Expert Editorial: Improve Wine Quality with Cleaning and Passivation of Stainless Steel Tanks

To maintain desired neutrality, stainless steel tanks must be properly cleaned. This is one job best left to the professionals.

By the Astro Pak team

For millennia, winemaking has been as much an art as a science. Even after the renowned enologist Emile Peynaud initiated the modernization of the industry in the 1960s, winemaking was about reducing the unknowns and letting artistry flourish. Not only did Peynaud push for using only the best grapes when fully ripe, he was also an advocate of using stainless steel. 

The two aspects are related. Tradition called for chateaux to harvest their grapes before they were fully ripened to take advantage of their high acidity, which acts as a preservative against spoilage in old oak vats. By switching to stainless steel fermentation vats (as Chateau Haut Brion did in 1961), wineries soon found they could use fully ripened grapes for richer, rounder flavors, with stabilized temperatures during fermentation, among other advantages. 

No Additional Flavors

On the surface of every stainless steel tank is a very thin (approximately one-millionth the diameter of a human hair) layer of highly stable metal atoms that do not easily corrode or rust. This passive layer, sometimes referred to as the passive film, is responsible for providing corrosion resistance. Because of this chemically nonreactive layer, aging wine in stainless steel adds nothing to the flavor. Advocates of aging in stainless steel maintain that this allows for the full expression of the variety and the environmental conditions in which the grape is grown. 

However, to maintain that desired neutrality, stainless steel tanks must be properly cleaned. Further, the passive layer must be maintained and enhanced so that it can continue to protect both the tank and the wine aging within it. “Passivation” is a chemical treatment for stainless steel and other alloys that enhances the ability of the treated surfaces to resist corrosion.

At first, cleaning would seem relatively straightforward, but doing it incorrectly can damage the steel, shortening its lifespan and (potentially) add unwanted flavor to the wine. On most surfaces and in most applications, a thorough cleaning with bleach and tap water would be acceptable. However, as winemakers know, the chlorine in bleach is the primary component that leads to cork taint in the wine. As a result, it’s simply not used in wineries. This avoidance also negates the corrosive nature of chlorine when it comes in contact with stainless steel components. 

What’s not as well known is that municipal tap water also contains chlorides that may similarly attack the passive layer with the same results. Using purified water and a proper detergent avoids that risk. But even this is only part of the solution. 

When exposed to air, stainless steel naturally oxidizes at the metal surface, creating a chemically resistant oxide film. However, environmental contaminants and free iron are still present near the surface. Studies have shown that this naturally occurring oxide film has a 1-to-1 ratio of chromium to iron. This surface does not provide maximum corrosion resistance, plus it may still contain contaminants that may alter a wine’s flavor profile. 

The acidity in wine also has an adverse effect on the steel’s ability to maintain its passivity, as it reduces the effectiveness of its self-healing property. Clearly, it needs to be strengthened as much as possible. This is where the process of passivation comes in. 

Building Protection

A chemical passivation treatment reduces the amount of free iron on the metal’s surface, allowing for a thicker, more resistant passive layer that’s better able to resist wine’s acidity and ensure that the steel will not interact with the wine as it’s fermenting and maturing. 

For best results, this is not a do-it-yourself job. Over-the-counter chemistry does not achieve the same chromium-to-iron ratio that a chemical cleaning and passivation provider can. Additionally, doing an in-house passivation raises several concerns, including: 

• What are the risks to personnel and equipment?
• Is this the best use of available manpower?
• How is the quality of the passive layer measured?

That last concern also applies to cleanliness: How is the quality of the cleaning process measured? 

The process of flipping a tank can take days and extensive cleaning. However, few wineries are able to document the effectiveness of the procedure. In fact, cleaning products themselves can pose a potential contamination risk if used incorrectly or not thoroughly flushed. This is in addition to the fining agents and preservatives used during the winemaking process. The latter can contain excess sulfur dioxide (SO2), which can corrode a tank’s surface. Not only can this reduce the useful life of the tank and associated plumbing, but it opens up the possibility of free iron interacting with the wine. If this occurs, the tank may need to be more thoroughly cleaned and treated (using a variety of more intense measures) and then passivated to restore its smooth, chemically neutral surface.

Ensuring that one’s facility is compliant with current Good Manufacturing Practices (cGMP) and current Good Documentation Practices (cGDP) is a full-time job in itself. Maintaining expert-level and up-to-date knowledge for verifying and documenting the success or failure of a sanitary cleaning and passivation process, on top of one’s other day-to-day responsibilities in the winemaking industry, may not be a realistic expectation. 

Engage the Professionals

Due to the extensive contact time between the wine and the tank, it is absolutely critical that the tanks are cared for. Contamination can upset the delicate balance of aromas and tastes.

Fortunately, there are professionals who know the details of cleaning and compliance and can tailor the procedure to the specific configuration and circumstances of the winemaking operation. A knowledgeable service provider will use surface active agents to break the water barrier and clean the actual surface of the metal, removing any contaminants that may be present. This could include biofilms that often form as a result of the malolactic fermentation process. 

Once all cleaning solutions are fully flushed away, the passive layer will be enhanced to make it resilient and reliable to ensure the stainless steel is stable from wine batch to wine batch. The last step is sanitization of the metal, followed by another thorough rinse to remove all chemicals from the vat. All procedures should be documented to establish compliance with industry and regulatory standards.

Like any preventive maintenance, sanitary cleaning and passivation is best performed on a regular, scheduled basis. A proactive plan that takes into account the specifics of your operation lets you focus on what you do best — making fine bottles of wine.

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Astro Pak provides high purity, precision cleaning services for process piping, equipment and tanks to the food and beverage industry. It services a wide range of clients from small to Fortune 100 companies, with cleaning and sanitization services that include pre-operational and maintenance cleaning to remove rust/rouge, organic contamination and bioburden using processes and solutions that are safer for both process equipment and personnel. Astro Pak’s skilled and dedicated crews provide customized solutions, faster turnaround and lower overall cost.