The process the team studied differs fundamentally from conventional brine fermentation. Traditional Chinese pickled vegetables such as paocai and suancai are typically submerged in prepared brine, allowing salts, sugars, and microbial metabolites to circulate freely between the plant tissue and the surrounding liquid. Vacuum dry-salted cowpeas, by contrast, are mixed directly with salt at 8 percent of the fresh weight, sealed in food-grade plastic bags without any added liquid, and left to ferment in the dark at room temperature. Only then does liquid gradually exude from the salted tissues, creating a fermentation environment with restricted oxygen exchange and a slowly developing aqueous phase. Because these conditions could shape microbial succession and flavor chemistry in unique ways, the researchers sampled the fermenting cowpeas at five time points: days 0, 3, 7, 14, and 21.

The physicochemical data tell a story of rapid acidification. The pH of fresh cowpeas stood at 5.94, but it plunged to 3.68 by day 7 and bottomed out at 3.46 by day 14 before rising slightly to 3.90 at day 21. Titratable acidity climbed in parallel, from 1.92 grams per kilogram initially to a peak of 7.62 grams per kilogram at day 14. Reducing sugars, the fuel for fermentation, collapsed from 1.92 grams per 100 grams to a mere 0.10 grams per 100 grams over the three weeks, indicating that fermentable carbohydrates were almost entirely consumed. Meanwhile, amino acid nitrogen nearly tripled, rising from 0.87 to 2.18 grams per 100 grams by day 14, a sign that proteins were being broken down into amino acids and peptides that contribute savory taste and serve as precursors for aroma compounds.

Food safety emerged as a reassuring subplot in the data. Nitrite, a compound of concern in fermented vegetables because of its potential health risks, rose transiently during the early stage, peaking at 0.49 milligrams per kilogram on day 3, then declined steadily to 0.22 milligrams per kilogram by day 21. That final value sits far below the Chinese national safety limit of 20 milligrams per kilogram, suggesting that the acidification and microbial dynamics of vacuum dry-salted fermentation naturally suppress nitrite accumulation. The researchers attribute this decline to a combination of factors, including the inhibitory effect of low pH on nitrite-producing microorganisms and possible microbial reduction of nitrite itself.

Beneath these chemical shifts lay an equally dramatic ecological upheaval. Amplicon sequencing of the 16S rRNA gene and the fungal internal transcribed spacer region showed that the bacterial community underwent near-total replacement. At day 0, the phylum Pseudomonadota dominated, accounting for 91.67 percent of bacterial reads, a signature of the raw plant surface and its processing environment. By day 7, Bacillota had surged from undetectable levels to 93.75 percent and remained dominant through the end of fermentation. At the genus level, the fresh cowpea microbiota of Acinetobacter, Sphingomonas, and Methylobacterium gave way to a transitional community at day 3 in which Weissella, Lactiplantibacillus, Klebsiella, Kosakonia, Enterobacter, and Pediococcus coexisted, before Lactiplantibacillus took command from day 7 onward, ultimately reaching 66.67 percent relative abundance by day 21.

The fungal community followed its own arc of succession. Plant-associated genera such as Plectosphaerella and Verticillium, abundant at the start, were suppressed under the acidic, saline, and oxygen-poor conditions inside the sealed bags. In their place, yeast-like fungi flourished in stage-specific waves: Debaryomyces peaked at 20.7 percent on day 7, while Pichia and Millerozyma rose later, with Millerozyma reaching 8.8 percent by day 21. These genera are known flavor engineers. Debaryomyces is notably tolerant of salt and acid and can generate alcohols and esters from sugars and amino acids, while Pichia is recognized for its strong capacity to produce aroma-active volatiles. Statistical analysis confirmed that fermentation stage significantly structured both bacterial and fungal communities, with PERMANOVA yielding p-values of 0.001 for both.

The volatile chemistry captured by headspace solid-phase microextraction gas chromatography-mass spectrometry revealed 95 distinct compounds and a clear narrative of replacement. Fresh cowpeas were dominated by C6 aldehydes and alcohols born of plant lipid oxidation: hexanal at 655.09 micrograms per kilogram, 2-hexenal at 734.88 micrograms per kilogram, and (Z)-3-hexen-1-ol, all carriers of green, grassy, fresh-cut-vegetable notes. These compounds collapsed within days of fermentation onset. In their place, fermentation-associated volatiles accumulated. Phenylethyl alcohol, a rose-scented compound derived from aromatic amino acid metabolism, climbed steadily to 272.42 micrograms per kilogram by day 21, becoming the third most abundant volatile. Esters increased roughly eightfold, from 38.98 to 312.91 micrograms per kilogram, shifting from a single green-note ester to a diverse array including ethyl hexadecanoate and 2-phenylethyl acetate, which lend fatty, sweet, floral, and fruity nuances.

The single most striking transformation involved 4-ethylphenol, a heterocyclic-adjacent phenolic compound that surged from 22.58 micrograms per kilogram at day 3 to 619.13 micrograms per kilogram at day 7 and ultimately reached 641.29 micrograms per kilogram at day 21, accounting for roughly 29.3 percent of the total volatile content. This smoky, spicy, phenolic-smelling compound effectively defines the mature product’s aroma profile, though the authors caution that its sensory impact is concentration- and matrix-dependent, and excessive levels could tip into undesirable territory. By intersecting statistical screening with odor activity values, the team distilled 11 candidate volatile markers of fermentation stage, including hexanal and 2-hexenal for the fresh stage, lipid-derived aldehydes such as (E)-2-heptenal and 1-octen-3-one for the early phase, and phenylethyl alcohol and 4-ethylphenol for the late stage. Such markers could eventually underpin process-monitoring and quality-control tools for manufacturers.

Untargeted metabolomics added a third dimension, detecting 4,257 annotated features dominated by lipids, organoheterocyclic compounds, organic acids, and phenylpropanoids. The differential metabolites were overwhelmingly stage-specific rather than shared across comparisons, indicating that flavor development proceeds through successive metabolic waves rather than a fixed set of continuously changing compounds. Early fermentation was marked by lipid remodeling and the accumulation of oxidized fatty acid derivatives that mirrored the lipid-derived volatiles. Middle and late stages saw the enrichment of phenyllactic acid and hydroxyphenyllactic acid, signature products of lactic acid bacterial metabolism of phenylalanine and tyrosine, alongside accumulating peptides such as gamma-glutamylleucine that signal active proteolysis. Organoheterocyclic compounds tied to tryptophan and amino acid degradation rose as plant-derived glycosides and carbohydrate-related metabolites declined, a molecular reflection of the raw material being progressively consumed and transformed.

Correlation analysis wove these threads together into a network of 47 nodes and 82 edges linking microbes, volatiles, and non-volatile metabolites. Early-stage bacteria such as Acinetobacter and Methylobacterium co-varied perfectly with the green-note compound (Z)-3-hexen-1-ol, while Klebsiella tracked the lipid-derived aldehydes of the early phase. In the late stage, Lactiplantibacillus co-varied with phenylethyl alcohol and 4-ethylphenol, and the yeasts Pichia and Oligophagozyma showed parallel temporal associations with these same aroma compounds. The authors are careful to stress that these are co-occurrence patterns, not proof of causation, and that semi-quantified volatile data and a single fermentation batch limit generalizability. Still, the study delivers something the fermented vegetable industry has lacked for this product: a temporal framework connecting who the microbes are, what chemistry they drive, and when each flavor milestone occurs. Future work involving targeted quantification, sensory panels, and inoculation experiments will determine whether these candidate markers can be translated into practical tools for ensuring that every batch of vacuum dry-salted cowpeas delivers the same beloved sour, floral, and fermented character.

Subject of Research: Microbial succession and flavor compound evolution during vacuum dry-salted cowpea fermentation

Article Title: Flavor development in vacuum dry-salted cowpeas: changes in volatile compounds and non-volatile metabolites during fermentation

Article References: Xie, Q., Zhou, Z.-G., Sun, Y., Li, T.-Y., Zhang, K.-X., Ou, H.-Y., Man, R.-J., Li, C.-B., & Xin, M. (2026). Flavor development in vacuum dry-salted cowpeas: changes in volatile compounds and non-volatile metabolites during fermentation. Food Chemistry: X, Article 104605. https://doi.org/10.1016/j.fochx.2026.104605

Image Credits: AI Generated

DOI: Not provided

Keywords: cowpea fermentation, lactic acid bacteria, Lactiplantibacillus, volatile compounds, metabolomics, 4-ethylphenol, phenylethyl alcohol, food flavor chemistry, fermented vegetables, microbial succession, nitrite safety, vacuum dry-salting