Freeze-drying qualities and state diagram of pulp and peel freeze-dried dragon fruits (Hylocereus spp.)

Freeze-drying has been proposed to preserve nutritional values and colour of foodstuff. Moreover, few studies disrobe the influence of freeze-drying on the quality characteristic of tropical fruit, like dragon fruit. In this work, freeze-drying characteristics, vitamin C, betacyanins content, L*, a* and b* colour parameters and state diagram of pulp and peel of white-flesh (Hylocereus undatus) and red-flesh (Hylocereus polyrhizus) dragon fruits were investigated after freeze-drying. The effect of pH level on the colour was monitored due to the pH sensitivity of the red pigment, betacyanins present in dragon fruit. The results show that dragon fruit can be successfully freeze-dried. For nutritional point of view, the retention of vitamin C was high and count up to 71.13% after freeze-drying. Freeze-dried pulp and peel of red-flesh dragon fruit were a potential source of betacyanins particularly the peel. The water solubility of freeze-dried pulp of both dragon fruit types was up to 7 times higher than that of the peel. The visual colour of reconstituted sample unchanged at their natural pH and shows only a small changes between pH 3 and pH 7. All cases of freeze-dried dragon fruits obtained low glass transition temperature that varied between -6.70 and 4.83 °C. However, it can be considered that the freeze-dried dragon fruits can probably be applied for food processing