Study of the Effects of Hot Air Drying and Ultrasound Pretreatment on Raisins Quality in the Packaging Plants

Introduction
Raisins are high-value-added products derived from grape processing. In 2022, Iran ranked fifth among major raisin-producing countries, with exporting over 63,000 tons. In Iran, raisins are primarily prepared using traditional methods, such as sun drying, shade drying, and lye solution dipping. Typically, these traditional methods (drying in open or sun-exposed areas) result in prolonged processing times, causing the raisins to darken. Sun-dried raisins may undergo additional processes such as re-washing and hot air drying before packaging and storage. Higher drying temperatures in re-washed raisins can lead to non-enzymatic browning. Therefore, it is essential to prevent browning reactions that cause color changes and undesirable flavors in raisins. Commercially, sulfur dioxide gas or sulfite solutions were previously used as pretreatment to prevent non-enzymatic browning during secondary drying. However, due to health concerns, the use of sulfites has been restricted. Consequently, newer methods, including alternative pretreatments and hybrid drying systems, have been proposed to enhance the quality of dried products. To date, the application of ultrasound pretreatment in the secondary drying of re-washed raisins has not been studied. Therefore, this research aims to explore methods for maintaining product quality during the current secondary drying process in the packaging plants by examining the effects of the different hot air temperatures and ultrasound pretreatment.
Materials and Methods
Samples (golden or green raisins) were obtained from a raisin processing plant in Bonab County and transported to the Laboratory of Biophysical and Mechanical Properties of Agricultural Products at the Faculty of Agriculture, University of Tabriz. The samples were stored at room temperature. Upon arrival at the laboratory, the samples were cleaned (removing all wood and waste materials) and then placed in appropriate packaging to prevent spoilage and moisture loss. For each experiment repeat, 120 grams of sample were used. Drying tests were conducted using a convective dryer at three different temperatures (45, 55, and 65 ℃) with a constant airflow rate of 1.5 m/s. The weight of the samples was measured every 10 minutes using a digital scale with an accuracy of ±0.01 grams. Measurements continued until the samples reached a moisture content of 12 to 13 percent. The quality characteristics of raisins (Color difference index, browning index, shrinkage, acidity, and rehydration ratio) were measured during experiments. In this research, experiments were conducted using a factorial design. The obtained data were analyzed with SPSS software, and the means were compared at a 5% significance level using Duncan's test.
Results and Discussion
The results from data analysis indicated that hot air treatment, both with and without ultrasound pretreatment, caused significant changes in the color index of raisins. Hot air at 65 ℃ without pretreatment and at 55 ℃ with ultrasound pretreatment were identified as effective methods for color preservation. The rehydration ratio depended on the duration the raisins were submerged in water. Given the short overall submersion time, the results were not significant. The average acidity levels across all treatments were acceptable. The lowest shrinkage rate was observed at 55 ℃ with ultrasound pretreatment. The best browning index results were obtained first with hot air at 45 ℃ without pretreatment, followed by 55 ℃ with ultrasound pretreatment. Using ultrasound pretreatment has enhanced the drying rate and reduced the drying time of the samples compared to those without pretreatment. Ultrasound waves induce cellular disruption and increase cell permeability, which boosts heat penetration and moisture removal. This process ultimately reduces the drying time of the product
Conclusion
The objective of this study was to propose methods to enhance the quality of raisins during secondary hot air drying, which is the prevalent system used in most packaging and processing facilities. This research examined the effects of hot air temperatures at 40, 50, and 60 ℃, with and without ultrasound pretreatment, on the quality indices of raisins. After considering all evaluated indices, hot air at 55 ℃ with ultrasound pretreatment was identified as the most suitable method. As a result of this study, it can be said that the ultrasound application may be used successfully for secondary drying of re-washed raisins.