he concentrations and compositions of anthocyanin are
responsible for the diverse and attractive colours observed in purple fruits and vegetables, such as red cabbage. However, anthocyanins are vulnerable to temperature, which is the primary problem with the application of anthocyanins as food colourants in food
processing (Cemerog˘lu, Veliog ˘lu, & Is ik, 1994). For example, anthocyanin content in the roots of Chinese red radish decreased during
the heat treatment (Jing et al., 2012). Moreover, in blank carrot
juice and concentrates, increased degradation rate of anthocyanins
was observed with the increasing temperature during heating or
storage. Similarly, in our work, the cooking treatments, such as
boiling, steaming, microwaving and conventional stir-frying,
which the cooking temperature reached 100C, resulted in the loss
of anthocyanin.
Compared with fresh-cut samples, total phenolic content was
reduced in all the tested samples after different cooking
treatments. However, no significant decline was found in the
microwave heating- and steaming-cooked samples, which was
probably associated with the inactivation of the polyphenol oxidase by heating, thereby inhibition of polyphenols degradation. It
has been also reported that leaching into the cooking water was
also contributed to the depletion of total phenolic content in broccoli after cooking (Vallejo, Tomás-Barberán, & García-Viguera,
2003). In our present study, the portions of red cabbage did not
contact with the cooking water thoroughly during steaming, therefore, the phenolic content was less affected by stir-frying and boiling, with only 29.4% and 28.2% losses, respectively. Our results
showed that DPPH radical-scavenging activity of fresh-cut red cabbage was 95.64% more than the raw broccoli florets (78.17%),
which were reported byTurkmen, Sari, and Velioglu (2005),
because of the higher amounts of anthocyanins in red cabbage than
that in other vegetables. There was no profound difference in the
DPPH radical-scavenging activity between fresh-cut and steaming
treatment, whereas the stir-frying, boiling and microwave treatments caused a significant decrease in antioxidant activity
(Fig. 2B), which may be associated with the changes of total phenolic and anthocyanin contents after various treatments. Similar
results were also found in raw broccoli florets (Zhang &
Hamauzu, 2004).
In general, thermal treatment is known to be able to accelerate
oxidation of ascorbic acid to dehydroascorbic acid (Gregory, 1996).
In this study, all the samples showed a reduction in vitamin C
content after cooked, regardless of the cooking methods (Fig. 3).
The reduction in vitamin C levels in red cabbage were most
pronounced when stir-fried or boiled, and the lowest when fresh-cut, microwaved or steamed. In broccoli, there was a 29% loss
of vitamin C level when boiled, while no change was observed
when steamed (Gliszczynska-Swiglo et al., 2006). Boiling and
stir-frying/boiling also caused a dramatic loss of vitamin C in broccoli, which was related to high temperatures, long cooking time,
enzymatic oxidation during the preparation and cooking process
and frequent stirring that expose the materials to atmospheric oxidation (Yadav & Sehgal, 1995). Besides, vitamin C is water soluble,
stir-frying and boiling might cause great losses of vitamin C by
leaching into surrounding water as well. Therefore, using minimal
cooking water and cooking for shorter time periods resulted in better vitamin C retention (Erdman Jr & Klein, 1982).
Few data were available on the influence of cooking treatments
on total soluble and reducing sugars in red cabbage, although soluble sugar was important chemical compound with nutritional
value for human. Our results showed that boiling and stir-frying
caused the dramatic loss of total soluble and reducing sugars. It
was reported that the concentration of soluble sugar in broccoli
was decreased after boiling and stir-frying/boiling with a great
amount of water due to sugars leaching into water(Yuan, Sun,
Yuan, & Wang, 2009), which was also the main reason for the loss
of soluble and reducing sugars in our study. Thus, it is necessary to
avoid adding excess water during cooking.
Glucosinolates are one of the most important health-promoting
compounds inBrassicaplants for the remarkable anticarinogenic
activity of their hydrolysis products, isothiocyanates and sulforaphane. In the current study, all the cooking treatments caused
the loss of total glucosinolates content. Boiling and stir-frying led
to the highest loss of total glucosinolates, while steaming presented the lowest, compare with the fresh-cut samples (Fig. 5).
Red cabbage suffers a series of stresses during postharvest handlings, which might trigger complex metabolism of glucosinolates.
For example, before cooking cut brings the degradation enzyme
myrosinase to contact with glucosinolates, which might lead to
glucosinolates hydrolysis at high degree (Jia et al., 2009). In addition, gluco