How Winemakers Craft Clean Natural Wines

The natural wine category is often associated with characteristics that are considered defects. Yet many natural wines are as clean and stable as conventionally produced wine.

Conventional winemakers can use a range of additives, including yeast nutrients, selected yeasts, acidity, microbial inhibitors, and sterilizers, to protect their wines. They can also filter their wines to remove unwanted microbes. Natural winemakers, however, must rely solely on their understanding of winemaking and grapes - and perhaps a small amount of sulfur dioxide (SO2).

Natural winemaking may appear simpler than conventional winemaking because it doesn’t involve the types of products and technologies that conventionally made wines often do, but the opposite is true: Crafting clean wine without these safety nets means that natural winemakers must possess even more expertise and execute their work even more precisely to avoid flaws in their wines.

Overall, it’s the choices made in the winery that typically lead to defects. However, there are many simple ways to avoid them.


Start with the Right Grape Chemistry

Healthy fruit is critical. When grapes are diseased or damaged, larger populations of microbes that can later cause defects will reach the fermenter.

It’s hugely beneficial to pick grapes at lower pH levels, particularly when winemakers add little to no SO2, because fewer microbes can survive in that environment.

Although conventional winemakers can simply add tartaric acid or more SO2 when pH is high, natural winemakers must work with a grape’s existing chemistry.


Maintain an (Over) Sanitized Winery

By keeping winery surfaces clean, winemakers minimize problem-causing microbes and lower the chances of defects that can come from Brettanomyces (Brett) and acetic acid bacteria. Maintaining a high level of cleanliness should be the practice at any winery, but conventional winemakers have a host of tools to combat problems when they arise, so hygiene is a bit less critical. For example, conventional winemakers can add SO2 early to inhibit yeast and bacteria that can lead to volatile acidity (VA), and they can take advantage of certain reverse-osmosis filters to remove VA. To combat Brett, conventional winemakers can use sterilizing agents and perform a sterile filtration, which can prevent the problem from occurring or worsening after bottling. Winemakers adhering to a natural philosophy don’t have these solutions at their disposal.

Maintaining a clean winery also helps reduce populations of fruit flies, which love finding their way into loosely sealed fermentations. It’s important to eliminate fruit flies because they carry acetic acid bacteria, which can lead to VA in wines.

All wines contain VA, though at low concentrations, it’s not perceptible. In fact, when VA is below or at sensory threshold levels, it may even contribute to a wine’s complexity, adding spice or unique fruit notes - “lift” is a common term used to describe VA’s contribution. However, once the concentration is firmly above that threshold, VA produces its hallmark vinegar aroma and may overshadow other aspects of the wine.


Manage Microbes During Fermentation

Fermentation is the most dynamic time in a wine’s life. Once fermentation is going strong - when yeasts (most commonly, Saccharomyces cerevisiae) have begun producing large amounts of carbon dioxide (CO2) - most wines are safe. However, before or early in the fermentation process, freshly pressed grape must is highly vulnerable to many bacteria.

Prior to fermentation, keeping musts cold - at 13 degrees Celsius or below - can inhibit bacteria while still allowing yeasts to slowly multiply and begin fermentation.

During the period prior to strong fermentation, other common problems that can arise include VA and ethyl acetate (the same compound as nail polish remover), which occurs when acetic bacteria has limited access to oxygen. To minimize or stop this, a winemaker can sparge (flush) the headspace of fermenters with inert gases that are heavier than oxygen, such as CO2 or argon, to minimize oxygen and prevent acetic bacteria from developing.

Sparging is an imperfect solution because it’s impossible to completely eliminate oxygen, so many winemakers couple the process with consistent punch-downs or pump-overs. By submerging the acetic populations that appear on the must’s surface back into the anaerobic mass of the must, they’ll cease to function. As fermentation begins producing significant amounts of CO2, saturating the must and filling its headspace, acetic pressure is greatly reduced. The frequency of these processes can be increased if necessary, though one daily pump-over or punch-down is usually sufficient.

Mousiness (an aroma that’s similar to a mouse’s cage) is a very common problem in natural wines. Although its exact causes remain somewhat mysterious, scientists believe that certain lactic acid bacteria (LAB) are one of the primary causes of mousiness. Adding a small amount of SO2 prior to fermentation - even at very low doses, about 10 to 15 ppm, perhaps regardless of pH - is often an effective way to prevent mousiness from occurring.

Another way to shorten this vulnerable period - and attempt to select for desirable yet still feral yeasts - is to prepare a pied de cuve. This is essentially a way of inoculating the wine with a larger, ready-to-go population of spontaneously occurring yeast, which is safer than allowing a wine to start fermenting on its own. (Conventional wineries can inoculate with selected yeast strains that reliably produce clean wines).


Protect Wines During Barrel-Aging

The three main problems most commonly associated with aging are oxidation, Brett, and VA production. Yet diligent, fundamental winemaking procedures are usually all that’s required to protect against all of them. When barrels aren’t topped frequently enough, the oxygen in the headspace can lead to oxidation; the standard topping-up prescription is every two weeks. Slight oxidation can add strange fruit character or a papery note, while extremely oxidized wines become reminiscent of oxidatively aged sherry.

There are also many microbes that can increase VA levels in insufficiently topped barrels; many of them form a film on the surface of the wine (and are sometimes incorrectly confused with flor yeast, also of Sherry fame). Simply minimizing the amount of oxygen and the amount of time these microbes have access to oxygen will reduce their contribution to VA levels. Conventional winemakers can maintain levels of SO2 during aging that keep these microbes in check, but natural winemakers can’t.

The burst of oxygen a wine gets from racking leads to an increase of VA concentration. When wines are aged in barrels for more than a couple of years, the oxygen that enters through the barrels’ pores slowly allows acetic bacteria to function, which slowly increases VA concentration and may eventually result in perceptible concentrations; long-aged Piedmontese reds are a famous example.


Lean Heavily on Lab Analysis

One might associate lab testing with conventional wines, yet it’s even more critical when crafting clean natural wines. Although it’s important to taste each barrel frequently to identify and isolate problems, lab analysis enables winemakers to stop problems earlier.

By using slides with grids, a winemaker can estimate the population of microbes in a sample. To determine which individual microbes are alive - and thereby pose a threat - and which are dead, he or she can do “gram staining” with a chemical called methylene blue (or simply observe which microbes are moving).


Balancing Risk and Talent

Of course, many other factors play a role in a winemaker’s ability to craft a clean natural wine. For example, wines made from grapes with high tannins are less susceptible to microbial spoilage and oxidation due to their ability to safely absorb and integrate oxygen into their structure.

In the end, even when winemakers institute best practices, natural winemaking is risky, and it’s not unusual for those who are interested in making only clean wines to be forced to dump a barrel or two down the drain.

Ultimately, although there is no exact formula for how to craft a clean natural wine, winemakers do have a lot of control over the process, and usually good fundamentals are all that is needed for cleanliness. Achieving success requires winemakers to have a profound understanding of all aspects of wine and the ability to work with limited tools and a small margin for error. Therefore, winemakers who craft clean natural wines are generally the industry’s most conscientious and talented.





Image credits: Rowan Heuvel