## 3 Results and Discussion
### Identification of Key Volatile and Non-Volatile Compounds in Raw Meat during Refrigeration
#### 3.3.2 Changes in Key Non-Volatile Compounds  
As shown in Table 2, the main free amino acids in raw meat were Glu (24.85-67.72 mg/100 g), Ala (22.67-41.55 mg/100 g), which were umami and sweet, respectively. The content of 5\({}^{\prime}\)-nucleotides and the corresponding nucleotides of raw meat at three chilling rates during refrigeration were shown in Table 3. 5\({}^{\prime}\)-GMP was the most abundant nucleotide in raw meat, ranging from 1797.17 to 2555.08 mg/100 g. The 5\({}^{\prime}\)-IMP content was ranged from 153.41 to 722.99 mg/100 g, and 5\({}^{\prime}\)-ADP content was ranged from 64.72 to 146.69 mg/100 g. AMP, Hx, and HxR were ranged from 16.78 to 47.79 mg/100 g, 16.71 to 33.35 mg/100 g, and 12.72 to 20.01 mg/100 g, respectively. Based on TAVs, Asp, Glu, 5\({}^{\prime}\)-GMP, 5\({}^{\prime}\)-IMP, and 5\({}^{\prime}\)-AMP were the key non-volatile compounds in raw meat during refrigeration with different chilling rates. As shown in Figure 5, the rate of decrease in IMP and GMP contents with a chilling rate of 32.4 \({}^{\circ}\)C/h was significantly lower (\(p<0.05\)) than that with 1.44 \({}^{\circ}\)C/h. When the chilling rate was 1.44 \({}^{\circ}\)C/h, the contents of UAAs at 24 h after slaughter were significantly lower (\(p<0.05\)) than that at other times. However, at 22.2 and 32.4 \({}^{\circ}\)C/h, the contents of UAAs were the lowest (\(p<0.05\)) at 72 h after slaughter. To get a better view of the umami intensity of raw meat, the EUCs of umami amino acids and taste nucleotides were calculated. As could be clearly observed in Figure 6, the EUCs and contents of UAAs showed the same trend at different chilling rates. The reduction in the contents of UAAs and FAAs might be related to the enhanced glycolysis and the rapid decrease in pH and ATP content during refrigeration. Several studies showed that rapid chilling rate could significantly inhibit pH decline, ATP consumption and suppress glycolysis [21; 58; 59]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Compound 1 **} & \multicolumn{2}{c}{**LRI 2 **} & \multicolumn{2}{c}{**Identification 3 **} & \multicolumn{2}{c}{**Compound 1 **} & \multicolumn{2}{c}{**Calculated 1 **} & \multicolumn{2}{c}{**Literature 2 **} \\ \hline Hexanal & 1076 & 1080 & MS, LRI, S & 2-3-Butanediol & 1571 & 1570 & MS, LRI \\ Heptanal & 1168 & 1170 & MS, LRI & 2-Octen-1-ol, (E)- & 1606 & 1603 & MS, LRI, S \\ Octanal & 1269 & 1275 & MS, LRI, S & 1-Nonen-4-ol & 1635 & -1 & MS \\ Nonanal & 1376 & 1380 & MS, LRI & 1-Nonanol & 1657 & 1656 & MS, LRI, S \\ (E)-2-Octenal & 1412 & 1416 & MS, LRI, S & 1-Nonen-3-ol & 1747 & 1555 & MS, LRI \\ Decanal & 1481 & 1483 & MS, LRI, S & acetic acid & 1442 & 1441 & MS, LRI \\ Benzaldehyde & 1502 & 1495 & MS, LRI, S & Pentanoic acid & 1729 & 1734 & MS, LRI \\ (E)-2-Nonanal & 1518 & 1514 & MS, LRI, S & Hexanoic acid & 1835 & 1834 & MS, LRI, S \\ cis+4-Decneal & 1525 & 1542 & MS, LRI & Heptanoic acid & 1966 & 1967 & MS, LRI \\ (E, E)-2-A Nonanal & 1682 & 1687 & MS, LRI, S & Octanoic acid & 2068 & 2072 & MS, LRI \\ 2-Undecanal & 1739 & 1755 & MS, LRI & Nonanoic acid & 2171 & 2174 & MS, LRI \\ (E, E)-2-4-Decadienal & 1752 & 1763 & MS, LRI, S & 3-hydroxy-2-butanone & 1265 & 1259 & MS, LRI \\ Tetradecanal & 1900 & 1908 & MS, LRI & 2-3-Octanedione & 1311 & 1325 & MS, LRI \\ Pentadecanal & 2037 & 2042 & MS, LRI & 3-Octen-2-one & 1393 & 1414 & MS, LRI \\ 1-Pentanol & 1262 & 1261 & MS, LRI & 3-Nonaden-2-one & 1801 & -1 & MS \\ 1-Hexanol & 1354 & 1356 & MS, LRI, S & Hexanoic acid, methyl ester & 1170 & 1177 & MS, LRI \\ 1-Octen-3-ol & 1447 & 1447 & MS, LRI, S & Nonanoic acid, methyl ester & 1477 & 1481 & MS, LRI \\ 1-Heptanol & 1452 & 1458 & MS, LRI, S & Furan, 2-pentyl- & 1202 & 1213 & MS, LRI, S \\ 1-Octanol & 1553 & 1554 & MS, LRI, S &styrene & 1234 & 1242 & MS, LRI \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Identification of volatile compounds in raw meat. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{ \begin{tabular}{c} **Content** \\ **(mg/100 g)** \\ \end{tabular}