Bread Crust Thickness Measurement Using Digital Imagining

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Bread crust thickness measurement using digital imaging and L a b colour system

Y.M. Mohd Jusoh a,b, N.L. Chin a,*, Y.A. Yusofa, R. Abdul Rahman a,c

a Department of Process and Food Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysiab Department of Bioprocess Engineering, Faculty of Chemical Engineering and Natural Resources Engineering, Universiti Teknologi Malaysia, 81310 UTM, Skudai, Johor, Malaysiac Department of Food Technology, Faculty of Science and Technology, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia

a r t i c l e i n f o

Article history:

Received 19 September 2008Received in revised form 20 March 2009

Accepted 4 April 2009

Available online 17 April 2009

Keywords:

L a bvalues

Crust thickness

Bread

Digital imaging

a b s t r a c t

A simple and new method was developed for the evaluation of baking process on bread quality through

the measurement of bread crust thickness. By distinguishing the crust and crumb regions of bread, the

system which uses digital imaging and the L a b colour system can predict bread crust thickness from

the colour measurements of bread surface browning. Standard baking tests were conducted at different

levels of temperature and time combinations to produce open breads with different crust thickness. The

results show that the crust thickness whichranged from 6.02 to 9.00 mmhas a negative relationship with

each of the L, a, and b values and a positive correlationwith thetotal colour difference (DE) of bread crust.

The data also demonstrated that crust thickness increases with the investigated baking temperatures of

185, 195, and 205 C more significantly (p< 0.0001) than baking times of 25, 30 and 35 min (p< 0.001).

2009 Elsevier Ltd. All rights reserved.

1. Introduction

During the baking process, dough experiences major physical

and biochemical changes due to heat exposure which lead to the

transformation from raw dough to bread with two distinctive

structures i.e. the crust and the crumb. The crust is associated to

the brown surface of bread while crumb is the inner white spongy

structure beneath the crust. The crust, which is formed through

Maillard reaction and caramelisation during baking, has several

important functions on bread properties. The thickness and charac-

teristics of the crust to a large extent define the product and give

its name (Wiggins, 1999; Cauvain, 1999). In general, bread crust

is referred as a marketing tool that attracts customers through its

appearance, aroma, and flavour (Zenthenbaur and Grosh, 1998;

Purlis and Salvadori, 2007). Food products which their appearances

have been used as a guide to estimate its overall quality are fruits,

vegetables, grains, meat, seafood and bakery products (Pearson,

1996; Chao et al., 2002; Blasco et al., 2007; Zou et al., 2007).

Abdullah et al. (2000)inspected the quality of muffins by using

an external property,i.e. colour. In terms of actual crust functions,

the formation of bread crust is imperative as it contributes to its

aroma, flavour and texture while influencing its baked volume, cell

structure and density (Zhang et al., 2007). An early crust formation

limits bread expansion, causing formation of coarser bread struc-

ture due to cell rupture and coalesces, and also causes densification

within the crumb. Besides that, bread crust is also known to be clo-

sely related with moisture loss of bread during and post baking

periods. The crust formation affects the amount of moisture evap-

orating from wet dough during the baking process as a thicker

crust is produced with a higher moisture loss in bread (Wiggins,

1999). This is because crust formation develops simultaneously

as moisture evaporates during the baking process. Moisture loss

during baking translates to weight loss of bread and this is less

favourable for breads sold by its weight. For post baking periods,

crust functions as an insulator that prevents moisture from migrat-

ing to surrounding thus may give an impact towards reducing

crumb staling. Detailed studies by Wahlby and Skjoldebrand

(2002) showed that crust functions as weight loss barrier since

moisture loss of crustless bun was higher compared to crusted

bun during a reheating treatment. Their observation was strength-

ened byPrimo-Martins et al. (2006), who showed that bread with

crust experienced lower moisture loss in comparison to crustless

bread during storage. In relating bread crust properties in terms

of its colour with moisture loss, Purlis and Salvadori (2007) pre-

sented a strong correlation between the moisture loss and the

crust colour formation in their study on browning kinetics of

bread. From these literatures, it seems viable to control moisture

loss from bread during baking and storage through the creation

of a desired crust with suitable properties.

There are various attempts to define and measure bread crust

although to date, there is not any widely accepted and approved

definition for bread crust and crumb. Attempts to define crust

and crumb based on breads physical structure, i.e. density and

behaviour are found.Jefferson et al. (2006) defined crust as a part

of bread near its surface where the density is highest in bread

structure. Lostie et al. (2004) interpreted crust and crumb based

on their behaviour where crumb behaves as viscous compressible

0260-8774/$ - see front matter 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.jfoodeng.2009.04.002

* Corresponding author. Tel.: +60 3 89466353; fax: +60 3 86567123.

E-mail addresses: [email protected], [email protected](N.L. Chin).

Journal of Food Engineering 94 (2009) 366371

Contents lists available at ScienceDirect

Journal of Food Engineering

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j f o o d e n g
mailto:[email protected]:[email protected]:[email protected]://www.sciencedirect.com/science/journal/02608774http://www.elsevier.com/locate/jfoodenghttp://www.elsevier.com/locate/jfoodenghttp://www.sciencedirect.com/science/journal/02608774mailto:[email protected]:[email protected]


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mixture while crust acts like a porous shell. Gallagher et al. (2003)

and Zhang et al. (2007), using the scanning electron microscope

(SEM), defined crust and crumb based on the difference of their

physical structure such that crust has a denser structure compared

to the crumb. In measuring crust thickness,Zanoni et al. (1992)

used quick-freezing to separate the crust from crumb since both

have different structure that easily tears apart from each other dur-

ing thawing. Other methods include the visual techniques using

scanning electron microscope (SEM) by Gallagher et al. (2003)

and the confocal laser scanning microscopy (CLSM) byPrimo-Mar-

tin et al. (2006). The disadvantage of quick-freezing method is that

it is prone to damage the bread structure while the SEM and CLSM

require high technology and expensive equipment.

The application of computer vision, digital imaging and colour

system to determine food qualities is becoming more popular

(Brosnan and Sun, 2004; Pedreschi et al., 2006). Images of food

can be conveniently captured and its colours are determined using

various systems, e.g. theL a b, RGB (red, green, blue), XYZ or the

CMYK (cyan, magenta, yellow, black) (Pedreschi et al., 2006) to

make correlations with food properties. Yam and Papadakis

(2004) designed a simple digital imaging to qualitatively and quan-

titatively analyze food surface and structure. The ability and reli-

ability of this method was tested by Larrain et al. (2008) who

used digital imaging to estimate colour coordinates of beef. Purlis

and Salvadori (2007)found a high correlation between bread crust

colour and moisture loss whileZhang et al., (2007) have estab-

lished relationships between the outer crust and the inner crumb

property. The aim of this research is to develop a non-destructive

method to measure crust thickness by integrating the digital imag-

ing and colour systems. Ultimately, bread crust thickness could be

obtained through measurement of surface browning of bread using

a chromameter as an indication of its inner properties or product

quality.

2. Materials and methods

2.1. Bread sample preparation

Open breads were produced using the straight dough method

following the standard baking tests using formulation as presented

in Table 1. All the ingredients were mixed in a vertical mixer

(SPN25053, Lian Huat, Malaysia) for a total of 16 min, i.e. 4 min

at low speed and 12 min at high speed. The whole dough was let

to rest for 5 min after mixing. After resting, the dough was weighed

and divided into 380 g dough balls, rounded and let to rest again

for another 5 min before moulded by an automatic moulding ma-

chine (CM750, Lian Huat, Malaysia). The moulded dough were

put into stainless steel baking tins with dimension of

10cm 19cm 10.5 cm and stored in the retarded proofer

(LRP36052, Lian Huat, Malaysia) for 90 min at 28 C and 85% rela-

tive humidity. The doughs were baked in tins without lids at com-binations of three temperatures, i.e. 185, 195 or 205 C and three

times, i.e. 25, 30, or 35 min in a convective deck oven (EO3050C5,

Lian Huat, Malaysia) to obtain various crust thickness. All tests

were conducted in triplicates and statistical analysis was per-

formed using Microsoft Excel (XP Edition, Microsoft Corporation,

USA).

2.2. Outer crust and inner crumb colour determination and

measurement

TheL a b colour system wasused for determining bread crust and

crumb colours because it is the most commonly used colour systemin colorimeter, data acquisition and image processing systems

(Pedreschi et al., 2006). Besides that, it also gives uniformity in col-

our distribution and closeness to human perception (Len et al.,

2006).TheL value represents lightness component on surface that

the value ranges from 0 to 100 while a andb values are chromatic

components of redness to greenness and blueness to yellowness

that ranges from120 to 120, respectively (Papadakis et al., 2000).

The colour of the outer crust and inner crumb from all 27 baked

samples were measured using a chromameter (CR410, Konica

Minolta, Japan) with xenon lamp as light source to determine its

colour of crust and crumb regions in terms L a bvalues. The scan-

ning of the outer crust colour wasperformed in a consistent manner

with sufficient lighting by placing the chromameter probe onto the

top surface of bread crust while for crumb, the probe was placed

onto the centre part of a central slice of bread (Fig. 1). The average

values ofL a b colours describing the outer crust and inner crumb

regions were obtained from all 27 baked open loaf samples. The to-

tal colour difference,DEof the bread slices from the reference is:

DE Lo L2 ao a

2 bo b2

h i12

1

whereLo= 100,ao= 0 andbo= 0.

2.3. Crust thickness measurement

Crust thickness in this study is defined as the distance between

outer crust and the point of inner crust where its colour satisfies

the colours as crust pre-determined in Section 2.2. This means

the point where the inner crust colour such that the L valueis the minimal and the a and bvalues are the maximal of the crumb

regions is taken as the thickness. For determining this crust thick-

ness, a sample slice of bread was placed on a transparency grid ona

scanner machine with a cold cathode fluorescent lamp (Scanjet

2400, HewlettPackard, USA) to be scanned simultaneously. The

scanned bread slice image in Tiff format was transferred to the

Adobe Photoshop software (Photoshop CS2, Adobe, USA) to allow

the user to determine the crust thickness by counting the number

of boxes which meets the requirement ofL a bvalues known as the

crust region (Fig. 2). The Info application in Photoshop CS2 reads

the originalL a bvalues of the scanned bread slice.Fig. 2 illustrates

that by moving the cursor slowly from the central crumb region to-

wards the crust manually, the thickness is obtained at the point

Table 1

Open bread formulation (based on 3000 g flour loading).

Ingredients Bakers %

Flour 100

Water 63

Sugar 6

Salt 1.5

Yeast 1

Shortening 5

Location of outer crust colour

measurement using chromameter

Location of inner crumb colour

measurement using chromameter

Centre slice

Fig. 1. Locations for measuring surface browning of outer crust and inner crumbcolours using a chromameter.

Y.M. Mohd Jusoh et al. / Journal of Food Engineering 94 (2009) 366371 367


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where the L a b values meet the requirement of the crust region

pre-determined in Sections 2.2 and 3.1which is L < 70, a > 0 and

b> 13. This method was adopted and improvised from the previ-

ous work ofYam and Papadakis (2004) andCollar et al. (2005).

3. Results and discussion

3.1. Outer crust and inner crumb colour regions

Table 2shows the average colour ranges for outer crust and in-

ner crumb regions from all the 27 baked open loaf samples. In

identifying the colour region for determining crust thickness, the

inner crust region was identified as the region lying between the

extreme colours of external crust and internal crumb.Fig. 3illus-

trates the distinguishable colour regions of the outer crust and in-

ner crumb in terms of L a b values for determination of colour

ranges for inner crust region as L < 70,a > 0 andb > 13.00. The in-

ner crust region is defined as the crust thickness where its colour

range identification is determined based on its location towards

the crust region and commences where at the point of minimum

Lvalue, and maximum values ofa andb. The lowerL value, higher

a and b values at the outer crust compared to the crumb, respec-tively indicate a darker shade, more reddish and yellowish pig-

mented region for the crust. The colour pigments formed on the

crust generally agree to the fact that crust experiences carameliza-

tion and Maillard reactions which are highly influenced by the

quality and quantity of the precursors, thermal processing param-

eters, pH and quantitative ratio of amino nitrogen to reducing su-

gar during baking (Martins et al., 2001).

3.2. Effect of baking temperature and time on crust colour and

thickness

Fig. 4 shows that the L a b values of outer crust and innercrumb decreases with increasing baking temperature and time.

Fig. 2. Measurement of crust thickness of a scanned bread slice with the aid of underlying 1 mm grid boxes using the Photoshop software, inset shows the toolbox forL a b

values.

Table 2

Colour ranges for outer crust, inner crust and inner crumb obtained from chromam-

eter scanning.

Bread regions L value a Value bValue

Outer crust 29.4148.66 8.7814.66 5.8026.65

Inner crumb 70.5079.00 0.81 to 0.00 5.9013.00

Inner crust 48.6670.50 >0.00 >13.00

29.4

48.7

70.5

79.0

8.8

14.7

5.8

26.7

5.9

13.0

-10

0

10

20

30

40

50

60

70

80

90

100

L value avalue bvalue

Crust Crumb Crust Crumb

0

- 0.8

Crust Crumb

Fig. 3. L a bvalues of the outer crust and inner crumb regions for determination of

inner crust regions and colour identification of crust thickness.

368 Y.M. Mohd Jusoh et al. / Journal of Food Engineering 94 (2009) 366371


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The similar decreasing trend ofL value as affected by increasing

baking parameters was observed byShittu et al. (2007). However,

the decrease ofL a b values are less prominent in the inner crumb.

This insignificant decreasing trend shows that the crumb colour is

not really affected by baking conditions and suggests that it has

been insulated by the crust layer. Referring literatures, the L value

has been a reliable colour parameter to describe the crust andcrumb regions (Shittu et al., 2007; Purlis and Salvadori, 2007;_Ibanoglu, 2002). The reporteda andb values for crust are less con-

sistent (_Ibanoglu, 2002) and this could be due to the difference in

the type of baked products and ingredients used.

Besides creating difference in colours and tone of bread surface

browning, baking temperature and time also produced bread with

different crust thickness. Using bread crust thickness measurement

established in Section2.3, the system showed capability to detect

small differences and gives reliable results. The bread crust thick-

ness from all the 27 baked samples ranged from 6.08 to

9.00 mm. The reliability of this crust thickness measurement

method is proven byFig. 5which shows that a higher baking tem-

perature and time produced bread with higher crust thickness.

These results are consistent with existing findings on crust thick-

ness as affected by its baking temperature and time (Zanoni

et al., 1992;Jefferson et al., 2006).Zanoni et al. (1992)found that

an extension of 5 min baking at a fixed baking temperature of

203 C caused bread crust thickness to increase 50% from its origi-

nal thickness whileJefferson et al. (2006)discovered that a 14% in-

crease in baking temperature caused a 10% increase in crust

thickness. A higher heat and mass transfer, and evaporation pro-

cess occur at higher baking temperatures which cause a thicker

crust formation.Fig. 6shows the image of crust thickening as af-

fected by baking temperature. The highest rate of crust formation

took place at 205 C with value of 0.108 mm/min. The ANOVA re-

sults showed that the baking temperature (p< 0.0001) causes more

significant crust variation compared to the baking time (p< 0.001).

0

6

12

18

24

30

24 28 32 36

Baking time (minute)

bv

alue

0

20

40

60

80

100

24 28 32 36


Lv

alue

-4

0

4

8

12

16

24 28 32 36


a

value

)c()a( (b)

Fig. 4. Colour trends for outer crust () and inner crumb ( ) in terms of (a) L, (b)a and (c)b values at three baking temperatures, 185 C (), 195 C (j), and 205 C (N).

8.25

7.92

9.00

6.92

7.33

7.75

6.08

6.42

7.00

y185C= 0.092x + 3.74, R2= 0.978

y195C= 0.083x + 4.84, R2= 1

y205C= 0.108x + 5.15, R2= 0.952

5

6

7

8

9

10

24 28 32 36


CrustThickness(mm)

205C

195C

185C

Fig. 5. Crust thickness trends as a result of three baking temperatures, 185 C (),

195C (j), and 205 C (N) and three baking times, 25, 30, or 35 min.

Fig. 6. Scanned bread crust images depicting thickness of 6, 7, and 9 mm obtained from respective baking temperatures, (a) 185 C, (b) 195 C, and (c) 205 C for a 25 minbaking period.



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3.3. Correlations between surface browning of crust and its thickness

Fig. 7shows clear negative correlations between each of the L abvalues with crust thickness. The strength of the relationships are

strong with high coefficients of correlations, R2, of 0.9549, 0.8515

and 0.9416 forL a b values, respectively. These highR2 values infer

that the colour components obtained from the outer crust could be

used to determine its crust thickness. In terms ofL value, a lowerL

value obtained from a darker crust surface infers a thicker crust. As

the crust thickens, the redness implied by the a value and the yel-

lowness implied by the b value, both decrease. However, as the

crust thickens, the Lvalue still dominates as the aand bvalues loss

their visibility on the surface, and replaced by the L value solely.

With the total colour difference, DE, calculated using Eq.(1), a po-

sitive correlation with the thickness was found (Fig. 8). With a high

R2, of 0.9467, this relationship could be used to predict bread crust

thickness. Similar linear relationships between the outer crust col-our and its thickness could be established for other types of loaves

or bakery products for the purpose of prediction of its crust prop-

erties. The crust properties are known to be useful in terms of

moisture loss and control hence may contribute a great deal inlowering its staling rate.

4. Conclusions

The thickness of bread crust was successfully determined by

distinguishing the crust and crumb colour regions via the applica-

tion of digital imaging and the L a b colour system. The system is

able to pick up small differences from all 27 samples which thick-

ness ranged from 6.08 to 9.00 mm. The strong positive correlation

between crust thickness and the total crust colour difference (DE)

was found and this allows prediction of crust thickness from the

brown surface colour of baked bread for bread quality evaluation.

The baking trials showed that the crust colour and thickness in-

crease with baking temperature and time. The temperature(p< 0.0001) has more significant impact on both crust properties,

the colour and thickness as compared with time (p> 0.001). This

crust thickness measurement method can be established for var-

ious bakery products and provides an alternative of cheap and

fast technique for more studies towards understanding crust

properties as crust has significant influences on bread quality

and shelf-life.

Acknowledgements

The authors would like to acknowledge the financial support

from Ministry of Higher Education of Malaysia for financial support

of this project (Vot. No. 5523214) and thank the Research, Develop-

ment and Commercialization Centre (RDCC) of Interflour Sdn. Bhd.

for providing baking facilities.

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Bread Crust Thickness Measurement Using Digital Imagining