## 3 Results and Discussion
### Vitamin A, D, E, K Contents of Fermented Samples  
The qualitative and quantitative determinations of fat-soluble vitamins A, D, E, K1, and K2 were carried out using the HPLC method developed by [PERSON] et al. [14]. The retention time (tR) value of each selected vitamin in the method was determined three times, and when the repeatability data were evaluated, the relative standard deviation percentage (%RSD) was calculated below 2%. In addition, chromatographic retention parameters were calculated to determine the compounds quantitatively. Accordingly, it was determined that the capacity factor (k) values were in the range of 1 \(\leq\) k \(\leq\) 10, the selectivity factor (\(\alpha\)) was \(\geq\)1.15, and the separation factor (Rs) value was \(\geq\)1.5. As a result of the perfect peak symmetry and method optimization of the analyzed vitamins, an external calibration method was applied for the quantitative analysis of the compounds [14]. Calibration curves were drawn with five different concentrations of each standard, and the linear working ranges were determined. The graphs are presented in Table 3.  
The developed HPLC method quantification was performed for the analysis of the samples. 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 production of vitamin K2, known as menaquinone-7, by _L. heleticus_ was determined to be the highest at 0.048 \(\upmu\)g/mL. The results obtained at different fermented samples are given in Table 2C. [PERSON] et al. [14] also reported that the vitamin content in milk was 15.79 \(\upmu\)g/100 g for vitamin A, 0.55 \(\upmu\)g/100 g for vitamin D, 1.11 \(\upmu\)g/100 g for vitamin E, 4.81 \(\upmu\)g/100 g for vitamin K1, and 0.97 \(\upmu\)g/100 g for vitamin K2 (MK-7). These values, when calculated per 100 g, can be interpreted as certain increases being attributed to bacteria. The data presented in the table illustrate the concentrations of various vitamins (A, D, E, K1, and K2) in different fermented milk samples. These findings are crucial for evaluating the effects of specific bacterial cultures on vitamin production. The highest vitamin A content was observed in the LA-p sample (31.083 \(\pm\) 0.041), suggesting that _L. acidophilus_ may play an active role in vitamin A production. The LC-p sample (13.397 \(\pm\) 0.044) exhibited the highest vitamin D level, indicating that _L. casei_ might be more efficient in vitamin D synthesis compared to other strains. The BB-p sample (1.135 \(\pm\) 0.004) contained the highest vitamin E concentration, implying that _B. bifidum_ could have a potential role in vitamin E production. The ST-p sample had the highest values for both vitamin K1 (0.869 \(\pm\) 0.003) and K2 (0.029 \(\pm\) 0.008), suggesting that _S. thermophilus_ significantly contributes to the production of K vitamins. In conclusion, the vitamin production capacities of different probiotic strains vary significantly. Therefore, selecting suitable probiotic strains based on specific vitamin requirements could be a strategic approach to enhancing the nutritional value of fermented dairy products. In a Finnish study quantifying carotenoid concentrations in various dairy products, \(\beta\)-carotene contents ranged from 3.0 to 186.5 \(\mu\)g/100 g [43]. LC-MS and LC-MS/MS analysis of whole milk and fresh cow's milk revealed vitamin D3 concentrations of 0.2 \(\mu\)g/L and 0.5-0.6 \(\mu\)g/L, respectively [44]. \(\alpha\)-tocopherol contents in dairy products with different fat contents were found to be in the range of 4.5-45.5 \(\mu\)g/100 g [45]. In a study investigating vitamin K contents in various animal products, dairy products were found to contain K1 and MK-4 in the range of 0.2-3.0 and 0.4-10.4 mg/100 g, respectively [46; 14]. As a result of the excellent peak symmetry of the compounds analyzed by HPLC and method optimization, an external calibration method was applied for the quantitative analysis of the compounds.