**8.1 Freeze drying**

Freeze drying/lyophilization is a technique in which water is removed from a product after it has been frozen and put under a vacuum, allowing the ice to convert directly from solid to vapor without going through a liquid phase. Probiotic stability is stabilized by freeze-drying (lyophilization), which is typically done with cryoprotectants present and with the water being sublimated under a vacuum [6]. The purpose of the cryoprotectants is to increase microbial survivability and stabilize the product during storage. Probiotics' viability and shelf life are also enhanced by lyophilization, although this does not affect how long they survive in the digestive system. As a result, it is widely used as a second step in microencapsulation processing to increase the shelf life of probiotic bacteria by drying them out after they have been encapsulated in emulsions or gels [6]. Sugars like glucose, mannose, lactose, trehalose are among the most popular low-molecular-weight cryoprotectants. Most of the molecules with large molecular weight are polysaccharides and proteins, such as soy protein, milk, gelatin, and maltodextrin [94]. Wang et al. [95] coated sodium alginate microcapsules containing *L. plantarum* CCTCC M 2014170 cells with an inner layer of inulin and an exterior layer of skim milk before freeze-drying the mixtures. The findings demonstrated that the probiotic's resilience to the GIT's simulated environment was enhanced by encapsulation. In the stomach, *L. plantarum's* viability remained unchanged after 2 hours, but in the 1% bile salt solution, there was a 1.21 log drop in cfu/mL. The microencapsulation of probiotics *L. acidophilus* and *L. casei* by freeze drying was also studied by Bora et al. [96] utilizing whey protein isolate, fructooligosaccharides, with a combination at ratio (1: 1). In contrast to lose cells, which had a low survival rate after 90 minutes of incubation at pH 2, the probiotic that had been encapsulated had a high encapsulation yield (98%) and was not significantly impacted by the simulated stomach juice [96].
