Abstract
Different household cooking techniques (boiling, steaming, stir frying, and microwave) were tested on the changes of targeted phenolic compounds, antioxidant property (ferric reducing-antioxidant power (FRAP) activity), α-glucosidase activity, antinutritive compounds, and sensory properties in commonly consumed traditional leafy vegetables in Southern Africa, the non-heading Chinese cabbage (Brassica rapa L. subsp. chinensis) and African nightshade (Solanum retroflexum Dun). Stir frying increased kaempferol-3-O-hydroxyferuloyl-trihexoside, kaempferol-dihexoside, sinapoyl malate, rutin, and isorhamnetin-O-dihexoside in Chinese cabbage leaves, followed by steaming. Similarly, stir frying increased kaempferol-3-O-rutinoside, chlorogenic acid, caffeoylmalic acid, and quercetin-3-O-xylosyl-rutinoside in nightshade, followed by steaming. Biomarkers, sinapoyl malate (Chinese cabbage) and caffeoylmalic acid (nightshade), separated the stir frying from the other cooking techniques. Steaming and stir-frying techniques significantly increased the FRAP activity; whereas boiling and microwaving reduced the tannin, oxalate, and phytate contents in both leafy vegetables and steroidal saponins in nightshade. Stir-fried nightshade leaf extract showed the most effective inhibition against α-glucosidase activity, with an IC50 of 26.4 μg ml−1, which was higher than acarbose, a synthetic compound (positive control; IC50 69.83 μg ml−1). Sensory panelists preferred the stir-fried Chinese cabbage and nightshade leaves, followed by steamed, microwaved, and boiled vegetables.
Highlights
Consumer preference for the intake of fruit and vegetables in the daily diet is increasing, and the World Health Organization [1] recommends a minimum of 400 g of fruit and vegetables, or five portions, per day excluding the starchy tubers
It is evident from the study that the dietary phenolic compounds, antinutrients, and associated bioactivities of Chinese cabbage and nightshade leaves are significantly altered by different household cooking techniques
Among the tested cooking techniques, the recommendation is stir frying for traditional African cuisine to maintain optimal health benefits of consumers
Summary
Consumer preference for the intake of fruit and vegetables in the daily diet is increasing, and the World Health Organization [1] recommends a minimum of 400 g of fruit and vegetables, or five portions, per day excluding the starchy tubers. The United States Department of Agriculture (USDA) [2] guidelines state that an individual must consume one cup (∼237 g) of raw or cooked vegetables or two cups of raw leafy greens. These recommendations help in the prevention of non-communicable diseases and micronutrient deficiencies. Nightshade (Ca 199 mg 100 g−1, Fe 12.8 mg 100 g−1) and Chinese cabbage (Ca 27–31 mg 100 g−1, Fe 0.5–3.5 mg 100 g−1) [5] contain higher levels of Ca and Fe than raw spinach [5, 6] These vegetables are consumed in cooked form. Various cooking methods, such as boiling and steaming, are adopted to improve their palatability and sensory properties (stir frying in hot oil has become a popular cooking method of vegetables due to its convenience, taste preference, and fresh cooking pattern) [7]
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