The making of wine is both a science and an art. From growing the grapes through the bottling of the final product, winemakers use careful techniques to protect the taste of their delicate product. Traditions and secret techniques are passed on through generations of winemakers. A vintner knows that proper care and maintenance of his equipment are as important as proper ingredients and techniques.
In modern commercial winemaking, wine is usually first fermented in stainless steel tanks. It is sometimes later transferred to wooden barrels for aging. Typically constructed from 304L or 316L austenitic stainless steel, these tanks are strong, durable, hygienic and easy to clean. Due of 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.
Tramp Metals Can Spoil the Wine
When a tank is brand new, it looks shiny, clean and pure. However, small amounts of “tramp” metal impurities can remain embedded in the surface. These small particles come from the tooling used to fabricate the tank. Or, they can come from other nearby projects in the same work area. Some of this metal can even come from the mechanical cleaning or polishing processes prior to commissioning. Because this material is not as corrosion-resistant as the stainless steel, they can dissolve in the acidic fermenting “must”. Even small amounts of iron, aluminum and other metals can change the activity of the yeasts. As they are not controlled or measured, their presence can create uncertain outcomes. This can be prevented by performing a passivation process on all new stainless steel equipment that will contact the wine prior to putting it into service.
Wine is fairly acidic with a pH typically between 2.5 to 4.5. This acidity is a combination of six natural acids. The grapes themselves contain tartaric, malic and citric acids. The fermentation process creates lactic, succinic and acetic acids. Many believe that this acidity passivates the stainless steel and making it non-reactive. That is not the case. The acids are not present in high enough concentrations to properly complete the process. Additionally, they do not chelate or neutralize the metal contaminants, leaving them to continue to react and change the outcome.
Passivating Once is Not Always Enough
As acidic as wine is, it does not aggressively attack the metal of the tank. However, metal alloy crystalline structures, such as stainless steel are not static. Over time, metallic atoms move through the voids between crystals. Iron will eventually migrate to the surface, coming in contact with the wine. Periodic re-passivation will prevent this from becoming a problem.
A very thorough deep cleaning is a normal part of a chemical passivation process, so the equipment will be cleaned of all other types of surface contaminants at the same time the stainless steel is prepared for chemical passivation. In many industries that are sensitive to biological contaminants such as bacteria and mold, a sanitizing process can be added to the passivation work so that all biofilms and living microbiological life forms are removed and exterminated. Although wines are typically about 12% ethyl alcohol, it is possible for biofilms to form and harbor certain alcohol-tolerant bacteria.
Regular Cleaning and Passivation is Optimal
In many industries where corrosive metal are present, periodic re-passivation is performed to maintain the stainless steel at peak performance. Since wines are not aggressively corrosive, this does not have to be performed as often. Prior to chemical passivation, the stainless steel is deep cleaned. Technicians will also sanitize the steel. This removes and exterminates biofilms and other unwanted microbiology.
The resulting passivated surface gives winemakers a blank slate to craft their product. They will know exactly what the tanks contain allowing them to focus on their art in pursuit of a classic vintage and achieve consistent results.