# Probiotics in Functional Foods: Survival Assessment and Approaches for Improved Viability
## 2 Probiotics in Food Products
### Survival Assessment and Viability  
When discussing the survival of probiotics two aspects should be considered. The first one is whether the selective strains are initially able to survive through the gastrointestinal tract and then, do they remain viable through the food manufacturing process. Most of the commercially implemented probiotics were isolated from milk and dairy products, so it is not surprising that these products are today considered the main functional food [51]. The low gastric pH tolerance and high bile salt concentrations are the most important criteria for the selection of the probiotic strains [52]. On the other hand, the choice of food matrix and further processing conditions are crucial for their survival. It is also critical to ensure that probiotic food products contain the minimum recommended level of probiotics (\(10^{6}\)-\(10^{7}\) CFU/mL product [53]) in the final product. This level of probiotics should be maintained throughout the product's shelf life because the number of viable and active probiotic cells per gram or milliliter of probiotic food products at the time of consumption determines their health effects [54]. Therefore, probiotics must meet severe technological standards in order to remain active throughout the entire manufacturing and storage process. So far, representatives of genera Lactobacillus and Bifidobacteriumium have long records in food implementation [55; 56; 57] because they are considered GRAS.  
During the manufacturing or storage process, the number of active probiotic bacteria declines by a fold factor of 10-100 or more. All of the factors that influence probiotics' long-term survival in meals include oxygen content, heating temperature, acidity, water activity, osmotic stress, moisture content, and packing material [40]. The number of viable and active cells per gram or milliliter of probioticic food products at the moment of consumption is the most critical value for the functional food, as it determines their efficacy [58]. Two general approaches exist in order to assess the vitality of individual bacterial cells in foods [59]. The golden standard remains the culture-dependent method for determination of numbers of colony-forming units per gram of original material (cfu/g) [60]. On the other hand DNA and flow cytometric-based methods are constantly improved in order to obtain culture-independent and reliable screening methods [59].  
Figure 2 summarizes the main factors that influence the viability of probiotics during manufacturing and consumption. During processing, the food component interactions as sugars, salt, antimicrobial substances can significantly affect the viability of probiotics. Together with the moisture content and acidity, these are the main requirements. Processing factors are also critical because they include often various mechanical forces are applied and most frequently different temperature regimes. The latter is one of the very limiting factors for the survival of probioticic bacteria. During storage, again the moisture content and acidity play an important role, as well as the type of packaging material [61]. Once the functional food is consumed then the environmental factors of different compartments of the GIT are applied.