Factors that affect alcoholic fermentation

by | Dec 1, 2023 | Practical in the cellar, Technical

The alcoholic fermentation of grape juice is affected by the yeast strain, sugar concentration, alcohol concentration, fermentation temperature, presence of oxygen, concentration of assimilable nitrogen, vitamins and minerals, survival factors, preservatives, carbon dioxide and volatile acidity. Yeast suppliers usually have a variety of yeast strains, which are selected to meet the conditions and needs of the cellar.

 

Yeast strains

The action of yeasts is partly determined by their genetic makeup, which depends on the species and strain. Yeast strains differ in their fermentation kinetics, nitrogen requirements, ethanol resistance, temperature tolerance, as well as glucose and fructose consumption. These differences are more evident under stress conditions, which are determined by the adaptability of yeast strains.

Yeast strains are selected for commercialisation based on the aromas and flavours they produce or enhance in wines, their temperature and alcohol tolerances, their nutrient demands and formation of volatile acidity, their suitability to ferment in barrels and their sensitivity to sulphur dioxide (SO2).

 

The sugar concentration of the grape juice

Yeasts are living organisms. Due to differences in genetic makeup, there are limitations to what some of them can and cannot do. The amount of sugar in grape juice determines the alcohol formed during alcoholic fermentation. If the sugar concentration of the juice is very high, it can result in high alcohol concentrations, which can slow down the fermentation. As a rule, many yeasts struggle to ferment 26°B to dryness under winemaking conditions, but exceptions do occur.

Glucose and fructose are the two main sugars found in grape juice. Yeasts differ in their preference to ferment glucose and fructose. Yeasts that prefer glucose are called glucophilic yeasts, and those that prefer fructose are called fructophilic yeasts. Saccharomyces cerevisiae strains are glucophilic yeasts, while Starmerella bacillaris strains can be fructophilic. Fructose tastes sweeter than glucose, and if the fermentation stops before all the sugars have been fermented, it can taste different depending on whether residual fructose or glucose is still in the wine. Fructose consumption can be promoted by good yeast nutrition management during fermentation.

 

Alcohol

Alcohol has an inhibitory influence on alcoholic fermentation, but yeast strains differ in alcohol sensitivity. The inhibitory effect increases with increasing alcohol concentration. However, alcohol concentrations of up to 17% can be obtained with specific yeast strains.

 

Fermentation temperature

The optimum temperature of yeasts is approximately 30°C. Temperatures lower and higher than this affect yeasts. When making white wine, fermentation is usually done at lower temperatures to limit the loss of aroma and flavour. If the temperature drops too low, the fermentation can become stuck. Fermentation of white wines in stainless steel tanks usually occurs at 12 to 18°C. In barrels, like in the case of Chardonnay, the temperatures can be higher. Red wines typically ferment with the skins at higher temperatures of 25 to 30°C to extract sufficient colour and tannin from the skins. If the fermentation temperature rises too high, the fermentation can lag or become stuck.

 

Presence of oxygen

Yeasts can multiply under aerobic (presence of oxygen) and anaerobic (absence of oxygen) conditions. However, the biomass increase is slower due to the lack of oxygen. At the beginning of fermentation, there may be dissolved oxygen in the juice, but the fermentation becomes anaerobic very quickly as soon as it is actively underway due to the absorption of oxygen by the yeast and the carbon dioxide that it forms. The initial bit of dissolved oxygen benefits the yeasts during their exponential growth phase, increasing the biomass needed to complete the fermentation.

Aeration (which contains oxygen) is sometimes used to restimulate lagging or stuck fermentations or to react with off-odours such as hydrogen sulphide (H2S) (which smells like rotten egg) and thereby remove the off-odour.

 

Assimilable nitrogen

Besides sugar, nitrogen is the most essential nutritional requirement of yeasts. Grape juice contains several nitrogen-containing components, but yeasts can only assimilate certain ones. That is why assimilable nitrogen is spoken of in winemaking terms. It is measured in mg/L YAN (yeast assimilable nitrogen). YAN consists of amino acids and ammonium. Proline, an amino acid, is excluded from YAN measurements because the yeast cannot absorb it under anaerobic fermentation conditions. During fermentation, yeasts take up amino acids and ammonium and use them for protein synthesis. Examples of proteins that yeast needs to survive are enzymes for sugar metabolism and permeases for transporting sugars and amino acids across the cell membrane. The rest of the proteins are used to form new yeast cells.

Nitrogen is also vital in wine aroma, especially in preventing sulphurous off-odours. It has been proven that a shortage of nitrogen during fermentation can lead to the formation of hydrogen sulphide (H2S) and other sulphur-containing off-odours. Grape juice often contains too little assimilable nitrogen to sustain the yeast. Yeasts differ in their nitrogen requirements and preferences for ammonium and amino acids. It is important to supplement the nitrogen during fermentation using commercially available yeast nutrients.

 

Vitamins and minerals

Vitamins and minerals act as cofactors in enzymatic reactions and can, therefore, be used repeatedly by the yeast. Grape juice usually contains enough vitamins and minerals, but certain circumstances can lead to a deficiency. An example is when grapes have been infected with Botrytis. Botrytis needs several nutrients to survive; one is thiamine (vitamin B1). Juice from Botrytis-infected grapes contains very little or no thiamine, an essential vitamin for yeast metabolism. A thiamine deficiency during fermentation can lead to the build-up of specific metabolites that can bind sulphur, and this is one of the reasons why it is so difficult to stabilise the free sulphur dioxide of noble late harvest wines. Other essential vitamins are biotin and pantothenic acid. Pantothenic acid is involved in the synthesis of the sulphur-containing amino acids methionine and cysteine. A pantothenic acid deficiency can lead to H2S formation, even if enough nitrogen is present. Vitamins can be added to fermentations in the form of rehydration nutrients, complex yeast nutrients and organic yeast nutrients.

 

Survival factors

In the case of white wine preparation, the juice is usually clarified after it has been separated from the skins. Alcoholic fermentation can be slow if the juice is excessively clarified. This happens because yeasts need sterols and long-chain fatty acids (survival factors) to strengthen their cell membranes for the high osmotic environment (juice) and for the alcohol formation that will follow. Yeasts can produce these themselves, but only in the presence of oxygen. Once fermentation starts, the environment quickly becomes anaerobic, and yeasts can no longer produce survival factors. Consequently, they absorb it from their external environment, like plant (grape) sterols and long-chain fatty acids in the juice. Very clear juice contains very few survival factors and consequently slows the multiplication of yeasts and the onset of fermentation. Specific rehydration nutrients can provide yeasts with sterols and long-chain fatty acids to compensate for the deficiencies in the juice.

 

Inhibiting factors

All preservatives have a negative influence on yeasts but differ in the extent to which they affect yeasts.

  • Sulphur dioxide, commonly used in winemaking, is also a preservative that can affect yeasts.
  • Some sprays sprayed for vine diseases negatively influence yeasts, and their safety periods must therefore be observed.
  • Carbon dioxide, formed during alcoholic fermentation, has little effect on yeasts except when pressure is applied together.
  • High volatile acidity can have a negative influence on alcoholic fermentation.

 

Conclusion

Winemakers must remember that yeasts are sensitive living organisms and that fermentations can sometimes lag or get stuck due to one or more of the abovementioned factors. It is important to act proactively to prevent lagging or stuck ferments rather than reactively when there may already be damage to wine quality.

 

References
  1. Coulon, J., Seabrook, A. & Erasmus, S., 2019. Avoiding stuck fermentations. Australian and New Zealand Grapegrower and Winemaker, December: 48 – 54.
  2. O’Kennedy, K. & Reid, G., 2008. Yeast nutrient management in winemaking. Australian and New Zealand Grapegrower and Winemaker, October: 92 – 100.

 

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