Protein degradation, lipid hydrolysis, and fat oxidation are crucial to the formation of the unique flavor of salted and dried fish, and endogenous and microbial enzymes play an important role [7,8]. The relative roles of microbial flora and endogenous enzymes in lipid and protein degradation and flavor formation in salted and dried fish products have been studied. Compared with microbial flora, fish lipase plays a major role in lipid hydrolysis, whereas microorganisms play a dominant role in lipid oxidation and flavor formation [9,10]. Microorganisms play a major role in protein degradation and amino acid metabolism, which is a key step in the formation of protein-derived flavor substances. The typical components detected in salted and dried fish can be divided into nine categories: aldehydes, ketones, alcohols, esters, alkanes, alkenes, aromatic compounds, amines, and other volatile substances [11].

Common flavor substances in salted and dried fish include aldehydes (e.g., hexyl aldehyde, nonal, octyl aldehyde, 3-methyl-butyral, and phenylacetaldehyde), ketones (e.g., 6-methyl-5-heptene-2-ketone; 2, 3-heptene-2-ketone; 3-heptene-2-ketone; 2-heptene-ketone; 2-nonone; 3-octyl ketone; 1-octene-3-ol; hexol; and 1-hexanol), esters (e.g., 2-methyl-butyl butyrate, methyl phthalate, ethyl acetate, and 3-methyl-1-butanol propionate), and olefins (e.g., terpenes). Common key flavor substances such as aldehydes usually have a low sensory threshold and a relatively high content in volatile flavor substances and mainly affect the flavor of dried salted fish (Figure 2) [12].

Microbial succession plays an important role in the formation of flavor during fermentation and is closely related to food safety. Currently, changes in the microbial community composition of salted and dried fish can be analyzed through amplicon sequencing and other methods. Changes in microbial communities have been attributed to changes in substrate composition and fermentation parameters, such as the salt concentration and fermentation temperature [27]. Proteobacteria and Firmicutes have been shown to dominate in salted and dried fish products [28]. Carbohydrate metabolism is reported to be closely related to the presence of Firmicutes, whereas amino acid and lipid metabolism is closely related to the presence of Proteus. Halophilic or halophobic microorganisms are more likely to exist in traditional fermented fish, such as halophilic o-omonas [29]. Cytoscape is used for bacterial correlation visualization, with more than four key flavor substances (ROAV ≥ 1), to identify key microorganisms by analyzing the correlation between bacteria and a volatile flavor via the Pearson correlation coefficient [30].

The complex microbial community plays an important role in flavor formation in traditional dry-cured fish. Based on the summary of the core microorganisms in the main salted and dried fish products (Figure 2), Halomonas and particularly Staphylococcus have been shown to be the most abundant genera detected in high-salt dried salted fish products. Specifically, to some extent, Staphylococcus species are the dominant microorganisms in these fish products. One study found Staphylococcus, with a relative abundance of 34.46%, to be the dominant bacterial genus [31].

Staphylococcus can affect the flavor and quality of dried salted and dried fish. When used as a starter culture, Staphylococcus promotes the rapid formation of flavor and delays the formation of fat oxidation [32]. Similar to LAB, Staphylococcus species have lipolytic and proteolytic activities and can slowly utilize carbohydrates and convert them into organic acids and aromatic substances, such as 2, 3-butanedione; acetaldehyde; and acetoin [33].

In the curing process of salted and dried fish, several metabolites produced by LAB and yeast are flavor substances. LAB can produce lactic, acetic, and propionic acids and can interact with alcohols and aldehydes in the fermentation process to form more complex flavor compounds and to promote the formation of flavor substances. LAB are used as starter cultures in most salted and dried fish products and degrade lipids and carbohydrates and act as biological preservatives [34]. A summary of the microbial community succession in salted and dried fish (Figure 2) shows that Lactobacillus strains (LAB) and Saccharomyces predominate. LAB includes Lactobacillus, Lactococcus, Connostrea, Enterococcus, Pediococcus, and Lactococcus among which Lactobacillus occupies a dominant position in the late curing process and is the dominant bacteria group. During salting, the organic acids (such as acetic and lactic acid) produced by LAB act as the main producers of acids and decrease the pH of the dried salted and dried fish system through glycolysis. Similar to Staphylococcus species, some LAB species have lipase and protease activities, contributing to the formation of dried salted and dried fish’s flavor compounds [35].

Yeast can not only hydrolyze lipids but also can use fatty acids to synthesize esters and therefore has an important role in the formation of salted and dried fish’s flavor. Fungal communities are often dominated by Saccharomyces cerevisiae, Kazachstania exigua, Torulaspora delbrueckii, Wickerhamomyces anomalus, and Pichia kudriavzevii. Demaria and Candida albicans are common yeasts in fermented fish products. They can reproduce in an acidic environment and have a strong perfuming ability [38,39]. The metabolic activities of numerous bacteria are directly related to the flavor of dried salted fish, and the metabolic pathway of flavor compound breakdown is intimately tied to microbial enzymes. Lipid decomposition and oxidation and protein degradation are considered to be the key processes in the formation of dried salted fish’s flavor [40,41]. Through fat decomposition and oxidation, fats become flavor compounds or flavor precursors [42]. However, the excessive oxidation of fat can also lead to undesirable odors in dried fish products. The interaction of endogenous proteases and microbial enzymes cause protein degradation [43]. According to a study on lipid oxidation in black carp during storage, Firmicutes, Proteobacteria, and Actinobacteria were the most common bacterial phyla, with several being strongly linked to meat rotting [38].

During the process of protein degradation, for example, Belleggia et al. demonstrated that flesh-eating Bacillus can decarboxylate amino acids and degrade proline, leucine, and phenylalanine into methylpropanal, 3-methylbutanal, and phenylacetyl, respectively [44].

The KEGG (the Kyoto Encyclopedia of Genes and Genomes) database can also be used to investigate the relationship between microorganisms and flavor. Molecular technologies, such as proteomics, macrotranscriptomics, and metagenomes, are combined with a KEGG analysis to construct the relationship networks between microorganisms and flavor compounds. A genomic analysis has been combined with the KEGG metabolic pathway, metabolic pathways and microbial correlations of key enzymes, and microorganisms to analyze the protein-derived flavor substances and fat-derived flavor substances in dried salted and dried fish [45].

Salted and dried fish products are rich in protein, and protein degradation and amino acid metabolism are consequently important for flavor formation (Figure 2). Microbial metabolism can form flavor substances in products in two main ways. One is through transamination, whereby flavor precursors are formed from amino acids, such as aromatic amino acids (phenylalanine, tyrosine, and tryptophan), branched amino acids (leucine, isoleucine, and valine), and sulfur-containing amino acids (cysteine and methionine) [46]. Branched amino acids are first converted into alpha-ketoacids under the action of aminotransferases; these compounds are then converted into aldehydes through the catalytic action of ketoacid decarboxylate (KdcA) and are further converted into alcohols (Alchol) with the aid of dehydrogenase. Aldehydes can also be converted from hydroxyacid dehydrogenase to organic acids [47]. The second pathway is the elimination reaction, whereby methionine generates sulfur-containing compounds. The associated microorganisms involved in the reactions include Bacillus with amino acid transaminase, Enterobacter with branched-chain amino acid transaminase and with aromatic amino acid transaminase, Macrococcus with alpha-ketoacid decarboxylase, and Lactococcus with phenylpyruvate decarboxylase. Further studies have demonstrated that the protein flavoring substances are related to specific microorganisms during the processing of salted and dried fish.

In salted and dried fish, the fat source is also one of the main sources of flavor (Figure 3). The initial reaction of flavor formation is the hydrolysis of fat into fatty acids by lipase and the release of free fatty acids (FFAs) for second-order fatty acid oxidation or reaction with proteins [50]. Lipoxygenases and lipoxygenases in adipose tissue or muscle fibers are the main endogenous lipases in adipose hydrolysis and oxidation. However, microorganisms have been shown to inhibit the activity of endogenous lipase through rapid fermentation to produce acid in the salting process. Therefore, in the process of fat oxidation, lipid lipoxygenase mainly oxidizes lipids, and automatic oxidation involving microorganisms also occurs. Although endogenous lipase is thought to be primarily responsible for lipolysis, the effect of microbial lipase on lipolysis cannot be ignored [51]. Microorganisms can also promote the release of FFA to a certain extent, and the relative action of endogenous and microbial enzymes can be studied by adding antibiotics to inhibit microbial growth [52].

Fat oxidation degrades unsaturated fatty acids into hydroperoxides (HPods), which can be further decomposed into low-molecular-flavor compounds. Moreover, HPods can react with amino acids or with Maillard-reaction intermediates to produce other odor substances, such as esters [53]. Arachidonic acid is metabolized by lipoxygenase (EC1.13.11) into 1-octylidene-3-alcohol, which can also be metabolized from linoleate to 13-HPOD by lipoxygenase (EC1.13.11) and then to hydroperoxide lyase (EC 4.1.2). In addition to the abovementioned linoleate ester that can generate 13-HPOD, this ester can generate 8-HPOD and 10-HPOD through lipoxygenase (EC 1.13.11). The 8-HPOD, 9-HPOD, 10-HPOD, and 11-HPOD produced via oleic acid metabolism by lipoxygenase (EC 1.13.11) are respectively metabolized by hydroperoxide lyase (EC 4.1.2) into capric aldehyde, heptal, nonal, and octyl, which are then, respectively, metabolized by alcohol dehydrogenase (EC 1.1.1.1) into n-decyl alcohol, heptal alcohol, nonyl alcohol, and octyl alcohol. Linolenic acid can be metabolized to 10-HPOD and 11-HPOD by lipoxygenase (EC 1.13.11) and to nonal and octyl compounds by hydroperoxide lyase (EC 4.1.2), respectively, and then by alcohol dehydrogenase (EC 1.1.1.1) to generate nonyl alcohol and octyl alcohol, respectively [48].