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

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To maintain the desired neutrality, stainless steel tanks must be properly cleaned. This is a 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 famed winemaker Emile Peynaud pioneered the modernization of the industry in the 1960s, winemaking was all about reducing the unknowns and letting the art flourish. Not only did Peynaud push to use only the best fully ripened grapes, but he was also a proponent of using stainless steel.

In the 1960s, Château Haut Brion switched to stainless steel fermentation tanks. [Astro Pak]
In the 1960s, Château Haut Brion switched to stainless steel fermentation tanks. [Astro Pak]

The two aspects are linked. It was tradition for chateaux to harvest their grapes before they were fully ripe to take advantage of their high acidity, which acts as a preservative against spoilage in old oak vats. By switching to stainless steel fermentation tanks (as Château Haut Brion did in 1961), wineries quickly discovered that they could use fully ripe grapes for richer, rounder flavors, with stabilized temperatures. during fermentation, among other benefits.

No additional flavor

On the surface of each stainless steel tank is a very fine (about one millionth the diameter of a human hair) layer of highly stable metal atoms that do not corrode or rust easily. This passive layer, ssometimes called passive film, is responsible for corrosion resistance. Because of this chemically unreactive layer, aging wine in stainless steel does not add flavor. Proponents of stainless steel aging argue that this allows for the full expression of the varietal and the environmental conditions in which the grape is grown.

Stainless steel composition [Astro Pak]
Stainless steel composition [Astro Pak]

However, to maintain this desired neutrality, stainless steel tanks must be properly cleaned. In addition, the passive layer must be maintained and improved so that it can continue to protect both the tank and the wine aging there. Passivation is a chemical treatment of stainless steel and other alloys that improves the ability of treated surfaces to resist corrosion.

At first, cleaning seems relatively simple, but doing it incorrectly can damage the steel, shorten its life and (potentially) add unwanted flavor to 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 causes corky taint in wine. As a result, it is simply not used in wineries. This avoidance also negates the corrosive nature of chlorine when it comes into contact with stainless steel components.

What is not as well known is that municipal tap water also contains chlorides which can similarly attack the passive layer with the same results. The use of purified water and an appropriate detergent avoids this risk. But even that is only part of the solution.

When exposed to air, stainless steel naturally oxidizes on the surface of the metal, 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 chromium to iron ratio of 1 to 1. This surface does not provide maximum corrosion resistance and may still contain contaminants that could alter the profile aroma of a wine.

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

Protection of buildings

A chemical passivation treatment reduces the amount of free iron on the surface of the metal, allowing for a thicker, stronger passive layer that is better able to withstand the acidity of wine and ensure that the steel will not interact with it. wine during its fermentation and maturation.

A competent service provider will use surfactants to break down the water barrier and clean the actual metal surface, removing any contaminants that may be present. [Astro Pak]
A competent service provider will use surfactants to break down the water barrier and clean the actual metal surface, removing any contaminants that may be present. [Astro Pak]

For best results, this is not a DIY job. Over-the-counter chemistry does not achieve the same chromium to iron ratio as a supplier of cleaning and passivating chemicals. Additionally, doing passivation in-house raises several concerns, including:

  • What are the risks for personnel and equipment?
  • Is this the best use of available manpower?
  • How do we measure the quality of the passive layer?

This 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 risk of contamination if used incorrectly or not rinsed thoroughly. This is in addition to fining agents and preservatives used during the winemaking process. The latter may contain an excess of sulfur dioxide (SO2), which can corrode the surface of a tank. Not only can this reduce the useful life of the tank and associated plumbing, but it opens up the possibility of free iron interaction with wine. If this happens, the tank may need to be cleaned and further treated (using a variety of more intense measures) and then passivated to restore its smooth and chemically neutral surface.

Ensuring your installation complies with current Good Manufacturing Practices (cGMP) and Good Documentation Practices (cGDP) is a full-time job in itself. Maintaining expert-level, up-to-date knowledge to verify and document the success or failure of a sanitary cleaning and passivation process, in addition to one’s other day-to-day responsibilities in the wine industry, may not be a realistic expectation .

Hire the professionals

Due to the prolonged contact time between the wine and the tank, it is absolutely essential that the tanks are maintained. Contamination can upset the delicate balance of aromas and tastes.

Professionals know the details of cleaning and compliance and can tailor the procedure to the specific setup and circumstances of the winemaking operation. [Astro Pak]
Professionals know the details of cleaning and compliance and can tailor the procedure to the specific setup and circumstances of the winemaking operation. [Astro Pak]

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

Once all the cleaning solutions are completely rinsed off, the passive layer will be upgraded to make it tough and reliable to ensure the stability of the stainless steel from batch to batch of wine. The final step is sanitizing the metal, followed by another thorough rinse to remove all chemicals from the tank. All procedures must be documented to establish compliance with industry and regulatory standards.

Like all preventive maintenance, sanitary cleaning and passivation must be carried out on a regular and planned basis. A proactive plan that takes into account the specifics of your operation allows you to focus on what you do best: making beautiful bottles of wine.

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Astro Pack provides high purity and precision cleaning services for food and beverage industry piping, equipment and process tanks. It serves a wide range of customers, from small businesses to Fortune 100 companies, with cleaning and sanitation services that include pre-operational and maintenance cleaning to remove rust/red, organic contamination and bioburden by using safer processes and solutions for process equipment and personnel. . Astro Pak’s skilled and dedicated teams provide customized solutions, faster turnaround time and lower overall cost.

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