Organic acids can keep apple base wine stable before distillation, but timing and acid type decide the flavor

Adjusting the acidity of apple distilled base wine with ordinary food-grade organic acids can suppress microbial growth during the weeks it sits in tanks before distillation, but the choice of acid and the timing of its addition produce sharply different effects on both microbial stability and the flavor of the final spirit. That is the central finding of a laboratory study in which researchers from Tai’an, China, treated Fuji apple juice and fermented apple base wine with malic, lactic, or citric acid at pH 3.0 and followed bacterial communities and volatile compounds over a 56-day storage period. The work also carries a caution: one of the three acids, lactic acid, stimulated abnormal accumulation of ethyl lactate and is not recommended by the authors.

The problem the study addresses stems from how apple brandy is made. After apple juice is fermented into a low-alcohol base wine, that wine is double-distilled and the distillate is matured in oak barrels. In actual production, however, the base wine is not distilled immediately after fermentation. It is typically held for days or weeks to match production scheduling, allow natural clarification, or accumulate sufficient batch volume for distillation. During this holding period the wine is vulnerable. Its modest alcohol content and abundant nutrients make it hospitable to spoilage yeasts such as Dekkera and Brettanomyces, which increase markedly and produce unpleasant off-flavors; film-forming yeasts that build surface layers and alter the aroma-active compound profile; acetic acid bacteria that oxidize ethanol into sharp, vinegar-like acetic acid; and lactic acid bacteria that can induce malolactic fermentation or produce biogenic amines. Critically, whatever these microbes produce can be carried over into the distillate during distillation and cannot be removed by subsequent processing.

The industry’s most common antimicrobial agent in fermented wine, sulfur dioxide, is of limited use in distilled spirits. SO₂ readily forms stable adducts with carbonyl compounds that evaporate into the distillate during distillation, significantly altering its flavor, and controlling residual SO₂ requires complex desulfurization that increases costs and risks introducing oxidative flavor defects. That leaves pH adjustment as an attractive alternative, because microbial growth in fermented products is highly pH-dependent. Organic acids work through a two-pronged mechanism: they lower the environmental pH, and their undissociated forms freely penetrate bacterial cell membranes, dissociating in the near-neutral intracellular environment to cause intracellular acidification and enzyme inhibition. The researchers chose malic, lactic, and citric acids, the three dominant organic acids in apple wine, partly because all three are non-volatile and therefore do not evaporate into the distillate or directly interfere with the aroma of the final spirit.

The experimental design compared two timing strategies at a uniform pH of 3.0, a value chosen because it represents a critical threshold for yeast growth and fermentation. In the pre-fermentation treatments, malic acid at 7.98 grams per liter, lactic acid at 10.15 grams per liter, or citric acid at 7.49 grams per liter was dissolved into apple juice before yeast inoculation. In the post-fermentation treatments, the same acid doses were added to clarified wine after fermentation, once the yeast sediment had been removed. All wines, along with an untreated control, were stored in the dark at room temperature for 56 days, with samples taken at 0, 7, 14, 28, 42, and 56 days. Fermentation itself was unaffected: finished wines contained between 2.6 and 4.4 grams per liter of residual sugar and 8.64 to 8.86 percent alcohol by volume, confirming that pH 3.0 did not hamper yeast performance.

The microbial results were striking. In the untreated control, bacterial counts increased rapidly during storage, particularly after day 14, reaching 7.77 log CFU per milliliter by day 56, accompanied by a 2.4 grams per liter drop in total acidity and a 0.34 rise in pH. Pre-fermentation acid treatment told a different story: no bacteria were detected in the treated wines at the end of fermentation, and all pre-treated groups maintained microbial stability for up to 42 days, with the citric and lactic acid groups staying below 4.00 log CFU per milliliter even at day 56. Post-fermentation treatment produced a twist. Lactic acid delivered a dramatic initial effect, with no viable microorganisms detected during the first 14 days, because the wine’s post-fermentation pH of 3.85 sat almost exactly at lactic acid’s pKa of 3.86, leaving a large proportion of the acid in its membrane-penetrating undissociated form. But as the lowered pH increased lactic acid dissociation, the bactericidal grip loosened and counts climbed to 5.76 log CFU per milliliter. Citric acid, whose pKa of 3.13 best matched the adjusted wine pH, kept counts below 3.00 log CFU per milliliter through 42 days of storage.

Sequencing of the 16S rRNA gene revealed which organisms the acids were actually suppressing. In the control wine, Firmicutes dominated at 92.28 percent relative abundance, with the genus Oenococcus accounting for 84.22 percent and Lactobacillus 8.06 percent. Oenococcus oeni converts L-malic acid to L-lactic acid during malolactic fermentation, and its proliferation explains the acidity decline in the control, making it the main microorganism affecting base wine quality stability during storage. Malic and lactic acid treatments, whether added before or after fermentation, reduced Oenococcus abundance but shifted the community toward Proteobacteria dominated by Acetobacter, a genus whose species Acetobacter malorum tolerates pH around 3.0 and maintains acetic acid production. Citric acid treatment produced the most profound community shift: the treated wines showed the highest Shannon diversity indices, 4.69 and 3.79, and Chao richness values of 507.56 and 514.15 among all groups, with no single genus dominating. Crucially, citric acid suppressed both Oenococcus and Acetobacter simultaneously.

Then came the flavor consequences. Gas chromatographic monitoring of 16 quantified target volatiles showed that the control wine remained relatively stable for the first 28 days, but after day 42, ethyl lactate, acetic acid, isoamyl alcohol, 1-butanol, hexanoic acid, and several other compounds increased markedly. The treated wines diverged in treatment-specific ways. Malic acid groups accumulated ethyl acetate, reaching 288.02 milligrams per liter in the post-fermentation malic group by day 56. Lactic acid groups diverged earliest, driven by rapid ethyl lactate and acetaldehyde accumulation; the post-fermentation lactic group ended with the highest total esters, 574.66 milligrams per liter, total aldoketones, 452.87 milligrams per liter, and total acids, 169.76 milligrams per liter. Citric acid groups, paradoxically, hyper-accumulated acetaldehyde, reaching 364.93 milligrams per liter in the post-fermentation citric wine, the highest of all groups, apparently through metabolic reprogramming of acid-stressed surviving microorganisms. The 42-to-56-day window emerged as the critical period for volatile divergence, dominated by the accumulation of total esters and total aldoketones.

After double distillation with head and tail removal, the differences persisted into the spirit itself. Acetaldehyde in the post-fermentation citric distillate reached 364.93 milligrams per liter, with a potential odor impact about 12.6 times that of the control, well above the 125 milligrams per liter level at which it imparts green vegetable and astringent notes. Acetal, formed when acetaldehyde condenses with ethanol, reached odor activity values of 446.41 in the pre-fermentation lactic spirit and 303.67 in the post-fermentation citric spirit, versus 47.89 in the control. Ethyl acetate, a key fruity ester, peaked at 517.38 milligrams per liter in the post-fermentation lactic spirit. Pre-fermentation acid treatment suppressed n-propanol formation to 75.08 to 84.13 milligrams per liter, likely by altering yeast metabolism, while the control retained the most isoamyl alcohol and 2-methyl-1-butanol. Volatile acids were largely stripped by tail removal, with acetic acid below 31.06 milligrams per liter in all groups, far under the 800 milligrams per liter threshold for perceptible vinegar-like flavor.

Correlation analysis tied the chemistry back to the microbes. In the control and malic acid groups, bacterial counts tracked positively with acetic acid, ethyl lactate, ethyl acetate, and other esters and acids, indicating that microbial metabolism drove flavor accumulation. The post-fermentation lactic group showed the strongest positive correlation network of all. But the post-fermentation citric group flipped the pattern: bacterial counts correlated negatively with isoamyl alcohol, n-hexanol, n-propanol, and 2-phenylethanol, suggesting its flavor compounds arose not from microbial metabolism but from non-microbial pathways such as acid-catalyzed esterification and non-enzymatic oxidation. A ten-member trained sensory panel found that post-fermentation treatments generally scored higher in fruity aroma, overall aroma intensity, body, and persistence, with the post-fermentation malic spirit showing the most balanced profile and the post-fermentation lactic spirit the highest fruity aroma, partially offset by elevated acetic acid and acetoin. The authors conclude that citric acid offers the broadest-spectrum inhibition but triggers acetaldehyde risks, that the antimicrobial effect at pH 3.0 is transient with an optimal storage window of 42 days or less, and that the community analysis relied on alpha diversity indices, so community-level interpretations of the flavor results should be considered preliminary.

Subject of Research: Effects of organic acid treatment timing and type on microbial stability and volatile profiles of apple distilled base wine during storage

Article Title: Effects of acid treatment on microbial stability and volatile profiles of apple distilled base wine

Article References: Han, Y., Su, Z., Du, J., Wang, Y., & Wang, W. (2026). Effects of acid treatment on microbial stability and volatile profiles of apple distilled base wine. Food Chemistry: X, 40, Article 104679. https://doi.org/10.1016/j.fochx.2026.104679

Image Credits: AI Generated

DOI: 10.1016/j.fochx.2026.104679

Keywords: apple brandy, base wine, organic acids, citric acid, malic acid, lactic acid, microbial stability, volatile compounds, acetaldehyde, ethyl acetate, 16S rRNA, distillation