## 3 Results and Discussion
### Properties of Convective and Infrared Dried Terephthal
#### 3.2.1 Effect of Pretreatments (US, BL, MW)
Bars show the differences between the sample with six replicates and the different letters differ significantly from each other at (\(p<0.05\)) as determined by the LSD test. Figure 2: Effect of different pretreatments (Untr: control, US: ultrasonic, BL: blanching, MW: microwave) and different conditions of drying methods temperature (50, 60 and 70 \({}^{\circ}\)C) for CV drying and infrared power (250, 500, and 750 W) on the total color difference of dried tereinth. Bars show the differences between the sample with three replicates and the different letters differ significantly from each other at (\(p<0.05\)) as determined by the LSD test. The highest rehydration rate occurred for samples treated with the US, regardless of methods of drying and its parameters. Based on the study by [PERSON] et al. [13], US pretreatment can enlarge the capillaries and loosen the internal structure of the product through cavitation and mechanical effects. Using US results in higher porosity and larger cell of the plant materials [45; 46]. This phenomenon is useful to improve the water absorption of the samples. [PERSON] et al. [6] also found that the use of US in drying while cabbage can improve the rehydration rate of the samples which can be due to the microstructural alterations by the application of ultrasound waves. [PERSON] et al. [18] also proved that a pretreatment before the drying process can accelerate the rehydration rate process in dried apples. They also expressed that the use of ultrasound pretreatment can lead to higher rehydration rates. Similarly, studies of [PERSON] et al. [26] confirmed that longer rehydration time of ultrasound treated and CV dried apples results in a higher rehydration rate. Moreover, it can be observed that the lowest rehydration rate occurred upon using MW pretreatment as the treebinth structure was relatively destroyed. This was linked with the irreversible physical and chemical changes and cellular deformation, which hindered the rehydration rate compared to the other two pretreatments (BL and US). A comparison of the results indicated that the lowest rehydration rate (1.75) occurred in the infrared method at a power of 250 W with no pretreatment. Thus the main factors in reducing the rehydration rate in this method are shrinkage, loss of cells, and long drying time. 2.3 Effect of Pretreatments (US, BL, MW) Applied before Convective and Infrared Drying on pH of Terephthal  
According to the results of Table 4, the highest pH value (3.96) was observed at 70 \({}^{\circ}\)C without pretreatment whereas the lowest pH (3.77) was obtained in microwave pretreatment and infrared drying at the power of 750 W. Some researchers attribute the higher acidity in dried samples at higher temperatures to less drying time and therefore less involvement of organic acids in the respiration process [47]. 2.4 Effect of Pretreatments (US, BL, MW) Applied before Convective and Infrared Drying on Total Phenol (TPC) and Flavonoid Contents (TFC) of Terephthal  
According to Table 4, among different drying methods and pretreatments, the convective drying at 60 \({}^{\circ}\)C with ultrasonic pretreatment led to the highest phenolic and flavonoid contents (153.35 mg GAE/g d.m. and 49.24 mg QE/g d.m., respectively) while the lowest phenolic and flavonoid contents (78.376 mg GAE/g d.m. and 49.24 mg QE/g d.m, respectively) were observed in samples dried by IR dry at a power of 250 W and without pretreatment. One of the most important factors influencing the variations of total phenol and flavonoid in convective drying could be the activation of enzymes such as polyphenol oxidase at 60 \({}^{\circ}\)C, which caused the browning reaction. Drying at high temperatures also destroyed the enzymes faster [48]. In the study of [PERSON] et al. [49], the antioxidant activity of roses with low frequency ultrasonic was higher than that of roses dried by high-frequency ultrasonic pretreatment. Thermal pretreatments, especially at high temperatures, reduced the phenolic content and quality of roses.