Abstract
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 C these values were in the ranges of 0.0310.057 g water/g product and
0.0890.210, 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.