Two independent variables, temperature and pH, were simultaneously evaluated to determine the stability of AA-2G. The experimental design based on the surface response methodology was shown in Table 2. The central point was set at 60 °C and pH 6.5. The upper temperature and pH was 85 °C and pH 9.0 respectively. The lower temperature and pH was 35 °C and pH 4.0 respectively. Samples were placed in sealed glass bottles and heated in a water bath for 10 h. After the heating process was complete, each sample was diluted to 1000