INFLUENCE OF DRYING TEMPERATURE AND PRETREATMENT METHODS ON THE DRYING CHARACTERISTICS AND QUALITY OF TIGER NUTS (Cyperus esculentus var. sativus)

Abstract

Tiger nut is one among the under-utilized crop due to lack of information on their nutritional potential and benefits. However, when harvested it can deteriorate rapidly due to physiological and temperature changes. This research investigated the influence of drying temperature and pretreatment methods on the drying characteristics and quality of tiger nut (Cyperus esculentus var. sativus). The experiment was conducted at four air drying temperatures i.e. 40⁰C, 50⁰C, 60⁰C and 70⁰C at constant air velocity 1.5 m/s utilizing a convective oven and cabinet dryer. The tiger nut samples were blanched for 2 mins at 100ºC and immediately cooled to 25⁰C, pre-conditioned by soaking for 5mins in 0.2% sodium metabisulphite solution and spread on a tray prior to drying in the oven and cabinet dryer and the open sun. The proximate and mineral composition of tiger nut was determined using the prescribed method by AOAC. The experimental data of moisture ratio of the tiger nut were fitted with seven mathematical models, and the effective moisture diffusivity and activation energy was calculated. The results show that drying air temperature and pretreatments influenced the drying process significantly. Blanching enhanced the drying process of tiger nut while sodium metabisulphite is considered suitable for drying tiger nut as there is no considerable loss of quality. The Midili-Kucuk model showed the best fit to experimental drying data for all pretreatments considered in this research for oven and cabinet dryer with high values of coefficient of determination R² (0.9932 to 0.9987), least value of Chi square X² (0.0001 to 0.0006), root mean square error RMSE (0.0091 to 0.0235) and sum of square error SSE (0.0073 to 0.0242). Where, Logarithmic model has the best fitting model for all pretreatments considered in this research for open sun drying with high values of coefficient of determination R² (0.9975 to 0.9980) and least value of Chi square X² (0.0001 to 0.0002), root mean square RMSE (0.0115 to 0.0137) and sum of square error SSE (0.0098 to 0.0142). The effective moisture diffusivity (𝐷𝑒𝑓𝑓) of the dried tiger nuts varied from 2.55 × 10−9m²/s to 2.86 × 10−7m²/s as the drying air temperature of the thin layer drying increased from 40⁰C to 70⁰C at air velocity 1.5 m/s with sodium metabisulphite treated sample as the highest averages effective moisture diffusivity. The activation energy for moisture diffusion in tiger nut was calculated and was found to be in range of 19.86 kJ/mol to 20.25 kJ/mol. However, Results shows that oven drying temperature of 60⁰C is considered safe for the drying of tiger nuts.