Investigation of the changes in physicochemical properties of dried collard using various drying techniques and modeling of drying kinetics

In this study, the effects of different drying techniques on the physical and chemical attributes of dried collard were investigated. Microwave drying (140, 364 and 511 W), oven drying (105 °C) and vacuum-oven drying (0,4-0,6-0,8 Pa) were used at different drying times. Water activity (aw), moisture loss (%), pH, acidity, total phenolic content, color values and FTIR spectrum analyses were conducted on dried collard leaves. The initial aw and moisture content of the fresh sample were 0,966 and 79,52% w.b., respectively. Minimum aw was 0,445 at 364 W – 4 min of microwave drying, while the highest moisture loss was 89,17% at 511 W – 3,2 min of microwave drying. The initial pH value was 6,40 and the pH values of the dried samples ranged between 5,57-6,80. The fresh samples contained a total phenolic content of 9,056 mg GAE/g and the highest phenolics concentration was determined as 1400,718 mg GAE/g at 511 W – 3,2 min of microwave drying. During drying, the L*, a* and b* values averaged at 18,79, -4,28, 9,64 at microwave drying, 16,35, -3,02, 7,99 in oven drying and 17,12, -3,85, 7,99 in vacuum oven drying, respectively. When compared to the initial color values of the fresh collard, the lowest ΔE* color difference value was 0.47 at oven drying for 32 min, while the highest ΔE* was 8.21 at 140 W – 6 min of microwave drying. The drying data was evaluated via kinetic modeling by obtaining the lowest RMSE and the highest R2 values. The most suitable model for drying of collard in this study was found as Midilli model. The findings of this study emphasizes that the most effective and rapid method of collard drying was microwave drying, while controlled conditions should be applied to prevent the chemical bonds from instant temperature changes. On the other hand, vacuum-oven drying comes into prominence by shortening drying duration compared to oven drying, as well as the vacuum-dried collard is the most close sample to the fresh collard.