## 2 Results
### Sensory Characteristics of Fermented Milk at Different Inoculum Ratios  
The use of an electronic nose as a rapid assessment tool for volatile flavor compounds in fermented dairy products offers numerous benefits. Figure 3 presents the electronic nose data for fermented milk prepared using different cultures, highlighting the outcomes of the preparation. The radar map demonstrates that, except for the control group and the 1000:1 ratio, the flavor profiles of the remaining four groups were similar. Furthermore, certain sensors (P10/1, P30/2, P30/1, T30/1, P40/2, and PA/2) exhibited an enhanced response within these four groups, indicating the presence of significant amounts of organic and alcoholic compounds in the samples (Figure 3A). The PCA chart in Figure 3B clearly depicts the differences in flavor attributes among the six groups. The PLS-DA results indicate that PC1 and PC2 account for 65.2% and 31.2% of the sample variation, respectively, collectively explaining 96.4% of the total variance (Figure 3C). In Figure 3B,C, we used R2Y (goodness-of-fit) and Q2 (predictive ability parameter) to assess the accuracy and predictability of the PLS-DA model. The R2Y and Q2 values were 0.90 and 0.68, respectively, indicating good accuracy and predictability of the model. The PLS-DA VIP analysis identified several sensors, including P10/1, P30/1, P40/2, PA/2, T70/2, TA/2, and P10/2, that were found to have a significant representation of over 1 (Figure 3D).  
A total of six sets of fermented milk samples were evaluated for their flavor, texture, aroma, and taste. In comparison to commercial starter cultures, the combined fermented milk exhibited improved flavor and aroma, as depicted in Figure 3E. The fermented milk with a 10:1 ratio achieved the highest overall score, closely resembling the sensory characteristics of the imported commodity starter, as shown in Figure 3F. Our findings indicate that the ratio of _S. thermophilus_ CICC 6063 to _L. helveticus_ CICC 6064 has a significant impact on the production of volatile flavor compounds and the sensory quality of fermented milk.  
Figure 2: Rheological results of fermented milk prepared with different proportions of co-culture and commercial starter culture: (**A**) aacroscopic viscosity index (MVI) values; (**B**) elastic index (EI) values; (**C**) fluidity index (FI) values; (**D**) solid-liquid balance (SLB) values.  
The 10:1 group had relatively higher scores in smell and flavor, but lower texture and taste characteristics compared with the CSC group. We recommend using a ratio of 10:1 for producing fermented milk with desirable flavor and storage stability. The sensory characteristics of the 10:1 and CSC groups were evaluated using quantitative descriptive analysis (QDA). The primary flavor attributes were defined by the first two sensory characteristics, with the 10:1 group exhibiting predominantly sour and fruity flavors, while the CSC group showed fruity and creamy flavors. It has been reported that increasing sourness intensity can mask the creamy flavor [20]. Additionally, the 10:1 group demonstrated stronger fruit, cheese, sour, and alcoholic flavors. In dairy products, the flavors of alcohol and fruit primarily originate from ester compounds [21]. The higher levels of ester compounds, particularly ethyl esters with alcoholic and fruity flavors, in the 10:1 group align with the results of volatile flavor compound determinations.