Characterization of water yam (Dioscorea Alata) for existing and potential food products

The yam species, Dioscorea alata has an advantage for sustainable cultivation due to its comparatively good agronomic characteristics. This research was aimed at identifying the major chemical and physical characteristics of D. alata tubers that affect food and industrial processing qualities. Tubers from 20 varieties of D. alata were compared to a reference variety, D. rotundata. The tubers were processed into flour for the determination of quality characteristics. Pounded yam, boiled yam and amala products were also prepared for sensory assessment against the reference. The influences of tuber maturity and length of storage on the quality characteristics were also evaluated, as well as the tuber tissue microstructure. On the average, most of the characteristics evaluated were relatively higher in D. alata varieties as compared to D. rotundata: moisture (72.2%), sugar (5.7%), protein (6.0%) and total dietary fibre (6.9%) contents; higher water binding capacity (163.3%), solubility (11.0%), and amylose (29.4%); breakdown (198.7 RVU), peak time (6.3 min) and pasting temperature (84.2 oC). However, dry matter (27.8%) and starch contents (68.4%), peak, setback, and final viscosities (215.7, 57.3 and 256.0 RVU respectively) were comparatively lower. D. alata varieties equally had appreciable contents of minerals. Based on the physicochemical and chemical properties, D. alata could be processed to flour for use in bakery products and diet formulations. The measured physicochemical properties in conjunction with pasting properties of the D. alata varieties suggest the presence of strong bonding forces within their starch granules. The relatively higher pasting temperatures required, and the lower pasting viscosities make D. alata suitable for producing weaning foods and other products that require low viscosity. Pasting properties of D. alata revealed relatively higher thermal and mechanical stability which will make it a good ingredient for processed foods, especially those that require thermal sterilization. The sensory analyses showed that D. alata varieties have comparative advantage over D. rotundata for amala production. However, specific promising varieties (TDa 98-159, TDa 291, TDa 297 and TDa 93-36) would need further genetic improvement to render them suitable for boiled yam and pounded yam dishes. Microstructural studies of the cooked cells in majority of the D. alata varieties (71%) showed incomplete disruption of the cells and consequently reducing exudation of starch contents. This feature partly accounts for less suitability of D. alata for sticky and doughy yam products in production areas. Moisture content of D. alata tubers decreased significantly from 5 to 9 months after planting (MAP) (74.53 to 71.76%) and throughout the 5 months storage period (71.76 to 68.06%) as a result of dehydration. Sugar and starch contents increased from the 5MAP to 9MAP (4.54 to 4.70%; 65.92% to 72.01% respectively), however, starch content decreased at the end of 5 months storage period (72.01 to 66.55%) with concomitant increase in sugar content (4.70-6.83%). The observations are due to photosynthetic materials accumulation during growth period and breakdown of starch to sugars as a result of respiration during storage. Amylose content generally decreased with the storage period but swelling power increased (6.26-9.02%). Pasting viscosities increased throughout the growth period to the end of storage. The increases in dry matter and sugar contents, swelling power and pasting viscosities of stored D. alata tubers would have significant improvement in their organoleptic and textural properties. Contribution to Science From this study, the following have been established and therefore add onto the scientific knowledge on D. alata yam species: 1. D. alata varieties had higher total dietary fibre than what is reported for brown rice (5%) (Best, 2005) with a few varieties having comparable values to whole wheat flour. The specific varieties with high TDF will be useful in diet formulation because of its health benefits 2. D. alata flour (elubo) is very good for amala product. This product should be promoted in Ghana due to the high flour storability and ease of preparation. 3. Significant mineral variations were observed among D. alata varieties which suggest the potential for improvement through breeding programmes.

Source: KNUST