The modified state diagram of freeze-dried grapefruit powder was obtained in order to determine the critical water content and critical water activity that cause the glass transition of the amorphous matrix at storage temperature. At 20 degrees C these values were in the ranges of 0.031-0.057 g water/g product and 0.089-0.210, respectively. Below those critical values, the glassy state of the amorphous matrix is guaranteed, thus avoiding an increase in the rate of the deteriorative reactions related to the loss of the bioactive compounds in the fruit (organic acids, vitamin C, main flavonoids, and total phenols) which contribute to the antioxidant capacity (AAO) of grapefruit. In the rubbery state, a certain time is needed for these degradative reactions to start. This time depends on the water content of the sample, the greater the water content the lower the time needed. In this study, the powder was stable for a relatively long storage time (3 months) regardless the relative humidity, due to the limited mobility of the molecular system. Between 3 and 6 months had to pass before a significant loss of bioactive compounds was observed; the higher the relative humidity, the greater the loss. (C) 2012 Elsevier Ltd. All rights reserved.