Acids in wine ripened grapes. While concentration varies among grape varieties, it is usually found in higher levels with red wine grapes. The acid is created as a byproduct of the metabolization of nitrogen by yeast cells during fermentation. The combination of succinic acid with one molecule of ethanol will create the ester mono-ethyl succinate responsible for a mild, fruit aroma in wines. Acidity is highest in wine grapes just before the start of "veraison", which ushers in the ripening period of the annual cycle of grape vines. As the grapes ripen, their sugar levels increase and their acidity levels decrease. Through the
Quinta classification of Port vineyards in the Douro that the wines that were available were of high quality and not something that could tarnish the reputation of the Douro. Over the years the system of licensing growers turned into a rating classification operated by the IVDP. The system only pertains the production of Port. A quinta that is growing grapes for both Port and table wines, can do whatever they like with the grapes destined for table wine production regardless of its classification rating. Each year the IVDP classifies and rates the vineyards on an A-F scale with A being the highest level. A quinta's rating will dictate