Effect of ultrasound pretreatment and microwave vacuum drying in the production of dried poniol fruit

Abstract This study suggested a combination of microwave and vacuum drying to meet the requirements of preserving the nutritional content of fresh poniol fruit, consuming lower energy, and having a faster drying rate. In this study, the influence of microwave power, vacuum level, and citric acid pretreatment level on the product quality of dried poniol was analyzed. The citric acid pretreatment was conducted in an ultrasonicator bath. The pretreated was dried by changing the microwave power () from 400 to 800 W and the vacuum level () from 300 to 600 mmHg. The quality of the dried product was analyzed in terms of total phenolic content (), antioxidant activity (), total change in color (), and rehydration ratio () of dried poniol fruit. The response range in the sequence was between 22.219–38.361 mg GAE/g, 69.037–92.578 %DPPH inhibition, 23.451–40.148, and 1.38–3.81, respectively, at various combinations of independent variables. The optimal combination of independent variables for maximal recovery of total phenolic content, antioxidant activity, rehydration ratio, and minimal color changes was obtained at 440 W power level, 600 mmHg vacuum, and pretreated with citric acid of concentration .8%. Under the optimum combination of independent variables, the total phenolic content, antioxidant activity, rehydration ratio, and total color change values were 37.904 mg GAE/g, 92.973%, 23.906, and 3.307, respectively. Because of the low temperature in the vacuum process combined with the rapid heating time of microwaves, the results indicate that microwave vacuum drying might be a viable alternative method for improving the quality of dried food products. Practical Applications Microwave vacuum drying is a technology that uses microwave radiation to generate heat in the absolute pressure range between the triple point of water and atmospheric pressure. Heat is generated in microwave vacuum drying by directly converting electromagnetic energy into kinetic molecular energy. Hence, heat is generated deep inside the product being dried. Because interior temperatures are greater than exterior temperatures, evaporating water is driven to the surface, resulting in higher partial pressure. As a result, the moisture evaporating within the product is released through the pore structure of the macro‐capillary system of the solid material, resulting in a faster drying rate. Since the superficial layer does not dry out completely, the surfaces remain porous. This method results in a superior end product that is also very nutritious with enhanced sensory properties. As a result, this approach is used in the chemical, pharmaceutical, and food industries.