Determination of the aroma changes of Zhengrong vinegar during different processing steps by SPME-GC-MS and GC-O

Zhengrong vinegar is one of the renowned popular condiments in China, which has been consumed around Hebei province for about 136 years. Its different processing steps are considered as important and critical steps that are responsible for the generation of representative flavors besides the acetic acid. An HS-SPME coupled with GC-MS method was used to evaluate the changes of the volatiles of the vinegar that were generated during the different processing steps including before adding salt (BAS), after adding salt (AAS), after filtration (AF) and final product (FP). In addition, aroma-active volatiles of the final vinegar product were characterized by GC-O coupled with detection frequency technique (modified frequency, MF%) that is a combination of aroma intensity and aroma frequency values that can be used to improve the description of a reliable data from GC-O. Among the 42 detected volatiles, 38 of them were positively identified by their corresponding authentic compounds. Besides, 24 volatiles were recognized as aroma-active volatiles. On the other hand, the content of individual volatile groups was decreased along with the continuation of the fermentation process from the BAS step to FP step, except the aldehydes group that displayed a slight increase. Of which, 2-pentylfuran and ethyl heptanoate disappeared in the AF and FP samples. The main contributors to the Zhengrong aroma profile that had MF (%) > 80 included isovaleric acid, 2-acetoxy-3-butanone, hexanoic acid, ethyl hexanoate, 3-(methylthio)propyl acetate, decanal and 2-phenyl-2-butenal, while the volatile compounds that had a middle MF in a range between 70 and 80 were 2-methylbutanoic acid, tetramethylpyrazine, nonanal, 1,2-dimethoxybenzene, 4-ethylphenol, ethyl benzoate, ethyl benzeneacetate, 2-phenylethyl acetate and 4-ethylguaiacol.