Effects of aerosolized citric acid–radio frequency as a pretreatment on hot‐air drying characteristics of banana

Abstract The effects of aerosolized citric acid–radio frequency (RF) pretreatment were evaluated on the quality characteristics of hot air‐dried banana. The results showed that increasing the RF intensity elevated the total phenolic content (TPC), shrinkage, and color changes, while the TPC and color changes decreased with increasing the RF exposure duration. A rise in the RF intensity reduced the rehydration ratio (RR) and firmness of the samples. Aerosolization of citric acid rendered the preservation of the phenolic compounds of the samples to a higher extent, and TPC decreased from 311 ± 3.4 mg/g in fresh banana to 252.1 ± 4.24 mg/g in the samples treated with a RF of 27.12 Hz for 40 min, 280.5 ± 8.1 mg/g in the ones treated with 1% aerosolized citric acid for 40 min, and 162.5 ± 10.8 mg/g in the ones with no pretreatment. According to scanning electron microscopy (SEM), the application of aerosolized citric acid pretreatment caused tissue softening and the formation of cell holes in the samples. Cell wall collapse and damage were severe when RF was in use, which caused the blockage of some microchannels within the tissue. The Page model with the highest determination coefficient (R 2) and the lowest root‐mean‐squared error (RMSE) and chi‐square (χ 2) was selected as the best model.

Browning is one of the most significant adverse changes that occurs in dried food products. It is caused by enzymatic and nonenzymatic reactions. Various acids such as citric acid, malic acid, and ascorbic acid are extensively used these days to reduce the enzymatic browning of dried products (Bonazzi & Dumoulin, 2011;Sarpong et al., 2018;Shah & Nath, 2008). Aerosolization is defined as the dispersion of a liquid or a solution in air in the form of fine mist (Oliveira et al., 2018). Therefore, instead of immersing the sample in the antibrowning solution, the aerosolization method can be used as a new antibrowning delivery technique.
Radio frequency (RF), with frequencies between 10 and 300 MHz, is a part of the electromagnetic spectrum. RF drying is an adequate means of rendering safe and high-quality food products, as fast and consistent heating patterns, high penetration depth, and stable processing temperatures are the features of this method.
Drying, thawing, disinfection, and pasteurization of food and agricultural products are some of the RF applications (Jiang et al., 2020;Wang et al., 2020;.
So far, no study has evaluated the simultaneous use of aerosolized citric acid and RF pretreatment in the HAD of banana. Therefore, in this study, the effects of aerosolized citric acid-RF pretreatment were investigated on the quality characteristics of the banana dried by HAD.

| MATERIAL S AND ME THODS
Cavendish bananas with the same size and degree of ripening were bought from local market in Mollasani, Iran. After peeling, the bananas were cut into 10-mm slices. The initial moisture content was determined using an oven at 95°C until reaching a constant weight.

| Aerosolized citric acid under radio frequency exposure
The RF system was composed of parallel electrodes with a gap of 20 cm. Citric acid (1% v/v) was aerosolized ( Figure 1). The simultaneous effects of aerosolized citric acid (1% v/v) and RF (13.56,27.12,and 40 MHz for 40 and 80 min), as a pretreatment, were evaluated on the quality characteristics of the banana dried by HAD at 50°C.

| Firmness
In order to evaluate the firmness of the samples, the penetration test was performed using a TA-XT Plus texture analyzer (Stable Micro Systems) equipped with a 25-N load cell and a cylindrical puncture probe, 6 mm in diameter, at a constant speed of 10 mm/min (Jafari et al., 2018).

| Shrinkage
Shrinkage was assessed based on the changes in the sample volume which was measured gravimetrically by the displacement of toluene in a pycnometer (Roueita et al., 2020).

| Rehydration ratio
The dried banana (10 ± 0.1 g) was added to distilled water (solid to liquid ratio of 1:40) and kept at ambient temperature for 16 h. The rehydration ratio (RR) of the samples was calculated based on the following equation (Noshad et al., 2012;Vega-Gálvez et al., 2012): where W r is the dried sample weight, and W d denotes the weight of the rehydrated sample.

| Color
A Minolta colorimeter CR-400 (Konica Minolta, Inc) was used to assess the color of the samples based on the CIELab color space. Total color difference (∆E) was used to examine the color changes in the samples (Jafari et al., 2018).

| Antioxidant activity
To evaluate the antioxidant activity, 0.

| Scanning electron microscopy
The microstructure of the samples was verified using a scanning electron microscope (LEO model 1455 VP, LEO Electron Microscopy Ltd) at 30 kV. After cutting the samples along the perpendicular axis, they were sputter-coated with gold (Sputter Coater Model SC 7620, Quorum Technologies Ltd) that had been connected to a doublesided sticky tape. The images were captured at 500× magnification.

| Drying kinetics
To investigate the effects of the aerosolized citric acid and RF pretreatment on the drying kinetics of banana, the samples were dried at 50°C. The moisture ratio (MR) of the samples during drying was calculated using the following equation: where, M, M 0 , and M e respectively represent the moisture content at any time, initial moisture content, and equilibrium moisture content (kg water/kg dry matter). The MRs were fitted to the equations presented in Table 1 using MATLAB (R2020a, Mathworks, Inc), and R 2 , RMSE, and χ 2 were evaluated to select the best model for drying the different samples, which should have the maximum R 2 and the minimum RMSE and χ 2 (Forouzanfar et al., 2020).

| Statistical analysis
Analysis of variance (ANOVA) and Duncan's multiple range test were conducted using SPSS 23.0 (SPSS Science) to investigate the differences between the treatments. The experiments were at least triplicated, and the average of the replications was reported.

| Total phenolic content and antioxidant activity
The results of the impacts of RF intensity and exposure duration on the TPC of the dried banana are summarized in Table 2, whereby raising the intensity of RF increased the TPC of the samples, while it decreased with a rise in the RF exposure duration. The highest TPC was associated with the sample treated with a frequency of 40 MHz for 40 min, and the lowest TPC was related to the one treated with a frequency of 13.56 MHz for 80 min ( Table 2). The RF pretreatment reduced the drying time, and hence the product was exposed to hot air for a shorter time; as a result, the degradation of TPC, caused by the contact with hot air during drying, was reduced. Consequently, the TPC of the dried banana was also reduced (Noshad & Ghasemi, 2020). Moreover, with increasing the RF exposure duration, TPC decreased in all the samples, which is probably due to further degradation of the phenolic compounds. Owing to the relationship between TPC and antioxidant activity, the amount

TA B L E 1 Mathematical models of drying presented by various authors
Model name Model equation Henderson and pabis MR = a exp(-k t) Logarithmic MR = a exp(-k t) + c

Wang and Singh
Note: a, b, and c indicate model's parameters, dimensionless. of the antioxidant activity decreased with an increase in the RF exposure duration.

| Color
The results of the impacts of the RF intensity and exposure duration on the color change in the dried banana are presented in  et al., 2020). As the duration of the RF exposure increased, the extent of enzymatic browning decreased in the samples because they were exposed to aerosolized citric acid for a longer time.
This reduced the amount of color changes in the samples (Moon et al., 2020).

| Texture
According to the results (Table 3)

| Rehydration ratio
Rehydration is employed to assess the extent of the damage caused by treatments in the drying or pre-drying processes. RR is significantly influenced by the textural properties of a product, and low RR denotes the collapse of its internal structures (Noshad et al., 2012).
According to the results (Table 3)

| Shrinkage
The results of the effects of the RF intensity and exposure duration on the shrinkage of the dried banana are displayed in Table 3. As can be seen, the use of the RF pretreatment had a significant effect on the increase in the shrinkage of the samples, so that the highest shrinkage was related to the sample treated with a frequency of 40 MHz for 80 min, and the lowest shrinkage was related to the one treated with a frequency of 13.56 MHz for 40 min (Table 3) According to the results (Table 4) Table 4 denote that the color changes in the pretreated samples were fewer than those in the control, TA B L E 3 Effects of RF intensity and exposure duration on color, firmness, rehydration ratio, and shrinkage of dried samples which was due to the reduction in the enzymatic and nonenzymatic browning reactions in the pretreated samples. According to the results (Table 4), the shrinkage and RR of the samples with pretreatment were significantly different (p < .05) from those of the control, as the samples with pretreatment had the lowest shrinkage and the highest RR, compared with the control. This difference indicated a better product quality in drying and less damage to the sample tissue.

| Drying kinetics
The results indicated that the use of aerosolized citric acid and RF pretreatment reduced the drying time of banana ( Figure 3) and eliminated the cell membrane resistance. Therefore, moisture could be TA B L E 4 Physicochemical properties of samples dried at 50°C and pretreated with a RF of 13.56 Hz for 40 min, 1% aerosolized citric acid for 40 min, and no pretreatment  the decomposition value of the membrane layers was elevated, the product tissue suffered more damage, and as a result, more moisture was released. Altogether, increasing the temperature intensified the molecular motion causing more water molecules to escape from the product and the drying time to be reduced (Forouzanfar et al., 2020;Noshad & Ghasemi, 2020). Different models were compared in terms of R 2 , RMSE, and χ 2 to model the drying kinetics of the treated and control bananas. Finally, the model with the highest R 2 and the lowest RMSE and χ 2 was selected as the best model (Noshad & Ghasemi, 2020). Based on the results of Table 5, the Page model had the highest R 2 and the lowest RMSE and χ 2 . Hence, it had the best fit with the experimental data at all the drying temperatures, the statistical results of which are given in Table 5.

| CON CLUS ION
The application of aerosolized citric acid and RF pretreatment improved the quality of dried banana. Aerosolized citric acid and RF pretreatment also increased the TPC and antioxidant activity of the dried banana by decreasing the drying time. The Page model can be used to model the kinetics of drying. Therefore, aerosolized citric acid and RF pretreatment might be used by the industry as a pretreatment in banana drying to improve its quality.

ACK N OWLED G M ENTS
The authors would like to express their sincere gratitude to the Vicechancellor for Research and Technology of Agricultural Sciences and the Natural Resources University of Khuzestan for supporting this study.

CO N FLI C T O F I NTE R E S T
The authors have declared no conflict of interest.