non-thermal effects of microwave oven heating on ground beef meat studied in the mid-infrared region...
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Non-Thermal Effects of Microwave Oven Heating onGround Beef Meat Studied in the Mid-Infrared Regionby Fourier Transform Infrared SpectroscopyEmanuele Calabrò a & Salvatore Magazù aa Department of Physics , University of Messina , Messina , ItalyAccepted author version posted online: 01 Aug 2013.Published online: 29 Apr 2014.
To cite this article: Emanuele Calabrò & Salvatore Magazù (2014) Non-Thermal Effects of Microwave Oven Heating onGround Beef Meat Studied in the Mid-Infrared Region by Fourier Transform Infrared Spectroscopy, Spectroscopy Letters: AnInternational Journal for Rapid Communication, 47:8, 649-656, DOI: 10.1080/00387010.2013.828313
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Non-Thermal Effects of Microwave OvenHeating on Ground Beef Meat Studied in
the Mid-Infrared Region by FourierTransform Infrared Spectroscopy
Emanuele Calabro,
and Salvatore Magazu
Department of Physics,
University of Messina, Messina,
Italy
ABSTRACT The effects of cooking by microwave oven on the secondary
structure of lipid and protein contents in bovine ground beef were investi-
gated in the midinfrared region by Fourier transform infrared (FTIR) spec-
troscopy to highlight the nonthermal effects of microwave oven heating.
Samples of bovine ground beef were cooked in a conventional electric oven
at the temperature of 175�C for 15min and in a microwave oven at 800W for
1½ min. Spectra analyses of bovine meat after cooking in the conventional
oven evidenced a relevant increase in intensity of the carbonyl band at
1742 cm�1 and of the methylene group at 2921 and 2853 cm�1 that can be
attributed to the Maillard reaction. In contrast, the increase in intensity of
these bands after microwave oven heating was less than that which occurred
after conventional cooking, showing that the temperature in ground beef
meat samples during microwave heating was less than that induced by con-
ventional heating. Spectral analysis in the amide I, II, and III regions showed
that a significant increase in intensity occurred in the region from 1660 to
1675 cm�1 and around 1695, 1635, 1575, and 988 cm�1 after cooking by
means of a microwave oven. These features, which can be attributed to
b-turns and b-sheet structures, are characteristic of disorder processes in meat
protein contents and increasing transition dipole coupling due to higher
contents in aggregated b-sheet structures. This result highlighted nonthermal
effects of microwave oven heating in the protein’s secondary structure.
KEYWORDS b-sheets, amide I, bovine meat, FTIR spectroscopy, microwave
oven
INTRODUCTION
Many physical and chemical reactions occur during food preparation.
These reactions are a result of the interaction between food components
and environmental conditions like heat, light, air, and materials that are used
during cooking process. Cooking improves the natural flavor and digestibility
Received 10 June 2013;accepted 21 July 2013.
Address correspondence toEmanuele Calabro, Department ofPhysics, University of Messina,Messina, Italy. E-mail:[email protected]
Color versions of one or more of thefigures in the article can be foundonline at www.tandfonline.com/lstl.
Spectroscopy Letters, 47:649–656, 2014Copyright # Taylor & Francis Group, LLCISSN: 0038-7010 print=1532-2289 onlineDOI: 10.1080/00387010.2013.828313
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of food and can destroy all harmful microorganisms
in food, but the quality of protein may be reduced
during cooking due to destruction of certain amino
acids. The most drastic changes in meat during heat-
ing are caused by changes in the muscle proteins.[1]
Water loss during cooking results from changes to
both myofibrillar and collagen muscle proteins.[2]
Indeed, after cooking, meat proteins denature,
producing structural changes in the meat, such as the
destruction of cell membranes, shrinkage of meat fibers,
and aggregation. Furthermore, nutritional properties of
the meat products depend on the method of cooking.
The cooking process in a conventional oven
consists of heating food by the surrounding hot air,
which is heated by a source of heat, either electricity
or gas. The physical processes of heat transfer in a
conventional oven are represented by radiation from
the source of heat to the metal wall at the base of the
oven, by conduction from the base to the other walls,
and by convection through the heated air currents
set up in the oven and delivered to the food. Gener-
ally, the temperatures that are used in a conventional
oven range from 130�C to 250�C.
The properties of food can change if the oven is
preheated before placing the food in it or when
cooking the food at different temperatures.[3–5]
The domestic use of the microwave oven occurred
during the 1970s and 1980s as a result of Japanese
technology transfer and global marketing.[6] The
microwave oven utilizes microwave (MW) energy to
cook food instead of forced-air convection heat in a
conventional oven. The power source magnetron
emits MWs that are absorbed by food, producing
rotation and collision of polar molecules such as
water and ions inside the food. In fact, the main mol-
ecular dipoles inside the food are water molecules
that will rotate very rapidly in food (about 2450
million times a second) following the frequency of
MWs, causing frictions that generate heat. Also, ionic
compounds in food can be accelerated by the MWs,
colliding with other molecules and disrupting hydro-
gen bonds with water, generating additional heat.[7,8]
Indeed, water and salt are the two major ingredients
that influence dielectric properties of food.
The heating process of foods by MWs is instan-
taneous, whereas conventional heating in an oven
can transmit thermal energy from the food’s surface
toward its center much more slowly, because the
low conductivity of food material. Some authors
reported that meat cooked in a microwave oven
can provide a product comparable to oven prep-
aration.[9] In contrast, other authors reported that
the aroma and flavor of hot-air oven-cooked pro-
ducts are better and more acceptable as compared
to microwave oven–cooked products.[10] However,
although cooking by microwave oven is widely
used for food preparation, insufficient information
is available on the effects of MWs on the composition
and nutritional quality of the food. For instance,
some studies revealed that microwave heating affects
fat oxidation and fatty acid isomer formations.[11–14]
The aim of this study was to investigate the effects
of the electromagnetic field (EMF) on meat produced
by microwave oven cooking apart from the effects of
the increase in temperature, using Fourier transform
infrared (FTIR) spectroscopy. FTIR spectroscopy has
been largely used in studies regarding the analysis of
the major components of milk, fat, and protein
composition of meats, as well as carbohydrates in
cereal samples.[15–17] Hence, this advanced analytical
technique may be able to detect molecular structural
changes that are associated with heat processing by
using a microwave oven.
MATERIALS AND METHODS
Ground Beef Samples
Bovine ground beef meat was collected from four
different commercial processing plants (SMA, Despar,
Sigma, Sidis, located in Messina, Italy), packed on ice,
and transported to the laboratory. The internal tem-
perature of the ground beef meat was maintained at
4�C before treatment. Bovine beef meat was minced
using a plate with 4.5-mm holes to obtain a more uni-
form cooking inside the meat samples. Also, minced
meat was divided into portions of 150 g with a thick-
ness of about 8 mm to allow a quick cooking in the
ovens. Meat samples were cooked immediately with-
out any additional treatment.
Experimental Design
A set of meat samples, prepared as explained
above, was inserted into a conventional electric oven
Indesit Mod. KG 8414 XES=I. Cooking began with
preheating the oven at the temperature of 175�C;
meat was cooked for 15 min at this temperature and
a thermocouple was used for temperature control.
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Another set of meat samples was subjected to heat-
ing in a domestic microwave oven model Whirlpool
AVM 541=WP=WH. Measurement of the power den-
sity generated by the microwave oven was performed
by means of a Narda SRM 3000 device. Spectrum
analysis mode showed two peaks around 2440 MHz
and 2480 MHz during microwave oven cooking,
as represented in Fig. 1. The integrated value of
the power density in the range 2350–2550 MHz
amounted to 217 mW=m2.
A set of samples was subjected to heating in
the microwave oven at the power level of 800W
for 1½ min. After cooking, each meat sample was
homogenized and stored in sterile jars.
Infrared Spectroscopy
FTIR absorption spectra were recorded at a room
temperature of 20�C by a spectrometer Vertex 80v
of Bruker Optics. The cooked meat sample of 12 mg
was placed between a pair of CaF2 windows. For
each spectrum, 64 interferograms were collected
and coadded by Fourier transform employing a
Happ-Genzel apodization function to generate a
spectrum with a spectral resolution of 4 cm�1 in the
range from 4000 to 1000 cm�1. Each measure was
performed under vacuum to eliminate minor spectral
contributions due to residual water vapor. However,
smoothing correction for atmospheric water back-
ground was performed and IR spectra were baseline
corrected and area normalized.
In addition, vector normalization was used, calculat-
ing the average value of the spectrum and subtracting
from the spectrum, decreasing the midspectrum. The
sum of the squares of all values was calculated and
the spectrum divided by the square root of this sum.
The automatic baseline scattering correction
function was used to subtract baselines from spectra,
which allows one to get spectra with band edges of
up to the theoretical baseline.
FIGURE 1 Spectrum analysis of the power density performed by a Narda SRM 3000 in the range 2350–2550 MHz of a microwave oven
Whirlpool Model AVM 541=WP=WH during ground beef cooking, acquired 25 cm from the door of the oven.
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Interactive baseline rubberband correction was
used. This method uses a rubberband that is stretched
from one spectrum end to the other, and the band is
pressed onto the spectrum from the bottom up with
varying intensity. This method performs iteratively,
depending on the number of iterations in the algor-
ithm and the baseline as a frequency polygon consist-
ing of n baseline points. The resulting spectrum
will be the original spectrum less the baseline points
manually set and a subsequent concave rubberband
correction. We used the default value of n¼ 64
baseline points and 50 iterations.
In order to enhance the fine spectral structure, the
Fourier self-deconvolution (FSD) technique was
used. The concept of FSD is based on the assumption
that a spectrum of single narrow bands is broadened
in the liquid or solid state and cannot be distinguished
in the amide envelope. A curve fitting procedure
can be applied to estimate quantitatively the area of
each component representing a type of secondary
structure.[18]
RESULTS AND DISCUSSION
Structural changes of vibration bands of ground
beef meat samples after cooking by conventional
electric oven and by microwave oven were studied
using FTIR spectroscopy. Raw ground beef samples
were used as the control.
Absorption spectra were acquired from several
samples, as described in the preceding section. Rep-
resentative FTIR spectra in the range 3000–1000 cm�1
are shown in Fig. 2; infrared spectra of raw ground
beef, ground beef cooked in the conventional oven
at 175�C, and ground beef cooked in the microwave
oven at the power of 800 W are represented by blue,
green, and red lines, respectively.
In the region from 3000 to 2800 cm�1 CH2 and CH3
stretching vibrations are observed, assigned to
methylene and methyl group that are the most
intense vibrations in the infrared spectra of lipid
systems. The band close to 2960 cm�1 originates from
the asymmetric stretching nasCH3 of methyl groups,
whereas vibration bands at 2921 and 2853 cm�1 are
assigned to symmetric and asymmetric bending
nsCH2 and nasCH2 of methylene, respectively.[19,20]
Both bands of the methylene group increased in
intensity after cooking by conventional electric oven,
whereas they increased in intensity after microwave
oven heating less than what occurred during cooking
in the conventional oven, as can be observed in Fig. 2
and in Table 1, where the integrated areas of the
main vibration bands are represented. Also, the CH2
bending vibration around 1460 cm�1 was observed
to be increased in intensity after conventional oven
heating more than what occurred after microwave
oven cooking, as indicated in Figs. 2 and 3.
A statistical analysis (t-test) was applied to the
integrated area of CH2 stretching and bending
vibration bands of different spectra, showing that
their intensities after microwave oven cooking were
significantly decreased in comparison to their
intensities after conventional oven cooking (p< 0.01).
The increase in intensity of the methylene group
after meat cooking in the conventional oven can be
attributed to lipid increases due to the Maillard
reaction. The Maillard reaction is one of the most
important effects of thermal processing of foods that
is generated by a series of reactions due to the
interaction between the carbonyl group of a reducing
sugar and a free amino group of an amino acid or a
protein, producing the formation of brown pigments
and numerous compounds responsible for browning,
texture, and flavor during baking and roasting by
means of complex reactions.[21]
A further relevant change in the midinfrared spec-
tral region upon conventional heat treatment was
observed around 1740 cm�1, where a broad band
can be observed in the spectrum from ground beef
meat cooked in the conventional oven. This band
can be assigned to the C=O mode of the first alkyl
chain with a trans-conformation in the C–C bond
adjacent to the ester grouping, representative of trigly-
ceride content.[19] The relevant increase in intensity of
this carbonyl band after cooking in a conventional
oven can be attributed to the Maillard reaction as well.
Indeed, the Maillard reaction follows the formation
of the initial intermediates, including Amadori and
Heyn’s products, among which there are CH2 and
C=O compounds.[22,23]
After cooking for 1½ min in the microwave oven,
this vibration band also increased in intensity signifi-
cantly less than what occurred during conventional
heating, as can be observed in Figs. 2 and 3, and
the corresponding integrated areas are compared in
Table 1.
The results that the symmetric and asymmetric CH2
vibrations and the C=O carbonyl band at 1742 cm�1
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after microwave oven heating were less intense than
after conventional heating can be considered a proof
that the Maillard reaction does not occur completely
after cooking in a microwave oven. Moreover, as
previous literature showed,[24,25] the higher the tem-
perature, the greater the Maillard browning reaction.
Therefore, spectroscopic results reported above high-
lighted that the temperature of the ground beef
during cooking with the microwave oven was less
than that which was induced by cooking with the
conventional oven at 175�C. Indeed, direct measure-
ment of the temperature inside the food during
cooking with a microwave oven cannot be carried
out by using conventional methods, since either a
thermocouple or a mercury thermometer would
interact with microwaves, altering the measure.
The spectra exhibited also an intense amide I band,
centered at 1648 cm�1, corresponding mainly to an
a-helix structure content due to a C¼O stretching
vibration and an N–H bending mode, and a low-
intensity amide II band, corresponding to coupling
of the N–H bending and C–N stretching modes. These
bands are related to the protein content in beef meat,
whose major component in meat is represented by
myoglobin. Indeed, myoglobin represents a myofiber
found in the muscle tissue of vertebrates.
TABLE 1 Average Integrated Area of the most Relevant Vibration Bands of Ground Beef Samples After
Exposure to Conventional Heating and Microwave Oven Heating
Vibration bands
Average integrated area
Raw
ground beef
Ground beef cooked
in a conventional oven
Ground beef cooked
in a microwave oven
asCH2 2925 cm�1 1.13� 0.06 7.01� 0.6�� 3.52� 0.3��sCH2 2853 cm�1 0.62� 0.05 4.87� 0.4�� 2.43� 0.3��
1740 cm�1 0.70� 0.05 6.95� 0.5�� 3.41� 0.4��
1695 cm�1 0.59� 0.03 0.68� 0.04� 1.07� 0.05��
1664 cm�1 0.84� 0.05 0.67� 0.05� 1.12� 0.05��
a-helix-1648 cm�1 2.00� 0.06 2.14� 0.05 2.20� 0.05
b-sheet-1635 cm�1 0.81� 0.05 0.86� 0.05 0.97� 0.05�
b-sheet-1572 cm�1 0.34� 0.03 0.35� 0.03 0.53� 0.04�
CH2 bend. 1460 cm�1 0.82� 0.07 3.12� 0.2�� 1.92� 0.15�
988 cm�1 0.14� 0.05 0.69� 0.08� 3.28� 0.3��
Each value reported represents the mean �SEM of 10 samples. Significant differences in comparison to raw groundbeef samples and ground beef samples cooked in the conventional oven are pointed out (��p< 0.01, �p< 0.05).
FIGURE 2 Representative FTIR spectra in the range 3000–970 cm�1 of ground beef meat. Blue, green, and red lines refer to raw ground
beef, ground beef cooked in a conventional oven, and ground beef cooked in a microwave oven, respectively. The CH2 methylene group
bands at 2921, 2853, and 1465 cm�1 and the carbonyl band at 1742 cm�1 of the ground beef sample cooked in a conventional oven are
indicated by arrows. Also, the b-sheet component at 988 cm�1 of the ground beef sample cooked in a microwave oven has been evidenced.
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Typical spectra acquired after the two different
cooking processes are shown in Fig. 3, in which it
appears that ground beef cooking in the microwave
oven at the power level of 800 W for 1½ min pro-
duced alterations in the amide I and amide II modes,
above all in the region from 1665 to 1695 cm�1 and
around 1635 and 1575 cm�1.
In order to enhance the microwave-induced
alterations in the amide I region of meat protein’s
content, the Fourier self-deconvolution (FSD) tech-
nique was applied using a Lorentzian shape, with
bandwidth¼ 6.78, deconvolution factor¼ 2, and
noise reduction factor¼ 0.5. Representative spectra
are shown in Fig. 4, in which blue, green, and red
lines represent, respectively, the spectra of raw
ground beef, ground beef cooked in the conventional
oven, and ground beef cooked in the microwave
oven after applying the FSD technique.
This analysis highlighted an increase of the
vibration band around 1665 cm�1 after cooking in a
microwave oven, which can be attributed to b-turns,
characteristic of disorder processes in theprotein.[26,27]
FIGURE 4 The Fourier self-deconvolution analysis in the region from 1700 to 1570 cm�1 highlighted a relative increase in intensity of
b-turn structure around 1665 cm�1 and an increase in intensity of the b-sheet components at 1695, 1635, and 1575 cm�1 in samples’ spectra
after cooking in the microwave oven (these vibration bands are indicated by arrows). Blue, green, and red lines refer to raw ground beef,
ground beef cooked in a conventional oven, and ground beef cooked in a microwave oven, respectively.
FIGURE 3 The FTIR spectra of Fig. 2 were zoomed in on in the region from 1800 to 1400 cm�1. Besides the carbonyl band at 1742 cm�1
and the CH2 bending at 1465 cm�1, the alterations in the amide I and amide II regions after cooking in a microwave oven are highlighted
and indicated by arrows. They are represented by an increase in intensity of the b-turn structure around 1665 cm�1 and increases of
b-sheet components at 1695, 1635, and 1575 cm�1.
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FSD analysis evidenced also significant increases
in intensity in b-sheet bands at 1635, 1695, and
1575 cm�1, comparing sample spectra after micro-
wave oven cooking and conventional oven cooking
(p< 0.05), which can be attributed to an increasing
transition dipole coupling due to a higher content
in aggregated b-sheet structures.
Peak intensity was also observed to be increased at
988 cm�1 after microwave oven cooking, as can be
observed in Fig. 5. This vibration band can be attrib-
uted to b-sheet structures in the amide III region.[28,29]
Such a significant increase when comparing micro-
wave oven cooking and conventional heating
(p< 0.01) confirmed that microwave oven heating
produces an increase in the b-sheet=a-helix ratio in
meat protein content. The average integrated area
values of the most relevant vibration bands of raw
ground beef, ground beef cooked in the conventional
oven, and ground beef cooked in the microwave
oven, along with their standard deviation of the mean
(SEM), are reported in Table 1. Statistical analysis was
applied to a set of 10 sample spectra, considering
significant p< 0.05. Otherwise, alterations of pro-
teins’ secondary structure in the amide I region
produced by exposure to mobile phone microwaves
was evidenced.[30–34]
The result of an increase in b-sheet structure in the
meat’s protein due to microwave oven heating should
be taken into account. In fact, as has been shown
extensively in the literature, proteins’ nutritive
effectiveness, digestive behavior, and utilization can
be influenced by their a-helix and b-sheet ratios in
the secondary structure, as a high b-sheet–to–a-helix
ratio can induce low access to gastrointestinal
digestive enzymes, resulting in a low protein value
and availability.[35–37]
CONCLUSIONS
The FTIR spectroscopy technique was applied
to investigate the effects of cooking ground beef meat
in a microwave oven at the power of 800 W for
1½ min. A relevant increase of methylene group
vibration bands and of carbonyl bands was evi-
denced after meat cooking with a conventional oven
at the temperature of 175�C, which can be attributed
to the Maillard reaction. After microwave oven cook-
ing, these bands increased significantly less than what
occurred after conventional heating. This result can
be considered proof that the temperature inside
ground beef during microwave oven cooking was
less than that which was induced by cooking with
the conventional oven at 175�C.
Relevant alterations in the amide I, II, and III
regions occurred after cooking in a microwave oven.
In fact, an increase in intensity around 1665 cm�1 was
observed, which can be attributed to b-turns, charac-
teristic of disorder processes in the protein. In
addition, significant increases of b-sheet content were
observed at 1695, 1635, 1575, and 988 cm�1 after
microwave oven cooking, which can be attributed
to aggregation processes of b-sheet structures.
FIGURE 5 FTIR spectra in the range 1130–970 cm�1 of ground beef, where blue, green, and red lines refer to raw ground beef, ground
beef cooked in a conventional oven, and ground beef cooked in a microwave oven, respectively. The significant increase in intensity of the
b-sheet component at 988 cm�1, indicated by arrow, was evidenced after cooking in a microwave oven.
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These results showed that nonthermal effects
occur after ground beef cooking with a microwave
oven.
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