## 2 Results
### Volatile Compounds Profiles during Fermentation and Storage  
The odor and flavor formation in milk products involves the breakdown of chemical components through protein, glycolysis, and lipolysis. In this study, a 10:1 co-culture sample was used to analyze volatile compounds at different time points (0 h (F), 2 h (F), 4 h (F), 0 d (S), 1 d (S), 7 d (S), 14 d (S), and 21 d (S)) during fermentation and storage. Figure 3: Electronic nose and sensory evaluation results of fermented milk prepared with different proportions of co-culture and commercial starter culture: (**A**) radar fingerprint chart of electronic nose; (**B**) principal component analysis (PCA); (**C,D**) scores by PLS-DA of E-nose data; (**E**) sensory scores of different sensory indexes; (**F**) total sensory scores of samples; (**G**) the average scores of sensory descriptors of 10:1 and CSC group. Table 1 presents the retention index (RI) of the DB-Wax column, which identified a total of 56 compounds. These compounds include ten acids, ten aldehydes, ten ketones, eleven alcohols, five esters, and ten aromatic carbohydrates. Mass spectrometry was used to identify these compounds, and their relative intensities (RIs) were calculated using an alkane mixed standard and cross-referenced with published research. During the fermentation and storage of fermented milk, ten different acids were detected. Notably, the short-chain fatty acids acetic, butanoic, and hexanoic acid were found to increase significantly, particularly towards the end of fermentation, reaching maximum concentrations of 39.44, 50.45, and 90.84 ug/L, respectively. Additionally, octanoic acid exhibited a fluctuating pattern, initially rising, then declining, and finally rising again. The highest concentration of octanoic acid, 14.34 ug/L, was observed at 0 d (S). In summary, the fermented milk contained a diverse range of acids in varying concentrations, which underwent changes throughout the fermentation and storage phases. During fermentation and storage, ten aldehydes were identified. The highest concentration was observed at 4 h (F) and 0 d (S). Acetaldehyde, in particular, exhibited high levels during fermentation, reaching its peak at 14 d (S) before gradually decreasing to 74.39 ug/L at the completion of the process. Additionally, benzaldehyde was detected at a concentration of 1.94 ug/L at the end of fermentation. Lastly, 2-methyl-2-butenal (0.31-1.11 ug/L) was detected during the later stages of storage. Ketone levels were higher in fermented milk compared to other materials during production and storage. Ten distinct types of ketones were identified. The concentration of 2-heptanone was highest during the early stages of fermentation (108.5 ug/L). The acetoin concentration increased and peaked at 0 d (S) during fermentation, then decreased at 1 d (S) and increased again. After fermentation, the mass concentration of 2,3-butanedione was 111.51 ug/L, with some variations observed during storage. After 21 days of storage, the concentration of 2,3-pentanedione increased to 51.36 ug/L. The concentration of 2-nononone increased again after decreasing during fermentation, following 21 days of storage. After seven days of storage, 4-methyl-2-hexanone was detected, with a mass concentration ranging from 1.02 to 3.03 ug/L. The total content of ketones, including 2,3-butanedione, acetoin, acetone, 2-heptanone, and 2-nononone, among others, was high (32.44-135.28 ug/L, 58.65-231.14 ug/L, 2.51-42.99 ug/L, 28.79-41.34 ug/L, and 1.46-7.79 ug/L, respectively). Eleven alcohols, including 3-pentanol, 2-hexanol, and methyl alcohol, were detected in the fermented milk. Initially, methanol had a high concentration of 97.33 ug/L during fermentation, which later decreased. The maximum concentration of 3-pentanol was 6.29 ug/L at the end of fermentation. The concentration of 1,2-ethanediol steadily increased during storage, ranging from 0.43 to 0.83 ug/L. On the other hand, the content of 1-propene-1-thiol ranged from 0.05 to 0.88 ug/L and gradually decreased after fermentation.