Kurzfassung - Justus-Liebig-Universität Gießen

Abstract
This research assignment investigates the influence of different cooking processes
on the concentration of (L)-ascorbic acid and mineral nutrients in two local
vegetables, broccoli and kohlrabi. Ash concentration is analysed to determine the
content of mineral nutrients.
Fresh crop kohlrabi is cut into equal pieces whereas broccoli is split into flowerets,
having approximately the same weight. Vegetables are steamed, stewed and
microwave cooked, using a cooking pot, a steamer and a microwave respectively.
The ascorbic acid content is measured using an enzymatic test from a company
called Boehringer/Mannheim. The results for fresh and cooked samples are
compared and the ascorbic acid losses are determined. The measurement data is
used to assess the value of the different type of cooking process. In addition to the
enzymatic test the samples are also analysed by HPLC to determine and compare
the ascorbic acid content.
All samples display a loss of ascorbic acid after cooking. Within the kohlrabi samples
the highest loss of ascorbic acid is found in stewed kohlrabi, whereas microwave
cooked kohlrabi preserves ascorbic acid the best. In contrast to the predictions
steamed broccoli samples show the highest loss of ascorbic acid. Microwave cooking
and stewing cause a smaller decrease to the content of ascorbic acid.
If only cooked samples are considered, ash analysis displays the highest nutrient
loss for stewed samples for both, broccoli and kohlrabi. The minimum loss of
minerals is caused by microwave cooking. Thereby kohlrabi shows a high variability,
whereas the deviation of broccoli is relatively small. The results tend to confirm the
common opinion, that cooking processes using a lot of water are unfavourable
because of the high loss of hydrophilic vitamins and mineral nutrients.
There are no significant distinctions between the methods used to analyse the
ascorbic acid concentration. Enzymatic results are almost similar to those of the
HPLC. Therefore, the perfect method depends on factors as time, number of tests
and equipment. Further investigations should include simultaneous ascorbic acid
examinations for a comparable assessment of different methods. It would be
interesting to test the broccoli analysis with alternative conditions and to verify the
received results with broccoli related vegetables.
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