Trends in Food Science & Technology 112 (2021) 720–734
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Trends in Food Science & Technology
journal homepage: www.elsevier.com/locate/tifs
Oral processing of bread: Implications of designing healthier
bread products
Jing Gao a, b, Weibiao Zhou a, b, *
a
b
Department of Food Science and Technology, National University of Singapore, Science Drive 2, Singapore 117542
National University of Singapore (Suzhou) Research Institute, 377 Linquan Street, Suzhou Industrial Park, Jiangsu 215123, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Oral processing
Bread structure
Bolus formation
Texture perception
Saltiness
Retronasal aroma
Background: Due to global health concerns, it becomes imperative to re-design bread products with reduced
glycaemic index, lower salt content and sustained consumer acceptability. This requires an in-depth under­
standing of the physics of the transformation of bread to bolus as well as people’s sensory perception.
Scope and approach: This article provides a critical review of bread oral processing, from ingestion to perception.
Structure-property-oral processing relationship is the key approach to understand sensory perception in relation
to bread characteristics and oral physiology. The significance of bread heterogeneity and changes in chewing
ability, especially the contrast between bread crust and crumb and ageing conditions, are highlighted.
Key findings and conclusions: Bread structure has a significant impact on people chewing behaviour, bolus for­
mation and sensory perceptions. In particular, bread crust increased chewing effort, saliva impregnation and
structure disintegration, which led to the desired complexity in texture and aroma perceptions but a slower
sodium release. Three major implications for re-designing bread of improved health benefits are: 1) bread need to
be considered as a heterogeneous matrix with both crust and crumb to be re-engineered; 2) transformation from
bread to bolus is the critical step to monitor and regulate the desired health benefits, be it slower digestion or a
faster sodium release; 3) human oral physiology especially changes in chewing ability due to ageing needs to be
considered for a balanced design for slow digestion and easy chewing.
1. Introduction
Bread has been widely consumed as a tradtional staple food in many
countries around the world. Globally, the average bread consumption
was 24.5 kg per person per year, and the annual sales revenue was USD
401.7 billion in the year 2019 (Statista, 2020). In general, white bread
has a high glycaemic index (GI) and is also a major source of salt in the
diet. Due to the increasing public health concerns, it becomes imperative
to develop bread with reduced GI, lower salt content and sustained
consumer acceptability. To deliver such products, an in-depth under­
standing of bread oral processing, especially the link between bread
characteristics and consumers’ sensory experience, is necessary.
Bread structure is formed through various steps of bread making
process (Cauvain & Young, 1998; Della Valle, 2014). The structure of
bread crumb determines its mechanical strength which ultimately af­
fects its manner of deformation during oral processing. A clear under­
standing of bread structure and its mechanical properties makes
re-designing of bread products with desired eating properties possible.
Reviews on the structure-texture relationship are available for both
bread crumb (Gao, Wang, Dong, & Zhou, 2018; Scanlon & Zghal, 2001;
Zghal, Scanlon, & Sapirstein, 2002) and crust (Altamirano-Fortoul,
Hernando, & Rosell, 2013).
Oral processing is responsible for converting bread to bolus through
complex manipulations, during which texture, flavour and aroma per­
ceptions are generated (Chen, 2020). However, the role of the eating
process has often been disregarded in the context of new product
development. To address this, a new field of study of the ‘structure –
oral-processing – sensory relationship’ has emerged (Chen, 2009;
Devezeaux de Lavergne, van de Velde, & Stieger, 2017; Koç, Vinyard,
Essick, & Foegeding, 2013). This review addresses four parts of litera­
ture pertaining to bread structure, people’s oral processing behaviour,
bolus formation and sensory perceptions (texture, saltiness and aroma).
Fig. 1 shows the framework of this review where food structure design,
oral processing and sensory perceptions are synergistically evaluated.
* Corresponding authorDepartment of Food Science and Technology, National University of Singapore, Science Drive 2, 117542, Singapore.
E-mail address: weibiao@nus.edu.sg (W. Zhou).
https://doi.org/10.1016/j.tifs.2021.04.030
Received 30 June 2020; Received in revised form 10 April 2021; Accepted 16 April 2021
Available online 26 April 2021
0924-2244/© 2021 Elsevier Ltd. All rights reserved.
J. Gao and W. Zhou
Trends in Food Science & Technology 112 (2021) 720–734
Fig. 1. Integration of bread structure and subject oral physiology in relation to bread oral processing process which consists of three major components (i.e., chewing
behaviour, bolus formation and sensory perceptions).
2. Bread structure and texture
and moisture content are the two main factors contributing to the
firming of bread crumb (Fadda, Sanguinetti, Del Caro, Collar, & Piga,
2014). Crumb firmness is normally assessed by Texture Profile Analysis
(TPA) with a cylinder probe (e.g., 36 mm in diameter) to indent a bread
slice to 40% deformation (AACC International, 1999).
2.1. Bread crumb
Bread is diverse in its ingredients, processing conditions and
appearance (Zhou, Therdthai, & Hui, 2014). French baguette has highly
aerated crumb and crispy crust. The U.S. bagel is doughy and chewy.
Sandwich bread is featured with a fine crumb cell structure. Rye bread is
usually baked with sourdough fermentation and has a hard and less
cohesive texture. Chinese steamed bread has a soft and moist skin.
Despite of the great variations in bread types, bread crumb is
considered as an open-cell foam consisting of highly connected pores on
a macroscopic level (Scanlon & Zghal, 2001). Fig. 2 shows the
three-dimensional structure of bread crumb reconstructed by computer
aided X-ray microtomography. For white bread, the 3D connectivity
index computed by the ratio of the largest cell volume to the total void
volume was close to 100% (Babin et al., 2006). Stereological analysis
allows quantification of the porosity, cell size and cell wall thickness of
white bread (Gao, Wong, Lim, Henry, & Zhou, 2015; Pentikäinen et al.,
2014), baguette (Gao et al., 2015; Mathieu et al., 2016), rye bread
(Pentikäinen et al., 2014), sourdough bread (Lomolino, Morari, Dal
Ferro, Vincenzi, & Pasini, 2017), durum wheat bread (Cafarelli, Spada,
Laverse, Lampignano, & Del Nobile, 2014) and steamed bread (Gao
et al., 2015). The porosity of bread crumb ranges from 30 to 90%, with
average cell size and cell wall thickness of 0.3–1.5 mm and 0.04–0.6
mm, respectively.
As a cellular material, the mechanical and textural properties of
bread crumb is governed by its porosity, cellular morphology, and me­
chanical properties of cell wall (Scanlon & Zghal, 2001). At macroscopic
level, bread density, which is inversely correlated with crumb porosity,
has a strong relationship with various textural properties. High-quality
bread made from refined flour is often characterized by a high volume
and soft and elastic crumb. At the microscopic level, the cell wall is a
heterogeneous material consisting of gelatinized starch granules
embedded in the coagulated protein matrix. Changes in starch polymer
2.2. Bread crust
Bread is macroscopically heterogeneous, consisting of crust and
crumb that are distinctly different in their appearance and mechanical
strength. Bread crust is darker and lower in moisture than the crumb,
and the Maillard reaction during its formation is the primary contributor
to the flavour of bread. During baking, the surface temperature of bread
crust quickly reaches 100◦ C and further approaches oven air tempera­
ture. The high temperature leads to quick evaporation of water, result­
ing in low water content of the crust (<20% wet basis) (Vanin, Lucas, &
Trystram, 2009). For example, a 1.7 mm thick crust was developed for
French baguette at a final local temperature of 160◦ C and moisture of
5% (Della Valle, Chiron, Jury, Raitière, & Réguerre, 2012).
Due to the proximity of bread surface to the oven, there is an
enhanced escape of gases. This leads to more elongated cells in bread
crust as compare to bread crumb (Fig. 2). The bread crust was 1-mm
thick with a porosity of 80–94% for baked bread (Altamirano-Fortou
et al., 2012), and 0.7–1.1 mm thick with a porosity of 61–68% and
average pore size of 0.19–0.24 mm for crispy bread (Primo-Martín et al.,
2010). The crust of baked bread, especially French baguette, gives its
unique texture property – crispiness. Chaunier et al. (2014) developed a
multi-indentation test to assess the crispy texture of baguette. Two pa­
rameters, an apparent modulus and an apparent relaxation stress, were
selected as the representative property of crust and crumb, respectively.
2.3. Breadmaking and formulation: impact on bread structure formation
Bread structure is formed through a series of dynamic physical and
biochemical transformations during bread making: (1) gluten
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Trends in Food Science & Technology 112 (2021) 720–734
Fig. 2. (A) Representative images of the bread crumb structure of baked bread, French baguette and Chinese steamed bread. (B) Crust and skin portions of baked
bread and steamed bread, respectively, are highlighted. The 3D image of bread crust is reproduced from van Dyck et al. (2014) with permission from Elsevier,
Copyright 2014.
development and gas nuclei inclusion during mixing, (2) bubble redis­
tribution during shaping, (3) bubble inflation during proofing, and (4)
bubble expansion and coalescence during baking/steaming. The impact
of bread processing conditions on bread quality and texture has been
widely investigated and reviewed (Cauvain & Young, 1998; Gao et al.,
2021; Gao, Wang et al., 2017; Zhou et al., 2014). Della Valle et al. (2014)
proposed the usage of basic knowledge model in bread texture design,
which is capable of predicting gluten formation, dough porosity, crumb
and crust formation during baking.
There is a growing interest in wholegrain and bran-rich bread due to
health benefits of dietary fibre, such as lowering the risk of diabetes,
cardiovascular disease and colon cancer (Okarter & Liu, 2010). Whole
wheat flour produced dough and bread with characteristics different
from refined wheat flour (e.g., increased density and crumb hardness),
mainly due to dilution of gluten protein by the bran, fiber-gluten
interaction, insufficient hydration of gluten protein, and physical ef­
fect of bran particles (Hemdane et al., 2016). The negative impact of
bran addition could be alleviated through particle size reduction (Jin
et al., 2020, 2021; Lin et al., 2020, 2021).
3. Chewing behaviour during bread oral processing
3.1. Electromyography assessment of chewing behaviour
Study of chewing behaviour includes the actions of teeth, tongue,
jaw and oro-facial muscles, from first bite until swallowing. Electro­
myography (EMG) is a non-invasive method used for monitoring muscle
activation and recruitment levels in clinical settings. It measures the
bioelectrical signal propagation along the muscle cell during muscle
contraction (Vinyard & Fiszman, 2016). EMG does not measure the
absolute force due to intrinsic (e.g. muscle length) and extrinsic (e.g.
electrodes placement) variations (Funami, Ishihara, & Kohyama, 2014,
pp. 283–307), but it provides an indicative assessment of muscular
effort. It is possible to compare muscle activity across subjects by nor­
malising the signal against individual’s maximum clenching activity
(Remijn, Groen, Speyer, van Limbeek, & Nijhuis-van der Sanden, 2016)
or their chewing activity for a reference product (Pentikäinen et al.,
2014).
EMG has been increasingly used in monitoring the activities of
masticatory muscles, mainly superficial masseter and anterior tempo­
ralis during eating. Gonzalez Espinosa and Chen (2012) offered a
comprehensive guide for applying EMG in oral processing studies.
Applying surface electrodes on subjects’ face draws their attention,
which results in a longer chewing time. But the trends among products
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J. Gao and W. Zhou
Table 1
Average chewing behaviour for bread samples with and without crust.
Bread characteristics
Baked bread
Baguette
Steamed
bread
Baked bread
Baguette
Steamed
bread
Baked bread
723
Baguette
Steamed
bread
Artisan
baguette
Serving
size
Moisture
content (%)
Density
(g/ml)
Crumb
Hardness
(N)
Crust
portion
(%)
Subjects
Chewing
time (s)
Chewing
cycle
Chewing
frequency
(cycle/s)
Burst
duration
(s)
Muscle
activity
(mV●s)
Total
work
(mV•s)
Ref.
With
curst
13 cm3
35.5
0.27
–
27.43
33
44
1.4
0.52
0.047
8.85
Gao et al.
(2017)
30.5
44.5
0.27
0.40
–
–
31.32
20.81
20 subjects
(13F, 7 M,
20–27 years old)
36
32
47
44
1.3
1.4
0.53
0.52
0.056
0.041
10.21
7.77
43.5
0.23
1.4
–
28
39
1.4
0.53
0.042
6.87
43.1
43.3
0.21
0.30
1.2
2.2
–
–
29
29
41
41
1.4
1.4
0.52
0.52
0.045
0.041
7.74
7.13
36.4
0.27
–
24.4
41
59
1.5
0.52
0.066
15.26
33.2
43.7
0.23
0.41
–
–
24.6
47.6
14 subjects (7F,
7M, 22–26 years
old)
44
36
63
52
1.42
1.46
0.53
0.50
0.072
0.055
17.80
11.45
23.2
0.19
30.8
69
36
34.2
0.96
0.39
0.05–0.20
6.00
29.6
0.18
23.7
68
5 subjects (2 F,
3 M, 32–50
years old)
35
34.9
1.01
0.38
0.03–0.22
5.58
31.3
42.1
0.27
0.27
16.4
93.4
24
0
26
27
28.3
29.4
1.09
1.10
0.37
0.37
0.25–0.13
0.04–0.13
2.62
2.91
43.0
0.20
1.7
–
11.6
17.5
1.51
0.39
59%a
60%a
54.5
0.50
7.4
–
13.8
20.3
1.47
0.38
65%a
76%a
47.9
0.31
6.4
–
12.1
18.1
1.50
0.37
67%a
68%a
47.0
0.32
5.9
–
13
18.6
1.43
0.37
68%a
70%a
Crumb
only
With
crust
With
crust
5g
5g
Industrial
baguette
Toast bread
Rye bread
Wholemeal
rye bread
Refined rye
bread
Rey bread
with gluten
a
Crumb
only
2×2×2
cm3
Normalized to corresponding values of a reference product.
15 subjects
(15F, 20–40
years old)
Gao et al.
(2015)
Tournier et al.
(2014)
Pentikäinen
et al. (2014)
Trends in Food Science & Technology 112 (2021) 720–734
Wheat bread
Chewing behaviour
Type
J. Gao and W. Zhou
Trends in Food Science & Technology 112 (2021) 720–734
Fig. 3. Picture of three types of bread (with crust or skin) and their bolus to illustrate the transformation at early (1/3 of total chewing cycles), middle (3/2 of total
chewing cycles) and late (swallowing point) stages of oral processing. Original serving size of bread samples was 13 cm3.
The direct link between bread hardness and EMG results makes
instrumental analysis valuable in predicting chewing efforts. Texture
profile analysis (TPA) worked well when differences in bread texture
were significant (e.g., between white bread and rye bread (Pentikäinen
et al., 2014)), but did not provide good prediction when differences were
small (e.g., between white bread crumb and baguette crumb (Gao et al.,
2017)). Furthermore, the relevance of TPA analysis in predicting hard­
ness perception has been challenged (Peleg, 2019). Large deformation
rheological analysis under controlled strain and/or stress condition
should be considered to eliminate the limitation of empirical analysis (e.
g., the great dependency of results on sample dimension and loading
conditions in TPA). Mathematical modelling of the compressive
stress-strain curve of bread crumb proposed by Liu and Scanlon (2003)
could be revisited to address the challenges of food oral processing
studies.
Bread moisture content is the most important factor that determines
chewing duration. Baguette required a longer mastication than toast
bread and rye bread due to its lower water content and a higher crust-tocrumb ratio (Tournier et al., 2012). Even for the same type of bread, an
increase in initial moisture content led to a decrease in the number of
chews for white bread, because of less amount of saliva needed (Motoi,
Morgenstern, Hedderley, Wilson, & Balita, 2013). Buttering and topping
of bread with cheese and mayonnaise significantly reduced the number
of chewing cycles due to enhanced lubrication (Engelen,
Fontijn-Tekamp, & Bilt, 2005; van Eck et al., 2019).
The way to standardise a mouthful serving of bread has a significant
impact on chewing behaviour. When a constant weight of the sample
was served, denser bread was swallowed faster than others (Gao et al.,
2015; Panouillé, Saint-Eve, Déléris, Le Bleis, & Souchon, 2014; Tournier
et al., 2014). When a constant volume of sample was served, chewing
time was not influenced by bread type (Jourdren, Saint-Eve, et al., 2017;
Le Bleis, Chaunier, Montigaud, & Della Valle, 2016; Pentikäinen et al.,
2014). Constant volume better represents the natural eating scenario
(Hutchings et al., 2009) and is recommended for oral processing studies,
especially when a big difference in crumb density is observed.
remain the same as the one without using EMG (Tournier, Grass, Septier,
Bertrand, & Salles, 2014). EMG provides information of chewing time,
the number of chewing cycles, chewing frequency, burst duration, and
muscular activities. As shown in Table 1, the time and number of cycles
required to chew a piece of bread are 12–45 s and 20–60 cycles,
respectively. The chewing frequency is rather constant within each
study and varies approximately between 1.0 and 1.5 cycle/s among
different studies. There is limited report on kinematic features of bread
oral processing, especially about the impact of bread characteristics on
chewing kinematics. This area could be better addressed in future
studies.
3.2. Impact of bread characteristics on chewing behaviour
Chewing effort is directly linked to the density, structure, moisture
content and texture of the bread. Crust is a minor component based on
its weight but dominantly affects chewing behaviour. With the presence
of crust, the number of chewing cycle, chewing time and muscle activ­
ities increased considerably (Gao, Ong, Henry, & Zhou, 2017; Tournier,
Grass, Zope, Salles, & Bertrand, 2012). The thicker the crust layer, the
more significant the effect (Gao et al., 2017). The substantial impact
brought by the crust is attributed to its high resistance to biting and low
moisture content.
For bread crumb, bread density is among the most important pa­
rameters that determine chewing effort. The denser structure requires a
longer chewing duration and greater muscular activities. Chewing rye
bread with a higher bulk density and a larger closed porosity elicited
greater total muscle work and work-per-bite than wheat bread (Pen­
tikäinen et al., 2014). Similarly, a French style country bread with a
dense structure and a harder texture required more chewing cycles and
longer chewing duration than aerated white bread (Le Bleis, Chaunier,
Della Valle, Panouillé, & Réguerre, 2013). With the breakdown of bread
structure, muscle activity decreased and was significantly larger at the
middle versus early or late stage of mastication (Kohyama & Mioche,
2004).
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Trends in Food Science & Technology 112 (2021) 720–734
Table 2
Parameters of bolus salivation of different types of bread.
Bread type
Crumb only
White bread
Country bread
Industrial
bread
Artisan
baguette
Baguette
Baguette
Wholemeal
bread
Wholemeal
bread
Toast bread
Steamed bread
Refined rye
bread
Wholemeal
rye
With crust
White bread
Wholemeal
bread
Baguette
Steamed bread
a
Serving size
Bread moisture
content (%)
Bolus moisture
content (%)
Saliva addition (g
saliva/g dry solid)
Chewing
time (s)
addition rate (g saliva/g
dry solid • min)
Ref
20 cm3, 3.5 g
4g
13 cm3, 3.0 g
20 cm3, 6.0 g
h: 3.5 cm; d:
3 cm
h: 3.5 cm; d:
3 cm
13 cm3, 2.7 g
h: 2.5 cm, d:
3 cm
h: 3.5 cm; d:
3 cm
h: 2.5 cm, d:
3 cm
5g
13 cm3, 3.8 g
8 cm3, 9.6 g
44.6
42.5–45.1
43.5
48.7
43.4
58.5
65
58.7
57
89.6
0.32a
0.37a
0.40
0.29a
0.51a
–
15.8–18.3
28.10
–
41
–
1.17–1.39a
1.55
–
0.74a
Le Bleis et al. (2013)
Panouillé et al. (2014)
Gao et al. (2017)
Le Bleis et al. (2013)
Joubert et al. (2017)
47.8
63.3
0.33a
40
0.50a
Joubert et al. (2017)
43.1
49.1–51.6
58.6
60.0–61.1
0.39
0.51–0.72
28.8
–
1.52
–
48.4
65.2
0.34a
44
0.46a
Gao et al. (2017)
Jourdren, Panouillé,
et al. (2016)
Joubert et al. (2017)
48.6
57.6
0.56
–
–
35.6
43.3
54.5
46.5
58.7
65a
0.24
0.40
0.3 g/g bread
–
29
12.1
–
1.47
3.3a
8 cm3, 15.1 g
47.9
59a
0.3 g/g bread
13.8
2.5a
13 cm3, 3.6 g
5g
h: 3.5 cm; r: 3
cm
13 cm3, 3.5 g
5g
35.5
35.5
33.9
56.5
61.1
47.8
0.52
0.97a
0.48
33
41.5
–
1.57
1.40a
–
30.5
30.5
55.5
61.3
0.62
0.88a
36.0
44.1
1.60
1.20a
h: 2.5 cm; r:
3.0 cm
13 cm3, 5.2 g
5g
33.4–37.2
49.4–51.1
0.49–0.51
–
–
43.3
43.3
59.0
63.5
0.35
1.04a
31.6
35.8
1.34
1.74a
Jourdren, Panouillé,
et al. (2016)
van Eck et al. (2019)
Gao et al. (2017)
Pentikäinen et al.
(2014)
Pentikäinen et al.
(2014)
Gao et al. (2017)
Gao et al. (2015)
Jourdren, Panouillé,
et al. (2016)
Gao et al. (2017)
Gao, Tay, Koh, and
Zhou (2018)
Jourdren, Panouillé,
et al. (2016)
Gao et al. (2017)
Gao et al. (2015)
Calculated based on the reported values of bread, bolus moisture content, and chewing time.
3.3. Impact of oral physiology on chewing behaviour
shown in Fig. 3, both bread crumb and crust are broken down into small
particles and mixed with each other to form a cohesive mass. Size
reduction and lubrication are the two most important aspects of bolus
formation. Analysing bolus formation offers a means to understand the
dynamics of sensory perception during chewing. In this section, saliva
impregnation, particle size distribution and bolus texture are discussed
for their instrumental analysis methods and relations to bread charac­
teristics and oral physiology.
People’s salivation and mastication efficiencies affect the way they
orally manipulate food. A study with healthy subjects eating melba toast
showed that both unstimulated and stimulated salivary flow rates were
negatively correlated with the length of chewing sequence (Engelen
et al., 2005). Subjects with more saliva needed fewer chewing cycles to
convert the dry toast into a swallowable bolus. On the other hand,
chewing efficiency, which was measured as the subject’s ability to
breakdown an experimental chewable material (e.g. Optosil) in a given
number of masticatory cycles, was not related to the individual EMG
activity or the number of chewing cycle (Tournier et al., 2014).
Collectively, salivation ability plays a major role while masticatory ef­
ficiency has minimum influence on chewing behaviour of healthy
individuals.
Ageing causes a decline in an individual’s masticatory function due
to teeth loss, decrease in muscle strength and reduced salivary flow rate
(Ketel et al., 2019). Designing bread products for the elderly must
consider their impaired oral physiology. Kohyama, Mioche, and Bour­
diol (2003, 2002)’s studies showed that the elderly had decreased
muscle activities, especially those with a smaller number of paired
post-canine teeth, and they compensated this weakness by increasing
the number of chewing cycle. Similarly, in studying brioche Assad-­
Bustillos, Tournier, Septier, Della Valle, and Feron (2019) found that the
elderly with satisfactory dental statues but lower salivary flow rate
achieved their swallowing point in a longer duration.
4.1. Saliva impregnation
4.1.1. Analysis of saliva impregnation
Saliva is made of 98% water (Ployon, Morzel, & Canon, 2017). Saliva
impregnation is quantified as the difference in water content between
bread and bolus. Oven drying is the universal method of water content
measurement applied to both food and its bolus. As shown in Table 2,
the moisture content of bread bolus varies within a narrow range of
50–60% for different types of bread at the swallowing point, regardless
whether crust is present. As a solid food, bread requires sufficient hy­
dration to be safely and comfortably swallowed. To reach this level of
hydration, various amount of saliva needs to be incorporated (0.3–0.7 g
saliva/g dry solid), depending on the initial moisture content of the
bread. The water content of bread bolus increased linearly during
chewing (Gao et al., 2017; Le Bleis et al., 2016) while the rate of saliva
impregnation was not affected by bread type (Gao et al., 2017; Motoi
et al., 2013). Hence, a greater amount of saliva impregnation is achieved
by chewing for a longer duration. This makes saliva impregnation the
rate-determining step in bread bolus formation, especially when crust is
present.
4. Bolus formation
Bread structure is continuously evolving during oral processing. As
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J. Gao and W. Zhou
Trends in Food Science & Technology 112 (2021) 720–734
Fig. 4. Variations of bolus water update during chewing with the ratio of theoretical saliva volume (Vs) to particle median width (w50), for three types of bread (□, ○,
Δ) at three mastication stages (open symbols, grey symbols and full dark symbols) and all subjects (n = 16). The inserts represent a schematic view of the evolution of
bolus during mastication. Reproduced from Le Bleis et al. (2016) with permission from Elsevier, Copyright 2016.
4.1.2. Effect of bread characteristics on saliva impregnation
Initial moisture content of bread is the most crucial factor that de­
termines saliva impregnation. In the study of Gao et al. (2015), baguette
with thick and dry crust had the lowest moisture content (30%) and
required the greatest amount of saliva addition (0.79 g/g bread) as
compared to baked bread and steamed bread (36–44%, 0.59–0.63 g/g
bread). Even for the same type of bread (e.g., white bread), an increase
in the starting moisture content from 20 to 60% led to a decrease in
saliva addition from 0.8 to 0.5 g/g bread (Motoi et al., 2013). Bread with
a higher moisture content was swallowed as a more hydrated bolus (Gao
et al., 2015; Joubert et al., 2017; Motoi et al., 2013). It suggests that the
moisture content of bread still had a significant impact on people’s
swallowing thresholds.
Mathieu et al. (2016) investigated how crumb structure affected its
hydration kinetics using an in vitro approach. They found that baguette
with a higher porosity had a higher hydration rate and capacity than the
denser one. A high proportion of small cells (diameter < 2 mm) gener­
ated a high interconnection probability and crumb hydration capability
due to enhanced capillary action. This is a good starting point to reveal
the underline mechanisms of bolus hydration. However, as the original
structure of bread is highly deformed during chewing, structure modi­
fications need to be added as another dimension in such mechanistic
studies.
computed by multiplying stimulated saliva flow rate (by chewing a piece
of parafilm) with the chewing duration.
ΔWC = α⋅[
Qs ⋅t n
]
w50
(Eq. 1)
where ΔWC is the water update of bolus (%), Qs is the salivary flow (mL/
min), t is the chewing time, and w50 is the median particle width at time
t. Based on curve fitting, the values of α and n were determined to be
7.12 mm− 2•n and 0.34 for fibre enriched bread (Le Bleis et al., 2016).
Including the information about particle size increased the predic­
tion power, as a reduction in particle size increases the total available
surface area and promotes saliva absorption (Le Bleis et al., 2016).
Moreover, a homogeneous distribution of small particles was preferred
for efficient hydration (Jourdren, Panouillé, et al., 2016). Despite the
observed impact of particle size, subject’s masticatory efficiency was not
significantly correlated with saliva uptake (Le Bleis et al., 2016).
4.2. Structure disintegration
4.2.1. Characterization of structure disintegration
A piece of bread is comminuted to a high volume of small particles
through oral processing. Conventional sieving method works well for
rigid food particles that do not significantly absorb water, but not for
bread. Laser diffraction and image analysis methods are most frequently
employed methods for bread bolus analysis (Gao et al., 2015; Jourdren,
Panouillé, et al., 2016; Le Bleis et al., 2013; Pentikäinen et al., 2014).
Laser diffraction is suitable for analysing particles smaller than 3 mm.
Image analysis works for larger particles that can be separated for image
capturing. To separate particles, boluses need to be dispersed in a liquid
medium, such as glycerol (Assad-Bustillos, Tournier, Feron, et al., 2019;
Jourdren, Panouillé, et al., 2016; Le Bleis et al., 2013) and ethanol (Gao
et al., 2015; Gao, Tay, Koh, & Zhou, 2018). A certain extent of breakage,
deformation or shrinkage of the particles is not evitable. Thus, results
obtained from different dispersing methods need to be compared with
caution.
Chewed bread particles do not have a clearly defined shape, which
4.1.3. Effect of oral physiology on saliva impregnation
Large individual variability in bolus moisture content have been
observed. This is directly related to intra-individual and inter-individual
variabilities of salivary flow rate. Jourdren et al., 2016 study of baguette
showed that subjects with a high salivary flow, a big oral cavity and a
long in-mouth duration had a greater saliva incorporation.
Assad-Bustillos, Tournier, Septier, et al. (2019)’s study of the elderly
also showed that those with a higher stimulated salivary flow rate pro­
duced more hydrated brioche boluses. As shown in Fig. 4, Le Bleis et al.
(2016) proposed that saliva absorption could be predicted based on the
theoretical volume of saliva present in the mouth and the median par­
ticle size of bread bolus (Eq. (1)). The theoretical volume of saliva is
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leads to approximations in determining their size. The degree of frag­
mentation can be assessed by the evolution of the mean/median particle
size or area. The spread of particle distribution could be quantified as the
75%–25% interquartile or by fitting into Gompertz model (Assad-Bus­
tillos, Tournier, Feron, et al., 2019). Bolus particles usually are smaller
than 2 mm for a wide range of solid foods (Jalabert-Malbos,
Mishellany-Dutour, Woda, & Peyron, 2007). For bread, the average
particle size was reported to be 1.6 mm, 1.4 mm, 3.2 mm for white bread
(Gao et al., 2015; Le Bleis et al., 2013; Pentikäinen et al., 2014), 1.5 mm
for country wheat bread (Le Bleis et al., 2013), 1.9 mm for bread
enriched with fibre (Le Bleis et al., 2016), 2.9 mm for Brioche (Assad-­
Bustillos, Tournier, Feron, et al., 2019), and 3.2 mm for steamed bread
(Gao et al., 2015).
When the crust is present, bread disintegration could be tracked
using image texture analysis due to the colour contrast between crust
and crumb. Tournier et al. (2012) used the grey level co-occurrence
matrix (GLCM) method to successfully discriminate 60–73% of bolus
images for its respective chewing cycles. Even though it does not provide
information about particle size, the GLCM method is useful to reveal
specific patterns of structural disintegration among bread and subjects.
through in vivo studies.
Moreover, eating rate is a behavioural characteristic that directly
links to food intake. Food with a longer chewing time and more chews
per bite leads to prolonged oral exposure, lower eating rate and a
decreased food intake (Hogenkamp & Schiöth, 2013). Modification of
food texture has proven to be effective in changing oral processing
behaviour and control the intake of semi-solid food, such as gel, yoghurt
and porridge (Lasschuijt et al., 2017; McCrickerd, Lim, Leong, Chia, &
Forde, 2017; Mosca et al., 2019). The scenario is more complicated for
bread products. Inclusion of a hard to chew component (e.g. a thick and
dry crust) leads to prolonged oral exposure and lower intake. But a
longer chewing lead to an extensive size reduction and promotes glucose
release. A balance between eating rate and size reduction is required for
designing healthier bread products.
4.3. Mechanical properties of bread bolus
4.3.1. Characterization of bolus mechanical properties
Oscillatory rheometry. Controlled stress rheometer equipment with
a four blades vane (10-mm in diameter, 8.8-mm in length) and an outer
cylinder (17 mm) were used to measure bread bolus deformation at the
linear viscoelastic domain (0.5% strain at 1 Hz) (Jourdren, Panouillé,
et al., 2016; Le Bleis et al., 2013; Panouillé et al., 2014). Bread bolus
behaved more like a solid than fluid as its storage modulus G′ is larger
than loss modulus G’’. The main limitation of oscillatory rheometry is
that it does not represent the large deformation applied in the mouth
during chewing.
Capillary rheometry. Bread bolus was loaded into a cylindrical
barrel (∅ = 10 mm) and extruded by a flat piston (∅ = 10 mm) through a
capillary die (∅ = 2 or 4 mm) at various speeds (20, 50 and 200 mm/
min) (Le Bleis et al., 2013). Based on the force-displacement curve,
apparent viscosity, consistency index (K) and flow behaviour index (n,
dimensionless, 0 < n < 1) were obtained. Bread bolus exhibited a
shear-thinning behaviour (n < 1) and behaved like a suspension of
numerous particles in saliva at the swallowing point (Le Bleis et al.,
2013, 2016).
Texture profile analysis (TPA). Bread bolus was compressed to the
strain of 65% at the speed of 0.83 mm/s (Jourdren, Panouillé, et al.,
2016) or to 50% at the speed of 5 mm/s (Gao et al., 2019). Bolus
hardness, adhesiveness, and cohesiveness could be obtained, just like
the conventional TPA analysis (Jourdren, Panouillé, et al., 2016; Peyron
et al., 2011). As an empirical method, TPA settings are not standardized,
and results are hardly comparable across studies. But it might be useful
in indicating the overall bolus texture and understanding sensory
perception in relation to bolus texture.
4.2.2. Effect of bread characteristics on structure disintegration
Bread composition has an impact on its disintegration. Wholemeal
bread had a higher fragmentation than baguette, which was explained
by the presence of fibres and milled wheat grains that disrupted gluten
network and led to a weak structure (Jourdren, Panouillé, et al., 2016).
Similarly, steamed bread made from low-protein flour was fragmented
more extensively within a shorter period of time as compared to steamed
bread and baked bread made from high-protein flour (Gao et al., 2015;
Gao, Tay, et al., 2018). Hence, well-developed gluten network and
cohesive structure help to preserve the integrity of crumb against
chewing.
Inclusion of bread crust resulted in an extensive breakdown.
Baguette with a dry and thick crust induced a higher chewing activity
and was broken down into smaller particles as compared to steamed
bread (Gao et al., 2015). Among three types of baguette, the one with a
thicker and drier crust was broken down to smaller particles (Jourdren,
Panouillé, et al., 2016). The thick crust also led to a peak of size het­
erogeneity (D75/D25) in the middle of the chewing sequence (Jourdren,
Panouillé, et al., 2016). It was reasoned that smaller particle size was
necessary to ensure safe swallowing of the originally dry and hard crust
pieces. These results reflect the difficulty of breaking down a hetero­
geneous bread sample composed of both hard and soft materials, and the
significance of crust thickness on disintegration mechanism.
4.2.3. Effect of chewing behaviour on structure disintegration
There is a large variability among individuals in the particle size of
bread bolus mainly due to differences in chewing time required (Le Bleis
et al., 2016). For the elderly, neither dental status nor salivary flow rate
affected the bolus particle size of brioche (Assad-Bustillos, Tournier,
Feron, et al., 2019). Up to now, understanding of the role of oral phys­
iology in bolus size reduction is still limited and not conclusive. This is
due to the limited ways to assess people’s physiological capability to
comminute soft solid foods, especially for the elderly. For young and
healthy subjects, the difference in chewing time explained the difference
in size reduction (Jourdren, Panouillé, et al., 2016). Faster eaters rapidly
broke down the bread, and continuously increased the heterogeneity of
particle size distribution. On the other hand, slower eater chewed bread
for a longer time and produced more homogenous bolus. This extensive
breakdown of bread structure was associated with a high glycaemic
response (Gao et al., 2015; Lau, Soong, Zhou, & Henry, 2015). The
positive correlation between the number of chews and the glycaemic
response was also reported for other cereal products such as rice and
pizza (Zhou et al., 2014; Tamura, Okazaki, Kumagai, & Ogawa, 2017).
However, the underlying mechanism attributed to the limited accessi­
bility of salivary amylase to the dense bread particles is yet to be verified
4.3.2. Effect of bread characteristics on bolus mechanical properties
Bread composition, density and texture contributed significantly to
the mechanical properties of its bolus. The apparent viscosity of bread
bolus was decreased from 2 to 10− 3 Pa s and its consistency index was
reduced from 10 to 1 Pa.sn during chewing, indicating a softening pro­
cess of bolus due to starch plasticization (Le Bleis et al., 2013). Bread
with fibre or a higher density was converted to bolus with a higher
consistency index (Le Bleis et al., 2013, 2016) and a higher value of G’
(Jourdren, Panouillé, et al., 2016; Le Bleis et al., 2013; Panouillé et al.,
2014). Bread crust significantly increased bolus hardness, especially at
the early stage of chewing when the relative intact crust strengthened
the bolus structure (Jourdren, Panouillé, et al., 2016).
Salivation and fragmentation together determined bolus texture.
Incorporation of saliva led to a decrease in bolus viscosity (Le Bleis et al.,
2013, 2016) and hardness (Jourdren, Panouillé, et al., 2016). For bread
crumb, increased hydration level exerted a major effect on the bolus
softening (Gao et al., 2017). For bread with crust, particle size reduction
had a more profound impact on bolus softening at the later stage of
chewing (Gao et al., 2017).
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unique to individuals.
Table 3
List of texture attributes used for bread oral processing studies.
Attribute
Definition
Perceived at
which stage
Ref
Soft
Less force required to bite through
sample between teeth
Present a resistance under the teeth/
requires a strong force to chewed
Longer time required to chew the
bread to reduce it to a consistency
suitable for swallowing
Describe a solid containing little cells
filled with gas; high density
Describe a solid containing cells filled
with gas; low density
Sensation of dryness due to lack of
saliva; absence of water
Sensation of wetness due to secretion
of saliva; presence of water
Adhere to the palate and the teeth
during chewing
Leads to presence of a large number of
pieces or particles when it is broken in
the mouth during mastication
Absence of separate particle
Presence of pieces or particles in saliva
With the consistency of a soft and thick
dough
Low pitch sound produced on crust
fracture during mastication
Produce makes noise during chewing
Product returns to its initial volume
after compression
Early and
middle
Early
1, 2,
3
2
Early and
middle
1, 3
Early
1, 3
Early
1, 2,
3
1, 2,
3
1, 3
Hard
Chewy
Dense
Aerated
Dry
Hydrated
Sticky
Crumbly
Homogeneous
Heterogeneous
Doughy
Crunchy
Crispy
Elastic
Middle
Late
Middle and
late
Middle and
late
1, 2,
3
1
Late
Middle
Late
2
2
2
Early
3
Early
Early
2
2
K = K0 ⋅exp[ − β ⋅ ΔWC]
(Eq. 2)
where K0 is the initial consistency index of bread, ΔWC is the water
uptake due to saliva incorporation, and β is the plasticization coefficient.
There was a large inter-individual variation in the value of β, which
could not be explained by the salivary amylase activity but might be
related to salivary mucin content (Le Bleis et al., 2016).
Among the elderly, those with satisfactory dental status produced a
bolus with a higher apparent viscosity than those with less number of
teeth (Assad-Bustillos, Tournier, Septier, et al., 2019). The elderly with a
higher saliva flow rate achieved the bolus viscosity needed to trigger
swallowing in a shorter duration. In contrast, those with a satisfactory
dental status, but a low salivary flow rate, still needed a longer chewing
duration. Hence, dental status determines the absolute value of bolus
viscosity while salivation efficiency determines oral processing time
required to achieve it, regardless of the dental status.
5. Texture perception during bread oral processing
5.1. Temporal profiling of texture perception
Texture perception is a dynamic process due to the continuously
changing properties of food during chewing. Dynamic sensory analysis
methods, such as Temporal Dominance of Sensation (TDS), are ideal for
oral processing study. TDS method requires subjects to select the most
striking perception or the new sensations that pops up as the dominant
sensation, without evaluating the intensity (Schlich, 2017). Dominance
rate is calculated as the percentage of selections of an attribute as
dominant at a particular time point. A higher dominance rate means a
better consensus among subjects. TDS curves illustrate how dominance
rates of all attributes change against the standardized chewing time.
Although performing a TDS task increases chewing time due to the
extra time required for completing the evaluation task, it does not
change the relative differences among products as observed in natural
chewing behaviour (Cheong et al., 2014). Dominant texture perceptions
at the early, middle and late stages of bread oral processing are indicated
in Table 3. The evolution of texture sensations during bread oral
1 Panouillé et al., 2014, p. 2. Jourdren, Saint-Eve et al., 2016, p. 3. Gao et al.,
2017.
4.3.3. Effect of oral physiology on bolus mechanical properties
Bolus hardness was proven to be negatively correlated with its
moisture content and was well predicted based on linear regression (Gao
et al., 2017). As shown in Eq. (2), changes in bolus consistency could be
modelled by its initial consistency and water uptake (Le Bleis et al.,
2016). By substituting the ΔWC of Eq. (1), bolus consistency could be
explained by salivary flow rate and particle size distribution that are
Fig. 5. Standardized TDS curves of baked bread, steamed bread and baguette and their crumb. The dotted line indicates the chance level (α = 0.05) and the dash line
indicates the significance level (α = 0.05). Reproduced from Gao et al. (2017) with permission from Elsevier, Copyright 2017.
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processing is illustrated in Fig. 5. At the early and middle stages, texture
perception depended on the original characteristics of bread. At the end
of the chewing sequence, perceptions were similar among bread where
‘hydrated’ and ‘sticky’ were the dominant ones, highlighting the
importance of hydration level and bolus texture in triggering the swal­
low of bread (Gao et al., 2017; Panouillé et al., 2014).
Progressive Profiling (PP) is a quantitative tool to evaluate intensities
of texture attributes at chosen discrete time points during chewing.
Jourdren, Saint-Eve, et al. (2016) evaluated nine texture attributes of
baguette at 10%, 40% and 100% of chewing time, and used the Multi­
block Partial Least Square (MB-PLS) regression to relate the sensory
scores to the initial bread properties and bolus properties at corre­
sponding time points. A similar approach might be useful for revealing
how bread structure and its transformations through oral processing are
related to texture perception.
6. Saltiness perception during bread oral processing
6.1. Measurement of sodium release and taste perception
Bread is one of the main contributors (Belz, Ryan, & Arendt, 2012) to
the daily sodium intake, which makes saltiness the most well-studied
taste of bread. Saltiness is mainly evoked by the free sodium ions
which are carried by the saliva from the bread matrix to the receptors.
To quantify sodium release during eating, either saliva sample (Tournier
et al., 2014) or bread bolus was collected (Konitzer et al., 2013; Pflaum,
Konitzer, Hofmann, & Koehler, 2013). There is also a continuous mea­
surement technique which pulls a strain ribbon through the mouth
during chewing (Konitzer et al., 2013). The ribbon was then cut into
pieces according to the chewing duration and extract with Tris buffer for
sodium-ion quantification (Konitzer et al., 2013). The overall perceived
saltiness intensity was evaluated using two-alternative forced choice
(2-AFC) test while the changes of saltiness perception over the time was
measured using Time-Intensity (TI) method (Konitzer et al., 2013;
Panouillé et al., 2014; Pflaum et al., 2013).
5.2. Effect of bread characteristics on texture perception
Texture is an important factor contributing to consumer acceptance
of bread. Texture perception arises from continuous sensing of changes
in food properties through oral mechanoreceptors and the force and
position of the mandibles (Nishinari & Fang, 2018). As shown in Fig. 5,
crumb of baked bread, steamed bread and baguette had a similar TDS
profile, even though they were different in density and microstructure
(Gao et al., 2017). Jourdren, Saint-Eve et al. (2016) also reported a
similar texture sequence for three types of baguette: ‘soft’ and ‘aerated’
were dominated at the beginning while ‘doughy’ and ‘sticky’ were
perceived before and after swallowing, respectively. Therefore, crumb
samples are less distinguishable for their texture properties.
Bread crust has contrasting mechanical properties as compared to its
crumb. Gao et al.‘s study of baguette and baked bread showed that crust
did not make the TDS sequence more complex but made it completely
different from its crumb counterpart until the swallowing point (Gao
et al., 2017) (Fig. 5). Jourdren, Saint-Eve et al. (2016)’s study of
baguette showed that the presence of crust increased the complexity of
the temporal sequence of texture perception. In both studies, bread crust
introduced the ‘crispy’ or ‘crunchy’ perceptions at the early stage of
chewing. The ‘crunchy’ and ‘crispy’ attributes were only perceived
during eating baguette but not baked bread (Gao et al., 2017), and the
perception was more significant for baguette with thicker crust (Jour­
dren, Saint-Eve et al., 2016). Bread crust introduced ‘heterogeneous’
perception and reduced the ‘aerated’, ‘doughy’, ‘sticky’, ‘soft’ percep­
tions at the middle stage of chewing sequence (Jourdren, Saint-Eve
et al., 2016).
6.2. Effect of bread characteristics on saltiness perception
Salt release during oral processing is a complex process, which de­
pends on bread formulation and its structure, as well as the physiology
and experience of the individuals. Pflaum et al. (2013) investigated the
role of bread density, controlled by the amount of yeast added and the
proofing duration, on the velocity of sodium release and the intensity of
saltiness perception. About 57% of the total sodium ions were extracted
from bread within the first 5 s of chewing. Nearly 100% was extracted at
60 s. Sodium was better extracted from coarse-pore bread crumb than
from fine-pore crumb at the first 5 s of chewing (74% vs 55%). This
agrees with the sensory analysis that bread with a coarse and aerated
crumb elicited a more intense salty taste than those with a fine and
denser crumb. Hence, the rate of sodium release during the first few
seconds is critical in explaining the difference in salt perception. A larger
pore size and a softer crumb favour a faster sodium release. Panouillé
et al. (2014)’s study confirmed that denser bread was perceived as less
salty. Using the TDS method, they found that saltiness was perceived at
the middle and later stages of chewing aerated bread, but it was never a
dominant sensation for dense bread.
Tournier et al. (2014) showed that baguette had a lower sodium
release than toast bread and rye bread, when all samples were served in
a constant mass. This might be explained by the lower moisture content
of baguette due to its thick and dry crust. Salt concentrated on the crust
needs more effort to be extracted, including a greater masticatory effort
and more saliva impregnation. Also, variations in formulation may lead
to different interactions between sodium ions and bread matrix (e.g.,
protein) which affect sodium release.
Salt distribution in the crumb also matters. Konitzer et al.’s study
showed that the use of coarse-grained salt led to an inhomogeneous
spatial distribution of sodium in the crumb (Konitzer et al., 2013). Both
TI and 2-AFC tests showed that people rated the bread with
coarse-grained salt significantly saltier. The distribution of salt inside
the crumb also affected bread saltiness. This was explained by the
increased contrast in sodium concentration, and a faster sodium release
during chewing resulted from the inhomogeneous distribution of salt
granules. A 25% reduction of sodium content was achieved without
affecting bread taste quality by using the coarse-grained salt. In
conclusion, the salt content can be reduced by modifying the crumb
structure that favours fast release of sodium.
5.3. Effect of bolus formation on texture perception
The constant transformation of bread structure during oral process­
ing contributes to the complexity of texture perception. Dynamic anal­
ysis of bolus properties provides a way to reveal the underline
mechanism that governs texture perception. The ’heterogeneous’
perception was linked with heterogeneous distribution of baguette bolus
particles (Jourdren, Saint-Eve et al., 2016). ‘Soft’, ‘dry’, ‘doughy’, and
‘sticky’ perceptions were more influenced by bolus properties than by
initial bread properties (Jourdren, Saint-Eve et al., 2016). In particular,
‘soft’ perception was explained by water content and bolus hardness.
Moreover, the threshold values of bolus moisture content that triggered
‘soft’ and ‘hydrated’ sensations were lowered due to the presence of
bread crust (Gao et al., 2017). Saliva has a profound impact on texture
experience. It is the food-saliva mixture rather than only the food that is
sensed in the mouth. Mosca and Chen (2016) and Devezeaux de Lav­
ergne et al. (2017) provided an in-depth review of how oral physiology
could be measured to understand the role of saliva in texture perception.
6.3. Effect of oral processing on saltiness perception
Sodium delivery is a temporal process that is related to chewing
behaviour and saliva flow. The maximum sodium concentration in the
saliva was reached when subjects applied a large number of chews and a
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long chewing duration (Tournier et al., 2014). A rapid initial sodium
release was linked to a high EMG activity, which indicates a greater
breakdown of bread due to the application of a larger biting force.
Different from muscular activity, the salt release is less impacted by
saliva impregnation. In a study of toast bread, rye bread and baguette,
the level of saliva impregnation varied greatly among individuals during
the chewing of bread (15–65%), but it was not significantly correlated to
salt release from the bread (Tournier et al., 2014). The role of salivation
has not been revealed. Further study is required to understand the sig­
nificance of variations in human oral physiology on saltiness perception.
A detailed profiling of a large number of subjects for their physiological
parameters, especially during the mastication process, is indispensable.
Table 4
List of aroma attributes and their definition used in bread oral processing
studies.
Attributes
Definition
Ref
Acetic
Alcoholic
Butter
Aroma associated with acetic acid (white vinegar)
Characteristic odour of item containing alcohol (ethanol)
Aroma associated with croissant/melted butter
Bakery
Caramel
Cereal
Cocoa
Dairy
Typical aroma of freshy baked bread
Aroma associated with caramel
Aroma associated with cereal derived produce like malt
Aroma associated with cocoa
Aromatic associated with products produced from cows
milk
Aroma associated with fermentation/fermented dough
Aroma associated with fruits like apple or pineapple
Aroma associated with wholemeal flour; A light, baked
wheat flour aromatic; An aromatic impression of cereal
derived products, typically rye, wheat, oats, cornmeal and
barley
Characteristic odour of fresh earth or wet soil
1
2
1, 2,
3
4
5
1
6
7
Fermented
Fruity
Grain
7. Aroma perception during bread oral processing
7.1. Instrumental analysis of aroma compounds
Aroma perception rises from the interaction of volatile compounds
with olfactory receptors. Volatile profiles of bread products have been
extensively studied and comprehensively reviewed (Pétel, Onno, &
Prost, 2017; Pico et al, 2015, 2016; Starr, Hansen, Petersen, & Bredie,
2015). More than 300 volatiles have been identified in the wheat-bread
family, while only a small number (about 30) has an impact on aroma
perception (Pico, Bernal, & Gómez, 2015). The volatile compounds are
usually low in concentration (e.g. parts per million) (Makhoul et al.,
2015), which makes volatile profile analysis a challenging task.
Instrumental analysis of volatile compounds consists of isolation and
characterization. Solvent extraction and distillation, dynamic headspace
extraction (DHE) and solid-phase microextraction (SPME) have been
applied to isolate bread aroma (Pico et al., 2016). However, none of
them consider bolus formation which alters the diffusion of volatiles
from bread matrix to the headspace. ‘Artificial mouth’ that simulates
mastication, salivation, airflow and temperature has been developed.
Retronasal aroma simulator (RAS) is the first one that is made from a 1 L
blender (Roberts & Acree, 1995). It incorporates temperature control
(37◦ C), synthetic saliva addition (at 1:5 saliva to food ratio), regulated
blending (10-500 s− 1 shear rate), controlled gas flow (N2, 20 mL/s) and
vapour phase sampling (purge-and-trap). Its operation parameters have
been optimized to simulate retronasal aroma release from white bread
(Onishi, Inoue, Araki, Iwabuchi, & Sagara, 2012). Another model
developed by (Poinot, Arvisenet, Grua-Priol, Fillonneau, & Prost, 2009)
uses a notched plunger (12 lines of sharp teeth, 5 mm in height) to
compress and shear bread crumb at precisely controlled speeds, with a
controlled gas flow (He, 200 mL/s) at regular intervals. In the artificial
mouth, the volatile compounds generated are usually trapped by an
SPME fibre or injected directly into an analytical unit. The volatile
extract is separated by gas chromatography (GC), and identi­
fied/quantified by mass spectrometry (MS) and flame ionisation (FID)
detectors (Pico et al., 2016).
To measure aroma release in real-time, fast analysis is required.
However, the length of chromatographic separation prevents its use for
such applications. On-line MS detectors such as proton-transfer reaction
mass spectrometry (PTR-MS) (Poinot, Arvisenet, Ledauphin, Gaillard, &
Prost, 2013) and atmospheric pressure chemical ionisation-MS
(APCI-MS) (Taylor, Linforth, Harvey, & Blake, 2000), are direct and
sensitive ways to monitor volatile compounds in bread during process­
ing (Makhoul et al., 2015) and eating (Jourdren, Masson, et al., 2017;
Jourdren, Saint-Eve et al., 2017; Pu et al., 2019). The direct sampling
capability of the on-line MS detectors also allows sampling of exhaled air
from nose (Jourdren, Masson, et al., 2017; Pu et al., 2019; Pu et al.,
2020) or mouth (Pu et al., 2019) during eating. In vivo studies of human
mastication showed that 32 volatile compounds were dominant ones
during the chewing of white bread and 9 of them contributed to aroma
perception after chewing (Pu et al., 2019). Ethanol (m/z 47) was the
most abundant ion detected during chewing the crumb of white bread
(Pu et al., 2019) and baguette (Jourdren, Masson, et al., 2017).
Greenearthy
Grilled
Global
flavour
Hay-like
Honey
Malty
Molasses
Musty
Nutty
Raw grain
Roasted
Roasted
cereal
Rye
Stout
Toasted
Toasted
grain
Wheat
Yeasty
3, 8
1
6
2
Aroma associated with grilled crust (black colour)
The overall retronasal perceptions
3
4
Aromatic reminiscent of dried grass and characteristics of
the bran layer that has been completely separated from the
rest of the grain kernel
Aroma associated with clear forest honey
Sweet aromatic typically of condensed milk, toffee and/or
malt
The sweet, caramelized aromatic reminiscent of molasses.
The aromatics associated with wet grain and or damp earth
The aromatic characteristics of mixed nuts, e.g. walnuts,
hazelnut, brazil nuts and pine nuts
Aromatic reminiscent of general cereal grains including the
light, dusty, musty aromatic associated with grains
Roasted odour of bread crust
Aroma associated with roasted cereals
9
Characteristic aromatic of rye flour
Aroma associated with stout (e.g. Guinness stout)
Aroma associated with toast crust (golden colour)
Aromatic associated with grain products that have been
browned by being subjected to heat sufficient to cause some
burnt notes
Characteristics aromatics of product made from entire
wheat grain
Aroma associated with fermented yeast-like
9
5
3
9
7
7
9
7
7
9
8
3
3
6
1.Campo et al., 2016, p. 2. Curic et al., 2008, p. 3. Jourdren, Saint-Eve et al.,
2017, p. 4. Raffo et al., 2018, p. 5. Hayakawa et al., 2010, p. 6. Starr et al., 2015,
p. 7. Heenan et al., 2008, p. 8. Callejo, 2011, p. 9. Carson et al., 2000.
7.2. Sensory analysis of bread aroma perception
Aroma perception occurs when volatile compounds enter the nasal
cavity through the nostrils (orthonasal olfactory) or through the
oropharyngeal airway during swallowing or exhalation (retronasal ol­
factory) (Bojanowski & Hummel, 2012). Many studies reported ortho­
nasal aroma perception of bread (Heenan, Dufour, Hamid, Harvey, &
Delahunty, 2009; Morais, Cruz, Faria, & Bolini, 2014) where descriptive
sensory analyses are commonly used (Callejo, 2011; Campo, delArco,
Urtasun, Oria, & Ferrer-Mairal, 2016; Carson, Setser, & Sun, 2000; Curic
et al., 2008; Hayakawa, Ukai, Nishida, Kazami, & Kohyama, 2010;
Heenan, Dufour, Hamid, Harvey, & Delahunty, 2008; Kihlberg,
Johansson, Langsrud, & Risvik, 2005; Raffo et al., 2018; Sinesio et al.,
2019). Retronasal aroma perception is the result of a chain of events.
Aroma compounds are released during mastication and transferred
through the pharynx during exhalation or swallowing. They are deliv­
ered to the nasal cavity and detected by the olfactory epithelial re­
ceptors. An intense retronasal perception is generally found in the first
expiration after swallowing, which is known as the ‘swallow-breath’
(Linforth & Taylor, 2000).
730
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Trends in Food Science & Technology 112 (2021) 720–734
Fig. 6. Standardized TDS curves and mean release curves of eight target ions in the crumb-with-crust (CC) and crumb-only (CO) samples of bread type B1. The mean
swallowing time (ST) is indicated by the vertical lines. S.L. = significance line; C.L. = chance line. Reproduced from Jourdren, Masson et al. (2017) with permission
from American Chemical Society, Copyright 2017.
Dynamic profiling methods are increasingly used for retronasal
aroma perception analysis, including dynamic quantitative descriptive
analysis (D-QDA) (Pu et al., 2020), PP (Jourdren, Saint-Eve et al., 2017)
and TDS methods (Jourdren, Masson, et al., 2017; Pu et al., 2019).
Table 4 shows the aroma attributes commonly used for bread. A typical
aroma perception during bread chewing captured using TDS method is
shown in Fig. 6. For baguette samples, bread crumb had a dominant
‘wheat’ note before swallowing and a dominant ‘wet flour’ note after
swallowing (Jourdren, Masson, et al., 2017). Bread crust introduced a
dominant ‘toasted’ note at the beginning of consumption and ‘roasted
cereals’ and ‘cardboard’ notes during consumption (Jourdren, Masson,
et al., 2017; Jourdren, Saint-Eve et al., 2017). For white bread, the
dominant aroma sensations were ‘fermented’, ‘sour’ and flour’ in
sequence (Pu et al., 2019). In general, the intensity of retronasal aroma
perception increases over time (Jourdren, Masson, et al., 2017; Pu et al.,
2020).
mastication and contributed to ‘toasted’, ‘roasted cereals’ and ‘card­
board’ sensation (Jourdren, Masson, et al., 2017). Study of bread crumb
showed that initial volatile profile and crumb texture were the key pa­
rameters influencing aroma perception. Crumb with a firmer texture
exhibited a greater aroma release, which could be attributed to a higher
muscle activity (Jourdren, Masson, et al., 2017).
The large degree of inter-individual variabilities, including in-mouth
air cavity (IMAC) volume changes after swallowing (Mishellany-Dutour
et al., 2012), chewing behaviour (Blissett, Hort, & Taylor, 2006), rate of
saliva incorporation (Doyennette et al., 2014), respiratory rate (Pion­
nier, Chabanet, Mioche, Le Quéré, & Salles, 2004), has a great impact on
in vivo aroma release during food consumption. In general, a greater
respiratory rate and a higher muscle activity lead to a faster transfer of
aroma compounds from food to the upper airways. Salivation reduces
aroma perception due to its dilution and interaction effects.
Salivation was proved to have a significant impact on the volatile
composition of bread extract in artificial mouth (Poinot et al., 2009).
Similarly, ex vivo study of masticated bread bolus showed that the
headspace aroma release decreased over time, which could be explained
by the dilution effect of saliva (Jourdren, Saint-Eve et al., 2017). How­
ever, in vivo nose-space analysis of the same type of bread showed that
the intensity of the key ion fragment increased during chewing and the
peak was reached at the swallowing point (Jourdren, Masson, et al.,
2017). Thus, the enhancing effect of mastication appears to have a
greater effect on aroma release than salivation for bread oral processing.
7.3. Factors affecting retronasal aroma release and perception
Kinetics of volatile release during chewing are related to food
composition and structure, as well as individual physiology and chewing
behaviour. Bread crumb and crust showed different dynamics of volatile
release. Crumb volatiles (e.g. acetaldehyde, ethanol, and 2-methyl-1propanol) were progressively released and reached the maximum in­
tensity at the swallowing point, which explained the higher ‘wheat’, ‘wet
flour’, and ‘fermented’ sensation at the end of mastication and post
swallowing. In contrast, crust volatiles (e.g. 2- and 3-methylbutanal, and
2-methylpropanal) were released in larger quantities at the beginning of
731
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J. Gao and W. Zhou
8. Bread structural design and its impact on bread nutritional
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Developing healthier bakery products with acceptable functional
and sensory characteristics has increasingly been demanded by con­
sumers. Increasing evidence has demonstrated that food structure plays
an equal, if not more important, role in the digestion and absorption of
nutrients. The aerated structure of bread has a direct impact on its high
GI. Burton & Lightowler (2006) reported the possibility to alter the GI of
white bread by manipulating bread structure. By lowering the loaf
volume, there was a significant reduction in peak plasma glucose level
and an increase in satiety. Later, Saulnier et al. (2014) confirmed this
relationship on a larger variety of breads. In another study, bread pre­
pared using the same formulation but different processing methods
(baking vs. steaming) elicited different glycaemic responses (Lau et al.,
2015). Further investigation revealed that this might be attributed to
their distinctly different bread structures which lead to different levels of
structure disintegration during oral processing (Gao et al., 2015, 2021).
The direct impact of oral processing on people’s glycaemic response was
also demonstrated in the studies of other cereal products, such as rice
and pizza (Ranawana et al., 2010a; Ranawana et al., 2010b; Zhou et al.,
2014; Tamura et al., 2017). It should be noted that it is still not definitely
established whether the GI decrease with increasing density is due to less
destructuration of starch during baking or to less accessibility of salivary
α-amylase of denser bread pieces fragmented during oral processing.
9. Conclusion
The current market trends of re-designing traditional bread products
for health concerns, especially reduction in glycaemic index and salt, has
bought to light the importance of understanding its oral processing.
Developing bread with the optimal structure that delivers the desired
organoleptic outcomes remains a very active research field. A basic
understanding of bread oral processing is established while a holistic
and integrated analysis is required. Some challenges are highlighted for
future studies:
1) Chewing behaviour needs to be characterized beyond orofacial
muscle activity. Kinematic analysis of jaw and tongue movement will
provide valuable insight on bolus formation and help to improve the
design of artificial mouth.
2) The link between bolus formation and texture perception remains
unclear. Innovative sampling and characterization methods are
required for bolus analysis to allow it to be profiled in a temporal
manner.
3) The role of oral physiology in bolus formation and sensory percep­
tion is not fully understood. Studies involving a large number of
subjects with a wide range of physiological conditions are required.
4) Bread products need to be designed for targeted consumer groups.
With challenges of the ageing population, bread products that are
softer in texture, easier to hydrate and swallow, and less sticky will
be welcomed.
Acknowledgement
The authors gratefully acknowledge the research funding from the
Agency for Science, Technology and Research (A*STAR), Singapore
through IAF-PP FSENH grant H18/01/a0/G11.
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