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

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