Which yeast?

Karien O’Kennedy - 21 Feb 2018

active dried yeast 300p

The two most important considerations for an optimal fermentation are: 1 - the yeast must have the ability to ferment to dryness in the given conditions without becoming sluggish or stuck and within the time frame required for process optimisation; and 2 - the sensory profile of the yeast must complement and promote the varietal character of the grapes.

These two aspects go hand in hand. However, the yeast’s fermentation capabilities are always more important than its aromatic profile.

Important yeast facts:

  • The majority of white wine yeasts cannot ferment colder than 15°C. The best practice to follow is to decide on a certain 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 musts, even if you do a nutrient addition. The highest quality commercial nutrients can never offer the equivalent of a naturally high YAN must. Rather use a yeast with a low demand on initially low YAN juice.
  • Some yeasts can produce elevated levels of volatile acidity (VA) 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 Balling juice (the higher the Balling, the higher the VA concentration formed)
    • Low YAN juice
    • Juice with a turbidity lower than 50 NTU
  • Some yeast rehydration nutrients can lower VA formation in an indirect way 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 in terms of their affinity for fructose. The fructose utilisation ability of yeasts is a very important consideration in the case of Chardonnay and late ripeners such as Cabernet Sauvignon and Shiraz. It has been observed in South Africa that as early as at harvest Chardonnay already can have higher fructose than glucose concentrations. Cabernet and Shiraz are usually picked at fairly high sugar levels and, because the discrepancy between glucose and fructose concentrations becomes bigger the longer the fermentation, it can become a contributing factor to lagging or stuck fermentations.
  • A glucose:fructose imbalance is only when the ratio of glucose to fructose exceeds 1:10, e.g. 0.5 g/L versus 9 g/L fructose (0.5 x 10 = 5). It is not considered an imbalance if the ratio is smaller.
  • Glucose addition to adjust the ratio is illegal.
  • “Prevention is better than cure.” By fermenting red wine at lower temperatures, the toxicity of alcohol becomes less. Yeasts are sensitive to high temperatures at the start and at the end of fermentation and it therefore is advised to start and end fermentation at temperatures below 25°C. The use of 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, seeing that they are even 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 is not possible to distinguish a difference in mouthfeel at the concentrations found in dry wines.
  • Use thiol enhancing yeasts on grape varieties such as Sauvignon blanc, Chenin blanc and Colombard.
  • Use ester enhancing yeasts on grape varieties such as Chardonnay, Viognier and Riesling.
  • Use the recommended yeast dosage – in South Africa it is normally between 20 and 30 g/hL due to our specific conditions (cold fermentation, high sugars, etc.)
  • Rehydrate correctly and avoid temperature shocks. The latter affect yeast viability and aroma formation negatively.

 

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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