Colour change kinetics of pumpkin (Cucurbita moschata) slices during convective air drying and bioactive compounds of the dried products

Abstract

The high contents of bioactive compounds make the pumpkin fruit an important crop for the development of novel functional foods for improving food security, nutrition and health. This study investigated the effect of drying air temperatures (50, 60 and 70 °C) and slice thicknesses (3 and 5 mm) on the drying behaviour, colour change kinetics and bioactive compounds content of the dried pumpkin slices. The effective moisture diffusivity of pumpkin increased from 2.860×10 −10 to 9.815×10 −10 m 2 /s as drying temperature increased while activation energy values ranged between 47.14–51.60 kJ/mol. After the drying process, Lightness ( L* ) and yellowness ( b* ) values of pumpkin decreased from 74.61 ± 1.18 to 56.50–70.15 and 61.95 ± 2.03 to 51.90–56.10, respectively whereas redness ( a* ) and total colour difference ( ΔE ) increased from 8.47 ± 0.09 to 9.98–11.07 and 0 to 10.01–17.12, respectively. Changes in L * and b* values during drying were adequately predicted by the first-order reaction kinetics while a* and ΔE followed zero-order reaction kinetics (R 2 = 0.9765 to 0.9978). The reaction rate constants for colour change greatly depended on the drying air temperature. Contents of β-carotene, ascorbic acid, total phenols, flavonoids and antioxidant activity of the dried pumpkins varied between 43.80–58.15μg.g −1 , 37.62–50.13 mg/100g, 109.60–155.92 mg GAE/100g, 49.68–67.74 mg kaempferol/100g and 61.45–80.72%, respectively. Predominantly, an increase in pumpkin slice thickness prolonged drying time and caused a greater loss of bioactive compounds and antioxidant activity. Moreover, β-carotene and ascorbic acid contents were higher in 60 °C dried pumpkin than those dried at 50 °C and 70 °C. Nonetheless, increasing the drying air temperature increased the total phenolic compounds and flavonoid contents in dried pumpkin products. The study results could be useful for the food industry and further research work. • Colour change kinetics of pumpkin during convective drying at 50–70°C was studied. • Drying air temperature and slice thickness affected colour change rate constants. • Degradation of yellow pigments (b*) of pumpkin followed first-order reaction model. • Reducing pumpkin slice thickness increased drying rate and β-carotene retention. • Increasing drying air temperature improved total phenols content of dried pumpkin.