Jane Anson, Decanter’s chief Bordeaux reviewer, mentioned there could have been ‘potential issues with stuck fermentations’ in her Bordeaux 2018 vintage preview – in this case, due to high sugars and high pH levels.
‘A stuck fermentation is essentially an alcoholic fermentation that stops before the winemaker wants it to,’ said Matt Walls, DWWA regional chair for Rhône.
There are a range of factors that can cause this to happen, and it can be more of a problem for anyone not using temperature-controlled fermentation vats.
‘These days it’s often caused by a lack of nitrogen in the grapes, which yeast cells need to grow and develop,’ said Walls.
‘Very ripe grapes can also cause problems, because high sugar levels lead to high levels of alcohol, which can also pose a challenge to yeasts,’ said Walls.
What does this mean for the wine?
‘It’s a serious problem, because the part-fermented must is prone to bacterial spoilage and oxidation,’ said Walls.
‘Stuck fermentations can be very difficult to restart, particularly because when yeast dies it secretes a compound that inhibits the future growth of yeast cells in that batch.’
How can stuck fermentation be prevented?
As well as temperature control mentioned above, the ‘addition of nitrogen and cultured yeasts that are resistant to high temperatures and high alcohol levels can help prevent stuck fermentations’, said Walls.
‘But these may have undesirable effects on the finished wine, like affecting the flavour.’