Effect of cooking methods on the nutritional quality of selected vegetables at Sylhet City

The study was carried out to analyze the impacts of boiling, steaming, and microwave cooking on the physicochemical properties, the content of bioactive compounds, and boiling effect on mineral and heavy metal content of six widely consumed vegetables in Bangladesh's north-eastern region. In comparison to raw, boiled, and microwave-cooked vegetables, those that are steam-cooked retain a higher percentage of β-carotene with the exception of carrots. Boiling vegetables led to the most substantial reduction in ascorbic acid content (from 9.83 % to 70.88 %), with spinach experiencing the greatest decline. In contrast, microwaving had the mildest effect on ascorbic acid, preserving over 90 % of the initial content. The decrease in carotene content may be associated with color changes (decreasing greenness and increasing hue angle) in the chosen vegetables. The colorimeter shows the L* value (lightness/darkness) of all cooked vegetables significantly decreased. In terms of total polyphenol content (TPC) and total flavonoid content (TFC), boiling had a higher negative effect on most vegetables than the other two cooking methods, with losses of up to 70.3 % and 82.27 %, respectively. All cooked vegetables, with the exception of carrot and microwave pumpkin, had substantial reductions in free radicals scavenging activity, with losses ranging from 8.48 % to 56.73 %. In comparison to raw vegetables, boiled vegetables significantly lost minerals like potassium (K), magnesium (Mg), zinc (Zn), copper (Cu), and manganese (Mn). On the other hand, the calcium (Ca) and iron (Fe) content of all cooked vegetables, except for carrots and peas, exhibited an increase, ranging from 6 to 17 % and 6–12 %, respectively. The Cr concentration in all vegetables and the Zn, Fe, Mn, and Cd content in the spinach sample was higher than the FAO/WHO recommended maximum permissible level (MPL), whereas the accumulation of Cu and Ni content was lower in all vegetables. In conclusion, this study demonstrated that microwaving was the most effective method for retaining the nutritional value of vegetables, while steaming had a moderate impact.