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J Nutr Sci Vitaminol, 47, 78-83, 2001
Various Cooking Methods and the Flavonoid Content in Onion
Kana IOKU, Yuka AOYAMA, Ayaka TOKUNO, Junji TERA01, Nobuji NAKATANI2 and Yoko TAKEI
Faculty of Education, Osaka Kyoiku University, Kashiwara 582-8582, Japan
1 Course of Nutrition
, School of Nutrition, The University of Tokushima, Tokushima 770-8503, Japan
2 Department of Food and Nutrition
, Faculty of Human Life Science, Osaka City University, Osaka 558-8585, Japan
(Received March 13, 2000)
Summary Onion is a major source of flavonoids and is cooked in various ways in the
world. The major flavonoids in onion are two quercetin glycosides, quercetin 4•L-O-ƒÀ-gluco
side (Q4•LG) and quercetin 3,4•L-O-ƒÀ-diglucosides (Q3,4•LG), which are recognized as bioac
tive substances that are good for our health. We have investigated the effect of cooking pro
cedures on the content of antioxidants. We selected quercetin conjugates, total phenol com
pounds, and ascorbic acid to estimate the amount of flavonoid ingestion from onion. We ex
amined the following cooking methods: boiling, frying with oil and butter, and microwave
cooking. Various cooking methods do not consider the degradation of quercetin conjugates
when cooking onion. Microwave cooking without water better retains flavonoids and ascor
bic acid. Frying does not affect flavonoid intake. The boiling of onion leads to about 30% loss
of quercetin glycosides, which transfers to the boiling water. At that time, the effect of addi
tives on the quercetin conjugates is different according to the compounds. The hydrolysis of
quercetin glycosides for daily cooking might occur with the addition of seasonings such as
glutamic acid. Additional ferrous ions accelerated the loss of flavonoids.
Key Words cooking, onion, flavonoid, quercetin
Onion (A11ium cepa L.) is thought to be a major source
of flavonoids, especially quercetin, and continues to at
tract widespread attention as a potentially useful agent
for preventing cancer and cardiovascular diseases
(1, 2).
The main quercetin glycosides in onion are
quercetin-4•L-O-ƒÀ-glucoside (Q4•LG) and quercetin-3,4•L
O-ƒÀ-diglucosides (Q3,4•LG) (3, 4). These two flavonoids
,
Q4•L G and Q3,4•LG, account for 85% of the total amount
of onion flavonoids. Although the relation between
onion intake and the risk of cancer was not clarified in
previous studies (5-7), it has been found that the
flavonoids in onion have a good effect on human
health. Recently, the significance of food functionality
in our health has been discussed as a new field of nutri
tion and flavonoids are recognized as an important food
factor for cancer prevention (8).
Generally, flavonoids in vegetables and fruits exist in
the form of glycosides. During the process of digestion,
most flavonoid glycosides pass into the small intestine,
and are decomposed by the intestinal microflora in the
colon (9). Some of them are absorbed from the small in
testine after hydrolysis (10) and work to prevent the ox
idation of LDL and oxidative damage (11-13). Onion,
which is a very popular vegetable around the world, is
important to the flavonoid intake. The consumption of
300g of onion per day was shown to clearly increase
the amount of quercetin in plasma (10). In addition,
onion is cooked by various methods. In the cooking
process, antioxidative vitamins such as ascorbic acid
and a-tocopherol are decomposed (14, 15). It is also
suggested that flavonoids are decomposed in cooking.
For understanding of flavonoid absorption and diges
tion, it is essential to determine the loss of flavonoids in
cooking processes. Recent research has used fried onion
as an onion supplement for the study of quercetin me
tabolism (12, 16). Some researchers have suggested
that boiling affects the content of quercetin glucosides
in onion, but the effects were not significant (17,18).
By comparing the effects of each cooking method on
the content of flavonoids in onion, we identified the
most effective cooking method for ingesting quercetin
conjugates from onion. In this study, we demonstrate
the effect of several cooking methods on the content of
flavonoid compounds in cooked onions and the effect of
additives on boiling.
MATERIALS AND METHODS
Materials. Yellow onions grown in Awaji, Hyogo
Prefecture, were obtained from a wholesale market in
Osaka, Japan. Methanol and ethyl acetate were ob
tained from Katayama Chemicals (Osaka, Japan) and
Wako Pure Chemicals (Osaka, Japan), and used after
distillation. Phenol reagent was purchased from
Nacalai Tesque (Kyoto, Japan). Distilled water for HPLC
analysis was filtered through a membrane filter (cellu
lose acetate, pore size 0.2ƒÊm, Advantec, Tokyo, Japan)
and degassed before use. Standard quercetin and pectin
were from Sigma Chemical Co. (Poole, UK) and gallic
acid was from Katayama Chemicals. Other solvents and
reagents were obtained from Wako Pure Chemicals.
Cooking methods
Frying: About 2kg of onions (4 or 5 bulbs) were
cleaned without water, peeled, and minced in a food
78
The Cooking Methods and the Flavonoid Contents in Onion 79
processor. An open frying pan (diameter 32cm) was
pre-heated to 150•Ž on an electric cooker (1,200W),
and the minced onion (300g) was put into it. Cooking
times were 5, 20, and 40min. Corn oil and butter were
added to 7% of the onion. As a control, minced onion
was fried without oil on the pan. Corn oil and butter for
frying were purchased from a local market in Osaka.
Microwave cooking: Onions were prepared and minced
in a food processor the same as for frying. The mi
crowave oven used was a Sharp Model RE-122 (500
W, Osaka, Japan). The frequency of radiation was
2,450MHz. Minced onion (50g) was placed in a petri
dish (diameter 9.5cm) covered with commercial plastic
food wrap (polyvinilidene chloride, 10ƒÊm in thickness,
Kureha Chemicals, Tokyo, Japan) and microwaved for
30, 60, and 180 seconds.
Boiling: Distilled hot water (200mL) and minced
onion (100g) were put into a beaker (300mL), covered
with a watch glass, and were individually boiled for 5,
20, and 40min on an electric heater (1,200W).
Additional reagents were used to the weight of 1% of
water.
Extraction and analytical methods
Extraction: We prepared two types of onion extract.
One was prepared for the estimation of ascorbic acid
content, and the other was for the assay of the total
phenol compounds and flavonoids. The extract solution
for ascorbic acid measurement was prepared as follows:
Cooked onions (2.0g) were triturated with 5%
metaphosphoric acid solution. The supernatant was
collected after centrifugation (3,000rpm, for 5 min
utes). The total volume was filled up to 50mL with 5%
metaphosphoric acid solution. The sample for ascorbic
acid assay was kept at 4•Ž in a refrigerator. After
preparing the extract for total phenol compounds and
flavonoids analysis, some of the cooked onion (30.0g)
was freeze-dried. In the case of frying with corn oil and
butter, cooked and freeze-dried onions were washed
with n-hexane (50mL) three times before extraction.
The freeze-dried cooked onion (1.0g) was extracted
with 70% methanol (50mL) at 30•Ž for 20h in the
dark. After being filtered through a filter paper (No. 2,
Advantec, Tokyo, Japan), the extract was evaporated
and filled up to 20mL with 70% methanol. After
adding twice the volume of methanol, the filtered boil
ing water was kept at -20•Ž. The boiling water was fil
tered through a No. 2 filter paper.
Analysis condition of HPLC and hydrolysis: Total phe
nol contents were evaluated by a slightly modified
Folin-Denis method (19). Gallic acid was used as a stan
dard. Ascorbic acid and flavonoids were measured
using HPLC analysis. For measuring compounds, two
HPLC apparatuses were used: One was a Gulliver intelli
gent system with UV detector (UV-970, Jasco, Tokyo,
Japan), and the other system consisted of a pump (PU
980, Jasco), UV detector (638-41, Hitachi, Tokyo,
Japan), and integrator (870-IT, Jasco). Ascorbic acid
was analyzed using an Inertsil ODS-3 column (4.6•~
150mm, pore size 5 ƒÊm, GL Sciences, Tokyo, Japan).
The eluent used was 20mM phosphate buffer, pH 2.5,
and monitored at 245nm. The analysis conditions of
quercetin glucosides were the same as in a previous
paper (20).
The total quercetin content was determined after
acid hydrolysis of extracted flavonoids with 2N-HCI for
30min in boiling water, and extracted with ethyl ac
etate. The analysis of quercetin by HPLC followed a pre
vious method (20). HPLC chromatograms of flavonoid
compounds were quantified by fast atom bombard
ment-mass spectrometry (FAB-MS) after peak prepara
tion.
Statistics: Statistical analysis was carried out using
Stat View 4.5 (Hulinks, Tokyo, Japan) on a Macintosh.
One-way repeated-measures ANOVA was used with
Scheffe's multiple analysis of variance. Multivariate
analysis was performed using hierarchical cluster
analysis with Pirouette 2.3 (InfoMetrix, Woodinville,
WA, USA) on Windows 95. The significance level was at
0.05.
RESULTS
Analysis of cooking methods
We examined the effects of three cooking methods on
the amount of antioxidative compounds in onion. In
case of frying, we used three methods: without oil (con
trol), with corn oil, and with butter. Ascorbic acid,
which is a hydrophilic antioxidant, is unstable under
heat treatment. Degradation of ascorbic acid during
cooking reflected the precision of our cooking (Table 1).
The main flavonoids in onion were quercetin glycosides.
The total amount of quercetin in fried onion is shown in
Fig, 1. The content of total quercetin after acid hydroly
sis was not significantly altered after 40min of frying.
More than 80% of flavonoid compounds remained in
the fried onion after heating. This indicated that the
method of frying had little effect on the quercetin con
tent in onion, regardless of the type of oil or butter. In
addition, total phenol content was suggested to increase
reluctant substances in cooking. During all frying of
onion in our cooking, there was no significant brown
ing. The temperature measured at the center of the pan
during frying was below 150•Ž.
On the other hand, the total quercetin content was
increased 1.5 times by microwave heating for 1min. It
was supposed that quercetin glycosides were extracted
from cooked onion more easily under microwave heat
ing than from fresh onion. The contents of total phenol
and ascorbic acid did not change significantly (data not
shown).
As shown in Fig. 2, the content of Q4•LG, one of the
main flavonoid glucosides in onion, was decreased by
boiling. It was observed that Q4•LG in onion was trans
ferred to the water without decomposition, although
ascorbic acid was clearly decreased by boiling (data not
shown).
Effect of additional reagents on the content of flavonoid glu
cosides in boiled onion
The effect of additives on the amounts of quercetin in
the form of free and quercetin glucosides in onion is
shown in Figs. 3-1 and 3-2. Cooking time was set as
80 IOKO Ket al.
Table 1. Degradation of the content of ascorbic acid in fried onion.
The addition of oils to frying onion is 21g (7% of onion). The amount of ascorbic acid includes the content of dehydroascorbic
acid and ascorbic acid. Data were expressed as mg per 100g fresh onion bulbs. Values are the means•}standard deviation of
7-10 determinations. Those in the same column with different superscript letter are significantly different at p<0.05.
Fig. 1. Changes of total quercetin in onion by differ
ent frying method. Onion bulbs (300g) were fried
without (•›) and with 7% of corn oil (•œ) or butter (•¢).
The amount of total quercetin includes quercetin
glycosides and free quercetin. Values are the
means•}standard deviation of 7-10 replicate analy
ses. Data was expressed as mg per 100g fresh onions.
Fig. 2. Changes in quercetin 4•L-O-ƒÀ-glucoside during
onion boiling. The hatched and shaded columns show
the relative amounts of Q4•LG compounds in cooking
water and onion, respectively, at various boiling times.
Values on a column with different superscript letters
are significantly different at p<0.05. Each value is the
mean•}standard deviation of three replicate analyses.
20 min referring to the data of Fig. 2. In this experi
ment, we had selected substances used in domestic
cooking. A higher content of Q3,4•LG remained than
that of quercetin with the addition of gelatin, sodium
chloride, ferrous sulfate (II), and ferrous sulfate (III)
(Fig. 3-1). The supplement of ferrous compounds re
markably decreased the content of flavonoids. Q3,4•LG
was more stable than other flavonoids under the addi
tion of ferrous compounds. After the addition of ferrous
sulfate (III), quercetin could not be detected in our sys
tem. Additional gelatin improved the amount of Q3,4•LG
more than that of other flavonoids. On the other hand,
quercetin content remained higher than that of the
other glucosides with the addition of calcium chloride,
glutamic acid, and oxalic acid (Fig. 3-2). The solution of
1% calcium chloride showed a pH of 6.6. The solution
of oxalic acid showed a pH of 1.6. Thus, oxalic acid was
thought to cause acid hydrolysis for quercetin glyco
sides. The addition of daily seasonings such as sodium
chloride and soy sauce caused a lowering of pH, too.
With the addition of sodium chloride, the solution
showed pH 5.2, and in the case of glutamic acid, it was
pH 3.3.Quercetin was increased by the addition of glu
tamic acid as shown in Fig. 3-2.
To clarify the co-relation of each additional com
pound, we applied the hierarchical cluster analysis to
the survival rate of Q4•LG, as shown in Fig. 4. The addi
tion of ferrous ions caused different effects than the
other reagents. The dendrograms patterns of the other
flavonoids were not so different.
DISCUSSION
Onions are very common vegetables and are cooked
by various methods. Moreover, they are recognized as a
The Cooking Methods and the Flavonoid Contents in Onion 81
Fig. 3-1.
Fig. 3-2.
Fig. 3. Effect of additives on the contents of onion flavonoids in boiling. The amount of additive was 1% of the boiling
water. The top, middle and bottom columns for each additive indicates the contents of Q3,4•LG, Q4•LG, and free
quercetin, respectively. The shaded and hatched areas of each column show the relative amounts of quercetin and its
glucosides in onion and cooking water. For the group of additives shown in Fig. 3-1, Q3,4•LG is better retained than
Q4•LG and quercetin. In addition, the group of additives for which quercetin is better retained than Q4•LG and Q3,4•LG is
shown in Fig. 3-2. 'N.D.' means 'not detected'.
Fig. 4. Dendrogram of hieratical cluster analysis based on the data of Q4•LG in cooked onion and water.
82 IOKU K et al.
good source of quercetin. We investigated the effect of
different cooking techniques on quercetin conjugates.
A previous study showed that frying with sunflower
oil had remarkable effects on the content of quercetin
glucosides. Furthermore, an overall loss of flavonol de
rivatives was observed in the tissue after more than
15 min, frying, which caused extensive browning (17).
We tried three types of frying for 40min with corn oil
and butter, but there was no significant difference. In
addition, browning of onion did not occur to any re
markable extent during prolonged frying. The increase
of temperature on the surface of the pan took place
slowly and there were few triggers for browning in
cooking onion. Considered that the use of fluorocarbon
coated frying pans is very popular in Japan (21), this
data is useful in daily cooking. Thus, we concluded that
flavonoid glycosides resist heating with oil and butter.
In the case of microwave cooking, 64% of quercetin
had been lost in previous data (22). However, they
added water for cooking before the microwave heating.
The addition of much water in microwave cooking is
considered the same as cooking by boiling. By covering
the top of the dish, onion can be cooked efficiently with
out the addition of water. Other studies have shown
equal or better retention of some vitamins (B1, B2, B6,
B12, C and folic acid) after microwave heating than after
other domestic heating methods (23-25). Taking into
account the nutritional and bioactive substances in
cooked foods, microwave cooking is the recommended
method.
As shown in Figs. 3-1, and 3-2, we demonstrated
that the residual amounts of Q3,4•LG and Q4•LG in boiled
onion were 30-40% of fresh onion. Considering these
amounts to be in the boiled water part, we concluded
that more than 70% of quercetin glucosides remain
after boiling. As shown in Fig. 2, we discovered the
amount of Q4•LG was not changed significantly by boil
ing. We can detect Q4•LG in boiling water in the form of
glucoside. Therefore, it is suggested that onion soup is a
good source of flavonoids. Previous data also showed
that domestic cooking of onion bulbs in the U.K. does
not result in gross changes in quercetin glycosides (17).
Our data also indicated the same results.
Cooking onions at home, it is usual to add some sea
sonings or other foodstuffs. Thus, we examined the ef
fect of additives on the flavonoid content in boiled
onions. Consequently, extra compounds were divided
into two groups. The first group was composed of
sodium chloride, ferrous compounds, and gelatin (Fig.
3-1). It was elucidated that C3-OH and C4•L-OH with the
conformation of quercetin were important to scavenge
radicals and chelate ferrous ions for antioxidative activ
ity (26, 27). Q3,4•LG substituted two glucosides with the
C3-OH and C4•L-OH positions, so the reactivity toward
the ferrous ions might not be high. The addition of 1ƒÊM
of ferrous chloride (II) to boiling water did not facilitate
the oxidation of Q4•LG and Q3,4•LG in onion (18). The
concentration of 1% ferrous sulfate (III) corresponded
to a mole concentration of 25mM. At a high concentra
tion of ferrous ions, it is possible to produce flavonoid
oxidation. In our HPLC systems, we could not measure
the compounds that chelate forms with flavonoids. In
general, the chelate form of flavonoids and binding
flavonoids exist in tissues. It is interesting to compare
the bioactivity with that of the free form of flavonoids.
On the other hand, there are many types of ferrous
compounds in food. Further study will be needed to
clarify the effect of foodstuffs containing ferrous com
pounds. Substantial work was carried out on gelatin
and polyphenol interactions and it indicated that it
might be possible to combine more OH groups in the
molecule with gelatin (28). Accordingly, Q3,4•LG has the
possibility to be combined easily with gelatin compared
with other flavonoids.
The second group of compounds summarized in Fig.
3-2 was composed of calcium chloride, glutamic acid,
and oxalic acid. The oxalic acid was caused by the acid
hydrolysis of quercetin glucosides, causing a strongly
acidic condition. Consequently, not only Q4•LG and
Q3,4•LG but also other quercetin glycosides in onion
were hydrolyzed, and a high amount of quercetin was
detected. The relationship of pH and hydrolysis of
flavonoid glucosides in daily cooking is not clear, but
the hydrolysis of flavonoid glycosides may depend on
the cooking method.
We used hierarchical cluster analysis to investigate
the relations and consider the characteristics of additive
compounds (Fig. 4). However, we did not find clear dif
ferences in this experiment, although the group with
the addition of ferrous ions was distinguishable from
the others.
The composition of flavonoids in the Japanese diet
(29) has been investigated, but it is very difficult to de
termine the quantity of flavonoids in a dish. In the case
of cooking onion for various dishes, it is necessary to
study the effect of cooking method on the flavonoid con
tent when cooking onion under several conditions.
Acknowledgement
We wish to express our special thanks to the Center
for Instrument Analysis in Osaka Kyoiku University.
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