## 1 Introduction  
Over the past years, wine production has been dramatically affected by climate changes. The main effects detected in grape composition and production are the increase in sugar content, the earlier shift of the ripening period and the higher degradation of organic acids during the late stages of ripening [1]. Acidity decrease is a serious concern for the wine industry. The level of titratable acidity of musts, along with changes in other parameters such as the concentration of potassium cations and pH, directly affects the quality of wines because when the values of these parameters fall outside their optimal ranges, they have detrimental effects on the microbial and sensorial quality of the finished wines. More specifically, wines with high pH values are more susceptible to microbic contamination [2], and sulfur anhydride to avoid microbial and oxidative risks is required at higher concentrations [3]. Therefore, acidity is a primary driver of important management decisions related to contamination risks [4]. Acidity and pH are determinants also for the sensorial properties of wines, although a direct correlation between pH and sensory attributes is not possible because of several factors, including the human physiology that determines the sourness perception, the importance of the buffering power and the whole gustative equilibrium during the wine tasting [5, 6].  
Several acidification practices, such as the addition of organic acids, the use of cation exchange resins and bipolar membrane electrodialysis, are permitted by the O.I.V. [7] as well as by specific country regulations. Across the European Community, acidification is allowed only in certain winemaking regions (Regulation EU 1308, 2013). However, this practice can be carried out only if the initial acidity content of the wine is not increased by more than 1.5 g/L for musts and 2.5 g/L for wines (expressed in tartaric acid) (respectively 20.0 and 33.3 meq/L) (Regulation EU 1308, 2013).  
A crucial decision for winemakers is the timing of the acidification treatment, especially as to whether it would be better to acidify musts or wines. In the first case, the proliferation of spoiling lactic bacteria and other bacteria during alcoholic fermentation is prevented [2], but the levels of pH and acids in musts cannot be easily regulated because they could change in finished wines. In fact, during fermentation, great variations in pH and tittatable acidity occur as a result of the activities of yeasts and bacteria [2] and of the precipitation equilibria linked to potassium bitartrate formation [3]. In addition, high levels of pH during fermentation could affect the metabolism of yeasts and the concentration of yeast-related compounds such as those involved in the formation of new pigments and of new polymeric tannic structures [8].  
In this regard, the most important compound for wine quality is acetaldehyde, which is differently involved in both fermentation and aging. Besides being produced during fermentation through microbial metabolism, acetaldehyde can also be formed by oxidation reactions [9]. Acetaldehyde is a highly reactive compound, and in wines it undergoes several reactions, including those with sulfur dioxide, with flavonoids thus altering wine pigments, with polymeric tannins and with several other nucleophiles. Although reactions in which acetaldehyde is involved are of paramount importance for wine characteristics such as color and tannins [10], there are no studies yet about the effect of acidification management on acetaldehyde, pigments, color and tannins and on their evolution over time.  
In previous studies, the impact of pH on the anthocyanin chemistry in model solution and real wine was investigated [11; 12]. Herein, the effect of both early (on musts, during fermentation) and late (on wines 7 days after the end of the fermentation) acidification on the acetaldehyde content, wine phenolics and color was evaluated over a two-year aging.