Microstructure and Optical Properties of Salak Fruit Under Different Drying and Pretreatment Conditions

The effect of heat treatment on microstructural and optical properties of salak fruit was investigated. Blanching pretreatments were performed at three temperatures (50–70°C) and drying experiments were conducted by using a hot air dryer (40–90°C), a heat pump–assisted dryer (constant and intermittent modes, 26 and 37°C), and a freeze dryer. The microstructure of the sample was examined with scanning electron microscopy (SEM) and the color change was assessed based on total color change value (ΔE*). In general, the freeze-dried product showed a porous structure and the hot air–dried product had a dense structure. Heat pump–assisted drying (both constant and intermittent modes) can maintain the original shape of parenchyma cells better compared to other drying methods, where the cells remained intact and showed less wrinkling after drying. However, pretreatment had an adverse effect on the product's microstructure. It was observed that elongation and disruption of parenchyma cells increased significantly with blanching temperature. On the other hand, browning indices were different among the samples depending on the pretreatment and drying conditions. Heat pump–assisted drying in constant mode can not only can prevent a high browning rate during drying but minimizes and delays color degradation during storage.