Fact #1 – The majority of wine yeasts belonging to the species Saccharomyces cerevisiae subsp. cerevisiae struggle to ferment colder than 15°C.
Practical advice – A good practice to follow is to decide on a certain drop in sugar per day, i.e. 2°Balling for white wine fermentations, and to control the temperature accordingly.
Fact #2 – 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 ripening cultivars, 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.
Practical advice – If you have a history of glucose fructose imbalances in grapes coming from particular vineyards:
- Make sure that the yeast chosen to conduct the fermentation has a good fructose utilisation. Yeast suppliers may have this information available.
- Ferment red musts at moderate temperatures (22 – 26°) instead of high temperatures (+30°C). It has been observed that yeasts’ fructose utilisation is better at moderate temperatures.
- Consider a yeast rehydration nutrient. These nutrients are an additional source of sterols and long chain fatty acids and can enhance yeast cell membrane integrity, thereby also enhancing proper sugar transport across the membrane.
- Measure the grape juice YAN. If necessary add a complex yeast nutrient one third into the fermentation to ensure enough building blocks (ammonia and amino acids) for sugar transport enzymes and co-factors (vitamins and minerals) to assist proper enzyme function.
Fact #3 – Some yeasts can produce elevated levels of volatile acidity (VA) during fermentation.
Practical advice – 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.