Effect of pretreatments and solar tunnel dryer zones on drying characteristics and quality of pumpkin (Cucurbita maxima) pulp slice and powder

Abstract

The objective of this study was to investigate the effects of pre-drying treatments and solar tunnel dryer zones on drying characteristics and quality parameters of pumpkin pulp powder. Pumpkin pulp slices (2 mm, thickness) were pretreated with 1% citric acid solution,2% salt solution, blanched at 650C water containing 1% salt solution for and untreated, then dried at three zones of solar tunnel dryer with average temperature ,45.64, 54.62 and 64.970C for zone I, zone II and zone III respectively. As quality parameters, physical, functional, chemical and nutritional parameters of dried pumpkin were studied. Pumpkin slice dried at zone III achieved high drying rate and short time. In addition, salt blanched and citric acid treated also increase moisture removal. Effective moisture diffusivity at zone I, control (7.63*10-9 m 2 /s) was found to be lower than that of zone III, citric acid treated (2.34*10-8 m 2 /s). Among the seven thin layer drying models tested, the diffusion approximation, was found best fitting whereas the logarithmic, two term exponential and Henderson and Babis were found to be suitable models to describe the drying behavior of pumpkin slice. The highest bulk density value was recorded at zone III for salt blanched sample and lowest is at zone II for control,0.78±0.02 (g/ml) and 0.52±0.006(g/ml), respectively. Pumpkin which dried at zone III with salt blanched treatment exhibited high values of water hydration capacity, shrinkage ratio, rehydration ratio and low water activity. Chemical parameters such as; titrable acidity, pH and total soluble solid was found between ;2.7±0.01%-1.1±0.003%, 6.1±0.01-4.6±0.007, 11.3±0.04-7.6±0.3 respectively. Some proximate composition of the pumpkin powder values recorded as follow 8.2±0.03 to 6.4±0.04% moisture content, 6.2±0.006 to4.7±0.02% total ash 4.2±0.05 to 1.1±0.03% fat and 4.2±0.05 to 2.4±0.02% protein. The highest total phenolic content, beta carotene, Ascorbic acid and antioxidant activity were recorded for sample dried at zone II and treated with 2% salt solution. Overall, Pumpkin dried at zone III combined and salt blanched, experienced high moisture removal, short drying time and high effective moisture diffusivity that lead to high shrinkage ratio, rehydration ratio, bulk density and water absorption capacity where only high shrinkage ratio is undesirable. Whereas, Pumpkin dried at zone II and salt treated exhibited better retention of bioactive components.