Impact of different cooking methods on food quality: Retention of ...

<strong>Impact</strong> <strong>of</strong> <strong>different</strong> <strong>cooking</strong> <strong>methods</strong> on food quality: Retention
<strong>of</strong> lipophilic vitamins in fresh and frozen vegetables
Simone Bernhardt *, Elmar Schlich
Institute <strong>of</strong> Agricultural Engineering, Department <strong>of</strong> Home Engineering, Justus-Liebig-University, Stephanstrasse 24, 35390 Giessen, Germany
Abstract
Accepted 6 June 2005
Available online 8 August 2005
The bioavailability <strong>of</strong> all-trans-b-carotene from vegetables depends among other things on the molecular linkage and the food
matrix in which it is incorporated. It is assumed that <strong>cooking</strong> can increase the bioavailability by disruption <strong>of</strong> the plant cell wall
and releasing from protein complexes. But it can also lead to isomerization and degradation <strong>of</strong> all-trans-b-carotene. In this investigation
the influence <strong>of</strong> <strong>different</strong> domestic <strong>cooking</strong> <strong>methods</strong> on the all-trans- and cis-b-carotene as well as the a-tocopherol content
in fresh and frozen broccoli and red sweet pepper is examined. While in fresh broccoli all <strong>cooking</strong> <strong>methods</strong> lead to a significant
release <strong>of</strong> all-trans-b-carotene and a-tocopherol in the frozen broccoli no change or a decrement occurs. In the fresh and frozen
peppers no change or a significant loss <strong>of</strong> a-tocopherol and all-trans-b-carotene is observed. A slight increase in the cis-isomers
<strong>of</strong> b-carotene can only be found by <strong>cooking</strong> fresh broccoli.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Cooking; Vitamins; b-carotene; a-tocopherol
1. Introduction
Journal <strong>of</strong> Food Engineering 77 (2006) 327–333
Food quality <strong>of</strong>ten deals only with the influences <strong>of</strong>
primary production and industrial processing. Food
preparation at home as final step <strong>of</strong> the chain has also
great influence on quality determining parameters like
sensory attributes and the content <strong>of</strong> vitamins and minerals.
It can change them both in a positive and negative
way.
In this study mainly the influences <strong>of</strong> <strong>different</strong> domestic
<strong>cooking</strong> <strong>methods</strong> on lipophilic vitamins by means <strong>of</strong>
the all-trans- andcis-b-carotene as well as the a-tocopherol
content in fresh and frozen vegetables are examined
and discussed.
*
Corresponding author. Tel.: +49 641 9939353; fax: +49 641
9939359.
E-mail address: simone.bernhardt@ernaehrung.uni-giessen.de (S.
Bernhardt).
0260-8774/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jfoodeng.2005.06.040
Finally some data about the retention <strong>of</strong> ascorbic
acid and minerals are given to evaluate the single <strong>cooking</strong>
<strong>methods</strong> more precisely.
1.1. b-Carotene
www.elsevier.com/locate/jfoodeng
In green vegetables b-carotene is incorporated in the
carotenoid–protein-complexes in the chloroplasts. These
carotenoproteins have an inhibitory effect upon carotenoid
digestion and absorption. In orange or red fruits bcarotene
is dissolved in oil droplets in the chromoplasts
and can be readily extracted during digestion (Castenmiller
& West, 1998; de Pee, West, Permaesih, Muhilal, &
Hautvast, 1998; Vishnevetsky, Ovadis, & Vainstein,
1999).
Food preparation e.g. mincing and <strong>cooking</strong> can increase
the extractability and therewith the bioavailability
<strong>of</strong> b-carotene from the food matrix by s<strong>of</strong>tening
or disruption <strong>of</strong> plant cell walls and the destruction
<strong>of</strong> carotenoid–protein-complexes (Dietz, Kantha, &

328 S. Bernhardt, E. Schlich / Journal <strong>of</strong> Food Engineering 77 (2006) 327–333
Erdman, 1988; Erdman, Poor, & Dietz, 1988; van het
H<strong>of</strong> et al., 2000; Howard, Wong, Perry, & Klein, 1999).
But <strong>cooking</strong> can also lead to an isomerization <strong>of</strong> the
natural mainly in the all-trans-form occurring b-carotene
to its cis-isomers. Cis-isomers are less bioavailable
and the provitamin A-activity is lower (Deming, Teixeira,
& Erdman, 2002; Deming, Baker, & Erdman,
2002; Zechmeister, 1949).
Oxidation promoted by the presence <strong>of</strong> light, heat
and oyxgen is the main reason for destruction <strong>of</strong> carotenoids
(von Elbe & Schwartz, 1996; Rodriguez-Amaya,
1997).
1.2. a-Tocopherol
a-Tocopherol is the most important natural occurring
compound with vitamin E activity (Esterbauer &
Hayn, 1997). In the leaves <strong>of</strong> higher plants vitamin E occurs
mainly as a-tocopherol which is situated inside
chloroplasts. Its concentration is higher in dark than
in light green leaves (Booth, 1963; Bramley et al., 2000).
In pepper the a-tocopherol synthesis occurs in the
chromoplasts (Arango & Heise, 1998). During fruit ripening
colour changes from green to red and the chloroplasts
are converted to chromoplasts. At the same time
the synthesis <strong>of</strong> a-tocopherol rises (Lichtenthaler, 1969).
Only a few studies about the change <strong>of</strong> vitamin E
content by <strong>cooking</strong> vegetables exist. Most <strong>of</strong> them are
antiquated and the authors mistrust their own results
(Brubacher, 1966).
1.3. Ascorbic acid
Ascorbic acid (vitamin C) is one <strong>of</strong> the most sensitive
vitamins. For this reason it is <strong>of</strong>ten used to evaluate
the influences <strong>of</strong> food processing on vitamin contents
(Bognár, 1989).
Cooking losses depend upon degree <strong>of</strong> heating, leaching
into the <strong>cooking</strong> medium, surface area exposed to
water and oxygen, pH and other factors (Eitenmiller &
Landen, 1999).
1.4. Minerals
Different from vitamins minerals are not destroyed by
light, heat and oxygen but only removed from the food
by leaching or physical separation (Miller, 1996).
2. Materials and <strong>methods</strong>
2.1. Vegetables
A green vegetable (broccoli) and a red vegetable fruit
(sweet pepper) were chosen to examine if differences in
localisation and binding form <strong>of</strong> b-carotene and a-
tocopherol in the plant tissue have an effect on their
behaviour during <strong>cooking</strong>. Further differences between
fresh and frozen vegetables should be examined.
The vegetables were bought as one batch at a wholesaler
in order to avoid variations in vitamin content because
<strong>of</strong> <strong>different</strong> locations, seasonal or climatic
influences. The fresh broccoli was grown in Germany,
the fresh red pepper originated from the Netherlands.
The frozen vegetables were from Portugal and Spain.
2.2. Cooking <strong>methods</strong>
The fresh vegetables were boiled and stewed in a
stainless steel pot on a hot plate. For steaming and pressure
steaming a special steam oven (Imperial Ò DLG
5664) respectively pressure steam oven (Imperial Ò DG
4664) was used. Temperature was about 100 ° C in the
steam oven and 120 °C in the pressure steam oven.
The frozen vegetables were boiled in a stainless steel
pot, prepared in a steam oven (Imperial Ò DLG 5664)
and in a Miele Ò Microwave (M 625 EG).
Before <strong>cooking</strong> the fresh vegetables were cleaned,
washed and cut into pieces. After removing the main
part <strong>of</strong> the stem the broccoli was cut into flowerets with
a weight <strong>of</strong> 30 g. The red pepper was divided into quarters
and cut into horizontal stripes <strong>of</strong> 1 cm.
The frozen vegetables were ready to use. Broccoli was
already cut into flowerets and red pepper into stripes.
For each <strong>cooking</strong> procedure 300 g <strong>of</strong> vegetables were
weighed out. The <strong>cooking</strong> parameters are shown in
Table 1. These parameters were evaluated with the help
<strong>of</strong> sensorial tests. All vegetables should be well-done but
firm to the bite.
For boiling the vegetables were covered completely
with cold water. For stewing only a small amount <strong>of</strong>
water (about 50–100 g) was added to avoid scorching.
Table 1
Cooking parameters (Loh, 2004)
Cooking method Type <strong>of</strong> vegetable Heating up
time (min)
Cooking
time (min)
Boiling Broccoli, fresh 9.5 16.0
Red pepper, fresh 6.3 6.0
Broccoli, frozen 12.0 2.0
Red pepper, frozen 9.5 5.0
Stewing Broccoli, fresh 4.0 8.0
Red pepper, fresh 4.0 6.0
Steaming Broccoli, fresh 10.0
Red pepper, fresh 10.0
Broccoli, frozen 6.0
Red pepper, frozen 8.0
Pressure steaming Broccoli, fresh 2.0
Red pepper, fresh 1.0
Microwave Broccoli, frozen 4.0 4.0
Red pepper, frozen 4.0 3.0

The temperature was controlled with the help <strong>of</strong> a temperature
sensor which was placed into the pot through a drill
hole in the lid. Heating up was carried out on a high energy
level until a temperature <strong>of</strong> 98 °C was reached. After
this energy was reduced to the lowest level and the <strong>cooking</strong>
process was continued until the vegetables are done.
For the steam ovens it is only necessary to select the
desired <strong>cooking</strong> time and to start the program. Steam is
created outside the food chamber. When it has reached
the required temperature it is injected into the chamber.
In the microwave no additional water was used.
Heating up occurred with a power <strong>of</strong> 850 W then it
was reduced to 450 W.
All <strong>cooking</strong> <strong>methods</strong> are repeated five times.
After <strong>cooking</strong> the vegetables were vacuum sealed.
Weight was determined again to calculate mass changes
during <strong>cooking</strong>. Afterwards the samples were cooled in
ice water and stored at 29 °C for two days maximum
before they were sent together with dry ice by express
delivery to the laboratory for vitamin analysis.
2.3. Vitamin and mineral analysis
Analysis was done by an external laboratory which is
accredited after DIN EN ISO/IEC 17025. The following
<strong>methods</strong> are used:
S. Bernhardt, E. Schlich / Journal <strong>of</strong> Food Engineering 77 (2006) 327–333 329
b-Carotene: DIN EN 12823-2 2000-07: Determination
<strong>of</strong> vitamin A by high performance liquid chromatography—Part
2: Determination <strong>of</strong> b-carotene
(DIN EN 12823-2, 2000).
a-Tocopherol: VDLUFA Methodenbuch Band III,
13.5.4 Determination <strong>of</strong> vitamin E, HPLC-method
(VDLUFA, 1993).
Ascorbic acid: Hausmethode Lufa-Kiel (IZ-Lufa-
ITL, 1999).
Minerals: The mineral content was determined by
measuring the ash content like it was done in other
studies before (e.g. Herrmann, 1994). Ash content
was measured by the method <strong>of</strong> VDLUFA (1976).
To calculate the true retention <strong>of</strong> vitamins and minerals
the weight changes during <strong>cooking</strong> have taken into
account. As aforementioned changes in weight were
determined and the retention was calculated on original
fresh weight basis.
2.4. Statistics
All <strong>cooking</strong> procedures were repeated five times. Results
are given as arithmetic mean ± SD (standard deviation).
Comparisons <strong>of</strong> means are performed using the
Lord-test with an error probability <strong>of</strong> 1% (Kesel, Junge,
& Nachtigall, 1999).
3. Results and discussion
Tables 2–5 show the b-carotene (all-trans- and cis-isomers)
and a-tocopherol contents in raw and cooked
vegetables:
Table 2
b-Carotene and a-tocopherol contents (mg/100 g) in fresh broccoli ðx SDÞ (Loh, 2004)
Before <strong>cooking</strong> Boiling Stewing Steaming Pressure steaming
All-trans-b-carotene 0.05 ± 0.01 0.27 ± 0.04 0.23 ± 0.03 0.21 ± 0.03 0.26 ± 0.03
Cis-b-carotene 0.02 ± 0.004 0.10 ± 0.012 0.08 ± 0.009 0.07 ± 0.015 0.09 ± 0.015
a-tocopherol 0.32 ± 0.05 1.54 ± 0.16 1.61 ± 0.07 1.58 ± 0.16 1.70 ± 0.08
Table 3
b-Carotene and a-tocopherol contents (mg/100 g) in fresh red pepper ðx SDÞ (Loh, 2004)
Before <strong>cooking</strong> Boiling Stewing Steaming Pressure steaming
All-trans-b-carotene 0.58 ± 0.07 0.38 ± 0.12 0.35 ± 0.07 0.37 ± 0.11 0.36 ± 0.05
Cis-b-carotene 0.16 ± 0.03 0.19 ± 0.03 0.16 ± 0.04 0.15 ± 0.04 0.14 ± 0.02
a-tocopherol 3.93 ± 0.47 3.59 ± 0.30 3.45 ± 0.07 3.39 ± 0.24 3.70 ± 0.37
Table 4
b-Carotene and a-tocopherol contents (mg/100 g) in frozen broccoli ðx SDÞ (Loh, 2004)
Before <strong>cooking</strong> Boiling Steaming Microwave
All-trans-b-carotene 0.37 ± 0.02 0.32 ± 0.02 0.30 ± 0.04 0.28 ± 0.03
Cis-b-carotene 0.09 ± 0.01 0.08 ± 0.02 0.07 ± 0.01 0.07 ± 0.01
a-tocopherol 1.26 ± 0.11 1.20 ± 0.06 1.43 ± 0.08 1.36 ± 0.07

330 S. Bernhardt, E. Schlich / Journal <strong>of</strong> Food Engineering 77 (2006) 327–333
Table 5
b-Carotene and a-tocopherol contents (mg/100 g) in frozen red pepper ðx SDÞ (Loh, 2004)
Before <strong>cooking</strong> Boiling Steaming Microwave
All-trans-b-carotene 0.90 ± 0.06 0.68 ± 0.04 0.78 ± 0.10 0.72 ± 0.09
Cis-b-carotene 0.18 ± 0.04 0.27 ± 0.03 0.28 ± 0.05 0.17 ± 0.02
a-tocopherol 4.48 ± 0.11 4.62 ± 0.06 4.29 ± 0.31 4.65 ± 0.17
3.1. b-Carotene
All <strong>cooking</strong> <strong>methods</strong> lead to a significant release <strong>of</strong>
all-trans-b-carotene and its cis-isomers in fresh broccoli.
Similar results have already been reported by Hart and
Lessin in cooked broccoli and by Howard in microwaved
broccoli (Hart & Scott, 1995; Howard et al., 1999;
Lessin, Catigani, & Schwartz, 1997). Previous own investigations
have already shown a release <strong>of</strong> b-carotene in
fresh broccoli prepared with <strong>different</strong> <strong>cooking</strong> <strong>methods</strong>
(Schlich, Boeker, Dietrich, Loh, & Ziems, 2001).
In broccoli b-carotene is incorporated in the carotenoid–protein-complexes
in the chloroplasts. These socalled
carotenoproteins have an inhibitory effect on
extractability <strong>of</strong> b-carotene from the vegetable matrix.
Cooking leads to a s<strong>of</strong>tening <strong>of</strong> the plant tissue and
the denaturation <strong>of</strong> proteins so that the carotenoids
can be extracted a lot easier (Rodriguez-Amaya, 1997).
It is assumed that an increased extractability may be
associated with improved bioavailability (Erdmann
et al., 1988).
In frozen broccoli all <strong>cooking</strong> <strong>methods</strong> led to significant
losses <strong>of</strong> all-trans-b-carotene, which range between
14% and 25%. Broccoli was blanched before freezing.
The blanching step leads already to a disruption <strong>of</strong> the
cell membrane and a s<strong>of</strong>tening <strong>of</strong> the vegetable matrix
(Herrmann, 1996). This causes already a better extractability
<strong>of</strong> b-carotene so that higher contents can be
proved (Oruña-Concha, González-Castro, Lopéz-
Hernández, & Simal-Lozano, 1997). In the following
<strong>cooking</strong> process no further release seems to be possible.
In fresh red pepper stewing, steaming and pressure
steaming lead to significant losses in all-trans-b-carotene
content. By viewing the average means it seems that also
<strong>cooking</strong> leads to losses. A change in the cis-b-carotene
content cannot be proven. In frozen red pepper boiling
and microwave <strong>cooking</strong> lead to significant losses <strong>of</strong>
all-trans-b-carotene. Only boiling leads to a significant
increase <strong>of</strong> the cis-isomers.
In summary it is shown that <strong>cooking</strong> has rather a negative
effect on the b-carotene content in fresh and frozen
red pepper. The red pepper is a vegetable fruit, from
botanical view it is a berry (Franke, 1997). In orange
or red fruits b-carotene is dissolved in oil droplets in
chromoplasts and can be better extracted than from
green vegetables (Castenmiller & West, 1998; de Pee
et al., 1998; Vishnevetsky et al., 1999). On the other
hand it seems that in this form b-carotene is much more
vulnerable against oxidation which is the main reason
for the destruction <strong>of</strong> carotenoids (Rodriguez-Amaya,
1997).
To estimate better the effects <strong>of</strong> <strong>cooking</strong> on the alltrans-
andcis-b-carotene contents the cis-/all-trans-ratio
is calculated (Loh & Schlich, 2003). Therefore the contents
<strong>of</strong> cis- and all-trans-b-carotene are set proportionally
to each other into relationship (Table 6). In this way
it is easier to recognise if and how the ratio from cis-bcarotene
to all-trans-b-carotene changes after <strong>cooking</strong>.
For visualisation the arithmetic means are illustrated
in Fig. 1.
It strikes that by <strong>cooking</strong> fresh broccoli the ratio does
not change respectively declines although you can observe
a significant increase <strong>of</strong> the all-trans and the cisforms.
This increase <strong>of</strong> the cis-isomers could probably
be caused by the release <strong>of</strong> the natural in small amounts
existing cis-forms and not by conversion <strong>of</strong> all-trans-bcarotene.
Cooking <strong>of</strong> frozen broccoli does not lead to significant
changes. The loss <strong>of</strong> all-trans-b-carotene by <strong>cooking</strong>
fresh and frozen red pepper is nearly always
combined with a significant increase <strong>of</strong> the ratio.
This could be a further hint that <strong>cooking</strong> <strong>of</strong> fresh
broccoli has positive effects because all-trans-b-carotene
is released and the cis-/all-trans-ratio decreases. Whereas
Table 6
Cis-/all-trans-ratio (in %) in the vegetables ðx SDÞ (Loh, 2004)
Before <strong>cooking</strong> Boiling Stewing Steaming Pressure steaming Microwave
Broccoli, fresh 42.0 ± 17.6 36.7 ± 6.9 33.6 ± 3.4 33.3 ± 4.3 32.2 ± 3.6 n.d.
Pepper, fresh 27.9 ± 3.47 54.9 ± 19.89 47.0 ± 5.52 42.1 ± 6.21 40.0 ± 1.66 n.d.
Broccoli, frozen 24.6 ± 3.6 25.0 ± 4.4 n.d. 24.5 ± 3.3 n.d. 23.3 ± 4.1
Pepper, frozen 20.3 ± 3.6 39.5 ± 3.4 n.d. 36.1 ± 3.2 n.d. 23.3 ± 2.0

<strong>cooking</strong> <strong>of</strong> red pepper leads to a degradation <strong>of</strong> the alltrans-b-carotene
content and an increase <strong>of</strong> the ratio.
3.2. a-Tocopherol
mg/100g
All <strong>cooking</strong> <strong>methods</strong> lead to a significant release <strong>of</strong>
a-tocopherol in fresh broccoli. Whereas in frozen
broccoli no significant changes occur. Previous own
investigations have shown the same results with the
exception that <strong>cooking</strong> <strong>of</strong> frozen broccoli leads to a
small release <strong>of</strong> a-tocopherol (Schlich et al., 2001).
Further literature is scarce. Some authors report
about losses <strong>of</strong> 3.7% by boiling <strong>different</strong> vegetables oth-
mg/100g
g/100g
60.0
50.0
40.0
30.0
20.0
10.0
0.0
180
160
140
120
100
80
60
40
20
0
0.7
0.6
0.5
0.4
0.3
0.2
0.1
S. Bernhardt, E. Schlich / Journal <strong>of</strong> Food Engineering 77 (2006) 327–333 331
0
Before
<strong>cooking</strong>
Before
<strong>cooking</strong>
Ascorbic acid
Boiling Steaming Microwave Pressure
Steaming
ers have found increases <strong>of</strong> 18% by pressure steaming
(Booth & Bradford, 1963; Brubacher, 1966).
Cooking fresh red pepper does not lead to a significant
change in a-tocopherol content. In consideration
<strong>of</strong> the arithmetic means it seems that <strong>cooking</strong> rather
causes a decrease. Cooking frozen red pepper also shows
no significant changes.
3.3. Ascorbic acid and mineral content
The influences <strong>of</strong> the single <strong>cooking</strong> <strong>methods</strong> on the
ascorbic acid and mineral content is exemplified in Figs.
2 and 3 for fresh and frozen pepper. The results show
pepper, fresh
pepper, frozen
Stewing
Fig. 2. Ascorbic acid content (arithmetic mean and standard deviation).
Before
<strong>cooking</strong>
cis-/all-trans-ratio
Boiling Stewing Steaming Pressure
Steaming
Microwave
Fig. 1. Cis-/all-trans-ratio (arithmetic mean).
Minerals
Boiling Steaming Microwave Pressure
Steaming
pepper, fresh
pepper, frozen
Stewing
Fig. 3. Mineral content (arithmetic mean and standard deviation).
broccoli,
fresh
pepper,
fresh
broccoli,
frozen
pepper,
frozen

332 S. Bernhardt, E. Schlich / Journal <strong>of</strong> Food Engineering 77 (2006) 327–333
clear that boiling leads to high losses while stewing,
steaming, microwave <strong>cooking</strong> and even pressure steaming
cause only small losses (Loh, 2004). Losses <strong>of</strong> minerals
during <strong>cooking</strong> are not caused by destruction but
only by leaching into the <strong>cooking</strong> water (Miller, 1996).
Leaching is also the main reason for the high losses <strong>of</strong>
ascorbic acid during <strong>cooking</strong>. Different studies have
found an decrease <strong>of</strong> ascorbic acid by boiling up to
75% (Gould & Golledge, 1989; Petersen, 1993; Schnepf
& Driskell, 1994).
4. Conclusions
By comparison <strong>of</strong> the results <strong>of</strong> b-carotene and atocopherol
it seems that similarities in localisation lead
to a similar behaviour during <strong>cooking</strong>.
Cooking a green fresh vegetable promotes the release
<strong>of</strong> lipophilic vitamins. Whereas in a red vegetable fruit
respectively in the frozen vegetables which have passed
a blanching step a decrease or no change occurs.
For b-carotene it is known since longer that <strong>cooking</strong>
can increase the extractability and thereby probably improves
the bioavailability <strong>of</strong> b-carotene from the vegetable
matrix.
Whereas for a-tocopherol it is still unclear how the
food matrix influences absorption and bioavailability.
The given results let assume that <strong>cooking</strong> leads also to
a better availability <strong>of</strong> a-tocopherol from sources which
are difficult to access.
Between the single <strong>cooking</strong> <strong>methods</strong> no significant
differences can be observed if only the results for the
lipophilic vitamins are considered. But if there is also
paid attention in preserving hydrophilic compounds like
ascorbic acid and minerals boiling should be avoided in
favour <strong>of</strong> a <strong>cooking</strong> method with less water.
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