Microwave Baking of Bread; A Review on the Impact of Formulation and Process on Bread Quality

Abstract

Microwave heating (MW) is the most important method in bread baking owing to its efficiency in terms of space, speed, and ability to produce crustless bread. However, MW has some unfavorable effects on breadcrumb and staling. Additives are a promising solution to reduce the negative impacts of MW. In this review, the impact of MW baking on bread characteristics is investigated in terms of starch properties, mechanical properties, the staling phenomenon, crust formation, and bread aromas. Additionally, the effects of the addition of enzymes and technical enhancers in MW bread baking are studied in terms of bread properties, quality, and texture.

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Microwave Baking of Bread; A Review on the
Impact of Formulation and Process on Bread
Quality
Roua Bou-Orm, Vanessa Jury, Lionel Boillereaux & Alain Le-Bail
To cite this article: Roua Bou-Orm, Vanessa Jury, Lionel Boillereaux & Alain Le-Bail (2021):
Microwave Baking of Bread; A Review on the Impact of Formulation and Process on Bread Quality,
Food Reviews International, DOI: 10.1080/87559129.2021.1931299
To link to this article: https://doi.org/10.1080/87559129.2021.1931299
Published online: 23 Jun 2021.
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REVIEW
Microwave Baking of Bread; A Review on the Impact of Formulation
and Process on Bread Quality
Roua Bou-Orm , Vanessa Jury, Lionel Boillereaux, and Alain Le-Bail
ONIRIS, Nantes, France
ABSTRACT
Microwave heating (MW) is the most important method in bread baking
owing to its eciency in terms of space, speed, and ability to produce
crustless bread. However, MW has some unfavorable eects on breadcrumb
and staling. Additives are a promising solution to reduce the negative
impacts of MW. In this review, the impact of MW baking on bread character-
istics is investigated in terms of starch properties, mechanical properties, the
staling phenomenon, crust formation, and bread aromas. Additionally, the
eects of the addition of enzymes and technical enhancers in MW bread
baking are studied in terms of bread properties, quality, and texture.
KEYWORDS
Baking; microwave heating;
starch granules; staling;
amylose leaching
Introduction
Most large conventional baking lines are operated with natural gas owing to their availability and the
possibility of obtaining a large energy supply with a minimal infrastructure compared to electricity
(gas pipes vs. expansive electrical cables). Regardless, conventional baking is the most energy-
demanding method, with an energy consumption that can reach 6 MJ/kg for bread dough and less
than 40% of the energy directly used for dough baking and water loss of the bread.
[1,12]
This high-
energy consumption increases production costs; accordingly, auditing energy consumption in bread
baking can significantly reduce production costs. The bakery industry needs to provide answers to
market demand given a low investment in time, equipment, or extra labor costs.
[3]
Continuous
improvement in baking technology is worth investigating, primarily for better-quality products, the
development of nutritionally superior products, and reduced production costs.
[4]
There are many types of nonconventional ovens that use infrared, electrical resistance (ERO),
microwave (MW), radiofrequency (RF), and air impingement (forced air convection) or
a combination of these technologies. The technologies aim to reduce the baking time and energy
efficiency, and eventually improve the product quality.
[5–8]
Baking only with infrared waves produces
a thick crust layer; hence, infrared baking must be combined with other baking methods to be more
effective.
[5]
In the case of electrical resistance, the breads produced were found to be of inferior quality
because the starch swelling was less.
[9]
Hence, microwaves could be the best alternative for improved
quality products with reduced time and energy.
For several decades, microwave (MW) heating has had various applications in the field of food
processing. The application of MW heating in food processing includes drying, pasteurization, steriliza-
tion, thawing, baking, and reheating.
[10]
Today, MW baking has gained popularity in the processing of
food products through to its ability to achieve volumetric heating, higher heating rates, a significant
reduction in baking time, homogeneous heating, safe handling, ease of operation, and reduced main-
tenance. However, the number of factories that are MW-baking cereal products is minimal.
[11–13]
On
the other hand, MW-baked breads may have disadvantages such as unacceptable texture, high moisture
loss, and rapid staling.
[13]
This review highlights the various aspects of MW baking with regard to bread
CONTACT Alain Le-Bail alain.lebail@oniris-nantes.fr ONIRIS, Bp 82225 44322, Nantes, France
FOOD REVIEWS INTERNATIONAL
https://doi.org/10.1080/87559129.2021.1931299
© 2021 Taylor & Francis

characteristics. First, the MW heating mechanism and dielectric properties are discussed. Then, the
impact of MW baking compared to conventional baking on bread qualities such as starch and texture,
the stalling phenomenon, crust formation, and bread aromas are investigated.
Figure 1. Heat generation by a) ionic interaction and b) dipole rotation under the action of an electromagnetic field Reprinted with
permission from Sahin et al 2005.
[15]
.
Figure 2. Schematic representation of starch granule structures A. Starch granules and a schematic description of the growth ring
model. B. Chain distribution model of the starch hilum region. Amylose molecules and amylopectin molecules are not organized into
crystalline arrays. Amylopectin molecules are distributed randomly between the amylopectin crystalline clusters. C. Model of
crystalline lamellae and amorphous lamellae in an alternate distribution. D. Aligned double helices (comprising amylopectin side
chains) within crystalline lamellae and amylopectin branch points within amorphous lamellae. E. Amylose distribution in the chain
distribution model. F. Partial molecular structures of amylose and amylopectin. Reprinted with permission from Yuan et al 2020.
[30]
.
2R. BOU-ORM ET AL.

Microwave heating mechanism of food materials
MW heating of food materials is volumetric heating, which occurs mainly because of dipole rotation
and ionic interaction (Figure 1). The conversion of MW to thermal energy yields a progressive decay
of the magnitude of the electromagnetic field while the wave is crossing the material. The decay is
ruled by Beer’s law in the case of a matrix of large dimensions or with low dielectric properties, as
discussed below. The depth of penetration corresponds to the distance at which the wave amplitude is
reduced by 63% (95% for three times the depth of penetration).
[14]
Only dielectric ingredients are able
to heat food under the action of MW. These dielectric compounds are characterized by their zero
electrical charge. In other words, they are electrically neutral entities, one part of which is positively
charged and another is a negative entity. These constituents form electrical dipoles in food. Water is an
important component of most food products. This is the main component that interacts with MW
because of its strong dipole rotation (presence of positively charged hydrogen atoms and a negatively
charged oxygen molecule).
[11,15,16]
Food may not be considered a good electrical insulator or good electrical conductor, but it can be
classified as “dielectric material with losses.” In order to understand the heating patterns of food in an
MW oven and the interaction of the electric field with the food matrix, it is important to determine its
dielectric properties. The dielectric properties can be classified into two categories: the dielectric
constant (real part) and the dielectric loss factor (imaginary part). The capacity of a food to absorb
and store MW energy is its constant dielectric, while the ability of a material to dissipate MW energy
into heat represents its loss factor.
[15]
In addition, the two parameters of the food dielectric property
constitute the relative electrical permittivity of the material (the link between the material and electric
field), which is given by the following equation:
εr¼ε0jε (1)
where j = √1, ε′′: loss factor, and ε′: dielectric constant (F/m).
Indeed, the dissipation factor reflects a phase-shifting state (offset) of polarized materials when they
orient toward the direction of the electric field owing to intermolecular friction. The dissipation factor δ
is the ratio between the dielectric loss of the samples or the loss factor ε′′ and the dielectric constant ε′.
tan@¼ε00
ε0(2)
The dissipation factor and penetration depth of the MW energy at a given frequency are closely
correlated. Therefore, more MW energy is dissipated in the form of heat. In fact, the penetration depth
(Z in m) represents the distance for which power decreases to 63% of its initial value. This is given by
Eq. (3):
Z¼0
2πf 20
ð Þ0:5ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1þ00
0
2
1
s
2
43
5
1=2
(3)
where 0is wavelength-free space (m), and f is the frequency in Hz.
The dielectric properties of food matrices and therefore the depth penetration depend on many
factors such as frequency and temperature but also on the water content and internal composition of
the product (especially the quantities of salt and fat). Indeed, the dielectric properties increase with an
increase in the water content of food materials (increasing the dielectric polarization). The ratio of
bound to free water present in food material can also affect the dielectric properties with variations in
temperature. The dielectric properties in the case of bound water increase with a rise in temperature
and decrease in free water.
[15]
In addition, the presence of solubilized ions such as sodium chloride can interact with warming
under the effect of MW, as described by Anwar et al.
[17]
In such cases, an ionic conduction heating
effect is considered. Anwar et al. (2015) investigated water samples with and without inorganic ions
FOOD REVIEWS INTERNATIONAL 3

exposed to microwaves in a personalized oven under similar conditions. A decrease in the temperature
rise was observed with an increase in the quantity of ions. In addition, the temperature rise was directly
proportional to the size of the ions. These authors concluded that under their conditions (domestic
microwave oven at 2450 MHz), the presence of ions tends to suppress the dielectric polarization,
resulting in a reduced efficacy in the conversion of the MW energy in heatup compared to diluted
aqueous solutions or pure water.
[17]
Microwave heating mechanism during bread baking
The heat generation in microwave baking results from losses by the conduction of free electrons and
by polarization during dielectric dipole alignment in the direction of the electric field. In microwave
baking, the energy equation including heat generation, which determines the central temperature of
the sample, is given by the following relation:
ρCp
@T
@t¼div:kÑTð Þ þ Qabs (4)
where ρ is the density, Cpis the specific heat capacity, k is the thermal conductivity, andQabs is the
quantity of heat dissipated per unit of volume.
To estimate the amount of internal microwave energy transformed into heat, it is necessary to
calculate the microwave power absorbed by the bread, which is converted into thermal energy per
volume unit (Qabs). This is given by Maxwell’s equation:
Qabs ¼σE2(5)
Furthermore, the electrical conductivity that characterizes the ability of a material to lose electrical
charge is given by the following relationship:
εr¼ε0jε (6)
where σ is in S/m (S: Siemens);, the angular frequency in rad/s; f is the frequency in Hz; ε0 is the
permittivity in vacuum; and ε′′ is the loss factor.
During MW baking, material transfer phenomena within the bread are all more complex because
the phenomena take place in a porous medium. The mass transfer mechanisms during microwave
baking involve diffusion by evaporation and condensation. However, these transfers of material in
MW bread baking rely mainly on the transport of water in the vapor state between the crumb and the
crust.
[18]
The phenomena of heat and mass transfer during MW bread baking are strongly coupled and
linked to the bread dielectric properties.
[19]
Tong et al. (1993) developed a coupling equation between the simultaneous transfer of heat and
moisture for the microwave baking of dough. In this study, the mass transfers caused by temperature
were neglected, and the coefficient of heat transfer to the surface was considered constant. In addition,
the temperature and moisture content were initially uniform with an absorption of the electric field at
the surface. In addition, C. H. Tong et al.
[20]
reported that the dielectric properties of a porous material
in the case of bread might be a function of the pore size distribution and physical structure.
[20]
Few studies exist on mass transfer during microwave baking of bread after the study of
C. H. Tong.
[20]
In 2006, Zhang and Datta developed a model for coupling heat and mass transfers.
The objective of this model was to study the effects of conventional and microwave baking on the
browning of the crust and the volume of the bread.
[21]
Purlis et al.
[22]
developed a model to study the
phenomena of evaporation and condensation of water during the baking of bread and their effect on
the gelatinization of starch and browning of the crust. This model resulted in optimized baking
conditions by identifying the parameters that have the most positive effects on the starch and sensory
state of the bread.
4R. BOU-ORM ET AL.

Recently, Nicolas et al.
[23]
studied the thermohydric behavior of bread dough during baking using
three simultaneous modes (convection, infrared, and contact conduction). A simulation of bread
baking using a multidimensional 2D model was developed. This model was used as a tool to predict
changes in temperature, water content, gas pressure, and the values of the bread porosity and gas
fraction.
The values listed in Table 1 show the depth of penetration of various food products at 20–25°C and
2450 MHz. The value for bread is very high (over 1 m), indicating no a priori overrun phenomena
(excess temperature rises on selected spots depending on the interaction between the electromagnetic
waves and the load in the cavity) were observed when considering breads of standard size (that is,
10 cm × 10 cm × 30 cm).
Applications of Microwave heating in bread baking industry in order to obtain a crustless bread
The use of microwave energy in the bread baking process is an interesting alternative for researchers
because of its benefits in terms of time and room-saving. Today, this innovation arouses some baking
industries to produce crustless bread due to the increased demand.
To produce crustless loaves, industrial producers trim bread loaves obtained by conventional
baking by mechanical means. Since bread loaves are parallelepiped shaped, a sophisticated six side
knife system is utilized to cut the crust. This procedure is carried out at low temperature to facilitate
cutting the crust. This mechanical crust cutting procedure generates a considerable loss of raw
material that may reach 45% of the dough weight. The cited method was registered under the patent
number EP 1 586 428 B1 by the company Harry’s France.
[25]
BIMBO and PANRICO have patented a dielectric heating technique for bread baking by Radio
Frequency (Patent number: EP 2 204 096 B1). This invention lies in the application of RF in to produce
bread (baked in its mold) with a very fine outer layer that has the same color as the bread crumb. The
baking stage is carried out in a way that the dough is uniformly baked by dielectric heating in an RF
oven by passing the dough between lower and higher electrodes so that the electrodes generate an
oscillatory electric field. Once the bread leaves the oven, it is dried before depaning to remove the
condensed water from the pan wall without increasing the surface temperature and yield crust
formation. Besides, the drying stage is carried out in a conventional oven at a temperature below
220°C for less than 10 min. After the drying stage, a depaning step involving a roll-over operation of
the pan and an air blowing operation between the baked bread and the pan, so that the bread
contained in the pan is depaned by gravity. Finally, the baked bread goes through a final stage of
cooling. This step is carried out in a well isolated cooling enclosure to prevent any source of air born
microbial contamination. Finally, the cooled bread is sliced, packaged and stored in a clean and
appropriate place.
[7]
Table 1. Dielectric properties of food at 20–25°C and 2450 MHz. Reprinted with permission from Fellow, 2009
[24].
Material Dielectric constant (Fm
−1
) Loss Factor Penetration Depth (cm)
Banana (raw) 62 17 0.93
Beef (raw) 51 16 0.87
Bread 4 0.005 1170
Brine (5%) 67 71 0.25
Butter 3 0.1 30.5
Carrot (cooked) 71 18 0.93
Cooking Oil 2.6 0.2 19.5
Distilled Water 77 9.2 1.7
Fish (cooked) 46.5 12 1.1
Glass 6 0.1 40
Ham 85 67 0.3
Ice 3.2 0.003 1162
Paper 4 0.1 50
Polyester tray 4 0.02 195
Potato (raw) 62 16.7 0.93
FOOD REVIEWS INTERNATIONAL 5

The company Jacquet Panification (France), one of the most well-known European bakeries,
patented (patent number: US 9,648,883 B2) a baking system to produce crustless bread using MW
technology (2450 MHz). In this context, one of the essential objectives of this invention was to provide
a new method to produce bread products in an MW oven, in which the depanning stage can be easily
carried out.
[26]
The existing industrial processes using MW (Jacquet) produce crustless loaves of bread,
demonstrating that electromagnetic baking is a viable technique.
Despite a lack of commercial applications for microwave baking, microwave baking has aroused the
interest of some bakers. The Stalam Company in Italy manufactured some microwave baking equip-
ment for producing bread with white crust. This equipment had high production capacity and great
energy efficiency.
[27]
In a conclusion, the 2 existing industrial processes using MW (Jacquet) and RF (PanRico) are
producing crustless bread, demonstrating that electromagnetic baking is a viable technology. Both
industries had to find means to depan the bread and filled up two specific patents. Indeed, during
baking, the moisture is diffusing from the warmer zone to the colder zone. In the case of conven-
tional baking, the warmer zone is the pan whereas the opposite occurs for MW or RF baking. In
turn, the moisture will condensate on the wall of the pan, resulting in difficulties for depaning the
bread.
Eect of MW heating on bread properties
The use of microwave energy in bread baking is an interesting alternative because of its benefits in
terms of saving time and space, its low energy cost, and the production of crustless bread. Indeed, in
microwave-baked products, the temperature rises more rapidly (higher heating rates), which explains
the shorter overall baking time (about one-fourth) and greater energy savings.
[9]
Moreover, microwave
baking has another advantage of producing breads with a higher nutritive value, in particular, the high
availability of some essential amino acids (lysine) and a lower browning reaction compared to
conventional baking.
[28]
On the other hand, several studies reported some disadvantages related to
microwave baking. These problems include unacceptable texture, low volume, high moisture loss, and
rapid staling.
[13]
Most physicochemical reactions during microwave baking are not fully completed
owing to the short baking time; this may affect the bread quality. It was stated that microwave baking
induces a higher moisture loss, insufficient gelatinization, changes in the gluten structure, and
significant amylose leaching. As a result, crumbs obtained by microwave heating are more frequent
in the bread firmness, and these potential modifications might cause rapid staling.
[9,13]
Moreover, the
cause of rapid staling in microwave-baked bread remains unclear and requires a deep understanding
of the effect of microwaves on each physicochemical reaction.
Eect of MW heating on starch granules morphology and crystalline structure
The starch granules (Figure 2) are semi-crystalline entities consisting of amylose (26–28%) and
amylopectin (2–74%) belonging to the family of polysaccharides and composed of
D-anhydroglucopyranose units. The 2 entities differ in the nature of the bond between the
D-anhydroglucopyranose chains. These chains can be linear (from 17% to 28% of the starch) with α
links (1–4) and constitute amylose, or branched with α links (1–6) and constitute amylopectin.
[29]
The
semi-crystalline structure consist of amorphous region (amylopectin with branched points and
amylose), and crystalline region composed of micro-crystalline bundles formed by amylopectin
double helix structures.
[30]
During bread baking, starch undergoes several hydrothermal transformations grouped under the
term gelatinization (Figure 3). The phenomenon of gelatinization involves the loss of the birefringence
of the starch grains (disappearance of the “Maltese cross” observed in polarized light). The swelling of
the grains (begins at around 55°C) followed by the solubilization of the amylose (leached out into the
aqueous intergranular phase) also modifies the consistency of the environment causing a rapid
6R. BOU-ORM ET AL.

increase in the viscosity of the continuous phase. The rate of gelatinization in water-limited systems
depends mainly on the amount of water present or available and the baking temperature reached by
the product. Depending on these factors, gelatinization will be more or less important.
[32,33]
MW treatment largely alters the morphology of starch granules in baked bread. The microstructure
of starch granules belonging to microwave-baked bread observed by microscopy was reported in a few
studies. In the study of Marta et al. (2019), the starch was extracted from conventionally baked fruit
bread, then mixed with 30% water and directly exposed to microwave heating at 1 W/g for 20 min.
Microscopic observations presented in Figure 4 showed that most of the starch granules were
completely deformed and broken under MW heating. Its granular structure and round shape
disappeared with the presence of small holes on the surface. These results could be related to the
rearrangement or transfer of the molecular structure; hence, the formation of a porous surface and
a cavity at the center granules.
[34]
The results presented previously are in agreement for MW baked bread combined with IR where
MW heating had a predominant effect on starch granules because of its internal heat transfer. SEM
observations showed that starch granules in breads baked with IR-MW (Figure 5b) were not com-
pletely distorted with the presence of granular residues associations. Hence, this observation may be
related to the high moisture loss during microwave heating and the high heating rate leading to an
incomplete starch swelling and gelatinization. On the other hand, in the conventional baked sample,
the swelling and the gelatinization of starch granules (Figure 5a) was more completed with
a continuous and uniform structure
.[35,36]
In addition to the effect of microwaves on starch granules, the increase in MW power and
processing time could significantly affect the starch microstructure. According to Xie et al.,
[37]
a rise
in the duration of potato microwave heating (2450 MHz, 750 W) from 5 to 20 s at 40°C to 95°C,
respectively, will provoke severe deformations such as cracks and the destruction of most starch
granules [with a final solid concentration of 33% (w/w) on a dry weight basis]. These findings were
explained by the fact that microwave heating strongly disrupts starch granules by affecting the trapped
water molecules in starch crystals, leading to the severe destruction and rupture of most starch
granules.
[37]
In fact, after initial swelling, the starch granules were broken into granular residue during
MW heating. This is probably owing to the strong vibrational motion of the polar molecules after the
application of MW energy. Furthermore, the nature of the residue after MW heating was different
Figure 3. Hydrothermal changes of starch granules occurred during conventional bread baking (I) Native starch granules; (II)
gelatinization, associated with swelling [a] and amylose leaching and partial granule disruption [b], resulting in the formation of
a starch paste; (III) retrogradation: formation of an amylose network (gelation/amylose retrogradation) during cooling of the starch
paste [a] and formation of ordered or crystalline amylopectin molecules (amylopectin retrogradation) during storage [b]. Reprinted
with permission from Goesaert et al., 2005
.[31]
.
FOOD REVIEWS INTERNATIONAL 7

from that observed after conduction heating, in which the granules retained their integrity even at 90°
C. These attributes influence the properties of dough gel that uses MW energy as a source of heat.
[38]
The severity changes caused by microwave treatment is represented by the increase in the gelatiniza-
tion temperature and the decrease in the degree of crystallinity of the treated starches. Previous studies
showed that microwave heating increases the gelatinization temperature compared to untreated
samples. It has been suggested that the rise in the gelatinization temperature was owing to the
association of amorphous amylose or configuration in a granular structure with greater stability.
[39,40]
The swelling of starch granules after MW baking did not occur before the loss of birefringence of
the starch granules as reported by Palav and Setharman.
[41]
In this study, experiments were conducted
on wheat starch baking with MW at 1.3 kW until reaching 65°C, 75°C, 85°C, and 95°C at different
starch concentrations (1%–8%) and solid concentrations of 33%, 40%, or 50% on a dry weight basis.
Based on polarized light microscopy images, the loss of birefringence indicates that the starch granules
have lost their crystallinity, which is one of the starch gelatinization steps. This behavior is different in
starch suspensions heated by the conduction mode, in which the swelling of granules and loss of
birefringence occur almost simultaneously.
[41]
Furthermore, microscopic images showed that the
Figure 4. Granules starch morphology of native and modified breadfruit starch irradiated by MW. Reprinted with permission from
Marta et al., 2019
[34]
.
8R. BOU-ORM ET AL.

increase in MW temperature exposure contributes to enhance the amount of leached amylose and the
complete rupture of starch granules with the presence of dark clumps of staining visible.
[42]
The effects of MW heating on the crystalline structure of bread starch granules using X-ray
diffraction (XRD), the amylopectin retrogradation by differential scanning calorimetry and setback
viscosity by Rapid ViscoTM Analyzer (RVA) during storage was compared to conventional baking by
Ozkoc et al 2009 and Patel et al 2005.
[35,43]
According to the statistical analysis (ANOVA), a positive interaction between the starch retro-
gradation and the crystallites formation was reported by Ozkoc et 2009.
[35]
The starch retrogradation
enthalpy and crystallinity values were significantly higher in MW baked bread due to the rapid staling
problem of microwave heating. It was reported that starch swelling, hydration, gelatinization granule
are all different in MW than that of the other samples baked in conventional and combination ovens.
X-ray diffraction patterns presented in Figure 6 showed an additional peak at 15.8 in MW baked bread
compared to conventional baking. This can indicate the more crystalline structure in MW baked
bread. This could be related to the faster increase of temperature by MW heating which can lead to the
formation of a more starch crystalline structure with a great water holding capacity.
[34]
The crystalline
structure may result from the complex bonds between amylose in the crystalline region and amylo-
pectin in the amorphous region.
[44]
In fact, under MW heating starch granules are significantly
disrupted which might result from an increase in the leached starch amount in bread (amylose) in
addition to the modification of the physical orientation of branched amylopectin molecules.
[35,45]
These results are in agreement with the X-Ray diffraction Patterns in starch exposed to MW and
extracted from breadfruits baked in CV baking. It was shown that MW treatment changed the XRD of
native starch from B to A + B type pattern. MW heating seems to induce the formation of double-helix
chains with the starch crystallites shifting which may result in a more crystalline structure than in
native starch.
[34]
Moreover, MW radiation significantly affects starch crystallite reorientation and
rearrangement and leads to an increased degree of crystallinity.
[46]
Setback viscosity measurements showed that microwave baked bread had the highest values.
According to several studies, viscosity is related to staling and its increase is linked to the increase of
amylose and amylopectin chains aggregation.
[26]
The results obtained could be related to the fact that
starch granules in bread baked in the microwave oven was not completely disintegrated.
Indeed, during conduction heating, the destruction of the radial arrangement of the amylopectin
chains is facilitated by the swelling of granules in the amorphous region. On the other hand,
microwave heating induces a loss in the crystalline arrangement of amylopectin. This phenomenon
was observed at a lower temperature (65°C) than conventional cooking. In fact, the dipolar rotation of
Figure 5. Starch microstructure observed in breads baked in a) IR-microwave combination and b) conventional ovens (white arrows
represent starch granule residues, whereas black arrows represent deformed starch structure. Reprinted with permission from Ozkoc
et al 2009
[36]
FOOD REVIEWS INTERNATIONAL 9

polarized molecules in amylopectin chains directly influences the crystalline lamella of amylopectin,
disorienting their radial association. Therefore, the vitreous transition of the amorphous region
appears after the total destruction of the crystalline structure of the amylopectin chains, resulting in
partial swelling.
[47]
Lewandowicz et al.
[38]
found that the greatest amount of soluble amylose was detected in MW-
treated samples at 0.5 W/g for 1 h with 30% moisture content. Indeed, the amylose content in the
bread was more affected by MW treatment than by conventional treatment.
[38]
After MW treatment,
the interaction between amylose and amylopectin and the improved dispersion of amylose results in
an increase in the “soluble” amylose content in the matrix.
Keskin 2004 proposed the Higo effect to explain the enhanced amount of leached amylose during
MW bread baking. Based on this “Higo effect” hypothesis, when MW bakes the bread, amylose is
increasingly leached out of the starch granules. Additionally, amylose is less bound to water and is
more disoriented during staling, which the opposite of conventional is baking. This promotes its
crystallization and therefore its crumb firmness
.[35]
In this part of our review, it is noted that there are several studies that investigated the effect of
microwave radiation on the different types of starch (corn, rice, wheat) microstructure presented in
a form of suspension mixed with water. In addition to the effect of MW heating combined to different
heating modes (infrared, jet-impingement, halogen lamp) on starch properties of baked products
(cake or bread). More studies should be done on bread baked only with MW without the combination
of other heating modes and compared to conventional baking with a deep study of starch granules
using scanning electron microscopy. The starch leached suspensions (soluble amylose) can be
observed using microscopy and by the exposition of starch granules to iodine vapor. Besides, the
effect of microwaves on the water molecular mobility between starch and gluten can be studied by the
mean of nuclear resonance magnetic. Differential scanning calorimetric could be also a new insight to
study the evolution of amylose crystals during staling. Studies should be supported by some
Figure 6. X-ray pattern change after 1 and 120 h storage for control breads baked in different ovens (a conventional 1 h;
b conventional 120 h; c microwave 1 h; d microwave 120 h). Reprinted with permission from Ozkoc et al., 2009
[35]
.
10 R. BOU-ORM ET AL.

microscopic image analysis such as SEM to verify the complete disruption of MW bread starch
granules and to observe the lixiviate amylose suspension by the exposition of starch granules to iodine
vapor. An in-depth study on the effect of microwaves on starch granules, their structure, dielectric
properties, the evolution of their molecular composition, and their bound water content could
constitute a method to better control the formulation and the process to reduce the properties defects
of microwaves such as crumb hardness and staling kinetics. This proposition confirms with Higo
effect, and constitute a new solution to better control the formation of amylose crystallization and
improve bread quality.
Eect of MW baking on mechanical properties of bread
Ribotta et al.
[48]
observed that the firmness of bread is related to the retrogradation of amylopectin and
the heating rate during baking. Using a miniaturized baking system based on Peltier elements, Ribotta
observed that increasing the heating rate yields a faster staling rate and a firmer bread crumb texture.
Many studies have reported on the effect of MW baking on bread firmness. It has been demonstrated
that MW bread baking may develop several textural defects.
[48]
Patel et al.
[49]
reported that breads baked at a higher heating rate, as in the case of MW baking, have
a firmer and stiffer texture. Ovadia and Walker
[50]
found that an increase in MW energy and its power
greatly increased the crumb firmness. Indeed, the force required to compress a given area of crumb by
25% can characterize firmness. However, the force required to separate a slice of bread characterizes
tenacity. Therefore, the outer parts of the crumb have a rubbery and resistant texture, while the inner
parts are firm and difficult to chew.
[50]
In addition, the authors suggested that the increased crumb firmness during storage is potentially
related to the hydration of starch granules, its swelling, dispersion, and the reassociation of amylose
molecules, which are all affected by MW heating during baking. Therefore, the increase in the melting
enthalpy of amylopectin observed during bread staling appears to be strongly related to the baking
conditions.
[50]
According to Ghiasi et al.,
[51]
firming was a result not only of amylopectin retro-
gradation. Many factors other than the retrogradation of amylopectin can contribute to the firmness of
MW-baked bread. First, the MW-gluten interaction has an adverse effect on the firmness and
toughness of MW-baked breads.
[9]
Indeed, MW heating could have aligned the gluten proteins with
strong hydrogen bonds, resulting in increased firmness during cooling. In addition, the fact that the
major part of amylose is leached outside the granule during MW baking could explain the initial
firmness of the crumb compared to conventional baking.
[5,52,53]
Ozkoc et al.
[35]
reported that bread
firmness can also result in hydrogen bonds between the gelatinized starch and the gluten network
mediated by amylose molecules leached out during baking. Moreover, this can involve hydrogen
bonds between retrograded starch molecules and the gluten network. In addition, the higher heating
rate in MW might increase moisture loss. This can accelerate protein-starch interactions (gelatinized
or retrograded) as well as starch-starch interactions, thus producing a firmer texture. Therefore, the
moisture of the crumb and the firmness are closely related.
[45,54]
Eect of MW baking on bread organoleptic properties
Bread aromas are important characteristics that influence the quality of bread. More than 500 volatile
compounds have been detected in several types of breads. The bread aroma is attributed to a complex
mixture of aromatic substances generated by enzymatic reactions and the fermentative activity of the
yeasts as well as thermal reactions during baking. Aromatic substances include alcohols, esters,
aldehydes, ketones, pyrazines, lactic acids, and pyrrolines in addition to furans, hydrocarbons, and
lactones. According to Martínez-Anaya et al.,
[55]
the enzymes during bread baking that allow for the
generation of aromatic components are amylases, proteases, and lipoxygenases in addition to lipases,
proteases, and invertases. In classic baking, heat treatment induces an increase in volatile and
nonvolatile compounds, which contributes to the bread aroma.
[55]
FOOD REVIEWS INTERNATIONAL 11

There are two types of coloring reactions: caramelization and Maillard. Caramelization involves
the sugars present on the bread surface. The action of heat causes the hydrolysis of sucrose, which
produces reducing sugars such as galactose, glucose, and fructose.
[55]
These reducing sugars are then
degraded and form colored compounds with a characteristic scent of caramel, acid, and bitterness.
At higher temperatures, this reaction also generates carbonyl compounds such as esters and
aldehydes. On the other hand, the Maillard reaction leads to the formation of brown pigments
and volatile compounds. This reaction results from the interaction between reducing sugars and
amino acids from 145°C. This reaction takes place in three essential steps: Maillard condensation,
rearrangement of amadori, and dehydration. Each stage results in the formation of glucosamines,
cétosamine, and carbonyl compounds, respectively, according to the baking temperature of the
bread.
[55]
MW bread baking causes the complete volatilization of different aromatic compounds at different
speeds and proportions from those observed during conventional baking. The biggest difference
between the two heating modes lies in the inability of MW ovens to cause browning because the
surface temperature does not exceed 100°C. In fact, the cold ambient temperature in the MW oven
causes surface cooling of MW-baked products, and a low surface temperature prevents the formation
of Maillard browning reactions. These are essential for the production of many flavored and colored
compounds.
[13]
In the MW oven, short-term exposure and a lack of hot and dry air (MW-clear air) surrounding the
bread surface not only prevent crust formation but also promote dryness owing to condensation
phenomena. On the other hand, the rapid release of moisture and its evaporation at the food center
lead to the formation of volatile substances added to temperatures below their boiling points. As
a result, the temperature and vapor pressure gradients increase the internal concentration and thus
keep the volatile aromatic compounds inside the bread.
[13]
These aromatic compounds do not have the possibility to be developed easily and are bound by
starches and proteins. Possible solutions to these problems include the substitution of modified
starches with low-amylose starches, thus eliminating the number of potential binding sites. Special
aromas were developed for baked products in MW ovens to improve the final taste of the product. On
the other hand, surface and unbonded aromatic components are subjected to distillation losses owing
to the absence of crust.
[]
Changing the formulation of foods can minimize the volatility of aromatic compounds, reducing
their evaporation during MW baking. This can be realized by adding an oily phase or by increasing the
oil content in the bread recipe. Flavoring agents can also be encapsulated in order to reduce the
volatility of aromatic compounds.
[56]
Table 2 summarizes the various studies related to the effect of MW heating on bread qualities in
terms of starch microstructure, moisture content, crumb firminess, porosity, specific volume and
bread color.
MW heating and bread formulation
The MW heating rate and mechanism are major problems in the formulation of MW-baked products.
Physicochemical changes and interactions of the main ingredients are not always completed during
the short baking time of an MW system. Moreover, each component of the bread formulation interacts
specifically with the MW. The addition of certain ingredients and the optimization of the bread recipe
are essential for obtaining baked products in MW systems with organoleptic and mechanical qualities
similar to those obtained in conventional baking.
When bread is baked in an MW oven, a firmer crumb is obtained after cooling than in conventional
baking. In fact, the firmness of the breadcrumb is related to the starch gel formed during baking. MW-
baked products with acceptable texture can be obtained by proper control of the gluten protein
network, degree of starch gelatinization and starch granule disruption, and final moisture level of
the baked product during microwave heating. The problems faced in breadcrumb firmness can be
12 R. BOU-ORM ET AL.

s-
Table 2. Summary of remarkable results on MW baking. ** Starch Morphology and crystallinity, Moisture content and loss, Firmness, Porosity, Viscosity, specific bread volume, bread color and flavor.
Baking condition and Microwave
parameters Structure index (corresponding detection methods) MW heating effect on bread properties** References
●MW – 706 W, 2 min (14.12 g /W) ●Texture Analyser (TA)
●gelatinization and retrogradation of starch in bread (RVA)
●Crystalline structure (XRD)
●Retrogradation enthalpies (DSC)
●Increase of staling rate of MW baked breads
●A higher values in crumb hardness, viscosity, and starch crystalline mass
were reported in breads baked in MW ovens
[35]
●1 W/g 20 min ●Morphology (SEM)
●Crystalline structure (XRD)
●Gelatinization and retrogradation of starch in bread (RVA)
●An alteration in the structure of starch granule
●Formation of a more crystalline structure
●Decrease in PV and relative crystallinity
[34]
●MW at 706 W for 30%, 50%,
70%
●The water binding capacity of bread crumbs by using the
method of Medcalf & Gilles
●The increase in microwave power resulted in higher water binding
capacity values.
●Microwave-dried bread crumbs had higher water binding capacities
compared to conventionally baked ones
[65]
●Conventional Baking – 200°C
for 13 min.
●MW baking – 706 W for
0.75 min
●50 g dough
●Firmness (Texturometer, after 1 h baking and 2 days storage)
●Specific volume
●Crust color of the bread samples was measured using a Minolta
color reader (L*, a*, and b* color scale, total color change (∆E)
was calculated, dough is the reference point
●Higher Firmness and specific volume of bread baked in MW oven
compared to conventional baking
●Significant increase of bread firmness baked in MW after 2 days storage
●Lowest ∆E for breads baked in microwave oven → MW baking do not
promote maillard reaction and the production of flavor and color
compounds
[58]
●MW heated breads, after fro-
zen storage 3 min at 70 W
●Moisture content and weight loss determination
●The textural characteristics of both crust and crumb
●Porosity and specific volume
●Decrease of 20% to 40% in the specific volume of samples treated by
microwave treatment
●water loss during microwave heating was also low (!1%)
[66]
●Conventional baking – 225°C,
20 min
●Microwave baking (635 W) –
100%,
●Firmness
●Bread specific volume
●Weight loss
●MW heating increased the bound water of bread in accordance with the
rapid hardening.
●MW heating increase the starch-gel in bread,
[67]
●Conventional baking (19 min
at 218”C),
●Microwave baking (8 min at
0.3 kW and 0.2 milliamperes,
2450Mh.
●Color measurments
●Sensory anaylysis for aromatic attributes and texture
●Less crust and crumb browning and less reduction in lysine availability in
Microwave baking breads
●Non-significant difference for flavor between conventionally baked and
microwave breads
●Lowest score for texture was registered for MW baked bread
[28]
●20% MW (706 W) for 6.5 min
and an air injection at 10 m/
s,200°C
●IR-combined: 20% MW
(706 W), 8 min combined to
IR (70%)
●Moisture content
●Specific bulk volume and porosity
●The addition of MW heating increased the specific Volume, porosity of
baked bread (void porous structure)
●MW heating increase moisture loss due to internal heating and pressure
[6]
(Continued)
FOOD REVIEWS INTERNATIONAL 13

Table 2. (Continued).
Baking condition and Microwave
parameters Structure index (corresponding detection methods) MW heating effect on bread properties** References
●Breads reheated in conven-
tional and microwave ovens
(707 W) after 7 days of storage
●Moisture Content
●Toughness Analysis.
●Specific volume
●Extractable Amylose
●Microscopic Methods : Light Microscopy
●MW heating causes a significant toughening of the bread crumb
●decrease in Moisture content in bread reheated by MW
●Increase of bread specific volume, and a considerable increase in extrac-
table amylose
●Amylose, selectively stained by the iodine solution, was observed at the
outside the starch with a considerable accumulation at the air-cell wall
interface
[6869]
●Combination of forced convec-
tion with microwave energy
●Hybrid – 2.0 kW air heater,
1.2 kW microwave heating
●Small bread (80 g) and large
bread (520 g)
●Moisture content and water holding capacity
●Firmness
●Soluble Amylose
●Pasting properties
●Enthalpy of amylopectin retrogradation
●The highest Heating rate due to the applied microwave energy has
influence on starch properties which causes firmness development.
●The enthalpy of amylopectin recrystallization, rate of bread firmness and
the level of soluble amylose were all higher at the higher heating rate.
●Lower moisture content, water holding capacity and pasting properties
in hybrid oven due to the high heating rate induced by MW which can
vary the kinetics, and extent of disordering of the amylopectin crystals,
granule swelling, amylose leaching.
[49]
14 R. BOU-ORM ET AL.

Table 3. Studies relevant to effects of formulation on MW-baked bread properties.
Additive type Additive name and composition Baking Conditions Major Findings Reference
Emulsifier ●3 types of emulsifiers: DATEM (0.4%): diace-
tyl tartaric esters of Monoglycerides
●Lecimulthin (1%): soybean lecithin, wheat
flour, hydrogenated vegetable oil (3%)
●Purawave: lecithin, Soy protein, monoglycer-
ides, vegetable gums
●Conventional baking at 220°C, 20-
min MW baking (635 W) – 100%,
3.5 min
●Lipid effect (8–12%), emulsifier (-
1–3%), and dextrose (1–5%)
●Gluten content (+-)
Optimum Conditions:
Minimum weight loss, maximum firmness, and specific volume:
●For low gluten concentration → 15.12% lipid, 2.28% emulsifier, and
3.38% dextrose
●For higher gluten concentration → 12% lipid, 2.25% emulsifier, and
3.48 dextrose
●Flour with 8.7% protein is best for MW-baked bread
[
68]
Purawave,
DATEM
Lecimulthin M-45
●MW baking at 100% power for
3.5 min.
●Bread specific volume and firm-
ness were measured as quality
factors
●The specific volumes were significantly lower in breads prepared with
DATEM and Lecimulthin M-45
●Purawave and Lecimulthin M-45 reduce microwave breads firmness
●Non-significant difference between control breads and thoses formu-
lated with DATEM on bread firminess
●Purawave reduced bread firmness of breads and increase bread spe-
cific volume
Purawave is the most effective emulsifier on microwave bread
quality
[67]
Enzymes Xylanase,
α-amylase, protease
●MW: Power 706 W efficiency 74%.
●Independent Variables: MW power
and baking time
●50% power for 0.5, 0.75, 1.0, 1.5,
and 2.0 min;
●100% power for 0.5, 0.75, and
1.0 min.
●Conventional baking: 200°C for
13 min
●Non-significant difference of enzyme effect on moisture content
●Alpha-amylase and protease had significant effect on bread specific
volume
●Additional action of amylase to produce dextrins leading to genera-
tion of more CO2 → increase in dough expansion in oven and bread
specific volume
●Proteases increase extensibility of protein networks, leading to higher
gas retention and bread specific volume
●Xylynase enhances water redistribution from pentosan phase to glu-
ten phase, increases protein network extensibility, water retention,
and bread volume
●Positive effect of enzymes on bread firmness and enhancement of
texture softening
●α-amylases decreases amount of starch available for amylose leaching
and decreases firmness
●Xylanase increases water release; this ensures complete starch
gelatinization
●Increase in free water content reduces crumb firmness and raises
protein network extensibility
[58]
(Continued)
FOOD REVIEWS INTERNATIONAL 15

Table 3. (Continued).
Additive type Additive name and composition Baking Conditions Major Findings Reference
Hydrocolloïdes Carrageenan ●IR combined with MWP: 70% halo-
gen lamp, 20% MW, 8 min
Bread prepared with carrageenan gums:
Dielectric properties of carrageenan are significant and may increase
●Internal heating
●Internal pressure
●Ensure complete starch gelatinization
●Decrease in crumb porosity and specific bread volume
[62]
Xanthan-guar Bread prepared with xanthan-guar gums:
●Increase in bread specific volume and crumb porosity
●Decrease in firmness
With or without addition of xanthan-guar at
0.5%
●Conventional baking at 200°C,
13 min
●MW (707 W) 100% power, 2 min
●IR-MW combination: 70% halogen
lamp (1500 W), 20% MW, 8 min
Addition of xanthan-guar gum can retard kinetics of staling:
●Decrease in retrogradation rate of amylopectin and amylose
●Increase in viscosity
●Decrease in crystallinity, water loss, and firmness
●Decrease in water migration from crumb to crust
●Gum-starch interaction and gum-gluten interaction
Non-significant effect on water content between different heating
modes
●Important perception of viscous and slippery mouth texture
[35]
Carrageenan
Xanthan
Guar
xanthan-guar blend at 0.5% concentration
●Conventional Baking at 200°C for
13 min.
●IR combined with: 70% halogen
lamp(70% of 3000 W), 20%
(706 W) MW, 8 min
●Xanthan-guar blend increase pore area fraction and 70% of the pores
had diameter above 1000 mm (highest percentage among
formulation)
●lowest pore area fraction with k-carrageenan in both baking heating
mode
●Xanthan gum permits a more uniform micro-structure of air cell walls
●Distortion of starch granules in bread prepared with carrageenan (high
dielectric properties →ensure a more efficient heat →complete starch
gelatinization)
●In xanthan-guar and guar gum added samples granular boundaries of
starch molecules is observed
[36]
Xanthan and guar gum added at ‘low’ and ‘high’
concentration (0.16 and 0.65 g/100 g flour,
respectively)
●Bread baked in conventional oven
●for 20 min at 198°C (average air
velocity 1.1 m/s) : – Fresh samples
●Frozen sample for 7 days and
defrost by MW for 3 min at 70 W
●Increase in the specific volume of the control sample by adding
xanthan gum (0.16% flour basis)
●Increase in xanthan concentration or guar gum addition decrease the
specific volume and bread porosity
●Higher failure force (lower firmness) by the addition of Guar gum than
xanthan
●Porosity reduction and Viscous crumb in all samples
●Major softening and Rubbery crust especially in samples containing
guar gum
●Xanthan addition gave better bread properties at ‘low’ concentration
[66]
16 R. BOU-ORM ET AL.

olved by using lipids and emulsifiers that are expected to interact with starch polymers during baking,
resulting in softer crumb and delayed staling. The gelatinization of starch is known to affect the
volume of bread. If the starch is gelatinized too early during baking, then small volumes of bread are
obtained because of the rapid setting of the dough-crumb transitions. Reducing the water activity in
the crumb by using salts or dextrose helps to obtain a more uniform texture in baked breads by
MW.
[57]
Eects of emulsiers on microwave-baked bread
The addition of an emulsifier to the MW-baked bread formulation significantly affects the crumb
firmness and final bread volume. In general, MW baking increases moisture loss compared to
conventional baking. Moreover, some emulsifiers used in the bread formulation have been proven
Figure 7. Evolution of bread: (a) firmness and (b) specific volume baked in MW and conventional oven, prepared with different types
of enzymes. Reprinted with permission from Keskin, S. et al 2004
[60][30==]
.
FOOD REVIEWS INTERNATIONAL 17

very effective in moisture retention during MW baking. These emulsifiers include soya lecithin,
monoglycerides, and diacetyltartaric esters of monoglycerides. This efficiency allows for the inhibition
of bread firmness, softens its texture, and increases its specific volume in the presence of MW. Indeed,
the addition of emulsifiers and lipids can be correlated to a soft crumb texture owing to their high
moisture content. This further promotes water retention owing to the high binding capacity of
emulsifiers to water molecules.
[57]
Seyhun et al.
[53]
reported that the addition of emulsifiers in the
formulation of MW-baked cakes significantly decreased the soluble amylose content by forming
insoluble complexes with emulsifiers. A similar result was obtained for crumb firmness because its
variation during storage could be related to the leaching of soluble amylose. Therefore, the presence of
emulsifiers in bread formulations during MW baking delays the firmness rate during storage. Finally,
an optimum lipid content is strictly necessary for the proper functioning of emulsifiers; hence, fat
content of 25% appears to be the most effective for all types of emulsifiers.
[53]
Eects of enzymes on microwave-baked bread
The effects of different enzymes (xylanase, α-amylase, and protease) on the specific volume, moisture
content, and firmness of MW-baked breads were studied by Keskin et al.
[58]
Regarding the moisture
content of the breads, no significant difference was found between samples prepared with different
types of enzymes and control, regardless of the mode of heating. It was shown that all enzymes studied
significantly increased the bread specific volumes (Figure 7). Alpha-amylase and protease had the
greatest effect on the variation in the specific volume of MW-baked products compared to conven-
tional baking. The positive effect of α-amylase on the specific volume of breads was attributed to its
influence on starch. During fermentation, amylases allow the damaged starch to be degraded into
dextrins, which are then fermented by the yeast and generate additional carbon dioxide (CO
2
),
resulting in an increase in the bread volume.
[58]
On the other hand, proteases act on gluten, increasing
the dough extensibility, which may ensure better gas retention and oven rise. As a result, breads treated
with proteases had a larger volume.
[59]
Emulsifiers comprising mono- and diglycerides were shown to
increase the volume of MW-baked breads.
[57]
Since lipases act on lipids by producing mono- and
diglycerides. The impact and interest of lipase in reducing breadcrumb firmness can thus be explained.
Xylanase is another enzyme that can significantly affect the specific volume of bread and the staling
of breads during MW baking. This might be owing to its effect on the water transfer from petosane and
gluten, which might increase the gluten extensibility and therefore the final bread volume. In terms of
firmness, the enzymes studied had a significantly positive effect while improving the softening of the
bread texture during MW baking (Figure 7). The softening effect of xylanase was related to the mono-
and oligosaccharide resulting from the hydrolysis of polysaccharides by xynalase. These substances
might increase the amount of water released and affect protein-starch interactions. Indeed, water
realized can affect starch gelatinization, causing the formation of an amyloidose-lipid complex, and
free water makes the dough more extensible and less firm.
In addition, during MW baking, the firmness is related to the massive leaching of amylose
compared to conventional baking. Since α-amylases are starch-hydrolyzing enzymes, they can disrupt
the starch network and thus reduce the amount of starch available for amylose leaching, resulting in
a reduction in bread firmness. For this reason, alpha amylase appears to be one of the enzymes that has
a significant positive effect on reducing bread firmness during MW baking.
[58]
According to Keskin
et al.,
[5]
the firmness problem of bread baked in MW can be associated with gluten-MW interactions;
however, this mechanism is still not clear. Proteases are able to decompose gluten, and as a result, less
gluten is available to interact with MW, and breads will have a less firm texture. As explained
previously, lipases produce mono- and diglycerides, resulting in an enhancement of the crumb
softness owing to the formation of amylose lipid complexes, which are known to delay the crystal-
lization of the amylose component of crumb. Keskin et al.
[60]
described this phenomenon in the case of
MW baking of bread.
18 R. BOU-ORM ET AL.

Eect of hydrocolloids on microwave-baked bread
Gums are water-soluble polysaccharides with very high molecular weight (more than 1 million).
Several authors have reported that gums act as texture enhancers, emulsifiers, fixation agents, lipid
substitutes, stabilizers, and preservatives. It has been suggested that hydrocolloids are able to affect the
crumb microstructure (pore size and distribution) because of their interactions with bread constitu-
ents, especially starch granules and the type of heating mode.
[33]
Ozkoc et al.
[35]
analyzed the
microstructure of bread crumb formulated with two different types of gum (xanthan-guar and
carrageenan) and baked in an infrared combined microwave oven (IR-MW; 70% halogen lamp:
2100 W, 20% MW: 141 W, 8 min) and a conventional oven (at 200°C for 13 min).
[35]
The starch granules present in the conventional-oven-baked samples containing all types of gums
were more distorted than those baked in IR-MW. It was observed that the gums covered the surface of
the starch granules and formed a veil-like structure. Indeed, it was shown that breadcrumbs prepared
with xanthan gum have significant porosity regardless of the heating mode compared to crumbs
containing carrageenan. Moreover, carrageenan restricts gluten solubility owing to its sulfated struc-
ture, which selectively interferes with gluten (with a medium molecular weight) and forms
a hydrophobic complex.
[36]
Other authors suggested that the complex formed is insoluble in water,
which inhibits the complete development of the starch-gluten network and therefore its strength to
retain the gases and promote dough expansion.
[61]
As a result, any effect of gas expansion reduces the development of pores, their size, and finally the
overall crumb porosity. On the other hand, an image analysis by scanning microscopy indicated that
breadcrumbs prepared with xanthan gum did not have a uniform microstructure. Because xanthan
thickens the walls of the alveolar cells, the samples formulated with this gum can withstand the high
pressure induced by MW, which can lead to a uniform microstructure. Gums can form an adsorbed
layer of molecules around the bubbles, resulting in a thickening of the crumb walls surrounding the
gaseous cells.
[61]
However, the bread samples formulated with carrageenan gum were found to have
wider pores and more distorted starch granules. This may be owing to the high dielectric properties of
the carrageenan gum.
[62,63]
Breads formulated with this gum were heated more efficiently and had
a higher internal pressure, thereby increasing the size of the bread pores and ensuring complete starch
gelatinization.
Ozkoc et al.
[36]
reported that the addition of xanthan reduced the retrogradation enthalpy, which
meant that the amylopectin recrystallization and therefore the staling phenomenon were retarded. It
has been reported that XG distributes moisture consistently through bread dough and thus promotes
uniform heating and prevents hot spots in MW-baked breads. This was owing to the fact that MW
heating occurs through the dipolar vibration of water molecules; therefore, a uniform distribution of
water allows for a uniform distribution of MW energy in the sample.
[35]
According to Chandrasekaran
et al.,
[12]
xanthan gum also increases the viscosity of bread dough, facilitating the encapsulation of gas,
resulting in a higher volume and improved texture of baked products. Moreover, xanthan gum is
characterized by its resistance to high temperatures, which allows it to keep the dough very viscous at
higher temperatures and extend further before the structure is formed.
[12]
The beneficial effects of
xanthan gum can be obtained even at very low levels. Gimeno et al.
[64]
suggested that the positive effect
of xanthan on the expansion of corn flour pellets is attributed to the formation of a thin network
caused by the interpenetration of xanthan macromolecules into the starch matrix. This network is
distributed uniformly in the matrix, which serves as an additional nucleation site for expansion.
[64]
Table 3 presents the results of several studies on the effects of formulation on MW-baked bread
compared to conventional baking.
Concluding remarks
MW baking is a promising alternative to conventional bread baking with expected reductions in
baking energy compared to conventional baking. MW heating is more or less homogeneous, and
FOOD REVIEWS INTERNATIONAL 19

there is always some risk of overrun with excess heating on some local spots. MW baking can
achieve higher heating rates than conventional baking (up to 40°C/min compared to 6/10°C/min)
with a significant reduction in baking time. However, a rapid temperature rise has an impact on
the gelatinization mechanism of starch and the degree of granule disruption during baking, which
in turn affects the final texture of the crumb and staling kinetics. Consequently, the most available
study noted that MW-baked breads have a firmer crumb texture and faster staling than those
produced by conventional baking. In some cases, higher moisture loss was also observed for MW-
baked breads. The firmness of MW-baked bread seems to be related to three main factors:
moisture loss, total amylose leaching of starch granules, and gluten-MW interaction. To slow
the kinetics of staling and reach the organoleptic properties of conventional bread, the use of
additives is often considered to reduce the adverse effects of MW on the mechanical properties of
the breadcrumb and to slow down the staling kinetics. All enzymes have a significant positive
effect on crumb firmness, more specifically, alpha-amylase and protease, according to their direct
action on amylose leaching and protein networks. Indeed, gums (in particular, xanthan and guar)
can be helpful by allowing a uniform distribution of MW energy in the bread sample during
baking. This may ensure complete starch gelatinization and, therefore, can significantly retard the
stalling phenomenon with regard to such characteristics as crumb firmness and amylopectin
retrogradation even though the rapid baking observed with MW may affect their efficacy. In
addition, some emulsifiers can be very effective for moisture retention during MW baking. This
efficiency inhibits bread firmness, softens its texture, and increases its specific volume in the
presence of MW.
Improving microwave bread different recipe by the addition of improvers appears to be
a promising solution to reduce both the staling kinetics and crumb firmness or toughness of bread.
A deep study of starch gelatinization, protein relaxation, water mobility, and the interaction of the
different components by nuclear magnetic resonance and differential scanning calorimetry will
provide a better understanding of different phenomena induced by microwave in bread products.
Further research on the impact of MW heating rate on bread properties could be a new insight to
reduce the staling kinetics and bread firmness. In addition, to the investigation of the effect of the use
of different heating modes combined to the microwave on the bread properties and quality may reduce
the usage of certain additives.
In future, a combination of conveyor microwave system with convection or convection system will
be widely used in the bakery industry for energy and time savings and production of better quality
products.
Acknowledgments
This study was supported by National association of technical research of FRANCE (CIFRE thesis N° 2017/1712) and the
Nantes-Atlantic National College of Food Process Engineer ONIRIS.
Funding
This work was supported by the National association of technical research of FRANCE [CIFRE thesis N° 2017/1712].
ORCID
Roua Bou-Orm http://orcid.org/0000-0003-2445-6996
Alain Le-Bail http://orcid.org/0000-0001-6132-5392
20 R. BOU-ORM ET AL.

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