Effect of calcium ions on the freeze-drying survival of probiotic encapsulated in sodium alginate

Alginate-based encapsulation of probiotics is a well-recognized method. Crosslinking of the hydrogel occurs by ions and the most commonly used ion is calcium. However, dramatic reduction of viable probiotics during freeze-drying of ion-crosslinked alginate hydrogel is often reported, although this loss is often overlooked or taken for granted. In the present work, the role of calcium in this critical loss of activity was examined. Compared with sodium alginate, it was found that the freeze-drying survival of Lactobacillus rhamnosus GG (LGG) encapsulated in calcium alginate was at least 0.6 log/CFU lower or completely lost. The loss of survival in calcium alginate was mainly in the drying process, which was positively correlated with the increase of calcium ions concentration. Calcium ion solution modeling exhibited significant survival loss at calcium above 1.5 M and the survival loss was proportional to the contact period between LGG and calcium. Under the same ionic strength, three common ions (sodium, potassium, and magnesium) caused a lower loss of LGG survival compared with calcium, indicating that the osmotic pressure effect is not the main reason for LGG damage by calcium. Transmission electron microscopy combined with ultra-thin section technology was used to observe the damage of calcium ions to LGG. The destruction of cell wall/membrane integrity, metabolic activity suppression, and reduced intracellular calcium ion concentration were factors in the loss of probiotic viability. This study provides new insights into the selection and design of alginate-based probiotic encapsulation.