Impact of Water Activity on Physical Stability and Bioactive Compound Retention in Yellow Pitaya (Selenicereus megalanthus) Pulp Powder

Yellow pitaya (Selenicereus megalanthus) pulp is rich in phenolic compounds with antioxidant capacity and exhibits desirable sensory properties. Dehydration and grinding into powder may enhance stability and broaden the potential for export and industrial applications. In this study, freeze-drying was used to obtain yellow pitaya pulp powder, which was stored at 20 °C under different water activity levels (aw 0.113–0.750). Changes in physical properties (water sorption, glass transition, texture, and color) and bioactive compounds (antioxidant capacity and phenolic content) were assessed after 3 months of storage. Combining the Gordon & Taylor model with the GAB sorption isotherm, the critical water content (CWC) and water activity (CWA) related to glass transition were determined as 0.023 g water/g product and 0.110, respectively. Below these critical values, the glassy state of pitaya pulp powder was maintained, enhancing its quality and stability during storage. The greatest changes in color and bioactive compound content were observed at high aw levels (0.680 and 0.750, respectively). Due to its high nutritional value and antioxidant properties, this powder can be incorporated into formulations or dietary supplements, offering additional functional benefits and expanding its application in the food industry.