Evaluation of high power ultrasound porous cleaning efficacy in American oak wine barrels using X-ray tomography

Abstract

This study presents a novel cleaning application which investigates the ability of high power ultrasound (HPU) to remove crystalline potassium hydrogen tartrate sediment from the porous structure of American oak wine barrels. Tartrate deposits within stave samples were imaged by X-ray tomography. Volumetric reconstruction software enabled tartrate volumes to be quantified after successive treatments at varied solvent temperatures by re-imaging samples and modelling obtained tomographic projections. The removal of microscopic tartrate crystals from stave surfaces by HPU was also investigated. It was demonstrated that HPU can significantly remove tartrate deposits from the first two millimetres of oak surfaces however such ability at depth 2–8 mm was not reproducible. An average of 89% total tartrate volume was removed from the surface layer in the first treatment but was further increased to 98% by increasing solvent temperature and irradiation time. A highly significant removal of stave surface tartrate crystals with this cleaning technique was also demonstrated at temperatures studied. Fouling of the interior surface and porous membranes of wine barrels is a significant problem encountered by every wine maker. This problem occurs in white and red wine production by precipitated tartrate crystallisation which forms a robust scale inhibiting oak compound extraction by wine, additionally harbouring spoilage microorganisms and contaminants. Tartrate scale is typically removed by spray techniques, but often with less than optimal results. Ultrasonic cleaning is a well established practice in many industries; this industrial scaled study demonstrates a high power ultrasonic (4 kW/20 kHz) application in the effective and efficient removal of tartrate sediment from the surface of staves, and to a depth of two millimetres, with a relatively short treatment time (12 min) at mild temperatures (40–60 °C). Such outcomes are expected to benefit the wine industry, consumers and environment by increased barrel functionality and less frequent replacement, an improvement in oak compound extraction rate and concentration in wine, increased product quality, as well as the minimisation of cleaning water consumption and excessive use of preservatives. ► Potassium hydrogen tartrate precipitate in wine crystallises inside barrel staves, reducing functionality. ► High power ultrasound removal of oak barrel tartrates investigated. ► High power ultrasound removed 98% of tartrate volume to depth 2 mm at 60 °C. ► High power ultrasound removed > 85% of microscopic tartrates from stave surfaces. ► High power ultrasound tartrate removal at stave depth 2–8 mm is not reproducible.