The Flextank controlled permeation tanks for wine maturation have been used increasingly widely in Australia over several years. International sales are also increasing. The following reviews the concept of Flextank maturation vessels and summarises the outcomes of recent wine trials carried out in Australia and Europe. The results of these trials clearly show that taint, so often a criticism leveled at 'plastic' tanks, is not an issue with wine stored in Flextanks. The trials also confirm that Flextanks equal or exceed oak barrels for wine maturation for quality wine development. For best results, emphasis must be placed on the selection, origin, number and use of the oak used in the Flextanks, considerations similar to barrel selection. The ability to tailor vessels with specific oxygen permeation rates to suit a particular wine style is described.

The selection of the thermoplastic polymer used by Flextank is the result of substantial theoretical and trial work: it is an ultra-clean, food grade high-density polyethylene that has received approval for liquid food and wine use specifically. It has also been extensively trialled. The materials development process also required consideration of each production step to ensure that the polymer had the right sinter density and was not degraded during the moulding step. Other considerations included lifespan of the products and ease of use aspects such as internal surface finish and tartar non-adhesion. The precise makeup of the Flextank polymer is commercially sensitive.

Tanks made more than twenty years ago were in fact not thermoplastic, but were actually made from thermosetting polyester resin mixed with glass fibres. The solvents and catalysts used, if extracted into wine, could lead to a distinct taint. Food grade thermoplastic polymers like polyethylene are a different material entirely. They contain no free solvents or other reagents that can migrate into wine.

The absence of taint in wine stored in Flextank maturation tanks has been confirmed by winemakers from various trials with, in one case, the wine being stored for three years. Not all trials are fully replicated and this always limits the depth of information that can be extracted from the data. But, in the words of one Australian winemaker, "we found that they (Flextanks) don't taint".

Chemical analysis of wine stored in Flextanks has shown that there is no difference in the general wine quality parameters for wine stored in Flextank, oak or stainless steel. Of particular note, chlorophenols, a concern often expressed with the use of plastic materials in wine production, were not found in Flextank-matured wine.

 

WINE TRIALS IN AUSTRALIA:

Flextank maturation tanks require the addition of oak to allow the wine to develop oak-derived flavours that occurs in the barrel maturation process. High quality staves are recommended for this process. Further, the Flextank maturation process with staves will be less at risk from other problems, including losses due to evaporation and development of Brett off-flavours, than can occur during oak barrel maturation.

On the other hand, if oak staves are not added to the wine in Flextanks, the maturation process becomes similar to that occurring in a neutral or spent barrel: the advantage of Flextank maturation, apart from much improved hygiene over use of old barrels, is that oxygen ingress is closer to that of a new barrel, allowing wine development to occur to a more normal timetable.

TRIAL 1:

A 2006 Shiraz, after malo-lactic fermentation, was added to the following vessels:

• 3 x new American oak hogsheads
• 2 x 1000 Litre Flextanks, each containing one 'Flextank 8 QP' modular oak stave pack
• 3 x 2100 Litre Flextanks, each containing two 'Flextank 8 QP' modular oak stave packs
  

All vessels were kept in the same storage area (same temperature and humidity) for the eleven months duration of the trial. The intention of the trial was to monitor relative changes in various chemical parameters for the wines stored in the different vessels.

Free sulfur dioxide concentration (mg/litre) in 2100 litre Flextanks (BP01, BP02, BP03), 1000 litre Flextanks (BP05, BP06)
and new American oak hogsheads over 5 months.

The free sulfur dioxide concentrations were essentially the same in all storage vessels after the 11 month maturation period. Importantly, the rate of free sulfur dioxide loss in the first five months occurred at the same rate for the oak and Flextank vessels, showing the validity of Flextank's oxygen permeation physics and vessel designs.

There was no significant difference in wine colour density after 11 months, while the wine hue was higher in the Flextank vessels in comparison to the oak vessels. Levels of oak volatiles increased during the time course of the maturation study, irrespective of storage vessel type.

The concentrations of free dimethyl sulfide in the Flextank vessels were slightly, but not significantly, higher than in the oak vessels (8 to 10 microgram/litre compared to 6 microgram/litre). The measured concentrations of hydrogen sulfide were the same in all vessels and the levels of methanethiol were below detection.

TRIAL 2:

This trial focussed on maturation of a 2007 Shiraz wine with subsequent sensory assessment for classification into wines for two retail price points: $40 and $15. The wine was stored in the following vessels for 7 months of maturation:

• 2nd fill French oak hogsheads
• 5,000 l. oak foudre
• Flextank with oak staves 2,000 l with French oak staves
• Stakvat 900 litre
• Stainless steel tanks with French oak staves

The resulting wines were assessed blind by 5 winemakers using the International 20 point scoring system (maximum score = 20). The results were:

• Price point $15: Flextank 18/20; stainless steel 14/20; oak hogsheads 13/20
• Price point $40 (trial 1): Flextank 18/20; Stakvat 17/20; hogsheads 9/20
• Price point $40 (trial 2): Flextank 16/20; hogsheads 14/20; foudre 11/20; Stakvat 9/20

The results of this trial clearly demonstrate the success of Flextank vessels for the maturation of high value wine.

TRIAL 3:

A 2008 vintage Shiraz was placed in several different vessels near the end of the primary ferment for 4 months maturation. The vessels used were:

• Barriques (2nd fill Francois Freres)
• Flextank 1,000 litre with mixed French staves
• Stainless steel (no oak)
  

The wines were tasted for allocation by five winemakers and scored using the International 20 point scoring system. The wine in Flextank received a 'gold medal' (that is, a score of 18.5/20 or higher), the wine in barriques received a 'silver medal' and that from the stainless steel, a bronze medal.

In both of the above trial cases the winemakers judged the wine matured in Flextank to exceed the quality of the wine matured in French oak barrels. The outcome of both trials confirmed the ability of Flextank vessels to mature wine destined for the even the top price points in the market place.

WINE TRIALS IN GERMANY:

Winemaking trials using Flextank vessels are still new in to the German industry. One trial at the Staatliche Lehr und Versuchsanstalt für Wein und Obstbau in Weinsberg (State Wine School, Weinsburg) examined the impact of oxygen on the maturation of Cabernet Dorsa, a variety known for its deep colour and high tannin levels.

The various vessels used included oak barriques, Flextanks and stainless steel, with and without oak chips. Micro-oxygenation (MoX) was also applied to wine in some of the stainless steel tanks. The results are preliminary at this stage, with the wines still to be bottled and to undergo formal sensory analysis at that time. However, in-cellar tasting from each vessel showed that those wines with access to oxygen (Flextanks, barriques and steel with MoX) showed a better palate structure than wines without oxygen. This is consistent with trials in other countries.

WINE TRIALS IN SWITZERLAND:

The wine trials that have been carried out in Switzerland tend to use the smaller volume Flextank vessels, closer to barrel volume (300L and 600L). One 5 month maturation trial used a local variety, Gamaret, and compared the following vessels:

• Flextank 300 litre
• Barrique (2nd fill Vicard)
• Stainless steel

The wines were tasted 'in-cellar' (that is, not blind) by 2 winemakers. The Flextank wine was seen as soft, integrated and structurally 'complete' and the superior wine. The wine in barriques was similar, except that it was showing slightly advanced colour and was dominated by oak flavours. The wine in stainless steel was regarded as 'hard' and not showing comparable development to the wines stored in Flextanks and barriques. The wine in Flextank was unambiguously preferred by both winemakers.

Whilst the Flextank vessels used in Switzerland are smaller than those used in the Australian trials, the results in terms of colour improvement and wine integration are much the same. Oak staves were not used in the trials in Switzerland, whereas they were important in Australia. This probably reflects the different grape type, wine style and marketing culture in each country.

Although smaller volume Flextanks were used in the Swiss trials, the designed oxygen permeation value of 17 mg/litre was the same as that used in the Australian trials. On the other hand, the period of maturation used Switzerland was much less than in Australia. Importantly, winemakers understand that the driving force for the maturation process is the inherent permeability of the Flextank vessels.

This and other trials in Switzerland, South Africa and the US demonstrate the impacts of maturation of wine in Flextank vessels to be:

• Colour improvement
• Tannin softening and integration
• Enhanced complexity of the wine
  

FERMENTATION & MATURATION TRIALS WITH WHITE WINE

Oxygen ingress during fermentation and ageing on lees in barrels is vital to the successful development of white wine styles such as Chardonnay. Flextank vessels with staves offer an attractive option to oak vessels, allowing both controlled oxygen ingress and also manipulation of the oak level by variation of the number of staves. More staves can be added or some removed during the maturation process, should the oak level be regarded as too low or too high, respectively.

In one Australian trial with a 2008 Chardonnay, the blind tasting showed a slight preference for the wine in barrel (4 year old with new oak insert) to the Flextank wine, although both were allocated to the same final blend. The wine stored in stainless steel in this trial was allocated to a lower grade.

A fermentation trial [again, Australian] with Pinot Gris in 2008 showed that the wine resulting from fermentation in Flextanks without added staves retained excellent aromatic characters. The winemakers participating in the tasting noted that the wine fermented in Flextanks had 'excellent mouthfeel and showed a high degree of varietal character'.

A US Sonoma Valley winemaker conducting fermentation and maturation trials with sauvignon blanc in Flextank vessels placed an additional purchase order for additional vessels within 3 months, commenting "already the wine in the Flextank's is superior to barrel...".

These and other trials, while remaining preliminary, clearly demonstrate the potential for the application of Flextank vessels to the fermentation and development of white wines as well as red wines.

OXYGEN PERMEATION RATES

The construction of the Flextank maturation vessels occurs under highly reproducible conditions that result in well-defined oxygen permeation rates. The below graph shows oxygen permeation rates for various wine production technologies, including barrels, corks and Flextank vessels.

The data shows that Flextank maturation vessels have oxygen permeabilities similar to second or third fill barrels. Importantly, a Flextank vessel with a stated permeability of 17 mg/litre/year will be exactly that, whereas oak barrels are highly variable in the permeation rates. Much better control of oxygen ingress can be achieved with Flextanks. Data is from the Australian Wine Research Institute together with measurements obtained by Flextank.

The defined permeation rates in Flextank maturation vessels provide winemakers with an option to choose the permeation rate best suited for a particular wine. As noted above, the wine trials in Switzerland used Flextanks with a permeability of 17 mg/litre/year for a short period of time to achieve the desired level of maturation, whereas the Australian trials used longer maturation periods with oak staves present.

OTHER POLYETHYLENE VESSELS

Polyethylene vessels such as drums and IBC's, have been used for transport wine and for short term storage for many years. However it is a common experience that wine stored in these does not keep well.

In fact, common polyethylene containers like drums & IBC's are too oxygen permeable to mature wine correctly, they:

• are too thin walled for their volume, i.e. too 'oxidative'
• have unsuitable shapes, i.e. have recesses or projections which trap air
• are difficult to clean internally, having small openings
• are made from common grades of PE which are incorrect for maturation
• often exhibit a 'burnt wax' taint

On the other hand, Flextank HDPE Vessels, are engineered to mature wine by using a correct balance between material permeability and wall thickness, vessels surface area and volume.

FOR FURTHER INFORMATION CONTACT:

Peter Steer, peter@flextank.com.au, is Managing Director of Flextank International Ltd. He is a graduate in oenology from Charles Sturt University and is a highly regarded winemaker, with more than twenty years winemaking experience in Australia, USA and France.