Effect of physical properties on the stability of Lactobacillus bulgaricus in a freeze-dried galacto-oligosaccharides matrix

The ability of galacto-oligosaccharides (GOS) to protect Lactobacillus delbrueckii subsp. bulgaricus upon freeze drying was analyzed on the basis of their capacity to form glassy structures. Glass transition temperatures (Tg) of a GOS matrix at various relative humidities (RH) were determined by DSC. Survival of L. bulgaricus in a glassy GOS matrix was investigated after freezing, freeze drying, equilibration at different RHs and storage at different temperatures. At 32 °C, a drastic viability loss was observed. At 20 °C, the survival was affected by the water content, having the samples stored at lower RHs, the highest survival percentages. At 4 °C, no decay in the cells count was observed after 45 days of storage. The correlation between molecular mobility [as measured by Proton nuclear magnetic resonance (1H NMR)] and loss of viability explained the efficiency of GOS as cryoprotectants. The preservation of microorganisms was improved at low molecular mobility and this condition was obtained at low water contents and low storage temperatures. These results are important in the developing of new functional foods containing pre and probiotics.

► Effect of thermophysical properties of GOS on freeze-dried L. bulgaricus. ► GOS remain in the glassy state at room temperature until RH = 33%. ► Survival of microorganisms is affected by the water content and temperature of storage. ► The presence of GOS matrix is not sufficient to prevent membrane damage upon freeze drying. ► The increase of molecular mobility is correlated with loss of viability.