Search

Colour and texture are important features of amala, a traditional thick paste obtained from dry yam flour. Tubers from eight yam cultivars were blanched at 65 °C for 20 min and dried at 40 °C for 5 days to test biochemical, thermal and pasting changes occurring during traditional yam flour processing. Blanching reduced peroxidase activity and drying reduced polyphenoloxidase activity, but total phenol content and the brown index of flour and of amala increased dramatically during the latter operations. The brown index of amala was significantly correlated with the total phenol content of the flour (r=0.84) and the peroxidase activity of the fresh tubers (r=0.75). Amylose content and starch gelatinization enthalpy remained stable. For all cultivars, drying significantly increased the onset gelatinization temperature, suggesting the occurrence of starch annealing. The latter leads to a reduction in swelling power and solubility during pasting, and hence to a lower paste viscosity.

This study was undertaken to follow the kinetics of polyphenoloxidase (PPO), peroxidase (POD), and phenolic compounds during yam blanching at different temperatures and after drying and to identify by high-performance liquid chromatography (HPLC) the main phenolic compounds present in yam products. PPO activity was 50% higher in nonprocessed freeze-dried Florido (Dioscorea alata) than in nonprocessed freeze-dried Deba (Dioscorea rotundata). It decreased progressively during blanching. Forty-five percent of PPO activity remained after 50 min of blanching at 60 or 65 degrees C, whereas the POD activity dropped sharply to less than 20% of the initial activity after 10 min of blanching, whatever the blanching temperature. No anthocyanidins could be detected by HPLC at 520 nm in nonprocessed freeze-dried yam. Flavanols with a maximum absorption wavelength (lambda(max)) at 280 nm and cinnamic acid compounds with 320 nm lambda(max) were detected. Catechin was identified as the major flavanol with concentrations ranging from 0.26 to 0.41 microM g(-)(1) depending on cultivar. One cinnamic compound, ferulic acid, was identified and assessed in both cultivars (0.03-0.04 microM g(-)(1)). Total phenol, flavanol, and cinnamic contents decreased during blanching independently of temperature whereas some unresolved peaks were detected by HPLC in dried product. The latter was probably due to the consumption of coloring precursors and the appearance of polymerized or complex colored products.