Physicochemical properties, glass transition state diagram and colour stability of pulp and peel of two dragon fruit varieties (Hylocereus spp.) as affected by freeze-drying

Abstract

The aim of the current study was to investigate the freeze-dried characteristics: physicochemical properties, colour parameters, total colour change, total betacyanins content and glass transition state diagram, of pulp and peel of white-flesh (Hylocereus undatus) and red-flesh (Hylocereus polyrhizus) dragon fruits. The results show that all dragon fruits can be successfully freeze-dried. For nutritional point of view, the retention of vitamin C counts up to 71.13% after freeze-drying. The water solubility of freeze-dried pulp was significantly different compared to the peel (p<0.05), that was about 7 times higher than that of the peel. Freeze-dried pulp and peel of red-flesh dragon fruit were a potential source of betacyanins content that found up to 360.14 mg/100 g. All freeze-dried dragon fruits obtained low glass transition temperature, varied between -6.70 and 4.83 °C. The visual colour of freeze-dried dragon fruits after rehydration was similar to their original colour. Thus freeze-drying seems to preserve the nutritional and colour properties of dragon fruits after processing. Furthermore, the impact of pH on the colour was also monitored due to the pH sensitivity of the red pigment, betacyanins present in the dragon fruit. The total colour change of all reconstituted freeze-dried gave very small value between pH 3 and pH 7 particularly the peel while the colour seriously changed at out of this pH range. Regarding to the finding results, it can be considered that the freeze-dried pulp of red-flesh dragon fruit can probably be used as a natural colorant due to relatively high in betacyanins, water solubility, vitamin C, and colour stability at low acid condition