## 1 Introduction  
The chemical composition of grapes is influenced by various factors such as degree of maturity, variety, terror, and year. Organic acids, having essential effects on characteristic fruit flavor, play a significant role in grape quality criteria, and consequently in wine characteristics such as stability, color and flavor. Aidity and sugar balance is fundamental to enhance grape flavor, which determines the wine quality. Grape juice with low acidity often results in unstable musts and wines susceptible to organoleptic degradation. On the contrary, excessive berry acidity is undesirable [1]. Although having the same genotype, grapes harvested under a different climate have different organic acid contents [2]. During grape ripening, continuous warm conditions result in a lower acid content at maturity, primarily due to the increasing degradation of malic acid. Must corrections of the acid-base balance (most often) can improve wine quality, by increasing acidity through organic acid addition [3]. More recently, some research groups have evaluated different winemaking techniques to regulate the ethanol content and pH of wines in response to the effect of global warming on the composition of grapes. High interannual climate variability has been recorded during the ripening period, which strongly affects the composition of the grape. In particular, high temperatures during the ripening period cause an increased accumulation of sugars and degradation of acidity due to malic acid consumption, impacting the synthesis of polyphenols. In addition, thermal stress during the maturation period causes the degradation and inhibition of anthocyanins accumulation (compounds responsible for the color of grapes, and hence red wines) [4, 5]. According to the European Union rules and the International Code of Enological Practices [6], only organic acids can be added to musts and wines to increase the total acidity and thus decrease the final pH. Inorganic acids are forbidden. It is not just a matter of balancing the wine flavor but also promoting good biological evolution and good wine preservation. Indeed, wine acidity is mainly due to the organic acids from grapes, such as tartaric, malic, and citric acids. Among those, tartaric acid is the most stable and has a higher pH impact. Tartaric and malic acids account for 90% or more of the total acidity in grapes. Malic acid is metabolized by lactic acid bacteria during malalactic fermentation [7, 8, 9, 10]. The use of citric acid is only allowed in certain non-European winemaking countries. It is usually reserved for wine acidity correction, since lactic bacteria can metabolize it, thus increasing volatile acidity. Adding tartaric acid to the musts has been thought of as the traditional way to adjust the acidity. However, this methodology has limitations, including the costs of quality tartaric acid and the amount required to attain the desired pH decrease. Moreover, the added tartaric acid losses via precipitation of excessive potassium hydrogen tartrate, known as tartaric instability, leads to further acidity corrections. Other alternative strategies to adjust wine acidity include: (1) blending with higher acidity wines; (2) adding acids other than tartaric acid; (3) clustering; (4) the use of cation exchange resins; and (5) applying bipolar membrane electrodialysis [11]. Another option for pH modulation is to blend grapes that have been harvested at different ripeness stages, since it is well known that different grape varieties under the same edaphoclimatic conditions have different behavior, reaching a maturity level at different times [11]. Some studies have been published describing the applications of the unripe grapes in food and beverages [12, 13, 14, 15, 16, 17], and also different strategies to reduce the alcohol concentration and pH of wine using unripe grapes [18, 19, 20, 21, 22, 23, 24]. Unripe grapes can be picked from the period of bunch closure to _veraison_.