The E#ect of Gelatinized Starch on Baking Bread

Shigehiro NAITO+�, Shinji FUKAMI

,, Yasuyuki MIZOKAMI,, Rieko HIROSE

-, Koji KAWASHIMA.,

Hiroyuki TAKANO/, Nobuaki ISHIDA

+, Mika KOIZUMI+ and Hiromi KANO

0

+ National Food Research Institute, ,�+�+, Kannondai, Tsukuba-shi , Ibaraki -*/�20.,, Japan, Tsukishima Foods Industry Co., Ltd., -�+1�3 Higashikasai, Edogawa-ku, Tokyo +-.�2/,*, Japan- Tokyo Metropolitan Food Technology Research Center, +�3 Kanda-sakuma-cho, Chiyoda-ku, Tokyo +*+�**,/, Japan. Seitoku University, //* Iwase, Matsudo-shi, Chiba ,1+�2///, Japan/ Agriculture, Forestry and Fisheries Technical Information Society, Seifun-kaikan 0F, +/�0 Kabuto-cho, Nihonbashi,

Chuo-ku, Tokyo +*-�**,0, Japan0 Oak-hill Georgic Patch-work Laboratory, .�+-�+* Miyamoto, Funabashi-shi, Chiba ,1-�***-, Japan.

Received October 1, ,**. ; Accepted April +,, ,**/

The e#ects of adding gelatinized starches to a kneading dough (a process known as yukone in Japanese)

on the crumb grain of baked white breads were studied using magnetic resonance imaging (MRI) and using

scanning electron microscopy (SEM) with and without distilled-water soaking of the samples. MRI

revealed that pores became larger and rounder in yukone breads compared to control breads using the

sponge dough method, while the number of pores decreased in yukone breads. SEM revealed many starch

granules on the surface of pore walls in the control breads, whereas the yukone breads contained starch

gels cemented between starch granules. Gluten nets were found to be uniform and oriented in the control

breads and became thicker and coarser in the yukone breads. Comparing the SEM images of two commer-

cial white breads made by the yukone method, the fine gluten nets under the starch walls were found to be

considerably di#erent.

Keywords : gelatinized starch, white bread, yukone, MRI, SEM, image analysis

Introduction

The formation of starch gels in dough at moderate

temperatures plays an important role in the baking proc-

ess (Schoch, +30/ ; Iwata, ,**+). Starch properties are re-

lated to specific loaf volumes in breads ; starches gela-

tinized at lower temperatures, such as potato starch, pro-

vide better loaf volumes than those gelatinized at higher

temperatures, such as corn and rice starches (Sandstedt,

+30+). Amylase-treated starches were found to improve

loaves (Sandstedt et al., +3-3) by eliminating the detrimen-

tal surface layer of starch granules (Sandstedt, +30+) and

the digestive action on wheat flour starches (Sandstedt,

+30+). It is also known that various textures may be

obtained in baked products by the addition of starches or

modified starches (Schoch, +30/ ; Iwata, ,**+). In some

instances, gelatinized starches from corn or tapioca are

mixed with wheat flour in the kneading dough at a rate of

+/� to ,*� in bread baking. The use of potato, corn, or

bridged starches results in a hard, crisp feeling, while

tapioca starch, waxy corn starches, or esterified and ac-

etylated starches give breads a soft, sticky and moist

feeling (Iwata, ,**+). Beta-limit dextrin has similar e#ects

(Sandstedt, +30+).

The yukone method produces breads with a soft, sticky,

and moist texture. In this method, some of the wheat

flour is separately kneaded at temperatures higher than

0*� to gelatinize the starches, and then returned to the

dough mixture for additional kneading. The starches in

wheat flour kneaded with hot water are gelatinized, but

gluten sheets do not develop because kneading takes

place at a temperature higher than 0*�. Dough made by

this method contains large amounts of gelatinized star-

ches. In addition, this method increases the apparent

moisture retention of the crumb (Schoch, +30/) and

reduces the gluten nets that weaken the crumb structure.

Yukone has been established in Japan as a method for

making breads that have a soft and sticky texture and a

high tolerance to staling without increasing the thickness

of pore walls in the crumb grain.

In a previous study (Naito et al., ,**.a), we investigated

the fine architectural structure of the crumb-grain walls

of baked breads using a scanning electron microscope

(SEM). The skeletal framework of the pore walls was

found to consist of fine nets of gluten fibrils covered with

gelatinized starch granules. The gluten-fibril nets are

sensitive to bread-making methods and dough-prepara-

tion conditions such as mixing time, molder clearance,

and freeze-thaw cycles (Naito et al., ,**.b). Considering

the observed reduction of gluten and increase of gela-

tinized starch in the yukone dough, it is likely that the

architecture of the skeletal gluten-fibril nets in the pore

walls has specific characteristics which a#ect the textureE-mail : naito@a#rc.go.jp

Food Sci. Technol. Res., ++ (,), +3.�,*+, ,**/

of baked breads made by the yukone method.

We studied the e#ects of yukone on crumb-grain net-

works using magnetic resonance imaging (MRI) and its

e#ects on the skeletal gluten nets using SEM. Di#er-

ences in the gas-cell-wall surfaces and fine gluten nets of

white breads prepared by the yukone method and the

sponge dough method are discussed.

Materials and Methods

Preparation of Dough and Baking of Breads The dough

was prepared according to a modified sponge dough

method (Japan Yeast Industry Association, +330). For

the yukone bread, hard wheat flour (Kameria, Nisshin

Flour Milling Inc., Tokyo) was kneaded with the same

amount of boiling water at low speed (++2 rpm, Mighty-,/

mixer, Aicohsha Manufacturing Co., Ltd., Saitama, Japan)

for , min. The gelatinized dough was cooled from 0,� to

+*� in a refrigerator, and then mixed with the fermented

sponge dough and the remaining ingredients at the

dough mixing stage. The formulas and procedures for

white breads are summarized in Tables + and ,.

Market Bread Two brands of white bread made by the

yukone method were purchased at local shops. SEM images

of these breads (Samples + and ,) are shown in Fig. 0.

Magnetic Resonance Imaging The Fe-�-acetone method

devised by Ishida et al. (,**+) was used to obtain MR

images of the crumb grain. Ishida et al. (,**+) reported

that acetone was useful in this context because of its

single peak, which prevents chemical shift e#ects on

images ; it also facilitates retention of the bread structure

and allows dissolution of heavy metals. They further

reported that the use of Fe-� shortened the overly long

relaxation times of acetone to practical lengths for imag-

ing and stained the materials to provide high contrasts.

A nuclear magnetic resonance (NMR) imaging system

(NMR spectrometer DRX -**, Bruker, Karlsruhe, Germa-

ny) was used to measure +0-mm cubic pieces of baked

bread by the spin-echo -D-FT method. The -D images

were measured, processed, and analyzed according to pro-

cedures reported in a previous paper (Takano et al., ,**,).

ImageJ (Ver. +.--j) (http : //rsb.info.nih.gov/ij/index.html),

a public domain Java image processing program, was

used for image analysis.

Scanning Electron Microscopy The crumb grain and

the underlying gluten fibrils of baked breads were ob-

served using scanning electron microscope (JSM-/0** LV,

JEOL, Tokyo, Japan). SEM images of gluten fibrils form-

ing the skeletal framework of pore walls were acquired as

described in previous papers (Naito et al., ,**.a, ,**.b).

Cubic pieces of baked white bread (+* mm) were soaked in

distilled water (AutoStill WG/+*, Yamato Scientific Co.

Ltd., Tokyo, Japan) at ,*� for +* min, with no stirring,

which washed out some of the starch granules on the

surface of the crumb grain. The samples were then rapid-

ly frozen in liquid N, for - min, fractured with a hammer,

and set in the sample holder of the apparatus.

Results and Discussion

Figure + illustrates the crumb-grain networks observed

by MRI. These networks represent the distribution of

gas cells in baked breads prepared using the sponge

dough method (control) (A) and the yukone method (B).

Table +. Formulas for white breads.

The E#ect of Gelatinized Starch on Baking Bread 195

Table ,. Procedures for white breads.

Fig. +. Magnetic resonance images of the crumb-grain networks of white breads produced using sponge dough and

yukone methods.

A : Control, sponge dough method. B : Yukone, yukone method using boiling water for kneading a portion of wheat flour.

,D image sections sliced from -D image data of bread soaked in acetone containing 2 mM Fe-� ion are illustrated. The

three horizontal images are di#erent sections from the same cubic sample. Scale bar is / mm.

S. NAITO et al.196

There were recognizable di#erences between the two

breads. The networks of the control bread possessed

vertically ellipsoidal pores, while networks from the

yukone bread showed rather round pores, or large and

long pores. The ratio of pore area to total area in yukone

bread increased slightly but significantly (p�*.*+) com-

pared to the control bread (Table -). This indicates an

increase in the ratio of bread volume to dough materials

and a decrease in the average thickness of the crumb-

grain networks. Both average pore area and average

pore circularity were significantly increased (p�*.*+) by

the addition of gelatinized starches, because the large

number of pores (over /,***) greatly decreased the stand-

ard errors of pore area and circularity (Table -). The

distributions of pore area and circularity were checked by

making percentile graphs, because statistical tests using a

large amount of data sometimes detect meaningless

di#erences (Nagata, +330 ; Gardner and Altman, ,**+).

The pore area distribution shifted to the larger side (Fig.

,A). Pore circularity also shifted in favor of roundness

with the addition of gelatinized starches (Fig. ,B). These

shifts corresponded to the impression of the grain net-

works observed by the naked eye. The pores separated

into large and small sizes in the yukone bread. Smaller

pores occupied a large area in the control bread ; in con-

trast, the proportion of the small-pore area decreased and

that of the larger pore area increased in the yukone bread

(Fig. -). This change may be related to changes in the

balance between sticky gelatinized starch and adhesive

gluten in the dough.

Figure . illustrates the wall surface of pores in the

crumb grain of the control bread (A) and the yukone

bread (B), as observed by SEM without distilled-water

soaking treatment. Starch granules were in tight con-

tact in the control bread (Fig. .A), but buried by starch

gels in the bread made by the yukone method (Fig. .B).

This tight contact may result from gelatinization of

starch granules due to progressing under limited water

conditions.

In baked breads, gelatinized starch granules form the

surface of gas-cell walls by making contact at their

gelatinized parts (Fig. .A), while gluten acts as an adhe-

sive to bind the starch granules (Sandstedt, +30+). The

gelatinized starch granules in baked breads may be partly

gelatinized due to the limited amounts of water available.

Gelatinization of starches in the early stage of baking

sustains the gas pressure on pore walls during expansion

at low temperature. The walls then crack, producing

holes in the cell walls (Ishida et al., ,**+) and exhausting

excess gas pressure to adjacent cells (He and Hoseney,

+33+ ; Hayman et al., +332) near the melting temperature of

the starch granules (Leon et al., +331) and at maximum

viscosity (Yasunaga et al., +302 ; Varriano-Marston et al.,+32*). The gas pressure is subsequently contained by the

external crust, which results in a uniform gas pressure

among all gas cells under the thick crust. This process is

considered necessary for obtaining good quality bread.

It is thought that during the process, the gluten sheets are

stripped into thin strings in sheeted orientation, although

they maintain contact with the starch granules at moder-

ate temperatures because the extensibility of the gluten

sheets is not as high as that of the gelatinized starches.

Consequently, the segregated gluten fibrils are covered

with starch gels, and fine gluten nets maintain the tight-

Table -. Image analysis of the networks using sponge dough and yukone methods.

The E#ect of Gelatinized Starch on Baking Bread 197

ness of starch walls in a similar manner to wires in plastic

sheets. These starches and gluten may play individual

roles in bread baking. In this context, dough prepared

by the yukone method contains less gluten binder. How-

ever, increased amounts of gelatinized starches provide

wall materials with stickiness and good expansion char-

acteristics, and they will bind starch granules.

The skeletal structures made of gluten fibrils under the

starch walls in the crumb grain sustain the gelatinized

starch granules. These skeletal structures are shown for

the control bread (Figs. /A and /C) and the yukone bread

(Figs. /B and /D) using SEM with distilled-water soaking

Fig. ,. E#ect of bread-making methods on the pore size

and circularity distributions of white bread.

A : Pore area distribution is expressed in percentile. B :

Pore circularity distribution is expressed in percentile.

Pores with an area of less than *.+* mm, or with a

circularity of less than *.. and an area exceeding +.** mm,

were neglected. The circularity threshold for removing

multi-fused pores was determined by considering the

relationship between circularity and pore area and by

checking pore shapes detected. Pore circularity, defined as

.p(pore area)/(perimeter),, equals one when the pore is a

perfect circle. A value approaching *.* indicates an

increasingly elongated pore.

Fig. -. E#ect of bread-making methods on pore area

distribution in relation to pore size.

The graph shows the relationship between the proportion

of pore area of individual size to total pore area, and

equivalent diameter (� � ���� ����� ). The equivalent dia-

meter indicates an assumed diameter based on pore size.

Fig. .. Scanning electron microscope images of pore wall

surfaces in the crumb grain of white breads made from

sponge dough (A) and yukone dough (B).

Deformed circles are starch granules at various stages of

deterioration in the bread from sponge dough ; starch gels

bury starch granules in the bread from yukone dough.

Scale bar is /* mm (-**-fold). These images were acquired

by SEM without distilled-water soaking treatment.

S. NAITO et al.198

treatment. Nets of fine gluten fibrils with small meshes

were observed on the eluted surface of the starch walls of

the control bread (Figs. /A and /C). The gluten-fibril

nets were thicker and coarser for the yukone bread (Figs.

/B and /D) than for the control bread (Figs. /A and /C),

and more starch gel was eluted for the latter. In the

control bread, many starch granules were observed on the

remaining wall surface under the fine gluten nets (Fig. /

C), but few starch granules were detected in the yukone

bread (Fig. /D). Easy elution of gelatinized starch gran-

ules from the crumb-grain walls of the yukone bread (Fig.

/D) indicated that the contact between gelatinized starch

granules and gluten sheets was weak (Sandstedt et al.,+3/. ; Sandstedt, +30+ ; Bechtel et al., +312), which may be

due to the inability of gelatinized starches to form a

complex with gluten. This could result in tight inter-

faces with adhesive gluten (Sandstedt, +30+) ; it is known

that the organization of starch granules at various stages

of distortion, including the membrane systems containing

starch granules, plays a role in causing tight contact with

gluten sheets (Khoo et al., +31/). We speculate that the

development of gluten sheets between starch granules

requires firm starch granules to put pressure on gluten in

the mixing dough. Gelatinized starches in yukone

dough are probably too soft to press gluten firmly. The

large mesh sizes of the gluten nets compared to the starch

granules (Naito et al., ,**.b) may result in di$culty in

fastening starch granules between the gluten fibrils of the

nets. If so, it is likely that only blocks of aggregated

starch granules are in contact with the gluten fibrils, and

the aggregated starches may be bound by starch gels.

This is consistent with the observation of coarser skeletal

gluten nets in the breads produced using the yukone

method compared to breads produced using the sponge

dough method. Furthermore, the gluten fibrils were not

of uniform thickness, indicating that the arrangement of

the block of starch granules regulating the gluten matrix

mesh was not uniform in the yukone bread.

Defects in the gluten sheets can be compensated for

with flexible starch (Sandstedt, +30+). Furthermore, the

addition of gelatinized starches may increase the flexibil-

ity of starch walls. The modification of starches improves

bread by eliminating the detrimental surface layer of

starch granules (Sandstedt et al., +3-3) and increases the

usefulness of starches (Iwata, ,**+). This property of

gelatinized starches has been utilized in modern tech-

niques to obtain breads that are easy to eat because of

their smooth texture.

Many types of bread have been produced by yukone

methods. Two brands of commercial white bread pro-

duced using the yukone method (commercial bread

samples + and ,) were studied by SEM. In sample +,

made by the typical yukone method (Figs. 0A and 0C),

starch granules were observed with a thin cover of starch

film in the image of the crumb-grain surface without

distilled-water soaking (Fig. 0A, /**-fold). In the image

Fig. /. Scanning electron microscope images of white breads from sponge dough (A, C) and yukone dough (B, D).

A, B : Low-magnitude (/*-fold) images showing pore surfaces with gelatinized starches eluted. Scale bar is /** mm. C, D :

High-magnitude (-**-fold) images showing the nets of gluten fibrils that form the skeletal architecture of pore walls

covered by gelatinized starch granules. Scale bar is /* mm. Nets were coarse and the mesh was large in the bread from

yukone dough. These images were acquired by SEM with distilled-water soaking treatment.

The E#ect of Gelatinized Starch on Baking Bread 199

of crumb grain with distilled-water soaking, thick gluten

fibrils formed rather uniform nets, but the mesh of the net

was coarse and not oriented (Fig. 0C, -**-fold). The

bread exhibited the characteristics of yukone bread (Fig.

/D), but the e#ect was not significant, indicating that the

amount of gelatinized starch mixed into the dough might

have been small. Commercial bread sample , was made

by a di#erent yukone method (Figs. 0B and 0D). A thick

covering of starch film was observed in the image of the

crumb-grain surface without distilled-water soaking (Fig.

0B, /**-fold). The fine gluten-fibril nets observed in the

image of the crumb grain with distilled-water soaking

(Fig. 0D, -**-fold) were considerably di#erent from those

found in bread made by the yukone method in the current

investigation (Fig. /D) and those of commercial bread

sample + (Fig. 0C).

In summary, it was found that starch characteristics are

important, and changes to the rigid plate at higher tem-

peratures (near +**�) can easily cause gas to break the

plate during baking. The characteristics of starches

allow gelatinization to form gas cells and enable them to

swell at moderate temperatures (0*�). Gluten sheets

orient the expansion of gas cells, facilitate uniform pro-

cesses, and provide pore wall strength. If the gluten

sheets are too strong, the gas cells do not expand enough.

The addition of gelatinized starches may compensate for

defects due to weak gluten sheets and the reduced tension

of gluten sheets. However, if the gluten sheets are too

thin or weak, their e#ects are minimal : pores expand

evenly to form circles (not ellipsoids), and the pore walls

become too soft and lose their elasticity. The gluten

sheets are thought to provide pore orientation, firmness,

and elasticity.

Acknowledgments This work was partly supported by research

funding in ,**+ from the Iijima Memorial Foundation for the

promotion of food science and technology (Japan).

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The E#ect of Gelatinized Starch on Baking Bread 201