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Cleaning and disinfection of oak barrels and oak adjuncts with high-power ultrasonics

by Charl Theron, guest lecturer in Oenology, Tim Rypstra, professor in Wood Science and Jan Swart, director: Wood and Fibre Institute.

The November 2007 edition published an introductory article about the use of ultrasonics to clean and disinfect barrels. This article develops the theme.

Storage of wine in barrels goes hand in hand with the deposit of tartrates on the inside of the barrels. Depending on the wine and barrel cleaning programme, the deposit might accumulate as much as 1 cm thick. As a result the wine will not be able to receive any exposure to wood, oxygen will not move through the pores of the wood to the wine and the deposit also serves as a breeding ground for undesirable micro-organisms. While a new barrel may enable the dissolved oxygen level in the wine to increase by 0.3 to 0.5 mg/l, the possibility is reduced to 0.1 mg/l if barrels are used 3 to 5 times. The barrels then simply become containers of wine. Current practice to clean barrels with warm water and/or chemicals is ineffective.

Barrels deriving from different size cellars, that were used for 3 to 5 years and cleaned with various temperature and water pressure treatments during that period, were inspected and the findings may be summarised as follows:

  • The tartrate deposits in the barrels may be flat (up to 1 mm thick) or occur in blisters (up to 1 cm high).
  • The deposits may range from soft to very hard, or crystalline to powdery.
  • The flat deposits are firmly attached to the wood surface and do not flake when dried.
  • Tartrate deposits occur not only on the dowel surfaces, but also in the croze of the barrels where the barrel heads fit into the dowels.
  • The extent of the tartrate deposits in the cleaned barrels increases with repeated usage of the barrels.
  • The effectiveness of the washing processes differed depending on the various interior surfaces of the barrels.
  • None of the water treatments of barrels removed all the tartrate deposits from the croze.
  • The cold and warm water treatment of barrels only removes the deposit on the wood surfaces, while that which occurs in the pores remains intact.
  • Municipal water pressure was too weak to remove tartrate deposits from the barrels.
  • Some staves that appeared clean visually, nevertheless had impregnated deposits in the wood tissue.

It appears that existing barrel cleaning methods do not efficiently remove tartrate deposits from barrels.

Various micro-organisms are adapted to surviving in wine barrels. Bacteria and yeasts are protected by the tartrate deposits occurring on the interior surface of wooden barrels and are therefore able to survive in the wood pores, so that wine pumped over into the barrels may be infected. Barrels infected with acetic acid bacteria or moulds are usually discarded, because these are difficult to remove from the wood pores. Biofilms constitute another form of barrel contamination. They are created when a liquid is in contact with an inactive surface and any microbial cells in the liquid attach themselves to the surface. Biofilms may occur on the surface of stainless steel tanks, for example, and therefore one expects them also to occur on barrel surfaces. Californian research showed that 50% of the Brettanomyces/Dekkera yeast cultures may form biofilms. Biofilms are far more resistant to biocides, chemical cleaning and sanitation agents. The influence of the formation of biofilm on the structure of barrels and the consequences thereof are unknown and the effectiveness of cleaning aids should be measured against their ability to break down biofilms. Ultrasound offers an effective method of creating stress factors for biofilm organisms and neutralising their attachment to the fixed surface. Brettanomyces/Dekkera spoilage of wines is a widespread occurrence throughout the world and has increased dramatically over the past 30 years. The insufficient sanitation of wine barrels and increase in the second-hand trade in wine barrels are some of the reasons that gave rise to this phenomenon.

High energy ultrasonics is a powerful new aid to address the cleaning problems of wine barrels. Laboratory analysis has confirmed that this technology is able to kill Brettanomyces/Dekkera cells effectively. The application of high energy ultrasound to clean and sanitise oak barrels and oak adjuncts such as staves has the following cost savings potential for cellars:

  • The elimination or prevention of microbiological spoilage problems.
  • The constant, uniform cleaning of barrels.
  • Reduction in the use of sulphur dioxide or other preservatives to restrict microbiological spoilage.
  • The elimination of the use of dangerous chemicals.
  • The simultaneous cleaning and sanitation of barrels.
  • The possible extension of the barrel’s lifetime.
  • The use of environmentally friendly technology that does not result in runoff residue.
(Andrew Yap, Vladimir Jiranek, Terry Lee, Paul Grbin, Mark Barnes & Darren Bates, 2007).

Reference

Yap, Andrew; Jiranek, Vladimir; Lee, Terry; Grbin, Paul; Barnes, Mark & Bates, Darren. 2007. Cleaning/disinfection of oak barrels/oak adjuncts with high-power ultrasonics. Practical Winery & Vineyard 29(4): 6 - 23.

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