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- ItemColour change kinetics of pumpkin (Cucurbita moschata) slices during convective air drying and bioactive compounds of the dried products(Amsterdam : Elsevier, 2022) Chikpah, Solomon Kofi; Korese, Joseph Kudadam; Sturm, Barbara; Hensel, OliverThe 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 m2/s as drying temperature increased while activation energy values ranged between 47.14 and 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 (R2 = 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 and 58.15 μg 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.
- ItemThe Effect of Pre-Drying Treatment and Drying Conditions on Quality and Energy Consumption of Hot Air-Dried Celeriac Slices: Optimisation(Basel : MDPI, 2021-7-29) Nurkhoeriyati, Tina; Kulig, Boris; Sturm, Barbara; Hensel, OliverCeleriac is a good source of fibre, trace minerals, and phenolic compounds; it has a pleasant aroma but is a perishable material, prone to discolouration. This research investigated the optimisation of the quality and energy demand in hot-air dried celeriac slices. The experiment utilised the I-optimal design of response surface methodology with 30 experiment runs. Pre-drying treatments (blanching at 85 °C, three minutes; dipping in 1% citric acid solution, three minutes; no pre-drying treatment), drying temperatures (50, 60, and 70 °C), air velocities (1.5, 2.2, and 2.9 m/s), and thickness (three-, five, and seven-mm) were applied. The drying conditions affected drying time significantly (p < 0.0001). The model by Midilli and others and the logarithmic model fitted best with celeriac slices drying kinetics. Blanched samples had a higher ΔE*ab (total colour difference) and BI (browning index) but lower WI (whiteness index) than samples with other pre-drying treatments. The rehydration ratio decreased with the increase of sample thickness and blanching (p < 0.0001). A quadratic model described the specific energy consumption (Es) best. The dried samples compared with fresh samples had increased antioxidant activity but decreased total phenolic compound value. The optimisation solution chosen was 58 °C drying temperature, 2.9 m/s air velocity, and 4.6 mm sample thickness with acid pre-drying treatment.