The Riserva (Reserve) qualification is found in many DOCG and DOC wines in Italy. This qualification is meant for wines that are aged for at least two years (red wines) and one year (white wines). The Riserva aging time is also one year for sparkling wines produced with the Charmat method and three years for sparkling wines produced with the classic method of secondary refermentation in the bottle. Often, these wines may have an alcohol content greater than non-Riserva wines, the exact percentage of which (alcohol by volume) is determined by the disciplinare guidelines according to the specific characteristics of the wine. For DOCG wines, the Riserva (Reserve) qualification is related to mandatory ageing periods by three to five years. In simpler terms, a Riserva wine almost always refers to a wine that is supposedly of better quality and is aged for a longer amount of time in oak (usually one additional year). As the wine was supposedly made from higher quality grapes and is richer and more concentrated than the non-Riserva wine, it ought to be able to withstand time and benefit from more time in oak. Clearly, it is not always so and many Riserva wines are just overly oaked and fruit-challenged.
Source: Italian Wine Unplugged (Positive Press, 2017), p.31-32.