Investigation of ultrasound pretreatment time and microwave power level on drying and rehydration kinetics of green olives

Olive (Olea Europea) is one of the most consumed fruits in Mediterranean countries with its superior properties such as high concentration of vitamin E and phenolic compounds including oleuropein, tyrosol and hydroxytyrosol (Aydar et al., 2017a). It is evaluated and consumed mostly as table olive and olive oil after harvesting. Fresh olives are high in nutrients but can be easily spoiled due to high water activity until processing (Kailis, 2016).


Introduction
Olive (Olea Europea) is one of the most consumed fruits in Mediterranean countries with its superior properties such as high concentration of vitamin E and phenolic compounds including oleuropein, tyrosol and hydroxytyrosol (Aydar et al., 2017a). It is evaluated and consumed mostly as table olive and olive oil after harvesting. Fresh olives are high in nutrients but can be easily spoiled due to high water activity until processing (Kailis, 2016).
Drying is a widely applied method which aims reducing water activity (aw) of foods to storage for long-term as well as to lower the weight of food and volume to reduce shipping costs (Antal et al., 2017). Hot air drying is one of the commonly used methods for drying of fruits and vegetables, but, it has many disadvantages, such as long drying time, loss in nutritional content, undesirable product deterioration and low energy efficiency (Horuz et al., 2017). Freeze drying obtains high quality products, however it associates with high energy consumption and long drying time. Microwave drying or radio frequency are also used by many researchers to replace conventional drying and shortened the drying time and improved product quality, but they still have drawbacks over conventional hot air drying such as their non-uniform heating characteristics and unstable temperature control (Rodríguez et al., 2007;Tang et al., 2005;Yildiz & İzli, 2019). Therefore it is necessary to develop new combine drying methods or pretreatments to attain improved drying process and enhanced product quality (Rawson et al., 2011).
Ultrasound is a novel technology has been used in many food processing and applications such as dairy and beverage technology (Ahmad et al., 2019;Guimarães et al., 2018Guimarães et al., , 2019a, oil extraction (Aydar et al., 2017b;Jiménez et al., 2007) and as a pretreatment in drying of foods (Huang et al., 2019). Ultrasound pretreatment accelerates the mass transfer in drying mainly due to breakdown of cells and formation of micro channels (Sun, 2014). In the last decade, ultrasound has been applied as a pretreatment in hot air drying of many fruits and vegetables such as tomato (Horuz et al., 2017), kiwifruit (Wang et al., 2019), okra (Sunil et al., 2017), garlic (Bozkir et al., 2018), apple (Fijalkowska et al., 2016) and mushroom (Zhang et al., 2016). However there is no study investigated the combined effect of ultrasound and microwave drying on quality characteristics and drying parameters of green olive slices. Therefore, in this study different minutes of sonication was applied as a pretreatment to the microwave drying of olive slices to describe the effect of ultrasound process on enhancement of the microwave drying and rehydration process.

Olive samples
Fresh olives (Domat variety) were acquired in a local olive company (Aydar Inc, Akhisar, Manisa) and processed at the same day. In order to confirm the sample uniformity, olives which has 20 mm ± 2 of diameter were chosen for this study and the olives were cut in slices of 5 mm thickness.

Ultrasound treatment
The green olive slices were put in a 250-mL glass beaker. The distilled water was used as the medium and the ratio of olive slices to water was 1:5 (w/w). The 5 and 10 minutes of ultrasound were applied to samples using ultrasonic bath (AlexMachine, PR-6711, Turkey, 150 kW and ultrasonic frequency 25 KHz; Tank volume 4.5 L) in this study. An infrared thermometer (Benetech, GM300, China) was used to measure the surface temperature of the olive slices and ultrasonic bath for 1 minute intervals during sonication process. The temperature inside the bath and surface of olive slices did not exceed to 25 °C during sonication.

Microwave drying
A microwave oven (GE83X, Samsung, Turkey, 2450 MHz and 23 L capacity) was used for drying of green olive slices. 5 g of sliced olives were weighed and put on a glass drying tray (10 cm diameter). Then they were left to dry at 180, 450, and 800 W microwave power levels. The weight of olive slices were recorded in every 1 min during microwave drying until the final moisture content of samples reached approximately %10 (w/w). The experiments were performed in triplicate.

Mathematical fitting
Both drying and rehydration kinetics of all samples, with the different treatments, were evaluated using the appropriate mathematical models. In order to determine the best model for describing the drying kinetics behavior, four empirical mathematical models (Table 1) were evaluated. These were selected considering its simplicity and expressive use in the literature (Wang et al., 2019). Where M t is the moisture at a specific time (w/w), M o is the initial moisture content (w/w), M e is the equilibrium moisture content (w/w), When moisture at a specific time and initial moisture content are compared with equilibrium moisture content, M e value approaches to a very small number. Therefore, MR is simplified by many researchers and MR of green table olive slices were calculated as this simplified Equation 2 (Midilli et al., 2002): Drying rates of different drying processes were computed according to Equation 3 (Akpinar et al., 2003): Where t 1 and t 2 are the different drying times and M t1 and M t2 are the moisture contents of olive slices at time t 1 and t 2 , respectively.
Henderson and Pabis, Logarithmic, Wang and Sing, and Diffusion models have been applied to describe the changes in moisture content and physicochemical degradation of olive slices during drying are shown Table 2 (Aregbesola et al., 2015;Simal et al., 2005;Wang et al., 2019). A graph of moisture ratio (MR) and drying rates (DR) against time (t) at the different microwave power levels and ultrasound times was plotted. Drying data of the green olive slices were fitted to four thin drying models (Table 2). SAS 9.4. (SAS Institute Inc., Cary, NC) was used to perform regression analyses. To identify the goodness of fit coefficient of determination (R 2 ), χ2 (reduced chi-square parameter) and root mean square error (RMSE) between the predicted and experimental values were applied (Akpinar et al. 2003).
The rehydration kinetics of olive slices dried at different microwave power levels were investigated by Peleg's model which is calculated with the Equation 4: Where X 0 is initial moisture content (g water/g dry matter) and X is the moisture content at time t, t is time, K 1 is the Peleg rate constant, and K 2 is the Peleg capacity constant. When process is an absorption/adsorption ± turns '+' if the process is drying/desorption, ± turnes to '-' (Planinić et al., 2005). Root mean square error (RMSE), reduced chi-squared value (χ2) and coefficient of determination (R 2 ) were calculated to evaluate the goodness of model fitting.

Effective diffusivity
Effective diffusivity of olive slices was calculated by using the Fick's second diffusion law for slab geometry which is shown in Equation 5: Where D eff is the effective moisture diffusivity (m 2 /s), M is the moisture content (dry basis), t is the time(s) (Yağcıoğlu et al., 2014). Fick's second diffusion law presumes that moisture removal is caused by diffusion, temperature, shrinkage and constant diffusion coefficients (Aregbesola et al., 2015;Sun, 2014) Where: ΔL*, Δa*, and Δb* are the differences of these values between the control sample and samples after drying treatment

Statistical analysis
The data was shown as means ± standard deviation (SD). SAS 9.4. (SAS Institute Inc., Cary, NC, USA) was used to determine the effect of the microwave power level and ultrasound time on the qualitative parameters of olive samples by one-way ANOVA. Tukey's honestly significant differences (HSD) test (α = 0.05) was applied as post-hoc test. Coefficient of determination (R 2 ), χ2 (reduced chi-square parameter) and root mean square error (RMSE) were calculated to interpret the adequacy of each model. Figure 1 shows the moisture rates of green olives during drying at different microwave power levels and ultrasound pretreatment times. The total drying time for olive slices was 849 second at microwave level of 180W and when 10 minutes of sonication applied before drying at 800W microwave level it was reduced to 488 seconds. This demonstrates that increasing of microwave level and ultrasound exposure time decreases the total drying time up to % 42.5. Horuz et al. (2017) also observed that application of ultrasound pre-treatment reduced the drying time of tomato slices by 7.38% when they were dried at 120 W microwave power. It was also determined by the researcher that ultrasound pretreatment caused shorter drying times (Bozkir et al., 2018;Rodrigues & Fernandes, 2007;Seidi Damyeh et al., 2016

Drying kinetics
Where t is time (s), L is half thickness of samples (m) and n is a positive integer. For longer drying times, Equation 6 can be converted to a further formula shown in Equation 7 consisting of only the first set of terms without significant influence on the correctness of the supposition.
From Equation 8, moisture effective diffusivity can be determined by plotting ln(MR) versus drying time (t); which provides a linear line and the slope of this line is explained as Equation 9:

Total Phenolic Content (TPC)
Total phenolic compounds of green olive slices were determined using Folin-Ciocalteau method (İçier et al., 2015). According to this method, 2 gr of olive slices extracted in a 50 mL of methanol solution (80:20), then extract was filtered. 50 µL of filtered extract was reacted by 250 µL Folin-Ciocalteau reagent for 5 minutes. After 750 µL of Na 2 CO 3 was added the volume was completed with 3.95 ml of distilled water. Final solution was kept at 2 h in dark and the absorbance value was measured using an UV-VİS spectrophotometer at 760 nm.

Color parameters
The color parameters of green olive slices was measured with a Chroma Meter (Konica Minolta, CR 300 Model, VA) based on 5 color coordinates (L*, a*, b*, C, h°). After the calibration  Table 4 demonstrates the effective moisture diffusivity (D eff ) values for each treatment. Among all samples, Deff value observed highest at 10 min ultrasound pretreated 800 W microwave drying (2.21 × 10 −8 m 2 /s) and lowest at 180 W microwave drying (9.13 × 10 −9 m 2 /s).

Effective diffusivities and rehydration kinetics
A higher D eff value demonstrated that the moisture removal rate in the green olive slices was greater, which would reduce the drying time to obtain the final moisture content. It was observed that the D eff values increased with increase in both with ultrasound pretreatment time and microwave power The results of nonlinear analysis of the fitting of four selected models to the rying data of green olive slices at the different ultrasound pretreatment times (5 and 10 min.) and microwave power levels (180, 450 and 800 W) are shown in Table 3. According to the evaluation criteria (R 2 , χ2 and RMSE), most of models well fitted with the thin layer drying characteristics (R 2 >0.9746, RMSE<0.0773 and χ2<0.0065) of olive slices which are subjected to 5 minute of ultrasound before being dried at 180 W power level in microwave. Wang & Sing and Midilli et al., were the best fitted models in describing the thin layer drying characteristics of olive slices which are not pretreated with ultrasound with the lowest RMSE and X 2 values and highest R 2 values. 0.9767, 0.9954 and 0.9999 were found the highest R 2 values for 180W, 450W and 800 W microwave dried olive samples, respectively.  . In ultrasound  Table 4. It was found that both k1 and k2 values increased as ultrasound time and microwave power have risen. Horuz et al. (2017) observed that k1 value of dried tomato slices increases and k2 value of dried tomato slices decreases as ultrasound time increases. Differences in rehydration behavior in this study may result from being studied at higher microwave power levels and lower ultrasound times.

Color parameters and total phenolic content
L* (lightness) values of olive slices dried at 180 W, 450 W and 800 W power level were not significantly different from control sample however they were lower than control sample in all 3 treatment which can be explained by non-enzymatic oxidation reaction. However, the decrease in L* value was not observed in most samples which ultrasound applied. In most of these drying conditions which are pretreated by ultrasound, L* value was higher than control except the 5 and 10 minute ultrasound pretreated microwave dried olive samples at 180W power . Bozkır et al., also studied the ultrasound pretreatment effect on quality and drying parameters found that lightness was lowest at microwave dried samples and highest at hot air dried samples (Bozkir et al., 2018).
level. Since, high microwave power levels cause an increase in the water molecule activity at elevated drying temperatures, moisture diffusion increases in samples.
Highest drying rates were observed in microwave drying at 800 W power level when combined with sonication 10 and 5 minutes, respectively. The drying rate of ultrasound non-treated samples was lower in comparison to pretreated samples and effective diffusion coefficients were increased as ultrasound time increased at same power level. Figure 2 demonstrates rehydration curves of olive slices dried at different microwave powers combined by ultrasound. As it can be seen in this figure, the when microwave power increased in drying procedure the rehydration ratio decreased during time.
The smaller values for k2 values of rehydration demonstrates better water absorption properties was confirmed for olives dried at lower microwave power levels. It was also resulted that increasing microwave power up to 800 W and ultrasound induced lower rehydration capacity of dried samples. Although ultrasound pretreatment forms micro channels which promotes a faster dehydration, microwave drying cause irreversible cell rupture in fruit tissue that reduces the water absorption at higher microwave power levels. Rehydration kinetics of olive slices calculated by Peleg's model were fitted in all drying treatments

Conclusions
This study concluded that ultrasound application as a pretreatment for microwave drying of green olive slices is a feasible method which reduced the drying time and enhanced the quality of product. Effective diffusion coefficients of 10 min ultrasound pretreatments were higher than those 5 min ultrasound pretreated and non-ultrasound treated samples. Rehydration ratios were higher for with and without ultrasound-pretreated olive slices dried at 180W microwave power. Midilli et al. and Wang & Singh models well explained the drying characteristics of MW dried samples (R 2 >0.9767), on the other hand Diffusion, Henderson & Pabis and Midilli et al. models described US-MW combined drying process more successfully (R 2 >0.9746). Rehydration ratios were higher for with and without ultrasound-pretreated olive slices dried at 180W microwave power than samples dried at 450 and 800 W microwave power. US-MW drying obtained acceptable olive quality with high lightness, and phenolic compounds. It was determined that ultrasound was a promising pretreatment in microwave drying method for green olive slices.
Lowest a* (redness-greenness) value observed in 5 minute ultrasound pretreated microwave dried olive samples at 450W power among all treatments. a* values of microwave dried samples were not significantly different (p > 0.05). It was also found that ultrasound pretreatment did not make significant differences in b* (yellowness-blueness) and c* values of all samples (p > 0.05). While total color difference (ΔE) values of ultrasound treated olive slices were between 1.26-18.23, it was 1.32-9.22 for non treated samples. Horuz et al. (2017) reported that ΔE values of all microwave drying conditions combined by ultrasound was not higher than 10 in tomato samples. It was found that the color of olive slices differed significantly in ultrasound pretreated olives dried at 450 W compared to control sample. The changes in L* value of olive slices were more clearly observed than in Hue angle (Table 5).
Total phenolic contents of green table olive slices reduced significantly in all microwave and ultrasound pretreated drying treatments compared to control sample (p < 0.05). The decrease in phenolic compounds might be related to deterioration of these compounds by electromagnetic waves of the microwave. In addition, microwave power induced to the internal temperature of food to rise due to the friction of the water molecules. Thus, thermal degradation and irreversible oxidative reactions caused phenolic compounds to be damaged during drying. Total phenolic compounds of olive slices obtained by ultrasound pretreatment were slightly higher when compared to those samples were not ultrasound pretreated. The degradation could be due to sonochemical and oxidation reactions, increased interaction with free radicals during sonication. It can be seen from Table 4 ultrasound pretreated samples preserved phenolics better compared to non-treated samples at each microwave power levels. Highest loss in phenolic compounds were observed when samples were dried at 800 W microwave power level. When olive slices were dried at 800 W microwave power, total phenolic compounds were decreased from 92.06 ± 1.17 mg GA/100g olive to 46.56 ± 2.84 mg GA/100g olive. However when samples subjected to 5 and 10 minutes of sonication before microwave drying at 800 W, the phenolic contents were 56.14 ± 2.81 mg GA/100g olive and 49.63 ± 2.27 mg GA/ 100g olive, respectively. Deterioration of phenolic compounds was also lower in ultrasound pretreated olives dried at 450 W and 180 W microwave levels when compared to non pretreated olives dried at same power levels.