Amino Acids/ Proteins
Nitrogen is taken up by grape vines in the form of nitrate, ammonia, or urea. Nitrate is reduced to ammonia and incorporated into the amino acids glutamate and glutamine. Nitrogen is then converted into other amino acids for protein synthesis. Amino acids act as building blocks for polypeptides and proteins. Of the twenty most-common amino acids, proline is found in the highest concentrations in must and wine because yeast do not utilize this amino acid. Arginine is also present in high concentrations in must, but it is not found in wine at significant levels because it is consumed by yeast.
Protein synthesis proceeds rapidly after veraison in the grape berries. Soluble protein levels tend to parallel accumulation of sugars with the most mature grapes having the highest levels. The protein level of the fruit is frequently higher in warmer regions. Higher levels of protein and total nitrogen are commonly associated with lower cropping vines. Protein levels in grapes have been found to be between 100 and 800 mg/L, depending on the cultivar.
Wine proteins have molecular weights in the range of 20- 40 kDalton. According to Boulton, et al, wine proteins seem to be subunits of cellular proteins from grapes that were disrupted by the pH change when berries were crushed. The solubility of wine proteins is highly dependent on the temperature, alcohol level, and pH. Changes in any of these parameters can result in protein precipitation within the wine. Proteins form complexes with polyphenols and polysaccharides in wines and are therefore more easily removed from wines with higher polyphenol concentrations. Wines low in phenols, such as whites and rosés, should be checked for protein stability before bottling.
Boulton, R., V. Singleton, L. Bisson, R. Kunkee. (1999). Principles and Practices of Winemaking. Gaithersberg, Maryland: Aspen Publishers.
Zoecklin, B., K. Fugelsang, B. Gump, F. Nury. Wine Analysis and Production. New York: Chapman & Hall. 1995.