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Page 1: Non-Thermal Effects of Microwave Oven Heating on Ground Beef Meat Studied in the Mid-Infrared Region by Fourier Transform Infrared Spectroscopy

This article was downloaded by: [Nipissing University]On: 17 October 2014, At: 12:51Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

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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

To link to this article: http://dx.doi.org/10.1080/00387010.2013.828313

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Page 2: Non-Thermal Effects of Microwave Oven Heating on Ground Beef Meat Studied in the Mid-Infrared Region by Fourier Transform Infrared Spectroscopy

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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