Storage water activity (a(w)) affects the stability of freeze-dried food. The sugar crystallization and storage stability of beta-carotene in freeze-dried mango powder was investigated following storage under various relative vapor pressures (11.3-80.9%). Sugar crystallization was revealed by the loss of sorbed water in the water sorption experiment. However, the sorption isotherms showed unclear divergence between the experimental and predicted values. X-ray powder diffraction and scanning electron microscopy were used to confirm the crystallization. The results showed that increased a, resulted in higher sugar crystallization. The loss of beta-carotene was monitored by high-performance liquid chromatography with a diode-array detector and fitted to a first-order reaction. The rate constant decreased as aw increased up to 0.43, due to the collapse of the mango powder. An increase in the rate constant above this value of aw coincided with pronounced sugar crystallization. Choosing the appropriate value of a for storage can prevent sugar crystallization and enhance beta-carotene stability in freeze-dried fruit powder. (C) 2011 Elsevier Ltd. All rights reserved.