The physicochemical properties and volatile compounds analysis in watermelon soy sauce, a Chinese traditional fermented condiment

Watermelon soy sauce (WSS) is a liquid condiment used in Chinese culinary art. Mainly used in southern China, WSS is usually prepared using watermelon juice, soybeans, and wheat flour as the main raw materials through the process of koji making and natural fermentation. This work was undertaken to characterize physicochemical properties, free amino acid composition and volatile compounds of WSS using SPME-GC/MS and E-nose. We noticed that WSS had the highest total acids content, but the lowest amino nitrogen and reducing sugar contents, as compared to that of commercial soy sauce. Moreover, the highest amount of glutamic acid and proline were observed in WSS. 173 volatile compounds were identified in WSS, including alcohols, hydrocarbons, esters, ketones and aldehydes. Compared to commercial soy sauce, WSS exhibited a distinctive aroma characteristic owing to significantly higher relative contents of phenylethyl alcohol, 2-furanmethanol, 2,4-dimethylbenzaldehyde, 4-hexenoic acid, butanoic acid and 4-methyl-pentanoic acid. E-nose analysis showed a good capacity of differentiating braised samples mainly through W5S, W1S, W1W, W2W and W3S sensors. The analysis of relationships between flavor components and free amino acids in soy sauce samples showed that Ser, Gly, Val, Ile, Leu, Phe and Lys had a great positive correlation with alcohol and acidic compounds. Moreover, Pro was found to correlate significantly with aldehyde, ketone, heterocyclic compounds, sulfur compounds and benzene. Additionally, glutamic acid also showed an apparent positive association with various aromatic compounds, including hydrocarbons, aldehyde and benzene. This study could provide an important information regarding WSS quality control, characterization and aroma improvement.