California Study Finds Ion Exchange Delivers the Biggest pH Drop in Red Wine

A new study from California’s Central Coast is offering winemakers a clearer comparison of three ways to manage acidity in red wine as warmer growing conditions continue to push pH higher and natural acidity lower.

The research, published Wednesday in OENO One by Charlotte Drop, Sean Kuster, Biljana Petrova, Jesus Villalobos, Augusta Schulte and L. Federico Casassa, examined Nebbiolo and Tannat wines made under a coastal Mediterranean climate in California. The team compared wines made without a special acid intervention against three alternatives: blocking malolactic fermentation, using the non-Saccharomyces yeast Lachancea thermotolerans, and treating finished wine with ion exchange before blending it back at 30%.

For wineries, the issue goes beyond taste. In red wines, lower pH can help support microbial stability, improve the protective effect of sulfur dioxide and preserve color. Those points matter more in warm regions, where grapes often arrive at the winery with more sugar, less malic acid and more potassium, a combination that can lead to softer but less stable wines.

The strongest pH shift in the trial came from ion exchange. According to the study, that treatment lowered pH by 0.31 units compared with control wines. It also reduced potassium by 9% and calcium by 13%. Those changes are important because both ions play a role in buffering wine and in tartrate stability. By pulling down cation levels, ion exchange can make acid adjustments hold more effectively over time rather than being partly reversed by the wine’s natural chemistry.

The researchers reported that wines made with blocked malolactic fermentation and wines treated with ion exchange also showed better color stability than the controls. In the Nebbiolo wines, both treatments were rated as having the highest color saturation in sensory analysis. Chemically, blocked malolactic fermentation produced wines with 19% more tannins and 5% more total phenolics than the control, while ion exchange wines showed 19% more tannins and 11% more total phenolics.

Lachancea thermotolerans moved the wines in a different direction. The yeast increased titratable acidity by 24% through lactic acid production, giving the wines the highest perceived acidity in the trial. In Nebbiolo, those wines were also described as having the longest-lasting red fruit character on the palate. At the same time, the study found lower levels of tannins, polymeric pigments and total phenolics, along with lower odor activity values for several fruity esters, including ethyl butyrate, ethyl isovalerate, isoamyl acetate, ethyl hexanoate and ethyl n-octanoate.

That mix of results suggests that the different acid strategies are not interchangeable. Ion exchange delivered the largest direct correction to pH and reduced key cations. Blocking malolactic fermentation preserved acidity while also supporting phenolic structure and color. Lachancea, by contrast, appeared to steer the wines toward brighter acidity and a different sensory profile, even when some color- and structure-related measures were lower.

The work is notable because it compares the three approaches within the same experimental framework and across two red varieties with very different phenolic profiles. Nebbiolo and Tannat are both used for age-worthy wines, but they do not behave the same way during fermentation and aging. The authors said side-by-side trials like this have been limited, especially for red wines grown in California’s warmer areas.

The fruit came from two Central Coast vineyards during the 2024 harvest. Nebbiolo grapes were picked Sept. 26 from Mora Vineyard in the Santa Ynez Valley, and Tannat grapes were picked Oct. 10 from Sunnybrook Vineyard in Paso Robles. Both lots were processed at the Justin and Jerri Lohr Center for Wine and Viticulture at Cal Poly. Each treatment was carried out in triplicate.

The researchers fermented the wines in small stainless-steel tanks and used standard sulfur dioxide additions at crush. For the blocked malolactic treatment, they prevented the secondary bacterial conversion of malic acid to lactic acid, a step often used to soften red wines but one that can also raise pH. In the ion exchange treatment, finished wines were passed through a resin column and then blended back into untreated wine at 30%, a level chosen after bench trials showed it gave the largest pH reduction without creating a harsh result.

The study also helps frame why wineries are looking beyond the traditional answer of tartaric acid additions. Tartaric acid remains the most common tool because it is familiar, relatively inexpensive and easy to apply. But the paper notes that it does not always lower pH efficiently because wine has a strong buffering capacity. Large additions can raise titratable acidity more than they lower pH, which may leave wines tasting tarter without solving all the stability concerns. Tartaric additions can also create tartrate precipitation issues that often require cold stabilization, an energy-intensive step.

By comparison, biological and physicochemical methods offer different tradeoffs. Blocking malolactic fermentation can preserve sharper acidity but requires careful sulfur dioxide management and sterile filtration to reduce the risk of refermentation in bottle. Lachancea thermotolerans can build acidity during alcoholic fermentation, but its effects vary by strain and can reshape aroma and mouthfeel. Ion exchange can produce a large pH correction, but the treated wine is usually too acidic to bottle on its own, which is why it has to be blended back.

The authors followed the wines not only at bottling but also during aging. Color, anthocyanins, tannins, polymeric pigments and total phenolics were measured through fermentation, at bottling, after 3 months and 6 months in bottle, and after an accelerated aging test. For that test, the wines were sealed in small glass ampules and held for five weeks at 38°C, a protocol the researchers used to simulate roughly two years of bottle aging, before further analysis of color compounds such as anthocyanins, anthocyanin-derived pigments and flavonols.