β-Carotene encapsulation in a mannitol matrix as affected by divalent cations and phosphate anion

The effects of addition of divalent cations and phosphate buffer on the degree of β-carotene encapsulation in a mannitol matrix during freeze-drying were analyzed. The degradation rate of encapsulated β-carotene as a function of % RH and its relationship with the physical state of the matrix during storage at 25 °C was also studied. The presence of phosphate salts significantly delayed mannitol crystallization at a highly satisfactory degree during freeze-drying and, consequently, the degree of β-carotene encapsulation increased. This effect was maintained over quite long time during storage of the freeze-dried samples at 25 °C. Unavoidable local variations in water content during 3 years storage caused the decrease of Tg values and made the crystallization degree to increase. The divalent cations showed a synergistic effect and also modified the kinetics of β-carotene degradation during storage, increasing its stability. The mechanism of crystallization inhibition likely includes a change in hydrogen bond network or/and change in molecular mobility in the presence of divalent cations and phosphate anions. The degradation rate of β-carotene in a mannitol/KH2PO4 matrix increased as increasing % RH until a value at which the samples collapsed (75% RH), and then the degradation rate decreased. Collapse phenomena may affect diffusion of oxygen from the surface to the inside of the matrix and increase retention of β-carotene. Surface color was not an appropriate indicator for β-carotene degradation, because it was mostly dependent on the optical properties of the matrix, which changed with the degree of matrix hydration and collapse.