Effects of three conventional cooking methods on vitamin C, tannin, myo-inositol phosphates contents in selected Thai vegetables

Vegetables commonly consumed in Thailand were analyzed for their vitamin C, tannin and total phytate (inositol penta- (IP5) and hexasphosphate (IP6)) contents. Three conventional household cooking methods, namely blanching, boiling and stir-frying, were used to evaluate the effects of cooking. IP5 and IP6 content were determined using ion-pair reverse-phase chromatography. Vitamin C and tannin content were analyzed spectrophotometrically. Vitamin C content for raw and cooked vegetables ranged from 0.5–83.6 to 0.2–70.8 mg/100 g, respectively. Stir-fried pagwanpa (Melientha suavis Pierre.), pagwanban (Sauropus andogynus (L) Merr.) and cowslip creeper flower (Telosma minor Craib) were excellent sources of vitamin C (64.4–70.8 mg/100 g). High tannin content was found in lead tree (“Yod-kratin”, Acacia farnesiana Willd.; 1353 and 679 mg/100 g tannic acid equivalent for raw and blanched sample, respectively), while neem tree (Azdirachta indica A. Juss) contained high phytate (52 and 38 mg/100 g for raw and blanched sample, respectively). Blanching, stir-frying and boiling caused a decrease in the total vitamin C, with losses from 14% to 95%, the greatest loss being found in boiled bitter cucumber (Monordica charantia Linn.) (95%), whereas retention of total phytate and tannin was around 58–79% for phytate and 44–93% for tannin. Although conventional boiling method was an effective method to reduce tannin and phytate content in vegetables, it also reduced the content of vitamin C.