Chapter 5

Yeast and Winemaking

Wine Yeast Nutritional Requirements

Yeast nutrition is an essential factor in the overall health and success of fermentation. Managing nutrient requirements not only allow for regular and complete fermentations but enhances sensory quality. Nitrogen, the most important yeast nutrient, is a key factor that has a significant impact on wine fermentation. Nitrogen is essential for yeast protein synthesis throughout fermentation. Vitamins are organic compounds essential for the optimum growth of yeasts cells and for their influence to survive under stressful conditions. A deficiency in vitamins can induce sudden changes in the fermentation kinetics. The majority of them act as enzymatic cofactors, but they can also intervene in energy transfers or in supporting the membrane integrity. For many years, not enough attention was given to the role of minerals in fermentation performance of the yeast. Nevertheless, minerals such as magnesium are absolutely essential for the growth and metabolism of the yeast, as well as zinc and potassium.

Nitrogen

Nitrogen is essential for yeast growth and yeast metabolism. It represents an important nutritional factor for yeasts during alcoholic fermentation due to its function in protein synthesis and sugar transport. Furthermore, the metabolism of nitrogen, and in particular that of amino acids, generates the formation of many aroma compounds involved in the aroma matrix of wine: higher alcohols, esters, and thiols by wine yeast.

YAN Requirements

The total nitrogen content of grapes is affected by variety, rootstock, climatic conditions, soil composition, vineyard management practices, fertilization, irrigation, rot incidence and grape maturity. Sometimes grapes naturally contain more than enough nitrogen for a satisfactory wine fermentation. More often they do not, especially with the higher Brix typical of today's harvests. Some grapes from some vineyards have notoriously low YAN levels almost every year. Most musts contain between 80 and 400 mg/L of YAN depending on grape variety, vintage, and microclimate (Reynolds, 2010).

Ammonium and Amino Acid Uptake

Amino acids composition of the must is quite important in the formation of numerous aroma compounds involved in the aroma matrix of wine including higher alcohols (fusel alcohols), acetate and ethyl esters (the main compounds conferring fruity and floral notes to wines) and thiols. When yeast is inoculated into grape juice, it immediately starts taking up whatever nitrogen is available. Ammonium ions are taken up the fastest and are the simplest to utilize. Yeast also takes up amino acids at this early stage with the accompanying hydrogen ions.

Vitamins

Vitamins are growth factors involved in yeast metabolism. Some vitamins are synthesized by the yeast itself and some need to be obtained from the medium. Grape must commonly have an ample supply of vitamins for fermentation, but deficiencies may occur under certain conditions. Various situations may result in vitamin deficiencies such as in the case of mold infestation (e.g., Botrytis) that deplete the on the grape berries of various nutrients or excessive DAP additions (leading to a vitamin imbalance rather than a deficiency).

Minerals

Minerals are components of the yeast cell membrane and help maintain fermentation metabolism activities. The elemental concentration of the soil, irrigation, yield, and canopy management play a role in mineral availability. For example, the concentration of these minerals is lower in overripe grapes and in those compromised by fungus or rot, which consume large quantities of minerals. Grape musts usually contain a sufficient mineral supply to ensure satisfactory fermentation.

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