Abstract
Kiwifruit slices were dried at four different air drying temperatures of 50, 55, 60 and 70ºC and at 2 m s-1 air velocity by using a cabinet dryer in this study. The drying, rehydration and colour characteristics were significantly influenced by pretreatment and drying temperature. The drying time decreased with the increase in drying temperature. The drying rate curves showed that the entire drying process took place in the falling rate period. Five well-known thin-layer models were evaluated for moisture ratios using nonlinear regression analysis. The results of regression analysis indicated that the Midilli & Kucuk model the best to describe the drying behaviour with the lowest c2 and RMSE values, and highest R2 value. The effective moisture diffusivity of the dried kiwifruit slices was calculated with Fick’s diffusion model, in which their values varied from 4.19×10–10 to 6.99×10-10 m2 s-1 over the mentioned temperature range. The dependence of effective diffusivity coefficient on temperature was expressed by an Arrhenius type equation. The calculated values of the activation energy of moisture diffusion were 10.37 and 19.08 kJ mol-1 for citric acid and control samples, respectively
Highlights
The purpose of the present work was to investigate the effect of temperature and citric acid solution on the drying, rehydration and colour characteristics of kiwifruit slices in the cabinet dryer, to fit the experimental data to five thinlayer models, and to calculate the effective moisture diffusivity and activation energy
The difference in drying time was close to 28%. This result shows that citric acid solution contributed to increase the permeability of the cell membranes of kiwifruit slices, leading to an increase in water diffusivity
Drying characteristics of kiwifruit slices were investigated by using a cabinet dryer at various temperatures of 50, 55, 60 and 70°C
Summary
Kiwifruit (Actinidia deliciosa) originates from China, and become a favourite fruit because of its flavours and taste. New Zealand, Chile and Greece are the leading kiwifruit growing countries (Food and Agricultural Organization [FAO], 2017). Kiwifruit is rich in vitamin A, vitamin C, vitamin E, potassium, calcium, iron, magnesium, proteins, minerals and it has strong antioxidant capacity due to a wide number of phytonutrients including carotenoids, lutein, phenolic, flavonoids and chlorophyll (Izli, Izli, & Taşkın, 2017). Kiwifruit can be consumed fresh as well as dried, frozen and processed for juice, jams, jellies, syrups and confectionery. Since kiwifruits soften quickly and lose vitamin even when refrigerated, they tend to have very short shelf-life. To prolong its shelf-life, the water inside the kiwifruit should be removed in order to have a long-term product without a significant change in quality. The most common method for this purpose is drying (Izli et al, 2017)
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