Yeast and culture media The freeze-dried commercial yeast S. cerevisiae MERIT.ferm (Chr.-Han., Horsholm, Denmark) was propagated in a sterile synthetic medium (2% w/v glucose , 0.25% w/v yeast extract, 0.25% w/v bacteriological peptone and 0.25% w/v malt extract, pH 5.0) for 48 h at 25°C. The pure culture was stored at -80°C until the preparation of fresh juice pre-cultures described below.