Effect of Thermal Processing Conditions on the Physicochemical and Moisture-Sorption Characteristics of Two Cocoyam (Colocasia esculenta) Varieties

Two cocoyam varieties (Colocasia esculenta) known as taro grown in Cross-River State and Bendel were subjected to different processing conditions (boiling and drying) and the effect of boiling temperature, boiling time and drying temperatures were investigated. They were processed into flour using standard methods, packaged in low-density polyethylene bags and kept in the laboratory for analysis. The results obtained showed that crude fat, crude protein, ash, moisture, crude fibre and carbohydrate contents ranged from 0.42-0.92%, 6.03-9.01%, 2.13-3.90%, 5.20-12.07%, 0.43-0.77% and 73.32-81.73%, respectively. The anti-nutrients contents of the samples showed that oxalate ranged from 0.01-0.99 mg/g, alkaloids (0.12-0.73 mg/g), flavonoids (0.00-0.83 mg/g), phytate (0.01-1.90 mg/g), saponin (0.00-0.83 mg/g) and tannin (0.00-0.01 mg/g). The investigation revealed that there were significant (p<0.05) variations in the functional properties of the cocoyam samples with the water absorption capacity ranging from (1.56-3.01 mg/g), bulk density(0.58-0.82 mg/g), swelling index (1.54-2.91 mg/g), Oil absorption capacity (1.32-1.67 mg/g), while porosity ranged from 0.30-0.76 mg/g. There were also significant (p<0.05) variations in the thermal diffusivity of the samples with the samples of Cross River Cocoyam having higher thermal diffusivity compared to sample of Bendel Cocoyam. The higher thermal diffusivity observed in the cross river cocoyam could be due to the lower moisture content of the samples. The result of the pasting properties showed that the peak viscosity, peak time, final viscosity, breakdown and set back viscosities ranged from 10.88-15.81 N/m2, 7.87-20.87 mins, 8.00-9.97 N/m2, 15.10-17.90 N/m2, 6.0-7.5 N/m2 and 2.3-3.8 N/m2. The research discovered that sample with less pasting temperatures and high peak viscosity had better thickening effect as seen in sample of Cross River Cocoyam. The moisture sorption isotherms had sigmoid-shaped profiles for all of the three temperatures. The hysteresis effect at the three temperatures was distinctly expressed. The increasing temperatures resulted in less hysteresis effect on taro flour which meant the adsorption and desorption curves were closer.