Structural collapse prevents beta-carotene loss in a supercooled polymeric matrix

The kinetics of degradation and surface color changes of beta-carotene encapsulated in a polymeric matrix (PVP-40) and its relationship with physical changes (manifested as structural collapse) of the matrix were studied during storage of samples at several water activities at constant temperature. The degradation rate constants obtained decreased with an increase in the relative humidity (RH) of the storage atmosphere. Beta-carotene losses were observed mainly at RHs below the glass transition temperature (Tg) of the corresponding systems, and the lower degradation constant rates were observed under conditions where the matrices were fully plasticized (i.e., rubbery) and collapsed (RH, 64 and 75%). An inverse correlation was observed between collapse and degradation rate constants. The results presented here indicated that the molecular mobility of the matrix is not rate limiting for the degradation of beta-carotene. Factors such as microstructure and porosity of the polymeric matrix may be more important as modifiers of kinetic reactions. Surface color was not a sensitive indicator of beta-carotene retention, because it was mostly affected by the degree of matrix hydration and collapse phenomena.