Abstract
Abstract For the first time, the impact of ultrasound-ethanol immersion as a non-thermal pretreatment (NTPT) and coating-blanching in starch solutions as a thermal pretreatment (TP) on the convective drying of beetroot was evaluated. The beetroot was exposed to ethanol immersion (E), ultrasound (U), and ultrasound-ethanol immersion (UE). Besides, TP pretreatment was performed by blanching the beetroot at steam (SB), water (WB), starch-coating solutions, and microwave (M). The hot air drying was conducted at 90 °C and air velocity of 1.2 m/s. The maximum decreases in the drying time were observed at UE30 (64.29 %) and the sample blanched at native corn starch solution (60.17 %). Moisture diffusion coefficients ranged from 0.851 to 2.312 × 10−9 m2/s. The friction drag force, convective heat, and mass transfer coefficients were 2.840 × 10−6 N, 59.368 W/m2 K, and 0.0492 m/s, respectively. The thermal conductivity, specific heat, and density ranged from 0.464 to 0.615 W/m. K, 3164–4071 J/kg. K, and 798.9 to 1055.9 kg/m3, respectively. The maximum values of rehydration ratio at non-thermal (NTPT) and thermal pretreatments (TP) were observed at U30 and the sample blanched at the modified starch solution (MCS), respectively. The total phenolic contents of the NTP sample decreased while those for the TP samples increased due to boosted polyphenol synthesis at high temperatures. Both U and E samples caused a decrease in the total antioxidant activity, while they increased the anthocyanin content of beetroot samples. Pretreatments reduced the hardness owing to changes in the microstructure of the sample. U, E, and M pretreatments increased the brightness of samples, and the minimum color change compared with control samples was observed by UE30 pretreatment. The UE and blanching at a starch solution could be selected for improving the drying characteristics of beetroots at an industrial scale.
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