Stainless Steel in the Wine Industry
Traditionally wine has been fermented and stored in oak barrels for hundreds of years but more recently stainless steel tanks have become more popular for winemakers.
Stainless steel is now often selected as the material for the wine production equipment because of its excellent corrosion resistant properties, durability and ease-of-cleaning. When the wine is fermenting the stainless steel tanks don’t impart additional flavours to the wine, meaning that more of the original palates from the grape are imparted.
Another benefit is the longevity of the stainless steel tanks compared to the traditional oak barrels. Oak will last around 3-5 years and will then need to be replaced. The deterioration of the oak due to the acidity of the wine is also an issue when it comes to regulating the oxidization of the wine.
But stainless steel is involved much earlier in the wine process with the stainless steel secateurs cutting the grapes from the vines when done by hand or by using specialist harvesting equipment which shakes the vices and causes the grapes to be collected and transported and this threshing equipment is made from 304 stainless steel.
There are several stages in the wine making process from pressing, to maceration and of course the process will vary for red and white wines. For those that know their grapes, for red wine the juice (or ‘must’) is macerated with all aspects of the grape such as skin and pulp. For white wine however the grapes pass through stainless steel rollers where the juice is extracted, leaving behind the skin and the seeds and the uncrushed pulp.
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Staying with white wines, the fermentation of the sugars from the grapes and the addition of yeasts creates the ethanol. In large stainless steel vats the fermentation of white wines needs to be regulated and depending on the desired outcome will take hours or days. The fermentation produces acids and these acids - tartaric, acetic, tannic, malic and citric - are not corrosive to stainless steel at the concentrations found in wines. But sulphur dioxide can be an issue and choosing between 304 or 316 stainless steel is an important consideration and often composite tanks are designed.
Residue from the yeasts and acids will sink to the bottom of the tank during fermentation and ease of cleaning of the tanks is a consideration when choosing stainless steel and its hygienic properties is one reason that stainless steel has proved so popular. Both white wine and red wine create different residual properties in the tanks and a simple rule of thumb is that 304L is suitable for most red wine fermentation tanks and 316L is more suited for white wines.
316L has molybdenum which increases corrosion resistance particularly against sulphur which is added in some white wines which then creates sulfur dioxide as a by-product.
Identifying which tank is 304L and which is 316L is often done by manufacturing the tanks from differently polished finishes – typically a circle polish or a smooth 2K finish both of which are suitable and which ensure that the white wine isn’t fermented in a tank made from 304L.
The internal surface of the tank should also have a suitable finish for the fermentation process that is hygienic and doesn’t trap the deposits. A mill finish such as 2B is suitable but the roughness average of this is generally around 0.8µm whereas a polished finish is between 0.05µm and 0.4µm if you choose either a Bright / Mirror Finish or a 2K smooth grained finish. It is important to note that if you use a grained finish then this should be vertical and not horizontal.
Stainless steel tanks don’t add tannins or other flavours from the oak barrels to the wine and this can be both a positive or a negative depending on whether you prefer your wine tannin rich or otherwise. But stainless steel is a huge part of the wine industry from collection of the grapes, to pressing, fermenting and bottling and is very likely to increase.
Whether you prefer red or white, chances are that next glass of wine you drink will have travelled extensively though stainless steel processes.