Storage Stability of Freeze-dried Starter Cultures (Streptococcus thermophilus) as Related to Physical State of Freezing Matrix

The temperature dependence for loss of acidification activity during storage of a freeze-dried skimmed milk based starter culture of Streptococcus thermophilus in a matrix of ascorbic acid, casein and sucrose or mannitol showed Arrhenius behaviour below the glass transition temperature (Tgas determined by differential scanning calorimetry), with an energy of activation depending on the sugar with 57 kJ/mol for sucrose and 40 kJ/mol for mannitol, but not on the initial concentration of sugar before freezing (60 g/kg or 120 g/kg of dry matter). For storage at or above Tg(Tgwas around 50 °C), loss of activity increased dramatically with non-Arrhenius temperature dependence. Mannitol based glasses yielded better protection for aerobic storage and this was probably due to better antioxidative properties.