Changes in the thiamine and riboflavin contents of rice during artisanal parboiling in Ghana

Abstract

Rice samples mildly parboiled at various combinations of soaking temperatures and steaming times were analysed for their thiamine and riboflavin contents. The relative parboiling index was calculated and this provided a reliable measure of the overall effects of different conditions on the degree of parboiling. The thiamine content of parboiled rice samples increased gradually as parboiling intensity was increased from initial soaking temperature at 30°C and steaming for 4 min to soaking at 70°C and steaming for 12 min. There was a sharper rise in thiamine when the soaking temperature was increased to 90°C, and the sample that was steamed for 12 min after soaking at 90°C had a thiamine content of 0.612 µg g−1. The level of riboflavin also initially increased with parboiling, but it peaked at 0.278 µg g−1 in the sample that was soaked at 70°C and steamed for 12 min, and then decreased as parboiling intensity was increased further. The commercially parboiled rice sample had a riboflavin content of 0.206 µg g−1. This pattern of change in riboflavin levels with parboiling was probably due to thermal breakdown of the vitamin at higher soaking temperature