*3.2.2.1 Alcohols*

(*O*-α-L arabinofuranosyl-β-D-glucopyranoside, 6-*O*-α-L rhamnosyl-β-Dglucopyranoside and 6-*O*-β-D apiosyl-β-D-glucopyranoside) [66].

*Chemistry and Biochemistry of Winemaking, Wine Stabilization and Aging*

its optimal activity is developed in other pH values [67].

*3.1.2.5 Volatile phenols*

in other wines [68].

**3.2 Secondary aroma**

*3.2.1 Pre-fermentative aroma*

the type of variety [70].

*3.2.2 Fermentative aroma*

**58**

methyl vanilla or homovainyl alcohol [69].

variety [46].

these compounds during winemaking, under normal winemaking conditions, glycosylated precursors have a poor effect on the development of the aroma since

Although grapes contain enzymes (β-glycosidases) capable of releasing some of

These compounds can be responsible for the originality or wine aroma defects. Phenolic compounds are non-odorous compounds, which can be submitted to different enzymatic reactions, transforming into volatile phenols and contributing to unpleasant aromas of pharmacy, smoke, forest, leather or pepper. These compounds are characteristic of carbonic maceration wines, although they are present

Among phenolic acids, caffeic, ferulic and *p*-coumaric are found in the pulp and skin in the form of tartaric esters (caftaric acid, feruloyl tartaric acid) and coutaric (*p*-coumaroyl tartaric). Its olfactory threshold is very low and they strongly decrease during the ripening of the grape, with large variations within the same

During vinification, free acids and then volatile phenols are formed. The process of hydrolysis causes the appearance of other phenolic compounds such as vanillin,

This type of aroma is developed during the winemaking process, more specifically, between the harvest of the grapes and the beginning of the fermentation. The

Fermentation is the main phase in the transformation of grape juices into wine, with two biological transformations occurring at this stage: alcoholic (yeast transform sugars into alcohol and other secondary products) and malolactic fermentation (lactic acid bacteria cause the degradation of malic acid in lactic acid) [71]. While malolactic fermentation subtly modifies the aroma of wine, alcoholic fermentation is of great importance in the aroma since it is responsible for the winey note that constitutes the common aromatic base for all wines. Moreover,

mechanical processes performed during this stage, such as grape transport, crushing, de-stemming, pressing and even carbonic maceration have a great influence in the pre-fermentative aroma. All these processes involve grape cellular rupture, allowing enzymes to come into contact with the aroma precursors. Thus, the availability of aroma precursors is directly proportional to the degree of grape rupture. Moreover, the mechanical process also favors the incorporation of oxygen, leading to enzymatic oxidation reactions. Throughout the process of crushing grapes, relatively large amounts of aldehydes and alcohols of 6 carbon atoms are formed. In some cases, these compounds give the smell of cut leaves, bitter taste and low olfactory threshold. The formation of C6 aroma compounds varies depending on the ripeness of the grape, although the greatest potential occurs before the ripening date, where grape lipids levels are higher and also depends on

Main alcohols synthesized in alcoholic fermentation are 2- and 3-methylbutanol, 1-propanol, 2-methylpropanol, 1-butanol, 1-pentanol, 2-phenylethanol, 3-methylthio-propanol, tyrosol and tryptophol, being its average total content between 400 and 500 mg/L. These compounds are produced at the metabolism of amino acids, so their concentration depends on the nitrogen content of the grape juice, the yeast species and the factors that influence their development [73].
