Various cellars experienced problems with stuck fermentation, in red wine especially, during the 1998 season. What were the causes and consequences thereof and which measures may be taken in the coming season

Problems with slow and stuck fermentation in wines are not new. Since 1978 these problems have been occurring to quite an extent and various research projects have been initiated to identify the problems. In the past fermentation problems occurred mostly in white wines. Research has indicated that the following factors in particular are the most important causes:

  • Low assimilable nitrogen concentrations (FAN) in juice. Certain cultivars contain inherently lower levels of FAN, e.g. Chenin blanc, Shiraz and Cinsaut. Viticultural factors such as excessive growth during ripening and bad weed control contribute to lower FAN concentrations. It is also known that levels of FAN are reduced by dry conditions during ripening and Botrytis infection.
  • Spraying residue on grapes that may inhibit the fermentation cells.
  • Bad handling of yeasts, in particular incorrect rehydration techniques and inoculation concentrations.
  • Yeast strains not adapted to specific conditions.
  • Inadequate temperature control during fermentation, especially during the last phase of fermentation.
  • Microbiological factors such as killer yeasts.

Specific causes of stuck fermentation in red wine are not known, however. If conditions during the 1998 season are taken into account, as well as provisional research results at the University of Stellenbosch, the following factors may be highlighted as possible causes:

  • Higher sugars than normal in grapes. The heat in February and the rapid ripening during the latter part of the season meant that Ballings of higher than 23 were not unusual. Alcohol concentrations of higher than 13 vol% are common in some 1998 wines. High alcohol concentrations such as these inhibit yeast cells, especially when other conditions are also unfavourable.
  • Lower doses of SO2 or none whatsoever applied before fermentation. Results have indicated that such wines contain more bacteria after fermentation that may easily cause spoilage if stuck fermentation occurs.
  • Occurrence of unusually high counts of natural micro-organisms on grapes, especially bacteria.
  • Higher fermentation temperatures used for red wines, especially >30C.
  • Inadequate control of fermentation temperature, especially during the first few days of fermentation.
  • Smaller berries during the 1998 season. Not all berries are crushed in the press. During separation of must and skins and at the time of post-fermentation pressing, however, the berries are crushed, resulting in the release of unfermented sugars trapped in the berry. Results have indicated that the Balling may rise by up to 2 degrees under these circumstances. These increased sugar concentrations together with higher alcohol levels may have an inhibiting osmotic effect on the yeast cells towards the end of fermentation, which may contribute to stuck fermentations.

The consequences of stuck fermentation, in red wines especially, are mostly that the wines develop high volatile acid and later spoil microbiologically. The quality is such that the wines cannot be marketed and this may sometimes entail great financial losses for cellars.

Recommendations for the 1999 season

The following measures will definitely help to reduce or prevent the occurrence of stuck fermentation in red wines:

  • Better control over all fermentation factors in the case of grapes with sugar concentrations higher than 23Balling. Recommendations in this regard include stronger inoculations with pure yeast (50 g/hl) and better temperature control.
  • The use of SO2 before fermentation. Dosages of 30 mg/kg in healthy and 50 mg/kg in grapes that have been slightly subject to rot. The bacteria population is thus suppressed.
  • Increase of the nitrogen levels before fermentation in the case of cultivars with low FAN.
  • Temperature control, especially in the first two to four days of fermentation. Also keep the skin crust moist by regular racking, rotation or pressing through during that period. Temperatures above 32C may be dangerous.
  • Do not use unknown yeast strains on all your wines. Ascertain yourself of the expiry dates and conditions under which the preparations have been stored.
  • Maintain hygienic conditions in your cellar so as to limit the growth of undesirable and potentially dangerous micro-organisms.

The Author:
LP Ellis, Department of Viticulture and Oenology, University of Stellenbosch

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