Glycative Stress Research
Online edition : ISSN 2188-3610
Print edition : ISSN 2188-3602
Received : March 8, 2022
Accepted : April 17, 2022
Published online : June 30, 2022
doi:10.24659/gsr.9.2_55

Original article

Effects of varieties and cooking on the glycative stressinhibitory effect of eggplant
Masayuki Yagi 1), Kanako Baba 1), Chieko Sakiyama 1), Mari Ogura 1, 2),
Wakako Takabe 3), Yoshikazu Yonei 1)
1) Anti-Aging Medical Research Center and Glycative Stress Research Center,
Graduate School of Life and Medical Sciences, Doshisha University, Kyoto, Japan
2) Department of Food and Nutrition, Kyoto Bunkyo Junior College, Kyoto, Japan
3) Department of Materials and Life Science, Faculty of Science and Technology,
Shizuoka Institute of Science and Technology, Shizuoka, Japan

Abstract

Accumulation of advanced glycation end products (AGEs) in the body due to glycative stress is a factor in the progression
and development of aging and lifestyle-related diseases. Oxidation is one of the accelerators of glycative stress because
it promotes the formation of AGEs. On the contrary, glycative stress suppression includes suppression of postprandial
hyperglycemia, suppression of AGE generation, and degradation and excretion of AGEs. Various vegetables and herbs havealready been reported to have antiglycative effects. In this study, for the purpose of verifying the usefulness of glycative
stress suppression materials, we focused on eggplant as a vegetable that can be consumed as a familiar food, and verified
its antiglycative effect and antioxidative effect. Seven commercial eggplant varieties were used as samples. Samples were
tested for changes in effects among eggplant varieties and after, four types of cooking (baked, fried, boiled, and nukazuke[rice bran pickles]). In addition, the amounts of chlorogenic acid and anthocyanin, the major components of eggplant, were
measured and their relationship to the antiglycative and antioxidative effects were verified. All eggplant varieties tested
showed antiglycative and antioxidative effects. The difference between the varieties was 3.5-fold for antiglycation and 4.9fold for antioxidative activity. The changes in the effects of eggplant after cooking were small for baked and fried eggplant,
less than 5 % change in antiglycation and less than 15 % change in antioxidative activity. In contrast, after eggplant wasboiled and nukazuke, the antiglycative and antioxidative effects decreased by more than 35 % and 60 %, respectively. It wasestimated that the difference in the antiglycative and antioxidative effects of eggplant was largely influenced by the amountof chlorogenic acid. It was considered that the choice of cultivar and cooking method is important for the use of eggplant
while focusing on glycative stress.

KEY WORDS: antiglycative effect, antioxidative effect, chlorogenic acid, anthocyanin, eggplant (Solanum melongena)

Introduction

The production and accumulation of advanced
glycation end products (AGEs) caused by glycative stressis a risk factor that promotes aging, diabetic complications,
osteoporosis, dementia, and other diseases 1). The formationof AGEs is also accelerated by oxidation and UV exposure 2).
Therefore, oxidation is one of the accelerators of glycative
stress. Suppression of glycative stress includes suppression of
postprandial hyperglycemia, suppression of AGE generation,and degradation, and excretion of AGEs 3). We have reported
that tea 4), vegetables, herbs 5), fruits 6), and fermented foods 7)
are natural products that inhibit AGE formation. 187
varieties of vegetable and herb extracts were tested for theirinhibitory effects on AGE formation, and the inhibitory
effects on fluorescent AGE (F-AGE) formation were found tobe significantly reduced by eggplant (Solanum melongena),
tomato (Solanum lycopersicum), chile pepper (Capsicum
annuum), paprika (Capsicum annuum var. 'grossum' ) andother vegetables of the Solanaceae family (Solanaceae) 5).

Contact Address: Professor Masayuki Yagi, PhD
Anti-Aging Medical Research Center / Glycative Stress Research Center,
Graduate School of Life and Medical Sciences, Doshisha University
1- 3, Tatara Miyakodani, Kyotanabe-shi, Kyoto, 610- 0394, Japan
TEL &FAX: + 81-774-65- 6394 e-mail: myagi@mail.doshisha.ac.jp
Co-authors; Baba K, bmq2003@mail4.doshisha.ac.jp;
Sakiyama C, csakiyam@mail.doshisha.ac.jp; Ogura M, m-ogura@po.kbu.ac.jp;
Takabe W, takabe.wakako@sist.ac.jp; Yonei Y, yyonei@mail.doshisha.ac.jp_ 55 _

Glycative Stress Research 2022; 9 (2) : 55- 62(c) Society for Glycative Stress ResearchGlycative stress inhibitory effect of eggplant

Eggplant is a crop which has been consumed in countries
and regions around the world since ancient times. It is believed
to have been introduced to Japan via China and cultivated inJapan around the 8th century (Nara Period) 8). There are morethan 170 varieties of eggplant in Japan, with different sizes,
shapes, and colors, and distinctive varieties cultivated to suit
local climates and customs 9). The Ministry of Agriculture,
Forestry and Fisheries has recognized eggplant as one of thedesignated crop vegetables with high consumption in Japan,
and as a familiar vegetable with a close relationship to thediet 10).

Materials and Methods
1) ReagentsThe reagents used were purchased from the following
manufacturers. Human serum albumin (HSA, lyophilized
powder, ≥ 96 %, agarose gel electrophoresis) was from SigmaAldrich Japan (Meguro-ku, Tokyo, Japan). Aminoguanidinehydrochloride (AG), (±)-6-Hydroxy-2, 5,7,8-tetramethylchroman-2-carboxylic acid (trolox), chlorogenic acid, (-)-quinine sulfate
dihydrate (-)-quinine sulfate dihydrate), and trifluoroacetic
acid (TFA) are manufactured by Fujifilm Wako Pure
Chemical Industries, Ltd. (Osaka, Japan). 2-( N-morpholino)ethanesulfonic acid (MES) is from Dojin Chemical Laboratory(Kamimashiki-gun, Kumamoto, Japan). 1, 1- diphenyl-2 picrylhydrazyl (DPPH) is from Cayman CHEMICAL (AnnArbor, Michigan, USA). Delphinidin is from Tokiwa
Phytochemical Laboratory (Sakura, Chiba, Japan). Other
reagents were purchased from Fujifilm Wako Pure Chemical
Industries or Nacalai Tesque (Kyoto, Japan).

2) Samples
The seven varieties of eggplant used as samples were
Naga-nasu (A), Oonaga-nasu (B), Ko-nasu (C), Maru-nasu(D), Mizu-nasu (E), Bei-nasu (F), Shiro-nasu (G) ( Table 1 ).
Eggplants were used after removing the hull and calyx.
Eggplants were purchased at a supermarket in Kyoto
Prefecture.

3) Cooking method
Eggplant samples were cooked according to commonly
practiced household methods.
a) Grilling
Eggplants were cut in half lengthwise and roasted in afrying pan over high heat (>100 °C ) from the peel side. Oncebrowned, then flipped and grilled from the pulp side for a
total of about 5 minutes until cooked all the way through.
b) Deep frying
The eggplant was cut into round slices, a grid was
cut on both sides, the moisture was removed with kitchen
paper, 800 mL of canola oil (Ajinomoto, Chuo-ku, Tokyo)
was poured into an electric deep fryer ( Zojirushi Mahobin,
Osaka, Japan), the eggplant was fried at 170 °C for 3 to 5
minutes, and then removed when they turned golden brown.
c) Boiling
Eggplants were cut into approximately 3 cm cubes andboiled in 800 mL of boiling water for 4 minutes.
d) Nukazuke (rice bran pickles)
After eggplants were cut in half lengthwise, they were
soaked in fermented Nuka-Doko ( Muji, Toshima-ku, Tokyo)
for 24 hours at 4 °C according to the product's directions foruse.

4) Preparation of sample extracts
Raw eggplants and cooked eggplants were sliced, driedat 65 °C for 100 hours using a warm-air dryer Petit Mini
(Daikisangyo, Okayama, Japan), and powdered in a foodprocessor. After deep-frying and cooking, the eggplant
was defatted as follows. For deep-fried eggplant, 12 g ofthe powder after warm air drying was immersed in 30 mL
butanol for 20 minutes, and the supernatant was removedby centrifugation. Then, 18 mL acetone was added andimmersed for another 20 minutes, and the supernatant wasremoved by centrifugation. This defatting process was
repeated twice. Finally, the defatted powder was then airdried to dry powder.
The sample extract was obtained by placing 2 g of thedried powder in 40 mL of 70 % ethanol, extracting at 50 °C
for 4 hours, and collecting the supernatant by centrifugation.
The solid concentration (mg/mL) of the sample extract wascalculated from the weight of 5 mL of the extract after
drying at 120 °C for 2 hours.

5) Measurement of antiglycative effect
The antiglycative effect was verified using the HSAglucose reaction model with reference to a previous report 11),and the inhibitory effect of fluorescent AGEs (F -AGEs) at asolid concentration of 0.3 mg/mL of the sample extract wasverified. F-AGEs were evaluated by measuring AGE-derived
fluorescence (excitation wavelength: 370 nm /fluorescence

Table 1. Characteristics of eggplant samples.
Samplevarietygeneral shapecolor of skincolor of calyx

A
B
C
D
E
F
G

Naga-nasu
Oonaga-nasu
Ko-nasuMaru-nasu
Mizu-nasu
Bei-nasu
Shiro-nasu

long ovoid
cylindrical
ovoid
globularpear shaped
obovatepear shaped

violetvioletvioletvioletvioletvioletwhite

violetvioletvioletvioletvioletgreen
green

Purchased at several supermarkets in Kyoto prefecture.

_ 56 _Glycative Stress Research

wavelength: 440 nm). F-AGEs were calculated as theconcentration in the reaction solution after incubation, relative
to the fluorescence value of 5 µg/mL quinine sulfate in 0.1 N
sulfate solution, which was 1,000. AG was used as a positive
control substance for inhibition of glycation reaction, andthe inhibition rate (%) of F-AGE formation was calculated
according to a previous report 11).

6) Measurement of antioxidative effectwas not correlated. For statistical analysis results, a risk
rate of less than 5 % (p < 0.05) was considered significant.
Statistical analysis was performed using the statisticalanalysis software BellCurve for Excel (Shakai Joho Service,
Shinjuku-ku, Tokyo).

Results

1) Antiglycative and antioxidative effects

For antioxidative activity, DPPH radical scavenging
activity corresponding to the sample solution (solid
concentration 1 mg/mL) was measured using the slope ofthe regression line created by Trolox, referring to a previous
report 7).
DPPH radical scavenging activity was determined byadding 1 mg/mL of sample extract (25-100 µL), 50 µL of
200 mmol/L MES buffer ( pH 6.0), 50 µL of 800 µmol/L
DPPH solution and 50 % ethanol solution (0-75 µL) to thewells of a microplate, so that the mixture volume reached
200 µL. After 20 minutes of reaction at room temperature,
absorbance was measured at 520 nm. The slope of the linear
regression line between the amount of sample extract added
and absorbance was then determined. The DPPH radical
scavenging activity value was calculated as the Trolox
equivalent (µmol-TE/ L) of the sample extract with a solid
concentration of 1 mg/mL by dividing the slope of the linear
regression line of absorbance obtained from the 0 -16 nmol/L
Trolox solution reacted simultaneously with the sample by
the slope of the regression line of the sample extract.

7) Determination of chlorogenic acid
Chlorogenic acid concentration in the extract wasmeasured by UV absorbance method referring to a previous
report 9). For chlorogenic acid concentration, 200 µL of thesample extract was mixed with 50 µL of 1 % TFA, added to
the wells of a microplate, mixed, and the absorbance at 330
nm was measured. The chlorogenic acid concentration (µg/mL) of the extract was calculated from the chlorogenic acidcalibration curve.

8) Anthocyanin measurement
Anthocyanin concentration in the extract was measured
by UV absorbance method with reference to a previous
report 12). Anthocyanin concentration was determined bymixing 200 µL of the sample extract with 50 µL of 1% TFA,
adding the mixture to the wells of a microplate, and measuring
the absorbance at 540 nm. Anthocyanin concentration was
calculated as delphinidin equivalent (µg-DE/mL) from thecalibration curve of delphinidin measured concurrently.

Statistical Analysis
Measurements were expressed as the mean ± standard
deviation (SD) of triplicate measurements. Tukey's test orDunnett's test was used to compare measurements. Pearson's
product-rate correlation coefficient was used to analyze
correlation. A high correlation was considered as 0.7 < |r|≤ 1. 0, from 0.4 < |r| ≤ 0.7 was correlated, and 0.2 < |r| ≤ 0.4

The solid concentration of the extract was 25. 2 ±2. 2 mg/mL in seven eggplant varieties (A-G). At a solid
concentration of 0.3 mg/mL, C showed the greatest inhibitionof F-AGE formation (88.7 ± 0.9%), while B showed the least(25.4 ± 1.2 %, Fig. 1 ). Similarly, DPPH radical scavenging
activity showed a maximum value (165 µmol-TE/L) for C,
while G and D exhibited the same minimum value (33.7 µmolTE/L).
The inhibition of F-AGE formation in A showed amaximum value before cooking (74.6 ± 2.6 %) to fried (75.5± 1.4%) and decreased to nukazuke (36.4 ± 3.4%, Fig. 2).
Similarly, DPPH radical scavenging activity went from 117.8µmol-TE/L before cooking (117.8 µmol-TE/L) to a maximum
value at baking (134.7 µmol-TE/L) and a minimum value atboiling (47.1 µmol-TE/L).
The percentage inhibition of F-AGE formation and
DPPH radical scavenging activity of the extract showed apositive high correlation ( y = 1.8413x − 13. 598, r = 0.756,
p < 0.05, Fig. 3).

2) Chlorogenic acid content and glycative stress
suppression
The highest value of chlorogenic acid in 1 mg/mL of
extract of seven eggplant varieties was 86. 5 ± 1.8 µg (C) andthe lowest value was 15. 5 ± 0. 2 µg (B), a 5.7- fold difference.
Chlorogenic acid in the extract of A showed the highest value
in grilled (51. 2 ± 1. 4 µg) and the lowest value in nukazuke(11.1 ± 0.1 µg) compared to before cooking (46.5 ± 0.4 µg).
There was a 4.6 - fold difference in chlorogenic acid due tocooking. Chlorogenic acid concentrations in all sample
extracts were highly positively correlated with bothantiglycative ( y = 0.2846x − 5.5247, r = 0.900, n = 11, p <0.05) and antioxidative ( y = 0.3393x + 4.8972, r = 0.784,
n = 11, p < 0.05) effects ( Fig. 4 ).

3) Anthocyanin content and glycative stress
suppression
The highest value of anthocyanins in 1 mg/mL of
extracts of seven eggplant cultivars was 0.722 ± 0.015 µgDE (B) and the lowest value was 0.106 ± 0.006 µg-DE (G),a 6.8-fold difference. Anthocyanins in the extract of A
showed the highest value (1.374 ± 0.029 µg-DE) in fried
and the lowest value (0.070 ± 0.002 µg-DE) in nukazuke,
compared to before cooking (0.673 ± 0. 017 µg-DE). There
was a 19.6 -fold difference in anthocyanin content bycooking. Anthocyanin concentration in all sample extracts
was positively and weakly correlated ( y = 0.002x + 0.0236,
r = 0.353, n = 11, p < 0.05) with antiglycation and positively
correlated ( y = 0.0033x + 0.1543, r = 0.429, n = 11, p < 0.05)
with antioxidative activity ( Fig. 5 ).

_ 57 _

100

F-AGEs

DPPH

200

80

160

60

120

40

80

20

40

0

C

A

F

G

E

D

B

DPPH radical scavenging activity(µmol-Trolox equivalent/L)

Inhibitory ratio (%) on F-AGE formation

Glycative stress inhibitory effect of eggplant0

Fig. 1. Antiglycative and antioxidative effects of eggplant varieties.

100

F-AGEs

DPPH

200

80

160

60

120

40

80

20

40

0

Raw

GrilledFried

BoiledNukazuke

Fig. 2. Post-cooking antiglycative and antioxidative effects of eggplant.

DPPH radical scavenging activity(µmol-Trolox equivalent/L)

Inhibitory ratio (%) on F-AGE formation

Data are expressed as mean ± SD, n = 3 (F-AGEs) or mean, n = 2 (DPPH). The concentration of extract,
0. 3 mg/mL (inhibitory ratio of F-AGE formation), 1 mg/mL (DPPH radical scavenging activity); A – F,
Eggplant varieties are shown Table 1. F-AGEs, fluorescent advanced glycation end products; DPPH,
1, 1-diphenyl- 2 -picrylhydrazyl; SD, standard deviation.

0

Data are expressed as mean ± SD, n = 3 (F-AGEs) or mean, n = 2 (DPPH). The concentration of extract,
0. 3 mg/mL (inhibitory ratio of F-AGE formation), 1 mg/mL (DPPH radical scavenging activity); Eggplantvarieties; A. F-AGEs, fluorescent advanced glycation end products; DPPH, 1,1- diphenyl-2-picrylhydrazyl;
SD, standard deviation.

_ 58 _Glycative Stress Research

200

DPPH radical scavenging activity(µmol-Trolox equivalent/L)

160

C

F
GrilledA (Raw)

120

Fried
B

80

E

BoiledNukazuke
40

0

G

D

0

20

40

60

80

100

Inhibitory ratio (%) on F-AGE formation

Fig. 3. Relationship between antiglycative and antioxidative effects.

Ⅰ30

Chlorogenic acid concentoration (µg/mL)

Chlorogenic acid concentoration (µg/mL)

y = 1.8413x − 13. 598, n = 11, r = 0.756, p < 0.05. The concentration of extract, 0.3 mg/mL (inhibitory
ratio of F-AGE formation), 1 mg/mL (DPPH radical scavenging activity); A – F, Eggplant varieties areshown Table 1; Cooked eggplant varieties was A. F-AGE, fluorescent advanced glycation end product;
DPPH, 1, 1- diphenyl -2 - picrylhydrazyl.

C

25
20
Grilled

15
Boiled

10
B

5
0

D

F

A (Raw)Fried

G

E
Nukazuke

0

20

40

60

80

100

Inhibitory ratio (%) on F-AGE formation

ⅡⅠ

100

C

80

60

GrilledA (Raw)
F

40
G

Boiled

D

20

E

Fried

B
Nukazuke

0

0

50

100

150

200

DPPH radical scavenging activity(µmol-Trolox equivalent/L)

Fig. 4. Relationship between chlorogenic acid concentration and antiglycative or antioxidative effect of eggplant extract.

Ⅰ, Antiglycative effect, y = 0.2846x − 5.5247, n = 11, r = 0.900, p < 0.05;
Ⅱ, Antioxidative effect, y = 0.3393x + 4.8972, n =11, r = 0.784. p < 0.05;
The concentration of extract, 0. 3 mg/mL (inhibitory ratio of F-AGE formation), 1 mg /mL (DPPH radical scavenging activity); A – F,
Eggplant varieties are shown Table 1; Cooked eggplant varieties was A. F-AGE, fluorescent advanced glycation end product; DPPH,
1, 1- diphenyl - 2 - picrylhydrazyl.

_ 59 _Glycative stress inhibitory effect of eggplant

Ⅰ
Fried

0.4

0.3
B

A (Raw)

0.2
Boiled0.1
0.0

DNukazuke
0Ⅱ

1.6

Anthocyanin concentoration(µg-delphinidin equivalent/mL)

Anthocyanin concentoration(µg-delphinidin equivalent/mL)

0.5

20

40

Grilled
F

E

C

G
60

1.4
1.2
1.0

100

B

0.8
0.6

A (Raw)

BoiledGrilled

0.4
0.2
0.0

80

Fried

E

D
G
0

FC

Nukazuke
50

100

150

200

DPPH radical scavenging activity(µmol-Trolox equivalent/L)

Inhibitory ratio (%) on F-AGE formation

Fig. 5. Relationship between anthocyanin concentration and antiglycative or antioxidative effect of eggplant extract.

Ⅰ, Antiglycative effect, y = 0.002x + 0.0236, n = 11, r = 0.353, p < 0.05;
Ⅱ, Antioxidative effect, y = 0.0033x + 0.1543, n = 11, r = 0.429, p < 0.05;
The concentration of extract, 0.3 mg/mL (inhibitory ratio of F-AGE formation), 1 mg/mL (DPPH radical scavenging activity); A – F,
Eggplant varieties are shown Table 1; Cooked eggplant varieties was A. F-AGE, fluorescent advanced glycation end product; DPPH,
1, 1- diphenyl-2 -picrylhydrazyl.

Discussion1) Inhibition of glycative stress in eggplant
Chlorogenic acid and anthocyanins are characteristic
components in eggplants 13). Chlorogenic acid is a polyphenol
found in many plants and is an ester compound of caffeic
acid and quinic acid. Chlorogenic acid has been reported tohave antioxidative and antiglycative effects 14). The purple
component of eggplant peel is anthocyanin, which is also
known as nasunin (delphinidin 3-( p- coumaroylrutinoside)5-glucoside), delphinidin 3 -rutinoside, delphinidin
3- (p-coumaroylrutinoside)- 5 - glucoside) , and delphinidin
3 - rutinoside. rutinoside- 5-glucoside, and their content
and amounts vary by variety 15). These anthocyanins haveantioxidant properties 16, 17).
The 70 % ethanol extracts of the seven eggplant varieties
tested showed both antiglycative and antioxidative effects.
However, the intensity of the effects varied among thevarieties, with 5.7 - and 6.8- fold differences in chlorogenic
acid and anthocyanin concentrations in the extracts among
the seven varieties, respectively. The chlorogenic acidconcentration in the extract was highly correlated withthe antiglycative and antioxidative effects. However, theanthocyanin concentration in the extracts correlated betterwith antioxidative activity than with antiglycative activity.
On the other hand, the chlorogenic acid concentrationin each extract was 21 to 242 times higher than that ofanthocyanins, suggesting that the chlorogenic acid content
may be responsible for the differences in the antiglycative
and antioxidative effects.
Vegetables and herbs contain a variety of polyphenolic
components; a study of the relationship between polyphenol

concentrations in 187 vegetable and herbal extracts andtheir antiglycative activity showed that the strength of theantiglycative effect did not correlate with total polyphenol
concentration 18). This result indicates that components
other than polyphenols are also involved in the antiglycative
effect of plant extracts. It has also been reported that the
antiglycative activity of catechins depends on the number
and position of hydroxyl groups in the molecule 19). Although
the types and amounts of components contained in the
eggplants vary from one cultivar to another, the effects ofchlorogenic acid content were considered to be significant in
eggplant's antiglycative and antioxidative effects.

2) Effect of eggplant cooking on glycative stressinhibition

All extracts of eggplant variety A after cooking by four
different methods showed antiglycative and antioxidative
effects. The change in antiglycative activity during cooking
was less than 5 % increase or decrease in baked and fried
eggplant compared to before cooking. However, in the caseof boiled and nukazuke, the antiglycative effect decreased bymore than 35 %. Similarly, antioxidative activity increased
by less than 15 % in the baked and deep-fried samples anddecreased by 60 % in the boiled and nukazuke samples. The
decrease in antiglycative and antioxidative effects in the
boiled and nukazuke were thought to be due to the leachingof eggplant components into the boiling water and the bedof bran.
In nukazuke, the fading of the rind was observed,
suggesting that the decomposition of eggplant components
by lactic acid bacteria and other bacteria in the bed was_ 60 _Glycative Stress Researchalso present. Chlorogenic acid and anthocyanins may beaffected by heat during cooking. However, chlorogenic acidin eggplant increased by 10 % when grilled and decreasedby 5 % when fried compared to raw eggplant. Anthocyaninsdecreased by 26 % when grilled and increased by 104 %when fried compared to raw. Increases in chlorogenic acidand polyphenols due to cooking have been reported for
potatoes 20) and kale 21). The increase in components in theextract due to cooking may be attributed to a change in theextraction rate due to the breakdown of the eggplant tissue
by heating 22).
There are no reports examining changes in theantiglycative effects of vegetables and herbs due to cooking.
The results of antioxidative activity before and after cooking,measured by the β-carotene fading method, showed increased
activity with steaming spinach, microwaving eggplant, andbaking carrots compared to raw 23). Alternatively, polyphenol
content decreased with boiling spinach, eggplant, andleafy green onions compared to raw, and increased withmicrowaving eggplant and baking carrots compared to raw.
When onions were cooked, microwaving resulted in smallchanges in quercetin and ascorbic acid content 24). Thechange in quercetin content was also small in fried onions.
While in boiling, 30 % of the quercetin in the onion leachedinto the boiling water.
In nukazuke of four eggplant varieties, the moisture
content decreased and the amount of fructose increased 25).
The amounts of chlorogenic acid and anthocyanins in
eggplant varied with cooking. However, baked and friedcooking methods of eggplant may have been less likely toattenuate the antiglycative and antioxidative effects thanother cooking methods.

3) Use of eggplant as a countermeasure against
glycative stressthe addition of cooking oil 27).
It is estimated that factors that reduce astringency
include the transfer of astringent components to cooking
oil and the suppression of contact with the tongue surface
by cooking oil. In addition, an increase in guanylic acid,an umami component, has been reported in eggplant whencooked 28). In contrast, in boiling cooking, it was highly
likely that the components leached into the boiling water.
Therefore, miso soup or soup in which the whole boiled
broth is consumed was considered preferable as a way ofeating eggplant to compensate for its reduced antiglycative
and antioxidative effects.
Nukazuke showed more than 50 % less antioxidative
and antiglycative activity, 76 % less chlorogenic acid, and
90 % less anthocyanins than raw. Therefore, it may not besuitable for cooking focused on glycative stress inhibition.
In contrast, nukazuke (pickles in rice bran) is considered
to be a useful cooking method that provides vitamin B1,γ-oryzanol, and ferulic acid contained in rice bran 29) and
improves the balance of intestinal flora by lactic acidbacteria, and thus provides effects other than glycation stress
suppression.

Research limitations
The antiglycative and antioxidative effects of different
eggplant varieties and cooking methods tested in this study
were in vitro results. Further validation is needed for theirusefulness when consumed by humans on a continuous basis.

Conclusion

Eggplant is a crop that is produced in more than 50
million tons worldwide and is a vegetable that is consumed
in many different countries and regions 13). Eggplants
have a water content of approximately 90 % and containvitamins, phenols, and carotenoids 26). Eggplant is considereda phytochemical food due to its antioxidant properties.
The eggplants used as samples in this study were sevenvarieties available in supermarkets. Eggplant was found tohave antiglycative and antioxidative effects. However, theseeffects differed among the varieties.
Eggplant varieties C, A, and F showed high antiglycative
and antioxidative activity with F-AGE formation inhibitionof more than 70 % and DPPH radical scavenging activityof 120 µmol-TE/L. On the other hand, B showed the lowest
inhibition of F-AGE formation (25.4 %), while G and D
showed the lowest DPPH radical scavenging activity (33.7µmol-TE/L). It was considered that the variety was importantin selecting eggplants for glycation stress inhibition.
The changes in antiglycative and antioxidative effectsof cooking eggplant variety A were smaller for baked andfried than for raw. In particular, fried eggplant showed thesmallest change in F-AGE formation inhibition rate (1.1%)
and DPPH radical scavenging activity (2.9 %). Chlorogenic
acid and polyphenols in eggplant are one of the astringent
components. It has been reported that the intensity of
astringency of eggplant and chlorogenic acid is reduced by

Antiglycative and antioxidative effects were observed in
eggplants. These effects differed among eggplant varieties.
The antiglycative and antioxidative effects of eggplantchanged less after baking and frying compared to raw
eggplant. Whereas, these effects decreased after boilingand nukazuke. Changes in the amount of chlorogenic acid
were presumed to be responsible for the antiglycative andantioxidative effects of eggplant. Selection of cultivars andcooking methods is important for the use of eggplant with afocus on glycation stress.

Conflict of Interest Declaration
There are no conflicts of interest in conducting thisresearch.

Acknowledgments
The publication of this study was supported by the
Isyoku-Dogen Research Foundation.

_ 61 _Glycative stress inhibitory effect of eggplant

Reference1) Ichihashi M, Yagi M, Nomoto K, et al. Glycation stressand photo-aging in skin. Anti-Aging Med. 2011; 8: 23-29.
2) Yagi M, Yonei Y. Glycative stress and anti-aging: 1. Whatis glycative stress? Glycative Stress Res. 2016; 3: 152-155.
3) Yagi M, Yonei Y. Glycative stress and anti-aging: 13.
Regulation of Glycative stress. 1. Postprandial blood glucose
regulation. Glycative Stress Res. 2016; 6: 175-180.
4) Hori M, Yagi Y, Nomoto K, et al. Inhibition of advanced
glycation end product formation by herbal teas and its
relation to anti-skin aging. Anti-Aging Med. 2012; 9: 135148.
5) Ishioka Y, Yagi M, Ogura M, et al. Antiglycation effect ofvarious vegetables: Inhibition of advanced glycation end
product formation in glucose and human serum albumin
reaction system. Glycative Stress Res. 2015; 2: 22-34.
6) Parengkuan L, Yagi M, Matsushima M, et al. Anti-glycation
activity of various fruits. Anti-Aging Med. 2013; 10: 70-76.
7) Okuda F, Yagi M, Takabe W, et al. Anti-glycative stresseffect of yogurt whey. Glycative Stress Res. 2019; 6: 230240.
8) Weese TL, Bohs L. Eggplant origins: Out of Africa, into
the Orient. TAXON. 2010; 59: 49-56.
9) Tateyama C, Igarashi K. Anthocyanin and chlorogenic acidcontents of some selected eggplant (Solanam melongena L.)
cultivars, and the radical scavenging activities of their
extracts. Nippon Shokuhin Kagaku Kogaku Kaishi. 2006;
53: 218-224. (in Japanese)10) Yoneyasu A. Japanese vegetables. J Integr Stud Diet Habits.
1996; 7: 7-14. (in Japanese)
11) Hori M, Yagi M, Nomoto K, et al. Experimental models
for advanced glycation end product formation using albumin,collagen, elastin, keratin and proteoglycan. Anti-Aging Med.
2012; 9: 125-134.
12) Ogawa T, Ikegami M, Miyoshi A, et al. Characteristicof anthocyanin pigment from purple-black rice
“ Murasakiinomai. Bull Hyogo Pre Tech Cent Arg Forest
Fish (Agriculture). 2006; 53: 13-16. (in Japanese)13) Gürbüz N, Uluişik S, Frary A, et al. Health benefits andbioactive compounds of eggplant. Food Chem. 2018; 268:
602-610.
14) Kim J, Jeong I, Kim C, et al. Chlorogenic acid inhibits
the formation of advanced glycation end products andassociated protein cross-linking. Arch Pharm Res. 2011;34: 495-500.
15) Ichiyanagi T, Kashiwada Y, Shida Y, et al. Nasunin from
Eggplant Consists of Cis−Trans Isomers of Delphinidin
3- [4-(p-Coumaroyl)-l -rhamnosyl (1 6)glucopyranoside]-5-glucopyranoside. J Agric Food Chem. 2005; 53: 94729477.
16) Azuma K, Ohyama A, Ippoushi K, et al. Structures andantioxidant activity of anthocyanins in many accessionsof eggplant and its related species. J Agric Food Chem.
2008; 56: 10154-10159.
17) Noda Y, Kneyuki T, Igarashi K, et al. Antioxidant activityof nasunin, an anthocyanin in eggplant peels. Toxicology.
2000; 148: 119-123.
18) Ishioka Y, Yagi M, Ogura M, et al. Polyphenol content ofvarious vegetables: Relationship to antiglycation activity.
Glycative Stress Res. 2015; 2: 41-51.

19) Otake K, Yagi M, Takabe W, et al. Effect of tea (Camelliasinensis) and herbs on advanced glycation endproductformation and the influence of post-fermentation. Glycative
Stress Res. 2015; 2: 156-162.
20) Blessington T, Nzaramba MN, Scheuring DC, et al. Cooking
methods and storage treatments of potato: Effects oncarotenoids, antioxidant activity, and phenolics. Am J Potato
Res. 2010; 87: 479-491.
21) Murador C D, Mercadante AZ, Rosso VV. Cooking
techniques improve the levels of bioactive compounds andantioxidant activity in kale and red cabbage. Food Chem.
2016; 196: 1101-1107.
22) Reeve R M, Hautala E, Weaver M L. Anatomy andcompositional variation within potatoes II. Phenolics,
enzymes and other minor components. American Potato
Journal. 1969; 46: 374-386.
23) Kubota A, Yamashita S. Antioxidative activity of freshand heated vegetable extracts. Bull Fukuoka Agric Res
Cent. 2000; 19: 81-84. (in Japanese)24) Ioku K, Aoyama Y, Tokuno A, et al. Various cooking
methods and the flavonoid content in onion. J Nutr Sci
Vitaminol. 2001; 47: 78-83.
25) Ohnishi H, Miyazawa D, Yoshioka H. Differences inphysical properties of eggplant cultivars and their ricebran pickles. Bulletin of Osaka Yuhigaoka Gakuen JuniorCollege. 2019; 62: 53-60. (in Japanese)
26) Niño-Medinaa G, Urías-Orona V, Muy-Rangel MD, et al.
Structure and content of phenolics in eggplant (Solanum
melongena) : A review. South African Journal of Botany.
2017; 111: 161-169.
27) Kurosawa S. Effects of edible oil on astringent taste of
cooked eggplants (Solanum melongena L.). Science of
Cookery. 1986; 19: 119-124. (in Japanese)
28) Horie H, Ando S. Eating-quality characteristics of eight
eggplant cultivars. Bull Nat Inst Veget Tea Sci. 2014; 13:
9-18.
29) Matsuoka H. Health benefit of Tsukemono. Bull Soc Sea
Water Sci Jpn. 2017; 71; 246-251. (in Japanese)_ 62 _