Merlot grapes were harvested with three maturity levels (21.1, 23.1, and 25.1 Brix), and processed with or without the application of microwave-assisted extraction (MW). The detailed phenolic composition and color were followed during winemaking. The MW treatment did not affect the basic chemical composition of the wines. Upon crushing, MW caused a 211% improvement in anthocyanins in the wines of the first harvest and an 89% improvement in the wines of the third harvest. At bottling, MW favored the formation of pyranoanthocyanins and tannin-anthocyanin dimers. Tannin extraction was not affected by MW just after application of this process, but improvements of 30, 20, and 10% on MW-treated wines of the first, second, and third harvest, respectively, were recorded at pressing. The formation of polymeric pigments during aging generally increased along with harvest date and was only favored in MW-treated wines of the first and third harvest, with preferential formation of small polymeric pigments, in accordance with enhanced anthocyanin extraction in these wines. Initial improvements of wine color upon application of MW in the wines of the first, second, and third harvest were of 275, 300, and 175%, respectively. Although these differences subsided or disappeared for the wines of the second and third harvest during aging, the wines of the first harvest treated with MW retained 52% more color than Control wines at day 150 post-crushing. Results suggest the MW treatment was more efficient in extracting and retaining phenolics and color when applied to unripe fruit.Fermentation 2019, 5, 15 2 of 16 varieties), and tannins (located both in skins and seeds) change during ripening, winemakers usually define harvest decisions based on a complex combination of factors that include weather forecast and disease pressure, fruit basic chemistry (including Brix, pH, and titratable acidity), and phenolic composition (including anthocyanins and tannins).Studies in Cabernet Sauvignon, Merlot, and Tempranillo have found a direct link between grape maturity, usually defined in terms of color and phenolic composition, and wine composition [1,3,8]. This accumulated knowledge has, in turn, allowed the production of wines with specific sensory features based on harvest time. Bindon et al. (2014) [9] studied the effect of five different harvest dates, with sugar levels spanning from 20 to 26 Brix, and corresponding alcohol contents in the finished wines spanning from 12 to 15.5% v/v. It was found that wine anthocyanins and tannins increased with harvest date, as well as an enhanced extraction of skin-derived phenolics [9]. From a sensory standpoint, later harvest dates produced wines with less green or vegetative attributes and more dark fruit attributes [1].Because grape maturity plays such a critical role in wine composition, it is expected that the outcomes of a given winemaking technique may be different based on when the fruit is harvested. In turn, variations in ethanol due to changes in fruit maturity may have an effect in phenolic e...