After storage at 70 °C: (e) VD
samples, (f) B/VD samples, (g) FF/FD samples, and (h) SF/FD samples.
(samples VD and B/VD) than in freeze-dried apple discs
(samples FF/FD and SF/FD). The freezing rate is a variable recognized as responsible for tissue damage (Fuchigami, Kato, & Teramoto, 1997). It is generally accepted
that high freezing rates retain the quality (and thus, the
structure) better than slow freezing rates (Partmann,
1975). Slow freezing rate causes severe changes in product
microstructure, SF/FD samples presented broken surfaces,
as shown in Fig. 4d, probably due to the higher ice crystals
formed during slow freezing before freeze-drying. This is
due the mechanical stress which is provoked by extracellular ice formation leading to membrane rupture and to the
cryoconcentration phenomena, which promotes membrane
denaturation and cell wall degradation. Membrane rupture
N.C. Acevedo et al. / Journal of Food Engineering 85 (2008) 222–231
results in enzyme and/or chemical activity that also contributes to the mechanical damage (Fuster, Prestamo, &
Cano, 1994; Tregunno & Goff, 1996). The different freezing
rate in samples FF/FD and SF/FD caused a significant
influence in water sorption properties, as previously discussed, and this change was also manifested at microscopic
level by ESEM observation (Fig. 4a–d). Samples SF/FD
presented deformed stretched pores due to the higher ice
crystals formed during slow freezing, which presumably
caused the broken surfaces visually observed. ESEM
images of the different dried samples subjected to further
high temperature storage at 70 °C are also shown in
Fig. 4e–h. After storage total loss of cellular integrity was
observed in discs VD and B (Fig. 4e and f), while in samples FF/FD and SF/FD (Fig. 4g and h) structural changes
were less drastic and remnants of void cells could be
observed. It is interesting to note that although thermal
treatment extended structural modifications, the most
important differences occurred already among the different
pre-treatments and/or dehydration methods.
3.2. Color development
Fig. 5 depicts a gallery of images of dehydrated apple
discs equilibrated at different RH as they underwent storage at 70 °C (note: at each RH the same specimen was photographed through time). Specimens at the initiation of
storage (time 0) in Fig. 5 are representative of the variation
in color and size after each drying treatment. Vacuumdried samples presented darker colors and more irregular
borders and shapes, evidencing a certain degree of shrinkage. Freeze-dried apple discs were appreciably lighter in
227
color, rounder and larger at the beginning of storage compared to vacuum-dried samples. Browning increased with
storage time at 70 °C, and was more evident at 64% and
75% RH, but no major changes in shape or size of specimens were appreciated upon further heating. Vacuum drying led to appreciable darkening of samples during storage
compared to freeze-drying. In summary, visual observation
of samples in Fig. 5 shows that the treatment that better
preserved structure and color was FF/FD, followed by
SF/FD, and that the appearance of VD samples were the
most affected. It is interesting to note that at the beginning
of storage blanched samples were lighter than unblanched
samples and that browning during the storage period was
less intense. This suggests that not only NEB but also enzymatic browning (EB) played a major role in color changes
during storage. In fact, the color difference at each time
(and constant RH) between VD and B/VD may be taken
as an EB contribution to browning.
Fig. 6 (a–d) shows the changes in lightness (DL*) or
browning index versus storage time at a given RH for all
samples. Data from samples powdered after freeze-drying
were also included for comparative purposes (dotted lines).
It has to be kept in mind that the index DL* measures differences in color with respect to time 0 and is not an absolute color value. An almost linear trend for DL* increase
was observed as a function of time for all groups but was
more pronounced at higher RH. The browning index, however, was different depending on the pre-treatment and/or
drying conditions of the samples. For example, at a given
time, vacuum-dried samples had developed higher browning indexes than freeze-dried samples, which retained a
light color until the end of the storage period. After drying
Fig. 5. Gallery of images of dehydrated apple discs during storage at 70 °C as a function of RH: (a) vacuum-dried, VD samples, (b) blanched and vacuumdried, B/VD samples, (c) fast-frozen and freeze-dried, FF/FD samples and (d) slowly frozen and freeze-dried, SF/FD samples.
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N.C. Acevedo et al. / Journal of Food Engineering 85 (2008) 222–231
Fig. 6. Change in color lightness (DL*) of apple discs stored at 70 °C as a function of time. Samples: VD (h), B/VD (*), FF/FD (N), SF/FD (d) and FD/
P (s), dotted line. (a) 33% RH, (b) 52% RH, (c) 64% RH and (d) 75% RH. Results from freeze-dried and powdered samples are shown in dotted lines.
VD samples were darker than B/VD samples, and after
storage they presented a browning index 25–42% higher
than B/VD samples. As mentioned before, the larger
browning indexes developed in VD samples are possibly
the consequence of the contribution of enzymatic browning
occurring in these non-blanched samples.
Rate constants for non-enzymatic browning development were calculated using linear regression analysis and
DL* as response variable. The regression coefficients of
the pseudo zero order reaction rate constants (slope of the
lines in Fig. 5a–d) varied from 0.89 to 0.99. Fig. 7 shows
the rate constants of browning for samples VD, B/VD,
FF/FD and SF/FD and also for the powdered samples
(FD/P) as a function of RH. Powdered samples showed
the highest browning rates at all RH, probably due their
higher proportion of broken cells that may increase the
effective release of reacting species and their diffusion, thus
accelerating the Maillard reaction. VD samples exhibited
higher browning rates than those of the B/VD group
(Fig. 7), which can be attributed to the effects of blanching
causing inhibition of the enzymes responsible for enzymatic
browning. Other enzymes, involved in the release of reducing sugars from pectins, may also affect browning of
Fig. 7. Browning rate of apple discs stored at 70 °C as a function of RH.
Group VD (h), Group B/VD (*), Group FF/FD (N), Group SF/FD (d)
and Group FD/P (s). Results from freeze-dried and powdered samples
are shown in dotted lines.
samples, as well as protein aggregation and solute mobilization during blanching (Greve, McArdle, Gohlke, & Labavitch, 1994).
N.C. Acevedo et al. / Journal of Food Engineering 85 (2008) 222–231
Up to a RH of 40%, FF/FD samples showed higher
browning rates than SF/FD samples, after which the rates
were almost similar. This result can be explained on the
basis of the effect of freezing rate on porosity and the sorption properties of the freeze-dried samples. The higher
water sorption capacity of FF/FD samples (see Fig. 3)
compared to that of SF/FD samples may also be a factor
preventing browning development, since it is known that
water exerts an inhibitory effect on the Maillard reaction
(Acevedo et al., 2006).
These results suggest that one way to control browning
is to preserve the structure of apple discs. The extreme case
is the freeze-dried powdered sample where all structure has
been obliterated and exhibited the highest browning rate. It
is interesting to note that independently of the pre-treatment and drying method, the browning rate followed parallel trends as increasing RH. There are evidences that the
maximum browning rate in vegetable tissues occurs around
70% RH (Acevedo et al., 2006; Hendel, Silveira, & Harrington, 1955; Lee, Chung, Kim, & Yam, 1991; Rapusas
& Driscoll, 1995). Thus, a decrease in browning rate is
expected at RH values higher than 75%, as was experimentally obtained for powdered apple samples (FD/P) at 85%
RH (Fig. 7).
229
3.3. Hardness of samples
Hardness values (force at fracture) obtained by the
puncture test are plotted in Fig. 8 as a function of RH
for the four sample groups. Grey bars represent data collected before storage of the samples while black bars correspond to data obtained after 36 h of storage at 70 °C.
Samples dried under vacuum at 50 °C (VD and B/VD)
showed higher hardness values than FD samples at similar
RH. Storage induced no change or a slight increase in
hardness of VD samples except at RH = 33%, where the
hardness of the stored samples appears to be larger than
initially. Maximum hardness was always observed at the
lowest RH (33%). Bourne (1986) showed that hardness of
apple tissue increased logarithmically as aw decreased (up
to aw values of 0.12), similar to what occurs in protein gels
(Beveridge, Arntfield, Ko, & Chung, 1980).
Fracture of dried food products is a complex phenomenon that depends largely on the material itself, the plasticizing effect of water as well as on the presence and
propagation of microscopic cracks induced by processing
(Castro & Aguilera, 2007). Based on these facts and the
previous structural information, the following interpretation of hardness results can be made: dehydrated cell walls
Fig. 8. Force at fracture (hardness, Newton) of apple discs equilibrated at different RH after drying (t = 0 h) and after storage at 70 °C (t = 36 h).
Segments over bars represent confidence intervals.
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Table 2
Effect of induced structural damage on browning and hardness development during thermal treatment of dried samples rehumidified at 33% RH
System
Structural damage
after dryinga
Browning rateb
(DL*/h)
L0 c
L *d
Hie (N)
Hff (N)
VD
B/VD
FF/FD
SF/FD
FD/P
++
+++
+/
+
NDg
0.89
0.46
0.70
0.28
0.90
51.8 ± 0.1
60.2 ± 0.1
73.3 ± 0.1
70.1 ± 0.1
80.8 ± 0.1
28.3 ± 0.1
42.9 ± 0.1
55.8 ± 0.1
56.7 ± 0.1
66.4 ± 0.1
16. ± 3
17. ± 3
7. ± 2
12. ± 2
NDg
32. ± 2
24. ± 3
8. ± 1
12. ± 1
NDg
a
Degree of structural damage, as evaluated by ESEM and correlated with visual macroscopically perceived changes. (/+) slightly damaged structure,
(+) low degree of structural change; (++) moderate structural changes; (+++) greatly damaged structure.
b
Browning rate during heat treatment at 70 °C.
c
L0 : initial lightness, after drying.
d
L*: final lightness after 36 h of thermal treatment at 70 °C.
e
Hi: hardness value after drying.
f
Hf: hardness value after heat treatment at 70 °C.
g
ND: not determined.
are the common material providing structural integrity and
mechanical resistance to all the dried apple discs. Due to
shrinkage (see Fig. 5) VD samples became denser than
FD samples, thus a larger fracture force was to be
expected. As water content increases (i.e., higher RH)
water plasticizes the cell walls and the material, which is
strong brittle at RH = 33%, becomes softer and more pliable, thus hardness decreases. Since water content is constant during storage the only possible effect of storage at
70 °C on hardness is that due to exposure to this temperature. Consequently, with the exception of the sample VD
(at RH 33%) there are no observable differences in sample
hardness between the initiation and the end of storage,
which is to be expected because major temperature effects
on hardness are not expected to occur at 70 °C. Some
authors have argued that blanching prior to dehydration
causes disruption of cell membranes and a concomitant
faster and more complete drying resulting in a harder material (Lazar & Rasmussen, 1964), but this phenomenon
could not be statistically demonstrated here because hardness differences between blanched and unblanched samples
were not significant. Beveridge and Weintraub (1995)
reported that blanching did not affect the texture of airdried apple samples equilibrated in a range between 70%
and 77% RH.
Freeze-drying introduces another variable beyond
shrinkage which is porosity and cell wall damage due to
the formation of ice crystals during freezing. Freeze-dried
apple discs (FF/FD, fast-frozen and SF/FD, slow-frozen)
showed lower hardness than VD samples. The hardness
values obtained for FF/FD apple discs were the lowest of
all samples and showed no appreciable differences as a
function of RH. On the other hand, the SF/FD samples
showed a decrease in hardness as the RH increased (both,
before and after storage). Unexpected low hardness values
for FF/FD samples at low aw may only be explained by the
presence of more fragile cell walls pierced by small ice crystals. During slow freezing on the other side, water can
migrate and large ice crystals are generated extracellularly.
Cell wall cracking and piercing by ice crystals has been
observed in frozen apple tissue by Fuster et al. (1994)
and Tregunno and Goff (1996).
Table 2 summarizes the results for samples humidified to
33% RH, which is a representative condition for dehydrated products. The degree of structural damage was evaluated through ESEM (Fig. 4a–d), and qualitatively
correlated to macroscopically observed changes (Fig. 5).
As shown in Table 2, in the fast-frozen freeze-dried samples, in which the global quality (structure, hardness and
color) was better preserved, the rate of browning development was higher than in SF/FD and B/VD samples, probably because their higher area for water sorption (shown in
Table 1) favors the reaction (being more water attached to
the active sites in the solids, its inhibitory is less evident).
However, the final color of the freeze-dried samples was
lighter than that of the vacuum-dried ones because of the
better conservation of structure (Table 2). After drying or
storage at 70 °C hardness values were also higher for the
vacuum-dried samples than for the freeze-dried ones, in
parallel to color changes and shrinkage. It is to be noted
that the hardness values of freeze-dried samples were not
affected by the heat treatment at 70 °C.
4. Conclusions
All treatments to which the apple discs were subjected
caused structural modifications at the macroscopic (i.e.,
size, shape) and microstructural levels (i.e., cell walls and
cell membrane). These structural changes affected the water
sorption properties and possibly led to the release of reacting species involved in browning, thus facilitating their diffusion and reaction. Changes were also directly related to
the final texture of the product. Color measurements
through the CVS system provided reproducible values of
browning changes without invasion of the sample. Particularly interesting is the fact that pre-treatment or processes
affected effective surface for water adsorption. Since water
plays an inhibiting role in the Maillard reaction, a higher
surface for water adsorption restricts water availability at
a microscopic level promoting a higher reaction rate. This
N.C. Acevedo et al. / Journal of Food Engineering 85 (2008) 222–231
observation is also in agreement with the findings of White
and Bell (1999), although their interpretation pointed out
to the presence of preferential sites of reactivity for the
Maillard reaction in the porous material. The better preserved structure of the freeze-dried samples avoided hardness development due to shrinkage and collapse during
storage at 70 °C.
The effects of suprastructural changes on the rate of
chemical reactions were scarcely explored in the literature
concerning food systems. This paper shows that the matrix
structure and its dynamics modifications play a decisive role
in determining the kinetics of physico-chemical phenomena.
In future steps it would be desirable to quantify microscopic
structural changes in order to include them as critical
parameters for improved quality changes predictions.
Acknowledgements
The authors acknowledge the financial support from
Research partly funded by Fondecyt Project 1030339
(JMA) and International Cooperation grant 2004 (JMA
and PB).
Financial support from ANPYCT (PICT 20545), UBACYT X226 and CONICET (PIP 3066) is also acknowledge.
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