Use of alginate and cryo-protective sugars to improve the viability of lactic acid bacteria after freezing and freeze-drying

Abstract

In the present paper, the effect of cryo-protective sugars on the survival rate of different strains of Lactic Acid Bacteria (LAB, Lactobacillus acidophilus, Lactobacillus delbrueckii subspbulgaricus, Streptococcus salivarius subsp.thermophilus), after freezing or freeze-drying procedures, was compared. The cells were incubated at 4 °C in 32% final concentration sugar solutions (trehalose, maltose, sucrose, glucose and lactose), and viability was evaluated by the enumeration of colony-forming units. All sugars tested showed a protective effect on cell viability as compared to isotonic solution, especially after freeze-drying procedures (log c.f.u./ml ranging between 1.16 and 2.08, P < 0.001). Furthermore, the resistance to different stress agents (lysozyme, pepsin, bile salts) was estimated. Trehalose was the most effective sugar in preserving bacterial viability [% (log c.f.u. trehalose/log c.f.u. isotonic solution) ranging between 124 and 175, P < 0.001] although each strain showed a different sensitivity. Finally, the protective effect of immobilization of LAB in Ca-alginate beads was compared to that exercised by trehalose. The immobilization induced a good survival rate but lower as compared to the trehalose effect, mainly after freeze-drying in the presence of the selective agents [% (log c.f.u. alginate/log c.f.u. trehalose ranging between 81.1 and 94.5, P < 0.0001]. The protective effect of trehalose was evident in particular for Lactobacillus delbrueckii subsp. bulgaricus in presence of lysozyme. Therefore, because of its chemical inertness and low cost, trehalose could be easily utilized as excellent bacterial preservative, both to improve the viability of starter cultures and to obtain probiotic formulations more resistant to a variety of stressful conditions.