Improvement of Mass Transfer Rate by Ultrasound Pretreatment in Convective Drying of Sour Cherry

Introduction: Sour cherries (Prunus cerasus L.) are relatively diverse and broadly distributed around the world, being found in Asia, Europe, and North America. Sour cherries have unique anthocyanin content, and they are rich in phenolic compounds. The fruits are generally used for processing purposes, such as for juice and jam. The fruits of sour cherries can also be frozen and dried (Doymaz 2007; Šumić et al 2013). One of the best methods for the preservation of agricultural products is drying, which consists in removing water from the manufactured goods (Salehi 2023). Dried sour cherries have a long shelf life and therefore may be a fine alternative to fresh fruit all year round (Wojdyło et al 2014). The effect of alkali ethyl oleate solution on the drying time of sour cherry was studied by Doymaz (2007). In this study, the thin-layer drying of sour cherries was carried out under two air temperatures of 55°C and 65°C. Their results showed that the effective moisture diffusivity of sour cherries based on the analytical solution of Fick’s second law ranged from 4.75×10−10 to 1.03×10−9 m2/s. Ultrasound pre-treatment as a non-thermal food processing technology could be a better pre-treatment technique for food processing, due to its benefits which comprise energy saving, preservation of original freshness and nutritional contents, keeping bioactive compounds, the decline in processing duration, and cost. Ultrasound pre-treatment accelerates the mass transfer in dehydration and drying of fruit and vegetable slices mostly due to the breakdown of cells and the creation of microchannels (Awad et al. 2012; Ghorbani et al. 2013). In terms of cost, ultrasonic is less expensive than other technologies, and the main cost of operating a sonication system is electrical energy, making it more cost-effective and environmentally friendly than other methods (Jalilzadeh et al. 2018). We found no report on the effects of ultrasound pretreatment on the hot-air drying kinetics of sour cherry in the literature. Hence, the purpose of this study was to estimate the impacts of ultrasound pretreatment on the drying time, mass transfer kinetic, effective moisture diffusivity (Deff), and rehydration of sour cherry. In addition, the moisture ratio changes of sour cherry during drying were modeled.
Material and methods: Sour cherries were purchased from the market at Hamedan, Hamedan Province, Iran. The average diameter of fresh sour cherries was 1.6 cm. In this study, the water content of fresh and dried sour cherries was calculated using an oven at 103°C for 5 h (Shimaz, Iran). In this research, the effect of ultrasound time on the drying time, effective moisture diffusivity coefficient, and rehydration of sour cherries were investigated, and drying kinetics were modeled. To apply the sonication treatments on the sour cherries, a Backer vCLEAN1-L6 ultrasonic bath (Iran) was employed with a frequency of 40 kHz and a power of 150 watts. The tank of the device was filled with 6L of distilled water and, then, after the temperature of the water reached 25°C, the sour cherries were placed directly in the bath. To apply ultrasound pre-treatment, the sour cherries were placed inside the ultrasonic bath device for 0, 4, 8, and 12 minutes, and after leaving the device and removing extra moisture, the samples in thin layers were placed in the hot-air dryer (with a temperature of 70°C). The dehydration kinetics of sour cherries have been explained using 6 simplified drying equations. Fick's second law of diffusion using spherical coordinates was used to calculate the moisture diffusivity of sour cherries at various hot-air drying conditions. The rehydration tests were conducted with a water bath (R.J42, Pars Azma Co., Iran). Dried sour cherries were weighed and immersed for 30 min in distilled water in a 250 ml glass beaker at 50°C.
Results and discussion: The results showed that sonication treatment, causes an increase in moisture removal rate from the sour cherries, an increase in the effective moisture diffusivity coefficient, and as a result, reduces the drying time. By increasing the sonication time from zero to 12 min, the average drying time of sour cherries in the hot-air dryer decreased from 465 min to 340 min. The average effective moisture diffusivity coefficient calculated for the samples placed in the hot-air dryer was equal to 3.86×10-10 m2/s. Increasing the sonication time from 0 to 12 minutes increased the average effective moisture diffusivity coefficient by 38%. The time of ultrasound treatment had no significant effects on the rehydration of dried sour cherries. Xu et al. (2022) reported that ultrasonic pretreatment changes the microstructure of pineapple slices, and the combination of ultrasonic and osmosis dehydration improves the effectiveness of the sonication, shortens the dehydration time, and produces a higher-quality product.
Conclusion: Kinetic modeling of sour cherries weight changes during drying was carried out by models in the sources, followed the Page model was selected as the best model to predict moisture ratio changes under the selected experimental conditions. Mean values of the sum of squares due to error, root mean square error, and r for all samples ranged from 0.003-0.010, 0.010-0.018, and 0.998-0.999, respectively. Generally, 12 minutes pre-treatment by ultrasound is the best condition for drying sour cherries.