Physicochemical Properties and Aroma Compounds Analysis in Watermelon Soy Sauce

Watermelon soy sauce (WSS) is a liquid condiment usually prepared using watermelon juice, soybeans, and wheat flour through the process of making koji and natural fermentation. It is widely used in Chinese culinary art, despite the lack of knowledge about its aromatic compound content. Here, we characterized the 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 acid content but the lowest amino nitrogen and reducing sugar contents compared with commercial soy sauce. Moreover, the highest amounts of Glu and Pro were observed in WSS. A total of 173 volatile compounds were identified in WSS, including alcohols, hydrocarbons, esters, ketones and aldehydes. The 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, Ph,e and Lys had a strong positive correlation with alcohol and acidic compounds. Moreover, Pro was found to be correlated with aldehyde, ketone, heterocyclic compounds, sulfur compounds, and benzene, while Glu was correlated with hydrocarbons, aldehyde, and benzene. This study could provide important information regarding WSS quality control, characterization, and aroma improvement.