## 3 Results and Discussion
### TSS and pH  
Figure 2 illustrates the changes in TSS and pH during the fermentation of Huaniu apple cider. The TSS content plays a significant role in the apple fermentation process. TSS during cider fermentation refers to the content of total solids that can be dissolved in cider, mainly composed of sugars, organic acids, amino acids, esters, and other dissolved substances in apple juice. The changes in soluble solids during cider fermentation are influenced by the metabolic activity and material transformation of yeast. As depicted in Figure 2A, the consumption of TSS content is relatively slow during the initial 4 days of fermentation, which can be attributed to the time required for the yeast to adapt to the fermentation environment of Huaniu apple juice. From day 4 to day 8, there is a rapid decrease in the TSS content, followed by a slower decrease or even stabilization from day 8 onwards. This pattern is related to the growth cycle and fermentation performance of the yeast. By the end of the 10-day fermentation period, the TSS content is reduced from the initial soluble solid 12.9 degrees Brix to 4.0 degrees Brix. The initial soluble solids content in the juice at the beginning of the cider fermentation process is an important factor in determining the sweetness and taste of the final product. It is usually measured in degrees Brix and represents the percentage of soluble solid substances based on glucose. During fermentation, yeast uses sugars from the fruit juice as substrates for fermentation. Yeast converts sugars into alcohol and carbon dioxide, resulting in a decrease in the soluble solids content in the fruit juice. As fermentation progresses, the concentration of sugars gradually decreases. During fermentation, organic acids, esters, and other metabolites produced by yeast gradually accumulate. These substances play a crucial role in the flavor and mouthfeel of cider. Their presence increases the complexity of cider and interacts with sugars, alcohol, and other components, affecting the soluble solids content and overall taste of cider [23]. At the end of fermentation, yeast converts most of the sugars into alcohol and other metabolites, resulting in a lower soluble solids content in cider. However, there may still be some residual sugar in cider, i.e., sugars that have not been completely converted by yeast, which will have an impact on the sweetness and taste of cider. It should be noted that the change in the soluble solids content of cider is affected by several factors, including the sugar/acid ratio of the raw fruit juice, the characteristics of the yeast strain, the fermentation temperature, the fermentation time, and more [24]. To control the change in soluble solids, adjustments can be made by modifying the amount of sugar added during fermentation and controlling the fermentation temperature and time, among other factors. This helps achieve the desired sweetness and taste while ensuring the quality and stability of the cider. pH is a critical factor in the fermentation process as it profoundly influences the quality attributes of the yeast and the final product, such as flavor, color, and aroma. The results presented in Figure 2B demonstrate that the pH of Huaniu apple cider fluctuates within a narrow range during fermentation and tends to stabilize within a smaller range after the completion of alcoholic fermentation. The pH initially exhibits a slight decrease in the first 4 days and then a slight increase in the last 4 days. However, the final pH falls within the suitable range for cider production, indicating that the yeast has adapted and influenced the composition of the apple juice. The pH value of cider fermentation undergoes a series of changes due to the accumulation of organic acids and other compounds produced by yeast metabolism during the fermentation process, as well as the growth and metabolic activities of yeast. At the beginning of the fermentation process, the initial pH of the apple juice typically falls between 3.0 and 4.0.