Effects of Drying on Quality and Drying Characteristics of Aerial Yam (Dioscorea Bulbifera)
Abstract
The preference for Dioscorea rotunda and other yams to the aerial yam, by consumers has led to its underutilisation and post-harvest losses. The quality attributes of the aerial yam processed into flour using various processing techniques (boiling, steaming, solar and oven drying) were determined. Drying characteristics of the aerial yam bulbis slabs were determined and fitted to twelve different drying models. Flour yields for the boiled flours were high (32.5%, 31.25%) for solar and oven drying methods. Protein content was high in steamed flour and low in boiled flour for solar drying and high in the fresh flours for the oven drying method. All the processed flours were significantly high in fat content which ranged from 0.96 to 1.40%. Oven dried flours were also high in ash content (0.24-0.51). Results suggested that aerial yam was a good source of calcium, iron, sodium and potassium. Oven drying did not significantly change P, K and Na contents relative to the other methods. The values of water binding capacity were different between boiled solar dried flour (303%) and all oven dried flours (241.05% -235.11%). Solubility was high in the solar dried flours with a range of 15.98% -22.01%. No significant differences occurred between drying methods for swelling power. For the drying methods, solar dried flours had the highest pH values of 5.90-7.25. Although the drying methods and pre-treatment showed no significant differences in the breakdown, final and setback viscosities of the aerial yam flours, pre-treatment of boiling and steaming and the two drying methods resulted in low peak and holding strength viscosities. The aerial yam flour, however, required higher temperature and time to cook with the exception of the boiled solar dried yam flour. Low final viscosity values of 0.50-46.00 BU indicated the inability of the aerial yam flour to form a firm gel after cooking and cooling making it appropriate for infant baby food formulation. Oven dried flours had lower pasting properties in all the pre-treated flour except for the breakdown and setback viscosities. Solar drying, boiling and steaming methods showed slight decreases in the amylose content of the yam flour while oven drying showed slight increases in the amylose content. Oven dried flours showed increases in yellowness whiles solar dried flours exhibited increased redness indicating a high potential for enzymatic browning. High temperatures resulted in shorter drying times, which increased with increasing thickness. Boiling decreased the rate of moisture movement which resulted in increased drying time. Midilli et al., 2002; Verma et al., (Dadali et al., 2007); Diffusion Approach, (Yaldiz and Ertekin, 2001), Wang and Singh, (Demir et al., 2007); Parabolic (Sharma and Prasad, 2001) and Simplified Ficks Diffusion (Babalis et al., 2004) models best described the thin-layer drying characteristics of aerial yam slabs. The effective moisture diffusivity values of aerial yam, varied between 1.401 ×10-10 m2/s to 6.720 x 10-10 m2/s for the fresh samples and 7.223 x 10-11 m2/s and 2.306 x 10-10 m2/s for the boiled samples over the temperature range of 50–70°C. Moisture diffusivity increased with increasing temperature. From the result aerial yam could be utilised in the food industry for baby food formulation and the above selected drying models will facilitate the design and manufacture of a suitable dryer for the aerial yam in the food industry.
Description
A thesis submitted to the Department of Food Science and Technology, College of Science, in partial fulfilment of the requirements for the degree of Master of Science in Food Quality Management,