Effect of various cooking methods on the proximate composition and nutrient contents of different rice varieties grown in Nigeria

The study investigated the effect of various cooking methods on the proximate composition and nutrient contents of some varieties of rice grown in Nigeria, using the methods of Association of Analytical Chemists (AOAC methods). Atomic Absorption Spectrophotometer and High Performance Liquid Chromatography were used to analyzed minerals and vitamins. The samples were cooked using microwave, pressure pot and parboiled methods and compared with the conventional method. The results showed that moisture, ash, fat, protein and carbohydrate ranged from 8.8 to 14.1, 0.88 to 1.67, 2.9 to 5.9, 6.1 to 6.6 and 72.6 to 78.2 %, respectively, and energy content from 1489 to 1636 kJ/100 g. Zinc, iron, copper and calcium ranged from 2.8 to 4.5, 4.2 to 10.6, 0.2 to 1.9 and 28.6 to 47.6 mg/100 g, correspondingly. Ascorbic acid contents (mg/kg) of raw samples ranged from 2.6 to 10.6, pyridoxine 306 to 534, thiamine 113 to 215, and folate from 62 to 88pg/kg. The mean percentage retention of ascorbic acid ranged from 17.5 to 49.8%, pyridoxine (44.7 to 93.2%), thiamine (54.5 to 89.5%) and folate (55.16 to 91.1%), conventional cooking recorded significantly higher (P< 0.05) retention values for pyridoxine, thiamine and folate whereas microwave and pressure cooking lead to substantial loss of all the vitamins. The study revealed that although cooking methods negatively impaired vitamin content of rice, however, the loss was more with microwave and pressure cooking which implied that these methods of cooking should be regulated in order to optimise nutrient intake from rice.