# Flavoromics Analysis of Passion Fruit-Roasted Chicken
## 2 Results and Discussion
### Evolution Patterns for Precursors of Flavor Substances
#### 2.5.1 Free Amino Acids and Their Contents during the Processing  
The types and variation patterns of free amino acids in passion fruit-roasted chicken are shown in Table S4. Free amino acids, as flavor precursors, play an important role in aroma and taste. During the processing, a total of 17 free amino acid components were identified, which significantly decreased from 545.25 mg/100 g in raw chicken to 505.19 mg/100 g in pickled chicken. The possible reason is that soaking in salt water dissolved many amino acids [33]. Free amino acids not only serve as precursors of volatile aroma components  
Figure 2: Changes in the estimated odor activity values (EOAVs) of characteristic flavor components during the processing of passion fruit-roasted chicken. The EOAVs of characteristic flavor components were normalized (0-1). but also enhance the taste of meat products. [PERSON] et al. (2018) found that the free amino acids that contribute significantly to the taste of chicken are alanine, glutamic acid, and arginine. From the perspective of flavor characteristics, amino acids are divided into UFAA (umami free amino acids: glutamic acid and aspartic acid), SFAA (sweet free amino acids: alanine, glycine, serine, and threonine), BFAA (bitter free amino acids: methionine, isoleucine, leucine, arginine, valine, tyrosine, phenylalanine, and histidine), and TFAA (tasteless free amino acids: lysine, proline, and cysteine). Figure 3 shows the changes in the content of amino acids with different flavors during processing. For raw chicken, the top three amino acids regarding content were glutamic acid, histidine, and arginine, with contents of 63.46 mg/100 g, 58.41 mg/100 g, and 57.74 mg/100 g, respectively. Among them, glutamic acid is an important UFAA, and its content significantly decreased (_p_ < 0.05) after 60 min of roasting, dropping to 56.50 mg/100 g. The content of SFAA significantly increased during 30 min of roasting, with serine being more prominent in the finished product, giving it a sweet and refreshing taste. In the finished product, BFAA, SFAA, and UFAA accounted for 43.42%, 24.48%, and 20.72% of the total amino acids, respectively, similar to the distribution of flavored amino acids in black Angus beef (S  
as follows: adenosine triphosphate (5\({}^{\prime}\)--ATP) continuously removes the phosphate group twice to form adenosine monophosphate (5\({}^{\prime}\)--AMP), adenosine monophosphate removes the amino group to form inosine monophosphate (5\({}^{\prime}\)--IMP), inosine acid is degraded to inosine (I) by removing phosphate groups, and inosine is degraded to hypoxanthine (Hx) by removing bases. In raw chicken, Hx and I nucleotides are the main components with higher content, which may be produced by refrigerated meat products. After pickling, the content of 5\({}^{\prime}\)--IMP and 5\({}^{\prime}\)--GMP sharply decreased (\(p<0.05\)), with a total amount of flavor nucleotides of only 11.71 mg/100 g. This is because organic acids such as acetic acid and hexanoic acid are produced during the pickling process, and the acidic environment causes the degradation of flavor nucleotides [37]. At the beginning of heating, the content of 5\({}^{\prime}\)--AMP and 5\({}^{\prime}\)--IMP significantly increased, being 11.5 times and 6.42 times higher than the post-pickling values, respectively. After heating for 60 min, the content of 5\({}^{\prime}\)--IMP increased to 77.09 mg/100 g, which was the highest point in the entire process. In meat products, flavor nucleotides have a significant impact on taste and flavor, but sometimes their content levels are lower. [PERSON] et al. [38] reported that chicken contains abundant compounds of 5\({}^{\prime}\)--nucleotides. [PERSON] et al. [39] indicated that the main nucleotide component of crab meat is AMP, followed by IMP, with a relatively low GMP content (2.3 mg/100 g). Consistent with the research findings of [PERSON] et al. [40], the concentrations of nucleotide ADP, AMP, and GMP in duck meat are relatively low.