## 3 Results and Discussion
### GC-MS Assay of Specimens before and after Fermentation
#### 3.4.2 Volatile Flavor Compounds of Four Groups of Unfermented Specimens
We hypothesized that part of the conjugated hexanal with enzymolysis can be transformed into a free state, accordingly strengthening the degradation of hexanal [2; 39]. This finding was in agreement with the result of previous literature [19]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sample** & **CP-0 h** & **CPB-0 h** & **CPY-0 h** & **CPBY-0 h** & **Sum** & **Shared** \\ \hline \multicolumn{6}{l}{**Type**} \\ \hline \hline \multicolumn{6}{l}{**Hydrocarbon**} & 0 & 1 & 5 & 7 & 12 & 0 \\ \multicolumn{6}{l}{**Alcohol**} & 3 & 2 & 3 & 4 & 6 & 2 \\ \multicolumn{6}{l}{**Aldehydes**} & 8 & 8 & 5 & 5 & 8 & 5 \\ \multicolumn{6}{l}{**Ketone**} & 0 & 0 & 0 & 2 & 2 & 0 \\ \multicolumn{6}{l}{**Esters**} & 0 & 1 & 1 & 0 & 2 & 0 \\ \multicolumn{6}{l}{**Acids**} & 0 & 0 & 1 & 0 & 1 & 0 \\ \multicolumn{6}{l}{**Phenols**} & 0 & 2 & 2 & 2 & 2 & 0 \\ \multicolumn{6}{l}{**Other**} & 2 & 2 & 2 & 2 & 2 & 2 \\ \multicolumn{6}{l}{**Total**} & 13 & 16 & 19 & 22 & 35 & 9 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Types of volatile odor substances in four groups of unfermented specimens. Figure 6: Comparison of volatile odor substances in four groups of unfermented specimens. CP-0 h, unfermented chickpea milk; CPB-0 h, unfermented chickpea milk with papain treatment; CPY-0 h, unfermented chickpea milk with yam addition; CPBY-0 h, unfermented chickpea milk with papain treatment plus yam addition. The primary non-beany-flavor ingredients in legume milk mainly include nonanal and (E)-2-ocrenal [13; 19], which present green broccoli, citrus-like, and cucumber flavor [12]. As illustrated in Table 3, nonanal was detected in all four groups of unfermented specimens. Compared with CP specimen (9.79%), the contents of nonanal in CPB, CPY, and CPBY specimens were increased to 13.69%, 11.12%, and 13.06%, respectively. Previous investigations demonstrated that nonanal was degraded by unsaturated fatty acids to form 9-hydroperoxide, which was then decomposed by HPL [19; 40]. It was speculated that enzymolysis promoted the degradation of fatty acids, leading to the increase of nonanal. (E)-2-Octenal, with a fatty aroma and strong peanut cake flavor, was obtained from 11-hydroperoxide during the degradation of linolenic acid by LOX [41; 42]. Similar to nonanal, compared with those in the CP sample (7.21%), the contents of (E)-2-ocrenal in CPB and CPBY samples with enzymolysis increased to 8.86% and 8.46%. The contents of heptanal in the CP sample were 7.23%, and that in the CPB sample hydrolyzed by papain was 0.24%. However, the contents of heptanal were not detectable in CP and CPB samples. In general, low aliphatic aldehydes possess unpleasant-smelling odorants. These compounds decrease with the increase of molecular weight. C\({}_{8}\)-C\({}_{12}\) saturated aldehydes have pleasant aroma characteristics, such as nonanal with rose fragrance and decanal with a refreshing sweet fragrance of flowers. Moreover, certain content of 3-hydroxy-2-butanone (1.52%) and 4-methoxyphenyl ethyl ketone (2.86%) was identified in the CPBY sample, which may endow it with a sweet flavor character. The main component of fresh frankinense is dimethyl sulfide, which has a low threshold and is formed by the degradation of proteins. Dimethyl sulfide was detectable in all four groups of unfermented specimens. Among them, the CPY specimen had the highest content of dimethyl sulfide (1.06%). Some of the flavor compounds in chickpea milk were in a free state, whereas others were in a binding state (conjugated flavor precursors). The conjugated aromatic precursors could be released by enzymatic treatment, resulting in the regeneration, enhancement, and variation of odorants [2]. In addition, compared with the CP specimen, alkane substances and phenol compounds with a slightly fruity aroma were detected in unfermented specimens with yarn addition (CPBY and CPY). The above results indicated that the flavor characteristics of the specimens could be improved by papain treatment and yarn addition.