The two most important considerations for optimal fermentation are, firstly, the yeast(s) ability to ferment to dryness in the given conditions without becoming sluggish or stuck and within the time frame required for process optimisation, and secondly, the sensory profile of the yeast.
These two aspects go hand in hand. However, the yeast’s fermentation capabilities are always more important than its aromatic profile.
Recommendations to consider:
- Most white wine yeasts cannot ferment colder than 15°C. The best practice to follow is to decide on a specific drop in sugar per day, i.e. 2°Balling, and to control the temperature accordingly.
- Do not use yeasts with a high nutrient demand on low YAN (Yeast Assimilable Nitrogen) musts, even if you add a nutrient. The highest quality commercial nutrients can never offer the equivalent of a naturally high YAN must. Instead, use yeast with a low demand on initially low YAN juice.
- Some yeasts can produce elevated volatile acidity (VA) levels during fermentation. There are times when this can be managed, and then there are times when the use of such yeasts must be avoided, i.e.:
- Any degree of Botrytis infection where the juice already has a baseline VA
- High sugar juice (the higher the °Balling, the higher the potential VA concentration formed)
- Low YAN juice
- Juice with turbidity lower than 50 NTU
- Some yeast rehydration nutrients can indirectly lower VA formation by strengthening the yeast cell membrane. Yeast-yeast co-inoculations can also lower the final VA concentration in the wine.
- Yeasts (Saccharomyces cerevisiae) always utilise glucose faster than fructose. However, there are differences between yeasts regarding their affinity for fructose. The fructose utilisation ability of yeasts is a crucial consideration in the case of Chardonnay and late cultivars such as Cabernet Sauvignon and Shiraz. It has been observed in South Africa that Chardonnay can already have higher fructose than glucose concentrations as early as harvest. Cabernet and Shiraz are usually picked at reasonably high sugar levels. Because the discrepancy between glucose and fructose concentrations becomes bigger the longer the fermentation, it can contribute to lagging or stuck fermentations.
- Glucose addition to adjust the glucose/fructose ratio is not permitted.
- “Prevention is better than cure.” Fermenting red wine at lower temperatures lessens the toxicity of alcohol. Yeasts are sensitive to high temperatures at the start and end of fermentation; therefore, it is advised to start and end fermentation at temperatures below 25°C. Using commercial enzymes during fermentation will ensure that the same amount of extraction or more can be obtained as in the case of high fermentation temperatures.
- Lower red wine fermentation temperatures are also more favourable for MLF bacteria, as they are more sensitive to alcohol than yeasts.
- Avoid yeasts that form SO2 during fermentation if MLF is being considered.
- Avoid yeasts with high nutrient demands on Shiraz, which has a greater tendency to reductiveness.
- Do not use glycerol formation as a criterion for yeast selection. It isn’t easy to distinguish a difference in mouthfeel at the concentrations found in dry wines.
- Generally, use thiol-enhancing yeasts on grape varieties such as Sauvignon blanc, Chenin blanc and Colombard and ester-enhancing yeasts on grape varieties such as Chardonnay, Viognier and Riesling.
- Use the recommended yeast dosage – in South Africa, it is generally between 20 and 30 g/hL due to our specific conditions (cold fermentation, high sugars, etc.)
- Rehydrate correctly and avoid temperature shocks. The latter affects yeast viability and aroma formation negatively.
- Finally – follow your yeast supplier’s usage instructions. Their recommendations are based on scientific facts and practical feedback from various winemakers over various harvests in many countries. Contact them at the first signs of a lagging fermentation. If you act timeously, the fermentation can be completed.
References:
Gafner, J., Hoffmann-Boller, P., Porret, N.A. & Pulver, D. 2000. Restarting sluggish and stuck fermentations. Paper: 2nd International Viticulture and Enology Congress, 8–10 November, Cape Town, South Africa.
Nieuwoudt, H.H., Prior, B.A., Pretorius IS & Bauer FF. 2002. Glycerol in South African table wines: an assessment of its relationship to wine quality. S Afr J Enol Vitc 23:22–30.
Ribéreau-Gayon, P., Dubourdieu, D., Donèche, B., Lonvaud, A. 2006. Conditions of yeast development. In: Handbook of Enology II ed. John Wiley & Sons, Ltd. 80 – 133.
Specht, G. 2010. Yeast fermentation management for improved wine quality. In: Managing wine quality, Woodhead Publishing Vol. 2. 3 – 33.
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