## 3 Discussion
### Liquid Chromatography  
The most abundant phenolic compound detected among all samples of balsamic vinegars was gallic acid (Figure 5), followed by other phenolic acids (i.e., caffeic acid, protocatechuric and \(p\)-coumaric acid; Table 2). The presence of these phenolic acids was also reported by [PERSON] et al. [8], and this is in good agreement with our results. Other compounds commonly found in balsamic vinegars are protocatechuric aldehyde, ellagic acid, vanillin, and coniferyl alcohol (with mean concentrations 6.41, 3.76, 5.62, and 4.76 mg/L, respectively [8]). The mean concentration of protocatechuric aldehyde found in our samples was 1.42 mg/L and 0.476 mg/L for vanillin. Coniferyl alcohol was not identified in our samples, and the analysis of ellagic acid was not performed in this work. Figure 5: Comparison of the concentration levels of gallic acid (orange) and its ester ethyl gallate (blue) for set of samples of balsamic vinegars. Error bars represents standard deviation (\(n=3\)). [PERSON] et al. [16] in their work also determined these four phenolic acids as the main compounds, but much higher mean values of the compounds were found in balsamic vinegars with respect to our samples. Furthermore, they quantified tyrosol as the compound with the highest mean concentration (i.e., 109.4 mg/L), contrary to [PERSON] et al. [8], who reported a mean concentration of tyrosol of 0.273 mg/L. Tyrosol had a mean concentration of 1.697 mg/L in our study. Tyrosol is formed from tyrosine during the fermentation process and is directly proportional to the amino acid content in the grape must [39]. [PERSON] et al. [14] determined gallic acid, protocatechuic acid, caffeic acid, and \(p\)-cumaric acid as the main compounds in BVM which agree with our results. Their concentration levels were similar to those of our work and the work by [PERSON] et al. [8]. [PERSON] et al. [24] determined phenolic acids by the GC method and found the following concentrations in TBVM samples (\(n\) = 4): gallic acid (18 mg/L); protocatechuic acid (18.8 mg/L); caffeic acid (10.9 mg/L) and \(p\)-cumaric acid (17.1 mg/L). Besides the phenolic acids, there were two compounds in the samples that were identified by both the HPLC and GC methods, i.e., eugenol and vanillin. Eugenol, which has a clove aroma, showed a higher content determined by HPLC in sample No. 5; however, semi-quantitative GC analysis indicated the highest content in samples No. 3 and No. 4. Vanillin was identified by GC only in samples No. 1 and 2 (the same trademark of the samples), which is consistent with the results of HPLC. Compared to the other balsamic vinegar samples, it is possible that vanillin was added artificially to samples No. 1 and No. 2 as a flavoring agent [40]. In our study, several other compounds (Table 2) with significant concentration levels were found. Salicylic acid, ethyl gallate, and protocatechuic aldehyde were determined in \"premium\" BVM and TBVM. Slight differences in the pyrogallol content were found between the types of balsamic vinegars. The variable content across the samples also showed tyrosol and syringic acid. There are significant differences in the composition of phenolic compounds, determined using the HPLC method, between the categories of balsamic vinegars (see Figure 6). The total phenolic content (mean \(\pm\) standard deviation) for each category was found as follows: Greek balsamic vinegar (samples No. 2-4) (14.69 \(\pm\) 5.01 mg/L); BVM (31.5 \(\pm\) 17.6 mg/L); \"premium\" BVM (75.6 \(\pm\) 13.4 mg/L) and TBVM (56.7 mg/L). From the results, it is obvious that the samples with the expected higher quality have a higher content of phenolic compounds. In sample No. 1 (white Greek balsamic vinegar), a significantly smaller total amount of phenolic compounds was detected.