Effect of tuber storage and pre- and post-blanching treatments on the physicochemical and pasting properties of dry yam flour

In West Africa, yams are often processed into dry flour that is consumed as a thick paste (amala), the main quality attributes of which are colour, texture and taste. The traditional process consists of peeling, blanching and drying the fresh tubers. The fresh tubers may also be stored for several months before processing. This study was undertaken to test whether fresh tuber storage and pre- and post-blanching treatments (1-h storage of sliced fresh yam pieces in air or water and steeping in the blanching water, respectively) affect the physicochemical characteristics and pasting behaviour of dry yam flour produced from D. rotundata cultivars. Both pre-blanching treatments significantly reduced only peroxidase activity, while the post-blanching treatment significantly reduced polyphenoloxidase activity. Despite the effects on enzymatic activities, neither pre- nor post-blanching treatments had any significant effect on the amala brown index or the flour total phenol content. However, peak areas of three major cinnamic acid compounds, detected by HPLC, decreased continuously with steeping time after blanching, while fresh tuber storage led to a dramatic reduction in amala paste viscosity.