Abstract
Infrared drying with a variable temperature strategy is a novel method introduced to improve energy efficiency and preserve the quality of agricultural products. The objective of this study was to evaluate the effects of different variable temperature patterns in an infrared dryer on the quality attributes of garlic slices, processing time, energy consumption, and to determine the optimal temperature variation pattern during drying.
Samples were dried in an infrared dryer using a constant temperature method at three levels (50, 60, and 70 °C) and a variable temperature method with two different patterns. In the first pattern (A), drying started at 60 °C and, after the product surface temperature reached 50 °C, the drying temperature was reduced to 50 °C. In the second pattern (B), drying began at 70 °C and, after 30 minutes, the temperature was reduced to 50 °C. Drying time, energy consumption, and quality indices including shrinkage, true density, total color change, antioxidant content, and polyphenol content were measured.
The results showed that energy consumption for constant drying at 70 °C and variable temperature pattern B was 0.76 and 1.16 kWh, respectively. However, pattern B resulted in significantly lower color changes in the final product at 70 °C (4.92 compared to 14.22), indicating superior quality preservation. In addition, antioxidant content under variable temperature pattern B was found to be acceptable. Therefore, the application of variable temperature drying using pattern B is preferable to constant temperature drying.
Introduction
Preserving the quality of the dried product and reducing energy consumption is an important issue in the drying process. Drying products at variable temperature is a new method used recently, in which the drying temperature changes during the process. This method is more suitable for the drying of heat-sensitive products. There is a temperature that causes damage to product quality, called the critical temperature, and varies across products. In the variable temperature drying method, the drying process starts at a high temperature. The drying temperature is reduced below the critical temperature when the temperature of the product increases to a critical temperature. It prevents the product temperature from increasing (Nalbandi et al., 2021). Through this technique, product quality loss caused by the temperature increase can be prevented, and it is possible to use the benefits of drying at high temperatures. Higher drying temperatures at the beginning of the process could accelerate the drying process and decrease the drying time. Reducing the temperature at the critical point preserves the product quality, such as the final color and nutritional component of the dried product. The lecture's review showed that this method is not used in infrared drying. Therefore, the aim of this study was to evaluate the effect of various variable temperature patterns in an infrared dryer on the drying time, and energy consumption, and quality properties of garlic slices such as the color changes, shrinkage, density, antioxidant and polyphenol content. Also, the best pattern for changing the drying temperature must be determined.
Material and Methods
The garlic cloves were peeled, sliced into slices of a thickness of 2.5 mm and put in a solution of KMS 5 % for 10 min. Then, the slices were dried in an infrared dryer at a consistent temperature of 50, 60, and 70 °C and at varying temperatures with two different patterns. In the first pattern (I), drying began at 60 °C and was lowered to 50 °C once the sample's surface temperature reached 50 °C. In pattern II, drying started at 70 °C and lasted 30 min before decreasing to 50 °C. Drying time, energy consumption, and qualitative characteristics such as shrinkage, true density, color changes, antioxidants and polyphenols content were evaluated.
Results and Discussion
The drying time of garlic slices to reach the moisture content of 0.3 g water/g dry matter at the drying temperature of 50, 60 and 70 °C was 122, 101 and 92 min, respectively. Increasing the drying temperature reduced the processing time, which agreed with the results reported by other researchers. The drying time of garlic samples was 119 and 109 min at the variable drying temperatures in patterns I and II, respectively. The drying time of garlic slices with constant and variable drying temperatures was analyzed based on a randomized complete block design. The analysis of variance indicated that the method and the drying temperature significantly affected the drying time at the 1 % probability level. The highest and lowest drying times were observed at constant drying temperatures of 50 and 70 °C, respectively. There were no significant differences between the drying times of samples at constant drying temperatures of 50 °C and pattern I. However, the differences between the drying time at constant drying temperature of 50 °C and pattern II were significant. Using the variable drying temperature (pattern II) caused a 13-min decrease in the drying time (10.5 %). The findings indicated that the drying method significantly affected the energy consumption during the process, total color changes and antioxidant content of the sample. The highest and lowest energy consumptions were observed at the constant drying temperature of 50 and 70 °C, respectively. Energy consumption was 1.16 kWh in pattern II, and it was 25 % lower than once obtained at the constant drying temperature of 50 °C. Therefore, application of variable drying temperature could reduce energy consumption. It also preserved the samples color so that the total color changes of slices in the pattern I and II were the same as the ones observed at the constant temperature of 50 °C. The sample dried at the constant temperature of 60 °C, pattern I and II had the higher antioxidant content as compared to the constant temperature of 50 and 70 °C. The highest antioxidant content was observed in the dried samples by the pattern I (4.49 %). It was about 6.63 % in fresh garlic. Shrinkage, true density and polyphenol content of sample were not affected by the drying methods and their mean values were 56 to 63 %, 1061 to 1626 kg/m3 and 1.667 to 2.229 mg garlic acid/g dried product.
Conclusions
The results showed that the application of variable drying temperature in the infrared dryer had a positive effect on drying time reduction and preserving the quality of samples. Comparison of the drying time, energy consumption and quality characteristics of dried garlic by the variable drying temperature (pattern II) showed that the quality of dried samples such as shrinkage, true density, color and polyphenols content were similar to those dried at the constant drying temperature of 50 °C. However, the antioxidants content of sample dried at this treatment (pattern II) was higher and energy consumption was lower that of observed at the constant temperature of 50 °C. In addition, color changes of sample dried by pattern II was lower that the color of samples dried at the constant temperature of 70 °C. Therefore, the variable drying temperature technique in the drying process of garlic in an infrared dryer could introduce as a successful method.
Author Contributions
Habibeh Nalbandi: Project administration; Conceptualization, formal analysis; writing– original draft.
Sadegh Seyedlou: Conceptualization; writing–review and editing.
Reza Hossein Nia: Data curation; formal analysis; investigation; methodology.

Data Availability Statement
The data that support the findings of this study are available from the corresponding author, upon reasonable request.
Ethical Considerations
This study was approved by the University of Tabriz. Research data are not shared. Written informed consent was obtained from all study participants