Evaluation of ascorbic acid and sodium metabisulphite as inhibitors of browning in yam (D. rotundata) flour processing

Abstract

The inhibition of polyphenol oxidase (PPO) and browning by ascorbic acid and sodium metabisulphite in the processing of yam flour (elubo) was investigated. The physicochemical properties and organoleptic score of flour meal (amala) samples were also evaluated using standard methods of analysis. The yam samples with or without inhibitors were steeped in water for 12hrs at 300C, 400C and 500C. Polyphenol oxidase activity and browning index were monitored at 2hrs interval. The study revealed that browning is temperature-dependent in all the yam samples. Sodium metabisulphite completely inhibited browning in the processed yam flour. Polyphenol oxidase activity and browning index were positively (53.2%) correlated. Browning and PPO activity, temperature and steeping time were also negatively correlated. However, a significant increase (p<0.05) in the calorific energy of yam sample with sodium metabisulphite was observed. The organoleptic score of amala produced from test samples revealed that there was no significant difference (p>0.05) in the flavour, taste and texture of amala made from yam samples processed with or without inhibitors. However, significant difference (p<0.05) in colour exist among the amala samples. Amala made from yam samples processed with sodium metabisulphite was white at all processing temperatures and time compared with the control samples. Those made from yam samples processed with ascorbic acid was darker compared to the control. This study thus, revealed that PPO and browning were effectively inhibited in processed yam flour by sodium metabisulphite as shown by white amala obtained after treatment with the inhibitor. Ascorbic acid did not inhibit PPO in yam flour. The work concludes that sodium metabisulphite completely inhibits PPO and produce amala that is white, and its acceptability was comparable to the normal dark amala produced by conventional process. Keywords: browning, physicochemical, polyphenol oxidase, yam, solubility, inhibition