How to handle high sugar musts

Karien O’Kennedy - 17 May 2018

Q: What classifies as high-sugar musts?

A: Any grapes arriving at the cellar at 24°Brix or higher

Q: Why is this a potential problem for yeasts?

A: Yeasts are living organisms and therefore have specific genetic capabilities and limitations. Very few yeasts can ferment to dryness at 26°Balling – no matter how good the conditions.

Q: What are the most important factors to bear in mind when fermenting high-sugar musts?

A: Initial grape sugar concentration, juice YAN, fermentation temperature, oxygen availability and potential alcohol. Botrytis infection is an additional factor that can make fermentation challenging.

Practical guidelines for dealing with high-sugar musts

  • It is only a minority of yeasts that can deal with high sugar levels. Select a yeast with a high alcohol tolerance, a low nutritional need and good fructose utilisation.
  • Depending on how high the sugar is, increase the yeast dosage to as high as 45 g/hL. This will compensate for the yeasts that will die as a result of the preservative effect of high sugar concentrations.
  • Use rehydration nutrition at the recommended dosage – usually about 30 g/hL. Rehydration nutrients provide the fermenting yeast with additional sterols and long chain fatty acids, which strengthen the yeast cell membrane, thereby enhancing viability and vitality.
  • Do not perform cold maceration and do not allow extended skin contact. The risk for microbial spoilage is too big. Use enzymes for flavour, colour and tannin extraction.
  • High sugars are often associated with high pH. Undertake the necessary pH/acid adjustments before fermentation.
  • Make sure the SO2 addition is sufficient (at last 50 ppm). If co-inoculation with bacteria is being considered (which is highly recommended), use a lower dose of SO2.
  • Avoid yeasts that form SO2 together with malolactic fermentation (MLF) co-inoculation.
  • If MLF co-inoculation will not be carried out, consider lysozyme addition to suppress bacterial growth during fermentation. This will allow the yeast time to finish fermentation at a natural pace, because competition with other microorganisms can hamper yeast growth. Take note that lysozyme is classified as an allergen. It also is of animal origin (hen egg).
  • Inoculate at 20°C and keep the fermentation temperature below 25°C. The closest to 20°C, the better. At these temperatures the alcohol is less toxic to the yeast cell membrane and the yeast therefore is able to function effectively for a much longer time, i.e. to take up sugar and nitrogen and to secrete hydrogen ions and alcohol into the medium. If the cell membrane starts struggling as a result of alcohol toxicity, these processes can no longer take place efficiently and the internal pH of the medium (6 – 7) falls to that of the surrounding medium (3 – 4). Most yeast enzymes do not function at wine pH and the fermentation therefore will stop, with associated cell death and a semisweet wine.
  • MLF bacteria are even more sensitive to alcohol toxicity, and co-inoculation will be affected negatively if the fermentation temperature is higher than 25°C.
  • Make sure that the temperature of the cap does not become too high by regularly punching down or pumping over. It must preferably not exceed 30°C.
  • Always use complex yeast nutrition for high-sugar grapes. It can be combined with DAP.
  • Ensure that the maximum admissible dosage (60 mg/L) of thiamine (a vitamin) is added.
  • If fermentation slows down towards the end, add pure yeast cell walls. These will bind to the medium-chain fatty acids that are formed by the stressed yeast in a desperate attempt to regenerate its cells membrane that was damaged. Long-chain fatty acid synthesis only takes place in the presence of sufficient oxygen and therefore is interrupted in an anaerobic fermentation. Medium-chain fatty acids in high concentrations are toxic to yeast and lactic acid bacteria.
  • Alternatively macro-oxygenation with molecular oxygen during fermentation can stimulate the production of sterols and long chain fatty acids and thus the regeneration of the cell membrane resulting in improved alcohol tolerance.  
  • Remove the wine from the skins as soon as possible. In sluggish fermentations the skins can be a source of spoilage organisms.
  • Also remove the dry wine from the yeast lees as soon as possible after fermentation and add sufficient amounts of SO2. The percentage of dead cells will be very high in the lees, which could potentially absorb colour. The lees can also become a source of sulphur-like off flavours. There are commercial products that rather could be considered to help with improving the mouthfeel and finish of the wine before bottling or bulk shipping.


To summarise:

Choose the correct yeast. Use enough yeast. Ferment at approximately 22°C (red wine). Use rehydration and complex nutrition.

Monitor the fermentation speed very closely. Contact your yeast supplier at the first sign of a slow or sluggish fermentation. A very close eye needs to be kept on volatile acidity in particular. Concentrations of 0.7 g/L and above that are accompanied by very slow fermentation can be an indication of bacterial growth (with the exception of certain yeasts for which this concentration can be normal under high sugar conditions).


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