Effect of polymers and storage temperature on the stability of freeze-dried lactic acid bacteria

Abstract

The effect of the addition of gelatin, xanthan gum or maltodextrins on the survival during freeze-drying and stability during storage of four lactic acid bacteria were studied. The polymers were added to a milk-sucrose base medium and mixed with the cell concentrate. The water activity of the cultures was adjusted to 0.22 prior to storage at −20, 4 or 20 °C. None of the polymers generated marked improvement of the milk-sucrose medium in terms of survival of bacteria during freeze-drying. There were significant differences in mortality rates between strains. Gelatin improved the storage stability of freeze-dried Lactobacillus casei ssp. rhamnosus RO11 and Bifidobacterium longum RO23 cultures kept at 20 and 4 °C, respectively. Also, bacterial populations of Lactococcus lactis ssp. lactis RO58, following 12 months at 4 °C, were higher than controls when gelatin was added to the freeze-drying medium. Overall, the additives had a detrimental effect on the stability of Streptococcus thermophilus RO57 during storage at 20 °C. The B. longum RO23 α- and β-galactosidase activity losses during storage at 20 °C were approximately twice those observed at 4 and −20 °C, while viable count losses at 20 °C were approximately 100× greater than those at 4 °C.