Wednesday, May 6, 2015

Non-Nitrogen yeast nutrition requirements during alcoholic fermentation

In addition to their utilization of nitrogen as a nutrient during alcoholic fermentation (AF), yeasts synthesize and/or assimilate a number of factors (growth, co, and survival) that help them compete and survive in the hostile environment that is the active fermentation vessel.

Vitamins are growth factors involved in yeast metabolism with some of the needed species synthesized while others are obtained from the juice/must. The most important vitamins in the AF process are indicated in the table below.

Vitamin Characteristics
Biotin Plays an important role in sugar, nitrogen, and fatty acid metabolism
Yeasts cannot grow without biotin (low concentrations will support growth but 10 microgram/L optimal)
Supplementation of musts with biotin increases the viable yeast population and increases fermentation rate
Thiamine Deficiency leads to accumulation of various products including pyruvic acid which is responsible for much of the non-acetaldehyde sulfite pool
Pantothenate Involved in the biosynthesis of the sulfur-containing amino acids cysteine and methionine
Involved in the formation of acetyl Co-A (acetate donor in esterification)
Deficiency results in increased hydrogen sulfide production as well as higher concentrations of acetic acid and glycerol (the latter two producing negative results when coupled)
Pyridoxine Involved in the biosynthesis of the sulfur-containing amino acids cysteine and methionine
Deficiency results in increased hydrogen sulfide production
Source: Kennedy and Reid

Adequate levels of vitamins are normally available to the yeasts but, in some cases (mold infestation, excessive diammonium phosphate addition), a deficiency is recorded and supplements are required. Inactivated yeasts are an excellent source of vitamins.

Yeast cell membrane integrity will be affected by the toxicity of ethanol and the increase in permeability associated with higher ethanol levels will have a negative impact on sugar and amino acid uptake. So-called survival factors, comprised primarily of sterols and long chain fatty acids, are responsible for cell membrane integrity and fending off the ethanol effects until fermentation is complete. Survival factors are formed only in the presence of oxygen and grape musts normally contain enough dissolved oxygen which, when combined with the use of active dry yeast, allows the synthesis of adequate amounts of these factors.

If ascorbic acid is added to the must for any reason, no additional survival factors will be synthesized. Naturally occurring sterols and fatty acids will be depleted by excessive must clarification. In the case of a deficiency of these survival factors, the addition of inactivated yeast cells or yeast hulls will provide a rich source of sterols and fatty acids. Such additions should be adde at the beginning of fermentation and should utilize fresh material to avoid the negative effects associated with lipid oxidation.

Minerals are used as co-factors in enzymatic reactions in the yeast cell with the most important ones being magnesium, potassium, manganese, zinc, iron, and copper. The grape must normally contains adequate levels of these minerals to support AF to its conclusion.

©Wine -- Mise en abyme

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