Water sorption properties, molecular mobility and probiotic survival in freeze dried protein-carbohydrate matrices

The moisture uptake and molecular mobility of freeze-dried powders containing whey protein isolate-carbohydrate matrices (1WPI:2maltodextrin; 1WPI:1maltodextrin:1d-glucose; and 1WPI:1maltodextrin:1l-glucose) and encapsulated Lactobacillus rhamnosus GG (LGG) in these matrices were investigated at 25 °C and 33% and 70% relative humidity (RH). The inactivation rate constant for probiotics in freeze-dried matrices were positively correlated (R(2) = 0.98) to moisture uptake and molecular mobility measured by NMR relaxometry. The stability of probiotics in glassy protein-carbohydrate matrices was dependent on the composition of the matrix. The partial substitution of maltodextrin with glucose (d- or l-) which improved microbial survival at 33% RH was related to the reduced molecular mobility and lower water uptake of the matrix. This study suggests that moisture uptake properties and molecular mobility of the matrix composition, as opposed to the relative humidity of the environment, are better determinants of probiotic viability during storage. Dynamic vapour sorption and NMR relaxometry are promising tools to assist in the selection of protein-carbohydrate matrices for enhancing probiotic viability during storage.