Comparative study of pretreatment on microwave drying of Gala apples (Malus pumila): Effect of blanching, electric field and freezing

Abstract

Microwave drying is particularly recommended for thermosensitive products such as food products in order to preserve all their properties. During drying, some fruits tend to oxidize and brown, losing flavor and vitamins A and C. To reduce oxidation and preserve the quality of the dried product, a pretreatment by placing the fruit in an acid solution (citric or ascorbic) is recommended. On the other hand, blanching, freezing and electric field treatment alters the cell structure, allowing moisture to escape more easily and products to rehydrate more quickly.An experimental study was conducted to see the effect of different pretreatment methods on the kinetics and quality of microwave-dried apple samples. For drying kinetics, no significant effect is observed for the blanched and electric field treated samples. However, a reduction in drying time by a value of 14.6–20.3% is observed for frozen samples. In other hand, the best quality (lowest ΔE, lowest BI, and highest WI) is observed for the sample blanched in 5% citric acid at 60 °C. Finally, the effect of the different pretreatment methods on the quality of the surface structure of the dried samples was studied by scanning the surface structure using Scanning Electron Microscope (SEM).