## 5 Lactic Acid Bacteria  
### General Characteristics  
Lactic acid bacteria (LAB) are Gram-positive, non-spore-forming, catalase non-producing, and non-motile microorganisms [60]. The main characteristic of these bacteria is lactic acid production, which is a key to fermentation products. In addition, they have a high tolerance to acidic pH, are aerotolerant, and can appear as cocci or rods [61]. Even though LAB can be present naturally in some dairy products, they are often deliberately incorporated as a starter culture, or sometimes as ingredients or additives to amplify the product's functionality, particularly their probiotic potential [62]. They can also be present in plants and the animal gastrointestinal tract [22]. Additionally, LAB present GRAS status. They do not pose a risk to human health, allowing for their use in technological and industrial processes in the food sector. In addition, LAB are acid-tolerant and can synthesize antimicrobial compounds [63]. Thus, in addition to its importance for promoting health, LAB presents good potential for use as a clean label ingredient for preserving foods, including dairy products.  
LAB are highly diverse, mainly classified under the phylum _Frimicutes_, class _Bacillus_, and order _Lactobacillus_. This order comprises six families (_Lactobacillus_, _Leuconostocaceae_, _Enterococcaceae_, _Aerococcaceae_, _Streptococcaceae_, and _Carnobacterium_) encompassing over 30 genera and 300 species. Additionally, the genus Bifidobacteriumium is also considered a LAB; however, it belongs to the phylum _Actinobacteria_[61], class _Actinobacteria_, order _Bifidobacterialies_, and family _Bifidobacteriaceae_[64].  
Bacteria from the genus _Bifidobacterium_ are Gram-positive, anaerobic or aerotolerant, non-spore-forming pleomorphic bacteria. They are present in the gastrointestinal tract of mammals, birds, and insects. Some examples of _Bifidobacterium_ species are _Bifidobacterium_ _Iongum_, _Bifidobacterium animalis_, and _Bifidobacterium bifidum_. Generally, bifidobacteria have an optimum growth temperature of around 37-41 \({}^{\circ}\)C, being considered mesophilic organisms. However, there are a few thermophilic species, such as _Bifidobacterium thermocidophilum_[65]. Moreover, bifidobacteria represent a significant group of probiotic cultures used in industry. They have shown various beneficial effects, such as a boost in lactose digestibility, a decrease in serum cholesterol level, synthesis of complex-B vitamins, and calcium absorption [66]. However, to recognize the benefits of probiotic intake, the viable cells must be at least 10\({}^{6}\) CFU/g of the food product [65]. Their optimum growth pH is around 6-7; nevertheless, some strains of _Bifidobacterium lactis_ and _Bifidobacterium animalis_ can grow even at pH 3.5 [66]. Nonetheless, the presence of high oxygen levels affects the growth and viability of these microorganisms. _Bifidobacteriumantimalis_ subsp. _lactis_ is broadly used in yogurt-like products due to its good tolerance to acid, low pH, and molecular oxygen [67].  
For example, _Streptococcus thermophilus_ and _Lactococcus lactis_ enhance flavors in foods like yogurts by producing a tangy, pleasantly acidic taste, characteristic of fermented dairy products [17]. Thus, this fact is positive for using LAB as a clean label ingredient to act as a natural preservative in yogurts, as it will not harm the characteristic acidic flavor of these food products, while also promoting health.