Continuous and intermittent microwave-vacuum drying of apple: Drying kinetics, protein, mineral content, and color

Abstract

The effect of microwave-vacuum drying on drying characteristics, protein, mineral content, and color of the apple was investigated in this study. Eight different drying treatments were applied with using levels of microwave power (200 and 300W), vacuum conditions (200 and 400mmHg), and pulse ratios (PR=1 [Continuous] and PR=2 [30s t(on)/30s t(off)]). Based on the drying settings used, increasing microwave power, vacuum treatments, and decreasing pulse ratio provided shorter drying time. The Page model gave a high fit of statistical values at all data points used to describe the drying kinetics of dried apple samples. At the 300W-200mmHg-PR=2 application, the quantity of potassium was found to be the highest compared with that of the other drying conditions. The protein rate of dried apples was higher than the fresh samples because of water loss. The lowest effect on L* (lightness [78.27]), a* (green/red [10.70]), and b* (blue/yellow [32.72]) values were observed in samples dried at the 200W-400mmHg-PR=2, 300W-200 mmHg-PR=1, and 200W-200mmHg-PR=2, respectively. As a conclusion, the treatment of 300W-400mmHg-PR=1 was found most effective with a shorter drying time (30min) and high protein content (2.5%).

Practical applicationsApple (Malus domestica L.) is one of the most important fruit due to its nutritional composition and the presence of biologically active compounds that provide positive effects on human health. Therefore, apple is grown and farmed extensively in most agricultural areas around the world. Drying is one of the oldest methods of food preservation. Dried apple can be not only stored longer, but also used as packaged snacks and integral breakfast foods. Combining microwave and vacuum drying offers rapid and uniform drying, which can be used to dry food products on a commercial scale. In addition, application of intermittent microwave-vacuum technology in the drying of apple can result in significant improvement in quality.