The flavor of fermented foods is mostly generated through microbial metabolism and enzymatic biochemical reactions
[111]. It is a key indicator of sensory qualities, which often directly determines acceptance by consumers
[112]. Characterization of fermented food flavor also contributes to determining the maturity of a product and the differences among products obtained from different production locations or fermentation strains
[112][113]. It mainly depends on the composition of volatile flavor compounds, which constitute a preferred flavor, or an unpleasant flavor based on different composition conditions. Flavor components are usually identified by GC-MS
[114][115]. The most relevant information obtained from metabolomic studies on the flavors of fermented foods is presented in
Table 2.
Distinctive flavors are essential factors in alcoholic beverages, including
baijiu,
huangjiu, beer, and red wine, which are all commonly consumed in Chinese society. Yang et al.
[116] applied GC-GC-TOF-MS to analyze
baijiu made from yellow, white, and black daqu, and captured 401 volatile compounds, mainly aromatic compounds and pyrazines. Among them, white daqu has the most types of esters and alcohols, but the levels of esters and alcohols in yellow and black daqu were higher than in white daqu, which may be related to microbial metabolism during fermentation. The flavor of Chinese
huangjiu from Shanxi, a yellow rice wine with zao-aroma, was also analyzed based on GC-MS and GC-olfactometry (GC-O) techniques, and ethyl cinnamate and ethyl 3-phenylpropionate were demonstrated to be key aroma compounds, which indicated differences from others such as Zhejiang Shaoxing and Fujian Hongqu
huangjiu [112][115]. Pu-erh tea is a unique tea obtained from the fermentation of
Monascus tea, and the understanding of the formation and evolution mechanism of its flavor-active compounds can improve its quality. Deng et al.
[117] summarized the metabolic evolution of key flavor-active compounds during Pu-erh tea fermentation systematically based on headspace solid phase microextraction (HS-SPME)-GC-MS. The volatile fingerprint and flavor profiles of fermented soybean foods have been investigated using metabolomics technologies. The flavor of fermented soy foods significantly reduces the proportion of beany flavor substances in the soybean itself, which is an unpleasant flavor affecting its consumption
[118]. Wangzhihe red sufu, a Chinese traditional fermented condiment, was analyzed using metabolomics during the fermentation process, and four typical commercial red sufu products were detected by GC-MS, GC-O/MS, and E-nose
[119]. The most prevalent volatile components (e.g., phenolics, esters, and alcohols) varied in the molded phetze, salted phetze, and post-fermentation stages of red sufu, and there were differences in the overall volatile profiles of the four sufu products, which were conducive to effective discrimination of the extent of sufu maturity and the type of sufu products. Notably, the flavor of products related to natto, and soybeans fermented by
B. subtilis, are usually undesirable to the Chinese population, whereas natto is very popular in Japan where it is an important side dish. The volatile metabolite composition of natto products has been investigated through GC-MS and NMR, and it has been shown that ammonia, 2,5-dimethylpyrazine, isovalerate, isobutyrate, and 2-methylbutyrate are increased in amount and proportion in natto, which may explain the undesirable flavor
[120][121][122]. Gao et al.
[122] studied soymilk fermented by
B. subtilis through NMR and not only found undesirable flavor compounds but also explained the changes in nutrients and functional components in fermented soymilk. In addition, metabolomics has played a key role in determining the maturity of cheese and other dairy products
[113], the analysis of the correlation between flavor components and microbial communities in the types of fermented mandarin fish
[123], and the elucidation of dynamic changes in flavor composition during the fermentation of rose vinegar and shrimp paste
[124][125].