Effects of ultrasound and microwave treatments on oenological yeast microbiota: comparison with a flash pasteurisation

The control of yeast biota in wine could be a challenge, as different genera could coexist with different effects on wine quality; generally, SO2 plays an important role in this context, while other approaches have been proposed. In this article, two Microwave (MW) and two Ultrasound treatments (US) were tested and compared to a flash pasteurisation on seven yeast strains (Saccharomyces cerevisiae, Saccharomyces bayanus, Brettanomyces spp., Schizosaccharomyces pombe, Hanseniaspora osmophila, Zygosaccharomyces bailii, Pichia membranifaciens); viable count (viability) and yeast growth kinetic (vitality) were assessed. The treatments reduced viable count by 27%, with a higher biocidal effect of US, depending on yeast and treatment conditions; they also impaired yeast growth causing a delay, which is a significant increase of the lag phase, while the effect on the growth rate or the maximum OD attained in the steady state was less significant. In conclusion, the US generally exerted a stronger effect than MW and flash pasteurisation and Schizo. pombe and Z. bailii were the less resistant yeasts. A partial uncoupling was found between vitality and viability for Z. bailii, while in Schizo. pombe the delay in the growth was generally related to the decrease in the cell count immediately after the treatment.