High quality onion powder production in infrared-assisted refractance window drying system

Onion (Allium cepa L.) is a significant agricultural product rich in bioactive compounds and is used globally in the preparation of various foods. Besides being consumed fresh, it is also used in the industrial production of processed foods in dried, semi-dried, and frozen forms. Traditional drying methods applied for this purpose cause significant color, aroma, and nutrient losses due to environmental contamination and oxidation. Additionally, new drying methods and approaches that can contribute to the international green deal, which includes measures to reduce carbon emissions and energy inputs worldwide, have gained importance. In this regard, innovative hybrid drying approaches that combine traditional and modern drying techniques have started to emerge. In this study, the diffraction window drying system, one of the innovative drying methods, was transformed into a hybrid technology using an infrared lamp controlled by a pyrometer. The target product of the project, high-aroma onion powder, was processed by optimizing the drying conditions of onion puree using an infrared-assisted diffraction window hybrid drying system under controlled conditions. In these optimal drying conditions, drying and foam drying approaches were also tested by adding a carrier to the puree. In the optimization study, Response Surface Methodology Box-Behnken experimental design was used in Design-Expert version 13 software (Design-Expert for Windows, Stat-Ease Inc., USA). Circulation water temperature (75-95°C), puree surface temperature (30-40°C), and airspeed (0-1.2 m/s) were selected as independent variables. Desired aroma active component amount (maximum total area), undesired aroma active component amount (minimum total area), quercetin amount (maximum), browning index (minimum), and drying time (minimum) were selected as dependent variables. The aim was to ensure the preservation of desired bioactive components and the reduction of undesired ones under conditions expected to provide rapid drying of onion puree. All samples were dried by spreading them in 2 mm thickness until they reached a final moisture content of 3.5 ± 0.5%, and drying times were recorded. The optimal drying conditions were determined as 89.71°C circulation water temperature, 30°C puree surface temperature, and 1.2 m/s airspeed. The drying time under optimal conditions was recorded as 14 minutes. Under the optimal conditions determined for drying onion puree in the developed hybrid dryer, mixture composition optimization with a carrier was also performed. This optimization study was conducted using the Mixture Optimal Experimental Design in Design-Expert version 13 software (Design-Expert for Windows, Stat-Ease Inc., USA). The components selected in an amount equal to 50% of the puree dry matter (4 g) were: β-cyclodextrin (0-1 g), Na-alginate (0-1 g), maltodextrin (0-4 g), and Arabic gum (0-4 g). The desired response variables were the amount of desired aroma active compounds (maximum total area), the amount of undesired aroma active compounds (minimum total area), the amount of quercetin (maximum), browning index (minimum), and drying time (minimum). The optimal mixture content was determined as 2.08 g maltodextrin and 1.92 g Arabic gum. However, the drying time for the optimal mixture content was recorded as 24 minutes. For the foam puree drying approach, the density and stability of onion puree foams created by adding different amounts of soapwort extract and at different foaming times were examined, and the foaming conditions were optimized. It was found that the minimum foam density and maximum foam stability were achieved by adding 0.5% soapwort extract and foaming the puree for 3 minutes. Before adding the soapwort extract, the onion was shredded for 1 minute, allowing enzyme activity and preventing the soapwort extract from remaining in the shredding chamber. The drying time in the foam puree drying approach was reduced to 10 minutes. The onion powder sample produced under optimal conditions was compared with its commercial imported and domestic counterparts through different model food trials. Sensory analysis results revealed that the onion powders produced from domestic onions using the developed hybrid technology were preferred more than commercial samples. Compared to convective drying and freeze-drying methods, the drying approaches in the thesis significantly reduced the drying time from hours to minutes and produced high-quality onion powder that meets the desired characteristics. Specifically, for the production of additive-free plain onion powder, the newly developed hybrid system, a pyrometer-controlled infrared-assisted diffraction window dryer, produced a final product that was superior in many physical and chemical quality attributes compared to onion powder produced using the commonly used traditional method of convective drying. The results obtained demonstrated that the developed hybrid drying method enabled the production of high-quality onion powder with a rich aroma content in a short time.