###### Abstract  
This study examines the fermentation performance of featured bacteria (_Lactobacillus acidophilus_-ATCC-4356, _Lactobacillus heleticus_-ATCC-15009, _Lactobacillus delbrucekii_ subsp. _bulgaricus_-ATCC-11842, _Lacticasibacillus casei_-ATCC-393, _Streptococcus thermophilus_-ATCC-19258 (ST), and _Bifidobacterium bifidum_-ATCC-29521 (BB)) used in fermented dairy products and their impact on product quality. The main focus is on evaluating the metabolic activities, organic acid production, viscosity values, and sensory properties of probiotic strains such as _L. acidophilus_, _L. bulgaricus_, _L. casi_, _L. heleticus_, _B. bifidum_, and _S. thermophilus_. The strains were activated in a sterile milk medium and incubated until they reached a pH of 4.6. Then, pH, microbial enumeration, organic acid, sugar composition, vitamins A, D, E, K1, and K2 (menaquinone-7), and viscosity values were measured in the bacteria. Organic acid, sugar composition, and vitamins A, D, E, K1, and K2 (menaquinone-7) were analyzed with the HPLC method. Additionally, sensory analyses were performed, and volatile compounds were examined. _L. casi_ demonstrated superiority in lactic acid production, while _L. heleticus_ showed high lactose consumption. _L. bulgaricus_ stood out in galactose metabolism. The highest viscosity was observed in products produced by _B. bifidum_. Differences in viscosity were attributed to exopolysaccharide charide (EPS) production and acid production capacity. A total of 62 volatile compounds were identified, with the highest levels of aromatic components found in products containing _B. bifidum_. The most preferred product, based on panel evaluations, was the fermented dairy product produced with _L. acidophilus_. As for aroma profiles, it was determined that the phenethyl alcohol, 3-methyl-1 butanol, and ethanol compounds are associated with _B. bifidum_, the hexanoic acid and 2-methylbutanal compounds are associated with the _L. acidophilus_, the hexanoic acid, 2-methylbutanal, 2-furanmethanol, and acetaldehyde compounds are associated with the _L. bulgaricus_, and the hexanoic acid, 2-methylbutanal, 2-heptanone, acetoin, and d-limonene are associated with the _L. casei_. On the other hand, the _L. heleticus_ strain is associated with the hexanoic acid, 2-methylbutanal, and 2-heptanone, and the _S. termophilus_ strain is associated with the hexanoic acid, hexanol, acetoin, 2,3-pentanedione, 1-butanol, and 3-methyl-2-butanone volatile aroma compounds. The determination of fat-soluble vitamins is particularly important for vitamin K1 and vitamin K2. In this study, the bacterial sources of these vitamins were compared for the first time. The menaquinone-7 production by _L. heleticus_ was determined to be the highest at 0.048 \(\mu\)g/mL. The unique metabolic capacities of these prominent cultures have been revealed to play an important role in determining the aroma, organic acid content, viscosity, and overall quality of the products as a whole. Therefore, the findings of this study will provide the right strain selection for a fermenteddairy product or a different non-dairy-based fermented product according to the desired functional properties. It also provides a preliminary guide for inoculation in the right ratios as an adjunct culture or co-culture for a desired property.  
Lactobacillus acidophilus; Lacticaseibacillus casei; Lactobacillus helveticus; Bifidobacteriumium bifidum; fat-soluble vitamins; rheology; aroma; sensory