## 3 Discussion  
The choice of suitable starter cultures is crucial for producing high-quality and consistent fermented milk products. Previous research has shown that _S. thermophilus_ CICC 6063 exhibits a high fermentation rate. In this study, we aimed to investigate the fermentation characteristics of fermented milk produced using different ratios of _S. thermophilus_ CICC 6063 to _L. heleticus_ CICC 6064. Furthermore, we assessed the production and storage stability of volatile flavor compounds under the optimal ratio. In selecting a starter with optimal fermentation properties, factors such as fermentation time, pH, tittatable acidity (TA), and live cell count are considered important [23]. In this study, a representative sample consisting of a 10:1 ratio was observed to reach the isoelectric point of casein after 4.2 h, indicating the potential to decrease fermentation time through the strategic combination of strains. The pH values of all samples rapidly declined within the initial 2 to 4 h post-inoculation, which is consistent with previous findings [24]. During the 21 days of storage at 4 \({}^{\circ}\)C, the pH and tittatable acidity of the yogurt samples were significantly influenced by the storage time. The pH decreased while the tittatable acidity increased, as observed in a study by [PERSON] on fermented milk co-cultured with _L. heleticus_ and _S. thermophilus_[25]. Additionally, samples with a 10:1 ratio and a commercial starter exhibited a more pronounced weak post-acidification effect, potentially extending the product's shelf life. Microrheology can reflect the rheological properties of fermented milk over time, which in turn affect the texture and taste of the product. The development of viscosity modulus can be measured directly using the MVI index, which captures changes in the lost modulus within a specific time period, providing insights into the variations in viscosity [26]. At the 2 h mark, co-culture samples showed an increase in viscosity, potentially due to the separation of casein micelles, development of emulsion gels, and swift viscosity alteration [27]. The MVI of each fermented milk sample peaked and transitioned into a phase of heightened viscosity [28]. An increase in the experimental investigation (EI) value indicates a stronger gel structure and improved stability in the sample. The gel strength in co-culture samples was observed to be lower compared to the control group, possibly due to the efficient proteolytic system of _L. heleticus_[29]. The fluidity index (FI) quantifies the velocity of particle movement within the system [30]. The SLB analysis revealed that acid production conditions influenced fermentation time variation, with the control group having the lowest SLB value at approximately 0.4. Co-culture samples showed a slightly higher SLB value of around 0.6, while the 10:1 group had a slightly higher SLB value compared to the other groups. Co-culture samples exhibited a higher liquid viscosity, while the control group had superior solid elasticity (CSC group). The flavor of fermented milk is influenced by various volatile compounds, which undergo significant changes in types and concentrations during fermentation and storage, as depicted in Table 1. This study is the first to examine the flavor fingerprints of _S. thermophilus_ and _L. heleticus_ throughout fermentation and storage, revealing that the concentrations of the majority of flavor compounds peaked at the end of the fermentation process, indicating superior flavor characteristics compared to other fermentation stages. Furthermore, Figure 5 illustrates the metabolic pathways of discriminatory volatile metabolites, and their content fluctuations during fermentation and storage. Carboxylic acids derived from lipolysis, proteolysis, or lactose fermentation in fermented milk enhance taste and contribute to sourness [17; 31]. These acids, including methyl ketones, alcohols, esters, and aldehydes, have minimal impact on the overall flavor due to their high thresholds [32].