effect of roasting conditions on the food quality of

J. Home Econ. Jpn. Vol. 51 No. 12 1115•`1125 (2000)

Effect of Roasting Conditions on the

Food Quality of Sesame Seeds

Tamami TAKEDA, Hiroko AONO,* Yasuko FUKUDA,* * Keiko HATAE* and Atsuko SHIMADA* * *

St. Catherine Women's Junior College, Hojo, Ehime 799-2496, Japan* Faculty of Life Science, Ochanomizu University, Bunkyo-ku, Tokyo 112-8610, Japan

* * Faculty of Education, Shizuoka University, Shizuoka 422-8529, Japan* * *

Graduate School, Showa University, Setagaya-ku, Tokyo 154-8533, Japan

The effects of roasting at 170•Ž, 200•Ž and 230•Ž for 5-40 min on the appearance, fine structure,

texture, saccharides and lignans of sesame seeds were examined.

Sesame seeds roasted at 170•Ž for up to 20 min were slightly browned with a cracked surface, but

had a poor texture. Seeds roasted at 230•Ž for 5-15 min burst and had a good texture. Distinct spaces

were apparent by SEM between the residual endosperm tissue and the cotyledon. However, strong

browning and cracks on the surface had already occurred after roasting for 5 min. In addition, sesamol

was significantly produced. Seeds roasted at 200•Ž had similar characteristics to those roasted at 230•Ž.

The free saccharides, glucose, fructose, sucrose, planteose and stachyose, were detected in the

sesame seeds. While stachyose was relatively stable, the other saccharides were rapidly degraded at the

higher roasting temperatures. A high correlation was apparent between the total amount of these

saccharides and the surface color (L value).

(Received September 24, 1999; Accepted in revised form August 24, 2000)

Keywords: sesame seed, roasting, texture, saccharide, lignan.

INTRODUCTION

Sesame seeds have been reported to play an

important role in the human diet around the world

(Fukuda and Takeda 1993; Takeda and Fukuda

1996). Roasted sesame seeds are most common in

East Asia, including Japan. Roasting sesame seeds

produces a unique aroma (Soliman et al. 1975; Takei

1988, 1989; Asai et al. 1994), and sesamol, a strong

antioxidant, is formed (Fukuda et al., 1986a). Asai et

al. (1994) have reported that roasted sesame seeds

with a nut-like aroma, which was produced before the

characteristic roasted sesame seed smell, were

particularly suitable for various kinds of cooking.

Takeda and Fukuda (1997) have investigated the

effect of roasting conditions on the taste of sesame

seeds, and demonstrated that when seeds were

roasted at 170•Ž, a relatively low temperature, they

were sweeter and tastier due to less loss of free amino

acids and free saccharides than when roasted at a

higher temperature. In contrast, at over 200•Ž, these

components were decomposed and the samples

became less tasty, although they had the characteris-

tic smell of sesame seeds. Thus, the properties of

roasted sesame seeds depended on the roasting conditions.

In Japan, roasted whole sesame seeds are sprinkled over food or crushed in an earthenware mortar with a wooden pestle (Takeda 1997). However, their cooking

properties have not been fully studied. In this study, we examined the effects of roasting

conditions on the color, shape, texture, and other characteristics of sesame seeds by instrumental and

sensory measurements. The behavior of an antioxid-ant contained in sesame seeds was also examined under the various roasting conditions.

MATERIALS AND METHODS

Preparation of the roasted sesame seeds and samples

White sesame seeds (Sesamum indicum L.) that had been cultivated in China in 1996 (supplied by Kadoya Oil Manufacturing) were used. Twenty grams of seeds

were roasted in a stainless-steel strainer inside a fixed-temperature dryer (Sanyo Oven, Mitamura Riken Manufacturing). The 14 roasting conditions

employed in this study were based on a preliminary sensory test to determine the edible range of the

(1115) 11

J. Home Econ. Jpn. Vol. 51 No. 12 (2000)

sesame seeds: i.e., roasting at 170•Ž for 5, 10, 15, 20,

30 and 40 min; at 200•Ž for 5, 10, 15, 20 and 30 min;

and at 230•Ž for 5, 10 and 15 min.

The roasted sesame seeds were crushed in a mill

(Shibata) for 1 min. The water content, crude lipids

and lignans were determined, and the content of total

and free saccharides was determined for seeds

de-fatted with hexane.

EXPERIMENTAL METHODS

Evaluation of the color

The color of the roasted or unroasted sesame seeds

was measured by a 1001DP color difference meter

(Japan Electronic Color Industry) with a round-type

quartz cell. The data are expressed by the L, a and b

values of the Hunter color system, and the color

difference between each roasted sample and the

unroasted sample was calculated.

Examination of the surface and cross section of

roasted sesame seeds by scanning electron mi-

croscopy

Both ends of a roasted or unroasted sesame seed

were cut off, and the sample was fixed with

glutaraldehyde and osmium tetroxide. The fixatives

were replaced by isoamyl acetate, and the specimen

was dried with a critical-point dryer. The surface of

the dried specimen was then observed by a

JSM-5800LV scanning electron microscope (JEOL)

with an accelerating voltage of 3.0 kV . The cross

section was then coated with platinum by JFC-1500

vapor deposition apparatus (JEOL) and examined

with an accelerating voltage of 20 kV.

Measurement of the thickness and breaking tests

Thickness measurements and breaking tests were

carried out with a KA3000PV Rheorobot (Kyowa

Precision Machinery Industry). A sesame seed was

placed on the platform, and the compression bar was

set up to touch the seed in order to measure its

thickness. The seed was then compressed, and the

load and displacement were evaluated. The degree of

displacement per unit thickness of the seed was

calculated and assigned as the strain value. The load

and the strain at the time of breaking were then

determined. The slope of the initially linear part of the

curve in the strain range of 0-0.1 is regarded as the

initial elastic modulus. Ten seeds for each roasting

condition were studied.

Quantification of the total saccharides and free

saccharides

The content of total saccharides was assayed by the

phenol-sulfate method (Dubois et al. 1956), and the

amount of each free saccharide by HPLC (Shimadzu

LC-6AD). The free saccharides were extracted by the

flow-back process at 80•Ž for 30 min with 80%

ethanol. A differential refractometer (Shimadzu RID-

10A) was used for detection. Glucose, fructose,

sucrose and stachyose used as standards were of

analytical grade, and planteose was supplied by K .

Kato (Gifu University).

Quantification of water and crude lipids

The water content was determined by drying at

atmospheric pressure with heating at 130•Ž for 2 h.

The content of crude lipids was measured by

Soxhlet's extraction method with hexane as a solvent.

Quantification of the lignans

Sesamine, sesamolin and sesamol were determined

by HPLC (Develosil SI-50 column, 4 mm 96 X 250 mm,

Nomura Chemicals). Each was extracted by ethyl

acetate from ground sesame seeds and passed

through 0.2-mm filter paper. Sesamine and sesamolin,

which had been confirmed to be pure by 1H-NMR

(Fukuda et al. 1986b), and sesamol (analytical grade,

Sigma) were used as standards. The eluting solvent

was hexane : ethyl acetate = 7 : 3 (1.0 ml/min) for

sesamine and sesamolin, and methanol : water= 3:7

(0.8-0.9 ml/min) for sesamol. A UV-VIS detector was

used at 290 nm. Quantification was made by an

integrator (Hitachi C-R6A, Shimadzu D-2500) and

calculations based on a two-point calibration curve

were performed.

Sensory test

Sesame seeds roasted under each of the test

conditions were evaluated by a pair-rated 5-point

scale test (Nikkagiren Kannokensa Iinkai 1990). Each

panelist seated in a laboratory with standard fluores-

cent lighting was served two samples of sesame seeds

each placed on a white dish. The panelists examined

the color and the number of cracks on the sesame

seeds before eating, and the brittleness, degree of

bursting and texture when bitten. Each panelist

chewed five seeds well, swallowed them, and then

rinsed out the mouth. The panelists scored each item

for the right-hand sample relative a score of 0 for the

left-hand sample. One of the pair of samples was

always unroasted sesame seeds as the standard . The

control sample and the other sample for each

comparison were served on the same right and left

positions to half of the panelists, and the same

samples were served in the opposite positions to the

other half of the panelists. The sensory evaluation was

conducted from 2 to 4 p.m. The panel members were

ten female food science students and staff members

12 (1116)

Effect of Roasting Conditions on the Food Quality of Sesame Seeds

of St. Catherine Women's Junior College ranging in

age from 20 to 42.

Statistical analyses

The difference in the correlation coefficient (r) of

two regression lines was tested according to the

method of Ichihara (1990). The regression analysis

and t-test were performed with Excel ver. 6.0 for

Macintosh (Microsoft), and the multiple-regression

analysis (Dempster et al. 1977) was carried out by

HALBAU (Yanai and Takagi 1988). The relationship

between the measured value in the breaking test and

the textural value in the sensory test was examined.

RESULTS AND DISCUSSION

Sesame seeds are usually roasted until the skin is

burst by the internal vapor pressure and the seeds

start jumping. In this study, the shortest roasting time

employed was that at 200•Ž which resulted in

recognizable cracks in the skin, and the longest

roasting time was decided by evaluating the color of

the scorched parts and the bitterness at each roasting

temperature in a preliminary sensory test.

Changes in the color and shape of sesame seeds by

roasting

The change in browning by roasting is shown in

Table 1. At 170•Ž, a small change was observed up to

20 min. After roasting for over 30 min, the color

difference had become greater due to the decrease in

L value and the increase in a value. At 200•Ž and 230

℃, the color difference had become greater after

roasting for 10 and 5 min, respectively. When the

samples were roasted for longer, the b value also decreased, and the color difference markedly in-creased.

Scanning electron micrographs of the surface of

sesame seeds roasted for 15 min are shown in Fig. 1. At 170t, only small cracks were apparent for any roasting time, but large cracks were seen at 200t

and 230t. The degree of cracking became greater at higher temperature and with longer roasting time. This change allowed the browned residual endosperm

tissue to be exposed through the cracks, which is not considered to be a preferable appearance.

The sesame seeds expanded and became thicker by roasting. Table 2 shows the thickness of the most

expanded part and the contents of water and lipids. While the thickness of unroasted seeds was increased

approximately 1.3 and 1.5 times by roasting at 230t for 5 and 15 min, respectively, it was increased 1.3 times by roasting at 200t and 1.2 times at 170t. The

Table 1. Changes in the surface color on sesame seeds by roastings

The L value represents lightness/darkness: white = 100, black = 0. The a value

represents red/green: red= (+), green= (-). The b value represents

yellow/blue: yellow= (+), blue= (-). ƒ¢E represents the color difference,

√(L1-L2)2+(α1-α2)2+(b1-b2)2.

(1117) 13


A B C

water content was decreased to 1.35% by roasting at

230•Ž for 5 min, this decrease being more rapid than

that at 170•Ž or 200•Ž, and to less than 1% after

roasting at 170•Ž or 200•Ž for 30 min. The lipid

content was around 56% in all samples and was not

changed by roasting. The vapor pressure is related to

the thickness of the roasted sesame seed.

Scanning electron micrographs of the cross section

of sesame seeds roasted at 230°C revealed distinct

spaces between the residual endosperm tissue and

the cotyledon. Voids in the center of the cotyledon

were also clearly observed (Fig. 2). The surface of the

sesame seed could be easily cracked under these

conditions. Itoh et al. (1989) have studied the effect

on the inner tissue of roasting sesame seeds, and

reported that the cell wall became thinner due to water evaporation and that the distance between oil droplets increased. These changes at the tissue level would affect the texture of sesame seeds and the

grinding process, in addition to the changes in the shape and surface of sesame seeds.

We concluded from these data that roasting at a

relatively low temperature of 170t is appropriate when a good external appearance is desirable, such as when whole or lightly crushed sesame seeds are

sprinkled on or mixed with food.

Fig. 1. Scanning electron micrographs of the surface of roasted sesame seeds

A: roasted at 170•Ž for 15 min, B: roasted at 200•Ž for 15 min, C: roasted at 230•Ž for 15 min.

Table 2. Changes in the thickness, water content, and crude lipids of roasted

sesame seeds

* The figures in parentheses indicate relative thickness value.

14 (1118)


Changes in the breaking properties of sesame seeds by roasting

A breaking test by pressure was performed,

because it was assumed that the change in shape or tissue structure just described, would greatly affect

the texture. At the beginning of compression, the change in shape resulted in almost a straight line, before the slope suddenly increased and the seed

burst (Table 3). The initial elastic modulus is

supposed to express the degree of inelasticity

(Mohsenin 1988; Watanabe 1998), and it tended to

decrease slightly up to 20 min of roasting at 170•Ž.

On the other hand, the values for the breaking force

and breaking deformation tended to increase slightly.

Under these or less severe roasting conditions, few

changes were observed in the color difference (Table

1) and thickness (Table 2), and only small cracks

developed on the surface. These results indicate that,

Fig. 2. Scanning electron micrographs of the sections of sesame seeds

A: unroasted, B: roasted at 230•Ž for 5 min; C, cotyledon; R, residual endosperm tissue .

Table 3. Breaking properties of roasted sesame seeds

A Rheorobot was used for the measurements with a 2-kg load cell , a

plunger with a 11.2-mm diameter and a compression speed of 678

μm/min.

(1119) 15


under these roasting conditions, no marked change in

the tissue was caused by the slight hardening due to

water loss. When the seeds were roasted at 170t for

over 30 min, however, the breaking force and

breaking deformation were both decreased. Under

these roasting conditions, the seeds turned brown

(Table 1), the water content decreased further (Table

2), and the tissue presumably became fragile by

roasting. At 200•Ž, the initial elastic modulus,

breaking force and breaking deformation tended to

decrease with increasing roasting time, and were

decreased markedly by roasting for 20 min or longer.

On the contrary, the initial elastic modulus at 230•Ž

was markedly decreased by roasting for 5 min, while

the breaking force showed a slight decrease. How-

ever, the breaking deformation increased greatly to

0.40 from 0.28, the value for unroasted sesame seeds.

With further roasting, the initial elastic modulus and

breaking force both markedly decreased. The break-

ing force remained almost constant for up to 10 min

and then markedly decreased. The amount of

remaining water (Table 2) might be attributed to the

increased breaking deformation observed in the

sesame seeds roasted at 230•Ž for 5 or 10 min.

Moreover, cracks on the surface and spaces in the

internal tissue weakened the tissue and would have

caused a rapid decrease in the initial elastic modulus

and breaking force.

The difference in physical properties of sesame

seeds with roasting greatly affected the texture of the

whole or crushed sesame seed, and the ease of

grinding.

Changes in the components of sesame seeds by

roasting

Many previous studies (Soliman et al. 1975; Fukuda

et al. 1986a; Takei 1988, 1989; Asai et al. 1994;

Yoshida 1994; Yoshida et al. 1995) have suggested

that the components of the internal tissue of sesame

seeds was also changed by roasting. Changes in the

color of roasted sesame seeds have been described

(Table 1), these changes being caused by such

chemical reactions as the caramelization of saccha-

rides and the aminocarbonyl reaction. The content of

total saccharides did not decrease much after

roasting at 170•Ž, but gradually decreased at 200t,

and decreased rapidly by roasting at 230•Ž for 10 min

or longer (Table 4).

The typical saccharides of sesame seeds were the

five types shown Fig. 3. Planteose, a triose, was the

most abundant, accounting for approximately 61% of

the free saccharides in unroasted sesame seeds

(Table 4). This was followed by sucrose (a di-

saccharide), and then by stachyose (a tetrasaccha-

ride). Gas chromatography was used by Dharmaraj

Table 4. Contents of total saccharides and free saccharides in roasted sesame seeds

* Dry weight. * * Not detected.

16 (1120)


et al. (1976) to find glucose, fructose, raffinose , stachyose, planteose and sucrose in this order of abundance. HPLC was used by Yamauchi et al. (1982)

to find 19.3 mg/g of planteose, 9.9 mg/g of sucrose and 0.5 mg/g of glucose in white sesame seeds. These

data are in agreement with the values we obtained. However, Yamauchi et al. (1982) did not detect stachyose. The types and amounts of saccharides

contained in sesame seeds may differ slightly with the variety. All of these saccharides were rapidly de-composed at a high roasting temperature. No fructose

could be detected in seeds roasted for 5 min at 200t or 230t, or for 30 min at 170t. Glucose could not be detected after roasting for 15 min at 200

℃, for 5 min at 230℃ or for 30 min at 170℃. Sucrose

was relatively stable in seeds roasted at 170•Ž, 87%

remaining after roasting for 40 min. At 200•Ž, sucrose

was gradually decomposed by roasting for 5 min, with

20% remaining after roasting for 30 min. Sucrose

was rapidly decomposed at 230•Ž and decreased to

around 10% after roasting for 15 min. Planteose

showed the same pattern as that of sucrose, but was

absent after roasting for 20 min at 170•Ž and 74%

remained after roasting for 40 min. Stachyose was

stable at any roasting temperature, and tended to

increase slightly after roasting for 30 min at 170•Ž or

for 30 min at 200•Ž. It is not clear whether or not this

phenomenon was derived from the decomposition of

polysaccharides.

A previous study has revealed that the content of

free amino acids decreased with higher roasting

temperature and longer roasting time (Takeda and

Fukuda 1997). These results suggest that the Maillard

reaction between saccharides and amino acids and

caramelization of the saccharides occurred inside the

skin such as in the residual endosperm tissue or in

cells of the cotyledon. There was a high correlation (r

=0 .98) between the total amount of free saccharides

and the L value for the surface of the samples (Table

1) to support this proposition.

The results of the analysis of lignans are shown Fig.

4. Sesamine, which has such physiological activity as

in vivo antioxidation, was hardly decomposed at any

roasting temperature. Similarly, as in the case of

deep-frying with sesame oil (Fukuda 1987), there was

little loss of sesamine during roasting. Sesamolin is

decomposed by heat into sesamol, a highly antioxida-

tive substance (Fukuda et al. 1986a; Takei and

Fukuda 1991). Sesamol was also detected at all

roasting temperatures examined in this study, with 12

Fig. 3. Chromatogram of the free saccharides

HPLC conditions: column, Shim-pack CLC-NH2; flow rate,

1.0 ml/min; mobile phase, acetonitrile : water =7 : 3-8 : 2;

temperature, 25•Ž; detection, differential refractometer .

Fig. 4. Sesamin, sesamolin and sesamol contents of roasted sesame seeds

Roasting temperature: •¡ 170•Ž; •œ, 200•Ž; •£, 230•Ž.

(1121) 17


mg yielded after roasting for 10 min. At 200•Ž, a

certain amount of sesamol was detected within 5 min

of roasting, the amount of detected sesamol increas-

ing markedly after roasting for 15 min or longer. No

sesamol could be detected after roasting at 170•Ž

for up to 20 min, although a small amount was

detected after roasting at this temperature for 30 min.

Evaluation of roasted sesame seeds by a sensory

test

The results just described showed that the degree

of change in the appearance and physical properties

of sesame seeds depended on the roasting conditions.

Sensory tests were therefore performed to examine

the relationship between these changes and the taste

of sesame seeds. The results are shown in Table 5.

The color evaluated as most desirable was developed

after roasting for 10 to 20 min at 170•Ž. It was a

slightly browned with the color difference between a

roasted and unroasted sample being less than 1.9

(Table 1). No significant difference was apparent in

the number of cracks in any seed after roasting at

170•Ž, and all seeds roasted at 200•Ž for over 15 min

or at 230•Ž had significantly more cracks. This

means that the small cracks shown in Fig. 1 could not

be recognized, only the large ones being recognized

as cracks in the sensory tests. The appearance after

roasting at a high temperature such as 200•Ž or 230

℃ was confirmed to be undesirable.

Seeds roasted at 230•Ž for all periods burst

favorably when bitten, as did seeds roasted at 200•Ž

for over 15 min and those roasted at 170•Ž for 40

min. Seeds roasted at 170•Ž had no significantly

improved in texture, while all seeds roasted at higher

temperatures such as 200•Ž and 230•Ž were evaluated

as being significantly more desirable.

GENERAL DISCUSSION

Seeds roasted at 170•Ž generally had a desirable

color and few cracks, but they did not burst favorably,

a desirable characteristic of roasted sesame seeds

when bitten, and their texture was not improved by

roasting. On the other hand, seeds roasted at 200•Ž

for over 10 min or at 230•Ž had more cracks and an

undesirable color. However, they burst favorably and

were breakable with the preferred texture. These

results show that it is necessary to select the roasting

conditions to suit the type of cookery with sesame

seeds.

A simple regression analysis was performed on the

texture evaluated by the sensory test and the

measured value from the breaking test (Table 3).

There was relatively high correlation between burst-

Table 5. Sensory evaluation of roasted sesame seeds

Color and texture: -2, not desirable; 2, desirable. Number of cracks: -2, many; 2, few. Bursting: -2, weak; 2, strong.

Brittleness: -2, not brittle; 2, brittle. Each value represents the average •} SD by a pair-rated 5-point scale test, using

unroasted sesame seeds as the standard (zero), n=10. Means are significantly different from that of unroasted

sesame seeds at * p< 0.05 and * *p< 0.01.

18 (1122)


ing and brittleness and the initial elastic modulus. A

lower, but still high correlation existed between these

parameters and the breaking force (Tables 6 and 7).

Further multiple regression analyses were carried out,

because the correlation coefficient (r) was small

between each of these texture-related parameters and

the breaking deformation which is related to break-

ability characteristics of roasted sesame seeds. The

values obtained in the sensory test were regarded as

dependent variables (Y), and the measured values in

the breaking test were regarded as explanatory

variables (X) . Bursting could be expressed by using

values for the two characteristic properties of

breaking force and breaking deformation with a

multiple correlation coefficient (R) of 0.80. The R

value between the brittleness and the combination of

the initial elastic modulus and breaking force was

0.88. This multiple correlation coeffient was larger

than the simple correlation coefficient. The character-

istics for the breakability of roasted sesame seeds

could be expressed more precisely in this study by

using two physical properties.

Takeda and Fukuda (1997) have reported that the

characteristic roasted sesame seed aroma was pro-

duced at over 200•Ž, a temperature at which

pyradines and pyrroles were produced. Free sac-

charides and free amino acids were relatively well

retained in sesame seeds roasted at 170•Ž for 10 or

15 min, and were evaluated as the best for

sweetness in the sensory test. Sesamol, browning

substances, 7-tocopherol and sesamine mutually

potentiated and created a strong antioxidative effect,

whereby roasted sesame seeds showed a stronger

antioxidative effect with browning (Fukuda et al.

1986a, 1996; Koizumi et al. 1996). Large quantities

of sesamol and browning substances were produced

by roasting at high temperature such as 200t and

230t.

We conclude from these results that sesame seeds

roasted at 170•Ž were very tasty and had a good

appearance, whereas those roasted at 200•Ž or 230•Ž

had the characteristic aroma and desirable texture,

being easily broken and highly antioxidative. It is

therefore necessary to select the roasting conditions

for sesame seeds according to which factor, appear-

ance, texture, taste or antioxidation, is most desired.

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Table 6. Simple- or multiple-regression analyses of values for bursting in

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**

p<0.01.

Table 7. Simple- or multiple-regression analyses of values for brittleness

in the sensory test and of measured values in the breaking test

**

p<0.01,***

p<0.001.

(1123) 19


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20 (1124)


ゴマ種子の性状に焙煎条件が及ぼす影響

武田珠美,　青野寛子*,　福田靖子**,　畑江敬子*,　島田淳子織

(聖カタリナ女子短期大学,*お茶の水女子大学生活科学部,*　*

静岡大学教育学部,*　*　* 昭和女子大学大学院)

原稿受付平成11年9月24日;原稿受理平成12年8月24日

ゴマ種子を170℃,200℃ および230℃ で5～40分間焙煎し,外観微細構造,テクスチャー,

糖およびリグナン類への影響を検討した.

170℃ で20分まで焙煎したゴマは表皮の着色や亀裂が少なく,外観上優れていたが,歯ざわ

りの評価が低かった.230℃ で5～15分焙煎したゴマはいずれもよくふくらみ,残存胚乳組織

と子葉問に空間が観察され,官能検査ではもろく,プチッと破断する感触が強く,好ましい歯

ざわりと評価された.しかし,焙煎5分ですでに表皮の着色が濃く,亀裂が目立った。セサモ

ールは著しく生成した.200℃ で焙煎したゴマは,230℃ 焙煎のゴマに類似していた.

遊離糖には,グルコース,フラクトース,スクロース,プランテオースおよびスタキオース

が含まれ,スタキオースは比較的安定であったが,他の糖は高温の焙煎ほど減少が速くみられ,

これらの総量と表面色(L値)には高い相関がみられた.

キ-ワード:ゴマ種子,焙煎,テクスチャー,糖リグナン.

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effect of roasting conditions on the food quality of

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