Abstract
Given that Persian shallot (mooser) is predominantly consumed in powdered form in the food industry, its drying is essential. Nowadays, due to the non-sterile conditions and low quality of products dried using the traditional sun-drying method, this approach is no longer employed, and industrial dryers are used instead. Among drying methods, the application of the impinging jet technique has attracted considerable attention from researchers in recent years. The advantages of this type of dryer include high heat transfer rates, reduced drying time, and improved quality of the final product. In the present study, the effects of air temperature at three levels (50, 60, and 70 °C), nozzle outlet air velocity at three levels (7, 10, and 13 m s⁻¹), and the ratio of nozzle-to-product surface distance to nozzle diameter (H/D) at three levels (4, 5, and 6) in an impinging jet dryer on the drying rate and color changes of Persian shallot slices were investigated. The experiments were conducted and analyzed using response surface methodology (RSM) based on a face-centered central composite design (CCD). The results showed that achieving a high drying rate occurred at higher air temperatures and velocities and lower H/D ratios. Moreover, the effects of temperature and air velocity on color change indices were significant, whereas the H/D ratio had no significant effect on these indices. The influence of air temperature on the drying rate of Persian shallot slices was greater than that of air velocity. The effects of air temperature and velocity on the color change indices ΔL* and Δb* were significant at the 1% level, while the H/D ratio had no significant effect on these indices. The optimal conditions for drying Persian shallot slices using the impinging jet dryer were determined to be an air temperature of 69 °C, an H/D ratio of 5.1, and an air velocity of 10.4 m s⁻¹.
EXTENDED ABSTRACT
Introduction
Allium hirtifolium is one of the most commonly used agricultural products in the food industry in Iran, because it is rich in minerals, vitamins, essential fatty acids, and antioxidant compounds. Due to the major consumption of powdered allium hirtifolium in products that may be stored on the shelf for weeks or months, drying is one of the essential steps in its processing to preserve it longer. Today, due to the non-sterility and low quality of the dried product resulting from the old method of using sunlight, this method is no longer used, and industrial dryers are employed to dry the products. There are numerous methods for drying agricultural products. Each method has some benefits and some limitations. Energy consumption of the process, drying time, quality of the final product, and available technology are the most important factors in selecting the method to be used for a specific product. Among the drying methods, the impact jet method has been considered by many researchers in recent years. A major part of the results that have been reported shows that this technique could be a suitable method for drying a wide range of agricultural products. The advantages of this type of dryer include high heat transfer, reduced drying time, and increased quality of the final product.
Material and Methods
In the present study, the relationship between some factors that can affect the drying process and the final reduction of humidity of Allium hirtifolium slices was studied. Also, some visual factors, such as the colour of the slices after drying time, were considered. The effect of air temperature at three levels of 50, 60 and 70 degreesCelsius, the velocity of the air coming out of the nozzles at three levels of 7, 10 and 13 meters per second and the ratio of the distance from the nozzle to the product surface to the nozzle diameter H/D Three levels 4, 5 and 6 in the collision jet dryer were investigated on the drying rate and color of allium hirtifolium leaves. The experiments were conducted using the response surface method and the central composite design in face-centered mode.
Results and Discussion
As expected, the air temperature of the dryer had a reverse relationship with drying time. With an increase in fluid temperature from 50° C to 70° C, the drying time was reduced from 95 minutes to 50 minutes. Of course, the relationship was not linear. At the first period of the drying process, in which the humidity content of the slices was higher, the water evaporation rate from the surface of Allium hirtifolium pieces was higher. Over time, the evaporation rate was reduced because of the reduction in the water content portion of the slices. Additionally, it was clarified that, although all three factors of air temperature, ratio of distance to diameter, and air velocity affected the drying process, the role of fluid temperature was the most significant. Increasing air velocity was also reduced during the period. At a constant 60° C temperature of fluid, by increasing the speed level of air from 7 ms^(-1) to 13 7 ms^(-1)Drying time was reduced from 90 minutes to 70 minutes. This relationship was not linear and had a more significant effect at lower speeds. The ratio of the distance from the nozzle to the product surface to the nozzle diameter, H/D, was another effective factor, the role of which on drying time was confirmed. When the H/D ratio was 4, the drying period time was 75 minutes. For an H/D ratio of 5, the drying period time was 80 minutes, and for an H/D ratio of 6, the drying period time was 90 minutes. So, it is clear that although the H/D ratio has an effect on the drying period time, its portion is minor. Another parameter that has been studied in this research is the final colour of Allium hirtifolium pieces. To quantify this parameter, three indices were used: (a*), (b*), and (L*). These indices were used to consider suitable situations and select the best combinations of effective factors in the drying process. To achieve final results and visualize complex data, the collected quantities of measured parameters were presented in 3D graphs, allowing for the analysis of holistic situations and effects. By studying these graphs, it can be seen that if one of the effective factors, such as the H/D ratio, were constant, another factor could affect the drying process by itself or in combination with the third one.
Conclusions
Many drying methods have been used by researchers in Iran to prepare Allium hirtifolium pieces as a widely used industrial product. Among them, the impact jet method is probably the best. Obviously, some factors, such as air temperature, air velocity, and H/D ratio, have a serious effect on the drying period time of the product and the quality of the final leaves. Finding the best combinations of these factors that yield the best outcome with minimum inputs is the big challenge that has been studied in this research. So many experiments were conducted to consider various situations. The results showed that high drying rates occur at high temperatures and air velocities, as well as low H/D ratios. The most effective factor was air temperature. Also, the effect of two variables, temperature and air speed, on the variation of color change indexes, 〖∆L〗^* and 〖∆b〗^* was significant, but H/D had no significant effect on these indices. Optimal conditions for drying Allium hirtifolium leaves with a jet dryer at 69 ° C, H/D ratio equal to 5.1, and air speed of 10.4 (m/s) were suggested.
Author Contributions
Esmaei Seidi: 34%
Aydin Salimi asl: 33%
Mohammad Hadi Mohammadi: 33%
Data Availability Statement
Not applicable.
Acknowledgements
Not applicable.
Ethical Considerations
This section states ethical approval details (e.g., Ethics Committee, ethical code) and confirms adherence to ethical standards, including avoidance of data fabrication, falsification, plagiarism, and misconduct.
Conflict of Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
Funding Statement
The author(s) received no specific funding for this research