## 3 Results and Discussions  
### Differences of Fermentation Biomass, Survival Rate of Bacterial Agent, and Acid Stress Tolerance  
Solid state fermentation (SSF) and liquid state fermentation (LSF) methods have their advantages and disadvantages in the production of LAB-related products, and both methods were widely applied. Substrates such as soybean meal, corn, and bran accounted for the majority of global feed consumption [38; 39]. Therefore, we used solid and liquid media prepared from bran and soybean meal as substrate to culture _Lacticaseibacillus paracasei_ Zhang, then prepared the powder by cold air drying and spray drying, then we detected the survival rate of powder bacterial agent prepared by the two methods (cold air drying and spray drying). The results showed (Figure S1) that the survival rate of the powder bacterial agent prepared by SSF and cold air drying (32.93%) was significantly higher than that prepared by LSF and cold air drying (22.13%). Similarly, the survival rate of the powder bacterial agent prepared by SSF and spray drying (0.21%) was significantly higher than that prepared by LSF and spray drying (0.15%). These results indicated that cold-air drying preparation method was more conducive to the survival rate of LAB than spray drying preparation method with the survival rate increased by two orders of magnitude.  
Whether it is consumed by human or by cultured animal, the bacteria agent would be digested by the stomach. Acid stress caused by gastric acid poses the biggest challenge for LAB to survive and function as probiotics. Tolerance to gastric acid has become an important indicator to determine whether a strain is a good probiotics strain or not. Therefore, we compared the acid stress tolerance of \(L\). _paracasei_ Zhang produced from two fermentation methods. The results showed that (Figure 1), after 10-min pH 5 acid stress treatment, the survival rate of solid-state and liquid-state fermented cells were not basically affected (98.30% and 99.39%, respectively), indicating that the cells had good tolerance to the pH 5. With the increasing acidity, survival rates began to differ. After 10-min pH 4 and pH 3 acid stress, the survival rates of the cells from liquid-state fermentation were 93.34% and 84.99%, whereas the survival rates of the cells from solid-state fermentation were decreased to 85.04% and 39.89%, suggesting that liquid-state fermentation contributed to enhancing the tolerance to acid stress of the cells, compared with solid-state fermentation. After 10-min pH 2 acid stress treatment, the survival rate of the cells from the two fermentation methods showed a sharp decline, and the survival rate of the cells from LSF was only 0.43%, but it was still significantly higher than that of the cells from SSF (0.04%). No viable bacteria were detected after 10-min pH 1 acid stress treatment, indicating bacteria from both fermentations were intolerant to pH 1.  
Solid state fermentation is an ancient biotechnology to promote nutrient utilization and reduce antinutritional factors (ANFs) levels [40]. The probiotics prepared by SSF have been reported to be superior to those from submerged fermentation (SMF) in improving the growth performance and intestinal microbiota balance in broilers and weaned pigs [41; 42], and the bacteria obtained from solid state fermentation exhibit better tolerance to various kinds of environment stress [43]. Our results showed that the survival rate of the cells from SSF was higher than that from LSF no matter whether they were prepared by spray drying or cold-air drying, suggesting an obvious bacterial product production advantages of SSF. However, our study also found that the cells from SSF had significantly lower tolerance to acid stress than the cells from LSF, thus leading to the reduction in live bacteria number, eventually weakening bacterial probiotic function. Therefore, we further explored the differences in the tolerance of the bacteria obtained by the two fermentation methods under acid stress.