EFFECT OF PROCESSING METHODS ON THE NUTRITIONAL COMPOSITION AND ANTI- NUTRITIONAL FACTORS OF THREE VARITIES OF BITTER YAM (Dioscorea dumetorum)

Abstract

Three varieties of Dioscorea dumetorum namely the yellow, pale yellow and white with purple layers bitter yam were divided into three portions: first portion was the peeled raw form (control) (RF), second was boiled before peeling (BBP) and while the third was boiled after peeling (BAP). All the samples were dried and evaluated with respect to proximate composition, mineral content, anti-nutrient and functional properties as well as vitamins using standard methods. Organoleptic /sensory evaluation of the differently processed bitter yams was carried out. On the average, the results in (g/100g) of proximate showed that moisture content in raw ranged (3.40 ±0.72 - 7.40 ±0.40), and (8.30 ±0.30 - 11.10±0.30) processed. Crude protein ranged (3.75 ±0.58 - 9.72 ±0.01) both raw and processed. Crude fibre ranged (1.66±0.06 – 4.97±0.16). Crude fat ranged (2.30±0.29 – 7.11±0.50). The ash ranged (3.20 ±0.03 – 3.38±0.21). Carbohydrate was highest both raw and processed ranged (73.18±1.30 – 81.26± 0.24). Gross energy (Kcal/100g) ranged (333.7 – 427). The result of the major minerals (mg/kg) analysed K, Na, P, Mg, and Ca had the highest values (BBP) was higher than (BAP). Among the minor minerals Fe was the most abundant especially in the yellow bitter yam while Cd, Cr; and Cu were the least abundant. Among the Na: K, Ca: P; Mg: Ca ratios calculated, the highest ratio was Mg: Ca ratio ranged 0.95 to 2.34. The least ratio was Na: K ranged 0.04 to 0.06. Vitamins B1, B2, and B3were the least abundant, processing increased their values especially yellow bitter yam ranged 0.04±0.00 to0.37±0.01 in raw, Vitamin A was the most abundant and ranged 320.00±1.00 – 1388.00±6.00mg/g. The anti-nutrients, tannin (10.36±0.02 to 14.53±0). Saponin (1.39±0.03 to 7.29± 0.07), Phytate (10.68±0.02 to 27.45±0.04) and Oxalate ranged (9.44±0.17– 38.90±0.28) they were all significantly (P ≤ 0.05) reduced after processing. The molar ratio of the anti nutrient to the contained minerals showed that phytate may hamper the availability of Ca BAP in yellow bitter yam because the value was greater than the critical values (> 6). The result on ratio of Phy: Fe ranged 0.78 to 0.89 respectively, lower than the standard (1.0). The result on Ox: Ca ratio ranged 1.06 to 3.44 (> 1). Phy: Zn ratio is rich on Zinc except raw and BAP of the white bitter yam with purple layers and raw yellow bitter yam which are greater than critical value (15). The result on {Ca}{Phy}{Zn}millimolar ratio were below 0.5 mol/kg which indicate that the tubers are rich in Zn and Ca.(WAC), (OAC), (FC), (FS), (EC), (BD) and (ES) were significantly P ≤ 0.05 different while the result on the least density (LD) was not significantly P ≥ 0.05 difference both raw and processed bitter yam. The result on sensory evaluation, boiled before peeling was preference to boiled after peeling of bitter yam tubers.