Abstract-- In this study, carrot samples with different slice thicknesses (2 and 4 mm) were dried by applying ultrasound pre-treatment at different time periods (0, 20 and 40 min) and the changes in colour, rehydration, pH and °Brix values were evaluated. Besides, microstructures of dried samples under different conditions were observed through scanning electron microscopy images. Drying was carried out in a modified oven at temperatures of 60 and 70 °C with an air velocity of 1 m/s. When compared to fresh potatoes, a decrease in yellowing values (b*) was observed for the different drying conditions employed. Carrots samples with a thickness of 2 mm, dried at 60 °C and treated with ultrasound for 40 min, presented the highest rehydration ratio (6.930). It was observed that pH and °Brix values changed from 6.705 to 6.120 and from 68.70 to 6.80, respectively. It was observed that the longer the duration of pre-treatment with ultra-sound, the greater the change in the structure of the product. The results showed that the pre-treatment by ultrasound can be used as an alternative method for drying carrot sample.
Keywords-- Rehydration, colour, microstructure, ultrasound technology.
I. INTRODUCTION
Food drying can be defined as a preservation method that reduces the moisture content and water activity, inhibiting the growth of microorganisms and the development of enzymatic and other reactions (Doymaz, 2017). Convective drying is one of the most used drying methods in the industry. The conventional method of convective drying consists of a heater to generate hot air and a protected fan to distribute the hot air. Simple structure and easy control are essential advantages of the convective drying method. However, there are some disadvantages associated with this technique, such as low processing rate, significant losses in product quality, and high energy consumption due to the use of high temperatures. The use of lower drying temperatures can reduce energy consumption and improve the quality of the dry product, but it increases processing time, causing an increase in drying costs (Oliveira et al., 2016). Thus, pre-treatment of samples has become an alternative to reduce processing time and costs. Many studies on pre-treatment have been reported in the literature. According to Deng et al. (2019), such studies can be divided into two groups: physical pre-treatment and chemical pre-treatment. Physical ultrasound pre-treatment was used in this study. During this application, pressure changes have occurred in the form of contraction and relaxation of the product. This effect is called the sponge effect. Due to the effect of contractions, the microchannel is formed in the product tissues and then these microchannel help discharging of water in the structure. With the accelerated drying process, the samples are exposed to heat for less time and thus, the characteristics and quality of the sample are preserved (Yang et al., 2018). In recent years, the effect of ultrasound pre-treatment on the drying of different agricultural samples such as banana, papaya, melon, apple, barley grass has been investigated by researchers (Azoubel et al., 2010; Azoubel et al., 2015; Da Silva et al., 2016; Fijalkowska et al., 2016; Cao et al., 2018). Ricce et al. (2016) studied the mechanisms that play a role in improving the drying and rehydration properties of carrot slices pre-treated using ultrasound technology. In our study, in addition to the determination of the rehydration properties of the ultrasound technology, colour, pH, °Brix, and microstructure factors were also examined.
Carrot (Daucus carota L.), one of the most important root vegetables, is cultivated all over the world. According to the Food and Agriculture Organization data, 42.831.958 tons of carrot were produced worldwide in 2017. China is the largest producer country with 20.274.393 tons (FAO, 2019). Carrot is considered a functional food rich in nutrients and important compounds for human health, such as β-carotene, ascorbic acid and tocopherol. It has a significant amount of carotenoid, which is a beneficial antioxidants that can enhance immune system and prevent cardiovascular diseases, muscle diseases, and cataract (Sharma et al., 2012).
This study evaluated quality parameters (color, rehydration, pH and °Brix) obtained during convective drying of carrot slices with different thicknesses, pre-treated with ultrasound. Also, the variation of cell structures in dried samples was examined with the help of scanning electron microscopy (SEM) images.
II. METHODS
A. Sample Preparation
Carrot samples used in the experiments were taken from a local market in Bursa province and stored at a temperature condition of 4 ± 0.5 °C until the experimental trials were completed. The samples were removed from the cooler and allowed to warm at room temperature. The moisture content of carrot samples was 8.75 (g water / g dry matter) on a dry basis (d.b.), determined by drying in an oven at 70 °C for 24 hours. For the drying process, mature and undamaged samples were selected. The selected samples were peeled and prepared at the thickness of 2 and 4 mm slices with the help of slicer (Börner, Germany). The drying processes and experiments were carried out in triplicate.
B. Convective Drying
The samples to be dried were placed on a 400 mm diameter glass plate as a thin layer. Drying was carried out in a modified oven (Whirlpool AMW 545) with a sensitivity of 0.1 °C, which can be controlled by a programmable logic controller (PLC) display. The samples were dried at 60 and 70 °C with an air velocity of 1 m/s. The oven was heated for 30 minutes to the level set before drying. The decrease in the moisture content of the samples until reaching the equilibrium moisture content was monitored with the aid of 0.01g precision balance which was placed under the oven.
C. Ultrasonic Pre-treatment Application
Ultrasound pre-treatment was per-formed with a frequency of 25 kHz in a 300 W ultrasonic bath at room temperature. The carrot slices were placed in a metal basket and then placed in an ultrasonic bath filled with distilled water. A 1:4 ratio of sliced carrot-distilled water was used in the ultrasonic bath. The samples, which were not pre-treated with ultrasound were kept in the same amount of distilled water. Ultrasound pre-treatment was applied to the samples at times 0 (control), 20 and 40 minutes. After ultrasonic pre-treatment, samples were filtered and the water was removed with filter paper (Horuz et al., 2017).
D. Changes of Colour Parameters
The colour changes of fresh and dried carrot samples were determined
by using a colorimeter (HunterLab, USA). A colour system was used in which 
, 
and 
represented
chromatic components of lightness, green/ red and blue/yellow, respectively.
The colour parameters for fresh carrot samples were defined as 
, 
and 
. During the
experiments, the colorimeter was calibrated via a standard white and black
plate prior to each colour measurement. Prior to each use, a glass cell
carrying a sample close to the nose cone of the colorimeter was placed on the
light source, and then the , , , , and values were
recorded. The colour reading process was done in five repetitions. Using the
colour parameters obtained, Chroma value (
), and hue
angle (
), total
colour differences (
) values were
calculated by using the Eqs. (1), (2) and (3) (Tian et al., 2016; Zhao et
al., 2017).
(1)
(2)
(3)
E. Calculation of Rehydration Ratio
So as to determine the rehydration values, dried carrot slices (10 ± 0.1 g) were placed in a 400 ml beaker containing distilled water at 20 °C for 14 hours. The solid/liquid ratio was adjusted to 1:50 (Vega-Gálvez et al., 2009). The carrot slices were then removed, emptied for 30 seconds, and weighed using an electronic digital balance (Shimadzu, Japan) with an accuracy of ± 0.001 g. This procedure was repeated in triplicate for each run. Finally, the rehydration rate (R) was calculated by using the Eq. (4) (Sunjka et al., 2008):
(4)
where 
and 
represent sample weights before and after
rehydration, respectively.
F. Measurement of pH Values
The pH values of fresh and dried carrot samples were measured using a pH meter (Jenco, USA). The dried samples were soaked in distilled water. The pH meter probe was then immersed in the solution, and the measurement was performed. The pH of fresh samples was measured in the juice extracted from the pulped carrots and filtered with filter paper (Mechlouch et al., 2012).
G. Determination of Water-soluble Dry Matter
Brix values of fresh and dried carrot samples were determined using a digital refractometer (Milwaukee, Romania). The dried carrot samples were soaked in distilled water. Then, the samples were filtered with the help of a multi-layer filter paper. °Brix values of fresh carrot samples were determined after removing the juice of the samples and filtering with the multi-layer filter paper (Meclouch et al., 2012).
H. SEM Analyses
SEM was used to observe the changes in microstructures of carrot samples dried under different conditions. The samples were cut from the center as cross-section and placed into the sample holders. For SEM analyses, samples were coated with a 40-50 nm thick gold-palladium and examined with a SEM device (EVO 40, Carl Zeiss, Oberkochen, Germany) at 20 kV accelerating voltage. Micrographs of samples were taken by using the device (Tian et al., 2015).
I. Statistical Analysis
MS-Excel program was used to process the data collected from
different drying conditions. Furthermore, in the statistical analysis of the
collected data, a LSD (least significant difference) with analysis of variance
(ANOVA) at 95% confidence level 
), using JMP
(Version 7, USA) software, was performed.
III. RESULTS
A. Colour Changes
The colour parameter plays a vital role in the consumer choice of
food samples. Homogeneous colour distribution on the product indicates the
quality of the product and has significant effects on consumer acceptance
(Ozturk et al., 2015). Colour values of fresh and dried carrot samples
are demonstrated in Table 1. The value in the parentheses of the results in
Table 1 is the standard deviation of the mean value obtained for each color
parameter. It was seen from the results that values of
carrot samples ranged between 47.796 and 58.110. The lowest value was
found in untreated 4 mm thick samples, which were dried at 70 °C
(70°C-4mm-US0). The increase in drying temperature and sample thickness de-
