Chapter 1

Chemical Components of Grapes and Wine

Nitrogen Compounds in Grapes and Wine

Nitrogen-containing compounds are naturally occurring in grapes and extremely important during their cultivation. 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. The total nitrogen concentration of the fruit increases during the maturation period and with extended maturity it declines. Nitrogen-containing compounds include proteins, polypeptides, amino acids, and ammonium. Several conditions can reduce must nitrogen content. For instance, nitrogen content may can be reduced in grapes when infected by Botrytis cinerea (Jackson, 2008).

Protein Stability

Winemakers are primarily concerned with proteins in regards to wine stability. A long list of factors, from grape variety and climate to protein molecular size and interactions with other wine components, effect the exact type and concentration of proteins in wine. The phenomenon known as protein haze occurs when soluble proteins precipitate in bottled wines (Section 9.1). Protein haze makes the wine appear cloudy or highly turbid, considered a defect by most producers and consumers. It is likely composed of several compounds: soluble proteins, polysaccharides, insoluble protein-polyphenol complexes, and metal-protein complexes (proteins act as nuclei for soluble iron, copper, etc.).

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