The Australian Wine Research Institute

Allowing a period of post-fermentation contact between wine and fermentation lees in barrel, is a traditional winemaking method in many parts of Europe. It is most commonly associated with Burgundian Chardonnay or Muscadet where the term ‘sur lie’ or ‘on lees’ originated to describe the practice.

Background

Fermentation lees are primarily made up of yeast cells, tartaric acid salts, bacteria, phenolic material and remnant grape solids. Over time, yeast cell components are released into wine due to both the passive transfer from cells and their autolysis. Those components, coupled with other mechanisms, result in an enhancement of the wine’s sensory characters, with the reported benefits including increased texture; greater oak, fruit, and yeast integration; more complexity; and improved oak aromas and flavours. The periodic stirring of lees, known as ‘batonnage’ in French, is often performed, although the practice has pros and cons.

The most important yeast-cell-derived compounds for improving wine sensory properties are amino acids, polysaccharides, mannoproteins and fatty acids. Amino acids have been related to enhancements in both wine texture and aroma, with a strong positive correlation between amino acid concentration and wine score. In addition, the presence of yeast cells can lead to the production of positive organoleptic compounds through oxidation reactions and modifications to oak-derived aroma and flavour.

Another benefit is the ability of lees to scavenge oxygen entering through the barrel staves and the bunghole, especially when they are stirred. This oxygen-scavenging capacity can result in lower concentrations of SO₂ being required to prevent oxidation, although excessive stirring can introduce too much oxygen leading to the loss of SO₂ and potentially the formation of acetaldehyde and acetic acid. Topping barrels will also introduce additional oxygen.

Practical considerations and helpful hints

Lees ageing is most commonly performed in small oak barrels, because lees ageing of wines fermented in tank is more likely to result in the formation of unwanted forms of volatile sulfur compounds (VSCs) because of the relatively lower oxygen concentration. However, this problem can be largely negated if the lees from tank-fermented wines are stored in barrels for a few days and later recombined with the wine in tank. The procedure can be repeated if VSCs subsequently form.

The stirring of lees encourages continued cell viability, and while it has been shown that cell viability is not an important factor in the improvement of organoleptic properties associated with lees contact, viability does encourage the assimilation of any residual sugar. Therefore, a small amount of stirring at the end of fermentation is recommended.

When unwanted VSCs are present in a barrel-fermented wine at the end of fermentation, transferring the lees into barrels for 48 hours and then recombining them with the wine has been found to completely eliminate ethyl and methyl mercaptan within 24 hours, with the concentration of hydrogen sulfide also falling by over 80% during the following four weeks (Lavigne-Cruège and Dubourdieu 2001).

Potential risks

of ‘leesy’, ‘cheesy’ sensory characters, or the formation of acetaldehyde and acetic acid. Unwanted VSCs are more likely to form when initial juice solids are above 200 NTU, or when insufficient oxygen is introduced by stirring, with acetaldehyde and acetic acid being more likely to form when excessive oxygen is introduced by stirring and topping.

In a study by Brajkovich (2008) a barrel-fermented Chardonnay was subjected to four treatments:

1. Neither stirred nor topped
2. Topped every two weeks without stirring
3. Stirred every two weeks without topping
4. Topped and stirred every two weeks

The wine that was neither stirred nor topped was preferred after four months, being described as “… fresher and with more varietal definition …. less signs of oxidation or the dullness that can be the result of excessive batonnage”.

The formation of biogenic amines in Chardonnay on lees for 180 days has been studied, with the concentrations of tyramine and histamine in particular, being higher than in the same wine stored in barrels without lees, particularly when the wine was stirred. The concentrations were found to be rising steadily at day 180, suggesting that the concentrations may have continued to increase to a point where they could cause problems for sensitive individuals (González-Marco and Ancín-Azpilicueta 2006).