## 3 Results and Discussion
### Visual Quality and Hardness  
The color changes of potatoes after unpacking are shown in Figure 4. As exhibited in Figure 4a, a small part surfaces of samples from day 0 to day 8 changed gradually from yellow to brown, whereas a large surface area of the day 12 sample turned pink quickly and then dark brown. The starting time of discoloration for these samples is recorded in Table 1. The surface discoloration of the 0-, 4-, 8- and 12-day samples started at about 31.88, 35.50, 20.35 and 5.71 min, respectively. A delay of browning on day 4 was observed  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Storage** & **pH** & **Lactic Acid** & **Acetic Acid** & **Hardness** & **Browning** \\
**Time (Day)** & **pH** & **(mg/g FW)** & **(mg/g FW)** & **(N)** & **Time (min)** \\ \hline
0 & 5.91 \(\pm\) 0.05 a & 0.04 \(\pm\) 0.01 d & 2.06 \(\pm\) 0.07 c & 37.76 \(\pm\) 5.53 a & 31.88 \(\pm\) 6.31 b \\
4 & 5.73 \(\pm\) 0.04 b & 0.24 \(\pm\) 0.04 c & 2.20 \(\pm\) 0.08 c & 37.53 \(\pm\) 3.62 a & 35.50 \(\pm\) 2.84 a \\
8 & 5.67 \(\pm\) 0.06 b & 0.45 \(\pm\) 0.03 b & 3.39 \(\pm\) 0.65 b & 37.13 \(\pm\) 3.96 a & 20.35 \(\pm\) 3.14 c \\
12 & 5.60 \(\pm\) 0.08 c & 0.79 \(\pm\) 0.09 a & 4.87 \(\pm\) 0.06 a & 31.55 \(\pm\) 6.70 b & 5.71 \(\pm\) 1.29 d \\ \hline \hline \end{tabular} Different superscripts in the same column indicate significant differences.  
\end{table}
Table 1: Changes in pH, lactic acid, acetic acid content, hardness and browning time of vacuum-packaged potatoes during storage.  
Figure 3: Mesophilic bacteria, psychrotrophic bacteria and molds and yeasts of vacuum-packaged peeled potatoes during storage. The different letters in mesophilic, psychrophilic bacteria and molds and yeasts indicate significant differences (\(p<0.05\)).  
compared to day 0, which could have been due to the inhibition of phenylalanine ammonia lyase activity by the increase of organic acid [38]. The discoloration of day 12 samples happened significantly earlier than other samples, which was also found in the results of the color change measured by the colorimeter (Figure 4b-d). Lightness (\(L^{*}\)), reddish-greenish (\(a^{*}\)) and yellowish-bluish (\(b^{*}\)) values also suggested a browning process of potatoes after opening the package. The samples of day 12 showed an obviously different trend compared to others. As presented in Figure 4b,d, a continual decrease was observed in the \(L^{*}\) and \(b^{*}\) values. In terms of \(a^{*}\) values, day 12 showed an increasing trend firstly, and then it kept constant after 60 min. The aggravation of color change on the 12 th day may have been due to cell damage, resulting in a mixing of enzymes with phenolic compounds, which was caused by microbial excessive proliferation and its metabolites' production [28; 38]. In addition, anaerobic bacteria (_Clostridium_) that multiplied during storage in this study have been reported to cause potato slimy rot, manifested by formation of pink pigments (aromatic polyketides metabolites), which can enable the anaerobic bacteria to survive when exposed to an oxygen-rich environment [39]. This could also explain why the potato turned pink rapidly after unpacking on day 12 compared with other samples.  
The hardness of potatoes did not change significantly during the first 8 days (Table 1). As the storage was prolonged to 12 days, the potatoes softened obviously in comparison with other samples. The softening of potato tissues could have been due to the accumulation of organic acids, causing an increase of water-soluble pectin [28] or an elevation of pectin-degrading enzymes' activities [40; 41] and then resulting in pectin degradation in the cell wall. Additionally, pectin-decomposing bacteria (_Erwinia_, _Clostridia_) can secrete pectin-degrading enzymes and lead to pectin degradation [39; 42].  
Figure 4: Changes of visual quality (**a**) and the values of \(L^{*}\) (**b**), \(a^{*}\) (**c**) and \(b^{*}\) (**d**) of vacuum-packaged peeled potatoes during storage within 180 min after unpacking.