Effect of maltodextrins in the water-content–water activity–glass transition relationships of noni (Morinda citrifolia L.) pulp powder

Water sorption behaviour at 25 °C and the water content (xw)–glass transition temperature (Tg)–water activity (aw) relationship was determined for noni pulp powder obtained by spray-drying with and without maltodextrin (DE 9-12), at three different MD/noni solid ratios of 1.17, 1.3 and 2.5, in order to know the effect of MD in the powder stability, preventing powder stickiness. The obtained isotherms were sigmoidal and the Guggenheim–Anderson–deBöer (GAB) model was fitted to the experimental data. The glass transition temperatures (Tg) of the different noni powders and maltodextrin in DE 9-12 were obtained using differential scanning calorimetry (DSC). The equilibrium water content of noni samples containing maltodextrin at a given water activity was lower than that obtained for the maltodextrin-free noni powder. At 25 °C, the critical water content that ensures the glassy state of the noni pulp during storage increased from 0.047 to 0.06 g water/g product when the maltodextrin/noni ratio was greater than 1.3, while the critical water activity increased from 0.13 to nearly 0.5.