By Bertus Fourie

Ageing or maturation of red wines in wooden barrels is a normal practice during production and is essential for the production of premium quality wines. The technique of maturing wine in barrels made from wood was introduced at the beginning of the 18th century. The cooperage profession has used different species of wood for vinification, transportation, ageing and storage of wine (Vivas, 1995). Although the demand for oak barrels is widely recognized, it remains nevertheless true that making wines in this fashion is an expensive operation and only a proportion of the wine, rarely 100%, can benefit from maturation in new oak. The challenge facing producers of fine wines is thus to decrease production costs, while maintaining organoleptic quality and preserving consistency in the product.

Incorporating oak in the maturation of fine wines is not an end in itself. It is merely a process through which the wine acquires the ability to age in the bottle or to set a specific style. The aim of using oak barrels is to flavor the wine and to provoke a number of chemical and physical changes, which are indispensable for the final product. Firstly the wine acquires clarity and stability during the maturation phase. Any solids, derived from the physical processes of alcoholic fermentation, malolactic fermentation and finings, chemical processes of hydrolysis and polymerization and tartaric precipitations (of the colouring matter) and microbiological matter for example dead yeast cells and bacteria, descends to the bottom under the force of gravity (Pontallier, 1992). Secondly the bouquet of the wine develops and becomes more refined and complex as the wood releases into the wine specific substances or toasting derived flavours (Sefton et al., 1990; Sefton 1991; Sefton et al., 1993). Thirdly oak barrels (and to a lesser extent the addition of staves, chips and blocks to wine in used barrels or stainless steel tanks), ensures the effect of micro-oxygenation estimated at 20-40mg O2 per year in new oak barrels (Pontallier, 1992), but this may drop to under 10 ml/L after usage. The oxygenation process stimulates oxidation of anthocyanin and tannin molecules, which make up the colour matter in red wine and can lead to precipitation under the effect of continued oxidation. Maturation of red wines in barrels results in a spectrum of phenolic transformations of which some can be organoleptically detected. An increase in the colouring intensity occurs, the hue evolves (the colour of red wine takes on ruby stabilization of colour takes place (i.e. tannin/anthocyanin pigments formed during polymerization reactions are more stable than the anthocyanin molecules which alone are responsible for the colour in young red wine), precipitation of large tannin molecules and a decrease in astringency and hardness takes place  (Somers, 1971; Somers and Evans, 1974; Somers 1978; Somers and Evans, 1986; Somers, 1990). It is therefor also important to recognize the significant role that oxygen plays in the development of these wines.

Now imagine having all of the above, with other very significant and valid advantages such as saving maturation space, saving energy, saving on labour, easier and more efficient cleaning and hygiene maintenance, preservation of fruit in a more significant way, no evaporation losses and ultimately, bottom line savings.

The technology…
The past few years saw the evolution of a new product called Flextank. Modern polymer catalyst technology has given rise to polyethylene  (PE) grades that are well-suited for the long term, no taint storage of premium wines and permit molecular oxygen influx. Flexcube is available in three O2 uptake versions: Low, Medium and High. Low passive oxygenation is suitable for all white wines and for elegant top quality red wines created for long ageing. Medium passive oxygenation is appropriate for a wide range of red wines.  High passive oxygenation allows approximately 30% more oxygen uptake than medium permeability cubes and is excellent for red wine maturation where a softer finish or earlier release is beneficial.

The polyethylene used by Flextank is free from taint. New vessels with can be treated with warm 2% solution of caustic soda (sodium hydroxide), or similar citric and rinsed with fresh water afterwards.

The lid is recessed and is screwed on firmly before filling the vessel via the bottom valve or the top lid outlet. For this purpose a lid spanner is supplied to tighten the lid in order to obtain a full hermetic seal. The sealing mechanism is so effective that lifting the cube with a forklift, and tilting the cube, will ensure efficient lees stirring.

During normal and stable temperature storage conditions, filling heights in Flextank vessels must be inspected every 4-8 weeks (as is done with oak barrels) as thermal expansion or contraction will always occur with wine temperature changes. The vessel should require very little topping. Flextank maturation vessels do not lose water, vapor or alcohol and have very substantially reduced losses of volatile compounds compared to oak barrels.

The vessels are UV stabilised, and when empty, can be stored out-of-doors for more than 25 years. Wine can be unfavourably changed though by exposure to strong visible light. Vessels in the natural translucent polymer are UV opaque, but do pass visible light (i.e. the wine level can be seen from outside the tank). If storing wine outside, the opaque grey colour option may be used or the can be tank insulating cover fitted.

Polyethylene is inert to almost all cleaning chemicals, as well as strong acids and alkalis. It is ozone safe and can be cleaned with hot pressure washing and steam cleaning. Polyethylene is easy to clean as its waxy surface prevents tartar adhesion. Cleaning should take less time and less water than equivalent oak, steel or concrete vessels. Pigment pickup is easily removed with alkali cleaning products, such as 2% caustic soda and is enhanced by use of spray balls and jets. Hardy residues can be removed by using plastic bristle-broom or mop.

Other important advantages worth mentioning is the fact that it is perfect for small batch fermentations, well suited for extended maceration and the fact that wine can be securely transported.

References

Pontallier, P. (1992). The intervention of oak wood in the making of great red wines. J. of Wine Research. 3, 241 – 247.
Sefton, M.A. (1991). How does oak barrel maturation contribute to wine flavour? PWV, November/December, 17 – 20.
Sefton, M.A., Francis, I.L., and Williams, P.J. (1990). Volatile norisoprenoid compounds as constituents of oak woods in wine and spirit maturation, J. Agric. Food Chem., 38, 2045-2049.
Sefton, M.A., Spillman, P.J., Pocock, K.F., Francis, I.L., and Williams, P.J. (1993). The influence of oak origin, seasoning, and other industry practices on the sensory characteristics and composition of oak extracts and barrel-aged white wines. The Aust. Grapegrower and Winemaker, No. 355, 17-15.
Somers, T.C. (1971). The polymeric nature of wine pigments. Phytochem. 10, 2175-2186.
Somers, T.C., and Evans, M.E. (1974). Wine quality: Correlations with colour density and anthocyanin equilibria in a group of young red wines, J. Sci. Fd Agric. 25, 1369-1379.
Somers, T.C. (1978). Interpretations of colour composition in young red wines. Vitis. 17, 161-167.
Somers, T.C. and Evans, M.E. (1986). Evolution of red wines.  I. Ambient influences on colour composition during early maturation.  Vitis25, 31-39.
Somers, T.C. (1990). An assessment of the “oak factor “ in current winemaking practice. The Aust. wine research institute tech. review. 67, 3-10.
Vivas, N. (1995).  The quality of oak wood and its use for the vinification and ageing of wine. J. Sci. Tech. Tonnellerie. 1, 103-121.